U.S. patent application number 11/834062 was filed with the patent office on 2008-07-31 for magnetic encoder having a stable output property with unsaturated magnetic sensor.
Invention is credited to Yasunori ABE, Hiroyuki HOSHIYA, Kenichi MEGURO, Kazuhiro NAKAMOTO, Yasuyuki OKADA.
Application Number | 20080180864 11/834062 |
Document ID | / |
Family ID | 39596685 |
Filed Date | 2008-07-31 |
United States Patent
Application |
20080180864 |
Kind Code |
A1 |
MEGURO; Kenichi ; et
al. |
July 31, 2008 |
MAGNETIC ENCODER HAVING A STABLE OUTPUT PROPERTY WITH UNSATURATED
MAGNETIC SENSOR
Abstract
The present invention provides a magnetic sensor suitable for
high resolution and having high reliability by achieving stable
output even at the occurrence of variations in a gap between a
magnetic medium and the magnetic sensor, and a magnetic encoder
using the magnetic sensor. The present invention uses a
magnetoresistive element having magnetoresistive properties that
satisfy the inequation, H10-50<H50-90, where H10-50 represents a
magnetic field required for a resistance change from
.DELTA.R.times.10% to .DELTA.R.times.50% with respect to a maximum
amount of resistance change .DELTA.R on a magnetoresitance effect
curve, and H50-90 represents a magnetic field required for a
resistance change from .DELTA.R.times.50% to
.DELTA.R.times.90%.
Inventors: |
MEGURO; Kenichi; (Kaisei,
JP) ; HOSHIYA; Hiroyuki; (Odawara, JP) ;
NAKAMOTO; Kazuhiro; (Ninomiya, JP) ; OKADA;
Yasuyuki; (Tokyo, JP) ; ABE; Yasunori; (Moka,
JP) |
Correspondence
Address: |
ANTONELLI, TERRY, STOUT & KRAUS, LLP
1300 NORTH SEVENTEENTH STREET, SUITE 1800
ARLINGTON
VA
22209-3873
US
|
Family ID: |
39596685 |
Appl. No.: |
11/834062 |
Filed: |
August 6, 2007 |
Current U.S.
Class: |
360/324.11 |
Current CPC
Class: |
G01R 33/093 20130101;
B82Y 25/00 20130101 |
Class at
Publication: |
360/324.11 |
International
Class: |
G11B 5/127 20060101
G11B005/127 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 31, 2007 |
JP |
2007-021372 |
Claims
1. A magnetic sensor, comprising 4n magnetoresistive elements
(where n denotes a natural number), each of the magnetoresistive
elements including a ferromagnetic pinned layer and a ferromagnetic
free layer stacked one on top of another with a non-magnetic
intermediate layer in between, each of the magnetoresistive
elements having a substantially rectangular shape, the magneto
resistive elements substantially the same magnetoresistive
properties, the ferromagnetic pinned layers having the same
direction of magnetization, the 4n magnetoresistive elements
constituting first and second element groups each formed of 2n
elements, the magnetoresistive elements that constitute each of the
element groups being equidistantly disposed as spaced a distance
.lamda. away from each other across the front-end and rear-end
elements in a direction of a short side of the element and being
series-connected in a direction of a long side of the element, the
rear-end element of the first element group and the front-end
element of the second element group being located as spaced a
distance .lamda./2 away from each other in the direction of the
short side of the element, the front-end element of the first
element group being connected to an electric power supply while the
rear-end element thereof being connected to the front-end element
of the second element group, the rear-end element of the second
element group being grounded, and an external magnetic field being
detected through midpoint electric potential from a connection
between the first element group and the second element group,
wherein the magnetoresistive element satisfies the inequation,
H10-50<H50-90, where H10-50 represents a magnetic field required
for a resistance change from .DELTA.R.times.10% to
.DELTA.R.times.50% with respect to a maximum amount of resistance
change .DELTA.R in process of a resistance change occurring
according to the external magnetic field parallel to the direction
of the short side of the element, and H50-90 represents a magnetic
field required for a resistance change from .DELTA.R.times.50% to
.DELTA.R.times.90%.
2. The magnetic sensor according to claim 1, wherein the
magnetoresistive element satisfies the inequation,
1.5<H50-90/H10-50<4.0.
3. The magnetic sensor according to claim 1, wherein the
non-magnetic intermediate layer has a locally nonuniform
distribution of layer thickness.
4. The magnetic sensor according to claim 1, wherein the
ferromagnetic pinned layer and the ferromagnetic free layer have
ferromagnetic interlayer interaction with the non-magnetic
intermediate layer in between, and the magnitude of the interlayer
interaction is locally nonuniform.
5. The magnetic sensor according to claim 1, wherein the direction
of magnetization of the ferromagnetic pinned layer is the direction
of the short side of the element.
6. The magnetic sensor according to claim 1, wherein the direction
of magnetization of the ferromagnetic pinned layer deviates from
the direction of the short side of the element within a range of
angles of 30 degrees or less.
7. A magnetic encoder, comprising: a magnetic sensor; and a
magnetic medium having magnetized areas, the directions of
magnetizations of which are periodically alternately reversed, and
in which the sum of the lengths of a pair of adjacent magnetized
areas is equal to 2.lamda., wherein the magnetic medium moves
relative to the magnetic sensor in a direction of arrangement of
the magnetized areas, facing the magnetic sensor with a
predetermined gap in between, the magnetic sensor includes 4n
magnetoresistive elements (where n denotes a natural number), each
including a ferromagnetic pinned layer and a ferromagnetic free
layer stacked one on top of another with a non-magnetic
intermediate layer in between, the magnetoresistive elements having
a substantially rectangular shape and substantially the same
magnetoresistive properties, the ferromagnetic pinned layers having
the same direction of magnetization, the 4n magnetoresistive
elements constitute first and second element groups each formed of
2n elements, the magnetoresistive elements that constitute each of
the element groups are equidistantly disposed as spaced a distance
.lamda. away from each other across the front-end and rear-end
elements in a direction of a short side of the element and are
series-connected in a direction of a long side of the element, the
rear-end element of the first element group and the front-end
element of the second element group are located as spaced a
distance .lamda./2 away from each other in the direction of the
short side of the element, the front-end element of the first
element group is connected to an electric power supply, the
rear-end element of the first element group is connected to the
front-end element of the second element group, the rear-end element
of the second element group is grounded, an external magnetic field
is detected through midpoint electric potential from a connection
between the first element group and the second element group, and
the magnetoresistive element satisfies the inequation,
H10-50<H50-90, where H10-50 represents a magnetic field required
for a resistance change from .DELTA.R.times.10% to
.DELTA.R.times.50% with respect to a maximum amount of resistance
change .DELTA.R in process of a resistance change occurring
according to the external magnetic field parallel to the direction
of the short side of the element, and H50-90 represents a magnetic
field required for a resistance change from .DELTA.R.times.50% to
.DELTA.R.times.90%.
8. The magnetic encoder according to claim 7, wherein the magnetic
sensor and the magnetic medium move relative to each other in a
magnetized direction of the magnetic medium, and the direction of
magnetization of the ferromagnetic pinned layer is the same as the
magnetized direction of the magnetic medium.
9. The magnetic encoder according to claim 7, wherein the
magnetoresistive element satisfies the inequation,
1.5<H50-90/H10-50<4.0.
10. The magnetic encoder according to claim 7, wherein the
non-magnetic intermediate layer has a locally nonuniform
distribution of layer thickness.
11. The magnetic encoder according to claim 7, wherein the
ferromagnetic pinned layer and the ferromagnetic free layer have
ferromagnetic interlayer interaction with the non-magnetic
intermediate layer in between, and the magnitude of the interlayer
interaction is locally nonuniform.
12. The magnetic encoder according to claim 7, wherein the
direction of magnetization of the ferromagnetic pinned layer is the
direction of the short side of the element.
13. The magnetic encoder according to claim 7, wherein the
direction of magnetization of the ferromagnetic pinned layer
deviates from the direction of the short side of the element within
a range of angles of 30 degrees or less.
Description
CLAIM OF PRIORITY
[0001] The present application claims priority from Japanese
application JP 2007-021372 filed on Jan. 31, 2007, the content of
which is hereby incorporated by reference into this
application.
BACKGROUND OF THE INVENTION
[0002] 1. Technical Field
[0003] The present invention relates to a magnetic sensor having a
Spin-valve type giant magnetoresistive element and a magnetic
encoder using the same.
[0004] 2. Background Art
[0005] Recently, there have been strong demands that a magnetic
encoder for use in consumer electronics equipment such as a digital
still camera and an ink jet printer achieves high resolution and
low power consumption in addition to small size and low price.
[0006] Heretofore, an anisotropic magnetoresitance effect
(hereinafter referred to simply as "AMR") film made of a NiFe
(nickel-iron) alloy film or the like has been used for a magnetic
sensor to be mounted to the magnetic encoder. The AMR effect is a
phenomenon in which electrical resistance changes according to a
relative angle between the direction of current passing through a
ferromagnetic film of a NiFe alloy or the like and the direction of
magnetization of the ferromagnetic film. By utilizing the
phenomenon, a change in resistance of an element according to an
externally applied signal magnetic field can be outputted through a
change in voltage or current. Specifically, when an AMR element is
disposed as separated by a predetermined gap from a magnetic medium
magnetized in alternating multipolar form as illustrated
schematically in FIG. 1, a change in output according to a periodic
signal magnetic field originating from the magnetic medium can be
detected.
