Atomic-scale determination of spontaneous magnetic reversal in oxide heterostructures

Abstract

Interfaces between transition metal oxides are known to exhibit emerging electronic and magnetic properties. Here we report intriguing magnetic phenomena for La 2/3 Sr 1/3 MnO 3 films on an SrTiO 3 (001) substrate (LSMO/STO), where the interface governs the macroscopic properties of the entire monolithic thin film. The interface is characterized on the atomic level utilizing scanning transmission electron microscopy and electron energy loss spectroscopy (STEM-EELS), and density functional theory (DFT) is employed to elucidate the physics. STEM-EELS reveals mixed interfacial stoichiometry, subtle lattice distortions, and oxidation-state changes. Magnetic measurements combined with DFT calculations demonstrate that a unique form of antiferromagnetic exchange coupling appears at the interface, generating a novel exchange spring-type interaction that results in a remarkable spontaneous magnetic reversal of the entire ferromagnetic film, and an inverted magnetic hysteresis, persisting above room temperature. Formal oxidation states derived from electron spectroscopy data expose the fact that interfacial oxidation states are not consistent with nominal charge counting. The present work demonstrates the necessity of atomically resolved electron microscopy and spectroscopy for interface studies. Theory demonstrates that interfacial nonstoichiometry is an essential ingredient, responsible for the observed physical properties. The DFT-calculated electrostatic potential is flat in both the LSMO and STO sides (no internal electric field) for both Sr-rich and stoichiometric interfaces, while the DFT-calculated charge density reveals no charge transfer/ accumulation at the interface, indicating that oxidation-state changes do not necessarily reflect charge transfer and that the concept of polar mismatch is not applicable in metal−insulator polar−nonpolar interfaces. magnetism | thin films | electron microscopy | oxide interfaces | density functional theory T ransition metal oxides (TMOs) exhibit a wide range of electrical, magnetic, and optical properties largely because they can be easily alloyed and the 10 slots in the transition metal atom d orbitals allow for very diverse spin arrangements. Adding strong electron−lattice coupling and easily formed oxygen vacancies to this mix, the result is a large number of coupled degrees of freedom. Therefore, it is not surprising that TMO heterostructures exhibit highly unusual behavior, induced by interfaces between different oxides where symmetry discontinuities occur, leading to properties that are absent in bulk (1-5). A wellknown example is the LaAlO 3 /SrTiO 3 (001) heterostructure, where a 2D electron gas with high mobility forms at the interface between two insulating oxides (6), including the appearance of superconductivity (7) and background ferromagnetic (FM) ordering (8). In many cases, the symmetry discontinuity at the interface strongly modifies the transition metal−oxygen octahedra network, inducing changes in local structure (bond geometry) and local stoichiometry. Thes...

Visualizing quantum phenomena at complex oxide interfaces: An atomic view from scanning transmission electron microscopy

“…As shown in Fig. 8a and 8b, it has been determined from STEM data that only aac 0 (c is perpendicular to the interface) rotation pattern can be responsible for the observed zig-zag oxygen arrangement [126,128]. Second harmonic generation is a technique that probes the electron symmetry at a region of broken symmetry [40].…”

“…La0.67Sr0.33 after the third atomic column from the interface. (d) L2,3 ratio and Mn nominal oxidation state as a function of distance from the interface[126]. Adapted from[126]…”

Exchange bias and inverted hysteresis in monolithic oxide films by structural gradient

“…Figure 4 shows a schematic that suggests an explanation for the hysteresis inversion based on the observed triple regions in STEM. The onsets of magnetizations show that FM-1 and FM-2 couple ferromagnetically, while at the interface, antiferromagnetic (AFM) coupling is expected where there is a considerable amount of excess Sr [26,46,47]. At high magnetic fields in the positive direction, all moments are aligned in the direction of the magnetic field.…”

“…Recently, it has been shown that the interfacial structure coupling together with composition variations near the interface leads to complex magnetic interactions [26]. In fact, the collaborating effects of change in bond geometry and composition are both necessary for observing spontaneous magnetic reversal and inverted hysteresis.…”

“…To visualize and understand the underlying mechanism of the observed complex magnetic behavior on the atomic scale, LSMO 10 , LSMO 40 , and LSMO 80 are studied using high-resolution transmission electron microscopy and spectroscopy. Figures 2(a) [26], (b) LSMO 40 , and (c) LSMO 10 . For each film, the HAADF and ABF images, OOT, OOP, and La and Sr concentrations are shown.…”