Metamagnetism and Exchange in Ferrous Bromide

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Abstract
The highly anisotropic behavior of several ferrous ion antiferromagnets (FeCl2, FeCO3) has suggested Ising‐model behavior. Sharp field‐induced (metamagnetic) transitions to a ferromagnetic (or nearly saturated paramagnetic) configuration have been observed. We find this transition in a single crystal of anhydrous FeBr2, occurring at a field of 31.5 kOe at 4.2°K, using pulsed fields to 200 kOe along the c axis. The spontaneous magnetization per Fe2+ ion is 4.05±0.2 Bohr magnetons. A differential molar susceptibility between 70 and 200 kOe of 0.8±0.2×10−2 is interpreted as a Van Vleck ``temperature‐independent'' paramagnetism in qualitative agreement with expectation. Susceptibility—temperature measurements along the c axis reveal an asymmetric shape characteristic of recent Ising‐model calculations. In the region just above the Néel point (11°K), the magnetization shows an initial upward concavity with field and a subsequent inflection point. This destruction of residual intermediate range order had also been predicted in Ising‐model calculations. The critical transition field determines the antiferromagnetic interlayer exchange and an estimate is made of the ferromagnetic intralayer exchange. The values for FeBr2 are compared with those for FeCl2, revealing a considerable drop in the ferromagnetic parameter with change of anion from Cl to Br. The antiferromagnetic exchange of FeBr2 is stronger than its ferromagnetic exchange in contrast to previous experience with layer structures.