U.S. patent number 3,607,460 [Application Number 04/776,619] was granted by the patent office on 1971-09-21 for first order transition films for magnetic recording and method of forming.
This patent grant is currently assigned to General Electric Company. Invention is credited to James M. Lommel.
United States Patent |
3,607,460 |
Lommel |
September 21, 1971 |
FIRST ORDER TRANSITION FILMS FOR MAGNETIC RECORDING AND METHOD OF
FORMING
Abstract
Thin films of iron-rhodium exhibiting a broadly hysteretic first
order transition between the ferromagnetic and antiferromagnetic
states are produced by sequentially depositing iron and rhodium
films upon a refractory substrate at a pressure in the range of
1.times.10.sup.-.sup.6 torr, annealing the structure in a vacuum of
1.times.10.sup.-.sup.6 torr at a temperature of approximately
700.degree. C. for 1 hour to produce a complete diffusion of the
iron and rhodium layers, and subsequently subjecting the diffused
layers to a second anneal in an atmosphere greater than 10 parts
per million oxygen in a thermal cycle that includes slowly heating
the structure to 400.degree. C., maintaining the 400.degree. C. for
approximately 10 minutes and slowly cooling to room temperature.
Films thus formed are advantageously employed in the recording of
digital information by electron beam heating individual regions
through a first order transition to the ferromagnetic state
whereupon the regions are permitted to cool to a biasing
temperature slightly higher than the temperature of transition back
to an antiferromagnetic state. A magnetic field then is applied to
the entire film to magnetize only those regions of the film in the
ferromagnetic state and readout of the recorded information can be
achieved by conventional electron beam microscopy. The
ferromagnetism of the film subsequently can be erased by cooling
the film below the transition temperature to the antiferromagnetic
state or by the application of a strain to the film.
Inventors: |
Lommel; James M. (Schenectady,
NY) |
Assignee: |
General Electric Company
(N/A)
|
Family
ID: |
25107921 |
Appl.
No.: |
04/776,619 |
Filed: |
November 18, 1968 |
Current U.S.
Class: |
148/306; 148/108;
365/171; 420/462 |
Current CPC
Class: |
H01F
10/002 (20130101); B82Y 25/00 (20130101); G11C
13/06 (20130101); H01F 10/325 (20130101) |
Current International
Class: |
H01F
10/00 (20060101); G11C 13/04 (20060101); G11C
13/06 (20060101); H01F 10/32 (20060101); H01f
010/00 (); H01f 001/14 () |
Field of
Search: |
;75/172 ;340/174TF
;148/31.55,108 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Rutledge; L. Dewayne
Assistant Examiner: White; G. K.
Claims
I claim:
1. A film of iron-rhodium and alloys thereof comprising:
alternately deposited layers of iron and rhodium, annealed
together, and totaling less than 1 mil in thickness;
said film composed of from 50 to 65 atom percent rhodium;
said annealed layers characterized by a first order transition
between ferromagnetic and antiferromagnetic states in excess of 50
percent of the film when the film is temperature cycled through the
thermal hysteresis loop of the film.
2. A film of iron-rhodium according to claim 1 wherein said film is
further characterized by a thermal hysteresis loop having a thermal
width between 10.degree. C. and 200.degree. C. at the mean
magnetization of said film.
3. A film of iron-rhodium according to claim 1 characterized by the
transformation of at least 90 percent of the film from the
antiferromagnetic state to the ferromagnetic state upon heating the
film to a temperature 70.degree. C. above the critical transition
temperature of the film.
4. A film of iron-rhodium according to claim 1 wherein said film is
less than 3,000A thick and exhibits a first order transition
between the ferromagnetic and antiferromagnetic states in excess of
90 percent of the film when temperature cycled through the thermal
hysteresis loop of the film.
5. A film of iron-rhodium and alloys thereof according to claim 4
wherein said film includes less than 10 percent of a metal,
codeposited with at least one of said iron and rhodium layers,
selected from the group consisting of ruthenium, osmium, iridium,
and platinum.
6. A film of iron-rhodium and alloys thereof according to claim 4
wherein said film contains less than 10 atom percent of a metal,
codeposited with at least one of said iron and rhodium layers,
selected from the group consisting of palladium, vanadium,
manganese and gold.
7. A film of iron-rhodium and alloys thereof according to claim 4,
wherein said film includes less than 10 atom percent of a metal,
deposited as a layer within at least one set of said iron and
rhodium layers, selected from the group consisting of ruthenium,
osmium, iridium and platinum.
8. A film of iron-rhodium and alloys thereof according to claim 4
wherein said film contains less that 10 atom percent of a metal,
deposited as a layer within at least one set of said iron and
rhodium layers, selected from the group consisting of palladium,
Vanadium, manganese and platinum.
Description
THE DISCLOSURE
This invention relates to iron-rhodium films having a substantially
complete first order transition between the magnetic and
nonmagnetic states on heating and the methods of forming such
films. These films are particularly useful in the recording of
digital information by a unique recording scheme wherein
information is stored by the conversion of individual regions to a
magnetic state rather than by the relative alignment of the
magnetization in selected regions of completely magnetic films. The
invention described herein was made in the course of or under a
contract or subcontract thereunder with the Department of the Air
Force.
