Structural origin of magnetic birefringence in rutile-type antiferromagnets

The microscopic origin of magnetic birefringence in the rutile-type antiferromagnets X${\mathrm{F}}_2$ (X=Mn, Fe, Co, or Ni) is analyzed on the basis of the theory of structural birefringence developed by Ewald and Born. The general principles of the Ewald-Born theory are reviewed. The magnetic birefringence can be explained by a small exchange-induced internal displacement of the fluorine atoms. Predictions from theory are compared with accurate crystal-structure analyses based on γ-ray-diffraction data. The agreement found between theory and experiment is excellent.