Spin-Dependent Transport in Symmetry-Broken Oxide Interfaces and Heterostructures

Abstract

Spintronics is a research field that exploits the spin degree of freedom of electrons, which has rapidly advanced since the early 2000s. Spin, representing the intrinsic angular momentum of electrons, provides an additional controllable parameter distinct from charge. As conventional charge-based electronic devices approach their physical limits in terms of miniaturization and power consumption, spintronics has emerged as a promising paradigm for next-generation information technology. The electron spin behaves as a tiny magnetic moment, and the collective alignment of spins gives rise to magnetization in solids. Ferromagnetic materials exhibit spontaneous magnetization originating from an imbalance of spin populations at the Fermi level, caused by exchange interactions in the 𝑑 or 𝑓 orbitals. Because electrons in ferromagnets are intrinsically spin-polarized, such materials serve as efficient spin generators and detectors. In sandwich structures of ferromagnetic (FM)/nonmagnetic (NM)/FM layers, a pronounced resistance change occurs depending on the relative alignment of the FM magnetizations—known as the giant magnetoresistance (GMR) effect—which underpins modern spintronic memory devices. Furthermore, manipulation of spin angular momentum through spin- transfer torque (STT) or spin–orbit torque (SOT) enables current-induced magnetization switching, extending the functionality beyond conventional field-driven magnetization control. In nonmagnetic heavy elements, strong spin–orbit interaction (SOI) couples the electron’s orbital and spin motions, giving rise to diverse spin-dependent transport phenomena. One representative example is the intrinsic spin Hall effect (SHE), in which a transverse spin current is generated from a longitudinal charge current with orthogonal spin orientation. The SHE enables charge-to-spin conversion, whereas its inverse counterpart (ISHE) allows the reciprocal spin-to-charge conversion, both forming the foundation of spin–orbitronic device concepts. The Rashba effect, another manifestation of SOI, arises at heterointerfaces lacking inversion symmetry, lifting spin degeneracy and inducing spin–momentum locking in the electronic band structure. Strontium titanate (SrTiO3, STO) is a versatile perovskite oxide where surface modification or heterostructure formation leads to a two-dimensional electron gas (2DEG) characterized by low carrier density and a pronounced Rashba effect. In this research, I demonstrate a strategic approach to enhance the Rashba interaction on STO surfaces generated by Ar⁺ ion irradiation. By introducing a high-work-function MoO3 capping layer, the Rashba coefficient was significantly increased, a result quantitatively validated through nonreciprocal charge transport measurements. Furthermore, I investigated the electrostatic gate control of the Rashba effect in AlOx/STO heterostructures, focusing on the Al-thickness dependence and the underlying tuning mechanism. This study suggests that the observed non-volatile gate response originates from defect-mediated charge trapping at the interface rather than intrinsic ferroelectricity in the STO substrate. Additionally, the interfacial coupling between the STO 2DEG and adjacent ferromagnetic layers was systematically examined to prove the spin-dependent scattering mechanisms at these oxide interfaces. Altermagnetism is an unconventional magnetic phase that combines features of both ferromagnets and antiferromagnets while maintaining zero net magnetization. Similar to antiferromagnets, neighboring spins are coupled antiparallel, resulting in magnetic compensation. However, the magnetic space-group symmetries in altermagnets permit a nonrelativistic, momentum-dependent spin splitting of the electronic bands even in a perfectly collinear, zero-moment state. The spin polarization alternates in momentum space with characteristic 𝑑-, 𝑔-, or 𝑖-wave patterns, producing spin-split Fermi surfaces without relying on spin–orbit coupling. This mechanism gives rise to spin-polarized transport properties despite the absence of macroscopic magnetization. Ruthenium dioxide (RuO₂) has recently been proposed as a prototypical altermagnetic material, although its magnetic nature remains under debate. While bulk RuO₂ is generally considered nonmagnetic, thin-film RuO₂ exhibits clear signatures of altermagnetism. Previous studies have explored its spin-split band structure through spin-torque ferromagnetic resonance (ST-FMR) and spin-thermoelectric measurements; however, direct spin- dependent transport such as tunneling magnetoresistance (TMR) had not been demonstrated. In this work, I experimentally realize spin-split TMR in RuO2-based magnetic tunnel junctions, providing the first direct evidence of spin-polarized tunneling transport in an altermagnetic system.