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Magnetic Insulator Thin Films and Induced Magneto-Transport Effect at Normal Metal / Magnetic Insulator Interface

  • Author(s): Lin, Tao
  • Advisor(s): Shi, Jing
  • et al.
Abstract

The discipline of spintronics with magnetic insulators (MI) has attracted extensive attention in both research and application interests. Yttrium iron garnet (YIG) is a ferrimagnetic insulator which is called the spin Seebeck insulator, for its supports of pure spin currents generation. Non-magnetic metals (NM) with strong spin-orbit interaction (e.g. Pd, Pt), are used as either spin current generator or detector based on the spin Hall effect (SHE) or the inverse spin Hall effect (ISHE). The combination of these two types of material plays a very important role in spintronics. One the other hand, a recent magneto-transport study shows strong evidence of a magnetic proximity effect in thin Pt films deposited on YIG. It is desirable to have more experimental evidence in distinguishing the two effects qualitatively and quantitatively. In my work here, the induced magneto-transport effects at NM/MI interface will be discussed in details, including induced independent spin moments at the interface and disentanglement of anisotropic magnetoresistance (AMR) and spin Hall magnetoresistance (SMR), as well as the fabrication of YIG thin films and devices.

Pulsed laser deposition (PLD) has emerged as a preferred technique to deposit complex oxide thin films, heterostructures, and superlattices with high quality. The general PLD techniques will be discussed in Chapter I, with examples of La0.7Sr0.3MnO3 and Fe3O4 thin films fabrication and characterization. In Chapter II, I will discuss our approach to grow YIG films on (110)- and (111)-oriented gadolinium gallium garnet (GGG) substrates. In both orientations, we have successfully grown epitaxial YIG thin films confirmed by the patterns of the reflection high-energy electron diffraction. The layer-by-layer mode was achieved with lower laser repetition rate than reported by other groups. For both orientations, the atomic force microscopy images showed that the YIG surface was extremely flat, and flat terraces were found with the atomic step height on film surface. The magnetic properties were measured with in-plane easy-axis and very narrow coercive field, which was almost independent of temperature. The in-plane magnetic anisotropy was explored as well. This work paves the way to engineering anisotropy of the thin films for YIG-based magnetic devices.

As a thin layer of NM (e.g. Pd, Pt) was deposited on the MI thin film like YIG, NM developed both low- and high-field magneto-transport effects that were absent in standalone NM or thick NM on YIG. While the low-field magnetoresistance (MR) peaks of NM tracked the coercive field of the YIG film, the much larger high-field MR and the anomalous Hall effect (AHE-like) did not appear to have any relationship with the bulk YIG magnetization. The high-field MR had a sign reversal at some intermediate temperature for both Pd and Pt, and the magnitude of AHE-like signal of Pd increased with decreasing temperature, while that of Pt had sign reversal. The distinct high-field magneto-transport effects in NM/MI structures had been investigated with different NM film thickness and some control samples, and were shown to be caused by interfacial local moments in NM. Besides, both a clear resistivity minimum and a ln(T) temperature dependence were observed at low temperatures in thin NM films, suggesting that the Kondo effect may be relevant. Detailed discussions about the origin of these effects will be presented in Chapter III.

The AMR effect, evidence for a proximity induced magnetic moment in Pt/YIG bilayer, was previously found absent in experiments with a rotating magnetic field applied at room temperature. Instead, the observed magnetoresistance was attributed to a new type of effect called SMR that is caused by magnetization-dependent spin current reflection at the interface. In both Pd/YIG and Pt/YIG bilayers, however, we have observed that SMR and AMR coexist over a wide range of temperatures with AMR dominating at low temperatures. Detailed experimental results were shown of the temperature dependence and field dependence of the two types of magnetoresistance, to disentangle AMR and SMR quantitatively. While AMR is likely caused by the proximity induced magnetization in NM that saturates at relatively low fields, at higher magnetic fields when additional interfacial magnetic moments are gradually aligned, the spin current reflection effect and consequently SMR are greatly enhanced. The detailed analysis and discussions will be presented in Chapter IV.

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