A calorimetric study of the magnetic ordering of microcrystalline nickelous oxide

Abstract

A calorimetric study of a finely divided NiO powder sample was carried out in order to investigate its magnetic ordering. Two different adiabatic calorimeters were built and test on calorimetric standard samples. The heat capacities of bulk-like NiO and a fine powder sample which exhibits an anomalously large paramagnetic susceptibility in the temperature range of interest were then measured. The magnetic heat capacity of the fine powder sample was obtained by analysis of the difference in their heat capacities and the magnetic entropy computed. The size of this magnetic entropy is consistent with the view that some of the nickel atoms on the surface of the particles undergo a magnetic order-disorder phase transition. The possibility of superparamagnetic ordering in this sample is not excluded but the magnetic entropy is too small to be the result of an order-disorder transition of a significant portion of the whole particles in the sample.