High-anisotropy nanocluster films for high-density perpendicular recording

This paper reports results on the synthesis and magnetic properties of L1/sub 0/:X nanocomposite films, where L1/sub 0/=FePt, CoPt, and X=C, Ag, etc. Two fabrication methods are discussed: nonepitaxial growth of oriented perpendicular media, and monodispersed nanoparticle-assembled films grown with a gas-aggregation source. The magnetic properties are controllable through variations in the nanocluster properties and nanostructure. The films show promise for development as recording media at extremely high areal densities.     

Self-assembled nano-size FePt islands for ultra-high density magnetic recording media

To find a method to form nano-size FePt alloy for ultra-high density magnetic recording media, this work concentrated on the formation mechanisms of nano-island FePt films on amorphous glass substrates. FePt films of different thicknesses (1-10 nm) were deposited on amorphous glass substrates and post-annealed at 700 °C for 10 and 30 min. The configuration of the film changed during the annealing process due to the surface energy difference between the glass substrate and FePt alloy. Investigation of the microstructures and magnetic properties of the ordered L1₀ FePt films revealed that the 1 nm FePt film annealed at 700 °C for 10 min had perpendicular magnetic anisotropy and good reproducibility of forming well-separated FePt nano-size islands for ultra-high density magnetic recording media.     

The effect of thickness on the texture and magnetic properties of single-layered FePt films by rapid thermal annealing

The single-layered FePt films with thickness in the range of 5 to 50 nm are deposited directly on Si(100) substrate without underlayer, then post annealed at 700 degrees C by rapid thermal annealing (RTA) technique. As the film thickness of FePt is over 20 nm, the L1(0) FePt(111) preferred orientation is presented and tended to in-plane magnetic anisotropy. However, the L1(0) FePt(001) texture is obtained and exhibited perpendicular magnetic anisotropy as the film thickness is decreased to 10 nm. Its perpendicular coercivity (Hc(perpendicular)), saturation magnetization (Ms) and perpendicular squareness (S(perpendicular)) are 14.8 kOe, 795 emu/cm3 and 0.79, respectively. On the other hand, both the grain size and domain size of FePt film decrease with decreasing the film thickness of FePt. The grain size for 10-nm FePt film is as small as 9.7 nm with domain size of 123 nm, which reveal its significant potential as perpendicular magnetic recording media for ultra high-density recording.     

L10 ordered FePt based double-layered perpendicular recording media with (002) oriented FeCo films as a soft magnetic underlayer

The introduction of the soft magnetic underlayer (SUL) in perpendicular recording technology is to further increase the recording areal density. However, problems such as growth of the uncontrollable recording layer and additional media noise contributed from the SUL could be resulted. In this work, a synthetic antiferromagnetically (SAF) coupled (002) oriented Fe₆₅Co₃₅ film as an SUL was developed for L1₀ ordered FePt based double-layered recording media. The crystallography of hetero-epitaxially grown double-layered media CrRu/(Ru/FeCo)₂/Pt/FePt/Ru was demonstrated. The L1₀ ordered FePt based double-layered perpendicular recording media with SAF coupled FeCo films as the SUL were developed.     

High-density perpendicular magnetic recording media of granular-type (FePt/MgO)/soft underlayer

Perpendicular magnetic recording media, composed of granular-type FePt-MgO films on Fe-Ta-C soft magnetic underlayer (SUL), have been fabricated on to 2.5-in glass disks. [001] textured FePt granular films with high-perpendicular magnetic anisotropy were obtained by annealing the FePt/MgO multilayer films. The FePt grain size, perpendicular coercivity, magnetic activation volume, and the exchange coupling between the FePt grains were found to be strongly dependent on the initial multilayer structures and the annealing conditions. The recording performance of the disks was evaluated by a spin-stand. The obtained results reveal a close correlation between the recording performance and magnetic properties. The thermal stability of the granular-type FePt media was studied using high-temperature magnetic force microscopy (MFM) technique, equipped with in situ sample heating, in the temperature range 25/spl deg/C-200/spl deg/C. The estimated signal decay at high temperature is ascribed to the temperature dependent magnetic anisotropy behavior.     

FePt-Ag nanocomposite thin films with longitudinal magnetic anisotropy

A well-controlled method to fabricate FePt thin films with the (200) texture and longitudinal magnetic anisotropy for high-density magnetic recording media is reported. FePt-Ag nanocomposite thin films with L1(0) ordered FePt grains embedded in an Ag matrix were deposited on the Cr90Ru10/glass by co-sputtering from Ag and FePt targets. The Ag doping suppressed the (001) texture but improved the L1(0) FePt (200) texture. The magnetic easy axis of FePt-Ag thin films changed from perpendicular to longitudinal in direction. In-plane coercivity of the films varied from 0.8 kOe to 6.5 kOe, depending on Ag contents in the films and under-layer thickness. The change from the (001) to (200) texture could be due to the competition of grain-boundary energy and epitaxial-strain energy.     