[0007] High resolution of the magnetic encoder can be achieved by
narrowing a magnetized pitch (or a length of a pair of the north
and south poles) of the magnetic medium and correspondingly
narrowing a pattern width of the magnetic sensor. However, it is
required that the magnetic sensor be of high sensitivity because
the narrowing of the magnetized pitch of the magnetic medium leads
to a decrease in the signal magnetic field from the surface of the
magnetic medium. Although an AMR film of NiFe or the like undergoes
a change in electrical resistance under a signal magnetic field of
relatively small magnitude, magnetoresistive ratio (hereinafter
referred to simply as "MR ratio") is of the order of a few percent,
which is not necessarily high. Thus, the sensitivity can possibly
be insufficient for the magnetic encoder to achieve high
resolution. Generally, the AMR film has a thickness of about 20 nm.
Thus, the narrowing of the pattern width of the magnetic sensor
corresponding to the magnetized pitch of the magnetic medium leads
to shape anisotropy, which can possibly cause an increase in an
anisotropy field and hence a reduction in the sensitivity to
magnetic field. Moreover, the thick AMR film means that the
resistance of the element is relatively low, and therefore the AMR
film has a problem also from the viewpoint of power
consumption.
[0008] Other magnetic sensors include an element utilizing an
antiferro-coupled giant magnetoresistive (hereinafter referred to
simply as "coupled GMR") film, as disclosed in Japanese Patent No.
2812042. The coupled GMR film is formed of a multilayer
superlattice film having ferromagnetic layers and non-magnetic
layers alternating with each other, which are stacked one on top of
another in a few layers to a few tens of layers. Antiferromagnetic
interlayer coupling (or interaction such that the magnetization
directions of adjacent ferromagnetic layers are antiparallel to
each other) occurs between the adjacent ferromagnetic layers with
the non-magnetic layer in between. In the GMR film, an electrical
resistance changes according to a relative angle between the
magnetization directions of the adjacent ferromagnetic layers with
the non-magnetic layer in between. More specifically, under no
external magnetic field, the magnetizations of the adjacent
ferromagnetic layers are antiparallel to each other, and the
resistance is maximized. On the other hand, under an external
magnetic field, the magnetizations of the adjacent ferromagnetic
layers are parallel to each other, and the resistance is minimized.
The MR ratio of the coupled GMR film is a few times higher than
that of the AMR film, and therefore the coupled GMR film is
advantageous in terms of high output. However, a transition of the
magnetizations of the adjacent ferromagnetic layers from an
antiparallel state to a parallel state requires a magnetic field of
such great magnitude that overcomes the antiferromagnetic
interlayer coupling between the ferromagnetic layers with the
non-magnetic layer in between. The coupled GMR film, in an aspect,
cannot be said to be suitable for use in the magnetic encoder for
detection of a signal magnetic field of relatively small magnitude.
Moreover, the coupled GMR film has difficulty in achieving low
power consumption because of having a thick sensor film and hence a
low element resistance, as in the case of the AMR film.
[0009] Japanese Patent No. 3040750 discloses a Spin-valve type GMR
film in use as a magnetic read head for a hard disk drive, as a
magnetic sensor film that responds to a signal magnetic field of
relatively small magnitude and exhibits a high MR ratio which is
about the same as that of the coupled GMR film. The Spin-valve type
GMR film is configured basically of a ferromagnetic pinned layer, a
non-magnetic intermediate layer, and a ferromagnetic free layer.
The direction of magnetization of the ferromagnetic pinned layer is
unidirectionally pinned by an antiferromagnetic layer, which is
formed adjacent to the ferromagnetic pinned layer to impart
unidirectional magnetic anisotropy to the ferromagnetic pinned
layer and do the like. On the other hand, the ferromagnetic free
layer changes the direction of magnetization according to an
external magnetic field. Thus, the Spin-valve type GMR film enables
the transition of the magnetizations of the two ferromagnetic
layers with the non-magnetic intermediate layer in between from the
antiparallel state to the parallel state, under a magnetic field of
relatively small magnitude. Moreover, the Spin-valve type GMR film
has electrical resistance a few times higher than that of the
coupled GMR film, and therefore the Spin-valve type GMR film is
advantageous also in terms of low power consumption. A bridge
circuit magnetic sensor using a Spin-valve type GMR element is
disclosed in Japanese Patent No. 3017061.
SUMMARY OF THE INVENTION
[0010] However, the use of the Spin-valve type GMR element in place
of the AMR element or the coupled GMR element has the problem of
reducing resolution by half. As is apparent from a magnetoresitance
effect curve shown in FIG. 2A, the AMR element undergoes
symmetrical resistance change in the positive and negative
directions of an external magnetic field (incidentally, the same
goes for the coupled GMR element). In other words, these elements
output resistance changes according to an increase or decrease in
the magnitude of a signal magnetic field, regardless of the
direction of the signal magnetic field. Thus, an output from the
magnetic sensor (or a resistance change of the element) is obtained
with a period .lamda. equal to the magnetized pitch .lamda. of the
magnetic medium. On the other hand, the Spin-valve type GMR element
has an asymmetrical magnetoresitance effect curve in the positive
and negative directions of the external magnetic field, as shown in
FIG. 2B. Thus, the output from the magnetic sensor (or the
resistance change of the element) is obtained with a period
2.lamda. relative to the magnetized pitch .lamda. of the magnetic
medium. FIG. 3 shows a difference between output changes of the AMR
element and the Spin-valve type GMR element relative to the
magnetized pitch of the magnetic medium as mentioned above. To use
the Spin-valve type GMR element as the magnetic sensor for a
high-resolution magnetic encoder, it is therefore essential that
the Spin-valve type GMR element be contrived to have
magnetoresistive properties such as are exhibited by the AMR
element or the coupled GMR element. Specifically, this is
accomplished by a superposition of the magnetoresistive properties
of two Spin-valve type GMR elements having the magnetoresistive
properties of having opposite phases.
[0011] However, this superposition has difficulty in achieving
sufficiently high output because it can possibly cause
unintentional output setoff. Moreover, a decrease in output due to
the output setoff depends greatly on a magnetic gap between the
magnetic sensor and the magnetic medium. In the magnetic encoder,
the magnetic gap has some fluctuations. It can be therefore said
that desirable properties are that the magnetic sensor undergoes no
output change even at the occurrence of variations in the magnetic
gap. Basically, larger magnetic gap leads to greater attenuation of
the signal magnetic field from the magnetic medium and hence to
lower output from the magnetic sensor. It is therefore necessary to
control the magnetic gap so as to avoid an excessively large
magnetic gap in order to ensure a required output level. On the
other hand, in process of a series of examinations, it has been
shown that too narrow a magnetic gap also causes a decrease in the
output from the magnetic sensor. In other words, the prior art has
difficulty in achieving the magnetic encoder suitable for high
resolution and also having high reliability, because of being
incapable of achieving stable high output at the occurrence of
fluctuations in the magnetic gap between the magnetic sensor and
the magnetic medium.
[0012] An object of the present invention is therefore to provide a
magnetic encoder having high resolution and having high reliability
with little change in output even at the occurrence of variations
in the magnetic gap between the magnetic sensor and the magnetic
medium.
[0013] To fabricate a magnetic encoder configured of a magnetic
medium periodically magnetized and a magnetic sensor formed of
plural magnetoresistive elements, each using a Spin-valve type GMR
film, the present invention uses the magnetoresistive element that
exhibits a characteristic response for magnetic field. Generally,
the magnetoresistive element is often desired to have linear
magnetic parametric performance for signal magnetic field. However,
the present invention fabricates and uses the magnetoresistive
element having nonlinear magnetoresistive properties for signal
magnetic field. Specifically, the present invention uses the
magnetoresistive element that satisfies the equation,
H10-50<H50-90, where H10-50 represents a magnetic field required
for a resistance change from .DELTA.R.times.10% to
.DELTA.R.times.50% with respect to a maximum amount of resistance
change (hereinafter referred to simply as ".DELTA.R") in process of
a resistance change occurring according to an external magnetic
field parallel to the direction of magnetization of the
ferromagnetic pinned layer, and H50-90 represents a magnetic field
required for a resistance change from .DELTA.R.times.50% to
.DELTA.R.times.90%. In other words, the magnetoresistive element
for use in the present invention has high sensitivity in a changing
region where the amount of resistance change increases from
.DELTA.R.times.10% to .DELTA.R.times.50% in process of a transition
of the amount of resistance change from zero to .DELTA.R, and has
low sensitivity in a changing region where the amount thereof
increases from .DELTA.R.times.50% to .DELTA.R.times.90%.
