Programmable bias field observed in graded ferromagnetic semiconductor films with broken symmetry

We report on the observation of a field-induced magnetic bias effect in a Ga0.94Mn0.06As1-yPy thin film with digitally graded phosphorus content. Although phosphorus concentration in the sample is changed in steps from y≈0.03 to y≈0.28, the magnetometry and magnetotransport data display a coherent magnetic response typical for single-layer in-plane magnetized films with cubic and weak [110] uniaxial anisotropy. Unexpectedly, low-temperature planar Hall resistance loops exhibit remarkable asymmetry tunable by application of strong in-plane initial magnetic field HIni. We discuss this unusual memory effect, resembling the magnetization asymmetry in the exchange biased magnetic bilayers, in terms of a unidirectional bias field Hb induced by HIni. We show that such bias field defines the delayed or accelerated nucleation of domains with chiral domain walls performing the magnetization reversal.