Magnetic Structure of MnP

Abstract

In magnetic measurements on a single crystal of MnP, Huber and Ridgley1 found a ferromagnetic Curie point at 291.5° and a transition at 50°K, below which the crystal was metamagnetic, i.e., it exhibited an antiferromagnetic-ferromagnetic transition which was a function of the applied field and temperature. We have carried out a neutron diffraction investigation at 4.2°K on this same crystal. Magnetic scattering was observed in pairs of satellite reflections displaced by ±δ from each nuclear peak in the [010] and [100] zones, with δ=0.112c*; in most cases the intensities of the two satellites of a single reflection differed markedly. Their existence and displacements along c* show that the spin structure is modulated in this direction, and the existence of only a pair of satellites to each reflection shows that the modulation is closely sinusoidal; however, the inequality of the pairs indicates that the structure cannot be a simple spiral. MnP has an orthorhombic B31 type structure, space group Pnma with 4 Mn in 4(c) (x, ¼, z). Detailed consideration of the satellite intensities shows that the spins all lie in the ab plane and are coupled ferromagnetically across the centers of symmetry of space group Pnma. The spiral is propagated by rotations of the spins by approximately 20° for each operation of the screw diad parallel to c. The field dependence of the satellites has also been studied: no change in spiral wavelength with field was detected. A complete account of this work will be published elsewhere.