  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 Hc ~ 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 Hc. The results, supported by electronic energy
calculations by the configuration interaction cluster method, provide
compelling evidence that above Hc LaCoO3 adopts a correlated low spin/high spin
state.
