Tomohito Maki

Effect of hydrogen plasma treatment on reduction process of iron oxide nanoparticles

Fe3O4 nanoparticles coated with oleic acid as a surfactant were reduced by hydrogen plasma generated by 13.56 MHz Radio-Frequency (RF). With increasing input RF power to the inductive coil, Fe3O4 nanoparticles could be transformed to almost single phase of Fe3C and finally to α-Fe. The average grain sizes of Fe3C and α-Fe nanoparticles are 19 – 23 nm and 52 -59 nm, respectively. Saturation magnetizations of Fe3C and α-Fe nanoparticles at 5 K were almost same as these of bulk Fe3C and α-Fe. Field cooling procedure from room temperature to 5 K in the magnetic measurement revealed a formation of FeO phase from Fe3O4 by weak reduction. The desired ranges of electron temperature and electron density in order to obtain Fe3C and α-Fe phases were determined by investigating plasma characteristics.

Analysis of squareness in demagnetization curve of Nd–Fe–B magnet produced by the hydrogenation-disproportionation-desorption-recombination process

An analysis of squareness in demagnetization curve of Nd–Fe–B fine-grained alloys prepared by the hydrogenation-disproportionation-desorption-recombination (HDDR) process was carried out in reference to the melt-spun and sintered magnets by measurements of their recoil curves. The recoil curves of the HDDR magnet and the melt-spun magnet suggested that the magnetization process associated with the recoil behavior of the HDDR magnet is dominated by rotation of magnetization, while that of the sintered magnet suggested that domain wall movement is involved. The recoil susceptibilities for recoil curves that returns to nearly demagnetized state were 0.095 and 0.048T−1 in the isotropic and anisotropic HDDR magnets, respectively, when normalized by saturation magnetization. The normalized susceptibilities could be reproduced quantitatively, assuming that the magnetization recoil process within a grain in the HDDR magnet is described by the coherent rotation of magnetization and a simple model of orientation di...