The heat capacity of cerous magnesium nitrate and some related materials between 0.3 and 4 K

Abstract

Cerous magnesium nitrate (CMN) is the preeminent electronic paramagnet in use in cryogenic physics for magnetic thermometry and adiabatic cooling. In demagnetization experiments designed to establish the thermodynamic temperature relations for CMN, an inexplicable heat capacity anomaly was found to occur above 20 mK and is shown here to persist to temperatures near 1 K. The anomaly is small but its presence interferes with and may cause errors in the analysis of thermometric data. We have measured the heat capacity of CMN, lanthanum magnesium nitrate (LMN), cerous nitrate hexahydrate, and a saturated aqueous solution of CMN (CMN liquor) in the temperature range 0.3–4 K in an attempt to find the source of the anomaly. The LMN heat capacity shows no anomaly and is used to approximate the lattice heat capacity of CMN. At low temperatures the CMN heat capacity, exclusive of the lattice contribution, is some 2 1/2 times larger than the magnetic heat capacity predicted by other investigations. At high temperatures an exponentially increasing heat capacity due to the first excited electronic level is observed and indicates a splitting which is in accurate agreement with the spectroscopic value. There is evidence that the lattice heat capacity in CMN is about 1% smaller than in LMN, which is probably the result of the crystal-field interaction with the electronic states of the cerous ions. The lattice terms and theT−2term of the magnetic heat capacity for cerous nitrate have been determined, the latter being 25 times larger than the predictedT−2term in CMN. The CMN liquor measurements indicate that this sample had probably become a glass on cooling. The lattice heat capacity is considerably larger than could be predicted from the separate components and there is no indication of the exponential term which would be observable if appreciable crystalline CMN were present. These measurements help to define the nature of the anomalous heat capacity and remove from consideration some possible explanations, but they do not reveal the cause of the anomaly.