LOCALIZED MAGNETIC MODE IN MNF2-FE2+ AND MNF2-CO2+

We have observed a magnetic dipole-active localized mode in MnF 2, doped with 0.04 mol% FeF2 at 94.8 cm-1 at 1.2°K, using an ir lamellar interferometer. To our knowledge, this is the first direct spectroscopic evidence using far ir techniques for a magnetic dipole-active impurity mode in an otherwise magnetically ordered system. The line has the following features: the half-width is 0.4 cm-1 and the absorption strength ∫αdv∼1 cm-2. With increasing temperature, the line shifts to lower frequencies and finally disappears at about 25°K. Magnetic fields up to 45 kG have been applied parallel to the c axis of the sample, yielding a linear splitting of the line. A g factor of 2.3 is derived from the data. The experimental results at 1.2°K can be understood satisfactorily both with a simple molecular field picture and, also, with the help of a simple spin-wave calculation. In the molecular field approximation, the line is correlated to a transition of the ground spin state to the first excited state of the orbital ground state of Fe2+. The observed value for the frequency is fitted by a Fe2+Single Bond signMn2+ exchange coupling constant J=1.9 cm-1 in both models. Preliminary results on a localized mode in MnF2:Co 2+ are also reported. © 1969 The American Institute of Physics.