Abstract: Phase Transitions in FeCl\u3csub\u3e2\u3c/sub\u3e·2H\u3csub\u3e2\u3c/sub\u3eO in External Magnetic Fields: Mössbauer Spectroscopy

Monoclinic FeCl2·2H2O orders antiferromagnetically at TN = 23 K, and the magnetic structure consists of two sublattices of -FeCl2-chains lying along the c-axis. The coupling along the chains is ferromagnetic with weak antiferromagnetic coupling between chains. Application of an external magnetic field along the easy axis (a) induces phase transitions at H1 = 39 kOe and at H2 = 46 kOe. We report the observation of the three phases using the Mössbauer effect in a single crystal of FeCl2·2H2O cut parallel to the c-axis and placed at 32° to the gamma ray beam and magnetic field Ho so that Ho was parallel to the easy axis alpha. The results may be summarized as follows: (1) For H0 \u3c H1 we observe two superposed spectra with equal intensities due to the external field adding and subtracting to the hyperfine field in the spin down and spin up sublattices respectively; (2) For H1 \u3c Ho \u3c H2 the relative intensities of the spin up to spin down spectra are roughly 3:1. (3) For H2 \u3c Ho, only one spectrum is observed. These observations are consistent with the antiferromagnetic==\u3eferrimagnetic==\u3eparamagnetic model of Narath1 from susceptibility measurements