Keywords:Magnetic Semiconductors, Magnetic Birefringence, Magneto-Optical Imaging
We present the direct imaging of 90-degree (90°) magnetization switching dynamics for a (Ga,Mn)As epitaxial layer (x = 0.02, TC » 50 K) with in-plane magnetic anisotropy by employing a home-made magneto-optical (MO) microscope. A small contrast between two different 90° domains is caused by magnetic birefringence (MB) which is primarily attributed to the difference in refractive index between light polarized parallel and light polarized perpendicular to the magnetization. A change in the in-plane magnetization vector is converted into the rotation of the polarization angle of reflected light, and thus into a change in the reflected light intensity. Digital imaging processing is utilized to improve the weak contrast in the domain images associated with this MO effect. Two consecutive 90° switchings are captured at temperature regimes below and above the half-value of the Curie temperature, namely, at 10K and 30 K. The dynamics are not the same for the first and the second switching, reflecting the influence of the á110ñ uniaxial anisotropy and spin-dependent pinning sites. At 10K, the first switching that passes via the relatively-easy uniaxial (REU) axis (the [1-10] axis) is dominated by smooth 90° domain wall (DW) motion, whereas the second switching that passes via the relatively-hard uniaxial (RHU) axis (the  axis) occurs through nucleation and coalescence of 90° domains together with the DW motion. Similarly, at 30K, we still observed a trace of two-step magnetization reversal: the first switching is initiated by nucleations and their rapid expansion, whereas, the second switching is dominated by relatively slow DW motion. The extracted DW velocity is analyzed by employing a thermally-activated depinning and flow models. The derived values of two key parameters, namely, the activation volume and DW mobility are found to be (28 nm)3 and 0.35 nm s−1 Oe at 10 K, respectively.