S. Hirosawa

Magnetotransport through the spin-reorientation transition in Tm2Fe14B

The electrical resistivity and Hall effect for a single crystal of Tm2Fe14B have been measured over the range of temperature (T) from 4 to 600 K in magnetic fields of up to 5 T. The resistivity exhibits a small step-like rise at the spin-reorientation temperature Ts, which is 311 K, and a broad minimum at 535 K. In addition, the Hall coefficient shows an anomaly at Ts, and drops sharply as T approaches the Curie temperature (549 K) from below. The lower-temperature anomalies, both in the resistivity and in the Hall coefficient, show that the spin-reorientation transition in Tm2Fe14B is of first order. The high-temperature Hall anomaly is probably produced by critical spin fluctuations near the Curie point. Dominant scattering mechanisms that underlie the Hall effect and magnetoresistance in Tm2Fe14B are inferred.

Direct formation of Fe3B/Nd2Fe14B nanocomposite permanent magnets in rapid solidification

Abstract Rapid solidification processing of Nd 4 Fe 77.5− x B 18.5 M x (M=Cr, Cu) alloys was studied in order to obtain better understandings of the “direct” process in which a Fe 3 B/Nd 2 Fe 14 B-type nanocomposite structure is formed during the rapid solidification of a melt. Effects of influential elements such as Cr and Cu on the critical cooling rate were also studied.

Direct observation of ferromagnetism in grain boundary phase of Nd-Fe-B sintered magnet using soft x-ray magnetic circular dichroism

We have investigated the magnetism of the grain boundary (GB) phase in a Nd14.0Fe79.7Cu0.1B6.2 sintered magnet using soft x-ray magnetic circular dichroism (XMCD) at the Fe L2,3-edges. Soft XMCD spectra were measured from the fractured surface that was confirmed to be covered with a thin GB phase by Auger electron spectroscopy. The magnetic moment of Fe in the GB phase was estimated to be mGB=1.4 μB at 30 °C using the sum rule analysis for XMCD spectra, which is 60% of that of Fe in the Nd2Fe14B compound. The temperature dependence of mGB evaluated with reference to Fe in the Nd2Fe14B phase indicated that the Curie temperature of the GB phase is more than 50 °C lower compared to that of Nd2Fe14B.

Influence of heating rate on the microstructure and magnetic properties of Fe3B/Nd2Fe14B nanocomposite magnets

The influence of heating rate on the microstructure and magnetic properties of the Fe3B/Nd2Fe14B nanocomposite permanent magnet materials produced by the crystallization of the Nd4.5Fe73B18.5Co2Cr2 amorphous alloy has been studied.

Mechanism of grain size refinement of Fe3B/Nd2Fe14B nanocomposite permanent magnet by Cu addition

Minor addition of Cu to Nd4.5Fe77B18.5 melt-spun alloy is effective on reducing the grain size of the Fe3B/Nd2Fe14B nanocomposite permanent magnet produced via the crystallization route from the amorphous phase, thereby improving hard magnetic properties. Three-dimensional atom probe and transmission electron microscopy observations have shown that Cu clusters with a number density of ∼1024 m−3 is formed prior to the nucleation event of the Fe3B primary crystal. In the nucleation and growth stage of the Fe3B primary crystals, Cu clusters are in direct contact with the primary particles, suggesting that Cu clusters serve as heterogeneous nucleation sites for the Fe3B primary particles, thereby increasing the number density of the particles.

Micromagnetic Simulations of Magnetization Reversal in Misaligned Multigrain Magnets With Various Grain Boundary Properties Using Large-Scale Parallel Computing

This paper reports on micromagnetic simulations of the magnetization reversal behavior in polycrystalline Nd-Fe-B sintered magnets using large-scale parallel computing. A multigrain model is introduced to calculate the grain alignment dependence of the coercivity of polycrystalline Nd-Fe-B magnets with various magnetic characteristics at grain boundaries (GBs). The magnetic domain wall motion is accurately treated by dividing the analyzed object into extremely small elements. The multigrain model with a soft magnetic GB phase and a reverse domain at the initial state well reproduces experimental results. The calculations of coercivity with several GB widths are also carried out to seek for the origin of the sudden decrease of coercivity with nearly perfect grain alignment.

