Magnetism Control by Doping in LaAlO3/SrTiO3 Heterointerfaces

Enhanced Rashba Spin Orbit Coupling and Magnetic Behavior at Oxide Heterointerfaces by Optical Gating

Complex oxide heterointerfaces have been a hot research spot due to their rich physical phenomena and broad quantum coherence that respond to multiple external stimuli. Among these external stimuli, light is a very powerful one to manipulate properties such as carrier density and spin characteristics. However, achieving a light-magnetic correlation is in high demand for multifield responding devices, and its intrinsic mechanism remains unclear. Here, by illuminating Nd0.86Sr0.14Al0.86Ni0.14O3-SrTiO3 heterointerfaces using 360 nm light, we observe a series of interesting physical phenomena, like enhanced magnetoresistance (MR). More interestingly, a band splitting and strong Rashba spin-orbit coupling (SOC) effect occur after illumination, accompanied by a magnetic feature and thus leading to an anomalous Hall effect (AHE). Upon optical gating, the magnetism can be caused by Rashba SOC induced spin-orbit torque (SOT). The work will be sure to have great importance in both theoretical studies and all-oxide devices.

High-Mobility Magnetic Two-Dimensional Electron Gas in Engineered Oxide Interfaces

The engineered interfaces of complex oxides have abundant physical properties and provide a powerful platform for the exploration of fundamental physics and emergent phenomena. In particular, research on the two-dimensional magnetic systems with high mobility remains a long-standing challenge for the discovery of quantum phase and spintronic applications. Here, we introduce a few atomic layers of the delta doping layer at LaAlO₃/SrTiO₃ interfaces through elaborately controllable epitaxial growth of SrRuO₃. After inserting a SrRuO₃ buffer layer, the interfaces exhibit a well-defined anomalous Hall effect up to 100 K and their mobility is enhanced by 3 orders of magnitude at low temperatures. More intriguingly, a large unsaturated positive magnetoresistance is created at interfaces. Combining with the density functional theory calculation, we attribute our findings to the electron transfer at interfaces and the magnetic moment of Ru⁴⁺ 4d bands. The results pave a way for further research of two-dimensional ferromagnetism and quantum transport in all-oxide systems.

Manipulation of 2DEG at double-doped high-entropy heterointerfaces

Chemical doping is a dominating method for manipulating oxide two-dimensional electron gas (2DEG). However, enhancing the doping level while maintaining the metallic conduction remains a challenge, which limits detailed knowledge of 2DEG manipulation. Herein, we propose a concept of high-entropy heterointerface, which consists of a complex oxide (containing at least 5 elements) at either or both sides of the interface. By doubly doping Sr and Mn elements in the Nd and Al sites of NdAlO₃, we grow Nd_1-xSr_xAl_1-xMn_xO₃ (NSAMO) films onto SrTiO₃ (STO) substrates to fabricate NSAMO/STO high-entropy heterointerfaces with different thicknesses (2-30 nm) and a wide range of doping ratios x (0.14-0.56). The 2DEG conducting behavior is maintained until x = 0.42, which is higher compared with similar studies. The varying x results in the coexistence of rich properties like a weak anti-localization (0.14-0.42), abnormal Hall effect (0.28 & 0.42), Lifshitz transition (0.42) and stable structure. These results confirm the potential of this strategy to tailor 2DEG in all-oxide interfaces.

Fully Optical Modulation of the Two-Dimensional Electron Gas at the γ-Al2O3/SrTiO3 Interface

Two-dimensional electron gas (2DEG) formed at the heterointerface between two oxide insulators hosts plenty of emergent phenomena and provides new opportunities for electronics and photoelectronics. However, despite being long sought after, on-demand properties controlled through a fully optical illumination remain far from being explored. Herein, a giant tunability of the 2DEG at the interface of γ-Al₂O₃/SrTiO₃ through a fully optical gating is discovered. Specifically, photon-generated carriers lead to a delicate tunability of the carrier density and the underlying electronic structure, which is accompanied by the remarkable Lifshitz transition. Moreover, the 2DEG can be optically tuned to possess a maximum Rashba spin-orbit coupling, particularly at the crossing region of the sub-bands with different symmetries. First-principles calculations essentially well explain the optical modulation of γ-Al₂O₃/SrTiO₃. Our fully optical gating opens a new pathway for manipulating emergent properties of the 2DEGs and is promising for on-demand photoelectric devices.