Preferably, the present invention uses the magnetoresistive element
that satisfies the equation, 1.5<H50-90/H10-50<4.0. This
enables suppressing the unintentional output setoff involved in the
superposition of outputs. Moreover, the suppression of the output
setoff involved in the superposition of outputs enables achieving
stable output characteristics, even at the occurrence of variations
in the magnetic gap between the magnetic sensor and the magnetic
medium.
[0014] The characteristic magnetoresistive properties as mentioned
above are accomplished by the following configuration: the
non-magnetic intermediate layer has a locally nonuniform
distribution of layer thickness, and the magnitude of ferromagnetic
interlayer coupling between the ferromagnetic pinned layer and the
ferromagnetic free layer with the non-magnetic intermediate layer
in between is locally nonuniform.
[0015] Used as other means for achieving the characteristic
magnetoresistive properties is a method that involves performing
annealing treatment under magnetic field so that the average
magnetization direction of the ferromagnetic pinned layer can
deviate from the magnetized direction of the magnetic medium within
a range of angles of 30 degrees or less.
[0016] The present invention enables achieving a magnetic encoder
capable of achieving stable output, suitable for high resolution,
and having high reliability, even at the occurrence of variations
in the magnetic gap between the magnetic medium and the magnetic
sensor.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017] FIG. 1 is a schematic illustration of the configuration of a
magnetic encoder according to the present invention.
[0018] FIGS. 2A and 2B are plots showing typical magnetoresitance
effect curves of an AMR element and a Spin-valve type GMR element,
respectively.
[0019] FIG. 3 is a graph showing a comparison of output
characteristic periods of the AMR element and the Spin-valve type
GMR element relative to a magnetized pitch of a magnetic
medium.
[0020] FIG. 4 is a schematic illustration of the configuration of a
magnetoresistive element using a Spin-valve type GMR film.
[0021] FIG. 5 is a cross-sectional view showing the relative
dispositions of the magnetoresistive elements and the magnetic
medium.
[0022] FIG. 6 is a perspective view illustrating a connection
method for the magnetoresistive elements.
[0023] FIG. 7 is a plot showing a typical magnetoresitance effect
curve of the Spin-valve type GMR element for use in a magnetic
encoder.
[0024] FIG. 8 is a plot showing a superposition output from two
magnetoresistive elements.
[0025] FIGS. 9A and 9B are plots showing a comparison of
superposition outputs from the two magnetoresistive elements, where
there are varying anisotropy fields.
[0026] FIGS. 10A to 10F are an illustration and charts of
assistance in explaining the structure and operation of the
magnetic encoder.
[0027] FIG. 11 is a plot showing a magnetoresitance effect curve of
the prior art Spin-valve type GMR element for use in the magnetic
encoder.
[0028] FIG. 12 is a plot showing the dependence of output from the
prior art magnetic encoder upon a gap between a magnetic sensor and
the magnetic medium.
[0029] FIG. 13 is a graph showing the spatial distribution of a
signal magnetic field from the magnetic medium.
[0030] FIG. 14 is a plot showing a comparison of the dependence of
the MR ratio of the magnetoresistive element upon the gap, where
the magnetoresistive element is at varying positions.
[0031] FIGS. 15A to 15C are an illustration and charts of
assistance in explaining a decrease in output, where the prior art
is used.
[0032] FIG. 16 is a graph showing a characteristic magnetoresitance
effect curve of the magnetoresistive element for use in the
magnetic encoder according to the present invention.
[0033] FIG. 17 is a plot showing a comparison of the dependence of
the MR ratio of the magnetoresistive element of the present
invention upon the gap, where the magnetoresistive element is at
varying positions.
[0034] FIG. 18 is a plot of the dependence, upon the gap, of a
difference between the maximum and minimum values of the MR ratio
reflected by output, showing a comparison between the prior art and
the present invention.
[0035] FIG. 19 is a plot showing the dependence of output from the
magnetic encoder of the present invention upon the gap.
[0036] FIG. 20 is a plot showing a usable gap region for the
magnetoresistive element having varying magnetoresistive
properties.
[0037] FIG. 21 is a plot showing the dependence of an interlayer
coupling field Hint upon a Cu layer thickness of a non-magnetic
intermediate layer.
[0038] FIG. 22 is a plot showing magnetoresitance effect curves,
where the Cu layer thickness is set to 1.70 nm, 1.75 nm, and 1.80
nm and the thicknesses are averaged.
[0039] FIG. 23 is a plot showing a difference in magnetoresistive
properties, where the direction of magnetization of a ferromagnetic
pinned layer varies.
[0040] FIG. 24 is a plot showing a change in the MR ratio with
respect to the direction of magnetization of the ferromagnetic
pinned layer.
[0041] FIG. 25 is a flowchart of a manufacturing method for
controlling the direction of magnetization of the ferromagnetic
pinned layer.
[0042] FIG. 26 is a schematic illustration of the configuration of
the magnetoresistive element using a ferromagnetic free layer of a
configuration of a synthetic ferri-magnet type.
[0043] FIG. 27 is a graph illustrating definitions of the
interlayer coupling field Hint and an effective anisotropy field
Hk* on a magnetoresitance effect curve.
[0044] FIG. 28 is a plot showing the dependence of the effective
anisotropy field Hk* upon a pattern width.
DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION
[0045] Embodiments of the present invention will be described below
with reference to the accompanying drawings. In order to simplify
an understanding of the embodiments of the present invention, the
same or similar functional parts in the several figures will be
given the same reference numerals.
First Embodiment
[0046] FIG. 1 illustrates, in schematic form, the configuration of
a magnetic encoder according to the present invention. The magnetic
encoder includes a magnetic medium 1, and a magnetic sensor 2 that
moves relative to the magnetic medium 1, facing the magnetic medium
1 with a predetermined gap in between, and the magnetic medium 1 is
magnetized in alternating multipolar form in the direction of
relative movement thereof with respect to the magnetic sensor
2.
[0047] The magnetic sensor 2 includes a magnetoresistive element 21
using at least a Spin-valve type GMR film. FIG. 4 illustrates, in
schematic form, the configuration of the magnetoresistive element
21. The Spin-valve type GMR film that constitutes the
magnetoresistive element 21 has a multilayer structure including at
least a ferromagnetic pinned layer 202, a non-magnetic intermediate
layer 203, and a ferromagnetic free layer 204, which are stacked
one on top of another. Furthermore, an antiferromagnetic layer 201
may be formed in contact with the ferromagnetic pinned layer 202
and on the opposite side to the non-magnetic intermediate layer 203
in order to unidirectionally pin the direction of magnetization of
the ferromagnetic pinned layer 202. Of course, a seed layer 200 and
a cap layer 205 may be appropriately formed as bottommost and
topmost layers, respectively. DC magnetron sputtering equipment was
used to form the Spin-valve type GMR film, from the viewpoint of
stability and efficiency of mass production.
[0048] The Spin-valve type GMR film is configured, for example, of
a substrate: the seed layer made of Ta (tantalum) of 2.5 nm thick,
a NiFeCr (nickel-iron-chromium) alloy of 3.2 nm thick, and NiFe of
0.8 nm thick; the antiferromagnetic layer made of a MnPt
(manganese-platinum) alloy of 14 nm thick; the ferromagnetic pinned
layer made of a CoFe (cobalt-iron) alloy of 1.8 nm thick, Ru
(ruthenium) of 0.8 nm thick, and CoFe of 2.2 nm thick; the
non-magnetic intermediate layer made of Cu (copper) of 2.3 nm
thick; the ferromagnetic free layer made of CoFe of 1 nm thick and
NiFe of 3 nm thick; and the cap layer made of Cu of 0.6 nm thick
and Ta of 3 nm thick, which are formed, as laid one on top of
another, on top of the substrate. Although MnPt was used for the
antiferromagnetic layer as given as an example, a material
represented as Mn--X, such as a MnIr (manganese-iridium) alloy or
MnRu, may be used for the antiferromagnetic layer (where X denotes
at least one of Ru, Rh (rhodium), Pd (palladium), Re (rhenium), Os
(osmium), Ir, Pt, Au (gold), Cr, Fe, and Ni.) What is known as a
"synthetic ferri-magnet type" is given as an example of the
configuration of the ferromagnetic pinned layer. This is the
configuration in which the two CoFe layers are
antiferromagnetically interlayer-coupled with the Ru layer in
between, and this configuration is expected to pin unidirectionally
the direction of magnetization of the ferromagnetic pinned layer
more firmly, and to achieve the effect of lessening the influence
of magnetostatic coupling on an end of the element by reducing an
effective amount of magnetization of the ferromagnetic pinned
layer. A typical CoFe single layer or the like may be used to form
the ferromagnetic pinned layer unless a particular problem arises.
Although an instance is herein given where the ferromagnetic pinned
layer is disposed toward the substrate, the layers may be stacked
in reverse order in such a manner that the substrate, the seed
layer, the ferromagnetic free layer, the non-magnetic intermediate
layer, the ferromagnetic pinned layer, the antiferromagnetic layer,
and the cap layer are formed in sequence.
