Magnetic properties of nanostructured γ-Ni–46Fe alloy synthesized by a mechanochemical process

Abstract

Magnetic properties and their relations to microstructures of nanostructured γ-Ni–46Fe alloy (nano γ-Ni–46Fe), synthesized by a mechanochemical process, were investigated at temperatures from ∼300 down to 8.5 K. The results indicated that the nano γ-Ni–46Fe with average grain sizes of 20–220 nm displayed ferromagnetism, with no superparamagnetic phenomenon being observed in the temperature range investigated. Coercivity increased with decreasing grain size obeying the law Hc∝1/d well in the grain size regime d>∼100 nm. Contrary to prediction from the random anisotropy model, however, the coercivity at both 300 and 8.5 K was not found to decrease with further decreasing grain size in the size range d=20–100 nm, although the grain sizes were then obviously smaller than the evaluated value (102 nm) of ferromagnetic exchange length, suggesting random anisotropy did not dominate its magnetic behavior. Instead, it was demonstrated that the further increase of coercivity with decreasing grain size as d<100 nm can be attributed to the effect of shape anisotropy in the nano γ-Ni–46Fe. Saturation magnetization Ms of the nano γ-Ni–46Fe was found to decrease with decreasing grain size, with a magnitude of ∼11% smaller than that of the coarse-grained standard being reached at d≈20 nm. Experiments revealed that this decrease of Ms was closely related to the increase of oxygen content in the specimens, implying oxidation would be responsible for the decrease of Ms. By assuming that the oxygen existed in the form of surface oxidation layers wrapping the nano-grains, a thickness of ∼0.8 nm for the oxidation layers was derived from the magnitudes of Ms corresponding to different grain sizes.