Effect of atomic distribution on the saturation magnetization of cobalt-chromium films

Abstract

The saturation magnetization Ms of CoCr films as a function of Cr content and atomic distribution has been modeled. For homogeneous films, two distinct cases have been calculated. The first case is a random array of atoms in an hcp lattice, while the second is an array with the condition that there be no Cr-Cr bonds. Ms is calculated by summing up the Co atom’s contribution, counting only nearest neighbor interactions, and assuming that the Cr atom is nonmagnetic. The model assumes that the contribution of the Co atom to Ms is determined by its Cr nearest neighbors; for zero Cr nearest neighbors, the moment is maximum with the moment linearly decreasing to zero for four or more Cr neighbors. These two cases describe the range of behavior of Ms for CoCr films produced by rf diode sputtering with applied rf bias. For cold substrates with no bias, Ms is close to that predicted for the random distribution, while as the bias level is increased, Ms approaches that predicted by the no Cr-Cr bond distribution. For 20 at. % Cr, the two models predict moments of 480 and 220 emu/cc, respectively. Experimental data supporting the model are presented for Cr concentrations of 11–20 at. %. This model explains why sputtered films tend to have higher Ms values than do bulk samples. In addition, the effect of heat treatment on the magnetic properties of CoCr films is determined by the combination of atomic distribution in the grain and Cr segregation at grain boundaries.