[0049] After having been formed, the Spin-valve type GMR film was
subjected to three-hour annealing treatment while being held at a
temperature of 270.degree. C. under a magnetic field in a vacuum,
in order that the direction of magnetization of the ferromagnetic
pinned layer 202 was pinned in a desired direction. The magnitude
of the magnetic field was set to 4 MA/m (50 kOe) so that the
ferromagnetic pinned layer 202 could be magnetized to sufficient
saturation. With the annealing treatment, the MnPt
antiferromagnetic layer undergoes phase transformation to form an
ordered structure, and thereby unidirectional magnetic anisotropy
can be imparted to the ferromagnetic pinned layer 202. Desirably,
heat annealing temperature and time are appropriately adjusted
according to a material for use (in particular, a material for the
antiferromagnetic layer), a film thickness, and so on.
Incidentally, the annealing treatment leads to the induction, into
the ferromagnetic free layer 204, of uniaxial magnetic anisotropy
such that the same direction coincides with the axis of easy
magnetization. If the ferromagnetic free layer 204 has a problem
with its coercivity or anisotropy field, the Spin-valve type GMR
film may be subjected to annealing treatment under a magnetic field
in the direction perpendicular to the above direction. In this
case, the annealing treatment temperature and time and the
magnitude of the applied magnetic field must be adjusted so as to
prevent the magnetization of the ferromagnetic pinned layer 202
from excessively deviating from the desired direction.
[0050] The Spin-valve type GMR film was subjected to patterning
into a substantially rectangular shape as shown in FIG. 4 by
photo-lithography process and ion milling process (the details of
which are omitted.) As employed herein, the substantially
rectangular shape refers to the general shape having a length and a
width and also permits the presence of concave and convex, curved
and other portions. This is for the purpose of narrowing a pattern
width W (or the width) of the magnetoresistive element, thereby
narrowing space occupied by the magnetoresistive element in a
magnetized pitch of the magnetic medium 1, and thereby increasing
spatial resolution of a sensed signal magnetic field. On the other
hand, a pattern length L (or the length) is increased, and
electrical resistance of the overall sensor is set high by the
passage of current along the length. This enables reducing power
consumption during constant-voltage driving. To achieve
sufficiently high resolution, the pattern width W can be set to
.lamda./4 or less, where .lamda. denotes the magnetized pitch.
Desirably, for example, the pattern width W of the magnetoresistive
element is approximately 5 .mu.m or less when the magnetized pitch
of the magnetic medium 1 is 20 .mu.m. In contrast, the pattern
length is as long as about a few hundreds of micrometers, because
longer pattern length enables lower power consumption, provided
that the pattern length falls within the width of the magnetic
medium 1 of the encoder.
[0051] In the Spin-valve type GMR film, the signal magnetic field
has to be parallel to the magnetization of the ferromagnetic pinned
layer. Accordingly, the direction of magnetization of the
ferromagnetic pinned layer 202 has to be identical with the width
direction of the magnetoresistive element and the magnetized
direction of the magnetic medium 1 (or the direction of relative
movement of the magnetic sensor 2 with respect to the magnetic
medium 1).
[0052] FIGS. 5 and 6 are a cross-sectional view and a perspective
view, respectively, showing the relative positions of the magnetic
medium 1 and the magnetic sensor 2. The magnetic medium 1 is
magnetized in multipolar form with the pitch .lamda., where the
pitch .lamda. is the length of each magnetized area in the
magnetized direction. In FIG. 5, the opposed magnetized areas each
having a length .lamda. alternate with each other, and the length
of each period (or the length of a pair of the rightward and
leftward magnetized areas) is 2.lamda.. The pitches of the
rightward and leftward magnetized areas are each not necessarily
limited to .lamda. but may be different from .lamda., provided that
the length of each period is 2.lamda.. For the sake of simplicity
of explanation, description will hereinafter be given with regard
to an instance where the opposed magnetized areas each having the
length .lamda. alternate with each other as shown in FIG. 5. The
magnetic sensor 2 is formed of four connected magnetoresistive
elements, is disposed as separated by a predetermined gap from the
magnetic medium 1, and moves relative to the magnetic medium 1. The
four magnetoresistive elements have approximately the same
magnetoresistive properties. The four magnetoresistive elements are
spaced at appropriate intervals in the direction of relative
movement with respect to the magnetic medium 1. More specifically,
two magnetoresistive elements 21 and 22 are spaced a distance
.lamda. away from each other in the direction of relative movement,
and are series-connected to form a first element group 25. Two
magnetoresistive elements 23 and 24 are also spaced a distance
.lamda. away from each other in the direction of relative movement,
and are likewise series-connected to form a second element group
26. As shown in FIG. 5, one end of the first element array 25 is
spaced a distance .lamda./2 away from one end of the second element
array 26. All the magnetizations of the ferromagnetic pinned layers
of the magnetoresistive elements 21, 22, 23 and 24 are oriented in
the same direction. Description will be given with reference to
FIG. 6 with regard to details of a connection method for the
magnetoresistive elements.
[0053] As shown in FIG. 6, the magnetoresistive elements have a
circuit configuration such that current passes through the elements
along the pattern length, and that all the elements are
series-connected. More specifically, the first element group 25 is
connected at one end to an electric power supply Vcc and is
connected at the other end to one end of the second element group
26. The second element group 26 is grounded at the other end, and a
midpoint electric potential Vout is detected through a connection
between the other end of the first element group 25 and one end of
the second element group 26.
[0054] Although the number of magnetoresistive elements that
constitute each element group is herein set to two, the number
thereof may be 2n (where n denotes a natural number.) Increasing
the number of magnetoresistive elements enables lessening the
influence of variations in element properties. Moreover,
superposition of signal phases of output signals from 2n
magnetoresistive elements enables lessening the influence of the
variations.
[0055] Description will now be given with regard to the properties
of the Spin-valve type GMR film required to achieve output
characteristics having a period equal to the magnetized pitch
.lamda. of the magnetic medium 1. FIG. 7 shows a typical
magnetoresitance effect curve of the Spin-valve type GMR film for
use in a high-resolution magnetic encoder. This curve is obtained
by sweeping a magnetic field parallel to the direction of
magnetization of the ferromagnetic pinned layer that is a
structural component of the Spin-valve type GMR film, and
expressing a change in resistance of the Spin-valve type GMR film
as an MR ratio. The curve is characterized in that a waveform is
intentionally shifted in one direction (in FIG. 7, the change in
resistance occurs only in a positive direction of the magnetic
field.) Such a waveform shift is accomplished by appropriately
setting an interlayer coupling field Hint between the ferromagnetic
pinned layer and the ferromagnetic free layer. Application of the
Spin-valve type GMR film having such magnetoresistive properties to
the magnetoresistive elements 21 and 22 shown in FIGS. 5 and 6
enables achieving superposition output as shown in FIG. 8
(incidentally, the MR ratio can be regarded as a synonym of output,
because output is determined by the MR ratio, provided that
constant-voltage driving takes place.) This results from the fact
that the magnetoresistive elements 21 and 22 undergo resistance
changes in opposite phases because of being subjected to opposite
signal magnetic fields 3. Thereby, symmetrical resistance changes
can occur in the positive and negative directions of an external
magnetic field, as in the case of an AMR element shown in FIG.
2A.
[0056] Incidentally, the superposition output is inevitably half of
output from the magnetoresistive element 21 or 22 alone because of
originating from the series-connected magnetoresistive elements 21
and 22.
[0057] Description will be further given below with regard to what
is important in preventing a decrease in the superposition output.
FIGS. 9A and 9B show magnetoresitance effect curves of the
magnetoresistive elements 21 and 22 and the superposed elements 21
and 22, which are observed when the ferromagnetic free layer has
varying anisotropy fields Hk. In FIGS. 9A and 9B, the ferromagnetic
interlayer coupling field Hint acting between the ferromagnetic
pinned layer and the ferromagnetic free layer was 1600 A/m (20 Oe),
and the anisotropy fields Hk were 800 A/m (10 Oe) (see FIG. 9A) and
3200 A/m (40 Oe) (see FIG. 9B) for comparison. In FIG. 9A, Hint is
greater than Hk (Hint>Hk), and both the resistance changes of
the magnetoresistive elements 21 and 22 occur only in a positive or
negative magnetic field region. The superposition output is 50% of
the output from the magnetoresistive element 21 or 22 alone. On the
other hand, in FIG. 9B, Hint is less than Hk (Hint<Hk), and
neither of the resistance change waves of the magnetoresistive
elements 21 and 22 is completely shifted to the positive or
negative magnetic field region. In this case, the superposition
output is 40% or less of the output from the magnetoresistive
element 21 or 22 alone due to the occurrence of output setoff in
the vicinity of a zero magnetic field. Accordingly, in order to
prevent a decrease in the superposition output due to the output
setoff, it is therefore important that each individual
magnetoresistive element undergo a resistance change by being
completely shifted to the positive or negative magnetic field
region. In short, this is accomplished by setting Hint and Hk so
that these values satisfy the inequation, Hint.gtoreq.Hk. However,
the values must be appropriately adjusted because too large a Hint
value makes it impossible to achieve a high MR ratio with respect
to the signal magnetic field. As previously mentioned, the
magnetoresistive element is formed in such a shape that the pattern
width W<< the pattern length L. This leads to the shape
anisotropy, which causes the induction, into the ferromagnetic free
layer, of the uniaxial magnetic anisotropy such that the pattern
length direction coincides with the axis of easy magnetization.
