Strain-Induced Atomic-Scale Building Blocks for Ferromagnetism in Epitaxial LaCoO₃

Abstract

The origin of strain-induced ferromagnetism, which is robust regardless of the type and degree of strain in LaCoO3 (LCO) thin films, is enigmatic despite intensive research efforts over the past decade. Here, by combining scanning transmission electron microscopy with ab initio density functional theory plus U calculations, we report that the ferromagnetism does not emerge directly from the strain itself, but rather from the creation of compressed structural units within ferroelastically formed twin-wall domains. The compressed structural units are magnetically active with the rocksalt-type high-spin/low-spin order. Our study highlights that the ferroelastic nature of ferromagnetic structural units is important for understanding the intriguing ferromagnetic properties in LCO thin films.

Tunable Ferromagnetism in LaCoO3 Epitaxial Thin Films

“…We carefully exclude the presence of oxygen vacancy ordering at the dark stripe patterns by electron energy loss spectroscopy (EELS) measurements (Figure S3, Supporting Information). While the dark stripe patterns have been frequently recognized for the high‐quality (100)‐oriented LCO thin films, [ 20,23,32,33 ] we report similar patterns in (110)‐ and (111)‐oriented thin films for the first time. The LCO thin films can be categorized into two regions considering the dark stripe pattern, i.e., Region I (near the interface) and Region II (above the interface).…”

“…Although bulk LCO is nonmagnetic with a rhombohedral crystal structure, the tensile strain in the thin film is known to induce monoclinic and tetragonal phases, resulting in the spin-ordered state. [20,22,23] The large and facile modification of the crystalline symmetry originates from the ferroelastic nature of LCO, which favors the formation of twin-domain structures under mechanical pressure or epitaxial strain. [24][25][26][27][28][29][30][31] Such twin-domain structures have been observed as dark stripe patterns in high-angle annular darkfield scanning transmission electron microscopy (HAADF STEM) imaging.…”

“…[24][25][26][27][28][29][30][31] Such twin-domain structures have been observed as dark stripe patterns in high-angle annular darkfield scanning transmission electron microscopy (HAADF STEM) imaging. [20,23,32,33] Detailed microstructure analyses using HAADF STEM on (100)-oriented LCO thin films enable classification of basic lattice units into compressed-(c-unit), tensile-stretched-(t-unit), and bulk-units (b-unit). [23] Among these, c-units with coexisting low (LS) and high spin (HS)-states of the Co spin configuration were found to be responsible for the FM phase.…”

“…[20,23,32,33] Detailed microstructure analyses using HAADF STEM on (100)-oriented LCO thin films enable classification of basic lattice units into compressed-(c-unit), tensile-stretched-(t-unit), and bulk-units (b-unit). [23] Among these, c-units with coexisting low (LS) and high spin (HS)-states of the Co spin configuration were found to be responsible for the FM phase. It should be noted that the FM phase, characterized by T C , in the (100)-oriented LCO thin film was mostly unchanged.…”

“…[ 43 ] Such unsusceptible behavior in T C suggests that the magnetically active c ‐units are highly resilient against orthogonal stress, again taking advantage of the ferroelastic nature of LCO. [ 23 ]…”