Compositions – Magnetic – With wax – bitumen – resin – or gum
Reexamination Certificate
1997-01-21
2002-09-17
Koslow, C. Melissa (Department: 1755)
Compositions
Magnetic
With wax, bitumen, resin, or gum
C252S062560, C252S062600, C252S062620, C252S062570, C252S062580, C252S062590
Reexamination Certificate
active
06451220
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to high density magnetic compositions and the use thereof in high density magnetic recording media, articles, and magnetic recording processes. More particularly, the invention relates to magnetic recording compositions and to processes for making and using thereof in, for example, analog and digital signal magnetic recording processes and applications thereof. The present invention provides high density magnetic recording media compositions comprising a nanocomposite, or nanocomposite particles comprising cobalt ferrite nanoparticles dispersed in an ionic exchange matrix. The compositions of the present invention are useful in a variety of magnetically responsive systems as indicated herein, and including, media, devices, and applications including but not limited to, for example, magnetic tape, magnetic discs, and related magnetic information storage materials.
The present invention also relates to a method for the preparation of high density magnetic recording media compositions having substantially only single domain cobalt ferrite nanoparticles as the active magnetic component. More particularly, the present invention relates to magnetic compositions possessing novel physical and magnetic properties including in embodiments: a primary particle size of the magnetic species of less than about 50 nanometers in diameter; a magnetic anisotropy constant of about 10
7
erg/cm
3
at about 300° K; a magnetocrystalline anisotropy and nanoparticle volume product (KV) greater than or equal to about 5×10
−12
ergs; a magnetic storage density from about 1,000 to about 100,000 Gigabits per square meter; a blocking temperature(T
b
) equal to or above room temperature, 25° C. to about 400° K; superparamagnetism at above about 350° K; and wherein the nanoparticles are substantially single domain. The magnetic nanoparticles of the present invention possess excellent magnetic storage lifetimes, for example, from about 10
10
seconds to about 10
20
seconds for particles on the order of about 50 nanometers in diameter.
In embodiments of the present invention, there are provided high density magnetic compositions, for example, nanocrystalline cobalt ferrite compounds contained or dispersed in an isolating matrix, such as ion exchange matrix, and which compositions or composite materials enable magnetic recording densities of from about 1,000 to greater than about 100,000 Gigabits per square.
The present invention, in embodiments provides high density magnetic recording compositions comprised of nanoscopic magnetic species generally of the formula CoFe
2
O
4
wherein the nanoparticles are dispersed in an ionic exchange matrix, for example, an ion exchange resin or ion exchange metal oxide.
A high density magnetic recording composition and methods of use refers for example, respectively, to a composition comprised of nanocomposite cobalt ferrite nanoparticles and an ionic exchange matrix, and the formulation of the nanocomposite compositions into articles and devices for magnetic recordation of signals or information.
PRIOR ART
Magnetic recording technologies are among the most important applications of permanent magnets today, reference for example, D. Jiles,
Introduction to Magnetism and Magnetic Materials,
Chapman and Hall, London, 1991, Chapter 14, Magnetic Recording. A typical storage magnetic medium consists of fine magnetic particles suspended in an organic binder adhering to a polymer substrate. During the recording process, different regions of the medium are briefly exposed to strong magnetic fields, so that each grain is magnetized in the desired direction. Each grain could thus in principle store one bit of data, so greater storage density could ideally be achieved by a medium containing many small grains than one containing a few large grains. However, in order to serve as reliable storage devices, the grains must be capable of retaining their magnetizations for long periods of time in weaker, arbitrarily oriented ambient magnetic fields, reference for example, Richards et al., in the
Journal of Magnetism and Magnetic Materials,
150 (1995) 37-50H.L, which describes magnetization switching in nanoscale ferromagnetic grains using a kinetic Ising model. Since experiments have shown the existence of a particles size at which the magnetizations are most stable, for example, E. F. Knetler and F. E. Luborsky, in the
J. Appl. Phys.,
34 (1963) 656, there is a tradeoff between high storage capacity and long-term data integrity which must give rise to an optimum choice of grain size for any given material. During both recording and storage, the relationship between the magnetic field, the size of the grain, and the lifetime of the magnetization opposed to the applied magnetic field is therefore of great technological interest.
Modern magnetic storage technology has been reviewed in, for example,
Physics Today,
April 1995, the disclosure of which is incorporated herein by reference in its entirety, wherein there is disclosed current storage capacity of the most advanced magnetic storage devices is on the order of from 1 to about 100 Gigabits per square meter.
Until recently fine particle magnetic materials were studied only as powders, which made it difficult to differentiate the statistical properties of single grain switching from effects resulting from distributions in particle sizes, compositions, and local environments, or from interactions between grains. Magnetic force microscopy (MFM) and Lorentz microscopy provides the means for overcoming the difficulties in resolving the magnetic properties of individual single-domain particles.
An important metric in the behavior of magnetic materials is blocking temperature (T
b
). The symbol T
b
refers to the relaxation time behavior of magnetic particles in an assembly. For magnetic particles with smaller volumes, the relaxation time is short and the particle response is fast under the influence of an external magnetic field. Larger particles are typically “frozen” and they do not respond superparamagnetically to an external field. Above the blocking temperature, the nanoparticle composition is superparamagnetic, that is, the composition exhibits zero hysteresis, and below the blocking temperature the nanoparticle composition is hysteretic, that is, the material has memory and hysteresis properties comparable to ordinary ferromagnetic materials. The T
b
is highly dependent upon the method used to measure the blocking temperature. The aforementioned terms and phenomena, are known, reference for example,
Science,
Vol 257, Jul. 10 1992, pages 219-223, and
J. Appl. Phys.,
Vol 73, No. 10, May 1993, pages 5109-5116.
Magnetic particles for use in, for example, magnetic storage media, that are known typically have a particle size of about 0.1 micrometer and a coercivity of about several hundred Oersteds. These particles readily retain magnetic patterns and thereby provide a method and means for magnetic information storage. However, these particles have an inherent functional performance limitation and problem in that these materials, when in the form of small particle sizes, for example, below about 0.1 micron, are superparamagnetic at room temperature, and are unsuitable for magnetic recording applications.
The following United States patents are noted:
U.S. Pat. No. 4,113,658, issued Sept. 12, 1978, to Geus, discloses a process wherein by applying certain controlled homogeneous precipitations techniques in the presence of a homogeneously distributed finely divided particulate supporting material, such as finely divided silica, there is effected a deposition precipitation of a metal or metal compound on the surfaces of the support particles, reference the working Examples.
U.S. Pat. No. 4,943,612, issued Jun. 24, 1990, to Morita et al., discloses an ultrafine particulated polymer latex having an average particle size of 100 nm or less, a crosslinked structure and a glass transition temperature lower than a value calculated by a weight fraction method.
U.S. Pat. No. 4,3
Kroll Elizabeth C.
Palacios Javier Tejada
Pieczynski Rachel
Zhang Xixiang
Ziolo Ronald F.
Koslow C. Melissa
Thompson Robert
Xerox Corporation
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