WO2006118677A2

WO2006118677A2 - HIGHLY ORDERED L10 FePT NANOMAGNETS FOR DATA STORAGE AND MAGNETIC SENSING AND METHOD OF MAKING

Info

Publication number: WO2006118677A2
Application number: PCT/US2006/009949
Authority: WO
Inventors: Rosa A. Lukaszew
Original assignee: The University Of Toledo
Priority date: 2005-04-29
Filing date: 2006-03-17
Publication date: 2006-11-09
Also published as: WO2006118677A4; WO2006118677A3

Abstract

A highly anisotropic magnetic composition which exhibits nano-regions of perpendicular magnetization, a method for making, and articles made therewith are disclosed.

Description

TITLE

Highly Ordered Ll₀ FePt Nanomagnets for Data Storage and Magnetic Sensing and Method of Making

Inventor: Rosa A. Lukaszew

FIELD OF THE INVENTION

The present invention is generally directed a highly anisotropic nano-magnetic structure comprising nanoparticles of a magnetic composition disposed on a substrate where the magnetic composition exhibits nano-regions of perpendicular magnetization, and to method of making the same. This invention was made with Government support under NSF contract 0355171 which may have certain rights in this invention.

BACKGROUND OF THE INVENTION

The requirements of the data storage industry for increased storage capacity continue to escalate. In particular, the magneto- recording industry has projected stored areal-data-densities in the tera-bit/inch-square range for the next few years.

As areal density grows at unprecedented rates, information is stored in smaller and smaller magnetized regions on the hard-drive disc. It is increasingly difficult to obtain stable smaller nano-magnets. One reason for this difficulty is due to the superparamagnetic limit, where thermal fluctuations can produce a 'flip' in the magnetization state 'stored'.

Research focused on circumventing the superparamagnetic effect concentrates on new approaches to both magnetic recording aspects and optically assisted data storage, such as perpendicular recording [1], exchange bias [2 - 4], patterned media [5], and optically assisted magnetic storage [6].

Magnetic binary alloys (e.g. Fe-Pt) are of significant interest in the field of magnetic data storage. Highly ordered LIo structures of these alloys exhibit very large magnetic anisotropy, and can be deposited with the anisotropy axis perpendicular to the film plane, making them suitable for perpendicular media. In this case, the system has an fee (face centered cubic) structure where the Fe and Pt (Pd, or Ir, similar to Pt) atoms occupy alternate plane along (001) directions. This leads to the formation of a superlattice with 2 atomic layers period and strong tetragonal distortion along the (001) direction. The chemical order alters the electronic structure of the system, thereby inducing strong magnetic anisotropy as well a magneto-optical activity. The highly ordered phase is obtained by annealing the alloy films at high temperature. Another important aspect for data storage applications is the fabrication of arrays of nanomagnets where each will constitute one bit of information. There have been several schemes proposed [7, 8] that consider self-assembly of molecules and lithographic techniques. None of these schemes is completely satisfactory. The arrays of self-assembled nano-magnets prepared do not have the desired magnetic properties (i.e. high magnetic anisotropy) and the involved fabrication processes requires too high temperatures. Further, the posterior thermal treatments needed in order to obtain the desired magnetic properties destroy the order in the array. On the other hand, the smallest features that can be obtained using lithographic techniques are still limited and are very expensive for nano-patterning of extensive areas.

Perpendicular recording are currently being considered as a candidate to supplant longitudinal recordings between ~200Gb/in² and 1 Tb/in2. In the industry, however there is a lack of optimized perpendicular head devices and perpendicular media. This lack of such optimized perpendicular media is hindering the progress of perpendicular recording.

