Thickness and composition effects on atomic moments and magnetic compensation point in rare-earth transition-metal thin films

Rare-earth (RE) transition-metal (TM) thin films have exhibited great promise for spintronic applications; however, the variation of their magnetic properties with composition and thickness has yet to be adequately quantified. In this paper, we investigate the atomic magnetic moments of few-nanometer-thick GdCo and TbCo via x-ray magnetic circular dichroism and find a significant decrease in both RE and Co average moments with increasing RE concentration, consistent with a local Co environment model description. We observe a shift in compensation composition to higher RE concentrations as the thickness of GdCo and TbCo films decreases $<10$ nm. Based on the dependence of the saturation magnetization $({M}_{s})$ on temperature and thickness, we posit the existence of an intrinsic unalloyed RE dead layer at room temperature that reduces the effective RE concentration in the remaining alloyed region, resulting in significant, nonzero ${M}_{s}$ in thin films at higher RE concentrations than is observed in thick films. We demonstrate a simple model for RE-TM films that accurately describes ${M}_{s}$ as a function of both RE content and film thickness. In addition, the atomic $g$ factors of GdCo and TbCo are found to vary significantly with RE concentration, largely deviating from their elemental values over the bulk of composition space.