Compensating intrinsic compositional grading in transition metal/rare earth ferrimagnets via power-modulated sputtering

Abstract

Understanding how intrinsic composition gradients during sputter growth modify perpendicular magnetic anisotropy (PMA) of transition metal/rare earth ferrimagnetic alloys remains a key challenge for spintronic device applications. In this work, we intentionally engineer ferrimagnetic CoTb thin films with composition gradients by varying the sputter target power during growth, enabling a systematic investigation of the role of composition gradients on coercivity and magnetic anisotropy. We find that reducing the rare earth element deposition power at the top interface significantly enhances the anisotropy strength and coercivity compared to nominally uniform films, with coercivity enhancements exceeding 100 mT. These results are counter to atomistic spin simulations, suggesting reduced anisotropy in compositionally graded systems. This contradiction is resolved through x-ray photoelectron spectroscopy depth profiling, which reveals that this enhancement arises from the compensation of an intrinsic Cobalt deficiency at the capping interface, which creates an asymmetry between the top and bottom film boundaries. By controlling the sputter power during deposition, the intrinsic composition gradient can be compensated, enabling adjustments of PMA and improving the performance of the ferrimagnetic thin films for spintronic devices.