Optical Absorption Spectrum of MnF2 at High Fields

Absorption spectra covering the range from 3800 to 6400 A have been obtained for single‐crystal samples of MnF2 cooled to 6.5°K in fields above 1 MG. The field and light propagation vectors were parallel to the c axis. Molecular field theory predicts that the magnetic moments of the sublattices should tilt toward the field direction as the magnetic field increases, achieving complete parallelism at twice the exchange field (1.10 MG). Drastic changes are to be expected in the absorption structure as the sample approaches the aligned state. The present data on MnF2 are the first obtained in this field region and show the expected behavior. Most of the absorption lines and diffuse bands fade out between 0.7 and 1.0 MG. The last of the changes occur at 1.01±0.02 MG, indicating that alignment is essentially complete at this field. The sharp line at 5410 A moves toward the red, the shift amounting to 31 cm−1 at 830 kG.