Calorimetric and Magnetic Studies on a Ferromagnetic Phase Transition of the Metal-Assembled Complex MnCu(obbz)·H2O

Heat capacities of the metal-assembled ferromagnet MnCu(obbz)·H2O (obbz = oxamidobis(benzoato)) have been measured in the temperature region between 0.1 and 301 K by adiabatic calorimetry. A broad phase transition due to the three-dimensional ferromagnetic ordering was observed around 17 K. Above the transition temperature, a heat capacity tail arising from the short-range order characteristic of a low-dimensional magnet was found. The enthalpy and entropy gained by the phase transition are 227 J mol-1 and 12.7 J K-1 mol-1, respectively. This entropy acquisition is very close to the value Rln 5 ( = 13.4 J K-1 mol-1) expected for the spin manifold of the spin quantum number S = 2 ground state, where R stands for the gas constant. The thermal anomaly above the transition temperature can be accounted for in terms of the S = 2 two-dimensional ferromagnetic Heisenberg model rather than the one- or three-dimensional models. This suggests that this complex can be regarded as a two-dimensional ferrimagnet, in which strong antiferromagnetic interaction exists between a pair of Mn(II) (S = 5/2) and Cu(II) (S = 1/2) but the magnetic coupling between the pairs is very weak. Magnetic measurements for the complex revealed a bend of the hysteresis loop above the transition temperature and a magnetic relaxation below the transition temperature. These results lead to the conclusion that the broadness of the phase transition may be attributed to superparamagnetism caused by the fact that the present complex is prepared only in a fine powder.