Spin injection and spin-orbit coupling in low-dimensional semiconductor nanostructures

Future spintronics devices will be built from elemental blocks allowing the electrical injection, propagation, manipulation and detection of spin-based information. Owing to their remarkable multi-functional and strongly correlated character, oxide materials already provide such building blocks for charge-based devices such as ferroelectric field-effect transistors (FETs), as well as for spin-based two-terminal devices such as magnetic tunnel junctions, with giant responses in both cases. Until now, the lack of suitable channel materials and the uncertainty of spin-injection conditions in these compounds had however prevented the exploration of similar giant responses in oxide-based lateral spin transport structures. In this paper, we discuss the potential of oxide-based spin FETs and report magnetotransport data that suggest electrical spin injection into the LaAlO(3)-SrTiO(3) interface system. In a local, three-terminal measurement scheme, we analyse the voltage variation associated with the precession of the injected spin accumulation driven by perpendicular or longitudinal magnetic fields (Hanle and 'inverted' Hanle effects). The spin accumulation signal appears to be much larger than expected, probably owing to amplification effects by resonant tunnelling through localized states in the LaAlO(3). We give perspectives on how to achieve direct spin injection with increased detection efficiency, as well on the implementation of efficient top gating schemes for spin manipulation.

Complex oxide heterointerfaces provide a platform to manipulate spin-orbit coupling under the broken inversion symmetry. Moreover, their weak antilocalization (WAL) effect displays quantum coherent behavior due to the strong spin-orbit coupling. Herein, we break through the limitation of lattice mismatch at ReAlO3/STO (Re = La, Pr, Nd, Sm, and Gd) heterointerfaces and obtain their two-dimensional electric gas (2DEG) by spin coating. The effect of different Re elements in the resulting quantum corrections on the conductivity is investigated. It is observed that the conductivity of heterointerfaces is reduced with larger atomic numbers due to the ionization potential of Re elements. Moreover, magnetoresistance (MR) measurements in a perpendicular or a parallel field distinctly uncover strong Rashba spin-orbit coupling (SOC) in ReAO/STO samples besides SAO/STO (Re = Sm) and GAO/STO (Re = Gd), and the effective fields of the SOC (Hso) gradually increase from LAO/STO (Re = La, Hso = 0.82 T) to NAO/STO (Re = Nd, Hso = 1.37 T) at 2 K. The competition between SOC scattering and inelastic scattering is revealed through a temperature-dependence study of MR, and the WAL-weak localization transition is at about 6 K. Furthermore, unambiguous results of the Kondo effect, nonlinear Hall, hysteresis loop, and Rashba SOC suggest the coexistence of WAL at the PAO/STO (Re = Pr) heterointerface with exchange coupling between the localized magnetic moment and the itinerant electron. These results pave a unique route for the exploration of spin-polarized 2DEGs at oxide heterointerfaces.

Recently, there have increasingly been debates on whether there exists a surface resonance state (SRS) in black phosphorus (BP), as suggested by recent angle-resolved photoemission spectroscopy results. To resolve this issue, we have performed temperature- and angle-dependent magnetoconductivity measurements on exfoliated, high-quality BP single crystals. A pronounced weak-antilocalization (WAL) effect was observed within a narrow temperature range of 8–16 K, with the electrical current flowing parallel to the cleaved ac-plane (along the a- or c-axis) and the magnetic field along the b-axis. The angle-dependent magnetoconductivity and the Hikami–Larkin–Nagaoka model-fitted results have revealed that the observed WAL effect shows surface-bulk coherent features, which supports the existence of SRS in BP.

The propagation of spin-polarized currents from a magnetic to a nonmagnetic material is fundamental to spintronics and, furthermore, has become a topic of great interest for basic research. The techniques of electrical spin injection and detection in bulk metals and thin metal films are reviewed, along with recent studies of spin injection between a ferromagnetic metal and a two-dimensional electron gas (2DEG). Electric current crossing the interface between a ferromagnet (F) and nonmagnetic metal (N) is also a current of magnetization, JM, and provides a source for spin diffusion in N. When the spin relaxation rate 1/T2 in N is relatively slow, a nonequilibrium population of polarized spins accumulates in a region characterized by the interfacial area and the spin diffusion length δs. The spin accumulation, equivalently called nonequilibrium magnetization M̃, can be detected by application of a transverse external magnetic field which causes a Hanle effect. Spin–orbit effects are the dominant spin relaxation mechanism in nonmagnetic metals. In a noncentrosymmetric 2DEG, unusual spin–orbit effects cause a spin splitting of the electron energy band. A bias current applied to such a 2DEG can generate nonequilibrium spin magnetization, which can be detected by a ferromagnetic electrode. Preliminary experiments on the injection and detection of spin polarized current at a F–2DEG are relevant to the realization of a spin injected field effect transistor (FET).

