Study of magnetic phase transition in LaCoO₃ and La_0.7Sr_0.3CoO₃ thin films grown by pulsed laser deposition

The rare-earth cobaltite, LaCoO₃ has attracted researchers’ interest because of its intriguing electronic and magnetic properties. In bulk LaCoO₃, Co³⁺ ions have the low spin configuration (S=0) at low temperatures. Thin films of LaCoO₃, however, exhibit a ferromagnetic ground state below ~85 K. There are several efforts to explain underlying physics in terms of multiple spin-state transitions, Jahn-Teller distortions, the Co-O-Co bond angle or the rotation of the CoO₆ octrahedra, oxygen vacancies, and strain induced by doping with Sr or the substrate-film lattice mismatch. A complete understanding of the mechanism, however, has not been achieved. In this presentation, we discuss the phase transition mechanism of LaCoO₃ and La_0.3Sr_0.7CoO₃ thin films less than 30 nm thick grown by pulsed laser deposition under compressive and tensile strain using SrTiO₃ and LaAlO₃ substrates. We demonstrate that strain plays an important role in the transition and discuss mechanisms related to substrate-induced strain and the formation of structural domains. We will also discuss the roles of effective dimensionality in the thinnest films and anisotropic exchange interactions.