Ultrafast magnetization dynamics in ferromagnetic thin films and heterostructures

by Guan, Yongfeng, Ph.D., COLUMBIA UNIVERSITY, 2008, 214 pages; 3299263


With the development of magnetic information storage technology, especially when data rates approach 1 GHz and above, new insight into the magnetization dynamics in ferromagnetic materials becomes a more pressing need. In this thesis, our recent studies of the ultrafast magnetization dynamics in ferromagnetic thin films and heterostructures using various measurement techniques are presented.

We present our static transmission-mode x-ray magnetic circular dichroism (XMCD) characterization of element-specific moments in ferromagnetic thin films. Sum rules analysis are further used to extract the projected element-specific spin and orbital moments. A very low projected Tb moment in the 6% Tb-doped Ni81Fe19 thin film, which nonetheless reverses with low applied fields, indicates a sperimagnetic alignment with respect to the Fe and Ni elements in the alloy. The nearly unchanged orbital-to-spin moment ratio of Fe over the measured range of 0 ≤ x ≤ 0.15 in the Fe1- xVx thin films, compatible with known magnetization behavior as well as spectroscopic splitting g-factor data in the alloy by means of a two-sublattice model, confirms that the very low Gilbert damping attained through the introduction of V into epitaxial Fe1-xVx thin films does not result from the reduction of orbital moment content in the alloy.

We also present our synchrotron-based development of time-resolved x-ray magnetic circular dichroism (TR-XMCD) technique. With this technique, we have demonstrated the first element- and layer-resolved magnetization dynamics with temporal resolution of 2–5 ps and angular resolution down to 0.1°. Coupled motion of Fe and Ni moments is verified in Ni81Fe 19 thin film, indicating a strong exchange coupling between Fe and Ni in the alloy. The influence of weak ferromagnetic interlayer coupling, difficult to identify in conventional FMR measurement, is clearly revealed in a pseudo-spin-valve structure of Ni81Fe19/Cu/Co93Zr7. Lagged phase behavior is observed between top and bottom bows in a (Ni 81Fe19)Tb1/Co93Zr7 bilaver. Moreover, Fe, Ni, and Tb moments are seen to be coupled within experimental resolution in a (Ni81Fe19)Tb1 alloy.

Moreover, it is described how we use all-optical pump-probe technique to investigate enhanced damping in Ni80Fe20/NM bilayer structures. The enhancement of both the damping constant α and the relaxation rate G are found to be dependent on the thickness of Ni 80Fe20 laver as well as the choice of normal metal (NM) capping layers. The data can be well explained by the existing theory for spin pumping from the Ni80Fe20 layer into the NM capping layer.

We finally describe our development of a novel dual-frequency ferromagnetic resonance (FMR) technique to explore the possibility of "gating" damping in a trilayer via pumped spin currents. In addition, compositional dependence of saturation magnetization and ferromagnetic relaxation in polycrystalline (Ni81Fe19)1-xCu x magnetic thin films has been measured using broadband CPW-based FMR technique, where both the ferromagnetic damping α and relaxation rate G increase despite the marked decrease in saturation magnetization µ0Ms. The results can he used to make an engineered trilayer structure of Ni81Fe 19/Cu/(Ni81Fe19)1-xCu x to get resonance frequencies to cross for further understanding of spin pumping mechanism using the dual-frequency FMB technique.

AdviserWilliam E. Bailey
Source TypeDissertation
SubjectsElectromagnetics; Condensed matter physics; Materials science
Publication Number3299263

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