Heat capacity measurements of the organic radical 2-tert-butylaminoxylbenzimidazole (BABI) were carried out in the 0.2 – 300 K temperature region. A heat capacity cusp at 1.7 K and a broad thermal anomaly centered around 2 K were observed. The former corresponds to the magnetic phase transition due to the onset of the long-range order, while the latter is due to the short-range order characteristic of two-dimensional system. The entropy gain estimated from these thermal anomalies is close to Rln2. The thermal anomaly around 2 K was explained well by the high-temperature expansion of the S = 1/2 two-dimensional antiferromagnetic Heisenberg model of square lattice with J/kB = −1.6 K.
Heat capacities of one-dimensional metal-assembled complex [NEt4]2[Mn(acacen)][Fe(CN)6] were measured from 0.1 to 301 K by adiabatic calorimetry. A magnetic phase transition was observed at Tc = 1.53 K, above which a heat capacity hump due to the short-range order characteristic of one-dimensional magnets was found. The transition enthalpy and entropy were determined to be ΔH = 104 J mol−1 and ΔS = 14.2 J K−1 mol−1, respectively. The value of the transition entropy is smaller than Rln10 (= 19.1 J K−1 mol−1), which is expected for the spin system composed of Mn(III) in the high spin state and Fe(III) in the low spin state, rather close to Rln6 (= 14.9 J K−1 mol−1). This may suggest that the complex has one-dimensional chains consisting of the strong alternation of Mn(III)-Fe(III) pairs or Mn(III) ions are in a low spin state S = 1. From temperature dependence of the magnetic heat capacities below the transition temperature, the complex can be regared as three-dimensional antiferromagnet below the transition temperature.
Heat capacity and magnetization measurements of the metal-assembled complex [Mn(cyclam)][Fe(CN)6] · 3H2O were performed. A magnetic phase transition was observed at 6.2 K. A magnetic thermal anomaly characteristic of low-dimensional magnets was also found around 14 K. The magnetic entropy was determined to be 2.32R, which agrees well with the theoretical value Rln(5×2) = 2.30R. Magnetic field dependence of heat capacity and magnetization reveals a field-induced metamagnetic transition.
Heat capacities of the single molecular magnet [Mn12O12(O2CEt)16(H2O)3] were measured without external magnetic field by adiabatic calorimetry. No distinct thermal anomaly was observed. Best fitting to the experimental heat capacities below 5 K with two uniaxial zero-field splitting parameters implies the existence of Jahn-Teller isomers. Heat capacities of the polycrystalline sample were also measured with applied magnetic fields by a relaxation method. A step-like heat capacity anomaly was observed around the blocking temperature TB ≈ 3.5 K. The magnitude of the anomaly showed a maximum at 0.7 T. The step-like heat capacity anomaly around TB under 0.7 T can be fairly well accounted for if one assumes that the conversion between spin-up and spin-down is allowed above TB by the resonant quantum tunneling while it is not allowed below TB.
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