MAGNETOELASTIC COUPLING BY DIPOLAR INTERACTIONS - SPIN ORIENTATION IN MNO

MnO is a low temperature antiferromagnet in which a spin orientation anisotropy within the plane of easy magnetization has been observed experimentally. The microscopic origin of this anisotropy has not yet been elucidated. As a consequence there is a certain uncertainty in the determination of the magnetic structure and in the interpretation of the magnon spectrum. We show here that magneto-elastic coupling by dipolar interactions leads to well defined anisotropy axis which are determined to be the [1, 1̄, 0] directions modulo 2π/3. We have calculated the values of coupling coefficients which are derivatives of the components of the dipolar tensor with respect to the components of the strain tensor. Moreover using the mechanism proposed in this paper it should be possible to interpret correctly the magnetic properties of MnO in presence of external fields (magnetic field and mechanical stresses).