U.S. patent application number 10/782356 was filed with the patent office on 2004-08-19 for magnetizable device.
This patent application is currently assigned to Nanomagnetics Limited. Invention is credited to Mayes, Eric Leigh, Tyler, Malvin Nicolas.
Application Number | 20040159821 10/782356 |
Document ID | / |
Family ID | 28676517 |
Filed Date | 2004-08-19 |
United States Patent
Application |
20040159821 |
Kind Code |
A1 |
Mayes, Eric Leigh ; et
al. |
August 19, 2004 |
Magnetizable device
Abstract
A composition that includes a plurality of uniformly sized
ferri- or ferromagnetizable particles, each particle having a
largest dimension no greater than about 100 nm and being at least
partially encased within an organic macromolecule, is disclosed
herein.
Inventors: |
Mayes, Eric Leigh; (Bristol,
GB) ; Tyler, Malvin Nicolas; (Bath, GB) |
Correspondence
Address: |
TESTA, HURWITZ & THIBEAULT, LLP
HIGH STREET TOWER
125 HIGH STREET
BOSTON
MA
02110
US
|
Assignee: |
Nanomagnetics Limited
Bristol
GB
|
Family ID: |
28676517 |
Appl. No.: |
10/782356 |
Filed: |
February 19, 2004 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10782356 |
Feb 19, 2004 |
|
|
|
09730117 |
Dec 5, 2000 |
|
|
|
6713173 |
|
|
|
|
09730117 |
Dec 5, 2000 |
|
|
|
09308166 |
Jun 25, 1999 |
|
|
|
09308166 |
Jun 25, 1999 |
|
|
|
PCT/GB97/03152 |
Nov 17, 1997 |
|
|
|
Current U.S.
Class: |
252/62.54 ;
252/62.55; G9B/5.306 |
Current CPC
Class: |
H01F 1/0063 20130101;
C07K 14/47 20130101; G11B 5/855 20130101; Y10T 428/257 20150115;
Y10T 428/2991 20150115; B82Y 25/00 20130101 |
Class at
Publication: |
252/062.54 ;
252/062.55 |
International
Class: |
H01F 001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 16, 1996 |
GB |
9623851.4 |
Claims
1. A magnetizable device which comprises a magnetic layer composed
of domain-separated, ferromagnetic particles each of which has a
largest dimension no greater than 100 nm.
2. Magnetic recording medium which includes a magnetizable layer
thereon, wherein said magnetizable layer comprises a plurality of
ferromagnetic particles each having a largest dimension no greater
than 100 nm, and each of which particles represents a separate
ferromagnetic domain.
3. Magnetic recording medium according to claim 2, wherein the
distance between adjacent ferromagnetic domains is at least 2
nm.
4. Magnetic recording medium according to claim 2 or 3, wherein the
distance between adjacent ferromagnetic domains is no greater than
10 nm.
5. Magnetic recording medium according to claim 1, 2, 3 or 4,
wherein each ferromagnetic particle is encased within an organic
macromolecule.
6. Magnetic recording medium according to claim 5, wherein each
ferromagnetic particle is encased within the cavity or opening of a
protein macromolecule.
7. Magnetic recording medium according to claim 6, wherein each
ferri- or ferromagnetic particle is encased within an apoferritin
protein.
8. A magnetic composition comprising a plurality of ferromagnetic
particles each of which is bound to an organic macromolecule, and
each of which ferromagnetic particles has a largest dimension no
greater than 100 nm.
Description
RELATED APPLICATIONS
[0001] This application is a continuation of U.S. Ser. No.
09/730,117, filed on Dec. 5, 2000, which is a continuation of U.S.
Ser. No. 09/308,166, filed on Jun. 25, 1999, which is a national
stage application of International Application No. PCT/GB97/03152,
filed Nov. 17, 1997, which claims priority to Great Britain
application No. 9623851.4, filed on Nov. 16, 1996.
[0002] This invention relates to a magnetizable device which
comprises a magnetic layer composed of domain-separated, nanoscale
(e.g. 1-100 nm) ferromagnetic particles. The magnetizable device of
the invention may be used as a magnetic storage device having
improved data storage characteristics. In particular, the invention
relates to magnetic storage media comprising single-domain,
domain-separated, uniform, ferromagnetic nanoscale (e.g. 1-100 nm)
particles which may be arranged into a regular 2-D packed array
useful in the storage of information.
