H. Okumura

CoPt and FePt nanowires by electrodeposition

In this study we have fabricated by electrodeposition CoPt(FePt) nanowires embedded inside an array of empty holes in anodized aluminum disks. By adjusting the current density and solution composition, one can control the composition of the film to 50:50 in order to obtain the high anisotropy CoPt and FePt face centered tetragonal (L10) phases. The as-made films are magnetically soft. Magnetic hardening is developed after annealing at 700u200a°C with coercivity typically in the range of 3–6 kOe. The scanning electron microscopy and transmission electron microscopy studies showed that nanowire structured CoPt and FePt with near stoichiometric composition inside an array of holes which are about 25–100 nm in diameter. A preferred perpendicular anisotropy is observed in the CoPt nanowires.

CoPt/Ag nanocomposites with (001) texture

CoPt/Ag nanocomposites with the tetragonal (L10) structure have been prepared by magnetron sputtering. The dependence of texture on film thickness, bilayer thickness, CoPt volume fraction, and annealing conditions is investigated. Films with a thickness below 15 nm consist of islands with (001) texture while as the thickness increases, the islands coalesce into a continuous film and the (111) texture appears. Microstrain is minimized in the range of film thickness where the (001) texturing is optimum indicating that strain energy provides the driving force of (001) growth texturing. The (001) texture improves with CoPt volume fraction for all annealing times but disappears above 95 volu200a% indicating that the existence of the Ag plays an important role in the development of the (001) texture.

FePt/BN granular films for high-density recording media

Abstract FePt particles with the high magnetocrystalline anisotropy L1 0 structure and particle sizes between 3 and 15xa0nm have been fabricated by annealing FePt/BN multilayers with bilayer thicknesses in the range of 2.5–200xa0A, at 600°C and 700°C. Films with thicker bilayer thicknesses were found to have a strong (1xa01xa01) texture, which is less developed in thinner bilayer films. For a particular bilayer thickness (FePt-20xa0A/BN-40xa0A), a special type of textured ordering is observed in the X-ray diffraction patterns, in which the c -axis of all the particles has a perpendicular component. Magnetic measurements showed that a wide range of coercivities (2–18xa0kOe) could be obtained in the films by varying the bilayer thickness and the annealing temperature and time. The concentration of BN was found to control the interparticle interactions, which for certain BN values led to decoupling of the FePt grains.

Magnetic properties and microstructure of mechanically milled Sm2(Co,M)17-based powders with M=Zr, Hf, Nb, V, Ti, Cr, Cu and Fe

The structure, microstructure, and magnetic properties of nanostructured Sm2Co17-based powders synthesized by mechanically milling and subsequent annealing have been systematically studied. It has been found that a nanoscale 2:17 phase with an average grain size of about 30 nm is developed within the powders, which have an average particle size of about 5 μm. Optimum magnetic properties of Ms=110.5 emu/g, Mr=66.2 emu/g, Mr/Ms=0.60, Hc=9.6 kOe, and (BH)m=10.8 MGOe have been obtained in stoichiometric Sm2Co17 powders milled for 6 h and annealed at 800u200a°C for 30 min. The observed magnetic hardening is believed to arise from the high anisotropy of the Sm2Co17 phase and its nanoscale grain size. A small amount of Zr substitution for Co significantly increases the coercivity by increasing the anisotropy field of the Sm2Co17 phase. Cu substitution in Zr-contained samples further increases the coercivity by introducing a nanoscale 1:5 phase which forms a uniform mixture with the 2:17 nanograins. The highest coerc...

Enhancement of magnetic properties of nanocomposite Pr2Fe14B/α-Fe magnets by small substitution of Dy for Pr

Nanocomposite (Pr,Dy)2Fe14B/α-Fe magnets with compositions Pr8−xDyxFe86B6 (x=0, 0.5, 1, 1.5, and 2) have been synthesized by melt spinning using low wheel speeds in the range from 18 to 21.7 m/s. It has been found that the coercivity is significantly increased by Dy substitution. An optimum coercivity of 6.6 kOe is obtained in the Pr7Dy1Fe86B6 magnet as compared to 4.3 kOe in the Pr8Fe86B6 magnet. As a result, the energy product is increased from 9.1 MGOe in the Pr8Fe86B6 magnet to 16.9 MGOe in the Pr7Dy1Fe86B6 magnet. The reason for this is the finer and more uniform 2:14:1/α-Fe nanoscale microstructure developed in the magnets with Dy substitution. Because of the low wheel speed spinning, the majority of the nanoscale microstructure is crystallized directly out of the melt. The enhancement of magnetic properties by Dy substitution is mainly due to the microstructure refinement which leads to an enhanced exchange coupling between the Pr2Fe14B and α-Fe, whereas the anisotropy increase by the Dy substituti...