The intermetallic compound iron-rhodium has been a source of
scientific curiosity because of the first order transition
exhibited by the bulk material in abruptly transforming from the
antiferromagnetic to the ferromagnetic states upon heating to a
temperature above approximately 60.degree. C. Films of the compound
heretofore produced by conventional film forming techniques
however, have been characterized by a broad thermal hysteresis
(contrary to the narrow thermal hysteresis of approximately
10.degree. C. exhibited by the well annealed bulk material) and
only a partial transition to the antiferromagnetic state upon
cooling. Such results have been reported in the Journal of Applied
Physics, Vol. 37, No. 3, 1483-1484, March 1, 1966. It is a
particular object of this invention to provide a thin film of
iron-rhodium exhibiting a broad thermal hysteresis with a high
percentage transition between the ferromagnetic and
antiferromagnetic states and the method of forming of such a
film.
The first order transition characteristics of the film also permit
the recording of digital information by novel techniques. For
example, prior to this time digital information has been recorded
upon magnetic materials by an orientation of the magnetic domains
at selected regions in a chosen direction. Illustrative of these
prior techniques is Curie point writing, wherein information is
stored by aligning the magnetization of regions selectively heated
above the Curie temperature of the magnetic film and cooling the
film in the presence of an aligning magnetic field. Information
then can be readout utilizing the magneto-optic Kerr or Faraday
effect. Similarly, in compensation point writing, heating of
selected regions of a gadolium iron garnet film by a laser or
electron beam decreases the coercive force of the heated regions to
permit alignment of the magnetization of the heated regions
utilizing an externally applied field.
It is an additional object of this invention to provide a novel
digital recording technique wherein information is thermally
recorded by a conversion of selected regions of a recording film
between the magnetic and the nonmagnetic states in a first order
transition.
A further object of this invention resides in providing a novel
recording medium for the storage of digital information in
accordance with the recording techniques of this invention.
It is still another object of this invention to provide a recording
medium wherein the ferromagnetism can be mechanically erased.
These and other objects of this invention generally can be achieved
utilizing a thin, e.g. less than 1 mil thick, iron-rhodium film
having a composition range between 50-65 atom percent rhodium and
characterized by a first order transition between the ferromagnetic
and antiferromagnetic states in excess of 50 percent of the film
when temperature cycled through the thermal hysteresis loop of the
film. To provide a suitable thermal tolerance for recording, the
film preferably has a thermal hysteresis loop having a width
between 10.degree. C. and 200.degree. C. at the mean magnetization
of the film.
Iron-rhodium films having these characteristics generally can be
formed by positioning a conventional iron-rhodium film, e.g. a
50-65 atom percent rhodium film characterized by a thermal
hysteresis in excess of 50.degree. C. and a transition of less than
50 percent of the film between the ferromagnetic and
antiferromagnetic state upon thermal cycling, in an atmosphere
containing oxygen in a quantity greater than 10 parts per million
and annealing the film in the partial oxygen atmosphere to increase
the portion of the film undergoing transition to an amount in
excess of 50 percent of the films. For a broad thermal hysteresis
in the film, the oxidation anneal preferably is conducted for a
period between 5 minutes and 4 hours at a temperature between
100.degree. C. and 800.degree. C. and the oxygen content of the
atmosphere wherein the anneal is conducted is less than
approximately 1,000 parts per million oxygen.
Information is recorded in digital form upon the medium by
selectively heating regions of the film in the nonmagnetic state to
a temperature whereat a first order transition to the magnetic
state is effected and applying a magnetic field to the film to
magnetize the regions of the film converted to the magnetic state.
Thus the digital information is stored by a unique method which
comprises the recording of information of a first magnitude as a
magnetic region in a film of homogeneous composition and recording
information of a second magnitude as a juxtaposed nonmagnetic
region in the film. The location of the magnetized regions of the
film then can be detected by conventional readout means such as
electron beam microscopy.
The novel features believed characteristic of the invention are set
forth in the appended claims. The invention itself, together with
further objects and advantages thereof may best be understood by
reference to the following description, taken in connection with
the accompanying drawings, in which:
FIG. 1 is a block diagram depicting a technique for the formation
of first order transition iron-rhodium films in accordance with
this invention,
FIG. 2 is a graph depicting the variation of magnetization with
temperature in an iron-rhodium thin film prior to an oxidation
anneal,
FIG. 3 is a graph depicting the variation of magnetization with
temperature in an iron-rhodium film subsequent to an oxidation
anneal in accordance with this invention,
FIG. 4 is a graph depicting the variation of magnetization,
hysteresis width and percent transition of an iron-rhodium film
with the oxygen content of the second annealing atmosphere,
FIG. 5 is an enlarged sectional view of a digital recording medium
employing the iron-rhodium film of this invention as a recording
film,
FIG. 6 is an enlarged pictorial illustration of one method of
readout from the film,
FIG. 7 is a graph illustrating the effect of stress upon an
iron-rhodium film,
FIG. 8 is a graph depicting the magnetic field strength required to
produce a given magnetization in an iron-rhodium film formed in
accordance with this invention, and
FIG. 9 is a graph depicting the variation of magnetization with
temperature at diverse locations along the thermal hysteresis loop
of the iron-rhodium film.
