Low-Temperature Magnetic Phase Transitions in CoCl2·6H2O and MnBr2·4H2O

Abstract
Apparatus for the continuous recording of quasistatic magnetization isotherms at low temperatures has been developed. It has been employed in the study of phase transitions in two antiferromagnets induced by magnetic fields up to 16 kOe applied along the preferred directions of spin alignment. Antiferro‐paramagnetic transitions have been detected in CoCl2·6H20 between the zero‐field Néel point, TN(0) = 2.29°K, and 2.06°K. Below 2.06°K, ``spin‐flopping'' is observed. The resulting magnetic H‐T phase diagram agrees well with that obtained by resonance techniques. Measurements on MnBr2·4H2O below TN(0) = 2.13°K show only antiferro‐paramagnetic transitions down to 1.15°K with no indication of spin‐flopping. The observed antiferro‐paramagnetic transitions appear to be of the λ‐type while the spin‐flop transitions in CoCl2·6H2O are probably of first order.