Antiferromagnetism in MnO—Calculation of Near-Neighbor Spin Correlation Functions forT<TN

Taking into account the exchange-striction effects, the near-neighbor spin correlation functions of MnO are calculated for temperatures below the Néel temperature by using Green's-function techniques with a random-phase Tyablikov decoupling approximation. The resulting theoretical calculations are compared to some experimental results, and it is concluded that the lattice-distortion-induced biquadratic exchange effects are sufficient to explain the experimental data. The trigonal distortion and isotropic volume contraction parameters are found to be $\frac{j}{J_2}=0.019\mathrm{}$ and $\frac{j_2}{J_2}=0.0021\mathrm{}$, respectively. The observed anomalous behavior of the next-nearest-neighbor spin correlation function near the Néel temperature is theoretically explained.