Magnetic binary alloys are currently being considered as candidates of significant interest in the field of high-density magnetic recording media. [15] FePt particles fabricated by chemical processes [16] and FePt composited films [17] offer possible avenues to increase anisotropy while decreasing particle size and size dispersion. Conventional CoPt and FePt films are fabricated by a sputtering technique or using multilayer precursors and subsequent annealing in order to form nanoparticles with ordered face-centered-tetragonal (fct) phase. However, films fabricated with this method showed low coercivity values compared with those predicted by the Stoner- Wohlfarth model for isolated single domain particles, thus indicating incomplete ordering. Other techniques such as chemical methods [18] and seed layers [19] have been developed recently to obtain films with perpendicular anisotropy. Patterned structures have also been proposed for obtaining even higher areal density. [20]

Also, thermally stable CoCrPtB media have been fabricated with low magnetic layer thickness. Such media are well oriented and have high magnetic anisotropy and small grain size. The magnetic hardness of the CoCrPt alloy enables more boron addition into the final composition, which results in better decoupled media with thermal stability and enhanced recording properties. [21]

Recently, a chemical procedure has been developed to synthesize monodispersive FePt nanocrystals with controlled size and composition. [16] The magnetocrystalline anisotropy of the particles after annealing was found to be of the order of 106 J/m³. [22] Synthesized FePt nanoparticles possess disordered fee structures and are superparamagnetic at room temperature. At 5 K the FePt nanoparticles show ferromagnetic behavior. It is observed that by raising the temperature the coercivity drops drastically. This is consistent with the small magnetocrystalline anisotropy of the disordered fee phase. Extremely high heat treatment had been used to induce the Fe and Pt atoms to rearrange in long range ordered fee structure that has good hard magnetic properties. For example, Fe₅₆Pt₄₄ nanoparticles annealed at the extremely high temperatures of 500°C, 550⁰C, and 58O⁰C showed a continuous increase of the coercivity with increasing annealing temperature. Transmission electron microscopy studies showed that the phase transformation occurred at 530 ⁰C. [23] The particles were randomly oriented. With increasing annealing temperatures, the monodisperse particles coalesced during annealing and form multiple twined nanocrystals. In the past, FePt had not been considered to be a viable medium material "as-it- is" because the processing temperatures required (>550 °C) [24] to achieve the desired face-centered-tetragonal (fct) ordered structure produced undesirable microstructural features, e.g., large, magnetically coupled grains. Modified Ll₀ phase transformation kinetics had been achieved by controlling the strain mismatch between the buffer layer and the FePt film [25, 26, 27] and by the addition of Cu [28] or Zr [29] but processing temperatures greater than about 400 °C are still required.

Many attempts have been made to produce FePt nanogranular films, [30, 31] in which decoupled nanoparticles OfLl₀ FePt phase are dispersed in a nonmagnetic matrix. Although Watanabe et al. [32] fabricated highly coercive nanogranular FePt films by annealing as-deposited superparamagnetic granular films, the FePt particles coalesced after annealing at 500 ⁰C.

In order to produce FePt nanogranular films suitable for recording media applications, there is a need for a method to make FePt nanoparticles ordered without coarsening and coalescence of the nanoparticles.

It was recently demonstrated that uniformly sized spherical FePt nanoparticles can be synthesized that will self assemble into highly ordered three-dimensional superlattices films. The films were thermally annealed to produce the high anisotropy Ll₀ phase while maintaining particle order. [16, 22] However, annealing above 550 ⁰C is necessary to obtain a high degree of ordering of pure FePt nanoparticles. [16, 33- 36] As such, one major disadvantage to these syntheses is that such high temperature processing is unsuitable for mass-production of magnetic recording media. Another disadvantage is that sintering of FePt nanoparticles occurs at high annealing temperatures. Recently, it had been found that the addition of Sn, Pb, Sb, Bi, and Ag into sputtered CoPt thin films promoted a disordered-ordered transformation, resulting in an appreciable reduction of the temperature for ordering. [37,38] Also, the ordering was promoted by defects produced by the additives during annealing, because those additives have very low surface energy and are easy to segregate. [39] Coffey et al. [42] achieved the hard magnetic properties of magnetron-sputtered Co_xPt_1-X and Fe_xPt_1-X films by extended postgrowth annealing at temperatures up to 700 ⁰C. As a manufacturing process this method has several disadvantages because the high temperatures can cause irreversible changes in substrates for magnetic media. Also, the very high temperatures cause an increase in manufacturing costs.

Thus, there is a strong need to find alternative ways to develop these highly ordered layered materials without high temperature treatments.

An object of the invention is to solve at least the problems and/or disadvantages associated with prior art magneto-recording media and to provide at least the advantages described herein.

It is also an object of the invention to provide a novel highly ordered nanomagnets for data storage.

