Structure and magnetic properties of compositionally modulated Tb-Fe thin films (abstract)

Abstract

Artificially layered structures have attracted attention because of their unique structural and magnetic characteristics. It has been reported that in these structures the anisotropic distribution of atomic pairs can play an important role in determining their magnetic anisotropy.1 Our previous work showed that the application of substrate bias in a rf diode sputtering system was effective in producing dense and stable amorphous compositionally modulated films (CMFs).2 In this work, we studied the interrelationships of the structural and magnetic properties of amorphous Tb-Fe CMFs deposited under various substrate bias conditions (0, −70, −90 V) in a multitarget rf diode sputtering system. Although considerable research has been conducted on the effect of ion bombardment during deposition on the film structure, it has been difficult to independently control the film structure and the composition in such experiments because of the preferential backsputtering of atoms. Accordingly, in this research we systematically prepared Tb-Fe CMFs by independently controlling the rf voltage ratio of both the Tb and Fe targets, thereby allowing the composition of the film to be held constant as the substrate bias was changed. In contrast to alloy films, atomic clustering was observed in ultrathin layered films and produced anomalous structural and magnetic property changes, e.g., a shift in the room-temperature compensation composition (25 at. % Tb for −70 V biased films and 23 at. % Tb for −90 V biased films), higher Curie temperatures, and an increased compositional range over which amorphous phase was stable. The atomic arrangements in layered films were modified by a variation of deposition parameters such as layer periodicity, substrate bias, and composition. Although all the bias-sputtered CMF films, except those with small Tb content (<18 at. % Tb), had a dense and featureless amorphous structure, electron diffraction revealed significant differences in atomic short-range ordering for different Tb compositions. The results indicated that the magnetic moments of the Fe atoms in the CMF were increased by an enhanced pair ordering of like atoms produced by the alternate deposition of ultrathin layers. This result was consistent with the observed anomalies in magnetic properties.