A preferred method of forming an iron-rhodium film having a broadly
hysteretic first order transition between the ferromagnetic and
antiferromagnetic states is depicted generally in FIG. 1 and
initially includes the preparation of a conventional iron-rhodium
film having a broad thermal hysteresis and an incomplete
transition, i.e. a minimum magnetization between one-half and
three-quarters the maximum film magnetization, by sequentially
vacuum depositing iron and rhodium films upon a refractory
substrate and annealing the deposited films in a vacuum better than
10.sup..sup.-5 torr at a temperature between 400.degree. and
700.degree. to diffuse the layers To obtain a first order complete
transition in the alloy film, a subsequent anneal of the film is
conducted in a flowing nitrogen atmosphere containing oxygen in
quantities greater than 10 parts per million.
The sequential vacuum deposition of the iron and rhodium films
preferably is accomplished by electron beam heating iron and
rhodium sources positioned upon a water cooled hearth within a
vacuum chamber which is evacuated to a pressure less than
approximately 10.sup..sup.-5 torr to reduce oxidation of the films
during the evaporation. Typically, the iron and rhodium are
evaporated in a vacuum of approximately 10.sup..sup.-6 torr range
with the sequence of the deposition of the alternate layers not
being important although preferably the iron film is deposited
initially and subsequently overlayed with rhodium to inhibit
oxidation of the iron during the subsequent anneal. Similarly, the
number of alternately deposited layers is not of significance
provided the respective thicknesses of the layers result in a 50-65
atom percent rhodium composition film when annealed to diffuse the
layers. For recording purpose wherein a low remanent magnetization
is desirable, a high rhodium percentage, e.g. approaching 65 atom
percent, is advantageously employed.
The deposition rate employed in forming the alternate iron and
rhodium layers generally is not critical with deposition rates of
4A per second to 25A per second being suitably employed for the
depositions. The higher deposition rates are preferred however
because of the reduced film contamination produced by the more
rapid deposition. During deposition of the alternate layers upon
the substrate, the substrate preferably is heated, e.g. to
300.degree. C. to reduce the stress in the rhodium film deposited
thereon and increase its adherence to the substrate. The substrate
employed for the deposition of the alternate layers thereon
desirably is a refractory material, e.g. fuse silica, alumina,
silicon or clear sapphire, to inhibit diffusion of impurities into
the film from the substrate during annealing and to withstand the
elevated temperatures of the diffusion anneal, e.g. up to
700.degree. C.
In general, the iron and rhodium sources employed in the vacuum
evaporation should be of high purity although minor amounts of some
impurities can be tolerated and often are beneficial to the
magnetic characteristics of iron-rhodium. For example, certain
materials, such as molybdenum, nickel, copper and niobium can
completely destroy the transition characteristics of iron-rhodium
if present in quantities in excess of 2 atom percent. Other
materials, however, serve to shift the transition temperature of
the iron-rhodium and can be beneficially employed to position the
magnetic hysteresis curve of the alloy at a desired thermal value,
i.e. less than 10 atom percent ruthenium, osmium, iridium, and
platinum tend to increase the critical transition temperature of
iron-rhodium while palladium, vanadium, manganese, and gold in
quantities of 10 atom percent or less tend to decrease the critical
transition temperature of iron-rhodium. When it is desired that one
or more of these impurities be incorporated into the film, the
impurity can be codeposited with the iron or rhodium layers as an
alloy or deposited as a separate alternate layer in a thickness
suitable for the desired percentage of impurity in the film. The
total thickness of the deposited layers however characteristically
is less than 1 mil with typical films having thicknesses between
200 A and 3,000 A.
After deposition of the iron and rhodium films upon the substrate,
the structure is annealed at a temperature between 400.degree. and
700.degree. C. in a vacuum below 10.sup..sup.-5 torr for the period
required to completely diffuse the layers together thereby forming
the intermetallic iron-rhodium compound. The diffusion anneal
preferably is conducted in a very good vacuum, e.g. 10.sup..sup.-6
torr, to inhibit an excessive oxidation of the film deleterious to
the film transition characteristics while annealing at temperatures
substantially above 700.degree. C. tends to produce island
structures in the iron-rhodium film reducing the adherence of the
film upon the substrate. Because a very fine island structure also
can reduce lateral heat flow between juxtaposed bit sites in the
writing of information into the film, the size of a bit site
advantageously could be reduced by a high temperature diffusion
anneal.
