Current-driven spin orbit field in LaAlO3/SrTiO3 heterostructures

Scientific Reports, 2015

The recent development in the fabrication of artificial oxide heterostructures opens new avenues in the field of quantum materials by enabling the manipulation of the charge, spin and orbital degrees of freedom. In this context, the discovery of two-dimensional electron gases (2-DEGs) at LaAlO3/SrTiO3 interfaces, which exhibit both superconductivity and strong Rashba spin-orbit coupling (SOC), represents a major breakthrough. Here, we report on the realisation of a field-effect LaAlO3/SrTiO3 device, whose physical properties, including superconductivity and SOC, can be tuned over a wide range by a top-gate voltage. We derive a phase diagram, which emphasises a field-effect-induced superconductor-to-insulator quantum phase transition. Magneto-transport measurements indicate that the Rashba coupling constant increases linearly with electrostatic doping.

Physical Review Letters, 2013

Possible ferromagnetism induced in otherwise non-magnetic materials has been motivating intense research in complex oxide heterostructures. Here we show that a confined magnetism is realized at the interface between SrTiO3 and two insulating polar oxides, BiMnO3 and LaAlO3 . By using polarization dependent x-ray absorption spectroscopy, we find that in both cases the magnetism can be stabilized by a negative exchange interaction between the electrons transferred to the interface and local magnetic moments. These local magnetic moments are associated to magnetic Ti 3+ ions at the interface itself for LaAlO3 /SrTiO3 and to Mn 3+ ions in the overlayer for BiMnO3 /SrTiO3 . In LaAlO3 /SrTiO3 the induced magnetism is quenched by annealing in oxygen, suggesting a decisive role of oxygen vacancies in this phenomenon. 75.47.Lx, 78.70.Dm The progress in oxide thin film technology is opening the possibility of electronic applications based on the peculiar physical properties of oxide interfaces [1, 2]. The best known example is the junction between two nonmagnetic band insulators, LaAlO 3 (LAO) and SrTiO 3 (STO), which hosts a quasi-two-dimensional electron gas (q2DEG) . The functional properties of LAO/STO heterostructures are indeed extraordinary, such as the possibility of driving an insulating to metal transition by electric field gating at room temperature . At the same time, the real ground state properties of this system are still hotly debated due to apparently conflicting observations of ferromagnetic ordering in some samples , and of superconductivity of the q2DEG below 0.3 K [6] in other samples. More recent studies [7-10] have suggested the coexistence of magnetism and superconductivity, quite intriguing for fundamental physics. However, the mere existence and the real nature of this magnetism in LAO/STO are still questioned and urge clarification.

Physical Review B

The spin Hall effect is considered a phenomenon in which a charge current is converted to a spin current due to the spin-orbit coupling (SOC). Recently, large values of the spin Hall angle (SHA), as the conversion efficiency between spin and charge currents, were observed in metal oxide structures in ferromagnetic/metal (FM/M) oxide bilayers, although their underlying mechanisms are ambiguous. The present study aims to analytically indicate that a mixed region of metal and oxide formed at the metal/oxide interface includes the surface oxide charge, which allows for introducing an SOC term in the electron Hamiltonian. Based on the results, the SHA increases by two orders of magnitude in a Cu/oxide thin film through the side jump and skew scattering mechanisms, reaching that of heavy metals. In addition, a comprehensive model is provided for the influence of oxidation on the SHA and spin-orbit torques (SOTs) in FM/M structures. We find that the magnitude of SOTs is greatly dependent on (i) the surface oxidation condition, (ii) the current flow path, and (iii) the electronic interface condition. The model represented in the present study is regarded as a promising model and prediction mechanism which explains recent observations. The findings can be implemented in generating spin current without any use of external magnetic fields and heavy metals in spintronic devices.

Physical Review B, 2020

Tailoring spin-orbit interactions and Coulomb repulsion are the key features to observe exotic physical phenomena such as magnetic anisotropy and topological spin texture at oxide interfaces. Our study proposes a novel platform for engineering the magnetism and spin-orbit coupling at LaMnO3/SrIrO3 (3d-5d oxide) interfaces by tuning the LaMnO3 growth conditions which controls the lattice displacement and spin-correlated interfacial coupling through charge transfer. We report on a tunable and enhanced interface-induced Rashba spin-orbit coupling and Elliot-Yafet spin relaxation mechanism in LaMnO3/SrIrO3 bilayer with change in the underlying magnetic order of LaMnO3. We also observed enhanced spin-orbit coupling strength in LaMnO3/SrIrO3 compared to previously reported SrIrO3 layers. The X-Ray spectroscopy measurement reveals the quantitative valence of Mn and their impact on charge transfer. Further, we performed angle-dependent magnetoresistance measurements, which show signatures of magnetic proximity effect in SrIrO3 while reflecting the magnetic order of LaMnO3. Our work thus demonstrates a new route to engineer the interface induced Rashba spin-orbit coupling and magnetic proximity effect in 3d-5d oxide interfaces which makes SrIrO3 an ideal candidate for spintronics applications.

