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Experimental and Modeling Study of Ferrimagnetic Rare-Earth Transition-Metal Thin Films

Li, Xiaopu
Thesis/Dissertation; Online
Li, Xiaopu
Poon, Joseph
As an important class of magnetic materials, ferrimagnets include a variety of substances, ranging from the oldest magnetite (Fe3O4) to yttrium-iron garnet (YIG), rare-earth transition-metal (RE-TM) alloy, and compensated Heusler compound. The existence of two or more antiferromagnetically coupled sublattices provides a pathway to tune magnetization via temperature, composition, crystal structure, or even ultrafast laser pulses. This freedom makes ferrimagnetic materials critical components in the state-of-the-art spintronic devices. In this study, ferrimagnetic thin films, particularly the amorphous RE-TM alloys, were deposited by a magnetron sputtering system. By tuning their compositions, the RE-TM thin films (RE = Gd, Tb, Sm) could effectively exhibit room-temperature compensation and perpendicular magnetic anisotropy (PMA). A thickness dependence of the compensation was revealed experimentally, which implies an existence of growth-induced heterogeneity within the amorphous samples. More interestingly, exchange bias (EB) and bistable magnetoresistance (MR) states have been uncovered in the co-sputtered amorphous Tb(Sm)FeCo thin films. Growth-induced nanoscale phase separation was proposed based on the characterization of transmission electron microscopy (TEM) and atom probe tomography (APT). With an increasing power of computers, numerical modeling has become an important tool for scientific research that examines physics in complex systems, especially the magnetic systems. In this study, an efficient magnetic modeling package (MMP) was designed and programmed in C++ from the ground up. This modeling package incorporates the atomistic magnetic modeling functionality based on both the Monte Carlo Metropolis sampling and the stochastic Landau-Lifshitz-Gilbert (LLG) equation. Moreover, the package has been extended with the parallel tempering algorithm and the micromagnetic Landau-Lifshitz-Bloch (LLB) algorithm to accommodate larger scale problems. With the help of the MMP, this study provides more insights into the static properties of the ferrimagnetic RE-TM heterostructures. For example, examination of a depth profile of short-range order, i.e. the relative ratio of RE-TM pairs, generated results consistent with experimental magnetization measurements. Additionally, tunable EB has been demonstrated in the atomistic ferrimagnetic core-and-matrix structure with compatible temperature dependence. Furthermore, a nanoscale phase-separated system has been modeled in the frame of micromagnetism, and this offers agreement with the experimental observations. Meanwhile, motivated by the recent discovery of all-optical switching (AOS) and Skyrmions in the RE-TM system, the ultrafast magnetization dynamics of the GdFe system was investigated by a phenomenological two-temperature model numerically. Quantitative dependence of the magnetization reversal probability was established in terms of laser fluence, atomic concentration, and Gd-Fe pair ratio. A deterministic reversal was confirmed within a window of these conditions achieving a reversal probability as high as 97%. Finally, an increasing laser fluence threshold has been demonstrated numerically in GdFe multilayers with increasing layer periods. This dynamic study implies a new platform for future ultrafast spintronic devices.
University of Virginia, Department of Engineering Physics, PHD (Doctor of Philosophy), 2017
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PHD (Doctor of Philosophy)
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