Experimental Study of the Magnetocaloric Effect in the Two-Dimensional Quantum System Cu(en)(H2O)2SO4

Magnetocaloric studies have been performed on a two-dimensional (2D) quantum system Cu(en)(H2O)(2)SO4 (en=ethylendiamine=C2N2H8) in the temperature range from 0.6 K to 4 K in magnetic fields up to 2 T by adiabatic magnetization and demagnetization measurements. The title compound has been previously identified as a potential realization of the quasi-two dimensional spatially anisotropic triangular Heisenberg antiferromagnet with spin 1/2 and effective intralayer exchange coupling, J/k(B) = -1.4 K. A phase transition to the magnetically ordered state has been observed in zero magnetic field at T-N = 0.91 K. The normal magnetocaloric effect (MCE) was observed in the temperature range from 0.6 K to 0.98 K. In contrast, at temperatures near the phase transition the character of MCE changes to inverse and above the temperature 2 K the normal MCE was observed again. This change can be ascribed to the onset of short-range magnetic correlations previously observed in specific heat and susceptibility studies.