Effect of interstitial carbon on the structural and magnetic properties of Nd(Fe,M) 12C y (M=Ti, V, Mo)

Abstract

Nd(Fe,M) 12C y carbides with M=Ti, V, and Mo have been prepared by heating fine powders of Nd(Fe,M) 12 in methane. The carbides retain the ThMn 12-type crystal structure of the parent alloys, but the unit cell volume expands by about 3%. Upon carbon absorption, the Curie temperatures, saturation magnetization, and anisotropy fields of these compounds were increased. First-order magnetization processes (FOMPs) were detected on textured samples of these compounds when an external field is applied perpendicular to the alignment direction. It has been found that these FOMPs depend on the net carbon concentration y, the temperature as well as the kind of element M. The FOMPs appear above a certain carbon concentration and at temperatures below 150–100 K. We have not found any FOMPs in the nitrides of the same parent alloys. An estimation of the crystal-field (CF) parameters shows that the higher-order CF parameters of the Nd ions play the key role in the origin of the observed FOMPs. The dependence of the FOMPs on temperature is due to the thermal evolution of anisotropy constants K i (i=1, 2, 3) related to corresponding CF parameters.