Neutron Diffraction Determination of the Intermediate-Field Magnetic Structure of CoCl2·2H2O

Recent magnetization measurements have shown that antiferromagnetic CoCl2·2H2O undergoes two metamagnetic transitions when a magnetic field is applied along the monoclinic b axis. At 4.0°K and 32 kOe there is an abrupt increase in magnetization to 1 μB per Co2+ ion followed at 46 kOe by a second increase to 3 μB per Co2+ ion, which is believed to correspond to the ferromagnetic state. Neutron diffraction data have been obtained with fields of up to 46 kOe applied along the b axis of a single crystal mounted within a split-coil superconducting magnet. At the first transition, magnetic reflections characteristic of the antiferromagnetic phase disappear, and a number of new reflections, based upon a unit cell in which the a axis is tripled, are observed. These are in good agreement with a simple collinear ferrimagnetic model recently proposed in which the moments are parallel within (100) planes, which in turn are ordered in the sequence −++−++ in the a direction. The observed moment is about 3.3 μB per Co2+ ion.