SUMMARY OF THE INVENTION To achieve the above objects, a magnetic structure is provided that comprises nano-particles of a magnetic composition (FePt) disposed on a substrate, for example, magnesium oxide; however, other materials may also be appropriate.

According to one aspect, the present invention relates to highly anisotropic nano-magnets fabricated without high temperature annealing by using ion implantation applied to films such as binary-alloy films.

In one aspect, the present invention relates to a highly anisotropic nano- magnetic structure having nanoparticles of a magnetic composition disposed on a substrate such that the magnetic composition exhibits nano-regions of perpendicular magnetization. In another aspect, the present invention relates to a method for making a highly anisotropic nano-magnet by ion implanting magnetic nanoparticles onto a film, and annealing the ion-implanted film at a temperature not greater than about 300⁰C to about 45O⁰C for a period of time which causes the nano-magnet to exhibit nano- regions of perpendicular magnetization.

In yet another aspect, the present invention relates to a magnetic composition comprising a magnet-optical composition exhibiting nano-regions of perpendicular magnetization.

In still another aspect, the present invention relates to a method for making highly anisotropic nano-magnetic compositions comprising ion implanting magnetic nanoparticles onto a film, and annealing the ion-implanted film at a temperature not greater than about 300⁰C to about 45O⁰C for a period of time which causes the nano- magnet to exhibit nano-regions of perpendicular magnetization.

In yet another aspect, the present invention relates to a magnetic storage medium including a film of magneto-optical material made according to the method described herein. The magnetic recording material can include such film deposited on a suitable substrate. In certain embodiments, the film is incorporated into a magneto- recording layer of a perpendicular magnetic recording disk.

Additional advantages, objects, and features of the invention will be set forth in part in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from practice of the invention. The objects and advantages of the invention may be realized and attained as particularly pointed out in the appended claims

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be described in detail with reference to the following drawings in which like reference numerals refer to like elements wherein: Fig. Ia shows the results of a symmetric x-ray diffraction (XRD) scan, comparing (cps) v. 2Θ (deg), for a sample 630 A Pt/MgO (001), annealed for 1 hour at 300⁰C.

Fig Ib is an enlargement of a section of Fig. Ia. Fig. 2a shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 2Θ (deg), for a sample FePt (001), annealed for 1 hour at 400⁰C.

Fig. 2b shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 2Θ (deg), for a Rocking FePt (001), annealed for 1 hour at 400⁰C. Fig. 3 a shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 2Θ (deg), for a sample FePt (002), annealed for 1 hour at 400⁰C.

Fig. 3b shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 2Θ (deg), for a Rocking scan for FePt (002), annealed for 1 hour at 400⁰C.

Fig. 4 shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 2Θ (deg), for a Rocking scan for Pt (200), annealed for 1 hour at 300⁰C.

Fig. 5 shows the results of an asymmetric XRD scan, comparing Intensity (cps) v. 2Θ (deg), for a sample 630 A PtMgO (001), annealed for 1 hour at 300⁰C.

Fig. 6 shows the results of asymmetric Phi scans, comparing Intensity (cps) v. 20 (deg), for a sample 630 A Pt/MgO (001), annealed for 1 hour at 300⁰C. Fig. 7 is a graph showing the Kerr rotation (a.u.) v. H(Oersted) before annealing and after annealing an ion-implanted Pt film.

Fig. 8 is a graph showing the Kerr rotation (°) v. energy (eV).

Fig. 9 is a magnetic force microscopy image showing highly ordered single domain magnetic nanocrystals.

DETAILED DESCRIPTION OF THE INVENTION

According to one aspect, the present invention relates to highly anisotropic nano-magnets fabricated without high temperature annealing by using ion implantation applied to films. In one aspect, the present invention relates to magnetic recording media with high magnetic anisotropy, high coercivity, which are made with intermetallic alloys such as Co_xPt_1-X and Fe_xPt_1-x, near the equiatomic composition x=0.5.[41] In one aspect, the present invention relates to a highly anisotropic nano- magnetic structure having nanoparticles of a magnetic composition disposed on a substrate such that the magnetic composition exhibits nano-regions of perpendicular magnetization. In certain embodiments, the magnetic composition comprises at least one of: FePt or FePd. In certain embodiments, the magnetic nanoparticles comprise Fe_xZy where Z comprises at least one of Pt or Pd, and x ranges from 40 to 50 and y =(100-x); for example FePt or FePd. The magnetic composition, after rapid annealing, exhibits a highly ordered Ll₀ phase. Also, the magnetic composition has a magnetic anisotropy which is, in general, perpendicular to a plane defined by a longitudinal surface of the structure. In particular, the annealed structure exhibits nano-regions of perpendicular magnetization as required for perpendicular magneto-recording media applications.

