More on spin orientation in MnO at 0 K and zero applied field : theoretical predictions

Above its Néel temperature TN (~ 110 K), MnO has the f.c.c. crystalline structure, the point group of Mn++ ions being m3m. At TN an antiferromagnetic order takes place. Some properties of that order are well enough described by taking into account the exchange and dipolar interactions and the magneto-elastic coupling from exchange interactions. So : 1) exchange interactions are responsible for the two-sublattice antiferromagnetic structure, 2) dipolar interactions set the spins (for Mn++ L = 0 and S = 5/2) parallel to the (111) planes of the f.c.c. structure and 3) magneto-elastic coupling due to exchange creates a 3m symmetry strain tensor. In this article we study the effects of the magneto-elastic coupling coming from dipolar interactions. Although they are small, these effects are qualitatively important. So, the dipolar magneto-elastic coupling removes partially the spin orientation degeneracy. We show there are twelve antiferromagnetic domains in MnO, the spin direction of a domain being contained in the plane defined by the [111] and [100] axes of the f.c.c. structure and being nearly orthogonal to the [111] axis. Furthermore, this interaction creates in each domain a strain tensor of symmetry lower than 3m. All the results agree with those one can deduce from the experimental studies of MnO in a null magnetic field and null stress tensor. The above results are not qualitatively changed by application of a hydrostatic pressure or an uniaxial stress along the [111] axis. But we show that one could favour one among the twelve antiferromagnetic domains by applying a stress with components on both the [111] and [100] axes. Such a result would be very interesting for the magnetic study of MnO.