Patterning of FePt for magnetic recording

Higher areal density for magnetic recording is needed to provide larger storage capacities on harddisk drives. However, as the recording bit size of traditional magnetic recording materials (such as Co/Cr) approaches 10 nm, the magnetic direction of each recording bit would become unstable at room temperature due to thermal fluctuation. To solve this problem, efforts have been made using two methods: one method is to replace the disk media with new materials possessing higher magnetic anisotropy which would lead to better thermal stability; and the second one is to employ different configurations for the recording layer. FePt with patterned media configuration is a combination of these two methods. In this paper we review some novel and interesting methods of patterning FePt for magnetic recording, including thermal patterning, self-assembly patterning, and lithography patterning.     

FePt纳米薄膜研究进展

由于FePt在超高密度磁存储材料方面的广阔应用前景及其局限性,研究人员对FePT薄膜进行了大量的研究及改性.根据国外近期在此领域的研究现状,综述了单相、复相及掺杂FePt薄膜的制备以及对结构和性质的影响.复相或掺杂主要是通过结构的改变来降低L10晶相转变温度和FePt颗粒的大小,通过其耦合作用来影响FePt薄膜的磁学性能,使其成为超高密度存储器材料.     

FePtAg-C纳米颗粒薄膜的制备及表征

采用磁控溅射法在硅基片上生长FePt纳米颗粒薄膜。在硅片表面生长MgO籽层用来引发FePt合金薄膜的fct织构,加入C来减小其颗粒尺寸,加入Ag来增强其L10有序度。采用X射线衍射仪（XRD）、超导量子干涉仪（SQUID）和高分辨率透射电镜（TEM）对FePt薄膜进行表征。结果表明制备的薄膜样品具有优良的L10相结构,其M-H曲线表明方形度很好,垂直矫顽力HC有2467 kA/m,颗粒大小为10.4 nm。该薄膜非常适合用做下一代高密度磁存储媒质,可有效提高信息存储密度。     

Nano-structure formation of Fe-Pt perpendicular magnetic recording media co-deposited with MgO, Al2O3 and SiO2 additives

Perpendicular magnetic recording media samples were prepared by sputter deposition on sapphire with a layer sequence of MgO seed-layer/Cr under-layer/FeSi soft magnetic under-layer/MgO intermediate layer/FePt-oxide recording layer. The effects of MgO, Al₂O₃ and SiO₂ additives on the morphology and orientation of the FePt layer were investigated by transmission electron microscopy. The samples exhibited (001) orientation of the L1₀ FePt phase with the mutual orientations of sapphire substrate//MgO(100)[001]//Cr(100)[11¯0]//FeSi(100)[11¯0]//MgO(100)[001]//FePt(001)[100]. The morphology of the FePt films varied due to the co-deposited oxides: The FePt layers were continuous and segmented by stacking faults aligned at 54° to the surface. Films with SiO₂ addition, beside the oriented columnar FePt grains, exhibited a fraction of misoriented crystallites due to random repeated nucleation. Al₂O₃ addition resulted in a layered structure, i.e. an initial continuous epitaxial FePt layer covered by a secondary layer of FePt-Al₂O₃ composite. Both components (FePt and MgO) of the MgO-added samples were grown epitaxially on the MgO intermediate layer, so that a nano-composite of intercalated (001) FePt and (001) MgO was formed.The revealed microstructures and formation mechanisms may facilitate the improvement of the structural and magnetic properties of the FePt-oxide composite perpendicular magnetic recording media.     

Film thickness dependence of microstructures and magnetic properties in single-layered FePt films by in-situ annealing

The FePt films with various thicknesses (t) of 5 to 50 nm are deposited on Si(100) substrate without any underlayer by in-situ annealing at substrate temperature (Ts) of 620 °C. A strong (001) texture of L1₀ FePt film is obtained and presents high perpendicular magnetic anisotropy as the film thickness increases to 30 nm. By further increasing the thickness to exceed 30 nm, the (111) orientation of L1₀ FePt is enhanced greatly, indicating that the quality of perpendicular magnetic anisotropy degrades when the thickness of the FePt film is greater than 30 nm. The single-layered FePt film with thickness of 30 nm by in-situ depositing at 620 °C shows good perpendicular magnetic properties (perpendicular coercivity of 1033 kA/m (13 kOe), saturation magnetization of 1.08 webers/m² and perpendicular squareness of 0.91, respectively), which reveal its significant potential for perpendicular magnetic recording media.     

