Calorimetric investigation of the antiferromagnetic transition in NiCl(2).6H(2)O

Abstract

The results of measurements of the specific heat of nickel chloride hexahydrate in zero and various applied fields are presented. The specific heats of two single-crystal samples were determined in zero field between 4.3 and 5.8 K with temperature resolution of 10(-6) K. The functional form of the specific heat near the Neel temperature is determined and compared with theoretical predictions. The specific heat maximum of both crystals is found to be rounded over several millidegrees. This non-ideal behavior is analyzed in terms of a complex transition temperature, the imaginary part of which is related to crystalline imperfections. Using data outside the rounded region, it is concluded that the Neel temperature of a perfect crystal would be 5.3475 +/- .001 K. The effect of magnetic fields applied along the easy axis on the specific heat peak was investigated. The peak was found to be slightly broadened by the field, and occurred at lower temperatures. The boundary between antiferromagnetic and paramagnetic phases was determined for fields to 20 kilo-oersteds.