Anisotropic AC Magnetic Susceptibility in (Ga,Mn)As Films

Abstract

Ferromagnetic semiconductor (Ga, Mn)As has emerged as the most thoroughly studied material for semiconductor spintronic applications, showing excellent magnetic properties that are usually described by the single-domain model [1]. However, certain uncertainties about magnetic phase structure and their dynamic properties of this system still remain unresolved. In this study, we systematically carry out both dc and ac magnetic measurements as a function of temperature and applied magnetic field in (Ga, Mn)As films with nominal Mn concentration of 5.0% grown by molecular beam epitaxy. The ac susceptibility vs. temperature shows similar results to those reported previously [2,3]: two peaks are observed with magnetic field H along [100], one close to T C and one close to T C /2, while one peak close to T C is observed with H along [1-10], and one peak close to T C /2 is seen for H along [110]. These results are consistent with dc magnetization measurements along these three axes. Careful analysis of hysteresis loops and of frequency-dependent ac susceptibility leads to the unambiguous conclusion that the ac susceptibility peak near T C occurs because of transition from paramagnetic to ferromagnetic phase; and the peak near T C /2 occurs because of the onset of a biaxial domain structure [4]. Specifically, ac susceptibility near T C involves a 180 degree magnetization flip along the [1-10] axis (i.e., the uniaxial easy axis), while the peak of the ac susceptibility near T C /2 originates from a magnetization flip between two biaxial easy axes with an angular separation of θ(T), arising from the competition between uniaxial and cubic anisotropy. Due to the difference between these two mechanisms, frequency dependences of ac susceptibilities in these two temperature regions are also quite different, providing additional information on both biaxial and uniaxial domain structures in (Ga, Mn)As films. In this connection, magnetic dynamic properties (such as the relaxation times) are analyzed and discussed using various dynamic models in order to allow us to get better insight into the evolution of magnetic domain transitions in (Ga, Mn)As films. The present observations on magnetic domain transitions and their dynamics provide new information relevant to spintronic applications such as memory and logic devices. This work was supported by the NSF Grant DMR14-00432.