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Direct Observation of Room-Temperature Stable Magnetism in LaAlO₃/SrTiO₃ Heterostructures

Along with an unexpected conducting interface between nonmagnetic insulating perovskites LaAlO₃ and SrTiO₃ (LaAlO₃/SrTiO₃), striking interfacial magnetisms have been observed in LaAlO₃/SrTiO₃ heterostructures. Interestingly, the strength of the interfacial magnetic moment is found to be dependent on oxygen partial pressures during the growth process. This raises an important, fundamental question on the origin of these remarkable interfacial magnetic orderings. Here, we report a direct evidence of room-temperature stable magnetism in a LaAlO₃/SrTiO₃ heterostructure prepared at high oxygen partial pressure by using element-specific soft X-ray magnetic circular dichroism at both Ti L_3,2 and O K edges. By combining X-ray absorption spectroscopy at both Ti L_3,2 and O K edges and first-principles calculations, we qualitatively ascribe that this strong magnetic ordering with dominant interfacial Ti³⁺ character is due to the coexistence of LaAlO₃ surface oxygen vacancies and interfacial (Ti_Al–Al_Ti) antisite defects. On the basis of this new understanding, we revisit the origin of the weak magnetism in LaAlO₃/SrTiO₃ heterostructures prepared at low oxygen partial pressures. Our calculations show that LaAlO₃ surface oxygen vacancies are responsible for the weak magnetism at the interface. Our result provides direct evidence on the presence of room-temperature stable magnetism and a novel perspective to understand magnetic and electronic reconstructions at such strategic oxide interfaces

The Effect of Polar Fluctuation and Lattice Mismatch on Carrier Mobility at Oxide Interfaces

Since the discovery of two-dimensional electron gas (2DEG) at the oxide interface of LaAlO₃/SrTiO₃ (LAO/STO), improving carrier mobility has become an important issue for device applications. In this paper, by using an alternate polar perovskite insulator (La_0.3Sr_0.7) (Al_0.65Ta_0.35)­O₃ (LSAT) for reducing lattice mismatch from 3.0% to 1.0%, the low-temperature carrier mobility has been increased 30 fold to 35 000 cm² V^–1 s^–1. Moreover, two critical thicknesses for the LSAT/STO (001) interface are found, one at 5 unit cells for appearance of the 2DEG and the other at 12 unit cells for a peak in the carrier mobility. By contrast, the conducting (110) and (111) LSAT/STO interfaces only show a single critical thickness of 8 unit cells. This can be explained in terms of polar fluctuation arising from LSAT chemical composition. In addition to lattice mismatch and crystal symmetry at the interface, polar fluctuation arising from composition has been identified as an important variable to be tailored at the oxide interfaces to optimize the 2DEG transport

Effect of Extrinsically Introduced Passive Interface Layer on the Performance of Ferroelectric Tunnel Junctions

We report the effect of the top electrode/functional layer interface on the performance of ferroelectric tunnel junctions. Ex situ and in situ fabrication process were used to fabricate the top Pt electrode. With the ex situ fabrication process, one passive layer at the top interface would be induced. Our experimental results show that the passive interface layer of the ex situ devices increases the coercive voltage of the functional BaTiO₃ layer and decreases the tunneling current magnitude. However, the ex situ tunneling devices possess more than 1000 times larger ON/OFF ratios than that of the in situ devices with the same size of top electrode

The Mechanism of Electrolyte Gating on High‑<i>T</i>_<i>c</i> Cuprates: The Role of Oxygen Migration and Electrostatics

Electrolyte gating is widely used to induce large carrier density modulation on solid surfaces to explore various properties. Most of past works have attributed the charge modulation to electrostatic field effect. However, some recent reports have argued that the electrolyte gating effect in VO₂, TiO₂, and SrTiO₃ originated from field-induced oxygen vacancy formation. This gives rise to a controversy about the gating mechanism, and it is therefore vital to reveal the relationship between the role of electrolyte gating and the intrinsic properties of materials. Here, we report entirely different mechanisms of electrolyte gating on two high-<i>T</i>_<i>c</i> cuprates, NdBa₂Cu₃O_7−δ (NBCO) and Pr_2–<i>x</i>Ce_<i>x</i>CuO₄ (PCCO), with different crystal structures. We show that field-induced oxygen vacancy formation in CuO chains of NBCO plays the dominant role, while it is mainly an electrostatic field effect in the case of PCCO. The possible reason is that NBCO has mobile oxygen in CuO chains, while PCCO does not. Our study helps clarify the controversy relating to the mechanism of electrolyte gating, leading to a better understanding of the role of oxygen electro migration which is very material specific

Decreasing the Hydroxylation Affinity of La_1–<i>x</i>Sr_<i>x</i>MnO₃ Perovskites To Promote Oxygen Reduction Electrocatalysis

Understanding the interaction between oxides and water is critical for designing many of their functionalities, including the electrocatalysis of molecular oxygen reduction. In this study, we probed the hydroxylation of model (001)-oriented La_1–<i>x</i>Sr_<i>x</i>MnO₃ (LSMO) perovskite surfaces, where the electronic structure and manganese valence were controlled by five substitution levels of lanthanum with strontium, using ambient-pressure X-ray photoelectron spectroscopy in a humid environment. The degree of hydroxyl formation on the oxide surface correlated with the proximity of the valence band center relative to the Fermi level. LSMO perovskites with a valence band center closer to the Fermi level were more reactive toward water, forming more hydroxyl species at a given relative humidity. More hydroxyl species correlate with greater electron-donating character to the surface free energy in wetting and reduce the activity to catalyze oxygen reduction reaction (ORR) kinetics in a basic solution. New strategies for designing more active catalysts should include design of electronically conducting oxides with lower valence band centers relative to the Fermi level at ORR-relevant potentials

Interface-Induced Enhancement of Ferromagnetism in Insulating LaMnO₃ Ultrathin Films

Engineering ferromagnetism, by modulating its magnitude or anisotropy, is an important topic in the field of magnetism and spintronics. Among different types of magnetic materials, ferromagnetic insulators, in which magnetic moment unusually coexists with localized electrons, are of particular interest. Here, we report a remarkable interfacial enhancement of the ferromagnetism by adding one unit-cell LaAlO₃ adjacent to an insulating LaMnO₃ ultrathin film. The enhancement of ferromagnetism is explained in terms of charge transfer at the interface, as evidenced by X-ray absorption spectroscopy and ab initio calculations. This study demonstrates an effective and dramatic approach to modulate the functionality of ferromagnetic insulators, contributing to the arsenal of engineering techniques for future spintronics