Switching distributions in Co-Ni nanopillars with perpendicular magnetic anisotropy.

摘要

This thesis reports on measurements of the switching distributions in Co-Ni nanopillars with perpendicular magnetic anisotropy. The Co-Ni nanopillars are incorporated into a spin-valve device - a two terminal device consisting of two ultrathin (1-3 nm) Co-Ni ferromagnets separated by a thin (4 nm) Cu spacer patterned into ellipses and circles with lateral sizes ranging from 40-300 nm. Magnetic fields applied along the uniaxial anisotropy axis can switch the alignment of the constituent ferromagnetic layers between anti-parallel and parallel. Electric currents flowing can also switch the nanopillar through the spin-transfer torque effect - an electric current transfers spin-angular momentum from conduction electrons to the background magnetization of a ferromagnet, ultimately manifesting as a torque on the magnetization.;Lateral geometry effects were studied on nanopillars with notches along the perimeter. Switching field measurements revealed an asymmetry between the anti-parallel (AP) to parallel (P) and P to AP switching field distributions. A phenomenological model that considers the spatially inhomogeneous dipole field from the polarizing layer explains this asymmetry.;In nanopillars with an 80 nm circular diameter, switching field measurements taken in a cryostat reveal non-uniform magnetization configurations during reversal. At the lowest temperatures (12 K), the transition between uniform states (P to AP) shows three consecutive hysteretic jumps. The thermal stability of the transition states was investigated for temperatures between 12 K and room temperature.;The thermally activated magnetization reversal model by Neel and Brown was tested on 75 nm diameter spin-valves between 20 and 400 K. The temperature dependence of the statistics of switching reflects enhanced thermal fluctuations and cannot be modeled by the Neel expression for the energy barrier. Taking into account the implicit temperature dependence of the energy barrier from the saturation magnetization and perpendicular anisotropy energy explains this discrepancy.;The effective barrier model for spin-torque thermally-activated switching of Co-Ni nanopillars was investigated. We extracted an effective energy barrier height for switching field distributions under several dc currents. The results mostly agree with the prediction that the current modifies the barrier height. However, rare switching events at the tails of the distributions reveal qualitative deviations from this model.