In general, the period of the diffusion anneal is not critical
provided the evacuation chamber is of a reduced oxygen content, for
example, at a vacuum of 4.times.10.sup..sup.-7 torr in a
dynamically pumped vacuum system employing a 2-inch oil diffusion
pump and a liquid nitrogen cold trap to prevent contamination of
the chamber, complete diffusion of the layers and a broadly
hysteretic transition in the magnetic properties of the film with
temperature was obtained when the diffusion anneal was conducted at
700.degree. C. for periods between 1 hour and 25 hours. When the
oxygen content of the diffusion anneal chamber is increased, an
excessively protracted diffusion anneal can substantially oxidize
the intermetallic compound formed by the anneal thereby destroying
the thermal hysteresis of the film. Annealing at 400.degree. C. for
periods as short as 1 hour has been found to produce films
exhibiting a magnetic hysteresis upon thermal cycling and a
physical structure identical to that of bulk samples of the
intermetallic compound iron-rhodium when examined by X-ray
diffraction.
The thermal hysteresis curve of an iron-rhodium film containing
0.56 atom fraction rhodium and formed by a diffusion anneal at
690.degree. C. for 1 hour at a pressure of 4.times.10.sup..sup.-7
torr when temperature cycled in the presence of a 1,000 oersted
field is characterized by the curve of FIG. 2, i.e. less than 50
percent of the film undergoes a transition between the
ferromagnetic and antiferromagnetic states with temperature
cycling. The thermal hysteresis of the film is broad, being in
excess of 150.degree. C. at the mean magnetization of the film in
comparison to well annealed bulk iron-rhodium which
characteristically exhibits a thermal hysteresis of 10.degree. C.
or less.
The fraction of film undergoing transition between the
ferromagnetic state and the antiferromagnetic state can be
determined approximately from the formula,
F=(M.sub.1 -M.sub.2)/M.sub.1
wherein F is the fraction of film undergoing transition
M.sub. 1 is the maximum magnetization of the film upon cooling
after heating the film above the transition temperature of the
film, and
M .sub.2 is the minimum magnetization of the film upon heating
after cooling the film below the film transition temperature.
For films which transform completely to the antiferromagnetic
state, the minimum magnetization (M .sub.2) on heating is zero and
F is equal to 1. It thus may be stated that the iron-rhodium
intermetallic compound films formed by the diffusion anneal are
characterized by a first order transition in which less than 50
percent of the film transforms between the ferromagnetic and
antiferromagnetic states and in which the thermal hysteresis is in
excess of 50.degree. C. at the center of the magnetic thermal
hysteresis loop of the film.
After annealing the layered film structure to form a homogeneous
iron-rhodium film having the broadly hysteretic incomplete
transition characterized by FIG. 2, the iron-rhodium film is again
annealed in an atmosphere containing oxygen in quantities greater
than 10 parts per million to produce a film having a substantially
complete transition such as is characteristically displayed by the
curve of FIG. 3. Preferably the anneal is conducted at a
temperature of approximately 400.degree. C. with the film being
raised from room temperature to 400.degree. C. in increments of
approximately 6.degree. C./min., held at 400.degree. C. for
approximately 5 minutes and cooled back to room temperature at a
rate similar to the heating rate. The atmosphere employed during
the anneal preferably is one atmosphere of flowing nitrogen
containing oxygen in quantities between 10 parts per million and
1,000 parts per million as measured by the oxygen sensor described
in U.S. Pat. application Ser. No. 554,443, and now abandoned filed
June 1, 1966 in the name of H. S. Spacil and assigned to the
assignee of the present invention. In general, the quantity of
oxygen present during the oxidation anneal is dependent upon the
temperatures employed for the anneal with higher temperatures
significantly reducing the quantity of oxygen required. Similarly,
the oxygen content needed in the annealing chamber varies as a
function of the time employed for the anneal. Annealing for periods
in excess of 1 hour at 400.degree. C. at a partial pressure above
10.sup.4 parts per million oxygen can substantially oxidize the
iron-rhodium intermetallic compound thereby destroying the magnetic
hysteresis of the film. The annealing desirably is conducted for
periods between 5 minutes and 4 hours at temperatures of
400.degree. C. when the oxygen content of the annealing atmosphere
is between 10 and 200 parts per million. In general, the oxygen
content, temperature and period of the oxidation anneal are
interdependent and controlled to produce an oxidation of the
iron-rhodium film sufficient to effect a transition in excess of 50
percent of the film without unduly oxidizing the film to reduce the
saturation magnetization below a recordable level. Iron-rhodium
films given a second anneal in one atmosphere nitrogen containing
oxygen in concentrations as high as 10.sup.4 parts per million
exhibited a nearly complete transition between the ferromagnetic
and antiferromagnetic states. The thermal hysteresis loop of the
film however was narrow, i.e. 10.degree. C. at the mean
magnetization, and the transition between the antiferromagnetic and
ferromagnetic states was gradual with a transition in approximately
90 percent of the film requiring an increase in temperature beyond
the film critical transition temperature approximately twice that
required for films annealed in one atmosphere nitrogen containing
less than 100 parts per million oxygen.
