M. Ohnuma

Cu clustering and Si partitioning in the early crystallization stage of an Fe73.5Si13.5B9Nb3Cu1 amorphous alloy

Solute clustering and partitioning behavior in the early crystallization stage of an Fe{sub 73.5}Si{sub 13.5}B{sub 9}Nb{sub 3}Cu{sub 1} amorphous alloy have been studied by employing a three-dimensional atom probe (3DAP) and a high resolution electron microscope (HREM). Results from the 3DAP have clearly shown that Cu atom clusters are present in the amorphous state after annealing below the crystallization temperature. The density of these clusters is in the order of 10{sup 24}/m{sup 3}, which is comparable to that of the {alpha}-Fe grains in the optimum nanocrystalline microstructure. In the early stage of primary crystallization, Cu clusters are in direct contact with the {alpha}-Fe nanocrystals, suggesting that the {alpha}-Fe primary particles are heterogeneously nucleated at the site of Cu clusters. In the early stage of crystallization, the concentration of Si is lower in the primary crystal than in the amorphous matrix phase, unlike in the late stage of the primary crystallization, where Si partitions into the {alpha}-Fe phase with a composition of approximately 20 at.%.

Size dependence of ordering in FePt nanoparticles

We have investigated the size effect of A1→L10 ordering of FePt nanoparticles in FePt–Al2O3 granular and FePt/SiO2 particulate films by transmission electron microscopy (TEM). The TEM results have shown convincingly that ordering does not progress when the particle size has a diameter of less than 4 nm. Calculation of the order parameter profile from the surface to the volume of the FePt nanoparticles based on diffuse-interface theory justified the experimentally observed size dependence of the ordering. The transition length from disorder to order depends on the interfacial energy, hence the critical particle size of ordering should vary depending on the type of matrix and substrate.

Size effect on the ordering of FePt granular films

We have investigated the A1→L10 ordering process of FePt granular films with different particle sizes. Although FePt particles larger than ∼7 nm ordered by annealing at 600 °C, the ordering did not proceed when the particle size was smaller than ∼4 nm in diameter. This suggests that there is a size dependence on the ordering of FePt particles. Calculation of the Helmholtz free energy using the Lennard-Jones potential suggested that Tc decreases below the annealing temperature when the particle size is decreased to a few nanometers suggesting that experimental observation is physically feasible.

Microstructure of Co–Al–O granular thin films

We have investigated the microstructures of Co–Al–O granular thin films, which were prepared by the sputter-deposition technique with various oxygen partial pressures. The constituent phases, grain sizes of granular particles, and width of insulating channels have been evaluated quantitatively. The specimen with the optimum giant magnetoresistance (GMR) is composed of nanoscale Co particles, and these are completely isolated by amorphous aluminum oxide channels. The GMR behavior observed in the Co–Al–O films has a close correlation with the width of the insulating channel and the grain size of the magnetic particles, which is consistent with the spin-dependent tunneling conduction mechanism of GMR

Microstructures of FePt–Al–O and FePt–Ag nanogranular thin films and their magnetic properties

The microstructure and magnetic property relationship in L10 ordered FePt–Al–O and FePt–Ag granular thin films have been studied. As-sputtered FePt–Al–O films, composed of isolated fine spherical FePt particles of ∼2 nm diameter with a disordered face-centered-cubic (fcc) structure, exhibit superparamagnetism. Annealing above 650 °C induces a transformation from a disordered fcc structure to ordered L10 phase and the films become magnetically hard. The microstructures of these films change greatly depending on the film compositions and annealing conditions, which are correlated with the magnetic properties. It was found that FePt particles smaller than 5 nm do not order at 500 °C, while the continuous FePt film orders perfectly at the same temperature, suggesting that the ordering temperature, Tc, decreases significantly when the particle size becomes less than 5 nm. In the FePt–Ag granular thin film, when the Ag composition is around 50 at. %, high coercivity (∼10 kOe) and fine uniform microstructure are...

