Random magnetocrystalline anisotropy in two-phase nanocrystalline systems

Abstract

In order to clarify the effect of the exchange stiffness in the intergranular phase on the exchange correlation length ${(L}_{\mathrm{ex}})$ and the random magnetocrystalline anisotropy $(〈{K}_{1}〉)$ of two-phase nanocrystalline soft magnetic materials, the hyperfine fields ${(}^{57}\mathrm{Fe}),$ coercivity and remanence to saturation ratio of nanocrystalline ${\mathrm{Fe}}_{91}{\mathrm{Zr}}_{7}{\mathrm{B}}_{2}$ have been studied in the temperature range from 77 to 473 K. We observe that the coercivity of the nanocrystalline ${\mathrm{Fe}}_{91}{\mathrm{Zr}}_{7}{\mathrm{B}}_{2}$ in the temperature range near the Curie temperature of the intergranular amorphous phase ${(T}_{C}^{\mathrm{am}})$ varies as approximately the $\ensuremath{-}6\mathrm{th}$ power of the mean hyperfine field of the intergranular phase. This indicates that ${L}_{\mathrm{ex}}$ near $T\ensuremath{\sim}{T}_{C}^{\mathrm{am}}$ is mostly governed by the exchange stiffness of the intergranular amorphous phase and $〈{K}_{1}〉$ of the Fe-Zr-B sample should vary as the $\ensuremath{-}3\mathrm{rd}$ power of the exchange stiffness constant in the intergranular region. These results are explained well by our extended two-phase random anisotropy model in which two local exchange stiffness constants are considered for the spin-spin correlation within ${L}_{\mathrm{ex}}.$