Dependence of magnetostrictive and magnetic properties on grain size was calculated numerically for an isotropic giant magnetostrictive material. The model magnet assumed is composed of 8000 cubic (Tb0.3Dy0.7)Fe2 grains. The direction of magnetization and the magnetostriction of each grain was determined for a given applied field under the consideration of the anisotropy, exchange, magnetoelastic, elastic, and Zeeman energies. Subsequently, the hysteresis loop and the magnetostriction versus applied field curve was obtained by varying the applied field. With decreasing the grain size D, the coercivity Hc and dλ/dH, where λ is the magnetostriction, decreased abruptly at D=25 nm. This improvement in soft magnetic properties exceeded the expected improvement from Herzer's calculation, in which Hc was proportional to D6. In order to study the significant improvement in detail, we calculated magnetic and magnetostrictive properties with varying magnetostriction and anisotropy constants. Resultantly, it was clarified that the critical phenomenon occurs when the total of elastic and magnetoelastic energies exceed the magnetic anisotropy energy in magnitude. (c)2006 American Institute of Physics
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