Random magnetocrystalline anisotropy in two-phase nanocrystalline systems

In order to clarify the effect of the exchange stiffness in the intergranular phase on the exchange correlation length (Lₑₓ) and the random magnetocrystalline anisotropy (〈K₁〉) of two-phase nanocrystalline soft magnetic materials, the hyperfine fields (⁵⁷Fe), coercivity and remanence to saturation ratio of nanocrystalline Fe₉₁Zr₇B₂ have been studied in the temperature range from 77 to 473 K. We observe that the coercivity of the nanocrystalline Fe₉₁Zr₇B₂ in the temperature range near the Curie temperature of the intergranular amorphous phase (TCᵃᵐ) varies as approximately the -6th power of the mean hyperfine field of the intergranular phase. This indicates that Lₑₓ near T~TCᵃᵐ is mostly governed by the exchange stiffness of the intergranular amorphous phase and 〈K₁〉 of the Fe-Zr-B sample should vary as the -3rd 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 Lₑₓ.