These alloys have desirable magnetic and magneto-optic characteristics. One especially desirable characteristic is that the intermetallic alloy films can undergo long-range chemical ordering to the LI₀ (like CuAu I) phase. This chemical ordering comprises alternating atomic planes of Co(Fe) and Pt along the c-axis. Fully ordered

FePt films have one of the largest known magnetic anisotropy energies (~1 .6x10 erg/cm³).

The substrate can comprise a material selected from the group of suitable heat resistant substrates such as oxides for example, MgO, ceramic, quartz, or plastic.

In another aspect, the present invention relates to a method for making a highly anisotropic nano-magnet by ion implanting magnetic nanoparticles onto a film, and annealing the ion-implanted film at a temperature not greater than about 300⁰C to about 450⁰C for a period of time which causes the nano-magnet to exhibit nano- regions of perpendicular magnetization. In certain embodiment, the ions of the magnetic nanoparticles are implanted onto films having a thickness in the range of about IOOA to about 400 A. The heavy ion implantation achieves a shallow formation of Fe nanoclusters on the Pt film. In certain embodiments, a very thin film of about 100 A to about 300A is irradiated or bombarded with the ions.

Rapid annealing constrains the size of the nanoclusters; further the rapid annealing prevents the accordance of fractile crystallization. In certain embodiments, the annealing is done by a suitable method which allows for both a rapid heating and a rapid cooling of the ion-bombarded film using suitable rapid thermal annealing instruments. After annealing, the Fe particles form a pattern of FePt nanoclusters oriented in a direction perpendicular to a surface substrate. As seen in Fig. 9, the nanoparticles are formed, are highly ordered with the desired Ll₀ structure.

In yet another aspect, the present invention relates to a magnetic composition exhibiting nano-regions of perpendicular magnetization. The magnetic composition comprises at least one of FePt or FePd. In certain embodiments, the magnetic nanoparticles comprise Fe_xZ_y where Z comprises at least one of Pt, Pd and x ranges from 40 to 50 and y =(100-x), or Fe_xZ_y where x ranges from 40 to 50 and y =(100-x). The nanoparticles typically have an average diameter from about 10 to about lOOnm, and can have an average diameter of about less than 50 nm.

The magnetic composition, after rapid annealing, exhibits a highly ordered Ll₀ phase. The Ll₀ ordered phase of the magnetic nanoclustered material overcomes the drawbacks normally associated with superparamagnetism

In still another aspect, the present invention relates to a method for making highly anisotropic nano-magnetic compositions comprising ion implanting magnetic nanoparticles onto a film, and annealing the ion-implanted film at a temperature not greater than about 300⁰C to about 45O⁰C for a period of time which causes the nano- magnet to exhibit nano-regions of perpendicular magnetization. The film comprises an fee structure and has a magnetic coercivity greater than about 2,000Oe, and up to about 5,000Oe. In yet another aspect, the present invention relates to a magnetic storage medium including a film of nanoclustered magnetic material made according to the method described herein. The magnetic recording material can include such film deposited on a suitable substrate. In certain embodiments, the film is incorporated into a magneto-optical recording layer of a perpendicular magnetic recording disk. The magnetic recording media made using the nanoclustered magnetic material of the present invention can have an areal density of 250 Gigabits or more per square inch (Gb/in²).

The nanoclustered magnetic material of the present invention is also useful in magnetic sensing.

In another aspect, the present invention relates to a method for making nano- magnets with high anisotropy by using ion implantation applied to Pt or Pd thin films.

