Materials-based control of ultrafast relaxation in ferromagnetic thin films.

摘要

As data rates in magnetic information storage approach 1GHz and above, strategies to control the magnetization dynamics in films become a more pressing need. Materials-based techniques to control relaxation can offer a straightforward implementation for this purpose.; Strategies to both increase and decrease the damping constant in ferromagnetic thin films are described in this thesis. By doping rare earth elements, both damping constant and precessional frequency of Ni81Fe19 (Permalloy) can be widely tuned. Sm, Tb, Dy, and Ho all contribute to damping in Ni81Fe19, among which the contribution of relaxation rate from Ho (1.9GHz/%) is the most, which is four times of that from Tb. The increased damping correlates well to the magnetic states of the rare earths. One element, Eu, does not contribute to damping, but it boosts the precessional frequency over a large range (>500 MHz) in Ni 81Fe19.; Fe has the lowest damping constant of all elemental ferromagnets. We demonstrate that by doping V into pure Fe, the damping constant can be further reduced. High quality MgO(100)/Fe1-xV x epitaxial thin films are deposited by UHV deposition, with the 35 GHz FMR linewidth (42 Oe) of MgO(100)/Fe film even smaller than the narrowest linewidth of Fe ever reported. As V is doped in, Gilbert damping G decreases. The minimum G value observed is only 14% of that of undoped Fe film, and is even only 34% of the lowest G value ever reported on metallic ferromagnets. The decrease in the Gilbert damping G is closely related to the reduced magnetic anisotropy in the system.; The results of this thesis will help advance the understanding of the damping mechanisms in ferromagnets and provide more freedom in engineering the GHz response of the magnetoelectronic devices.