The effect of oxygen during the second anneal upon the magnetic
properties of an iron-rhodium film given a second anneal for 10
minutes at 400.degree. C. subsequent to a diffusion anneal at
700.degree. C. for 1 hour in a vacuum of 5.times.10.sup..sup.-7
torr is depicted in FIG. 4. As can be noted from the percent film
transition curve, identified by reference numeral 14, transition in
over 80 percent of the iron-rhodium film occurred at oxygen levels
slightly above 7 parts per million oxygen in one atmosphere of
flowing nitrogen and the percent of film undergoing transition
increased from an initial value (identified by reference numeral
15) below 40 percent to a value of 90 percent when annealed for 10
minutes at oxygen concentrations above 50 parts per million. The
hysteresis width of the transition at the mean magnetization,
illustrated by curve 16, decreased from an original value
(identified by reference numeral 17) in excess of 160.degree. C.
prior to the second anneal to a value of approximately 80.degree.
C. when annealed in a nitrogen atmosphere containing 40 parts per
million oxygen. The maximum magnetization of the iron-rhodium film
(identified by reference numeral 18) remained essentially constant
at approximately 120 emu/gm. notwithstanding the diverse oxygen
contents of the second anneal.
In specifically forming one iron-rhodium film in accordance with
this invention, a 99.9 percent electrolytic iron source and a 99.9
percent rhodium source were positioned upon a water cooled crucible
in a conventional vacuum bell jar having a 4 inch oil diffusion
pump and an antimigration liquid nitrogen cold trap. The iron
source had been annealed in dry hydrogen for 1 inch at 900.degree.
C. to reduce the oxygen content of the iron. The vacuum system was
evacuated to approximately 8.times.10.sup..sup.-7 torr and both
sources were melted separately by a 2 KW electron gun to reduce
outgassing during evaporation. The iron source then was electron
beam evaporated at a pressure of 8.times.10.sup..sup.-6 torr and
deposited at a rate of approximately 12 A/sec. upon a clean fused
silica substrate positioned 25 cm. from the sources and heated to a
temperature of 275.degree.-300.degree. C. After deposition of the
iron layer, the rhodium source was evaporated at a pressure of
4.times.10.sup..sup.-6 torr and deposited at 15A/sec. atop the iron
film upon the heated substrate to a thickness sufficient to produce
a 0.54 atom fraction rhodium film upon subsequent diffusion of the
layered structure. The total thickness of the films was
approximately 550A. The structure then was given a diffusion anneal
at 690.degree. C. for 13 1 hour at 8.times.10.sup..sup.-7 torr in a
dynamically pumped vacuum system. After cooling the structure to
room temperature, the film exhibited a microstructure having as a
major component the CsCl structure phase typical of bulk FeRh
samples with an f.c.c. phase with a lattice parameter about the
same as elemental rhodium. The thermal hysteresis loop of the film
in a 1,000 oersted field is depicted in FIG. 2, and shows an
incomplete transition between the antiferromagnetic and
ferromagnetic states with a large thermal hysteresis at the mean
magnetization of the film.
The structure then was given a second anneal in a gaseous
environment of one atmosphere of flowing nitrogen containing an
oxygen concentration between 80 p.p.m. and 100 p.p.m. with the rate
of gaseous flow through the system being approximately 2 ft..sup.3
/hr. The structure was raised from room temperature to 400.degree.
C. in approximately 5 minutes, held at 400.degree. C. for about 10
minutes and cooled to room temperature the same rate. The film
exhibited an approximately 95 percent transition, as portrayed in
the thermal hysteresis loop of FIG. 3 when temperature cycled in a
1,000 oersted field, and a large thermal hysteresis of
approximately 80.degree. C. at the mean magnetization of the film.
The transition was sharper upon heating, occurring within
approximately 60.degree. C., then on cooling where a temperature
change of approximately 200.degree. C. was required to return the
film essentially to the antiferromagnetic state. Repeated
temperature cycling of the film between 125.degree. C. and
150.degree. C. indicated the transition to be stable.
The thermal hysteresis loop illustrated in FIG. 3 and produced by
an iron-rhodium film treated in accordance with the double-anneal
techniques of this invention is characterized by a first order
transition from the antiferromagnetic state to the ferromagnetic
state (as illustrated by a measured magnetization of approximately
115 emu/gm. in a 1000 oersted field when heated above 100.degree.
C.). Upon subsequent cooling of the film below 60.degree. C. the
measured magnetization of the film (and therefore the percentage of
the film in the ferromagnetic state) remains substantially constant
to a temperature of approximately 50.degree. C. whereafter the film
returns to an essentially antiferromagnetic state of 8 emu/gm.
(.+-.2 emu/gm. error in the measured film magnetization) in a first
order transition. Thus approximately 95 percent of the film
undergoes transition between the ferromagnetic and
antiferromagnetic states after annealing a conventional 35 percent
transformed film having a broad hysteresis in an atmosphere having
an oxygen concentration greater than 10 parts per million oxygen.
