Magnetic properties of FeCl2at high pressure

Using elastic and inelastic neutron scattering, we have studied Fe${\mathrm{Cl}}_2$ at low temperatures and pressures up to 4 kbar. It is confirmed that the high-pressure phase $h-\mathrm{F}\mathrm{e}{\mathrm{Cl}}_2$ at 4 °K is hexagonal and derives from the low-pressure form by a translation of the sandwiched chlorine-iron-chlorine sheets. The magnons simulate a two-dimensional ferromagnet with large anisotropy in both phases. The dispersion curves have the same shape at all pressures, and the in-plane exchange constants are independent of pressure. However, the energy gap shows a significant discontinuity at the structural transition. The renormalization of the magnon energy with temperature is due entirely to the decrease in gap energy. For the high-pressure phase, $h-\mathrm{F}\mathrm{e}{\mathrm{Cl}}_2$, the magnons collapse several degrees below the Néel temperature $T_N$, while in $r-\mathrm{F}\mathrm{e}{\mathrm{Cl}}_2$ they persist to nearly $T_N$, a result consistent with Dyson theory. Finally the magnetization in $h-\mathrm{F}\mathrm{e}{\mathrm{Cl}}_2$ has the same critical exponent and amplitude as $r-\mathrm{F}\mathrm{e}{\mathrm{Cl}}_2$.