Mechanism of spin crossover in LaCoO₃ resolved by shape magnetostriction in pulsed magnetic fields

In the scientific description of unconventional transport properties of oxides (spin-dependent transport, superconductivity etc.), the spin-state degree of freedom plays a fundamental role. Because of this, temperature- or magnetic field-induced spin-state transitions are in the focus of solid-state physics. Cobaltites, e.g. LaCoO₃, are prominent examples showing these spin transitions. However, the microscopic nature of the spontaneous spin crossover in LaCoO₃ is still controversial. Here we report magnetostriction measurements on LaCoO₃ in magnetic fields up to 70 T to study the sharp, field-induced transition at H_c ≈ 60 T. Measurements of both longitudinal and transversal magnetostriction allow us to separate magnetovolume and magnetodistortive changes. We find a large increase in volume, but only a very small increase in tetragonal istortion at H_c. The results, supported by electronic energy calculations by the configuration interaction cluster method, provide compelling evidence that above H_c LaCoO adopts a correlated low spin/high spin state.