Another significant characteristic in iron-rhodium films of this
invention is the broad thermal hysteresis of the film, i.e. at the
mean magnetization of the film i.e. approximately 60 emu/gm., the
film is characterized by a thermal hysteresis of approximately
80.degree. C. Although repeated cycling of the film through the
thermal hysteresis loop of the film produces some slight
diminishment in the thermal hysteresis, iron-rhodium films after
10.sup. 5 thermal cycles between -195.degree. C. and 100.degree. C.
still have been found to retain essentially the original thermal
hysteresis.
The iron-rhodium film of this invention also is characterized by a
rapid transition to the ferromagnetic state upon heating above the
critical temperature of the film. As can be seen from FIG. 3, the
magnetization of the film in a 1,000 oersted field increases from a
value of approximately 8 emu/gm. to a maximum value of
approximately 115 emu/gm. between 20.degree. C. and 90.degree. C.,
e.g. over 90 percent of the film is transformed between the
antiferromagnetic and ferromagnetic states within a temperature
span of 70.degree. C.
In general, the magnetic hysteresis loop of the iron-rhodium film
of this invention is relatively square exhibiting a remanent
magnetization to saturation magnetization ratio of approximately
0.7. A sample 550A thick iron-rhodium film given a double-anneal
treatment in accordance with this invention had a coercive force of
approximately 160 oersteds.
Transmission electromicrographs were taken of iron-rhodium films
formed under identical conditions except for the oxidation anneal.
One film was annealed in vacuum and produced an incomplete
transition characterized by the hysteresis curve of FIG. 2 while
the second film was annealed in a flowing nitrogen atmosphere
containing oxygen in concentrations between 10 and 200 parts per
million and produced a complete transition characterized by the
hysteresis curve of FIG. 3. Most aspects of the microstructure,
e.g. grain size, stacking faults, twins, etc., were qualitatively
identical although the film with the complete transition showed a
much more mottled structure within the grains than did the film
having the incomplete transition. Electron beam microscopic
examination indicates that the mottling arises from a discrete
array of particles very regularly arranged in a two dimensional
square network. The network appears to be uniform within a grain
and has a periodicity of approximately 100 A. It may be postulated
that the "dirty" appearance is a fine dispersion of an oxide
phase.
The films of this invention are particularly adapted for
utilization as a digital information recording medium 19 such as is
shown in FIG. 5 wherein an iron-rhodium film 20 less than 1 mil
thick and having a transition characteristic similar to that
illustrated in FIG. 3 is situated atop a thermally conductive
substrate 22 of a material such as silicon or quartz. The substrate
is secured to thermoelectric base 24, e.g. bismuth telluride, lead
telluride, antimony telluride, silver indium telluride, copper
gallium telluride, etc. by a suitable adhesive, e.g. solder layer
26, to permit temperature cycling of the iron-rhodium film by the
thermoelectric base through substrate 22 upon electrical
energization of the thermoelectric base from a DC source (not
shown) through leads 28 and 30. Because iron-rhodium film 20 is
thermally switched between the ferromagnetic and antiferromagnetic
states in a first order transition producing a variation in film
volume, adjacent recording media 19A and 19B forming a memory unit
are spaced by a suitable span, e.g. 2 percent of the film
dimension, to permit nondestructive thermal expansion. When the
dimensions of the recording medium are sufficiently small however,
the memory unit desirably is formed as a unitary structure to avoid
isolation of information storage sites.
To record information in selected sites of 1 mil diameter or less
along the iron-rhodium film thermoelectric layer 24 initially is
energized with DC current in a first direction to cool the
structure below the temperature, t.sub.e or approximately
-150.degree. C. in the hysteresis loop of FIG. 3, at which the film
becomes essentially antiferromagnetic thereby erasing any residual
magnetism in the film. Electrical energization of thermoelectric
base 24 than is terminated to permit the temperature of the
iron-rhodium film to increase to a biasing level, T.sub.b or
approximately 20.degree. C., whereat the film remains in an
antiferromagnetic state below the critical temperature producing a
first order transition of the iron-rhodium film to a ferromagnetic
state. An electron beam from an addressable electron gun, such as
is described in U.S. Pat. b. Ser. No. 671,353, and now U.S. Pat.
No. 3,491,236 filed Sept. 28, 1967, in the name of Sterling
Newberry and assigned to the assignee of the present invention,
then is irradiated upon selected bit sites 20A of the iron-rhodium
film to heat the irradiated bit sites above the transition
temperature of the film, e.g. above 120.degree. C., and the
irradiated bit sites are transformed to the ferromagnetic state in
a first order transition. Upon removal of the electron beam, the
irradiated bit sites, i.e. 20A, cool to the biasing temperature,
T.sub.b. After the application of a sufficiently large magnetic
field for a short time, e.g. a pulsed field greater than 300
oersteds, the irradiated bit sites possess a ferromagnetism
indicative of information of a first magnitude. Those bit sites 20B
not irradiated during recording remain in the antiferromagnetic
state thereby storing digital information of a differing magnitude.
