Magnetic ordering and exchange effects in the antiferromagnetic solid solutions ${\mathrm{Mn}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$

The antiferromagnetic phases of MnO, NiO, and the solid solutions ${\mathrm{Mn}}_x{\mathrm{Ni}}_{1-x}\mathrm{O}$ ($x=0.24, 0.48, 0.77$) have been examined by high-resolution powder neutron diffraction. The solid solutions have random cation distributions, and Néel temperature and mean cation-moment measurements indicate that the spins of ${\mathrm{Mn}}^{2+}$ and ${\mathrm{Ni}}^{2+}$ are effectively collinear owing to the strength of the 180° cation-anion-cation superexchange. As in MnO and NiO, the magnetic moments are confined to (111) planes by dipole-dipole forces, and the very small trigonal exchange strictions are interpreted in terms of a competition between antiferromagnetic ${\mathrm{Mn}}^{2+}$-${\mathrm{Mn}}^{2+}$ and ferromagnetic ${\mathrm{Ni}}^{2+}$-${\mathrm{Mn}}^{2+}$ nearest-neighbor exchange interactions.