Time Resolved X-ray Imaging of Magnetic Dynamics in Perpendicularly Magnetized Nanopillars

Spin Transfer Torque (STT) provides a novel means to manipulate magnetic bits. Previous time-resolved scanning transmission x-ray microscopy (STXM) experiments have shown that in-plane magnetized nanomagnets switch via vortex motion through or virtual vortex motion around a metallic pillar[1]. Here we present results for perpendicularly magnetized samples. It has been suggested that such samples have a smaller critical switching current than those magnetized in-plane[2]. We find that the samples switch via domain wall propagation across the sample. Pillars of 300nm width switch from anti-parallel to parallel over 700 ps when subjected to 2 ns pulses, yielding a domain wall speed of approximately 400 m/s once the domain wall has formed. The reset pulse revealed a slower propagation, taking 1.3 ns, perhaps due to the difference in spin-polarization reflected by and transmitted through the polarizing layer. \\[4pt] [1] J.P. Strachan, \textit{et. al.} Phys. Rev. Lett. 100, 247201 (2008)\\[0pt] [2] S. Mangin, \textit{et. al}, Appl. Phys. Lett. 94, 012502 (2009)