Long-Range Magnetic Order in MnO

Abstract

Neutron diffraction measurements on powder samples confirm that the magnetic moments in MnO are arrayed in ferromagnetic sheets parallel to {111} planes, with neighboring planes coupled in an antiferromagnetic fashion. The long-range order parameter $S$ increases progressively with decreasing temperature below the Néel temperature, but complete order is apparently not reached even at very low temperatures. The magnetic unit cell is rhombohedral, and the rhombohedral angle changes progressively as the temperature is lowered. The change in rhombohedral angle is shown to be related to the long-range order parameter, and it is deduced from this that the distance between antiparallel spins is less than the distance between parallel spins by 0.8%.