  In the bulk, LaCoO3 (LCO) is a paramagnet, yet in tensile strained thin films
at low temperature ferromagnetism (FM) is observed, and its origin remains
unresolved. Polarized neutron reflectometry (PNR) is a powerful tool to
determine the depth profiles of the structure and magnetization simultaneously
and thus the evolution of the interfacial FM with strain can be accurately
revealed. Here we quantitatively measured the distribution of atomic density
and magnetization in LCO films by PNR and found that the LCO layers near the
heterointerfaces exhibit a reduced magnetization but an enhanced atomic
density, whereas the interior shows the opposite trend. We attribute the
nonuniformity to the symmetry mismatch at the interface, which induces a
structural distortion related to the ferroelasticity of LCO. This assertion is
tested by systematic application of hydrostatic pressure during the PNR
experiments. These results provide unique insights into mechanisms driving FM
in strained LCO films while offering a tantalizing observation that tunable
deformation of the CoO6 octahedra in combination with the ferroelastic order
parameter.
