Asymmetry in the angular dependence of the switching field of GaMnAs film

Planar Hall effect measurements were carried out on two GaMnAs ferromagnetic films with different Mn concentrations (6.2 and 8.3 Mn). The switching fields of magnetization taken from field scans of the planar Hall effect showed significantly different angular dependences in the two samples. While the angular dependence of the switching field for the sample with 8.3 Mn had a symmetric rectangular shape in the polar plot, that of the other sample with 6.2 Mn exhibited a clearly asymmetric behavior, with large steps at the 110 crystallographic directions. This switching field behavior was analyzed by considering pinning fields for crossing the 110 directions. The fitting of step features appearing at the 110 directions revealed the presence of a new uniaxial anisotropy field H_u2 along the 100 direction, in addition to the commonly observed cubic H_c anisotropy field (along the 100 directions) and uniaxial anisotropy H_u1 fields (along either the 110 or the [1 1 0] direction) in the GaMnAs film.