Thus, the effective Hk* value becomes larger than the Hk value. The
Hint and Hk values must be appropriately adjusted, allowing for the
above.
[0058] Description will now be given with reference to FIGS. 10A to
10F with regard to the principle of operation of the magnetic
encoder according to the present invention. FIG. 10A is a schematic
illustration of the magnetic encoder using the magnetic sensor 2
and the magnetic medium 1 according to the present invention. The
magnetic medium 1 moves relative to the magnetic sensor 2, facing
the magnetic sensor 2, leftward as seen in the drawing. The
magnetizations of the ferromagnetic pinned layers of the
magnetoresistive elements 21, 22, 23 and 24 that constitute the
magnetic sensor 2 are oriented leftward as seen in the drawing.
Actually, each of the magnetoresistive elements has a limited
pattern width that cannot be neglected, and therefore, assuming
that the magnetoresistive element is divided into fine regions in
the pattern width direction leads to the result that the fine
regions detect magnetic fields of different magnitudes. For the
sake of convenience, description is herein given with regard to the
operation, regardless of the pattern width. FIGS. 10B and 10C show
changes in outputs from the magnetoresistive elements 21 and 22
with respect to a distance moved by the magnetic medium. FIG. 10D
shows a change in output from the first element group 25 configured
of the series-connected magnetoresistive elements 21 and 22. As
previously mentioned, output amplitude from the element group 25 is
half of output amplitude from the magnetoresistive element 21 or 22
alone, because the output from the element group 25 is a
superposition of the outputs from the series-connected
magnetoresistive elements 21 and 22. Likewise, FIG. 10E shows a
change in output from the second element group 26 configured of the
series-connected magnetoresistive elements 23 and 24. Since the
magnetoresistive element 23 is spaced a distance .lamda./2 away
from the magnetoresistive element 22, an output wave from the
second element group 26 is phase shifted .lamda./2 from an output
wave from the first element group 25. FIG. 10F shows a change in
the midpoint electric potential Vout at the connection between the
first element group 25 and the second element group 26. It can be
seen that the midpoint electric potential Vout that is an output
from the magnetic sensor 2 is a signal having a period of .lamda.
as shown in FIG. 10F.
[0059] Description will be given with regard to the results of
evaluations of the magnetic encoder fabricated of the above
configuration. The Spin-valve type GMR film was configured of a
glass substrate: the seed layer made of Ta of 2.5 nm thick, NiFeCr
of 3.2 nm thick, and NiFe of 0.8 nm thick; the antiferromagnetic
layer made of MnPt of 14 nm thick; the ferromagnetic pinned layer
made of CoFe of 1.8 nm thick, Ru of 0.45 nm thick, and CoFe of 2.2
nm thick; the non-magnetic intermediate layer made of Cu of 2.35 nm
thick; the ferromagnetic free layer made of CoFe of 1 nm thick and
NiFe of 3 nm thick; and the cap layer made of Cu of 0.6 nm thick
and Ta of 3 nm thick. After its deposition using sputtering method,
the Spin-valve type GMR film was subjected to annealing treatment
for three hours at 270.degree. C. under a direct current magnetic
field of 4 MA/m (50 kOe) in a vacuum, and thereby the direction of
magnetization of the ferromagnetic pinned layer was pinned. FIG. 11
shows a magnetoresitance effect curve of the Spin-valve type GMR
film used for the evaluations. Hint is about 1800 A/m (22.5 Oe),
and a change in resistance occurs only in a positive magnetic field
region. The Spin-valve type GMR film was fabricated in a width of 5
.mu.m and a length of 100 .mu.m to form the magnetoresistive
element. At this point, the pinned direction of magnetization of
the ferromagnetic pinned layer coincides with the pattern width
direction. Four magnetoresistive elements were series-connected to
form the magnetic sensor, as shown in FIG. 6. At this point, the
magnetoresistive elements 21 and 22 were spaced a distance of 20
.mu.m away from each other and the magnetoresistive elements 23 and
24 were spaced a distance of 20 .mu.m away from each other. The
magnetoresistive elements 22 and 23 were spaced a distance of 10
.mu.m away from each other. The magnetic sensor and the magnetic
medium having a magnetized pitch of 20 .mu.m were used to evaluate
the dependence of output on the gap between the magnetic sensor and
the magnetic medium.
[0060] FIG. 12 shows the dependence of output on the gap. When the
gap is equal to 10 .mu.m, the output has a maximum value. When the
gap is more than 10 .mu.m, the output decreases and can be
understood to be affected by attenuation of signal magnetic field.
It is therefore required that the magnetic sensor be disposed so as
to avoid an excessively large gap in order to achieve a signal
magnetic field of sufficient magnitude. On the other hand, when the
gap is less than 10 .mu.m, a decrease in the output takes place and
becomes a large problem in bringing the magnetic encoder into
operation. The reason is as follows: since the gap can possibly
have some fluctuations, the dependence of the output on the gap as
mentioned above can possibly impair reliability because of making
it impossible to achieve stable output characteristics. Desirably,
therefore, the magnetic encoder has improved output characteristics
capable of achieving high resolution and also achieving high
reliability with little change in output even at the occurrence of
some variations in the gap.
[0061] Description will be given with regard to the cause of the
decrease in the output in a region where the gap is narrow as
mentioned above. As previously mentioned, actually, each of the
magnetoresistive elements has a limited pattern width that cannot
be neglected, and therefore, assuming that the magnetoresistive
element is divided into fine regions in the pattern width direction
leads to the result that the fine regions detect magnetic fields of
different magnitudes. It is therefore necessary to take into
account the spatial distribution of the signal magnetic field from
the magnetic medium. FIG. 13 shows, in normalized form, the spatial
distribution of the signal magnetic field originating from the
magnetic medium, where the gap is a parameter. The magnetized pitch
.lamda. of the magnetic medium is 20 .mu.m. In FIG. 13, there are
additionally shown the relative positions of the magnetic medium
and the magnetoresistive element having a pattern width of 5
.mu.m.
[0062] As is apparent from FIG. 13, the narrower gap produces the
larger maximum value of the signal magnetic field and also produces
a sharper change in the magnetic field at a boundary between
opposite magnetic fields. As shown in FIG. 13, when the center of
the magnetoresistive element coincides with a boundary between
magnetized bits of the magnetic medium, the signal magnetic field
in the X direction directly at the boundary between the magnetized
bits (position_X=20 .mu.m) is zero regardless of the length of the
gap. However, the magnetoresistive element detects leftward and
rightward signal magnetic fields at its left end (position X=17.5
.mu.m) and right end (position_X=22.5 .mu.m), respectively,
provided that the magnetoresistive element has a pattern width of 5
.mu.m. In particular, when the gap is 0 .mu.m, the magnetoresistive
element detects maximum leftward and rightward magnetic fields at
its left and right ends, respectively. In other words, the fine
regions detect different signal magnetic fields according to the
position and correspondingly undergo resistance changes, assuming
that the magnetoresistive element is divided into the fine regions.
The resistance of the overall magnetoresistive element is the sum
of the resistance changes of the fine regions. For example when the
gap is 5 .mu.m, the magnetoresistive element likewise detects the
leftward and rightward magnetic fields at its left and right ends,
respectively. However, an increase in the length of the gap reduces
the strengths of the signal magnetic fields detected by the left
and right ends of the magnetoresistive element, thus reducing the
influence on the resistance change of the element.
[0063] FIG. 14 shows the dependence of the MR ratio of the
magnetoresistive element upon the gap, which is observed when the
center of the magnetoresistive element is at position_X in FIG. 13,
where X=20 .mu.m and X=30 .mu.m. When X=30 .mu.m, the strength of
the signal magnetic field detected in the X direction has a maximum
value in the case of each gap. Thus, the magnetoresistive element
detects the signal magnetic field of sufficient magnitude for the
ferromagnetic free layer to be magnetized to saturation.
Accordingly, the magnetoresistive element exhibits substantially
the same MR ratio close to 12% in the case of any gap. On the other
hand, when X=20 .mu.m, the signal magnetic field to be detected in
the X direction is zero, and it is therefore desirable that the MR
ratio of the magnetoresistive element be essentially zero. However,
it can be seen that the narrower gap leads to the higher MR ratio
of the magnetoresistive element. For example, when the gap is 0
.mu.m, some of the fine regions of the magnetoresistive element
detect signal magnetic fields of great magnitude and exhibit the
maximum MR ratio of no less than 12%, and averaging of the
resistance change of each individual region and summation of the
resistance changes of the regions lead to the result that the
overall magnetoresistive element exhibits a relatively high MR
ratio of 5% or more. As for the output characteristics of the
magnetic encoder, a final output level is determined by a
difference between the MR ratio at the position X=20 .mu.m and the
MR ratio at the position X=30 .mu.m. Desirably, therefore, there is
a larger difference between the MR ratio at the position X=20 .mu.m
and the rate of resistance change at the position X=30 .mu.m, and
more desirably, the difference has a fixed value with respect to
the gap.
