HUGE ANISOTROPIC MAGNETOSTRICTION IN LA1-XSRXCOO3-DELTA (X-GREATER-THAN-OR-EQUAL-TO-0.3) - FIELD-INDUCED ORBITAL INSTABILITY

Magnetostriction parallel and perpendicular to the applied magnetic fi eld has been measured for La1-xSrxCoO3 (x = 0.3, 0.5) over a wide temp erature 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 magn etostriction (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 larg e value of lambda(t) cannot be explained on the basis of the usual spi n-orbit-coupling contribution. We propose that such a huge anisotropic magnetostriction has its origin in the orbital instability of the tri valent Co ions under the magnetic field, giving rise to a transition f rom a nondegenerated low-spin state to an orbital degenerated intermed iate-spin state.