Magnetostriction and the two-spin correlation function in EuO

The longitudinal and transverse magnetostrictions along the [100] direction of EuO were measured at temperatures between the Curie point T/sub C/ = 69.25 and 245 K in magnetic fields H up to 145 kOe. The volume magnetostriction was obtained from these measurements. The data indicate that above T/sub C/ the isotropic magnetostriction (independent of the direction of H) is much larger than the anisotropic magnetostriction. The isotropic magnetostriction is attributed to the volume dependence of the dominant nearest-neighbor exchange constant J. The results are compared with the theory of Callen and Callen. Assuming only nearest-neighbor exchange interactions, this theory relates the isotropic magnetostriction to the H dependence of the two-spin correlation function < S/sub i/ x S/sub j/ > for nearest neighbors. An alternative theoretical approach, based on thermodynamic and statistical-mechanical arguments is presented and is shown to lead to equivalent results. Based on these theoretical results for the magnetostriction, numerical predictions are obtained using high-temperature series expansions. These numerical results point to the inadequacy of the molecular-field approximation, particularly near T/sub C/. The overall agreement between theory (evaluated using series expansions) and experiment is very good. However, there are small discrepancies near T/sub C/, consistent with other data which show that for EuO the model of a Heisenberg ferromagnet with nearest-neighbor exchange interactions is good, but not exact