Magnetic Structure of Magnesium Chromite

Abstract

The magnetic structure of the normal cubic spinel Mg${\mathrm{Cr}}_{2}$${\mathrm{O}}_{4}$ (${a}_{0}=8.335$ \AA{}) was investigated by means of neutron diffraction. Two distinct transitions to magnetically ordered states were found at \ensuremath{\sim}16 and \ensuremath{\sim}13.5\ifmmode^\circ\else\textdegree\fi{}K. The reflections associated with the 16\ifmmode^\circ\else\textdegree\fi{}K transition are explained by a class of noncollinear antiferromagnetic structures with a magnetic unit cell identical with the chemical cubic unit cell. The most symmetric of these high-temperature ($H$) structures belongs to the space group $P\overline{4}{2}^{\ensuremath{'}}{\mathrm{m}}^{\ensuremath{'}}$. The $H$-structure reflections appeared with different intensities relative to the nuclear reflections in three samples and did not appear at all in two other samples. The low-temperature ($L$) reflections associated with the 13.5\ifmmode^\circ\else\textdegree\fi{}K transition are explained by either of two nonequivalent noncollinear antiferromagnetic $L$ structures which belong to space groups ${P}_{2b}{2}^{\ensuremath{'}}{2}^{\ensuremath{'}}{2}_{1}$ and ${P}_{2b}222$ with a magnetic unit cell ($2{a}_{0}, 2{a}_{0}, {a}_{0}$). The intensities of the $L$ reflections relative to the intensities of the nuclear reflections varied somewhat among the five samples. It is suggested that the $H$ and $L$ structures represent two different phases which coexist below 13.5\ifmmode^\circ\else\textdegree\fi{}K. The intensities of the magnetic reflections ($H+L$) are accounted for by a magnetic moment of about $2.2{\ensuremath{\mu}}_{B}$ per ${\mathrm{Cr}}^{3+}$.