Exchange Striction in NiO

Abstract

Exchange-striction effects have been studied in both the antiferromagnetic and paramagnetic states of NiO. Near-neighbor spin correlation functions as functions of temperature for various choices of $\frac{{J}_{2}}{{J}_{1}}$ have been calculated using Green's-function techniques and have been compared with our experimental x-ray data measured between 6 and 698\ifmmode^\circ\else\textdegree\fi{}K. The temperature dependence of the calculated trigonal distortion using $\frac{{J}_{2}}{{J}_{1}}=1.5$ yields a good fit with the measurements which vary from 0 at ${T}_{N}$ to 4.5\ensuremath{'} at 0\ifmmode^\circ\else\textdegree\fi{}K. The isotropic volume contraction (strain) due to the exchange striction is small and never exceeds \ensuremath{\sim} 3\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}4}$ throughout the temperature range; hence, these latter measurements do not provide detailed information on the correlation functions as was the case for MnO.