Antiferromagnetism in the Face-Centered Cubic Lattice. II. Magnetic Properties of MnO

Abstract

The somewhat anomalous properties of MnO have recently been cited as evidence for the existence of significant biquadratic exchange in this salt. The present paper suggests that this conclusion is not justified and that the magnetic properties of MnO can be quantitatively explained in the complete absence of biquadratic exchange. MnO is a face-centered cubic antiferromagnet. The anomalous properties are shown to result from the sensitivity of bilinear exchange to inter-ion distance, which results (for MnO) in an anisotropic distortion of the cubic lattice for temperatures below the Néel point $T_N$. The random-phase Green's function theory of Part I is used to describe the magnetic susceptibility for $T>\mathrm{}{T}_N$, and to investigate the sublattice magnetization as a function of temperature for $T<\mathrm{}{T}_N$. Good agreement between theory and experiment is obtained, and values $J_1=10\mathrm{}°$K and $J_2=11\mathrm{}°$K are calculated for the nearest-neighbor and next-nearest-neighbor exchange parameters, respectively. The molecular-field theory is shown to be far too crude an approximation to give quantitatively satisfactory results. The ${\mathrm{Mn}}^{55}$ zero-field nuclear magnetic resonance (NMR) has been observed in the antiferromagnetic state of MnO and, from the temperature dependence of the resonance frequency, information regarding the spin deviation as a function of temperature is derived. A simple noninteracting spin-wave theory has been developed to describe the spin deviation, and a satisfactory description of the low-temperature properties is obtained. A calculation is also presented for the contribution of the indirect nuclear spin-spin interaction to the NMR linewidth.