Ruthenium dioxide (RuO2) is one of the novel altermagnetic materials. The interleaved antiferromagnetic ordering and the high conductivity provide a potential application for the development of spintronic devices. However, controversies remain regarding its canted magnetism. Therefore, high-quality RuO2 thin films are crucial for investigating electric and magnetic properties. In this work, RuO2 thin films were deposited on Si(100) substrates by reactive magnetron sputtering. Physical properties were tuned by optimizing the argon/oxygen ratio and growth temperature. The oxygen partial pressure could modify the carrier type from electron-dominated to hole-dominated. This result challenged the conventional understanding that N-type carriers dominate in bulk RuO2. X-ray photoelectron and Raman spectroscopy analyses suggested this transition might stem from variations in oxygen vacancy concentration, which could have induced either band structure distortion or changes in defect energy levels. X-ray diffraction and atomic force microscopy revealed temperature-dependent crystal orientation transitions and surface morphology evolution. Additionally, weak antilocalization effects were observed at 2 K, highlighting quantum interference dominated by spin–orbit coupling. As RuO2 was identified as a rare p-type metal oxide, this work may help to resolve controversies about its magnetism and offer a novel p-type magnetic material for spintronic applications.

Spin–orbit interactions in a two-dimensional electron gas were studied in an InAlAs/InGaAs/InAlAs quantum well. Since weak antilocalization effects take place far beyond the diffusive regime, (i.e., the ratio of the characteristic magnetic field, at which the magnetoresistance correction maximum occurs, to the transport magnetic field is more than ten) the experimental data are examined by the Golub theory, which is applicable to both diffusive regime and ballistic regime. Satisfactory fitting lines to the experimental data have been achieved using the Golub theory. In the strong spin–orbit interaction two-dimensional electron gas system, the large spin splitting energy of 6.08 meV is observed mainly due to the high electron concentration in the quantum well. The temperature dependence of the phase-breaking rate is qualitatively in agreement with the theoretical predictions.

$\mathrm{Ca}{\mathrm{Cu}}_{3}{\mathrm{Ru}}_{4}{\mathrm{O}}_{12}$ (CCRO) is considered a $d$-electron-based heavy-fermion metallic system with intriguing electronic properties. The magnetotransport measurements on metallic CCRO thin films reveal weak antilocalization (WAL) effects; however, it is not straightforward to infer whether the same originates from $e\text{\ensuremath{-}}e$ Coulomb interaction (EEI), spin-orbit interaction (SOI), or both present in CCRO. By evaluating quantum correction to sheet conductance for CCRO metallic thin films in the two-dimensional limit, it is observed that SOI gives rise to a dominant contribution to negative magnetoconductance (MC) in the weak-field regime. Based on weak localization (WL) and WAL analysis, SOI and inelastic scattering lengths (${l}_{so}$ and ${l}_{\ensuremath{\phi}}$) are obtained to be 30 and 74 nm, respectively, which indicates that WAL $({l}_{so}<{l}_{\ensuremath{\phi}})$ remains at play. The anisotropic effect of SOI is reflected in the field-direction-dependent in-plane and out-of-plane MC measurements. The presence of the EEI contribution is evidenced from (i) the linear increment (positive slope) of sheet conductance with $ln(T)$ in the quantum interference regime [only the WAL effect should otherwise show a negative slope with $ln(T)]$ and (ii) $ln(T)$ dependence of the Hall coefficient with a negative slope. Further, in the high-magnetic-field regime where WL or WAL is not valid, MC follows $ln(B)$-type behavior, indicating the presence of EEI. These findings have implications for the basic understanding of quantum magnetotransport properties in the presence of SOI and EEI in CCRO.

We demonstrate the monolithic integration of AlGaAs/GaAs 2DEGs with short carrier lifetime photoconductive LT-GaAs, allowing the investigation of THz frequency range magnetoplasmon resonances by the direct injection of ~picosecond duration pulses through the system. The evolution of time domain signals with magnetic field were measured, and the results found to be in good agreement with theoretical calculations made using a mode-matching approach for frequencies above 0.1 THz. Our experimental work introduces a generic technique suitable for the study of ultrafast carrier dynamics in low-dimensional semiconductor nanostructures at THz frequencies

Electronic bound states around charged impurities in two-dimensional systems with structural inversion asymmetry can be described in terms of a two-dimensional hydrogen atom in the presence of a Rashba spin-orbit interaction. Here, the energy levels of the bound electron are evaluated numerically as a function of the spin-orbit interaction, and analytic expressions for the weak and strong spin-orbit coupling limits are compared with the numerical results. It is found that, besides the level splitting due to the lack of inversion symmetry, the energy levels are lowered for sufficiently strong spin-orbit coupling, indicating that the electron gets more tightly bound to the ion as the spin-orbit interaction increases. Similarities and differences with respect to the two-dimensional Fr\"ohlich polaron with the Rashba coupling are discussed.

