Towards sub-nanometer real-space observation of spin and orbital magnetism at the Fe/MgO interface

Thomas Thersleff,Mirosław Werwiński,Björgvin Hjӧrvarsson,Olle Eriksson,Klaus Leifer,Shunsuke Muto,Jakob Spiegelberg,Yaroslav Kvashnin,Ján Rusz

doi:10.1038/srep44802

Abstract

While the performance of magnetic tunnel junctions based on metal/oxide interfaces is determined by hybridization, charge transfer, and magnetic properties at the interface, there are currently only limited experimental techniques with sufficient spatial resolution to directly observe these effects simultaneously in real-space. In this letter, we demonstrate an experimental method based on Electron Magnetic Circular Dichroism (EMCD) that will allow researchers to simultaneously map magnetic transitions and valency in real-space over interfacial cross-sections with sub-nanometer spatial resolution. We apply this method to an Fe/MgO bilayer system, observing a significant enhancement in the orbital to spin moment ratio that is strongly localized to the interfacial region. Through the use of first-principles calculations, multivariate statistical analysis, and Electron Energy-Loss Spectroscopy (EELS), we explore the extent to which this enhancement can be attributed to emergent magnetism due to structural confinement at the interface. We conclude that this method has the potential to directly visualize spin and orbital moments at buried interfaces in magnetic systems with unprecedented spatial resolution.

Highlights

In this letter, we use an angular-resolved Electron Energy-Loss Spectroscopy (EELS) technique known as Electron Magnetic Circular Dichroism (EMCD)[12] to experimentally probe spin and orbital magnetism at the Fe/ MgO interface in real-space with a spatial resolution of approximately 0.8 nm
While the Density Functional Theory (DFT) and Dynamical Mean-Field Theory (DMFT) simulations support the experimental observation of an enhanced mL/m∼S at the interface, there are some important discrepancies that need to be discussed in detail before conclusions are drawn
We note that the spatial extension of the interfacial region with increased white line ratio change is

Summary

Introduction

We use an angular-resolved Electron Energy-Loss Spectroscopy (EELS) technique known as Electron Magnetic Circular Dichroism (EMCD)[12] to experimentally probe spin and orbital magnetism at the Fe/ MgO interface in real-space with a spatial resolution of approximately 0.8 nm. First principles simulations based on Density Functional Theory (DFT)[13] combined with Dynamical Mean-Field Theory (DMFT)[14,15] show that such behavior could be partially explained by a reduction in crystalline symmetry caused by an atomically sharp interfacial bond between iron and oxygen atoms. While this interfacial model is supported experimentally by High Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) and Electron Energy Loss Spectroscopy (EELS) investigations, we note that the observed increase is larger than predicted by theory. We believe that further refinements to this experimental design could distinguish these effects, allowing researchers to directly explore the local magnetic properties of magnetic interfaces, opening the way for considerably more detailed investigations into the influence of emergent interfacial effects on spintronic materials in the future

Methods

Results

Conclusion