Effect of Rare-Earth Incorporation in Ferromagnetic Metals for Magnetic Domain Wall Devices.

摘要

Devices that utilize propagation of magnetic domain walls are of interest for memory and logic applications. These devices are inherently non-volatile and have the potential for major reductions in energy consumption and required power dissipation. However, the speed of these devices must increase to compete with conventional electronics and the energy required to switch and propagate magnetic domains must be reduced to achieve low-power operation.;While Permalloy wires have been widely studied in recent years for domain wall based device applications, domain walls propagated by a spin-polarized current through Permalloy require high critical current densities to initiate domain wall motion and exhibit low velocities. Domain wall dynamics caused by an applied current are not yet fully understood and must be further explored and optimized to establish domain wall device concepts as viable technologies. Magnetization dynamics depend strongly on material properties yet utilization of materials engineering to provide insight and enhancements in dynamics is an area that has not been adequately explored.;In this work, we study novel magnetic metal alloys created by doping ferromagnetic materials with rare-earth elements and evaluate their potential to enhance domain wall dynamics thereby improving domain wall device performance. We focus on Gd dopants that reduce magnetization of magnetic transition metals by antiferromagnetic coupling while still maintaining a low Gilbert damping factor. Dopant atoms should also act as scattering centers that reduce the spin-flip length thus enhancing the non-adiabatic spin torque factor. The static magnetic properties and crystallinity of various film compositions incorporating Gd dopants in Permalloy, Ni and Co are characterized by SQUID magnetometry and XRD measurements. The measurements confirm that Gd dopants decrease the film magnetization and destroy the film crystallinity creating amorphous films with low coercive fields.;Measurements analyzing the effect of Gd dopants on magnetization dynamics are limited to doped Permalloy films because Permalloy has the lowest Gilbert damping factor and has been extensively characterized. FMR measurements demonstrate a small effect on the Gilbert damping factor and verify that co-sputtering yields high-quality uniform thin films. The spin transfer velocity and current polarization are measured using a spin wave Doppler technique. Through measurements, it was found for the first time, that introducing Gd dopants results in a considerable reduction in current polarization which reduces the spin transfer velocity.;These results are verified with the first domain wall velocity measurements through PyGd wires performed using time-resolved MOKE magnetometry. The spin transfer velocity for Permalloy reduces by 30% for the Py0.921Gd 0.079 composition. The effect of the non-adiabatic spin torque contribution is determined through current-assisted domain wall depinning measurements. An unexpected negative spin torque effect is measured for two of the PyGd compositions implying a negative non-adiabatic factor.;In summary, this work explores new magnetic materials and provides insight into current-induced domain wall dynamics that can be used to enhance the domain wall velocity for device applications. These findings indicate that rare-earth dopants are useful for devices utilizing magnetic field propagation resulting in similar velocities as Permalloy but at lower magnetic fields. However, for devices that propagate domain walls using spin-polarized current, the rare-earth doping technique is not beneficial due to the unexpected degradation in spin torque efficiency. This work is the first of its kind to reveal this mechanism and provides insight into the impact of rare-earth incorporation on domain wall dynamics. Although these devices may not provide the expected increase in velocity, the use of rare-earth dopants in magnetic materials provides a platform to study domain wall dynamics by allowing the tuning of material parameters and correlating the impact on domain wall velocities.