Transport of spin angular momentum (spin currents) in magnetic thin films is important for non-volatile spin-based memory devices and other emerging information technology applications. It is especially important to understand how a spin current propagates across interfaces and how a spin current interacts with magnetic moments. The great interest in devices based on ferromagnetic metals generated intensive theoretical and experimental studies on the basic physics of spin currents for the last few decades. Of particular interest recently is the so-called "pure" electronic spin current, which is carried by electrons and yet unaccompanied by net charge flow, in part because of the prospect of transporting spin with minimal Joule heating. However, in contrast to ferromagnetic metals, spin transport in antiferromagnetic metals, which are promising materials for next-generation magnetic information technology, is not well understood yet. This dissertation addresses the mechanisms of transport by pure spin current in thin-film multilayers incorporating metals with antiferromagnetic order. We focus on two specific materials: (1) CoGd alloys with ferrimagnetic sublattices, which resemble antiferromagnets near the compensation composition, and (2) elemental antiferromagnetic Cr, which can be grown as epitaxial films and hence serve as a model system material. For both the CoGd and Cr studies, spin-valve-like structures of NiFe/Cu/CoGd and NiFe/Cu/Cr/CoFe are prepared to conduct ferromagnetic resonance spin pumping experiments. Precessing magnetization in the NiFe "spin source" pumps a transverse spin current to the adjacent layers. We measure the loss of the spin angular momentum in the "spin sink" layer by measuring the broadening of the resonance linewidth, i.e., the non-local damping enhancement, of the spin source. The antiparallel magnetic moments of Co and Gd sublattices partially cancel out the dephasing of a transverse spin current, thereby resulting in a long spin dephasing length of ≈ 5-6 nm near the magnetic compensation point. We find evidence that the spin current interacts somewhat more strongly with the itinerant transition-metal Co magnetism than the localized rare-earth-metal Gd magnetism in the CoGd alloy. We also examine spin transport via structurally clean antiferromagnetic Cr, epitaxially grown with BCC crystal order. We observe strong spin reflection at the Cu/Cr interface, which is surprising considering that thin layers of Cu and Cr individually are transparent to spin currents carried by electrons. Further, our results indicate other combinations of electrically conductive elemental metals (e.g., Cu/V) can form effective spin-reflecting interfaces. Overall, this thesis advances the basic understanding of spin transport in metallic thin films with and without magnetic order, which can aid the development of next generations of efficient spintronic devices.
This work was supported in part by the National Science Foundation, Grant No.
DMR-2003914. / Doctor of Philosophy / Manipulation of electronic flow, i.e., net charge flow, underlies modern electronic devices such as computers, mobile phones, and electric cars. However, the conventional charge transport inevitably results in wasted energy, due to resistive (Joule) heating in the devices. A new research area which uses the electron's spin has recently emerged, namely spintronics. Spintronics uses the spin of electrons rather than just the charge, thereby reducing the dependence on charge flow. The flow of spin angular momentum carried by electrons, i.e., "electronic spin current," underpins numerous phenomena in condensed matter physics. An important example is switching and excitation of magnetic order driven electrically by spin current rather than external magnetic field. Spin currents can interact not only with ferromagnetic order consisting of parallel magnetic moments – but also with antiferromagnetic order consisting of alternating magnetic moments that cancel the net magnetization of the material. Indeed, experiments from the last few years demonstrate the ability to rotate antiferromagnetic order (a.k.a. Néel vector) by spin current, which offers new physics not achievable in ferromagnets, such as ultrafast spin dynamics in the THz regime and superfluid spin transport analogous to superconducting electronic transport. However, interaction of a spin current with antiferromagnetic order is not well understood yet. The aim of this thesis is to build a better understanding of spin currents in antiferromagnetic metals. Specifically, we experimentally study basic spin-current physics in a ferrimagnet (CoGd) and an antiferromagnet (Cr). We choose CoGd because adjusting its chemical composition allows us to easily tune its magnetism from ferromagnet-like (uncompensated magnetization) to antiferromagnet-like (compensated magnetization). In antiferromagnet-like CoGd, we find that the oppositely oriented Co and Gd magnetic moments partially cancel the scrambling (dephasing) of spins, so that the spin current is able to propagate over a longer distance - about 3-4 times more than in ferromagnetic metals. The mechanisms behind the longer spin propagation is somewhat akin to the spin "rephasing" technique for lengthening the lifetime of spin-based qubits for quantum computers, but what is remarkable is that we observe this effect in rather disordered magnetic alloys at room temperature. In the other major project of this thesis, we investigate spin transport through multilayers that contain Cr, a structurally and chemically clean antiferromagnetic material. We find that Cr by itself is a good spin transmitter, i.e., effectively allowing a pure spin current to pass through. Surprisingly, when Cr and Cu (another good spin transmitter) are stacked together, we observe strong reflection of a pure spin current at the interface of Cr and Cu. We find that the antiferromagnetic order in Cr is not responsible for this peculiar spin reflection and that other pairs of spin-transmitting metals (for example, V and Cu) can form spin-reflecting interfaces as well. Our work shows an interesting example of "emergent" phenomena where the interface behaves in a way that is not intuitively expected from the properties of the constituent materials. The basic scientific findings from this thesis may help the development of more efficient information-technology devices that run on spin currents.
| Identifer | oai:union.ndltd.org:VTETD/oai:vtechworks.lib.vt.edu:10919/110379 |
| Date | 31 May 2022 |
| Creators | Lim, Youngmin |
| Contributors | Physics, Emori, Satoru, Khodaparast, Giti, Park, Kyungwha, Heremans, Jean Joseph |
| Publisher | Virginia Tech |
| Source Sets | Virginia Tech Theses and Dissertation |
| Language | English |
| Detected Language | English |
| Type | Dissertation |
| Format | ETD, application/pdf, application/pdf |
| Rights | In Copyright, http://rightsstatements.org/vocab/InC/1.0/ |
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