SPONTANEOUS MAGNETOSTRICTION IN THE ORDER ED PHASE OF NICL2

The spontaneous magnetostriction in the ordered state of NiCl2 has bee n measured for the first time, using two independent methods (powder x -ray diffractometry and dilatometry in crossed magnetic fields). These methods differ in that the quantity measured in one case (x-ray) is t he changes of the lattice parameters at the transition to the ordered polydomain (effectively isotropic within the basal plane) state, while in the other, changes in sample dimensions as a result of the magneti c-field-induced monodomainization. Despite the qualitative similarity (positive striction within the hexagonal plane of magnetic ordering an d constriction along the c axis), in the quantitative aspect the resul ts differ strongly. The spontaneous striction, as determined by the x- ray method, is u(xx) = u(yy) = (7.2 +/- 2,2).10(-4) within the plain a nd u(zz) = -(8.7 +/- 3,3).10(-4) along the c axis, which is approximat ely by an order of magnitude larger than the dilatometric values (besi des, the dilatometrically measured striction exhibits a strong anisotr opy within the basal plane, which is quite natural).