[0064] Description will be given with reference to FIGS. 15A to 15C
with regard to an influence exerted on the output characteristics
of the magnetic encoder by the dependence of the MR ratio on the
gap as mentioned above. Description will be given assuming that the
gap is 0 .mu.m, although in FIG. 15A, the magnetic sensor 2 and the
magnetic medium 1 are shown as disposed with a wide gap in between.
FIG. 15B shows changes in outputs from the magnetoresistive
elements 21 and 22 with respect to a distance moved by the magnetic
medium. As previously mentioned, when the center of the
magnetoresistive element coincides with the boundary between the
magnetized bits of the magnetic medium, the signal magnetic field
to be detected in the X direction is zero, and it is therefore
desirable that the MR ratio of the magnetoresistive element be
essentially zero. Actually, a resistance change that cannot be
neglected, however, occurs due to the spatial spread of the
element, when the center of the magnetoresistive element coincides
with the boundary between the magnetized bits of the magnetic
medium. FIG. 15C shows an output from the first element group 25
formed of a superposition of the outputs from the magnetoresistive
elements 21 and 22. Although the output amplitude from the first
element group 25 is inherently half of the output amplitude from
the magnetoresistive element 21 or 22 alone, signals represented as
the diagonally shaded areas in FIG. 15B are offset, resulting in a
significant decrease in the output amplitude from the first element
group 25. Consequently, too narrow a gap causes a significant
decrease in the amplitude of the midpoint electric potential Vout
that is the output from the magnetic sensor 2.
[0065] In order to solve this problem, the present invention uses
the magnetoresistive element having a magnetoresitance effect curve
as shown in FIG. 16. In FIG. 16, the vertical axis indicates, in
normalized form, the amount of resistance change .DELTA.R. The
prior art exhibits substantially linear response to magnetic field,
and a magnetic field required for a resistance change from
.DELTA.R.times.10% to .DELTA.R.times.50% is substantially the same
as that required for a resistance change from .DELTA.R.times.50% to
.DELTA.R.times.90%. The present invention uses the magnetoresistive
element having the magnetoresistive properties of exhibiting high
sensitivity in a changing region where the normalized amount of
resistance change increases from 0% to 50% and exhibiting low
sensitivity in a changing region where the normalized amount
thereof increases from 50% to its maximum value and reaches
saturation. Specifically, the present invention uses the
magnetoresistive element having the magnetoresistive properties
that satisfy the inequation, H10-50<H50-90, where H10-50
represents the magnetic field required for the resistance change
from .DELTA.R.times.10% to .DELTA.R.times.50% with respect to the
amount of resistance change .DELTA.R on the magnetoresitance effect
curve, and H50-90 represents the magnetic field required for the
resistance change from .DELTA.R.times.50% to
.DELTA.R.times.90%.
[0066] FIG. 17 shows the dependence, on the gap, of the MR ratio of
the magnetoresistive element having the magnetoresistive properties
in which H50-90/H10-50=2.8. In FIG. 17, there is shown a comparison
with a situation where the center of the magnetoresistive element
is at position X in FIG. 13 where X=20 .mu.m and X=30 .mu.m,
provided that the pattern width of the magnetoresistive element is
5 .mu.m, and that the magnetized pitch of the magnetic medium is 20
.mu.m. It can be seen that the absolute value of the rate of
resistance change at X=20 .mu.m decreases, as compared to that of
the magnetoresistive element having the magnetoresistive properties
in which H50-90/H10-50=1.0 as shown in FIG. 14. This suggests that,
because the magnetoresistive element has the magnetoresistive
properties of difficult magnetic saturation, the MR ratio of the
overall magnetoresistive element is not very high even when the gap
becomes narrow and thus allows partial detection of a signal
magnetic field of great magnitude. It can be also seen that,
because of the use of the magnetoresistive properties of difficult
magnetic saturation, the MR ratio at X=30 .mu.m also decreases
monotonically when the gap becomes great and thus reduces a maximum
signal magnetic field.
[0067] As previously mentioned, the output from the magnetic
encoder reflects the difference between the MR ratio at X=20 .mu.m
and the MR ratio at X=30 .mu.m in FIG. 13. FIG. 18 shows the
dependence of the difference in the MR ratio upon the gap, showing
a comparison between the prior art and the present invention. The
prior art (H50-90/H10-50=1.0) corresponds to the difference between
the MR ratio at X=20 .mu.m and the MR ratio at X=30 .mu.m in FIG.
14, and the present invention (H50-90/H10-50=2.8) corresponds to
the difference between the MR ratio at X=20 .mu.m and the MR ratio
at X=30 .mu.m in FIG. 17. With the use of the present invention,
the difference in the MR ratio has a substantially fixed value even
if the gap has varying values, as compared to the prior art.
Accordingly, the use of the magnetoresistive element of the present
invention enables preventing a decrease in output involved in the
narrowed gap, achieving stable output even at the occurrence of
variations in the gap, and thereby providing the magnetic encoder
having high reliability.
[0068] FIG. 19 shows the dependence of normalized output from the
magnetic encoder upon the gap, where the H50-90/H10-50 ratio
varies. The pattern width of the magnetoresistive element is 5
.mu.m, and the magnetized pitch of the magnetic medium is 20 .mu.m.
As previously mentioned, if H50-90/H10-50=1.0, the output is
maximized when the gap is 10 .mu.m, and the output decreases
greatly as the gap becomes narrower than 10 .mu.m. On the other
hand, as can be seen from FIG. 19, if H50-90/H10-50=2.1 or
H50-90/H10-50=3.8, a decrease in output is suppressed in a region
where the gap is narrower, and a change in output is slight over a
wide range of gaps from about 0 to 10 .mu.m. In other words, these
instances are preferable because they achieve stable output even at
the occurrence of variations in the gap. With the use of the
magnetoresistive element having the magnetoresistive properties of
excessively difficult saturation in which H50-90/H10-50=5.9, the
output decreases monotonically as the gap becomes larger. In other
words, this instance is not desirable because it suggests that the
output varies with respect to the gap. This is understood to result
from the fact that a great change in resistance does not take place
because a signal magnetic field is of excessively small magnitude
as compared to a magnetic field required for the magnetoresistive
element to reach magnetic saturation.
[0069] FIG. 20 shows the dependence of a usable gap region upon the
H50-90/H10-50 ratio, where the usable gap region is a gap region
that gives 80% or more of maximum output. It can be seen that when
the H50-90/H10-50 ratio is set to an appropriate value, a wide
region of gaps of about 10 .mu.m or more is usable. Specifically,
when the H50-90/H10-50 ratio is set to lie between 1.5 and 4.0, the
magnetic encoder having a sufficiently wide usable gap region can
be provided.
[0070] A means for achieving the magnetoresistive properties of the
present invention as shown in FIG. 16 includes the approach of
making, locally nonuniform, the magnitude of ferromagnetic
interlayer interaction Hint between the ferromagnetic pinned layer
and the ferromagnetic free layer with the non-magnetic intermediate
layer in between. When the magnetoresistive element locally has
different interlayer interactions Hint, the overall
magnetoresistive element exhibits the properties obtained through
the averaging of the interactions Hint, thus enabling achievement
of the magnetoresistive properties of the present invention as
shown in FIG. 16. To make the magnitude of Hint locally nonuniform,
the layer thickness distribution of the non-magnetic intermediate
layer can be made locally nonuniform. FIG. 21 shows the dependence
of Hint upon the Cu layer thickness of the non-magnetic
intermediate layer. The Spin-valve type GMR film examined is
configured of the glass substrate: the seed layer made of Ta of 2.5
nm thick, NiFeCr of 3.2 nm thick, and NiFe of 0.8 nm thick; the
antiferromagnetic layer made of MnPt of 14 nm thick; the
ferromagnetic pinned layer made of CoFe of 1.8 nm thick, Ru of 0.45
nm thick, and CoFe of 2.2 nm thick; the non-magnetic intermediate
layer made of Cu (t); the ferromagnetic free layer made of CoFe of
1 nm thick and NiFe of 3 nm thick; and the cap layer made of Cu of
0.6 nm thick and Ta of 3 nm thick. Positive Hint means
ferromagnetic interlayer coupling, and negative Hint means
antiferromagnetic interlayer coupling. It can be seen that Hint
changes relatively sharply according to a minute difference in the
Cu layer thickness. Thus, this purpose is accomplished by forming
the non-magnetic intermediate layer made of the Cu layer whose
thickness undergoes tiny variations in local regions. For example,
two regions of Cu layer thickness, give the appropriate Hint value
of 1.6 kA/m (20 Oe). Since the former exhibits a great change in
Hint with respect to the Cu layer thickness as compared to the
latter, setting the Cu layer thickness in the vicinity of 1.78 nm
facilitates achieving the magnetoresistive properties as shown in
FIG. 16.
