Fumihiko Hirata

Magnetic viscosity in strontium ferrite fine particles

The magnetic viscosity coefficient (S/sub v/) and the rotational hysteresis loss (W/sub r/) are measured for M-type hexagonal strontium ferrite (SrM) fine particles whose average diameter (D) and coercive force (H/sub cJ/) differ. The relationship between S/sub v/, H/sub cJ/, the activation volume (v) and W/sub r/ is studied, together with milling effects. This experiment shows that S/sub v/ is constant and independent of the magnetic field. Ln S/sub v/ is in proportion to In H/sub cJ/, and H/sub cJ/ approximately v/sup -2/ could be obtained in SrM fine particles that had less milling effects. It is found that Sv increases and v decreases with SrM fine particles (D=0.3 mu m) whose Wr curve increases in width with milling, even through their H/sub cJ/ significantly decreases. This can be explained as an increase in high magnetic field components of anisotropy field (H/sub A/) caused by milling. >

Magnetic Aftereffect and Magnetization Reversal of Sr-Ferrite Fine Particles

the magnetic aftereffect constant (S<inf>v</inf>), the anisotropy field (H<inf>A</inf>) distribution and the rotational hysteresis integral (R<inf>h</inf>) were measured for M-type hexagonal strontium ferrite (SrM) fine particles with various average particle diameters (D) and coercive forces (H<inf>cJ</inf>). The relations between S<inf>v</inf>, the activation volume (v), H<inf>cJ</inf>, H<inf>A</inf>, and R<inf>h</inf> were studied, as well as the effect of milling. S<inf>v</inf> is constant and independent of the magnetic field. In S<inf>v</inf> varies in proportion to In H<inf>cJ</inf>, and the relation H<inf>cJ</inf>¿v^¿2 was found to obtain in SrM particles not extensively subjected to milling. S<inf>v</inf> increases and v decreases for SrM fine particles (D=0.3 ¿m) whose H<inf>A</inf> is widely distributed by milling, even though H<inf>cJ</inf> decreases markedly. S<inf>v</inf> and v are related to the distribution of H<inf>A</inf>. As H<inf>cJ</inf> increases, R<inf>h</inf> and v decrease, which indicates that the magnetization reversal mechanism of SrM fine particles approaches that of a coherent rotation mode.

Crystal Distortion of Submicron Sr-Ferrite Particles by Milling

Submicron-size particles of M-type Sr ferrite were obtained from fine Fe2O3 and precipitated SrCO3. The magnetic properties of the powder were ¿s=71 emu/g and HcJ=5.5 kOe. The HcJ of the powder was reduced to less than 40% of its initial value by dry vibration milling. SEM observations showed that the primary particle size distribution was not much different from that in the initial state. Crystal distortion was determined by X-ray diffraction studies, using the method of Warren and Averbach. The torque curve of the milled powder was measured using a highly sensitive torque magnetometer. The dependence of K2/K1 on the measuring magnetic field was increased with milling-induced crystal distortion. The Mössbauer absorption effect was also measured. submicron particles were analyzed in comparison with coarse Sr ferrite particles (3 ¿m) by a conventional breaking-down process.