Origin of insulating weak-ferromagnetic phase in ultra-thin La0.67Sr0.33MnO3 films on SrTiO3 substrate

Yun Li,Shin-Ik Kim,Seung-Hyub Baek,Younghun Jo,Seung-Young Park,Euiyoung Choi,Jiwon Seo

doi:10.1063/1.4990448

Abstract

We investigate the origin of insulating weak-ferromagnetic phase in ultra-thin epitaxial La0.67Sr0.33MnO3 (LSMO) films on SrTiO3 substrate using density functional theory calculation together with X-ray linear dichroism (XLD). The calculations show that symmetry breaking of the crystal field at the LSMO surface largely lowers the energy level of Mn d3z2 orbital at the surface and leads to full occupancy of the d3z2 orbital in majority spin channel, and XLD spectra clearly show the preferential occupation of Mn d3z2 orbital at the surface. Such an orbital reconstruction and charge redistribution in the ultra-thin films largely suppresses double-exchange interaction and favors super-exchange interaction, resulting in G-type antiferromagnetic spin ordering and insulating state. The anisotropic exchange interaction due to spin-orbital interaction leads to spin canting, and thus the films show weak ferromagnetism.

Highlights

The development of fine growth techniques in complex oxide thin films promises to widen the range of possible research topics in the field of condensed matter physics
Calculated electronic structures clearly show that the ground-states are in insulating state and orbital reconstruction and charge redistribution occur in the ultra-thin films, which are suggested to lead to the AFM spin ordering
The X-ray linear dichroism (XLD) spectra show that the preferential occupation of Mn d3z2 orbital at the surface, confirming the orbital reconstruction observed in the calculations

Summary

INTRODUCTION

The development of fine growth techniques in complex oxide thin films promises to widen the range of possible research topics in the field of condensed matter physics. Charge, lattice, and orbital properties due to the confined d and f electron clouds in complex oxides trigger a variety of emergent phenomena when the geometrical thickness and dimensionality of oxide films are reduced. These emergent phenomena include the existence of polar discontinuities, oxygen vacancies, octahedral distortion, orbital reconstruction, charge redistribution, chemical disorders, and strain, together with the inevitable presence of surface and interface effects in the films. Calculated electronic structures clearly show that the ground-states are in insulating state and orbital reconstruction and charge redistribution occur in the ultra-thin films, which are suggested to lead to the AFM spin ordering. The XLD spectra show that the preferential occupation of Mn d3z2 orbital at the surface, confirming the orbital reconstruction observed in the calculations

Sample preparation and characterization

DFT calculations

Orbital configuration measured by XLD

CONCLUSIONS