Exchange-spring magnets are nanocomposites that are composed of magnetically hard and soft phases that interact by magnetic exchange coupling. Such systems are promising for advanced permanent magnetic applications, as they have a large energy product--the combination of permanent magnet field and magnetization--compared to traditional, single-phase materials. Conventional techniques, including melt-spinning, mechanical milling and sputtering, have been explored to prepare exchange-spring magnets. However, the requirement that both the hard and soft phases are controlled at the nanometre scale, to ensure efficient exchange coupling, has posed significant preparation challenges. Here we report the fabrication of exchange-coupled nanocomposites using nanoparticle self-assembly. In this approach, both FePt and Fe3O4 particles are incorporated as nanometre-scale building blocks into binary assemblies. Subsequent annealing converts the assembly into FePt-Fe3Pt nanocomposites, where FePt is a magnetically hard phase and Fe3Pt a soft phase. An optimum exchange coupling, and therefore an optimum energy product, can be obtained by independently tuning the size and composition of the individual building blocks. We have produced exchange-coupled isotropic FePt-Fe3Pt nanocomposites with an energy product of 20.1 MG Oe, which exceeds the theoretical limit of 13 MG Oe for non-exchange-coupled isotropic FePt by over 50 per cent.

Exchange-coupled nanocomposite magnets by nanoparticle self-assembly

This paper reviews recent advances in chemical synthesis, self-assembly, and potential applications of monodisperse binary FePt nanoparticles. After a brief introduction to nanomagnetism and conventional processes of fabricating FePt nanoparticles, the paper focuses on recent developments in solution-phase syntheses of monodisperse FePt nanoparticles and their self-assembly into nanoparticle superlattices. The paper further outlines the surface, structural, and magnetic properties of the FePt nanoparticles and gives examples of three potential applications in data storage, permanent magnetic nanocomposites, and biomedicine.

Recent Advances in Chemical Synthesis, Self‐Assembly, and Applications of FePt Nanoparticles

TOPICAL REVIEW: Nanomagnetics

▪ Abstract Continuing increases in the areal density of hard disk drives will be limited by thermal instability of the thin film medium. Patterned media, in which data are stored in an array of single-domain magnetic particles, have been suggested as a means to overcome this limitation and to enable recording densities of up to 150 Gbit cm−2 (1 Tbit inch−2) to be achieved. However, the implementation of patterned media requires fabrication of sub-50-nm features over large areas and the design of recording systems that differ substantially from those used in conventional hard drives. This review describes patterned media, including the fabrication of arrays of small magnetic particles and their magnetic properties, such as domain structure, reversal mechanisms, thermal stability, and interactions. The practical implementation of patterned media recording schemes is assessed.

Patterned Magnetic Recording Media

Nanophase hard magnets

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.

High energy products in rapidly annealed nanoscale Fe/Pt multilayers

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.

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Nanostructured FePt:B2O3 thin films with perpendicular magnetic anisotropy

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.

/pdf/structural-and-magnetic-properties-of-fept-sio2-granular-55j40g4k8y.pdf

Structural and magnetic properties of FePt:SiO2 granular thin films

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.

/pdf/fept-sio2-granular-thin-film-for-high-density-magnetic-4b3icgat00.pdf

FePt:SiO2 granular thin film for high density magnetic recording

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.

/pdf/magnetic-hardening-in-fept-nanostructured-films-3txf978ux3.pdf

C.P. Luo

Papers

High energy products in rapidly annealed nanoscale Fe/Pt multilayers

Nanostructured FePt:B2O3 thin films with perpendicular magnetic anisotropy

Structural and magnetic properties of FePt:SiO2 granular thin films

FePt:SiO2 granular thin film for high density magnetic recording

Magnetic hardening in FePt nanostructured films