Magnetic Properties of FeCl2in Zero Field. I. Excitations

A detailed neutron-scattering investigation of the magnetic properties of the antiferromagnet Fe${\mathrm{Cl}}_2$ in zero field has been carried out. In this paper we report inelastic studies of the spin waves and magnetic excitons both at low temperatures and around the phase transition. The spin waves are found to simulate those of a two-dimensional ferromagnet with large anisotropy. The magnon dispersion relations at 5 °K may be accurately described by simple $S=1\mathrm{}$ spin-wave theory with an anisotropy field $g{\mu }_B{H}_A=2.0\mathrm{}\pm 0.1$ meV, in-plane isotropic exchange interactions of $2J_1=0.68\mathrm{}\pm 0.02$ meV, $2J_2=-0.09\mathrm{}\pm 0.02$ meV, and an antiferromagnetic interplanar interaction of $2J_1^{\prime }=-0.03\mathrm{}\pm 0.01$ meV. The temperature dependence of these magnons is quite unusual. Up to 21 °K there is no renormalization at all of the exchange part of the spin-wave energy. However, between 21 °K and $T_N=23.55\mathrm{}$ °K the entire magnon branch collapses precipitously into a continuum of scattering. Magnetic excitons originating in transitions between the spin-orbit-split ${}_{}{}^5{}_{}{}^{}T_{2g}^{}{}_{}{}^{}\mathrm{}(J=1\mathrm{})\mathrm{}$ and $J=2\mathrm{}$ states have also been observed. However, these are complicated by coupling to the optical phonons so that only qualitative results are obtained.