In one embodiment, films are prepared and irradiated/bombarded with Fe ions. This bombardment also induces formation of nano-crystallites with a desired structure. In one method, the ion bombardment is carried out using a heavy ion accelerator which generates and accelerates positively charged ions with charge states primarily of charge one, and which includes an ion source for the production of ions with high energies and/or high charge with a large population of ions per pulse. Examples Ex-situ structural characterization of the films is determined with high resolution transmission electron (HRTEM) microscopy and X-Ray diffraction (XRD). Additional ex-situ surface characterization is performed using Atomic Force Microscopy (AFM).

Magnetic anisotropy analyses are carried out using longitudinal as well as polar Magneto-Optical Kerr effect (MOKE). [48]

The Figures Ia through 6 are X-ray diffraction scans showing clearly the appearance of the Ll₀ FePt ordered phase after annealing the ion-implanted sample. Fig. Ia shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 20 (deg), for an Fe implanted sample 630 A Pt/MgO (001), annealed for 1 hour at 300⁰C. The Figlb is an enlargement of a section of Fig. Ia.

Fig. 2a shows a graph of an X-ray diffraction pattern of a symmetric scan , comparing Intensity (cps) v. 20 (deg), for a sample FePt (001), annealed for 1 hour at 400⁰C.

Fig. 2b shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 20 (deg), for a Rocking scan for FePt (001), annealed for 1 hour at 400⁰C. Fig. Ic shows a symmetric FePt (002) scan. Fig. 3 a shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 20 (deg), for a sample FePt (002), annealed for 1 hour at 400⁰C.

Fig. 3b shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 20 (deg), for a Rocking scan for FePt (002), annealed for 1 hour at 400⁰C.

Fig. 4 shows the results of a symmetric XRD scan, comparing Intensity (cps) v. 20 (deg), for a Rocking scan for Pt (200), annealed for 1 hour at 300⁰C.

Fig. 5 shows the results of an asymmetric XRD scan, comparing Intensity (cps) v. 2Θ (deg), for a sample 630 A Pt/MgO (001), annealed for 1 hour at 300⁰C.

Fig. 6 shows the results of asymmetric Phi scans, comparing Intensity (cps) v. 20 (deg), for a sample 630 A Pt/MgO (001), annealed for 1 hour at 300⁰C. Fig. 7 is a graph showing the Kerr rotation (a.u.) v. H(Oersted) before annealing and after annealing an ion-implanted sample, and showing the magnetic characterization with polar MOKE. The red hysteresis curve clearly indicates that the ion-implanted sample has acquired perpendicular anisotropy after annealing. The green hysteresis curve shows the in-plane magnetic anisotropy of the ion-implanted sample before annealing.

Fig. 8 is a graph showing the Kerr rotation (°) v. energy (eV), and showing the magneto-optical characterization clearly indicating the enhancement of the magneto- optical response after annealing the ion-implanted sample. Fig. 9 is a magnetic force microscopy image showing highly ordered single domain magnetic nanocrystals. The magnetic force microscopy image clearly indicates that annealing of the ion-implanted sample produces nano-regions, or nanoclusters, of perpendicular magnetic anisotropy. The iron ion-implanted platinum thin-film samples after rapid annealing exhibit the presence of the highly ordered Ll₀ phase of the FePt alloys in the x-ray diffraction scans. In certain embodiments, the films having a thickness from about 100 A to about 300 A are modified by ion implantation

The magnetic composition can comprise, for example, FePt or FePd. The substrate can comprise, for example, MgO or quartz.

The magnetic and magneto-optical properties clearly indicate that the magnetic anisotropy of the annealed films is perpendicular, and also that the magneto-optical properties of the annealed sample are enhanced.

The annealed implanted samples exhibit nano-regions of perpendicular magnetization as required for media applications.

According to one embodiment, the method for making a highly anisotropic nano-magnet includes ion implanting magnetic nanoparticles onto a film, and annealing the ion-implanted film at a temperature not greater than about 300⁰C to about 45O⁰C for a period of time which causes the nano-magnet to exhibit nano- regions of perpendicular magnetization.

