# Understanding the Effects of Tensile Strain on the Structure and Magnetism of Stoichiometric LaCoO3 Films

## TL;DR

This study investigates how tensile strain affects the structure and magnetism of LaCoO3 films, revealing that strain stabilizes magnetic order.

## Contribution

The novel contribution is identifying that tensile strain from SrTiO3 substrates stabilizes magnetic properties in stoichiometric LaCoO3 films.

## Key findings

- Ferromagnetism is stabilized only in LaCoO3 films grown under tensile strain on SrTiO3 substrates.
- Tensile strain leads to specific structural distortions and stabilizes high- or intermediate-spin Co3+ ions, promoting magnetic order.
- Films on LaAlO3 substrates contain mostly low-spin Co3+ ions and lack magnetic order.

## Abstract

Despite numerous
reports of an insulating ferromagnetic state in
epitaxial LaCoO3 thin films, no consensus has been reached
on the details of ferromagnetism in these films. To better understand
the origins of magnetic order in such films, stoichiometric LaCoO3 films have been deposited on SrTiO3(001) and LaAlO3(001) substrates using off-axis sputtering. This technique
allows growth to occur in conditions that minimize deviations from
the ideal stoichiometry. SQUID magnetometry shows that ferromagnetism
is stabilized only in films grown under tensile strain on SrTiO3. The magnetic properties of these films (T
C ≈ 70 K, M
sat ≈
0.3 μB/Co, and H
C ≈
5 kOe) are essentially independent of thickness, consistent with nearly
uniform magnetization. At room temperature, strain induced by the
SrTiO3 substrate breaks the rhombohedral symmetry of the
bulk structure, leading to a
–
a
–
c
0 octahedral
tilting and an anisotropic distortion of the Co-centered octahedra.
Low-temperature (T = 36 K) X-ray absorption spectroscopy
reveals that tensile strain inherent to the SrTiO3 substrate
stabilizes a substantial fraction of high- or intermediate-spin Co3+ ions, facilitating magnetic order, whereas films grown on
LaAlO3 are made up nearly entirely of low-spin Co3+ ions.