We have constructed heat capacity measurement system, which can be used with dilution refrigerator at IMS. This system makes it possible to perform heat capacity measurements from a dilution temperature region to 10 K continuously. Using this system, we have measured heat capacity of a tiny single crystal with 45.5 µg of the magnetic superconductor κ-(BETS)2FeCl4 from 0.1 K to 1 K. A sharp thermal anomaly corresponding to the antiferromagnetic transition was observed. The antiferromagnetic transition temperature shifts to the lower temperature by applying magnetic fields perpendicular to conduction layer. However the peak shape of the thermal anomaly does not change so much. These behavior is consistent with that of κ-(BETS)2FeBr4.
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Fig. 1. Crystal structure of κ–(BETS)2FeX4 (X = Br, Cl). The donor layers are consisting of dimerized structure and insulate layer shows magnetic behavior due to d–electrons of Fe ions. |
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Fig. 2. CpT−1 vs T2 curves for κ–(BETS)2FeCl4. The sharp peak observed around 0.47 K at 0 T shifts to 0.38 K under 1 T. This behavior is a typical for antiferromagnets. |