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

The heat capacities of ferromagnetic Cu(CH₃NH₃)₂Cl₄ and Cu(C₂H₅NH₃)₂Cl₄ (S =12 role=presentation style=font-size: 90%; display: inline-block; position: relative;>$\text{S =}\text{1}\text{2}$$\text{S =}\text{1}\text{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.