Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion

TL;DR

This study investigates how epitaxial strain and resulting rhombohedral distortion in LaCoO3 thin films suppress ferromagnetism, revealing nano-twinning and strain relaxation effects through structural analysis.

Contribution

It demonstrates the suppression of ferromagnetic order in LaCoO3 films due to rhombohedral distortion caused by epitaxial strain and nano-twinning phenomena.

Findings

Nano-twinning occurs early during film growth.

Pseudo-tetragonal structure maintains constant volume over various thicknesses.

Significant lattice relaxation occurs only in films thicker than 130 nm.

Abstract

Epitaxially strained LaCoO3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7 (LSAT) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nano-twins with twin planes predominantly along the <100> direction. Nano-twinning occurs already at the initial stage of growth, albeit in a more moderate way. Pseudomorphic grains, on the other hand, still grow up to a thickness of at least several tenths of nanometers. The twinning is attributed to the symmetry lowering of the epitaxially strained pseudo-tetragonal structure towards the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudo-tetragonal structure is found to be nearly constant over a very large range of t. Only films with t > 130 nm show a significant relaxation of the lattice parameters towards values comparable to those of bulk LCO.