Magnetic Structure of Manganese Chromite

Abstract

An investigation of the magnetic structure and properties of Mn${\mathrm{Cr}}_{2}$${\mathrm{O}}_{4}$ has been carried out by means of neutron diffraction and an evaluation of the suitability of various proposed theoretical models has been made. From room-temperature diffraction patterns it is established that Mn${\mathrm{Cr}}_{2}$${\mathrm{O}}_{4}$ is a normal spinel with a $u$ parameter of 0.3892\ifmmode\pm\else\textpm\fi{}0.0005 and with less than one percent of the ${\mathrm{Mn}}^{++}$ ions present on $B$ sites. The Curie temperature, as determined from diffraction data, is \ensuremath{\sim}43\ifmmode^\circ\else\textdegree\fi{}K. Below this temperature the magnetic contribution to the fundamental spinel peaks arising from aligned spins increases as the temperature is lowered and is effectively saturated at about 20\ifmmode^\circ\else\textdegree\fi{}K. At 18\ifmmode^\circ\else\textdegree\fi{}K, additional sharp peaks appear at positions which cannot be indexed either on the original unit cell or on any reasonably enlarged cell. These extra reflections persist with unchanged intensities down to 4.2\ifmmode^\circ\else\textdegree\fi{}K. No change in either the positions or intensities of the fundamental lines is observed in going through the transition. Above 18\ifmmode^\circ\else\textdegree\fi{}K, a diffuse peak is present in the region where the principal extra lines develop. This diffuse peak decreases with increasing temperature, but is still observable above the Curie point. Application of a magnetic field along the neutron scattering vector decreases the magnetic contributions to the fundamentals, but increases the intensities of the extra reflections.The N\'eel model and the Yafet-Kittel model fail to account for major qualitative features of the diffraction results. On the other hand, the ferrimagnetic spiral model of Lyons, Kaplan, Dwight, and Menyuk is in good qualitative accord with all the observations. A detailed comparison with the theoretical predictions is presented and attention is called to broad areas of agreement as well as to certain discrepancies. The general conclusion reached is that the spiral model is a good first approximation to the ground-state spin configuration.