Mössbauer Study of Metamagnetic Transition in FeCl2and FeBr2

The metamagnetic (spin-flop) transition is studied in single crystals of Fe${\mathrm{Cl}}_2$ and Fe${\mathrm{Br}}_2$ using the Mössbauer effect and external magnetic fields up to 40 kG. From the changes which take place in the $m=\frac{1}{2}\to m^{*}=\frac{3}{2}$ and $m=-\frac{1}{2}\to m^{*}=-\frac{3}{2}$ transitions when Fe${\mathrm{Br}}_2$ passes through the spin-flop, the sign of the internal field at the nucleus is found to be positive. In addition, the difference between the interionic dipolar fields above and below the spin-flop transition is determined as -6.7 kG for Fe${\mathrm{Br}}_2$. The interionic dipolar fields are also calculated for Fe${\mathrm{Cl}}_2$, Fe${\mathrm{Br}}_2$, and Fe${\mathrm{I}}_2$ above and below the spin-flop. The calculated difference for Fe${\mathrm{Br}}_2$ is -6.8 kG, in good agreement with the experimental difference. The internal magnetic field at the iron nucleus in Fe${\mathrm{Cl}}_2$ is found to be -5.0 kG above the spin-flop. This result is used with the calculated lattice sums to determine the internal field below the spin-flop. A value of +2.8 kG is found for this field, in agreement with earlier observations. The sign of the internal field in Fe${\mathrm{I}}_2$ is predicted to be positive. Increasingly positive internal fields going down the series Fe${\mathrm{Cl}}_2$, Fe${\mathrm{Br}}_2$, and Fe${\mathrm{I}}_2$ are consistent with the interpretation that the iron-halogen bond is increasingly covalent. This is manifested in an increasing fraction of $4s$ electron character in the ${\mathrm{Fe}}^{2+}$ electronic configuration.