Thermal effects in spin-torque assisted domain wall depinning

We have investigated the magnetization reversal in V-shaped permalloy (Py) nanowires under high dc currents via anisotropic magnetoresistance (AMR) measurements. Utilizing a diamond substrate as heat sink, current densities up to $2\ifmmode\times\else\texttimes\fi{}{10}^{12}\phantom{\rule{0.16em}{0ex}}{\text{A}/\text{m}}^{2}$ can be applied. These high current densities enabled us to observe spin-torque assisted switching in thermal equilibrium, without the influence of transient effects. At high currents, the field driven magnetization reversal process is influenced by Joule heating, Oersted fields, and spin-torque effects. These contributions can be identified in our experiment due to their different symmetry properties under magnetization and current reversal. We obtain two important results when evaluating the spin-torque efficiency $\ensuremath{\epsilon}$, which is proportional to the nonadiabaticity parameter $\ensuremath{\beta}$. First, we find different values for $\ensuremath{\epsilon}$ within the same sample, obtained with just a slight variation of the experimental parameters. Second, within the temperature range of 77 to 327 K, $\ensuremath{\epsilon}$ is found to be constant.