[0003] Among the possible pathways to ultrahigh-density (>=1
Gbit/in.sup.2) magnetic media is the use of nanoscale (1-100 nm)
particles. Beyond the standard requirements for magnetic media, a
viable particulate media should have a small standard deviation in
particle size as well as the particles being exchange decoupled.
These requirements are necessary to avoid adverse media noise.
Current methods of fabricating nanoscale particles, such as
arc-discharge or multiple target ion-beam sputtering, have not
fully addressed these two requirements. Moreover, if the uniform
particles are arranged into an ordered array, each particle can
represent a "bit" of information at a predictable location further
increasing the media's efficiency. This invention details methods
of producing particulate media that meet these requirements for
ultrahigh-density recording. This invention is also an open system
which allows for the production of a variety of magnetic materials,
such that the media can be tuned for different applications.
[0004] In particular this invention details the use of an iron
storage protein, ferritin, whose internal cavity is used to produce
the nanoscale particles. Ferritin is utilised in iron metabolism
throughout living species and its structure is highly conserved
among them. It consists of 24 subunits which are arranged to
provide a hollow shell roughly 8 nm in diameter. The cavity
normally stores 4500 iron(III) atoms in the form of paramagnetic
ferrihydrite. However, this ferrihydrite can be removed (a ferritin
devoid of ferrihydrite is termed "apoferritin") and other materials
may be incorporated. Examples include ceramics, superparamagnetic
magnetite, acetaminophen, and even the sweetener aspartame. To
address magnetic media concerns, the invention incorporates
ferromagnetically ordered materials.
[0005] According to a first aspect of the present invention, there
is provided a magnetizable device which comprises a magnetic layer
composed of domain-separated, ferromagnetic particles each of which
has a largest dimension no greater than 100 nm.
[0006] According to a second aspect of the invention, there is
provided a magnetic recording medium which includes a magnetizable
layer, wherein said magnetizable layer comprises a plurality of
ferromagnetic particles each having a largest dimension no greater
than 100 nm, and each of which particles represents a separate
ferromagnetic domain. The magnetizable layer is preferably
supported on a non-magnetic substrate.
[0007] According to a third aspect of the present invention, there
is provided a magnetic composition comprising a plurality of
ferromagnetic particles each of which is bound to an organic
macromolecule, and each of which has a largest dimension no greater
than 100 nm. In this aspect of the invention, it is preferred that
said organic macromolecule is ferritin from which the normal core
ferrihydrite has been removed and replaced by a ferromagnetic
particle.
[0008] As used herein, the term "ferromagnetic" embraces materials
which are either "ferromagnetic" and "ferrimagnetic". Such usage is
common in the electrical engineering art.
[0009] The ferromagnetic particles used in the invention should be
of a material and size such that they possess ferromagnetic
properties at ambient temperatures (e.g. 15.degree. C. to
30.degree. C.),
[0010] Preferably, the ferromagnetic particles each have a largest
dimension no greater than 50 nm, more preferably less than 25 nm
and most preferably smaller than 15 nm. The largest dimension of
the ferromagnetic particles should not be so small that the
particle will lose its ferromagnetic property and become
superparamagnetic at the desired operating temperature of the
recording medium. Typically, for operation at ambient temperature,
this means that the magnetic particles will normally be no smaller
than about 3 nm in their largest diameter.
[0011] In the magnetizable device of the first aspect of this
invention and the magnetic recording medium of the second aspect of
this invention, the distance between adjacent ferromagnetic domains
is preferably as small as possible to permit the maximum number of
discrete domains in a given area, and provide the maximum storage
capacity for the recording medium. The actual lower limit will vary
for different materials and other conditions such as the
temperature at which the recording medium is to be used. The key
requirement, however, is that neighbouring domains should not be
able to interfere magnetically with each other to the extent that
the magnetic alignment of any domain can be altered by neighbouring
domains. Typically, the lower limit on the spacing of the domains
is about 2 nm. The distance between adjacent domains will be
determined by the density of discrete domains required. Typically,
however, to take advantage of the miniaturization possibilities
provided by the invention, the distance between adjacent domains
will be no greater than 10 nm.
