Intergranular Magnetic Coupling and Coercivity in Two-Phase Nanocrystalline Materials.

Abstract

Bcc/amorphous two-phase nanocrystalline samples with various volume fractions of the amorphous phase were prepared from a melt-spun precursor amorphous Fe91Zr7B2 alloy. The coercivity of the sample at 77 K, well below the Curie temperature of the intergranular amorphous phase (Tcam ≈ 370 K), shows a tendency to decrease with increasing the volume fraction of the amorphous phase (Vam). This suggests that the magnetocrystalline anisotropy energy of the sample is diluted by the volume of the residual amorphous phase when the intergranular coupling is stiff. On the other hand, the coercivity at 450 K, well above Tcam, increases dramatically with increasing Vam. This indicates that the intergranular magnetic coupling above Tcam depends on the intergranular distance. The stiffness of ferromagnetic coupling in the intergranular region above Tcam was estimated to be ∼ 10-14 to 10-13 J/m2. The coercivity at T ∼ Tcam appears to vary as the - 6th power of the spontaneous magnetization of the intergranular amorphous phase when Vam is relatively large.