[0071] FIG. 22 shows magnetoresitance effect curves, where the Cu
layer thickness was set to 1.70 nm, 1.75 nm, and 1.80 nm. Hint was
3.9 kA/m (48.8 Oe), 2.3 kA/m (28.8 Oe), and 1.0 kA/m (12.5 Oe). In
FIG. 22, there is additionally shown a magnetoresitance effect
curve, which was obtained through the averaging of thicknesses when
the Cu layer thickness of the non-magnetic intermediate layer
contained 20% of a thickness of 1.70 nm, 60% of a thickness of 1.75
nm, and 20% of a thickness of 1.80 nm. This is none other than the
desired magnetoresistive properties that satisfy the inequation,
H10-50<H50-90. In other words, when the Cu layer of the
non-magnetic intermediate layer has a locally nonuniform
distribution of layer thickness with tolerance of about plus or
minus 0.05 nm, the magnetoresistive properties that satisfy the
inequation, H10-50<H50-90, are achieved. In process of
examination of the conditions of formation of the Spin-valve type
GMR film, it has been shown that reducing a deposition rate of the
Cu layer to about 0.6 nm/min facilitates achieving the nonuniform
distribution of layer thickness as mentioned above. Conversely,
when the Cu layer thickness is set to a region of thicknesses, such
as the vicinity of 1.78 nm, where Hint changes sharply with respect
to the Cu layer thickness, wider tolerance of Hint may have to be
set for wafers or lots, and it is therefore desirable that the Cu
layer thickness be set to an appropriate value, allowing also for
controllability. As for Hint, the controllability is improved by
performing plasma process prior to formation of the non-magnetic
intermediate layer, introducing a trace of oxygen before and after
the formation of the non-magnetic intermediate layer, or
appropriately selecting a material for the cap layer. An
appropriate combination of these manufacturing methods enables
achieving the desired magnetoresistive properties with high
yields.
[0072] The description has been given assuming that the Hint value
is set to a plus (or the ferromagnetic interlayer coupling).
However, even if the Hint value is set to a minus (or the
antiferromagnetic interlayer coupling), the Hint value can be used
for the magnetic encoder of the present invention in precisely the
same way. As is apparent from FIG. 21, the negative Hint value
having an absolute value large enough for use in the magnetic
encoder is obtained in the vicinity of a Cu layer thickness of 2.0
nm. Since the configuration of such a Spin-valve type GMR film and
a manufacturing method therefor, except for the Cu layer thickness,
are identical with those mentioned above, detailed description
thereof is omitted.
[0073] Other auxiliary means for achieving the magnetoresistive
properties of the present invention as shown in FIG. 16 include the
approach of intentionally deviating the direction of magnetization
of the ferromagnetic pinned layer from the magnetized direction of
the magnetic medium (or the pattern width direction of the
magnetoresistive element). FIG. 23 shows a change in the
H50-90/H10-50 ratio with respect to a deviation angle of the
direction of magnetization of the ferromagnetic pinned layer. It
can be seen that the H50-90/H10-50 ratio increases monotonically as
the deviation angle of the direction of magnetization of the
ferromagnetic pinned layer increases. As shown in FIG. 24, however,
the MR ratio decreases as the deviation angle of the direction of
magnetization of the ferromagnetic pinned layer increases, and it
is therefore required that the deviation angle be appropriately
controlled. Preferably, the deviation angle is set at 30 degrees or
less, according to the degree of decrease in the MR ratio.
[0074] Description will be given with regard to a manufacturing
method that includes intentionally deviating the direction of
magnetization of the ferromagnetic pinned layer from the magnetized
direction of the magnetic medium. FIG. 25 shows a flowchart about
the manufacturing method. After having been formed, the Spin-valve
type GMR film is subjected to first annealing treatment under
magnetic field in a vacuum, while being subjected to a magnetic
field in the pattern width direction of the magnetoresistive
element (or the magnetized direction of the magnetic medium), the
magnetic field being of such magnitude that the direction of
magnetization of the ferromagnetic pinned layer reaches sufficient
magnetic saturation. By the annealing treatment, the direction of
magnetization of the ferromagnetic pinned layer is fixed in the
pattern width direction of the magnetoresistive element. The
annealing treatment can take place at a temperature of about 230 to
300.degree. C. for a duration of the order of a few hours. A
annealing treatment temperature exceeding 300.degree. C. is not
desirable because it can possibly cause interdiffusion on an
interface between the layers and hence cause a decrease in the MR
ratio. Then, the Spin-valve type GMR film is subjected to second
annealing treatment under magnetic field in a vacuum, while being
subjected to a magnetic field in a direction perpendicular to the
above-mentioned direction (or in the pattern length direction of
the magnetoresistive element). By the annealing treatment, the
direction of magnetization of the ferromagnetic pinned layer is
fixed in a direction deviating from the pattern width direction of
the magnetoresistive element. It is required that the conditions of
the second annealing treatment under magnetic field be
appropriately adjusted, because too great an angle of deviation of
the direction of magnetization of the ferromagnetic pinned layer
causes the decrease in the MR ratio as mentioned above. The most
important parameter is the magnitude of the applied magnetic field,
and the magnitude thereof can be set according to the resistance of
the magnetization of the ferromagnetic pinned layer to an external
magnetic field.
[0075] For our examination, the conditions of the first annealing
treatment under magnetic field were 270.degree. C. and three hours
under an applied magnetic field of 4 MA/m (50 kOe) in the pattern
width direction of the magnetoresistive element, and the conditions
of the second annealing treatment under magnetic field were
250.degree. C. and three hours under an applied magnetic field of
80 kA/m (1 kOe) in the direction perpendicular to the pattern width
direction (or in the pattern length direction). The second
annealing treatment under magnetic field induces into, the
ferromagnetic free layer, the uniaxial magnetic anisotropy such
that the pattern length direction coincides with the axis of easy
magnetization, resulting in also the effect of achieving good soft
magnetic properties. Incidentally, the second annealing treatment
under magnetic field can apply a magnetic field containing at least
a component in the direction perpendicular to the pattern width
direction of the magnetoresistive element to thereby, in the same
manner, deviate the direction of magnetization of the ferromagnetic
pinned layer from the pattern width direction. The same effect is
achieved, for example, by performing the annealing treatment under
magnetic field through the application of a magnetic field in a
direction that forms an angle of 45 degrees with respect to the
pattern width direction. Also in this case, it is required that the
magnitude of the applied magnetic field be appropriately adjusted.
Finally, the Spin-valve type GMR film is subjected to patterning
into a substantially rectangular shape to thereby yield the
magnetoresistive element. The order in which the processes are
performed may be changed so that the patterning can take place
immediately after the formation of the Spin-valve type GMR film.
However, there arises the need to appropriately set the conditions
of the annealing treatment under magnetic field, noting that, after
the patterning, the control of the direction of magnetization can
possibly become difficult under the influence of a diamagnetic
field on the end of the element.
[0076] Description will be given with regard to the results of
evaluations of the magnetic encoder fabricated by using the
above-mentioned configuration and manufacturing method. The
Spin-valve type GMR film was formed by use of sputtering method.
The Spin-valve type GMR film was configured of the glass substrate:
the seed layer made of Ta of 2.5 nm thick, NiFeCr of 3.2 nm thick,
and NiFe of 0.8 nm thick; the antiferromagnetic layer made of MnPt
of 14 nm thick; the ferromagnetic pinned layer made of CoFe of 1.8
nm thick, Ru of 0.45 nm thick, and CoFe of 2.2 nm thick; the
non-magnetic intermediate layer made of Cu of 1.75 nm thick; the
ferromagnetic free layer made of CoFe of 1 nm thick and NiFe of 3
nm thick; and the cap layer made of Cu of 0.6 nm thick and Ta of 3
nm thick. Then, the Spin-valve type GMR film was subjected to the
first annealing treatment under magnetic field, the conditions of
which were 270.degree. C. and three hours in a vacuum under an
applied magnetic field of 4 MA/m (50 kOe) in the pattern width
direction of the magnetoresistive element. Subsequently, the
Spin-valve type GMR film was subjected to the second annealing
treatment under magnetic field, the conditions of which were
250.degree. C. and three hours under an applied magnetic field of
80 kA/m (1 kOe) in the direction perpendicular to the pattern width
direction of the magnetoresistive element. Under this condition,
measurements were made to determine the magnetoresistive
properties, while sweeping the magnetic field. The results of the
measurements were an interlayer coupling field Hint of 1.9 kA/m
(23.8 Oe) or less and H50-90/H10-50=2.8. The Spin-valve type GMR
film was subjected to patterning in a pattern width of 5 .mu.m and
a pattern length of 100 .mu.m by photo-lithography process and ion
milling process (the details of which are omitted) to thereby yield
the magnetoresistive element. As shown in FIG. 6, four
magnetoresistive elements were series-connected to form the
magnetic sensor. Specifically, the magnetoresistive elements 21 and
22 were disposed as spaced a distance of 20 .mu.m away from each
other, the magnetoresistive elements 23 and 24 were disposed as
spaced a distance of 20 .mu.m away from each other, and the
magnetoresistive elements 22 and 23 were disposed as spaced a
distance of 10 .mu.m away from each other. As shown in FIG. 6, the
magnetoresistive element 21 was connected at one end to the
electric power supply Vcc, and the magnetoresistive element 24 was
connected at one end to a ground GND. The midpoint electric
potential was detected between the magnetoresistive elements 22 and
23 and was used for output evaluation. The magnetic medium was used
as magnetized in alternating multipolar form with a pitch .lamda.
of 20 .mu.m, as shown in FIG. 5. The magnetic sensor was disposed
in such a manner that the pattern width direction coincided with
the magnetized direction of the magnetic medium. Output evaluations
were performed for gaps of varying lengths between the magnetic
sensor and the magnetic medium. When the gap is laid between 0 and
11.5 .mu.m, the normalized output was 0.85 or more, and stable
output characteristics could be achieved even at the occurrence of
variations in the gap. In other words, the magnetic sensor suitable
for high resolution and having high reliability, and the magnetic
encoder using the magnetic sensor could be fabricated.
