The antiferromagnet ${\mathrm{FeBr}}_2$ has been studied by Mössbauer spectroscopy in external fields both in the metamagnetic region below the multicritical temperature ${\mathit{T}}_{\mathrm{MCP}}$ and in the second-order transition region above. The local magnetization shows that the metamagnetic transition occurs by spin flips, as in simple models. However, in the second-order transition region, the local magnetization of the sublattice oriented antiparallel to the external field varies continuously but remains parallel to the c axis. This can only be understood if the external magnetic field induces strong transversal spin precession of the moments on the antiparallel sublattice. This shows that the anomalous maxima in the imaginary part ${\mathrm{\chi }}^{\mathrm{″}}$ recently found in the ac susceptibility [M.M. Pereira de Azevedo et al., J. Magn. Magn. Mater. 140-144, 1557 (1995)] and denoted ${\mathit{H}}_{\mathrm{-}}$ below the critical field ${\mathit{H}}_{\mathit{C}}$(T), and ${\mathit{H}}_{+}$ above, can be understood as being caused by noncritical transversal spin fluctuations.

• Received 19 June 1995

DOI:https://doi.org/10.1103/PhysRevB.52.15372