Thus, the selectively recorded iron-rhodium film is of homogeneous
composition and characterized by a plurality of bit sites in either
a magnetic or a nonmagnetic state dependent upon the magnitude of
information recorded at the individual bit sites. In general, an 8
kv, 2.times.10.sup..sup.-7 ampere electron beam irradiation of a 10
micron diameter region of an iron-rhodium film for 4 milliseconds
has been found adequate to convert the irradiated bit sites from
the antiferromagnetic state to the ferromagnetic state. Adjacent
bit sites were not raised above the critical transition temperature
and remained essentially antiferromagnetic.
Readout of the recorded information from the iron-rhodium is
achieved by the application of a pulsed magnetic field in excess of
300 oersteds to the film to align the domains in ferromagnetic bit
sites 20A thereby producing a film characterized by ferromagnetic
bits with a magnetization in a given direction interspaced with
essentially antiferromagnetic bit sites 20B having zero net
measurable magnetization. Because detection of the alignment
direction of the individual bit sites is not required, visual
readout can be easily effected by coating the iron-rhodium film
with a colloidal solution of iron oxide particles (or Bitter
solution) which particles drift to the magnetized bit sites in the
recording medium. Thus a plurality of observable dark areas, e.g.
spots 50 shown in FIG. 6, are produced at the electron beam
irradiated ferromagnetic bit sites with the drift of iron particles
from the essentially antiferromagnetic bit sites 20B resulting in a
relatively clear liquid coating at such sites. when extremely high
speed is desired for readout, other conventional methods of
magnetic detection, e.g. electron beam microscopy can be employed
to located the magnetized bit sites. To erase the recorded
information, thermoelectric base 24 is again energized to reduce
the temperature of iron-rhodium film 20 to T.sub.e whereupon the
entire film returns to the antiferromagnetic state and the
previously recorded information is erased.
Erasure of the recorded information from the iron-rhodium film also
can be effected mechanically by the application of a strain to the
film thereby returning the strained portion of the film to
essentially the antiferromagnetic state, as is depicted by the
curves of FIG. 7. There thermal hysteresis curve was obtained by
temperature cycling a doubly annealed iron-rhodium film along
hysteresis loop 33 to 140.degree. C. to transform the film to the
ferromagnetic state and subsequently cooling the film to
approximately 15.degree. C. whereupon the magnetization of the film
returned along the hysteresis loop to a value of approximately 66
emu/gm. (identified by reference numeral 34). The film then was
hand rubbed with a cotton swab for less than 20 20 and the
magnetization decreased (as shown by dotted line 35) to a value of
approximately 9 emu/gm. with a variation of only 5.degree. C. in
the temperature of the iron-rhodium film. Continued rubbing of the
iron-rhodium film with the cotton swab reduced the measured
magnetization of the film at 10.degree. C. to less than 5 emu/gm.
and low magnetization state obtained by the rubbing induced strain
remained stable as the film was cooled to a temperature below
-180.degree. C. Upon subsequent heating the film to 140.degree. C.
however, the ferromagnetic characteristic of the film returned as
exemplified by solid hysteresis loop 36, although the thermal
hysteresis curve is somewhat narrowed by the effects of the strain
upon the film. In general, the quantity of strain applied to the
iron-rhodium film to effect an erasure of the recorded information
should be greater than 0.3 percent but not so great as to cause the
body centered cubic structure to transform to the paramagnetic face
centered cubic structure.
Desirably, a small quantity of palladium is added to the
iron-rhodium film to shift the critical temperature of the film to
the magnetic state to approximately 30.degree. C., e.g. 5.degree.
C. above room temperature to reduce the electron beam power
required to transform bit sites to the ferromagnetic state while
retaining the nonvolatile characteristics of the medium. Similarly
cooling apparatus, such as thermoelectric base 24, can be omitted
when sufficient iridium or platinum is introduced into the
iron-rhodium film to shift the hysteresis loop by an amount
positioning T.sub.e at 25.degree. C. The film then can be raised to
the biasing temperature T.sub.b at the threshold of the critical
film transition temperature by current through the film or by
electron beam impingement upon the entire film plane. Information
is recorded at selected bit sites by heating with a second electron
beam to increase the temperature of the irradiated sites above the
critical transition temperature of the film thereby converting the
irradiated sites to the ferromagnetic state. Similarly, other
conventional heat sources, e.g. visible or infrared light, can be
employed to raise an iron-rhodium-iridium film to the biasing
temperature of the film.
Although alignment of the magnetic domains within the ferromagnetic
bit sites of iron-rhodium film 20 has been described as being
produced by the application of a magnetic field to the film after
the selectively heated film has been cooled to a biasing
temperature, T.sub.b, the magnetic field also can be applied to the
film simultaneously with the selective heating of the film. In such
event, selective heating of the film bit sites utilizing a laser
beam is preferred to inhibit undersired deflection of the beam by
the field magnetizing the bit sites converted to the ferromagnetic
state. Thus, to write information the film is cooled to a
temperature T.sub.e whereat the entire film is converted to the
antiferromagnetic state and the entire film is thereupon heated to
a biasing temperature T.sub.b at the threshold of a first order
transition to the ferromagnetic state. A laser bean then is
selectively impinged upon individual bit sites of the film to raise
the bit sites above the transition temperature converting the bit
sites to the ferromagnetic state in a first order transition. Upon
removal of the writing laser beam from each irradiated bit site the
irradiated sites return to the biasing temperature along the
thermal hysteresis loop and remain in a ferromagnetic state
relative to the unheated bit sites.