Controlling transport properties at LaFeO3/SrTiO3interfaces by defect engineering

The formation of conductive LaFeO₃/SrTiO₃interfaces is first time reported by pulsed laser deposition via controlling the defects of SrTiO₃, which are closely related to the surface of substrate. It is found that the interfaces grown on SrTiO₃substrates without terraces exhibit the two dimensional electron gas. Moreover, the conductive interfaces show a resistance upturn at low temperatures which is strongly diminished by light irradiation. These interfaces favor the persistent photoconductivity, and the enormous value of relative change in resistance, about 60 185.8%, is also obtained at 20 K. The experimental results provide fundamental insights into controlling the defects at conductive interfaces of oxides and paving a way for complex-oxides based optoelectronic devices.

Effect of Rare Earth Elements at Amorphous ReAlO3/SrTiO3 (Re = La, Pr, Nd, Sm, Gd, and Tm) Heterointerfaces

Although the amorphous two-dimensional electron gas (a-2DEG) of oxides provides new opportunities to explore nanoelectronic as well as quantum devices, the intrinsic effect of rare earth (Re = La, Pr, Nd, Sm, Gd, and Tm) elements at ReAlO₃/SrTiO₃ heterointerfaces is still largely unknown and needs to be addressed systematically. Herein, we first propose that the ionization potential of Re elements is a critical factor for the 2DEG fabricated by chemical spin coating. Furthermore, the photoresponsive properties of heterointerfaces are investigated comprehensively with the ionization potential ranging from 35.79 to 41.69 eV. The results show that the sheet resistances significantly increase with increasing the ionization potential, and a resistance upturn phenomenon is observed at TmAlO₃/SrTiO₃ heterointerfaces, which can be attributed to the weak localization effect theoretically. The most important observation is the dramatic transition from negative (-178.3%, Re = La) to positive (+89.9%, Re = Gd) photoresponse at ReAlO₃/SrTiO₃ heterointerfaces under the irradiation of 405 nm light at 50 K. More remarkably, a unique recovery behavior of transient-persistent photoconductivity coexistence at low temperatures is discovered at the TmAlO₃/SrTiO₃ heterointerface. This work reveals an effective approach to tune the transport and photoresponsive properties by changing Re elements and paves the way for the application of all-oxide devices.

Enhancing Sunlight Control of Interfacial Magnetism by Introducing the ZnO Layer for Electron Harvesting

Recently, researchers have developed photovoltaic (PV) control of magnetism to provide a new way of manipulating spin states in an energy-effective manner, where the capability of magnetism manipulation is crucial. Here, we established a PV heterostructure of Pt/PV/ZnO/Co/Si to realize sunlight control of magnetism, where the ZnO layer is introduced to enhance the electron transportation as well as the interfacial optical-electromagnetic tunability. Compared to the PV heterostructure without the ZnO layer (245 Oe), a much greater ferromagnetic resonance shift (1149 Oe) and a saturated magnetization reduction (12.7%) were obtained with the optimal ZnO inserting layer under sunlight illumination. These results prove that the ZnO layer plays a key role in optimizing magnetic manipulation and opening a door toward PV spintronics in the future.

Tuning Magnetism and Photocurrent in Mn-Doped Organic-Inorganic Perovskites

Organic-inorganic perovskites have attracted increasing attention in recent years owing to their excellent optoelectronic properties and photovoltaic performance. In this work, the prototypical hybrid perovskite CH₃NH₃PbI₃ is turned into a ferromagnetic material by doping Mn, which enables simultaneous control of both charge and spin of electrons. The room-temperature ferromagnetism originates from the double exchange interaction between Mn²⁺-I^--Mn³⁺ ions. Furthermore, it is discovered that the magnetic field can effectively modulate the photovoltaic properties of Mn-doped perovskite films. The photocurrent of Mn-doped perovskite solar cells increases by 0.5% under a magnetic field of 1 T, whereas the photocurrent of undoped perovskite decreases by 3.3%. These findings underscore the potential of Mn-doped perovskites as novel solution-processed ferromagnetic material and promote their application in multifunctional photoelectric-magnetic devices.

