Strain-induced Ferromagnetism at LaFeO3/SrTiO3 Interface

Research in spintronics has made significant progress in the past decade by exploring magnetic state manipulation through interface engineering [1]. By joining two dissimilar materials, multiple degrees of freedom that control magnetic interactions, including structure, electronics, and orbitals, can be altered. However, each degree of freedom can play their unique role, and the modifications are typically confined to a few unit cells near the interface. To fully understand the origin of interfacial magnetism, comprehensive and precise characterization with high spatial resolution is necessary, as current knowledge is often incomplete. In this study, we first introduce a conductive ferromagnetic state at the LaFeO3/SrTiO3 (LFO/STO) interface. Next, using a combination of atomic-level imaging and spectroscopy in scanning transmission electron microscopy, and first-principal calculations, we precisely determine the structural and electronic origins of this state. The result not only demonstrates a new paradigm of creating interfacial magnetic states by combining multiple degrees of freedom, but also presents a routine to understand the atomistic origin of emergent magnetism. Antiferromagnetic LFO films with varying thickness were grown epitaxially on a diamagnetic TiO2-terminated STO substrate using off-axis magnetron sputtering. The magnetic properties of the films were analyzed using SQUID, as displayed in Fig. 1a. All films exhibit ferromagnetic-like hysteresis loops, with the saturated magnetic moment per Fe increasing as film thickness decreases, suggesting an interfacial origin for the magnetic moment. The crystal structure, as depicted in the Fig. 1b, was determined through a combination of high-resolution atomic images from three different orientations (Fig. 1c-f). The analysis showed that the interfacial FeO6 octahedrons are arranged in a checkerboard pattern with alternating large and small volumes to match the lattice parameters and octahedron rotations of the substrate. The delicate structural modifications have a significant impact on the electronic properties, as revealed by layer-resolved EELS shown in Fig. 2. The shape of the Ti edge (Fig. 2b) remains consistent from the bulk to the interface, while the Fe edge (Fig. 2a) displays an increased weight in the peak, indicating a decrease in valency from +3 to +2 due to the polar mismatch between LFO and STO [2]. On the other hand, the O edge (Fig. 2c) peak #1 splits into two shoulders due to crystal field splitting. However, near the interface, the two shoulders gradually converge (yellow vs. purple), which can be attributed to a high spin (HS) to low spin (LS) transition of the Fe atoms. The observations above are confirmed by density functional theory calculations, as shown in the Fig. 2k. The STEM-determined structure was first relaxed to its ground state, which replicated the checkerboard arrangement of FeO6 octahedrons with alternating large and small volumes as measured. The relaxed structure was then used to calculate the electronic and magnetic properties. The Fe atoms in the smaller interfacial octahedrons exhibit a LS Fe2+ state, while those in larger octahedrons exhibit a HS Fe3+ state similar to bulk LFO. The spin state transition is driven by a larger crystal field splitting [3], due to shorter Fe-O bond lengths near the interface, which further results in a change of superexchange interaction from antiferromagnetic to ferromagnetic [4, 5]. (a) SQUID data for LFO film with different thicknesses. (b) 3D model of the interface determined from STEM images. The gray octahedrons correspond to TiO6 and the blue ones to smaller FeO6 and the red ones to large FeO6 (c) HAADF image of the LFO/STO interface along [100] direction. The first unit cell is indicated by arrow and the La column is circled. (d) ABF image acquired simultaneously with (c). Every four oxygen atoms around Fe/Ti atom are connected to a square and the rotation of the square is represented with different colors according to the color bar. (e) HAADF image of the interface along [110] direction. (f) ABF image acquired simultaneously with (e), the O-O bond length comparing to its two horizontal neighbors is represented with different colors according to the color bar. Layer resolved (a) Fe L3,2, (b) Ti L3,2, and (c) O K-edges across the interface. The spectrums are color-coded based on their distances away from the interface. (d) HAADF image with a color bar showing the color for spectrums of each layer. Two Lorentzian functions are fitted to peak 1 and 2 of Fe edge. The fitting result of L7 and L1 layer is shown in (e) and (f), respectively. The orange profile is the fitted function for peak 1, and the red profile is the fitted function for peak 2. The shaded region indicates the area within FWHM. (g) The same fitting routine is repeated for all LFO layers, and the profile shows the ratio between FWHM areas for peak 1 and 2. (h) Heating map shows MLLF residual as a function of energy and spatial location. (i) Line profile of the MLLF residual from layer L1. (j) Line profile of the MLLF residual from layer L7. (k) Orbital occupation of larger (type #1) and smaller (type #2) FeO6 octahedrons calculated with DFT.