Ultrahigh-density Recording Media Research Articles

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.

Granular $\hbox{Co}_{3}\hbox{Pt}$ Thin Films With Perpendicular Hard Magnetic Properties for Perpendicular Magnetic Recording Media

The granular hcp Co3Pt nano-grains with large perpendicular coercivity (Hcperp) of 3800 Oe and high perpendicular squareness (Sperp) of 0.93 can be achieved by deposited 14 nm thickness of Co3Pt magnetic layer onto the Pt underlayer with thickness of 200 nm and annealing at low temperature of 300degC in a vacuum of 1 mTorr. It is found that the oxygen atoms would diffuse into Co3Pt layer through inter-grains to isolate Co3Pt grains and prevent grain growth that will decrease inter-grains interaction and enhance recording density of the magnetic film. This film revealed its prominent potential to be applied as perpendicular magnetic recording media for ultra high-density recording.

Synthesis of Ultrasmall Ferromagnetic Face‐Centered Tetragonal FePt–Graphite Core–Shell Nanocrystals

Chemically L10-ordered face-centered tetragonal (fct) FePt nanocrystals are of great interest because of their large magnetocrystalline anisotropy,[1,2] which makes them excellent candidates for applications in ultrahigh-density recording media and for use in biological separation.[3–5] There have been considerable efforts to organize FePt nanocrystals for such applications by nanocrystal self-assembly[3] and polymer templating.[6] It has been predicted that fct FePt particles as small as 2.8 nm exhibit sufficiently high ferromagnetic stability against superparamagnetism at room temperature.[7] These ultrasmall ferromagnetic particles could be useful for various high-density applications.

Size‐Dependent Chemical and Magnetic Ordering in L10‐FePt Nanoparticles

FePt nanoparticles have great application potential in advanced magnetic materials such as ultrahigh-density recording media and high-performance permanent magnets. The key for applications is the very high uniaxial magnetocrystalline anisotropy of the L10-FePt phase, which is based on crystalline ordering of the face-centered tetragonal (fct) structure, described by the chemical-ordering parameter S. Higher chemical ordering results in higher magnetocrystalline anisotropy. Unfortunately, as-synthesized FePt nanoparticles take a disordered face-centered cubic (fcc) structure that has low magnetocrystalline anisotropy. Heat-treatment is necessary to convert the fcc structure to the ordered fct structure. Several previous theoretical and experimental investigations have been reported on the size-dependent chemical ordering of FePt nanoparticles. It has been observed that the degree of ordering decreases with decreasing particle size of the sputtered FePt nanoparticles. Theoretical simulation predicted that the ordering would not take place when the particle size is below a critical value. However, there have not been systematic experimental studies on quantitative size dependence of chemical ordering of FePt nanoparticles due to the lack of monodisperse L10-FePt nanoparticles with controllable sizes. There are also few studies reported to date on the quantitative particle size dependence of magnetic properties, including the Curie temperature, coercivity, and magnetization of the L10-FePt phase, although it has been well accepted that there is a size effect on the ferromagnetism of any low-dimensional magnets. Additionally, the magnetic properties of FePt ferromagnets, as observed in thin-film samples, are affected by the degree of chemical ordering, which is in turn size dependent. It is therefore highly desirable to understand the size and chemical-ordering effects, and their influence on the magnetic properties of the nanoparticles. A major hurdle in obtaining the particle size dependence of structural and magnetic properties of the L10 phase is particle sintering during heat-treatments that convert the fcc phase to the fct phase. This long-pending problem has been solved recently by adopting the salt-matrix annealing technique. With this technique, particle aggregation during the phase transformation has been avoided so that the true size-dependent properties of the fct phase can be measured. In this paper, we report results on quantitative particle size dependence of the chemical-ordering parameter S and selected magnetic properties, including the Curie temperature, Tc, magnetization, Ms, and coercivity, Hc, with the particle size varying from 2 to 15 nm. Figure 1 shows the transmission electron microscopy (TEM) images of the FePt nanoparticles with different sizes before and after annealing in a salt matrix at 973 K for 4 h. The images, from left to right, show nanoparticles with nominal diameters of 2, 4, 6, 8, and 15 nm, respectively. The upper and lower rows are images of as-synthesized and salt-matrixannealed nanoparticles, respectively. As shown in Figure 1, the particle size is retained well upon annealing. Both the assynthesized and annealed nanoparticles are monodisperse with a standard deviation of 5–10 % in diameter. TEM observations also revealed that when the particle size is smaller than or equal to 8 nm, the fct nanoparticles are monocrystalline, whereas the 15 nm fct particles are polycrystalline. It is interesting to see that the L10 nanoparticles, tiny ferromagnets at room temperature, are dispersed very well without agglomeration despite the dipolar interaction between the particles, if a solvent with high viscosity is chosen and if the solution is diluted. Extensive TEM and X-ray diffraction (XRD) analyses have proved that the technique of salt-matrix annealing can be applied to heat-treatments of the FePt nanoparticles without leading to particle agglomeration and sintering, if a suitable salt-to-particle ratio and proper annealing conditions are chosen. Figure 2 shows the XRD patterns of the 4 nm, as-synthesized, fcc-structured nanoparticles and the particles annealed in a salt matrix at 873 K for 2 h, 973 K for 2 h, and 973 K for 4 h (from bottom to top), respectively. As shown in the figure, the positions of the (111) peaks shift in the higher-anC O M M U N IC A TI O N

