Low-temperature specific heat of the molecule-based magnet M[N(CN)2]2 (M=Co, Ni, Cu, Zn) series

Abstract

We report the specific heat as a function of temperature in zero applied magnetic field for three magnetic systems (α-Co[N(CN)2]2, β-Co[N(CN)2]2, Ni[N(CN)2]2) and two nonmagnetic systems (Cu[N(CN)2]2, Zn[N(CN)2]2). The anomalies revealed by the magnetic contribution to the specific heat are associated with the onset of long-range order. The transition entropies imply that the magnetic ordering originates from a ground-state doublet (J=12) for the Co2+ systems and a ground-state triplet (J=1) for the Ni2+ system. In addition, we extract and compare the exchange couplings J using several theoretical models: interacting spin waves, high-temperature series expansions, total magnetic energy and mean field. The J-values and the Ising-like anisotropy obtained here, are in quantitative agreement with earlier magnetization and neutron diffraction results. The comparison of the zero-field and in-field (8T) specific heat of the Cu2+ system (J=12) demonstrates the previously unknown ferromagnetic order at very low temperature (⩽1.7K).