Effects of Os inserted layers on the microstructures and magnetic properties of the FePt films

The microstructure and magnetic properties of multilayer [Os(t)/FePt(x)]n films on a glass substrate with a 10 nm Os buffer layer by ion beam sputtering have been studied as a function of the annealing temperatures between 300 and 800 degrees C. Here, t = 0.2, 1 or 5 nm and x varied from 10, 20, 25, 50, to 100 nm with its associated n value of 10, 5, 4, 2, and 1, respectively. No diffusion evidence was found in samples with a thin Os layer and t > or = 1 nm. The average grain size of the multilayer films can be well controlled by both annealing temperature and thickness of the FePt layer by a very thin Os space layer with t > or = 1 nm. The enhancement of H(c) can be understood from the fact that for a FePt film with an Os spacer layers, the increasing number of Os layer will inhibit the grain growth of FePt grains and enriches the grain boundary. We have experimentally demonstrated that even with a very thin 1 nm Os spacer layers, the [Os(t)/FePt(x)]n multilayer films can exhibit good hard magnetic properties and are attractive candidates for ultrahigh density magnetic recording media.     

Effects of Cu underlayer on the growth and magnetic properties of FePt thin films on MgO (001) substrate

During ordering process of face centered tegragonal (fct) L1(0) phase of the FePt alloy, there exist three growth variants of axes (001) from original disordered fcc structured phase. When FePt film was directly deposited on the MgO (001) substrate, the variant perpendicular to the film plane grew, resulting in a low out-of-plane coercivity of 1.3 kOe. By using Cu underlayer, two variants lying in the film plane got same chance to grow, which caused an in-plane perpendicular alignment of the tetragonal axes of FePt L1(0) phases. The crystallographic relationship between Cu and FePt layers is Cu (100)<100>//fct FePt (100)<100>. A high in-plane coercivity of 4.6 kOe was obtained due to the high density of micro-defects (mcro-twins, anti-phase boundaries, etc.) in the film plane. This work demonstrated a way of selecting the growth variants of ordering process to adjust the magnetic properties of the ordered FePt thin films.     

Effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers

FePt (50 nm) and [FePt(xnm)/AlN(1, 2, 3 nm)]₁₀ (x=2, 3 nm) films were prepared by RF magnetron sputtering technique, then were annealed at 550 °C for 30 min. This work investigates the effect of AlN layer thickness on structure and magnetic properties of FePt/AlN multilayers. Superlattice (0 0 1) peaks can be found in the grazing incidence X-ray diffraction of FePt and [FePt (3 nm)/AlN (1, 2, 3 nm)]₁₀ films, which indicate that the FCC phase has been partially transformed into ordered L1₀ phase. Compared with the single layer FePt film, superlattice (0 0 1) peaks of FePt/AlN multilayers are weak and wide, which indicates that the introducing of AlN hinders the growth of FePt particle, and also shows the introducing of AlN is not beneficial to the transformation from FCC phase to L1₀ phase. In addition, the low-angle XRD spectra show the layered structure of FePt/AlN has been broken after annealing. The coercivities, particle size, intergrain exchange interactions of FePt/AlN films are decreased with increasing AlN layer thickness.     

Low temperature ordering of FePt films by in-situ heating deposition plus post deposition annealing

FePt thin films with 40 nm thickness were prepared on thermally oxidized Si (001) substrates by dc magnetron sputtering at the nominal growth temperature 375 °C. The effects of annealing on microstructure and magnetic properties of FePt thin films were investigated. The as-deposited FePt thin films show soft magnetic properties. After the as-deposited FePt thin films were annealed at various temperatures and furnace cooled, it is found that the ordering temperature of L1₀ FePt phase could be reduced to 350 °C. For FePt thin films annealed at 350 °C, the in-plane and out-of-plane coercivities of the films increased to 510 and 543 kA/m, respectively, and the films had hard magnetic properties. A highly (001) orientation was obtained, when FePt thin films were annealed at 600 °C. And the hysteresis loops of FePt thin films annealed at 600 °C show out-of-plane magnetic anisotropy.     

