C.P. Luo

High energy products in rapidly annealed nanoscale Fe/Pt multilayers

Magnetic properties of nanocomposite Fe–Pt films with Fe concentration higher than 50 at % have been investigated in this study. Fe/Pt multilayers were produced by sputtering and magnetic hardening was observed after heat treatment including rapid annealing. The final nanocomposite films consisted of the hard face-centered tetragonal FePt phase and a soft face-centered-cubic phase. The maximum energy products of the optimally processed samples exceeded 40 MGOe. Evidence for exchange coupling of the hard and soft phases was found.

Nanostructured FePt:B2O3 thin films with perpendicular magnetic anisotropy

FePt/B2O3 multilayers were deposited by magnetron sputtering onto 7059 glass substrates. By annealing the as-deposited films at 550 °C, nanostructured FePt:B2O3 films consisting of FePt grains with L10 structure, embedded in a glassy B2O3 matrix, were obtained. The c axes of the FePt grains can be made to align with the film normal direction, which results in a perpendicular anisotropy constant of 3.5×107 erg/cc. The films remain layered structures after annealing when the B2O3 layer thickness exceeds 16 A. The nanostructure of the films was investigated by transmission electron microscopy. The coercivities and the average grain sizes of the films are dependent on the B2O3 concentrations, with coercivities varying from 4 to 12 kOe, while average grain sizes vary from 4 to 17 nm. Strong perpendicular anisotropy, adjustable coercivity, and fine grain size suggest this nanocomposite system might have significant potential as recording media at extremely high areal density.

Structural and magnetic properties of FePt=SiO2 granular thin films

Nanocomposite FePt:SiO2 films have been fabricated by annealing the as-deposited FePt/SiO2 multilayers at temperatures from 450 to 650 °C. These films consist of high-anisotropy tetragonal L10 FePt particles embedded in a SiO2 matrix. The structural and magnetic properties of these films were investigated. We have found that coercivity and grain size are highly dependent on the annealing temperature and SiO2 concentration. Films with coercivities in the range from 2 to 8 kOe and grain sizes of 10 nm or less were obtained. These films have considerable potential as high-density magnetic recording media.

Magnetic properties and structure of Fe/Pt thin films

The structural transformation from fcc to fct and relevant magnetic properties of initially multilayered Fe/Pt films were studied. When the as-deposited Fe/Pt films were annealed at the temperature of 300/spl deg/C and up, the fct-phase was formed. Rather square hysteresis loops and large coercivities were measured from the annealed films. The films annealed at 300/spl deg/C retain a layered structure.

FePt:SiO2 granular thin film for high density magnetic recording

Nanocomposite FePt:SiO2 thin films consisting of high anisotropy FePt particles embedded in a SiO2 matrix have been successfully fabricated by annealing the as-deposited FePt/SiO2 multilayers. By adjusting the annealing temperatures and compositions, films were obtained with coercivity of 3.8 kOe and grain size of 10 nm, which are suitable for high-density magnetic recording. Magnetic activation volumes were measured and thermal stability is discussed.

High-anisotropy nanocomposite films for magnetic recording

This paper presents results on the synthesis and properties of FePt- and CoPt-based high-anisotropy nanocomposite films. These films consist of high anisotropy L1/sub 0/ CoPt or FePt particles embedded in a nonmagnetic matrix such as C, SiO/sub 2/, or B/sub 2/O/sub 3/. The grain size and magnetic properties of these films ran he controlled by the processing temperatures and the film compositions. Particularly in the FePt:B/sub 2/O/sub 3/ films the c-axes of the FePt grains can be made to align along the film normal direction, resulting in films with perpendicular anisotropy. Temperature-dependence of magnetic properties and activation volumes are investigated to understand the magnetization reversal and thermal-activation effects. The potential of these films as high-density recording media is discussed.

Magnetic hardening in FePt nanostructured films

FePt films have been prepared by sputtering Fe/Pt multilayers onto glass or silicon substrates. The thickness of the Fe and Pt layers was adjusted with the Fe:Pt atomic ratio from about 1:1 to 2:1. Magnetic hardening is observed after heat treatment at elevated temperatures, which led to coercivity values exceeding 20 kOe in samples with an Fe:Pt ratio around 1.2:1. The hardening originates from the formation of the tetragonal FePt phase with high magnetocrystalline anisotropy and a favorable microstructure. Two-phase composite films containing hard and soft phases were obtained when the Fe:Pt ratio increased. Under optimized processing conditions, composite films with energy products larger than 30 MG Oe at room temperature have been successfully produced.

Magnetic viscosity and switching volumes of annealed Fe/Pt multilayers

Magnetic viscosity and the phenomenon of the sweep‐rate dependence of the coercivity are important for the characterization of magnetic recording media. The activation volume of magnetization reversal or switching volume is directly related to medium noise. The magnetic viscosity behavior of annealed Fe/Pt multilayers was studied and the switching volumes were obtained by both measurements of the sweep‐rate dependence of coercivity and time dependence of magnetization. It is found that samples with larger coercivities have smaller switching volumes, and an estimate of the magnetic grain diameter is about 9 nm. The coercivity mechanisms are also discussed.

Chapter 7 – Magnetism of nanophase composite films

Publisher Summary Magnetism in nanophase composite materials is a phenomenon that depends on several length scales, ranging from less than 1A to a length scale of the order of the size of the magnet. It has been seen that intrinsic properties such as magnetic anisotropy are highly sensitive to local atomic arrangements so that layered or superlattice structures can be exploited to produce exceedingly large anisotropy constants. These, in turn, are the parameters most important in controlling extrinsic properties, such as coercively that is all important for semi-hard magnetic recording media and hard permanent-magnet materials. The concept of enlightened nano structuring to achieve exchange decoupling of single-domain grains of characteristic length about 10 nm is emphasized. There is considerable potential to fabricate magnetic recording media with perpendicular anisotropy, grain sizes of 5–10 nm, and coercivities in the 10- to 15-kOe range because one can control both the properties of the embedded particles and their interactions. Such films are able to serve as recording media in the 1 terabit per square inch regime.

Correlation of Co[110]/Cr[002] texture and magnetic properties in CoCrTaPt granular films

Studies of the effects of substrate temperature T/sub s/ on the evolution of Co[110]/Cr[002] texture, magnetic and physical grain size, intergrain-interaction, anisotropy, orientation ratio of remanence and coercivity were investigated experimentally. It is found that the hcp-Co[110]/bcc-Cr[002] texture is improved with increasing T/sub s/ from 27/spl deg/C to 265/spl deg/C. The intergrain-interaction, magnetic and physical grain size decrease with increasing T/sub s/ and reach their minima at T/sub s//spl ap/265/spl deg/C where the magnetic grain size is close to the physical grain size. The correlation between the film microstructure and magnetic properties is studied systematically.