[0012] Generally the particles will be uniform in size, by which we
mean that the particles do not vary in largest diameter by more
than about 5%. One of the advantages of the use in the invention of
an organic macromolecule which binds a magnetic particle by
surrounding it is that this can be used to select particles of a
uniform size.
[0013] In the case where the particles are spheroidal, it will be
the diameter of the particles which must be no greater than 100
nm.
[0014] In preferred embodiments of all aspects of this invention,
each ferromagnetic particle is encased, or partially encased,
within an organic macromolecule. The term macromolecule means a
molecule, or assembly of molecules, and may have a molecular weight
of up 1500 kD, typically less than 500 kD. Ferritin has a molecular
weight of 400 kD.
[0015] The macromolecule should be capable of binding by encasing
or otherwise organising the magnetic particle, and may therefore
comprise a suitable cavity capable of containing the particle; a
cavity will normally be fully enclosed within the macromolecule.
Alternatively, the macromolecule may include a suitable opening
which is not fully surrounded, but which nevertheless is capable of
receiving and supporting the magnetic particle; for example, the
opening may be that defined by an annulus in the macromolecule. For
example, suitable macromolecules which may be used in the invention
are proteins, for example the protein apoferritin (which is
ferritin in which the cavity is empty), flagellar L-P rings,
cyclodextrins, self-assembled cyclic peptides. As an alternative to
encasing the magnetic particles within the macromolecule, they may
be organised on the macromolecule, such as on a bacterial
S-layer.
[0016] Other materials which may be used in the invention to
organise the ferromagnetic particles are inorganic-silica networks
such as MCM type materials, dendrimers and micellar type
systems.
[0017] The presently preferred macromolecule for use in the
invention is the apoferritin protein which has a cavity of the
order of 8 nm in diameter. The ferri- or ferromagnetic particles to
be accommodated within this protein should have a diameter no
greater than 8 nm.
[0018] The bound particles of this aspect of the present invention
with a coating that inhibits aggregation and oxidation, also
helping them to be domain-separated.
[0019] In the magnetizable device of the first aspect of this
invention and the magnetic recording medium of the second aspect of
this invention, the particles are preferably arranged in a 2-D
ordered array which would yield an ultrahigh-density magnetic
media.
[0020] The ferromagnetic material may be a metal, such as cobalt,
iron, or nickel; a metal alloy, such as an alloy which contains
aluminium, barium, bismuth, cerium, chromium, cobalt, copper, iron,
manganese, molybdenum, neodymium, nickel, niobium, platinum,
praseodymium, samarium, strontium, titanium, vanadium, ytterbium,
yttrium or a mixture thereof; a metal ferrite such as a ferrite
containing barium, cobalt, or strontium; or an organic
ferromagnetic material.
[0021] When generating nanoscale particles, one major concern is
that the particles produced are not superparamagnetic.
Superparamagnetic particles are those which have permanent magnetic
dipole moments, but the moments' orientations with respect to the
crystallographic axes fluctuate with time. This is not useful for a
practical magnetic storage media. Superparamagnetism depends on the
volume, temperature, and anisotropy of the particles. Via energy
considerations, one can derive an equation relating these
quantities. The volume at which a particle becomes
superparamagnetic (V.sub.p) is given by: V.sub.P=25 kT/K, where k
is Boltzman's constant, T the temperature of the particle in
degrees Kelvin, and K the anisotropy constant of the material.
Using this formula, it is possible to determine the temperature at
which a particle becomes superparamagnetic (the "blocking
temperature") for a given material at a fixed volume. In our
specific case, the fixed volume is 8 nm in ferritin. If a cobalt
metal particle with only crystalline anisotropy (that value being
45.times.10.sup.5) is a sphere with a diameter of 8 nm, the
blocking temperature is 353.degree. K. This is within the range of
temperatures experienced within a hard disk drive, and the cobalt
particles may prove to be a useful storage medium. Obviously, there
are other considerations such as the materials' coercivity, moment,
saturation magnetisation, and relaxation time. By tuning the
materials incorporated into the ferritin, though, these can be
addressed.
[0022] Ferritin is utilised in iron metabolism throughout living
species and its structure is highly conserved among them. It
consists of 24 subunits arranged in a 432 symmetry which provide a
hollow shell roughly 8 nm in diameter. The cavity normally stores
4500 iron(III) atoms in the form of paramagnetic ferrihydrite.