Evidence of weak-antilocalization phenomenon in Al-induced crystallization grown polycrystalline-SiGe thin film

Possible observation of Berry phase in Aharonov Bohm rings of InGaAs

Electron spin operation by electric fields: spin dynamics and spin injection

Two-dimensional materials (2D) and their van der Waals heterostructures provide unique opportunities for the exploration of various spin-related phenomena that emerge due to their combined properties and functionalities. Spintronic devices operate based on spin, the angular momentum of an electron, which is used to transfer and store information. Apart from the requisite preservation of the spin information over a micro-meter distance, there is a need for active electrical control of the spin signal for efficient spin-logic operations. In this regard, graphene is an excellent choice of material for 2D spintronic devices as it has high charge carrier mobility and weak spin-orbit coupling (SOC), allowing for long-distance spin transport. However, for the electrical generation and/or manipulation of the spin signal, materials with strong spin-orbit or exchange coupling are required. In this thesis, we experimentally show that it is possible to induce both spin-orbit and exchange couplings in graphene by the proximity to other 2D materials. We observe that the induced SOC in graphene in the vicinity of transition metal dichalcogenides results in a strong spin lifetime anisotropy and the emergence of spin-to-charge conversion mechanisms. Furthermore, we detect a strong exchange splitting in graphene in the proximity of a 2D magnetic material, CrSBr, that results in large spin polarization of conductivity. This allows for the active electrical and thermal generation of spin currents by the magnetic graphene itself. These experimental observations open the path for the direct applications of electrically-controlled 2D spintronic and spin-caloritronic circuitries with graphene-based heterostructures.

In this work, we investigate electronic and magnetic phenomena in thin films and heterostructures of transition metal oxides with strong spin–orbit coupling. Ultrathin films are prepared by pulsed laser deposition, a technique which enables layer-by-layer growth of complex materials on atomically flat crystal surfaces. The properties of these heterostructures, which include materials such as strontium iridate (SrIrO3) and strontium ruthenate (SrRuO3), are probed by applying electric and magnetic fields. By varying parameters such as temperature, magnetic field strength and layer thickness, we obtain information about spin and charge transport in these atomically engineered crystals. Chapter 1 provides an introduction to the field of transition metal oxides, followed by a brief overview of the materials studied in this dissertation. Chapters 2 to 5 are dedicated to SrIrO3, a material that displays unexpected physical properties owing to the strong spin–orbit coupling of Ir. Chapter 2 starts with the growth and thermodynamic stability of SrIrO3, which is essential to obtain high-quality films and study their properties in the ultrathin limit. We develop a method to grow stoichiometric films by measuring their transport characteristics as a function of the target condition. We discover that the properties of SrIrO3 are sensitive to degradation in air and develop an encapsulation procedure to protect the filmsurface. SrIrO3 displays an exotic semimetallic state due to the interplay between electronic correlations, spin–orbit coupling, and octahedral rotations. In Chapter 3, we combine thermoelectric and magnetotransport measurements to quantitatively determine the transport coefficients of the different conduction channels. Despite their different dispersion relationships, electrons and holes are found to have strikingly similar transport coefficients. Chapters 4 and 5 focus on the electronic and magnetic properties of SrIrO3 in the two-dimensional limit. In Chapter 4, we discover a metal–insulator transition occurring at a critical thickness of 4 unit cells and an enhancement of spin fluctuations near the transition point. We investigate the magnetic state in Chapter 5, showing that a fourfold symmetric magnetoresistance component appears above a criticalmagnetic field. In Chapter 6, we interface ultrathin SrIrO3 with SrRuO3, an itinerant ferromagnet with an unconventional anomalous Hall conductivity. We discover that the presence of two dissimilar interfaces results in the emergence of two spin-polarized conduction channels. Having explored the influence of epitaxial interfaces, in Chapter 7 we develop a method to detach thin films fromtheir growth substrate using an epitaxial buffer layer. Using this approach, we prepare nanomechanical resonators of freestanding SrTiO3 and SrRuO3 films. By measuring the temperature dependence of theirmechanical response, we observe signatures of structural phase transitions in the SrTiO3, which affect the strain and mechanical dissipation of the resonators. Chapter 8 summarizes the findings of the previous chapters and provide perspectives for future work. We discuss ongoing experiments regarding Berry phase engineering and the manipulation of freestanding films.

The interest in spin transport in nanoscopic semiconductor channels is driven by both the inevitable miniaturization of spintronics devices toward nanoscale and the rich spin-dependent physics the quantum confinement engenders. For such studies, the all-important issue of the ferromagnet/semiconductor (FM/SC) interface becomes even more critical at nanoscale. Here we elucidate the effects of the FM/SC interface on electrical spin injection and detection at nanoscale dimensions, utilizing a unique type of Si nanowires (NWs) with an inherent axial doping gradient. Two-terminal and nonlocal four-terminal lateral spin-valve measurements were performed using different combinations from a series of FM contacts positioned along the same NW. The data are analyzed with a general model of spin accumulation in a normal channel under electrical spin injection from a FM, which reveals a distinct correlation of decreasing spin-valve signal with increasing injector junction resistance. The observation is attributed to the diminishing contribution of the d-electrons in the FM to the injected current spin polarization with increasing Schottky barrier width. The results demonstrate that there is a window of interface parameters for optimal spin injection efficiency and current spin polarization, which provides important design guidelines for nanospintronic devices with quasi-one-dimensional semiconductor channels.