This invention is particularly useful for hard drive disc technology, which continues to experience dramatic increases in aerial density for stored information. The invention is also applicable to any device where magnetic sensing on a very small scale is desired. The foregoing embodiments and advantages are merely exemplary and are not to be construed as limiting the present invention. The present teaching can be readily applied to other types of apparatuses. The description of the present invention is intended to be illustrative, and not to limit the scope of the claims. Many alternatives, modifications, and variations will be apparent to those skilled in the art. In the claims, means-plus-function clauses are intended to cover the structures described herein as performing the recited function and not only structural equivalents but also equivalent structures. All scientific and patent publications referenced herein are hereby incorporated by reference. The invention having now been described by way of written description and example, those of skill in the art will recognize that the invention can be practiced in a variety of embodiments, that the foregoing description and example is for purposes of illustration and not limitation of the following claims.

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Claims

What is claimed is:

1. A magnetic structure comprising nanoparticles disposed on a substrate wherein the magnetic structure exhibits nano-regions of perpendicular magnetization.

2. The magnetic structure of claim 1, wherein the nanoparticles comprises at least one of: FePt or FePd.

3. The magnetic structure of claim 1 , wherein the nanoparticles comprises FePt.

4. The magnetic structure of claim 2, wherein the nanoparticles comprise Fe_xZy where Z comprises at least one of Pt, Pd, and x ranges from 40 to 50 and y = (100-x).

5. The magnetic structure of claim 1, wherein the substrate comprises a material selected from the group of: MgO, ceramic, quartz, or plastic.

6. The magnetic structure of claim 1, wherein the structure, after annealing, exhibits a highly ordered Ll₀ phase.

7. The magnetic structure of claim 1, wherein the magnetic nanoparticles have a magnetic anisotropy which is perpendicular to a plane defined by a longitudinal surface of the structure.

8. The magnetic structure of claim 6, wherein the annealed structure exhibits nano-regions of perpendicular magnetization as required for media applications.

9. The magnetic structure of claim 1, wherein the magnetic structure has a magnetic coercivity greater than about 2000 Oe.

10. The magnetic structure of claim 1, wherein the thickness of the substrate is in the range of about IOOA to about 400 A.

11. A thin film of magnetic material comprising the magnetic structure of claim 1.

12. A magnetic storage medium including a film comprising the magnetic structure of claim 1.

13. The magnetic storage medium of claim 12, wherein the magnetic structure comprises nanoparticles having an average size of less than about 50 nm.

14. The magnetic storage medium of claim 12, wherein the substrate has a thickness in the range of about IOOA to about 400 A.

15. The magnetic recording medium of claim 12, wherein the medium comprises a magneto- recording layer of a perpendicular magnetic recording disk.

16. A method for making a highly anisotropic nano-magnet, the method comprising: ion implanting magnetic nanoparticles onto a substrate, and annealing the ion-implanted substrate at a temperature not greater than about 45O⁰C for a period of time which causes the nano-magnet to exhibit nano-regions of perpendicular magnetization.

17. The method of claim 16, wherein the magnetic nanoparticles comprise at least one of: FePt or FePd.

18. The method of claim 16, wherein the magnetic nanoparticles comprise

FePt.

19. The method of claim 16, wherein the magnetic nanoparticles comprise Fe_xZ_y where Z comprises at least one of Pt or Pd, and x ranges from 40 to 50 and y = (100-x).

20. The method of claim 16, wherein the substrate comprises a material selected from the group of: MgO, ceramic, quartz, or plastic.

21. The method of claim 16, wherein the structure, after annealing, exhibits a highly ordered Ll₀ phase.

22. The method of claim 16, wherein the magnetic nanoparticles have a magnetic anisotropy which is perpendicular to a plane defined by a longitudinal surface of the structure.

23. The method of claim 16, wherein the annealed structure exhibits nano- regions of perpendicular magnetization as required for media applications.

24. The method of claim 16, wherein the magnetic nanoparticles have a magnetic coercivity greater than about 2000 Oe.

25. The method of claim 16, wherein the thickness of the substrate is in the range of about IOOA to about 400 A.

26. A thin film of magnetic material made according to the method of claim 16.

27. A magnetic storage medium including the film of claim 26.

28. The magnetic storage medium of claim 27, wherein the magnetic material comprises nanoparticles having an average size of less than about 50 nm.

29. The magnetic storage medium of claim 27, having a thickness in the range of about IOOA to about 400 A..

30. The magnetic storage material of claim 27, wherein the magnetic storage medium comprises a magneto- recording layer of a perpendicular magnetic recording disk.