Epitaxial Co1-xMox thin film alloys studied by 59Co NMR

Abstract

59Co NMR (Nuclear Magnetic Resonance) has been used to investigate the distribution of 59Co hyperfine fields in the Co1-xMox thin film alloys (0.01 < x < 0.1) co-deposited on V buffer. At x = 0 the films consisted of a mixture of fcc and hcp Co, and Mo was found to enter randomly the two Co fractions in the investigated concentration range. However, alloying with Mo was found to increase the amount of stacking faults, in agreement with the previously published results of theoretical calculations for binary alloys. A certain deviation from a random distribution of Mo, most probably due to the presence of magnetically induced miscibility gap for Co:Mo, has been identified for the alloys with Mo content larger than 1%. It was found that in the limit of a very small Mo concentration x ≈ 0, each Mo nearest neighbor lowers the 59Co NMR resonance frequency by 43.8 MHz, i.e. the corresponding hyperfine field is reduced by 4.34 T, implying that Co atoms would become nonmagnetic when 5 Mo atoms enter as the nearest neighbors. However, the effect increases with concentration, due to the contribution from Mo in the more distant shells. The extrapolation to Mo concentration higher than the one used in the experiment predicts that Co becomes nonmagnetic when surrounded by 3 Mo nearest neighbors for the concentration as low as x = 0.2–0.25.