Magnetic properties of nanocrystalline FeMCuNbSiB alloys (M: Co, Ni)

Abstract The magnetic properties and microstructure of nanocrystalline FeMCuNbSiB alloys (M: Co, Ni) annealed under a transverse field have been studied. Large induced magnetic anisotropy and good properties in the high frequency range were obtained in Co-rich nanocrystalline FeCo(Cu)NbSiB alloys.

Small-angle neutron scattering and differential scanning calorimetry studies on the copper clustering stage of Fe–Si–B–Nb–Cu nanocrystalline alloys

Abstract The kinetics of copper clustering and primary crystallization of FINEMET type alloys with the compositions Fe 74.5− x Si 13.5 B 9 Nb 3 Cu x and Fe 77 Si 11 B 9 Nb 3− x Cu x have been studied by small-angle neutron scattering (SANS) and high-sensitivity differential scanning calorimetry (DSC) in order to explain the different optimized Cu contents, x , for obtaining the highest permeability in these two alloys. SANS results have shown that the alloys with the optimized Cu contents have the finest nanocrystalline microstructures. Kinetic analyses of Cu clustering prior to primary crystallization have shown that the number density of Cu clusters becomes highest at the crystallization stage of α-Fe primary crystals in the alloy containing an optimized amount of Cu.

Microstructures and magnetic properties of Co–Al–O granular thin films

The microstructures of Co–Al–O thin films of wide varieties of compositions are studied by transmission electron microscopy and small angle x-ray scattering (SAXS). In the superparamagnetic specimens, high resolution electron microscope images reveal that isolated spherical Co particles are surrounded by an amorphous aluminum oxide matrix. However, in the soft ferromagnetic films, the shape of the Co particles is prolate ellipsoidal. SAXS intensities from the soft magnetic specimens decrease inversely with the wave vector, q, in a low wave-vector region, while an interparticle interference peak is observed for the superparamagnetic specimens. The scattering profiles of the soft magnetic films imply that the Co particles have a cylindrical shape and are randomly oriented. The correlation between the magnetic properties and the microstructures is discussed.

Optimization of the microstructure and properties of Co-substituted Fe–Si–B–Nb–Cu nanocrystalline soft magnetic alloys

The effect of Co replacement for Fe on the microstructure and soft magnetic properties of Fe78.8−xCoxNb2.6Si9B9Cu0.6 (x=5–60) nanocrystalline alloys has been studied for improving the soft magnetic properties of Fe–Si–B–Nb–Cu type alloys at a high frequency range. The magnetic anisotropy constant increases with x, but the coercivity increases when x exceeds 20, indicating that magnetic softness is degraded by replacing Fe with Co. Three-dimensional atom-probe observations have revealed that the number density of Cu-enriched clusters decreases with x, thereby decreasing the number density of the heterogeneous nucleation sites for bcc-Fe primary crystals. In addition, differential scanning calorimetry measurements show that the Cu clustering temperature shifts to a higher temperature with increasing x, suggesting that the kinetics for the Cu clustering decreases as Co content. These experimental results are discussed from the thermodynamical point of view, and the optimized Cu composition to achieve a low c...

Nanoquasicrystalline phase formation in binary Zr–Pd and Zr–Pt alloys

Abstract This paper reports nanoquasicrystalline phase formation in Zr 100−x Pd x (x=30 and 35) and Zr 80 Pt 20 binary alloys and the kinetics of the nanoquasicrystallization process. While the icosahedral phase ( i -phase) forms as a metastable phase in the transient stage during the crystallization of Zr–Pd amorphous alloy, it forms directly from the liquid during melt-spinning of Zr–Pt alloy. The isothermal kinetics studies show that i -phase forms from the Zr 70 Pd 30 amorphous alloy by the primary crystallization process with the Avrami exponent in the range of 1.5–2.5. Three-dimensional atom probe analysis results suggest that the i -phase is slightly enriched with Zr with respect to the matrix and its composition is close to Zr 75 Pd 25 . The tendency of quasicrystallization of Zr-based alloys appears to have correlation with the enthalpy of mixing of the system.