Magnetic Structure of Magnesium Chromite

Abstract

The magnetic structure of the normal cubic spinel Mg${\mathrm{Cr}}_2$ ${\mathrm{O}}_4$ ($a_0=8.335\mathrm{}$ Å) was investigated by means of neutron diffraction. Two distinct transitions to magnetically ordered states were found at ∼16 and ∼13.5°K. The reflections associated with the 16°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 $P\overline{4}{2}^{\prime }{\mathrm{m}}^{\prime }$. 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°K transition are explained by either of two nonequivalent noncollinear antiferromagnetic $L$ structures which belong to space groups $P_{2b}{2}^{\prime }{2}^{\prime }{2}_1$ and $P_{2b}222$ with a magnetic unit cell ($2a_0, 2a_0, a_0$). 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°K. The intensities of the magnetic reflections ($H+L$) are accounted for by a magnetic moment of about $2.2{\mu }_B$ per ${\mathrm{Cr}}^{3+}$.