Microwave dynamics in nanoscale multilayer CPP devices driven by spin-transfer torque

Nanopillar devices with an elliptical cross-section of about 130 nm × 70 nm were studied. The devices were made by ion milling of a sputtered multilayer Cu 80 nm/Py 20 nm/Cu 6 nm/Py 2 nm/Cu 2 nm/Pt 30 nm (Py=Ni₈₀Fe₂₀) followed by planarization with silicon oxide and the deposition of a Cu top contact. Current is applied perpendicular to the sample layers. We used Py magnetic layers due to its low magnetocrystalline anisotropy and small magnetostriction. As compared to previous studies of Co samples, the small magnetic moment of Py leads to smaller critical currents I_c for switching or excitation of dynamical states and smaller coercive magnetic fields H_c. This allows us to study nanopillar devices over an effectively larger range of current and field than in previous experiments.