Observation of two transition temperatures in the heat capacity of a two-dimensional ferromagnet

Abstract

The heat capacities of ferromagnetic Cu(CH 3NH 3) 2Cl 4 and Cu(C 2H 5NH 3) 2Cl 4 ( S = 1 2 ) have been studied b etween 1 and 25°K. Below 4°K the magnetic heat capacity is propertional to T, as expected from the (quadratic) layer-type crystal structure. Two transition points have been observed, one being ascribed to the transition temperature for the isolated two-dimensional ferromagnet and the lower one to the onset of long range ordering. The higher transition occurs at J/ kT c = 1.1.