Study of far-infrared excitations in metamagnetic FeCl2 2H2 O

The spin-wave spectra in the antiferromagnetic, ferrimagnetic, and ferromagnetic phases of FeCl2 2H2O have been examined at 2 and 6°K. These spectra are well described by a spin-wave calculation; it is shown that g=2.230.02, S=2, and that the exchange interactions are isotropic. The (large) longitudinal anisotropy is included in the Hamiltonian as a single-ion anisotropy (D=+9.580.05 cm-1), which is shown to make an anomolously large contribution to the spin-wave energies. These results are discussed and interpreted from the point of view of crystal-field theory. In all three metamagnetic phases, the magnetic resonance modes are observed to interact with a field-independent excitation with energy 31.5 cm-1, which is presumably an optical phonon. The measured value of the metamagnetic transition field Hc2=45.00.5 kOe compares quite well with Narath's value of 45.6 kOe, but our value of Hc1=35.00.5 kOe is in poor agreement with Narath's 39.2 kOe. Near Hc1 (the antiferromagnetic-to-ferrimagnetic transition field), the far-infrared spectrum appears to indicate that both antiferromagnetic and ferrimagnetic "domains" coexist over a certain range of field. The temperature dependence and hysteresis of these domains are also described and compared with Tinkham's microscopic description of these transitions. © 1970 The American Physical Society.