Physical Review B, 2017

Physical Review Letters, 2010

The superconducting transition temperature, Tc, of the SrTiO3/LaAlO3 interface was varied by the electric field effect. The anisotropy of the upper critical field and the normal state magnetotransport were studied as a function of gate voltage. The spin-orbit coupling energy ǫSO is extracted. This tunable energy scale is used to explain the strong gate dependence of the mobility and of the anomalous Hall signal observed. ǫSO follows Tc for the electric field range under study.

Radioelectronics. Nanosystems. Information Technologies, 2018

The magnetic properties of the heterostructures consisting of platinum Pt, epitaxially grown manganite optimally doped by strontium La 0.7 Sr 0.3 MnO 3 (LSMO), rare earth intermetallic superlattices consisting of exchange-coupled layers TbCo 2 /FeCo (TCFC), and the epitaxial film of yttrium iron garnet Y 3 Fe 5 O1 2 (YIG) were investigated. The TCFC material provides giant magnetostriction, large magnitude of the magnetomechanical coupling coefficient, and controlled induced magnetic anisotropy. In addition TCFC, as well as Pt, has strong spin-orbit interaction. Experimental studies have shown that the magnetic interaction of the heterostructure (TeCo 2 / FeCo)n/LSMO has an antiferromagnetic character. An increase of linewidth of the ferromagnetic resonance in the Pt/LSMO structure was observed and explained by the spin current flow induced in Pt film by the LSMO film at ferromagnetic resonance. In the TCFC/YIG heterostructure, an electric voltage induced in the TCFC film was observed and explained by the inverse spin-Hall effect under conditions of ferromagnetic resonance.

Physical Review B

The combined effect of charge transfer, broken symmetries, and spin-orbit coupling (SOC) yields interfacial magnetism and transport in spatially asymmetric quantum oxide heterostructures. The polar nature of such interfaces activates the Rashba effect which provides a platform to modify the electronic band structure, whereas the presence of competing magnetic anisotropies is essential for deriving the topologically nontrivial spin textures. Entanglement of these two effects wherein the Rashba physics can be utilized to manipulate magnetic anisotropy (MA) is contemplated as a pathway to alternative electronic states. Here, using the CaMnO 3 /CaIrO 3 heterostructure as a template, we demonstrate that Rashba SOC modulates the MA and controls the sign and magnitude of anomalous Hall conductivity consequent to the modification in the electronic band structure via the reconstruction of Berry curvature. This approach to tune and control the multipronged magnetotransport phenomena via Rashba SOC (without external bias voltage) is potentially relevant for spin-orbitronics functionalities.

Physical Review Letters, 2010

We report on a systematic study of a number of structurally identical but chemically distinct transition metal oxides in order to determine how the material-specific properties such as the composition and the strain affect the properties at the interface of heterostructures. Our study considers a series of structures containing two layers of ferromagnetic SrRuO3, with antiferromagnetic insulating manganites sandwiched in between. The results demonstrate how to control the strength and relative orientation of interfacial ferromagnetism in correlated electron materials by means of valence state variation and substrate-induced strain, respectively.

Physical Review Letters, 2013

We report the spin structure of an exchange-biased ferromagnetic oxide heterostructure, La 0:67 Sr 0:33 MnO 3 =SrRuO 3 , through magnetization and polarized neutron reflectometry measurements. We reveal that the magnetization reversal process of the La 0:67 Sr 0:33 MnO 3 biased layer critically depends on the frozen-in spin structure of the SrRuO 3 biasing layer during the cooling process. Furthermore, we observe unexpected double-shifted hysteresis loops of the biased layer that originates from the formation of lateral 180 magnetic domains within the biasing layer, a new mechanism not found in conventional exchange-bias systems.

Nature Communications, 2015

The discovery of two-dimensional electron gases (2DEGs) at oxide interfaces-involving electrons in narrow d-bands-has broken new ground, enabling the access to correlated states that are unreachable in conventional semiconductors based on s-and p-electrons. There is a growing consensus that emerging properties at these novel quantum wells-such as 2D superconductivity and magnetism-are intimately connected to specific orbital symmetries in the 2DEG sub-band structure. Here we show that crystal orientation allows selective orbital occupancy, disclosing unprecedented ways to tailor the 2DEG properties. By carrying out electrostatic gating experiments in LaAlO 3 /SrTiO 3 wells of different crystal orientations, we show that the spatial extension and anisotropy of the 2D superconductivity and the Rashba spin-orbit field can be largely modulated by controlling the 2DEG sub-band filling. Such an orientational tuning expands the possibilities for electronic engineering of 2DEGs at LaAlO 3 /SrTiO 3 interfaces.

Advanced Materials, 2021

2022

2012

Ferromagnetic heterostructures provide an ideal platform to explore the nature of spin-orbit torques arising from the interplay mediated by itinerant electrons between a Rashba-type spin-orbit coupling and a ferromagnetic exchange interaction. For such a prototypic system, we develop a set of coupled diffusion equations to describe the diffusive spin dynamics and spin-orbit torques. We characterize the spin torque and its two prominent--out-of-plane and in-plane--components for a wide range of relative strength between the Rashba coupling and ferromagnetic exchange. The symmetry and angular dependence of the spin torque emerging from our simple Rashba model is in an agreement with experiments. The spin diffusion equation can be generalized to incorporate dynamic effect such as spin pumping and magnetic damping.

arXiv (Cornell University), 2021