Exchange Striction Effects in MnO and MnS

Lattice constants of MnO and MnS have been measured as a function of temperature from 7 to 300^∘{}K. Below the N\'eel temperature, the fcc lattices undergo a trigonal distortion as well as a volume contraction. By subtracting the usual thermal expansion from this volume contraction, the isotropic exchange striction has been studied. For MnO below the N\'eel temperature (TN118^∘K), good agreement with the randomphase Green's-function theory previously developed to describe sublattice magnetization is obtained for the trigonal distortion; on the other hand, for the volume contraction, there are no theoretical results available for the region immediately below the N\'eel temperature to compare with the experimental next-nearest-neighbor spin-correlation curve which has been obtained. For MnS, below TN(147^∘K) the observed trigonal distortion was found to be a contraction along the [111] direction; the magnetization curves suggest, but do not positively prove, the absence of a biquadratic-exchange effect.