Suppression of the ferromagnetic state in LaCoO3 films by rhombohedral distortion

Epitaxially strained LaCoO3 (LCO) thin films were grown with different film thickness, t, on (001) oriented (LaAlO3)(0.3)(SrAl0.5Ta0.5O3)(0.7) substrates. After initial pseudomorphic growth the films start to relieve their strain partly by the formation of periodic nanotwins with twin planes predominantly along the < 100 > direction. Nanotwinning 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 pseudotetragonal structure toward the relaxed rhombohedral structure of bulk LCO. However, the unit-cell volume of the pseudotetragonal 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 toward values comparable to those of bulk LCO. Measurements of the magnetic moment indicate that the effective paramagnetic moment, m(eff), and thus the spin state of the Co3+ ion do not change for films with t <= 100 nm. However, the saturated ferromagnetic moment, m(s), was found to be proportional only to the pseudotetragonal part of the film and decreases with increasing rhombohedral distortion. The measurements demonstrate that ferromagnetism of LCO is strongly affected by the rhombohedral distortion while the increased unit-cell volume mainly controls the effective paramagnetic moment and thus the spin state of the Co3+ ion.