Huge anisotropic magnetostriction in La1-xSrxCoO3-δ (x≥0.3):: Field-induced orbital instability

Magnetostriction parallel and perpendicular to the applied magnetic field has been measured for La1-xSrxCoO3 (x = 0.3, 0.5) over a wide temperature range (4.2 less than or equal to T less than or equal to 300 K) and up to a maximum field of 14.2 T. We find a huge anisotropic magnetostriction (lambda(t)) with values as large as 2.2 x 10(-3) for x = 0.3 and 1 x 10(-3) for x = 0.5 at 14.2 T and temperatures below 100 K. The volume magnetostriction has been found to be negligible. The large value of lambda(t) cannot be explained on the basis of the usual spin-orbit-coupling contribution. We propose that such a huge anisotropic magnetostriction has its origin in the orbital instability of the trivalent Co ions under the magnetic field, giving rise to a transition from a nondegenerated low-spin state to an orbital degenerated intermediate-spin state.