Magnetization reduction due to oxygen contamination of bias sputtered Fe35Co65 thin films

Obtaining high magnetization sputtered thin films with soft properties requires small grains with good exchange coupling between the grains. Contamination of grain boundaries during deposition will significantly degrade magnetic softness and reduce magnetization. In this paper, we report on such contamination induced by substrate bias during sputter deposition of Fe₃₅Co₆₅ films. 100-nm-thick Fe₃₅Co₆₅ films were sputtered at 100 W target power in 3 mtorr of Ar, with varying substrate bias from 0 to -300 V. A maximum value of magnetization of 2.35 T was observed at intermediate bias values, but the saturation magnetization of the films decreased nearly 10% for the samples at the highest bias. TEM analysis of the samples indicated an increase in oxide content and a decrease of grain diameter with increasing substrate bias. The loss of magnetization with increasing bias is quantitatively consistent with a nonmagnetic oxide shell of approximately 1 nm in thickness around each grain for all biases. The effect is simply that of increasing surface to volume ratio as the grains get smaller. A nonmagnetic shell of this thickness is expected to be sufficient to break exchange coupling between the grains and yield very poor soft properties, which were observed in these samples.