Direct measurement of spin correlations using magnetostriction

We demonstrate that the short-range spin correlator Si ·Sj, a fundamental measure of the interaction between adjacent spins, can be directly measured in certain insulating magnets. We present magnetostriction data for the insulating organic compound NiCl2-4SCNH22, and show that the magnetostriction as a function of field is proportional to the dominant short-range spin correlator. Furthermore, the constant of proportionality between the magnetostriction and the spin correlator gives information about the spin-lattice interaction. Combining these results with the measured Young’s modulus, we are able to extract dJ/dz, the dependence of the superexchange constant J on the Ni interionic distance z.