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Heat capacity measurements of the two-dimensional metal-assembled complex, (NEt4)[{MnIII(salen)}2FeIII(CN)6] [Et=ethyl, salen= N,N’-ethylenebis(salicylideneaminato) dianion], were performed in the temperature range between 0.2 and 300 K by adiabatic calorimetry. A ferrimagnetic phase transition was observed at Tc1=7.51 K. Furthermore, another small magnetic phase transition appeared at Tc2=0.78 K. Above Tc1, a heat capacity tail arising from the short-range ordering of the spins characteristic of two-dimensional magnets was found. The magnetic enthalpy and entropy were evaluated to be Δ H=291 J mol-1 and Δ S=27.4 J K-1 mol-1, respectively. The experimental magnetic entropy agrees roughly with Δ S= Rln(5·5·2) (=32.5 J K-1 mol-1; R being the gas constant), which is expected for the metal complex with two Mn(III) ions in high-spin state (spin quantum number S=2) and one Fe(III) ion in low-spin state ( S=1/2). The heat capacity tail above Tc1 became small by grinding and pressing the crystal. This mechanochemical effect would be attributed to the increase of lattice defects and imperfections in the crystal lattice, leading not only to formation of the crystal with a different magnetic phase transition temperature but also to decrease of the magnetic heat capacity and thus the magnetic enthalpy and entropy. [ABSTRACT FROM AUTHOR]

Heat capacity measurements of the two-dimensional metal-assembled complex, (NEt4)[{MnIII(salen)}2FeIII(CN)6] [Et=ethyl, salen= N,N’-ethylenebis(salicylideneaminato) dianion], were performed in the temperature range between 0.2 and 300 K by adiabatic calorimetry. A ferrimagnetic phase transition was observed at Tc1=7.51 K. Furthermore, another small magnetic phase transition appeared at Tc2=0.78 K. Above Tc1, a heat capacity tail arising from the short-range ordering of the spins characteristic of two-dimensional magnets was found. The magnetic enthalpy and entropy were evaluated to be Δ H=291 J mol-1 and Δ S=27.4 J K-1 mol-1, respectively. The experimental magnetic entropy agrees roughly with Δ S= Rln(5·5·2) (=32.5 J K-1 mol-1; R being the gas constant), which is expected for the metal complex with two Mn(III) ions in high-spin state (spin quantum number S=2) and one Fe(III) ion in low-spin state ( S=1/2). The heat capacity tail above Tc1 became small by grinding and pressing the crystal. This mechanochemical effect would be attributed to the increase of lattice defects and imperfections in the crystal lattice, leading not only to formation of the crystal with a different magnetic phase transition temperature but also to decrease of the magnetic heat capacity and thus the magnetic enthalpy and entropy. [ABSTRACT FROM AUTHOR]

Heat capacity studies of the layered molecule-based magnetic material {N(n-C5H11)4[MnIIFeIII(ox)3]}∞ (ox=oxalato) have been made in the 2–280 K range. Two distinct heat capacity anomalies were detected at 27.1 K (TN) and 226 K corresponding to magnetic and structural phase transitions, respectively, along with a hump around 23 K. The magnetic heat capacities above TN can be well represented by S=5/2 two-dimensional antiferromagnetic Heisenberg model of a honeycomb lattice with intralayer exchange interaction J/kB=-3.3 K. Application of spin-wave theory indicated the existence of a very weak interlayer magnetic interaction below TN. The anomaly at 27.1 K is due to the antiferromagnetic transition, in conformity with the magnetic results reported earlier. The hump around 23 K might be associated with the existing uncompensated magnetic moments. The estimated magnetic entropy (33.22 J K-1 mol-1) is close to the expected magnetic entropy (R ln (6×6)=29.80 J K-1 mol-1) for the spin multiplicity of high spin MnII and FeIII ions. The heat capacity anomaly at 226 K may be assigned to a structural phase transition of order–disorder type due to increasing conformational change of the n-C5H11 chains in the organic cation. [Copyright &y& Elsevier]

