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 Ni Cl2 -4SC (N H2) 2, 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.
|Physical Review B - Condensed Matter and Materials Physics
|Published - Jan 9 2008