Mechanism of spin crossover in LaCoO(3) 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. LaCoO(3), are prominent examples showing these spin transitions. However, the microscopic nature of the spontaneous spin crossover in LaCoO(3) is still controversial. Here we report magnetostriction measurements on LaCoO(3) 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 distortion at H(c). The results, supported by electronic energy calculations by the configuration interaction cluster method, provide compelling evidence that above H(c) LaCoO(3) adopts a correlated low spin/high spin state.