Magnetocaloric effect in {[Mn(pyrazole) 4 ] 2 [Nb(CN) 8 ]·4 H 2 O} n molecular magnet is reported. It crystallizes in tetragonal I4 1 /a space group. The compound exhibits a phase transition to a long range magnetically ordered state at T N ≈ 22.8 K . Temperature dependences of the magnetic entropy change Δ S M as well as the adiabatic temperature change Δ T ad due to applied field change μ 0 Δ H in the range of 0.1–9 T have been inferred from the relaxation calorimetry measurements. A systematic approximate approach has been used to determine the lattice contribution to the heat capacity. The maximum value of Δ S M for μ 0 Δ H = 5 T is 6.83 J mol −1 K −1 (6.65 J kg −1 K −1 ) at 24.3 K. The corresponding maximum value of Δ T ad is 1.4 K at 23.8 K. The temperature dependence of the exponent n characterizing the field dependence of Δ S M has been estimated. It attains the value of 0.64 at the transition temperature, which is consistent with the 3D Heisenberg universality class. A hitherto unobserved two-peak structure has been revealed in the temperature dependence of Δ T ad . [ABSTRACT FROM AUTHOR]

Abstract: Studies on the magnetic properties of the molecular antiferromagnetic material {N(n-C5H11)4[MnIIFeIII(ox)3]}∞, carried out by various physical techniques (AC/DC magnetic susceptibility, magnetization, heat capacity measurements and Mössbauer spectroscopy) at low temperatures, have been presented. Different experimental observations complement each other and provide a clue for the observation of an uncompensated magnetization below the Néel temperature and short-range correlations persisting high above T N. It is understood that the honeycomb layered structure of the compound contains non-equivalent magnetic sub-lattices, (MnII–ox–FeIII A–...) and (MnII–ox–FeIII B–...), where different responses of the FeIII A and FeIII B spin sites towards an external magnetic field might be responsible for the observation of the uncompensated magnetization in this compound at T < T N. The present magnetic system is an S =5/2 2-D Heisenberg antiferromagnet system with the intralayer exchange parameter J/k B =−3.29K. A very weak interlayer exchange interaction was anticipated from the spin wave modeling of the magnetic heat capacity for T <0.5T N. The positive sign of the coupling between the layers has been concluded from the Mössbauer spectrum in the applied magnetic field. Frustration in the magnetic interactions gives rise to the uncompensated magnetic moment in this compound at low temperatures. [Copyright &y& Elsevier]

A study of magnetocaloric effect of a structurally unique molecular magnet built of spin clusters with the pentadecanuclear Ni(II) 9 W(V) 6 core forming a cube-like framework is reported. The compound crystallizes in the triclinic system. The clusters display anisotropy quantified with the axial D =0.07 K and rhombic E =0.023 K anisotropy constants. The temperature dependences of magnetic entropy change Δ S M as well as the adiabatic temperature change Δ T ad due to applied field change μ 0 Δ H ranging from 1 to 9 T have been determined by employing the relaxation calorimetry measurements in applied field. The maximum value of Δ S M for μ 0 Δ H =5 T is 18.38 J K −1 mol −1 (3.36 J K −1 kg −1 ) at 4.3 K. The corresponding maximum value of Δ T ad =4.6 K is attained at 2.2 K. The occurrence of the inverse MCE has been observed at high temperatures for small field change values. The temperature dependence of the exponent n characterizing the field dependence of Δ S M has been estimated and discussed. The study revealed that the compound may be treated as a possible candidate for cryogenic magnetic cooling. [ABSTRACT FROM AUTHOR]

Abstract: Phase transitions in —a mixed valence PB analogue have been investigated with the help of heat capacity calorimetry under 0–90kOe magnetic fields in the 1.8–300K temperature range. Two heat capacity anomalies at 2.1 and 7.5K have been detected, which correspond to ferromagnetic and ferrimagnetic transitions, respectively. The magnitudes as well as the position of these thermal anomalies have varied with changing magnetic field. The estimated zero-field magnetic entropy amounted to , which is close to the expected value . The uniaxial zero-field splitting parameter is estimated to be . A comparatively stronger antiferromagnetic interaction seems to lead to the magnetic transition around 7.3K, whereas a weaker ferromagnetic interaction gives rise to the thermal anomaly at 2.1K. The present results on magnetic phase transitions are in complete agreement with the earlier observations made through magnetic studies. Additionally, a glass transition at 194K has been observed in the heat capacity study presumably due to freezing of the orientational motion of the molecules present in the material. This phenomenon is reflected into the temperature dependence of the estimated and concentrations in the present material obtained earlier through Mössbauer spectroscopy. [Copyright &y& Elsevier]