Magnetic Structure of Magnesium Chromite

The magnetic structure of the normal cubic spinel MgCr₂{O}₄$ (${a}₀=8.335$ {}) 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 P4̄2^'m^'. 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 P2b2^'2^'2₁ and P2b222 with a magnetic unit cell (2a₀, 2a₀, a₀). 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 2.2μB per Cr³⁺.