Bimodal switching field distributions in all-perpendicular spin-valve nanopillars

Switching field measurements of the free layer element of 75nm diameter spin-valve nanopillars reveal a bimodal distribution of switching fields at low temperatures (below 100K). This result is inconsistent with a model of thermal activation over a single perpendicular anisotropy barrier. The correlation between antiparallel to parallel and parallel to antiparallel switching fields increases to nearly 50% at low temperatures. This reflects random fluctuation of the shift of the free layer hysteresis loop between two different magnitudes, which may originate from changes in the dipole field from the polarizing layer. The magnitude of the loop shift changes by 25% and is correlated to transitions of the spin-valve into an antiparallel configuration.