Gradient-Composition Sputtering: An Approach to Fabricate Magnetic Thin Films With Magnetic Anisotropy Increased With Temperature

Magnetic anisotropy is always known to decrease with temperature and so far there has been no work in the literature reporting about any experimental observation of the increment of magnetic anisotropy with temperature in any thin films other than the one reported in our previous papers. In this paper, we demonstrate that gradient-composition sputtering technique is a method to fabricate magnetic thin films having an in-plane magnetic anisotropy increased with temperature. In particular, our composition-graded magnetic thin films of FeCoX (X = Ta, Zr, Hf, Lu) show an increase of ferromagnetic resonance frequency when the temperature is elevated, which evidently indicates the increase of magnetic anisotropy with temperature. This peculiar behavior is ascribed to the stress-induced origin of the magnetic anisotropy present in the films grown by gradient-composition sputtering in which the stress of the films is increased with temperature. In order to gain a deeper understanding of the mechanism of this unusual thermal behavior, an additional set of composition-graded FeCoTa thin films were fabricated onto different substrates showing that the increase of magnetic anisotropy with temperature is present in all the films regardless of the substrates used. This experimental result unambiguously implies that the increasing of stress with temperature is not only due to the difference between the thermal expansion coefficients between the substrates and the films, but also owing to the intrinsic nature of the composition gradient of the films.