[0023] However, this ferrihydrite can be removed (a ferritin devoid
of ferrihydrite is termed "apoferritin") and other materials may be
incorporated. The subunits in ferritin pack tightly, however there
are channels into the cavity at the 3-fold and 4-fold axes. Lining
the 3-fold channels are residues which bind metals such as cadmium,
zinc, and calcium. By introducing such divalent ions one can
potentially bind ferritin molecules together, or at least encourage
their proximal arrangement.
[0024] One method of preparing a 2-D packed array of
ferromagnetically ordered particles of uniform size up to 8 nm
includes the removal of the ferrihydrite core from the native
ferritin in aqueous solution, the incorporation of
ferromagnetically ordered cobalt metal particles by sodium
borohydride reduction of the aqueous Co(II) solution into the
ferritin cavities, the generation of a narrow size distribution
through ultracentrifugation, the injection of particles into an
MES/glucose subphase solution upon which the 2-D array assembles,
and the transfer of the 2-D array to a substrate which is then
carbon coated. In this method, the ferritin source may be a
vertebrate, invertebrate, plant, fungi, yeast, bacteria, or one
produced through recombinant techniques.
[0025] In the method described, a metal alloy core may be produced
by sodium borohydride reduction of a water soluble metal salt.
Other oxidation methods include carbon, carbon monoxide, hydrogen,
or hydrazine hydrate solution. Alternatively, a suitable-solution
may be oxidised to yield a metal ferrite core. Oxidation may be
chemical or electrochemical to yield the metal ferrite.
[0026] In this method, other methods of selecting a narrow size
distribution may be employed such as short or long column meniscus
depletion methods or magnetic field separation.
[0027] Further, in this method, divalent metal salts containing
cadmium, calcium, or zinc may be added into the subphase solution
to aid in particle ordering.
[0028] Further, in this, other methods of arranging the particles
into a 2-D array may be employed, such as solution evaporation onto
a solid substrate.
[0029] Further, in this method, the 2-D array may be coated with
carbon-based films such as hydrogenated or nitrogen doped
diamond-like carbon, or with silicon-based films such as silicon
dioxide.
[0030] In the present invention, ferritin may be used to enclose a
ferromagnetic particle whose largest dimension is limited by
ferritin's inner diameter of 8 nm. The particles are produced first
by removing the ferrihydrite core to yield apoferritin. The is done
by dialysis against a buffered sodium acetate solution under a
nitrogen flow. Reductive chelation using thioglycolic acid is used
to remove the ferrihydrite core. This is followed by repeated
dialysis against a sodium chloride solution to completely remove
the reduced ferrihydrite core from solution. Once the apoferritin
is produced, ferr- or ferromagnetic particles are incorporated in
the following ways. The first is by reducing a metal salt solution
in the presence of apoferritin. This is performed in an inert
atmosphere to protect the metal particles from oxidation which
would lessen their magnetic benefit. A combination of metal salts
in solution can also be reduced to generate alloys or alloy
precursors. Sintering or annealing in a magnetic field may be
necessary to generate the useful magnetic alloys. Another method is
to oxidise a combination of an iron(II) salt and another metal
salt. This gives a metal ferrite particle which does not suffer
negatively from oxidation. The metal salts which are beneficial
include salts of aluminium, barium, bismuth, cerium, chromium,
cobalt, copper, iron, manganese, molybdenum, neodymium, nickel,
niobium, platinum, praseodymium, samarium, strontium, titanium,
vanadium, ytterbium, and yttrium.
[0031] A narrow size distribution of particles is necessary to
avoid media noise. Such a distribution can be obtained through a
variety of procedures including, but not limited to, density
gradient centrifugation or magnetic field separation.
[0032] While the production procedure detailed uses native horse
spleen ferritin, this invention should not be seen as limited to
that source. Ferritin can be found in vertebrates, invertebrates,
plants, fungi, yeasts, bacteria, or even produced through
recombinant-techniques. By creating mutant apoferritins lacking the
divalent binding site, others have found that the mutant proteins
assemble into oblique assemblies as opposed to the regular
hexagonal close-packed.