Photoresponsive properties at (0 0 1), (1 1 1) and (1 1 0) LaAlO3/SrTiO3 interfaces

We report the photoresponsive characteristics of (0 0 1), (1 1 0), and (1 1 1) LaAlO₃/SrTiO₃ heterointerfaces deposited at different oxygen pressures using a 360 nm light. The results show that LaAlO₃/SrTiO₃ interfaces with less oxygen vacancies exhibit a larger resistance change when illuminated by light and a slower recovery process when light is off. In addition, the (1 1 0) LaAlO₃/SrTiO₃ heterointerfaces present the smallest photoinduced change and residual photoinduced change in the resistance, which are related to the negligible polarization discontinuity at the interfaces. Our results provide a deeper insight into the photoinduced properties in the 2D electron gas system, paving the way for the design of oxide optoelectronic devices.

Polarization screening-induced epitaxial growth and interfacial magnetism of BiFeO3/PbTiO3nanoplates

Single-crystal BiFeO₃/PbTiO₃nanoplates have been synthesizedviaa hydrothermal method, where BFO films selectively grew on the negative polar surface of PTO with a saturation thickness of about 18–20 nm and a room-temperature ferromagnetism.

Sunlight Control of Interfacial Magnetism for Solar Driven Spintronic Applications

The inexorable trend of next generation spintronics is to develop smaller, lighter, faster, and more energy efficient devices. Ultimately, spintronics driven by free energy, for example, solar power, is imperative. Here, a prototype photovoltaic spintronic device with an optical-magneto-electric tricoupled photovoltaic/magnetic thin film heterojunction, where magnetism can be manipulated directly by sunlight via interfacial effect, is proposed. The magnetic anisotropy is reduced evidenced by the out-of-plane ferromagnetic resonance (FMR) field change of 640.26 Oe under 150 mW cm-2 illumination via in situ electron spin resonance (ESR) method. The transient absorption analysis and the first-principles calculation reveal that the photovoltaic electrons doping in the cobalt film alter the band filling of this ferromagnetic film. The findings provide a new path of electron doping control magnetism and demonstrate an optical-magnetic dual controllable logical switch with limited energy supply, which may further transform the landscape of spintronics research.

Strain-induced conductivity accelerated recoveries in LaAlO3/SrTiO3 heterostructure with millimeter scale

The transport and magnetic properties of LaAlO₃/SrTiO₃ (LAO/STO) heterostructure have been studied during cooling and warming. The strain gradient perpendicular to the surface of the heterostructure increases with the thickness of LAO film. The conductivity accelerated recoveries (CAR) are found at 80 K and 176 K in the interface of LAO/STO sample with millimeter scale, and are more obvious for thicker LAO layers during warming. These two recovering temperatures correspond to the migrating energies of oxygen single vacancy and divacancy trapped by polarized domain walls, separately. This indicated that domain walls diffuse along the longitudinal direction and expand to larger area due the strain gradient perpendicular to the interface. The stable and precise accelerating recovering temperatures make the sample at a larger scale a potential widely applied temperature standard reference. The magnetization measurements reveal the coexistence of paramagnetic and diamagnetic in the LAO/STO samples at whole temperature from 2 K to 300 K. The abnormal electric resistance rise is observed with the decreasing temperature below 25 K for the samples of 7 and 15 LAO layers. This anomaly is attributed to the Kondo effect below 25 K and weak anti-localization below 5 K due to the weightier paramagnetic content. The larger diamagnetic content suppresses these contributions in 25 LAO layers sample. This work provided an insightful view that the strain modified structure domain leads to the enhancement of CAR effect, which helps to achieve a better understanding of domain related physics in the LAO/STO system.

The transport and magnetic properties of LaAlO₃/SrTiO₃ (LAO/STO) heterostructure have been studied during cooling and warming.