Magnetic Structure of MnO at 10 K from Total Neutron Scattering Data

Total neutron scattering data from a powdered sample of MnO collected at 10 K have been analyzed using the reverse Monte Carlo method to refine the nuclear and magnetic structure. The results give the first unambiguous assignment of the average magnetic structure. The magnetic moments are aligned ferromagnetically within (111) sheets with the magnetization vectors of alternate sheets along axes parallel and antiparallel to the $⟨11\overline{2}⟩$ directions, albeit with a small modulated out-of-plane component. Small displacements of Mn and O (modulated with the same periodicity) accompany the magnetic ordering and both atomic and magnetic structures may be described in the monoclinic space group $C2$.

Figure 1

(a) Representation of the basic antiferromagnetic structure of the Mn sublattice in MnO. The spin orientations (arrows) are known to lie within the (111) ferromagnetic sheets, but their direction $\theta$ within these planes is unknown. (b) RMC goodness-of-fit values obtained for equilibrium RMC configurations with differing average spin orientations $\theta$.

Figure 2

(a) Observed and (b)–(d) expected RMC goodness-of-fit values for an equilibrium spin configuration rotated to various extents about the $[111]$ axis.

Figure 3

(a) Average displacements of the Mn (open circles) and O (solid circles) atoms, and (b) average out-of-plane spin components across the MnO unit cell. The $z$ axis lies parallel to the $[111]$ stacking axis and spans six (111) sheets of Mn atoms.

Figure 4

Rietveld refinement fit to low-$Q$ scattering data obtained using the program gsas, incorporating the modulated behavior shown in Fig. 3. The low-$Q$ $(\xi \xi \xi )$ reflections are indicated by “tickmarks,” labeled “N” or “M” to indicate nuclear or magnetic character, respectively. The largest isolated superlattice reflection $(\frac{5}{6}\frac{5}{6}\frac{5}{6})$ was found to contain less intensity than the scatter in data (inset).