  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.