Structure and magnetic properties of Nd/sub 2/Fe/sub 14/B/Fe/sub x/B-type nanocomposites prepared by strip casting

A series of isotropic nanocomposite permanent magnets composed of Nd/sub 2/Fe/sub 14/B (the majority phase) and Fe/sub 3/B and/or Fe/sub 23/B/sub 6/ are developed by converting phase formation behavior with partial replacement of Fe with Ti and B with C. The glass forming ability of the melt of the new series of alloys is great enough to allow application of the strip casting technique as the means of rapid solidification process. Powders exhibit excellent magnetic properties ranging from (B/sub r/,H/sub cJ/)=(0.88 T, 470 kA/m) to (0.80 T, 1010 kA/m), closely matching to the property range covered by the conventional isotropic Nd-Fe-B hard magnetic powders.

Effects of V and Si additions on the coercivity and microstructure of nanocomposite Fe3B/Nd2Fe14B magnets

The microstructure and the temperature dependence of the coercivity of nanocomposite Fe3B/Nd2Fe14B magnets with compositions of Nd4.5Fe77B18.5, Nd4.5Fe74B18.5V3 and Nd4.5Fe73B18.5V3Si1 (at. %) have been studied to understand the role of microalloyed elements of vanadium and silicon in improving hard magnetic properties. The results show that the increase in coercivity by adding vanadium and silicon is mainly caused by the increase in the mass fraction of the hard magnetic phase rather than by the decrease in the grain size. Vanadium atoms partition into the soft magnetic phase with a concentration of about 5 at. %, while silicon atoms partition into the hard magnetic phase with a concentration of about 2 at. %. The partitioning of vanadium and silicon causes the Curie temperatures of the both hard and soft magnetic phases to increase. This increase in the Curie temperature, combined with refinement of the microstructure in these nanocomposite magnets, contributes to the enhancement of the exchange couplin...

Micromagnetic simulation of the orientation dependence of grain boundary properties on the coercivity of Nd-Fe-B sintered magnets

This paper is focused on the micromagnetic simulation study about the orientation dependence of grain boundary properties on the coercivity of polycrystalline Nd-Fe-B sintered magnets. A multigrain object with a large number of meshes is introduced to analyze such anisotropic grain boundaries and the simulation is performed by combining the finite element method and the parallel computing. When the grain boundary phase parallel to the c-plane is less ferromagnetic the process of the magnetization reversal changes and the coercivity of the multigrain object increases. The simulations with various magnetic properties of the grain boundary phases are executed to search for the way to enhance the coercivity of polycrystalline Nd-Fe-B sintered magnets.

Microalloying effect of Cu and Nb on the microstructure and magnetic properties of Fe/sub 3/B/Nd/sub 2/Fe/sub 14/B nanocomposite permanent magnets

This paper reports the microalloying effect of Cu and Nb on the microstructure and magnetic properties of a Fe/sub 3/B/Nd/sub 2/Fe/sub 14/B nanocomposite permanent magnet. Optimum magnetic properties with B/sub r/=1.25 T, H/sub cJ/=273 kA/m and (BH)/sub max/=125 kJ/m/sup 3/ were obtained by annealing a melt-spun Nd/sub 4.5/Fe/sub 75.8/B/sub 18.5/Cu/sub 0.2/Nb/sub 1/ amorphous ribbon at 660/spl deg/C for 6 min. Compared with a ternary Nd/sub 4.5/Fe/sub 77/B/sub 18.5/ alloy, the grain size is much finer in the optimum microstructure of the Cu and Nb containing alloys, and the temperature range of the heat-treatment for obtaining optimum hard magnetic properties was significantly extended. The soft magnetic Fe/sub 23/B/sub 6/ phase coexists with Fe/sub 3/B and Nd/sub 2/Fe/sub 14/B phases in the optimum microstructure. Three-dimensional atom probe (3DAP) analysis results have revealed that the finer microstructure is due to the formation of a high number density of Cu clusters, which promote the nucleation of the Fe/sub 3/B phase. The Nb atoms appear to induce the formation of the Fe/sub 23/B/sub 6/ phase and stabilize it by partitioning into this phase.