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| - Low-Temperature Heat Capacity and One-Dimensional Ferromagnetic Behavior of theOrganic Free Radical 4-Benzylideneamino-2,2,6,6-tetramethylpiperidin-1-oxyl (BATMP)
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| - Heat capacities of the organic free radical ferromagnet 4-benzylideneamino-2,2,6,6-tetramethylpiperidin-1-oxyl (BATMP) crystal were measured in the temperature range between 0.1 and 300 K by adiabatic calorimetry.A ferromagnetic phase transition was found at TC = 0.19 K, and a broad heat-capacity anomaly was foundarising from the short-range ordering above TC characteristic of low-dimensional magnetic spin systems. Theenthalpy and entropy gains due to both the magnetic phase transition and the heat-capacity anomaly wereevaluated to be ΔH = 3.86 J mol-1 and ΔS = 5.64 J K-1 mol-1, respectively. The value of the experimentalmagnetic entropy agrees well with the theoretical value R ln 2 (5.76 J K-1 mol-1) expected for a spin quantumnumber S = 1/2 spin system (R is the gas constant). The magnetic heat capacity hump due to the short-rangeorder was well accounted for in terms of an S = 1/2 one-dimensional ferromagnetic Heisenberg model withthe intrachain exchange interaction J/kB = 0.95 K (kB is Boltzmann's constant). This fact suggests that BATMPcrystal is a one-dimensional ferromagnet above TC. The spin wave analysis of the magnetic heat capacitiesbelow TC revealed that BATMP crystal is in a three-dimensional ferromagnetic state below TC and the averagedinterchain exchange interaction is J‘/kB = 0.026 K.
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