Low-temperature heat capacity of the Mn(III)-porphyrin-TCNE ferrimagnetic complexes, [MnTXPP][TCNE]·2PhMe, (X=Cl, Br)

Heat capacity measurements by thermal relaxation method were performed under the magnetic field (up to 8 T) between 1.8 and 100 K for the quasi-one-dimensional ferrimagnetic complexes, meso-tetrakis(4-chorophenyl or 4-bromophenyl)porphyrinato-manganese(III) tetracyanoethenide, [MnTXPP]+[TCNE]−·2PhMe (X=Cl, Br; PhMe=toluene). These complexes are ferrimagnets consisting of S=2 spins of manganese (III) cations and S=1/2 spins of [TCNE]− organic radical anions that is arranged alternatively. At the critical temperatures (∼8 K), below which a long-range magnetic order takes place, ac magnetic susceptibilities have been reported to exhibit sharp peaks. In contrast, no distinct heat capacity anomaly was observed, which is probably attributable to the low-dimensional effects. However, a magnet-caloric behavior characteristic of the alternating ferrimagnetic chain was observed. The contrastively enhanced behavior of the magnetic susceptibility is explained by the previously reported model assuming formation of magnetic domains in the crystal.