Domain wall propagation in ferromagnetic semiconductors: Beyond the one-dimensional model

We have investigated experimentally the field-driven propagation of domain walls (DWs) in perpendicularly magnetized ferromagnetic semiconductor layers. The results were then compared with the historical one-dimensional (1D) DW propagation model widely used in spintronics studies of magnetic nanostructures. Anomalous velocity peaks, not predicted by the 1D model, were observed experimentally. Using micromagnetic simulations we show indeed that, in the particular regime of layer thickness (h) of the order of the exchange length, velocity peaks appear in the precessional regime, their shape and position shifting with h. Analyses of the simulations show a distinct correlation between the curvature of the DW and the twist of the magnetization vector within it and the velocity peak. Associating a phenomenological description of this twist with a four-coordinate DW propagation model, we show that the velocity peaks result from the torque exerted by the stray field created by the domains on the twisted magnetization. The position of the peaks is well predicted from the DW’s first flexural mode frequency and depends strongly on the layer thickness. Comparison of the proposed model with data obtained on GaMnAs and GaMnAsP shows that the anomalies observed close to Walker breakdown are indeed induced by the flexion resonance of the domain wall.