Nuclear orientation of 54Mn in magnetically ordered MnCl2∙4H2O

The antiferromagnetic, spin-flop, and paramagnetic phases of MnCl2∙4H2O have been investigated by observing the nuclear orientation of radioactive 54Mn in a single crystal of the salt. The directions of the sublattice magnetizations in the absence of an external magnetic field were determined. The field dependence of the sublattice magnetizations in the spin-flop state indicated that second order anisotropy is significant in this system. Furthermore, the measurements allowed the determination of the molecular fields in the molecular field theory. The spin-flop transition region was found to be described adequately by a domain structure in which regions of antiferromagnetic phase and spin-flop phase co-exist in the crystal. The temperature dependence of the spin-flop field was determined by magnetic susceptibility measurements. It was found that the spin-flop field decreases with decreasing temperature down to temperatures less than 0.1 K contrary to previous reports. The cooling of the specimen in contact with a copper heat sink was investigated and the temperature dependence of the heat transfer is discussed. An attempt to observe magnetic resonance of the oriented nuclei was unsuccessful.