Perpendicularly oriented FePt nanoparticles sputtered on heated substrates

Abstract Textured FePt/C nanocomposite films have been deposited onto heated substrates at a temperature up to 650°C. At ambient substrate temperature, FePt and carbon form a multilayered structure with a disordered FCC structure which is magnetically soft. Post-annealing in the range of 550–750°C leads to the formation of ordered FePt particles. FePt nanoparticles with strong perpendicular texture were obtained by depositing directly onto heated substrates at a temperature above 450°C. TEM data show well-separated FePt particles of unique elongated shape with the major axis (∼20xa0nm) oriented in a direction perpendicular to the film. The development of the texture is controlled by both the substrate temperature and bilayer film thickness. Kelly Henkel plots indicate small interactions between the particles in both the parallel and perpendicular directions.

Microstructure refinement and significant improvements of magnetic properties in Pr2Fe14B/α-Fe nanocomposites

Abstract Exchange coupled (Pr,Tb) 2 (Fe,Nb,Zr) 14 B/α-Fe nanocomposites have been produced by melt spinning. A trend for perpendicular and planar c -axis orientation of the 2:14:1 phase was observed in the free surface of ribbons spun at speeds below 10xa0m/s and at optimal speeds, respectively. Higher wheel speeds led to the formation of an amorphous phase that transformed to 2:14:1 phase around 680°C. Optimum magnetic properties were found in samples spun at 14–17xa0m/s and annealed at 700°C for 20xa0min. The loop squareness was also found to depend mainly on the microstructure that is very sensitive to the sample composition. A few percentage of Nb and Zr suppressed the grain growth, resulting in a drastic improvement of magnetic properties, with approximate 50% enhancement in the intrinsic coercivity and an increase in maximum energy product from 5.6xa0kOe and 14.7xa0MGOe for the (Nb,Zr)-free sample to 8.2xa0kOe and 20.3xa0MGOe for the (Nb,Zr)-substituted samples, respectively. The significant improvement in magnetic properties originated from a much finer and homogeneous nanocomposite microstructure with an average grain size of 20xa0nm, leading to a high remanence of 0.73 M s . Henkel plots indicate the enhancement of exchange coupling between hard and soft magnetic phases.

Microstructure refinement and magnetic property enhancement of nanocomposite Pr2Fe14B/α-Fe magnets by small substitution of M for Fe (M=Cr, Nb, Ti and Zr)

Abstract A comprehensive study on the effect of various substitutions of M on the magnetic and structural properties of melt-spun nanocomposite Pr8Fe84M2B6 (M=Cr, Nb, Ti and Zr) magnets has been performed with the aim of refining the microstructure and therefore enhancing the hard magnetic properties. It has been found that magnetic properties are improved by all the substitutions. The largest enhancement is obtained in Nb substituted Pr8Fe84Nb2B6 magnets where a coercivity of 6.5 kOe and a maximum energy product of 17.9 MGOe have been obtained, as compared to the coercivity of 4.3 kOe and the energy product of 9.1 MGOe in the Pr8Fe86B6 magnet. Microstructure studies revealed a finer and more uniform 2:14:1/α-Fe nanoscale microstructure with M substitutions. The most uniform microstructure with the smallest average grain size of 10–20 nm is developed in the Nb substituted magnets. The enhancement of magnetic properties by M substitution is believed to be due to the microstructure refinement which leads to an enhanced exchange coupling between Pr2Fe14B and α-Fe.

Microstructure and magnetic properties of (Pr, Tb)2(Fe, Nb, Zr))14B/α-Fe nanocomposites

The microstructure and magnetic properties of Pr2Fe14B/α-Fe nanocomposites with small amount of Tb, Nb, and Zr additions have been investigated. A c-axis texture in the 2:14:1 phase has been observed in ribbons spun at speeds below 14 m/s. Optimal magnetic properties were found in annealed samples spun at 14–17 m/s. With small additions of Nb and Zr, the coercivity significantly increases. Nb substitution leads to a poor loop squareness, which can be significantly improved with Zr addition due to a refinement in microstructure, resulting in a drastic increase in the energy product. Optimum magnetic properties with a room temperature coercivity of 8.2 kOe, and (BH)m=20.3u200aMGOe have been obtained in Pr7Tb1Fe87Nb0.5Zr0.5B4 ribbons. Transmission electron microscopy on this sample revealed a much finer and homogeneous microstructure with an average grain size of 20 nm.

Magnetic properties and granular structure of CoPt/B films

Granular CoPt/B films, consisting of high anisotropy face-centered-tetragonal (fct) CoPt particles embedded in a boron matrix have been prepared by heat treating cosputtered films. Transmission electron microscopy and x-ray diffraction studies show that the as-deposited films consist of a mixture of an amorphous-like phase and face-centered-cubic CoPt nanocrystallites. After a heat treatment at 700u200a°C for 5 min a microstructure consisting of interconnected particles of partially ordered fct CoPt with sizes ranging from 20 to 200 nm is formed. These samples have coercivities of 6 kOe and high reduced remanence (Mr/MS=0.9) due to the interparticle interactions. After prolonged annealing the particles become spherical and their size increases to around 400 nm. The coercivities of these overaged samples are higher but the hysteresis loops are constricted and the remanence is reduced. Measurements of irreversible susceptibility and viscosity as a function of temperature and applied field were used to determine...