Multiscale models of hard-soft composite media

The status of models of composite media for ultra-high density recording is reviewed. It is shown that the usual micromagnetic formalism may break down when dealing with interfaces where rapid spatial changes in magnetisation may occur. A multiscale approach is outlined, in which atomistic models are coupled with micromagnetic concepts to provide an improved physical model. The theory is applied to FePt/FeRh bilayers, which have potential as media for heat-assisted magnetic recording.

Micromagnetic analysis of bit decay in perpendicular recording media

The life time of stored data depends on the storage density as well as the physical properties and microstructure of the media. Perpendicular media for ultra-high density recording needs to have high anisotropy to withstand thermal instabilities over long time. In this study, we used a micromagnetic model based on a scaling method to numerically calculate the percentage of magnetic decay caused by thermal fluctuations after ten years. Realistic grain configurations for 1 Tbit/in/sup 2/ and 125 Gbit/in/sup 2/ recording were used to seek the critical grain magnetization and anisotropy field of magnetic media that are thermally stable for ten years. For less than 10% decay, an H/sub K/ of 20 kOe and M/sub S//sup grain/ of 600 emu/cm/sup 3/ is required. Alternatively, H/sub K/ can be reduced to 16 kOe if M/sub S//sup grain/ is 800 emu/cm/sup 3/.

Feasibility of FePt longitudinal recording media for ultrahigh density recording

The feasibility of chemically ordered, ultrathin FePt longitudinal media for ultrahigh recording density is discussed with the basic magnetic and structural properties which can be obtained under acceptable thin film processing condition, as well as preliminary read/write test. High coercivity (≫3000 Oe) could be achieved even in the film with very low Mrt (≪0.3 memu/cm2) at moderate annealing temperature (375–450 °C). However, rapid degradation of crystallographic texture with thickness, in the absence of proper template layer, results in very low coercivity squareness (S*∼0.2), which may be detrimental to recording application. Media noise, particularly exchange coupling induced transition noise, may not be a critical problem, by virtue of extremely small grain size (≃5 nm) and high Ku.

Investigation of patterned thin film media for ultra-high density recording

Micromagnetic modeling is used to study the recording characteristics of patterned polycrystalline thin film media. The dependence of medium noise on medium microstructure is quantitatively investigated. It is found that for patterned media the medium noise arises from magnetization fluctuation at the saturate remanent state. Forming the orientation preference of anisotropy easy axes along the recording direction significantly reduces the medium noise of patterned media. Large bit aspect ratio is preferred to reduce medium noise. For patterned media, a moderate exchange coupling helps to maintain medium thermal stability and the medium noise it introduces is negligible.

MFM study of the effects of thickness and composition in high recording density CoCrTa/Cr media

The effects of differing thickness and Cr concentration of CoCrTa/Cr thin-film media for ultrahigh-density recording were studied with magnetic force microscopy (MFM). Although the remanence-thickness product Brδ increases linearly with magnetic layer thickness δ, the readback signal at high linear recording densities does not change linearly, as expected from the thickness loss factor in the Wallace equation. Increased medium thickness broadens the transition width. Increasing the Cr concentration in CoCrTa films decreases Br, but weakens intergranular exchange coupling and so avoids the bridging between neighboring magnetization transitions and the breaking-up of magnetization patterns recorded at high linear densities. Therefore, the effects of δ and Cr composition must be simultaneously taken into account in the design of ultrahigh-density recording media.

AFM structure and media noise of SmCo/Cr thin films and hard disks

Longitudinal media for ultrahigh density recording require a high coercivity and a low medium noise. While the coercivity is controlled mainly by the chemical composition of the alloy, the medium noise is influenced significantly by the microstructure of the underlayer. We used atomic force microscopy (AFM) to study the microstructure of Cr underlayers and SmCo magnetic films. The AFM study shows that the Cr grains have elongated ‘‘rice’’-like granular features whereas the SmCo grains without an underlayer appear circular with nonuniform grain size. The Cr underlayer grows as well isolated columns with voids. When thin SmCo films (<20 nm) were deposited on the thin Cr (<35 nm) underlayers the SmCo grains were found to replicate the isolated, columnar structure of the underlayer. The intergranular exchange interaction and the recording media noise of SmCo/Cr thin films decreased with a decreasing thickness of the Cr underlayer. The medium noise of high coercivity SmCo/Cr hard disks prepared on thin Cr underlayers remained almost flat with no supralinear increase when tested up to 2760 fc/mm (70 kfci).

Modeling of Advanced Tape Media for Ultra-High Density Recording

Modeling of Advanced Tape Media for Ultra-High Density Recording