Study of Far-Infrared Excitations in Metamagnetic FeCl2· 2H2O

The spin-wave spectra in the antiferromagnetic, ferrimagnetic, and ferromagnetic phases of Fe${\mathrm{Cl}}_2$ · 2${\mathrm{H}}_2$O have been examined at 2 and 6 °K. These spectra are well described by a spin-wave calculation; it is shown that $g_{\parallel }=2.23\mathrm{}\pm 0.02$, $S=2\mathrm{}$, and that the exchange interactions are isotropic. The (large) longitudinal anisotropy is included in the Hamiltonian as a single-ion anisotropy ($D=+9.58\mathrm{}\pm 0.05$ ${\mathrm{cm}}^{-1\mathrm{}}$), 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 ${\mathrm{cm}}^{-1\mathrm{}}$, which is presumably an optical phonon. The measured value of the metamagnetic transition field ${\mathrm{H}}_{\mathrm{c}2}=45.0\mathrm{}\pm 0.5$ kOe compares quite well with Narath's value of 45.6 kOe, but our value of ${\mathrm{H}}_{c1\mathrm{}}=35.0\mathrm{}\pm 0.5$ kOe is in poor agreement with Narath's 39.2 kOe. Near ${\mathrm{H}}_{c1\mathrm{}}$ (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.