Crystallographic structure and magnetic properties in L1(0) FePt thin films deposited at different temperature

The crystallographic structure and magnetic properties of L1(0) FePt thin films deposited at different substrate temperature were investigated systematically in present paper. The ordered L1(0) FePt thin film was developed when substrate temperature is higher than 300 degrees C. The ordering parameter S, the degree of tetragonality c/a, and the epitaxial quality of the films, increase with increasing substrate temperature. The squareness and coercivity in the direction perpendicular to the film increase as the substrate temperature is increased and the perpendicular anisotropy is developed when the substrate temperature is higher than 300 degrees C. The magnetic anisotropy Ku increases with increasing substrate temperature and it might be concluded that the magnetic anisotropy of ordered L1(0) FePt thin films mainly stems from the magnetocrystalline origin and also possibly due to pair ordering mechanism.     

Effect of carbon additive on microstructure evolution and magnetic properties of epitaxial FePt (001) thin films

FePt:C thin films were deposited on CrRu underlayers by DC magnetron co-sputtering. The effects of C content, FePt:C film thickness and substrate temperature on the microstructural and magnetic properties of the epitaxial FePt (001) films were studied. Experimental results showed that even with 30 vol.% C doping, the FePt films could keep a (001) preferred orientation at 350 °C. When a FePt:C film was very thin (< 5 nm), the film had a continuous microstructure instead of a granual structure with C diffused onto the film surface. With further increased film thickness, the film started to nucleate and formed a column microstructure over continuous FePt films. A strong exchange coupling in the FePt:C films was believed to be due to the presence of a thin continuous FePt layer attributed to the carbon diffusion during the initial stage of the FePt:C film growth. Despite the presence of a strong exchange coupling in the FePt:C (20 vol.% C) film, the SNR ratio of the FePt:C media was about 10 dB better than that of the pure FePt media. The epitaxial growth of the FePt:C films on the Pt layers was observed from high resolution TEM cross sectional images even for the films grown at about 200 °C. The TEM images did not show an obvious change in the morphology of the FePt:C films deposited at different temperatures (from 200 °C to 350 °C), though the ordering degree and coercivity of the films increased with increased substrate temperature.     

Perpendicularly self-oriented and shape-controlled L10-FePt nanorods directly synthesized by a temperature-modulated process

The synthesis and self-assembly of tetragonal phase-containing L1₀-Fe₅₅Pt₄₅ nanorods with high coercive field is described. The experimental procedure resulted in a tetragonal/cubic phase ratio close to 1:1 for the as-synthesized nanoparticles. Using different surfactant/solvent proportions in the process allowed control of particle morphology from nanospheres to nanowires. Monodisperse nanorods with lengths of 60 ± 5 nm and diameters of 2–3 nm were self-assembled in a perpendicular oriented array onto a substrate surface using hexadecylamine as organic spacer. Magnetic alignment and properties assigned, respectively, to the shape anisotropy and the tetragonal phase suggest that the self-assembled materials are a strong candidate to solve the problem of random magnetic alignment observed in FePt nanospheres leading to applications in ultrahigh magnetic recording (UHMR) systems capable of achieving a performance of the order of terabits/in².      

新型磁存储介质L10有序FePt薄膜研究综述

L10有序FePt合金薄膜有大的各向异性能、矫顽力和饱和磁化强度,而且根据制备工艺条件的不同,其易磁化轴可以平行或垂直于膜面,因此极有可能成为下一代超高密度磁存储的介质,近年来引起了广泛的关注.详细介绍了Fept薄膜近年来的研究结果,分析了其大矫顽力的机制、降低有序化温度、控制易磁化轴取向、降低粒子间相互作用的方法等对磁存储至关重要的问题,并对其在磁存储中的应用前景作了分析.     

Lattice Expansion by Adding Oxygen to Control Crystallographic Orientations in Chemically Ordered L10 FePt Films

We fabricated L1₀ FePt thin films by sputtering in reactive oxygen on polycrystalline glass substrates, and we investigated the magnetic properties and crystallographic orientations of the films. Oxygen addition during the FePt deposition promoted heteoroepitaxial growth by decreasing the lattice misfit with the Ag underlayer. In an oxygen/argon ratio of 1.5-3.0 vol.%, the in-plane lattice parameter of the FePt films expanded, and the lattice misfit with the Ag underlayer decreased from 6.3 to 3.9% in the as-deposited state, as determined by grazing incidence X-ray diffraction (GIXRD). Annealing at 700degC for 1 min produced a heteroepitaxially grown L1₀ (001) texture with a large out-of-plane coercivity of 8.8 kOe and a nucleation field of kOe. Transmission electron microscopy showed that average grain size in the as-deposited films was about 4-5 nm and was in the range of 10-15 nm in the annealed films, indicating that there was some grain growth.