AC Heat Capacity Measurements
of Networked System
of Single-Molecule Magnets

A thermodynamic study of [Mn4(hmp)4(pdm)2{N(CN)2}2] (ClO4)2 · 1.75H2O · 2Me(CN), which shows magnetic field induced freezing of spin correlations has been performed. The relaxation calorimetry using a dilution refrigerator revealed a broad heat capacity peak associated with an ordering of ground state doublet of Sz = +9, −9 of the Mn4 cluster. Upon applying magnetic fields, the peak is suppressed and the field induced freezing of spin correlations appears around 0.1 T. The measurements of frequency dependence by the ac-calorimetry technique demonstrate that the freezing temperature tends to increase with the increase of modulation frequency. The observed frequency dependence data as well as a preliminary pressure dependence experimets are consistent with the glass-like freezing typically observed in single-molecule magnets.

(by O. Kubota & Y. Nakazawa)

	Fig. 1. Low-temperature heat capacity of [Mn4(hmp)4(pdm)2{N(CN)2}2] (ClO4)2 · 1.75H2O · 2Me(CN) obtained by a dilution refrigerator. A thermal anomaly which is associated with formation of an antiferromagnetic ordering was observed at 380 mK.
	Fig. 2. Frequency dependence of the ac heat capacity obtained under external field of 0.2 T. A systematic increase of the freezing temperature with the increase of the ac frequency is observable from the figure.