Strain in epitaxial MnSi films on Si(111) in the thick film limit studied by polarization-dependent extended x-ray absorption fine structure

A I Figueroa,D Gianolio,S L Zhang,G Van Der Laan,T Hesjedal,R Chalasani,A A Baker,A Kohn,S C Speller,C Pfleiderer

doi:10.1103/physrevb.94.174107

Abstract

We report a study of the strain state of epitaxial MnSi films on Si(111) substrates in the thick film limit (100-500~\AA) as a function of film thickness using polarization-dependent extended x-ray absorption fine structure (EXAFS). All films investigated are phase-pure and of high quality with a sharp interface between MnSi and Si. The investigated MnSi films are in a thickness regime where the magnetic transition temperature $T_\mathrm{c}$ assumes a thickness-independent enhanced value of $\geq$43~K as compared with that of bulk MnSi, where $T_\mathrm{c} \approx 29~{\rm K}$. A detailed refinement of the EXAFS data reveals that the Mn positions are unchanged, whereas the Si positions vary along the out-of-plane [111]-direction, alternating in orientation from unit cell to unit cell. Thus, for thick MnSi films, the unit cell volume is essentially that of bulk MnSi --- except in the vicinity of the interface with the Si substrate (thin film limit). In view of the enhanced magnetic transition temperature we conclude that the mere presence of the interface, and its specific characteristics, strongly affects the magnetic properties of the entire MnSi film, even far from the interface. Our analysis provides invaluable information about the local strain at the MnSi/Si(111) interface. The presented methodology of polarization dependent EXAFS can also be employed to investigate the local structure of other interesting interfaces.

Highlights

Skyrmions are topologically nontrivial whirls of the magnetization
We report a study of the strain state of epitaxial MnSi films on Si(111) substrates in the thick film limit (100–500 A ) as a function of film thickness using polarization-dependent extended x-ray absorption fine structure (EXAFS)
It was in particular the observation that skyrmions in bulk samples can be manipulated with current densities that are nearly six orders of magnitude smaller than those needed for conventional spin transfer torque (STT) based schemes that has generated great interest in skyrmions as a novel route towards applications [6]

Summary

INTRODUCTION

Skyrmions are topologically nontrivial whirls of the magnetization. They have been observed as individual objects as well as in the form of periodic, three-dimensional lattices with periodicities in the range ∼2–100 nm, covering nearly two orders of magnitude [1,2,3]. By exploiting the linear polarization of the synchrotron radiation and by orienting the sample with the surface normal either parallel or perpendicular to the electric vector of the impinging x rays, it is possible to probe either the out-of-plane or in-plane atomic bonds This analysis provides direct information about strain in well-oriented thin films such as MnSi on Si(111) and could be used to investigate other systems where the local strain at the interface affects their electronic and magnetic properties as in the case of superconducting lattices [31,32] or heterostructures incorporating topological insulators [33,34]. In order to clarify the precise mechanism causing the enhanced magnetic transition temperature detailed measurements of the structural properties of epitaxial MnSi film in the thick film limit are required as reported below

Sample preparation and characterization

EXAFS measurements and analysis

Bond-distance analysis

Lattice structure analysis

CONCLUSIONS