[0033] While ferritin seems to be an ideal system for generating
nanoscale particles, it is not the only system available. For
example, flagellar L-P rings are tubular proteins with an inner
diameter of 13 nm. By creating a 2-D array of these proteins, metal
films could be deposited into the tubular centres to create
perpendicular rods of magnetic material. Also metal reduction in
the presence of a microemulsion can be used to generate nanoscale
particles which are coated with surfactant. This invention is open
to other nanoscale particle production methods.
[0034] Finally an ordered arrangement of the particles is desired.
One way to accomplish this is by injecting an aqueous solution of
particles into an MES/glucose subphase solution contained in a
Teflon trough. The particles spread at the air-subphase interface,
and a portion denature to form a monolayer film. The 2-D
arrangement of encased particles occurs underneath this monolayer.
After 10 minutes at room temperature, the arrangement and monolayer
are transferred to a substrate by placing the substrate directly
onto the monolayer for 5 minutes. After withdrawing the substrate,
the attached arrangement is coated with a thin layer of carbon for
protection. Other methods such as solution evaporation onto a solid
substrate can also give 2-D arrangements, and this invention should
not be seen as limited in its arrangement methods.
EXAMPLE 1
[0035] This example illustrates the preparation of apoferritin from
horse spleen ferritin. Apoferritin was prepared from cadmium-free
native horse spleen ferritin (CalBiochem, 100 mg/ml) by dialysis
(molecular weight cut-off of 10-14 kDaltons) against sodium acetate
solution (0.2 M) buffered at pH 5.5 under a nitrogen flow with
reductive chelation using thioglycolic acid (0.3 M) to remove the
ferrihydrite core. This is followed by repeated dialysis against
sodium chloride solution (0.15 M) to completely remove the reduced
ferrihydrite core from solution.
EXAMPLE 2
[0036] This example illustrates the preparation of cobalt metal
within apoferritin. The apoprotein is added to a deaerated
TES/sodium chloride solution (0.1/0.4 M) buffered at pH 7.5 to give
an approximate 1 mg/ml working solution of the protein. A deaerated
cobalt(II) [for example, as the acetate salt] solution (1 mg/ml)
was added incrementally such that the total number of atoms added
was approximately 500 atoms/apoprotein molecule. This was allowed
to stir at room temperature for one day in an inert atmosphere.
This is followed by reduction of the cobalt(II) salt with sodium
borohydride to cobalt(0) metal. The final product yielded a
solution of cobalt particles, each surrounded by a ferritin
shell.
EXAMPLE 3
[0037] This example illustrates the preparation of a metal alloy
such as yttrium cobalt (YCo.sub.5) within apoferritin. The metal
alloy follows the same procedure as Example 2 but using a 1:5 ratio
of yttrium(III) [for example, as the acetate salt] to cobalt(II)
[for example, as the acetate salt]. The final product yielded a
solution of yttrium cobalt particles, each surrounded by a ferritin
shell.
EXAMPLE 4
[0038] This example illustrates the preparation of a metal ferrite
such as cobalt ferrite (CoO.multidot.Fe.sub.2O.sub.3) within
apoferritin. The apoprotein is added to a deaerated MES/sodium
chloride solution (0.1/0.4 M) buffered at pH 6 to give an
approximate 1 mg/ml working solution of the protein. A deaerated
solution of cobalt(II) [for example, as the acetate salt] and
iron(II) [for example, as the ammonium sulphate salt] in a ratio of
1:2 is added incrementally and allowed to air-oxidise. The final
product yielded a solution of cobalt ferrite particles, each
surrounded by a ferritin shell.
EXAMPLE 5
[0039] This example illustrates the 2-D arrangement of
ferritin-encased magnetic particles. An aqueous solution of
particles [from Examples 2-4, and whose uniformity in size has been
selected] is injected into an MES/glucose subphase solution (0.01
M/2%) contained in a Teflon trough. The particles spread at the
air-subphase interface, and a portion denature to form a monolayer
film. The 2-D arrangement of encased particles occurs underneath
this monolayer. After 10 minutes at room temperature, the
arrangement and monolayer are transferred to a substrate by placing
the substrate directly onto the monolayer for 5 minutes. After
withdrawing the substrate, the attached arrangement is coated with
a thin layer of carbon for protection.
* * * * *