Synthesis and magnetic properties of nanostructured γ-Ni-Fe alloys

Abstract

Nanostructured γ-Ni-xFe alloys with x ≈32, 46, 55, and 64wt% and with an average grain size of 30–40nm were successfully synthesized by a mechano-chemical process. Magnetic measurements indicated that these nano-alloys were ferromagnetic; no superparamagnetic phenomenon was observed at all the temperatures investigated (from ∼300K to 8.5K). The coercivity of these nano-alloys were much higher than that of their conventional coarser-grained counterparts, implying that random anisotropy did not dominate their magnetic behavior although their grain sizes were substantially smaller than the evaluated magnitudes of the ferromagnetic exchange lengths. The increase of coercivity and retentivity with iron content, as x < 55wt%, was found to be basically proportional to magneto-crystalline anisotropy in the Ni-Fe system, indicating magnetocrystalline anisotropy was the dominant factor. As x > 55wt%, however, the decrease of coercivity with iron content was far slower than one would expect based on the crystalline anisotropy. Microstructural analysis indicated that the high coercivity for the nano γ-Ni-Fe with higher iron content (x = 64wt%) can be explained by its larger shape anisotropy. The variation of the saturation magnetization of the nano γ-Ni-xFe alloys with iron content are basically similar to that of its polycrystalline counterpart. But, the absolute magnitudes were about 11–13% lower than those of the corresponding polycrystalline counterparts. This slight decrease in magnitude can be ascribed to oxidation of the nano γ-Ni-xFe powder after reduction.