Direct imaging of current-induced domain wall motion in CoFeB structures

By direct x-ray photoemission electron microscopy imaging, we probe current-induced domain wall motion in 20 nm thick CoFeB wires. We observe transverse walls for all wire widths up to 1500 nm as a consequence of the small saturation magnetization of the material. High critical current densities above 1×10¹² A/m² for wall displacement due to the spin transfer torque effect are found. The critical current densities j_c increase further with decreasing wire width indicating that j_c is governed by extrinsic pinning due to edge defects. In addition to wall displacements, we observe wall transformations to energetically favorable wall types due to heating. Owing to the high Curie temperature though, the sample temperature stays below the Curie temperature even for the highest current densities where structural damage sets in.