The magnitude of the magnetic field required to magnetize a
ferromagnetic, but demagnetized, iron-rhodium film is depicted by
the curves of FIG. 8 wherein curves 37 and 38 represent the
saturation magnetization and remanent magnetization of the film,
respectively. As can be seen from curve 37, an applied field of 800
oersteds is required for a ferromagnetic iron-rhodium film at room
temperature to reach 0.9 of the maximum saturation magnetization of
the film. Upon termination of the applied magnetic field to the
film, the magnetism of the film decreases to a remanent value,
identified by curve 38 of FIG. 8, approximately 60 emu/gm. below
the saturation magnetization obtainable with the given field.
Another advantageous attribute of iron-rhodium films in accordance
with this invention is the adjustable saturation flux density of
the film as can be observed from FIG. 9 wherein a major thermal
hysteresis loop 40 of an iron-rhodium film in a 1,000 oersted field
is depicted. Thus, if the iron-rhodium film upon temperature
cycling is cooled only partially in the return cycle, e.g.
interrupted at -10.degree. C. along hysteresis loop 40, and then
reheated, the saturation magnetization of the film, as depicted by
curve 42 remains substantially constant at 60 emu/gm. over a
temperature span between -10.degree. C. and 30.degree. C. When the
cooling cycle of the major hysteresis loop is interrupted at a more
reduced temperature, e.g. -58.degree. C. a constant saturation
magnetization of 23 emu/gm., as portrayed by curve 44, is
maintained when the temperature of the film is cycled between
-58.degree. C. and =30.degree. C. In general, the allowable
temperature excursion permissible without change in film
magnetization is somewhat less than the thermal span between the
temperature at which the cooling cycle of the film is interrupted
and the critical transition temperature of the film to the
ferromagnetic state, curve 48. Thus the magnetization of the
iron-rhodium film (indicative of the saturation flux density of the
film) can be adjusted merely by an interruption of the cooling
cycle of the film at a desired thermal location subsequent to the
transition of the film to the ferromagnetic state. Similarly,
alterations in the saturation flux density of the iron-rhodium film
can be effected during the heating cycle of the major hysteresis
loop by interrupting the heating cycle prior to the maximum
magnetization of the film, e.g. at 55.degree. c., and cooling the
film along minor hysteresis loop 46 to a temperature, T.sub.F,
producing the desired flux density, as indicated by the measured
magnetization of the film in the 1,000 oersted field. Subsequent
reheating and cooling of the film within a temperature span between
T.sub.F and the critical transition temperature T.sub.C of the
minor hysteresis loop produces a negligible variation in the
saturation flux density from the present value. Thus the flux of
the iron-rhodium film can be adjusted to a desired value namely by
an alteration in the thermal cycling of the film.
The iron-rhodium film of this invention also can be suitably
employed as a temperature sensor to indicate the maximum excursions
at the termination of a temperature cycle. For example, when a
material is cooled from a temperature above 150.degree. C., the
magnetization of an iron-rhodium film contacting the material is
indicative of the minimum temperature reached during the cooling
cycle notwithstanding a slight increase in iron-rhodium film
temperature upon removal from contact with the material. Because
the thermal sensitivity of the iron-rhodium film is dependent upon
the disposition of the thermal hysteresis loop of the film relative
to the abscissa, the slope of the iron-rhodium hysteresis curve
preferably is angularly disposed relative to the temperature scale
over the temperature range of interest. To obtain measurements over
a broad temperature range, the oxidation anneal preferably is
conducted at the upper limits of the allowed oxygen concentration
to reduce the squareness of the thermal hysteresis loop while
highly precise temperature measurements over a narrow range best is
effected by a relatively square thermal hysteresis loop exhibiting
a rapid change in magnetization over the temperature range of
interest.
Although the method of recording employing the first order
transition of film bit sites between the magnetic and nonmagnetic
states has been described herein by specific reference to
iron-rhodium alloy films, any material characterized by a first
order transition from the magnetic to the nonmagnetic states with
an associated thermal hysteresis, e.g. manganese bismuth, manganese
arsenide or chromium manganese antimonide, also can be employed for
recording in accordance with the techniques of this invention. The
technique of this invention however is to be distinguished from the
use of manganese bismuth in conventional Curie point writing
because of the reliance of the herewith disclosed wiring technique
on the thermal hysteresis loop of the material, e.g. with a biasing
temperature along the thermal hysteresis loop, while "Curie Point
Writing" with manganese bismuth ignores the thermal hysteresis of
the manganese bismuth and temperature cycles completely through the
thermal hysteresis loop. Curie point writing with manganese bismuth
then records information by the alignment of the magnetization at
the various bit sites.
* * * * *