Second Embodiment
[0077] As previously mentioned, the magnetoresistive element is of
such a shape that the pattern length L is very great as compared to
the pattern width W. The narrower pattern width W is more favorable
in particular for an increase in the resolution for detection of
the signal magnetic field from the magnetic medium. However, an
increase in an aspect ratio of the pattern length L to the pattern
width W results in the shape anisotropy, which causes the
induction, into the ferromagnetic free layer, of the uniaxial
magnetic anisotropy such that the direction of the pattern length L
coincides with the axis of easy magnetization. Thus, the effective
Hk* value of the magnetoresistive element becomes larger than the
Hk value of the ferromagnetic free layer in itself. This is not
desirable because it not only reduces the sensitivity of the
magnetoresistive element to the magnetic field but also causes
output setoff and hence a decrease in output at the time of
superposition of outputs from two magnetoresistive elements, as
shown in FIG. 9B. A uniaxial anisotropy field resulting from the
shape anisotropy increases in inverse proportion to the pattern
width W and in proportion to the amount of magnetization of the
ferromagnetic free layer, if the pattern length L is fixed. A
reduction in the amount of magnetization of the ferromagnetic free
layer is therefore effective for suppression of an increase in the
uniaxial anisotropy field resulting from the shape anisotropy.
However, a simple reduction in the thickness of the ferromagnetic
free layer for the reduction in the amount of magnetization thereof
is not desirable because of causing a decrease in the MR ratio, an
increase in the coercivity of the ferromagnetic free layer,
difficulty in controlling the interlayer coupling field Hint, and
so on.
[0078] To reduce the amount of magnetization of the ferromagnetic
free layer without causing such problems, the ferromagnetic free
layer of a configuration of a so-called "synthetic ferri-magnet"
type is effective. Specifically, a ferromagnetic free layer 204 is
configured of a first soft magnetic layer 2041, an
antiferromagnetic interlayer coupling layer 2042, and a second soft
magnetic layer 2043, as the configuration of the magnetoresistive
element is illustrated schematically in FIG. 26. In this
configuration, the first and second soft magnetic layers are
antiferromagnetically coupled with the antiferromagnetic interlayer
coupling layer in between. In other words, the magnetizations of
the two soft magnetic layers are oriented in antiparallel relation,
and the substantial amount of magnetization of the ferromagnetic
free layer corresponds to a difference between the amounts of
magnetizations of the first and second soft magnetic layers. For
example, the substantial amount of magnetization of the
ferromagnetic free layer can be 2 Tnm (4-2=2 Tnm), assuming that
the amount of magnetization of the first soft magnetic layer is 4
Tnm, which is determined by calculating the product of saturated
magnetic flux density and layer thickness, and the amount of
magnetization of the second soft magnetic layer is 2 Tnm. When
antiferromagnetic interlayer coupling between the two soft magnetic
layers with the antiferromagnetic interlayer coupling layer in
between is set much stronger than the strength of the signal
magnetic field from the magnetic medium, the two soft magnetic
layers integrally function as a single ferromagnetic free layer
having a small amount of magnetization. This configuration can
suppress a decrease in the MR ratio because of being able to keep
great the thickness of the soft magnetic layer in contact with the
non-magnetic intermediate layer.
[0079] A comparison will now be made, as to the relation between
the effective anisotropy field Hk* and the pattern width W of the
magnetoresistive element, between the ferromagnetic free layer of a
typical configuration and the ferromagnetic free layer of a
synthetic ferri-magnet type configuration. Firstly, definition of
Hk* will be described with reference to FIG. 27. Assuming that the
magnetoresistive element has a magnetoresitance effect curve as
shown in FIG. 27, and that the interlayer coupling field Hint is
the magnetic field at a point where the MR ratio is 1/2 of its
maximum value, the result of subtraction of Hint from the magnetic
field at a point of intersection of a tangent to the curve at the
above point and the maximum MR ratio is defined as Hk*.
[0080] FIG. 28 shows the dependence of the effective anisotropy
field Hk* estimated from the magnetoresitance effect curve of the
magnetoresistive element upon the pattern width W. The pattern
length is 100 .mu.m. Shown is a comparison between the use of the
ferromagnetic free layer of the typical configuration and the use
of the ferromagnetic free layer of the synthetic ferri-magnet type
configuration. The Spin-valve type GMR film is configured basically
of: the seed layer made of NiCrFe of 3.2 nm thick and NiFe of 0.8
nm thick; the antiferromagnetic layer made of MnPt of 14 nm thick;
the ferromagnetic pinned layer made of CoFe of 1.5 nm thick, Ru of
0.46 nm thick, and CoFe of 1.5 nm thick; the non-magnetic
intermediate layer made of Cu; the ferromagnetic free layer; and
the cap layer made of Cu of 0.6 nm thick and Ta of 2.0 nm thick.
The ferromagnetic free layer of the typical configuration was made
of CoFe of 1.0 nm thick and NiFe of 2.0 nm thick, and the
ferromagnetic free layer of the synthetic ferri-magnet type
configuration was made of CoFe of 1.0 nm thick, NiFe of 2.0 nm
thick, Ru of 0.46 nm thick, and NiFe of 1.0 nm thick. The effective
amount of magnetization of the ferromagnetic free layer of the
typical configuration was 3.6 Tnm, and the effective amount of
magnetization of the ferromagnetic free layer of the synthetic
ferri-magnet type configuration was 2.7 Tnm. In order for the
interlayer coupling field Hint to be 1.6 kA/m (20 Oe), the
thickness of the non-magnetic intermediate layer, namely, the Cu
layer, of the typical configuration was 2.4 nm, and the thickness
of the Cu layer of the synthetic ferri-magnet type configuration
was 2.35 nm.
[0081] From FIG. 28, it can be seen that the effective anisotropy
field Hk* increases as the pattern width decreases. The
ferromagnetic free layer of the typical configuration has the
larger Hk* value than the ferromagnetic free layer of the synthetic
ferri-magnet type configuration, and a difference between the Hk*
values increases particularly as the pattern width decreases. The
difference is understood to result from a difference in the
effective amount of magnetization of the ferromagnetic free layer.
For example, when the pattern width is 5 .mu.m, the ferromagnetic
free layer of the synthetic ferri-magnet type configuration has an
effective anisotropy field Hk* of 1.0 kA/m (Hk*=12.5 Oe), whereas
the ferromagnetic free layer of the typical configuration has an
effective anisotropy field Hk* of 1.8 kA/m (Hk*=22.5 Oe), which is
larger than the interlayer coupling field Hint. The effective
anisotropy field Hk* exceeding the interlayer coupling field Hint
is not desirable because of causing the output setoff at the time
of superposition of the outputs from the two magnetoresistive
elements, as previously mentioned. In other words, the synthetic
ferri-magnet type configuration is applied to the ferromagnetic
free layer to reduce the effective amount of magnetization thereof,
thereby making it possible to suppress an increase in the effective
anisotropy field Hk* even if the pattern width is reduced.
Accordingly, the use of the configuration of the present invention
enables achieving high response sensitivity even if the pattern
width is narrowed, thus achieving the magnetic encoder having high
resolution.
[0082] Consequently, the first embodiment is modified to use the
ferromagnetic free layer of the synthetic ferri-magnet type
configuration made of CoFe of 1.0 nm thick, NiFe of 2.0 nm thick,
Ru of 0.46 nm thick, and NiFe of 1.0 nm thick, in place of the
ferromagnetic free layer of the typical configuration made of CoFe
of 1.0 nm thick and NiFe of 2.0 nm thick. This enables making the
pattern width still narrower, as well as suppressing output
variations due to fluctuations in the gap between the magnetic
sensor and the magnetic medium as described for the first
embodiment, thereby providing the magnetic encoder having still
higher resolution. Since the basic configurations of the
magnetoresistive element, the magnetic sensor and the magnetic
encoder and the manufacturing methods therefor, except for the
configuration of the ferromagnetic free layer, are precisely the
same as those of the first embodiment, detailed description thereof
is omitted.
[0083] Using the configuration and manufacturing method as
described above makes it possible to fabricate the magnetic sensor
having high resolution and also having high reliability with little
change in output even at the occurrence of variations in the gap
between the magnetic sensor and the magnetic medium, and the
magnetic encoder using the magnetic sensor.
* * * * *