Magnetic Heat Capacity of Stage 2 Graphite-CoCl 2
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M. SHAYEGAN,t$ L.
SALAMANCA-RIBA,*$ J.
HEREMANS,#
G. DRESSELHAUS§ and J-P. ISSI#
Massachusetts Institute of Technology,
Cambridge,
MA 02139, USA
ABSTRACT The heat capacity C of graphite-CoC1 2 (stage 2) is measured at zero and high (up to 14 T) magnetic fields (H applied in-plane). By suppressing the magnetic contribution to Cp at the highest fields, we are able to decompose Cp into its electronic, lattice, and magnetic contributions. The magnetic heat capacity CM is seen to have a broad peak at = 9.1 K. The shape of this peak is consistent with the reported Monte Carlo calculations of CM based on a twodimensional xy model. INTRODUCTION The magnetic graphite intercalation compounds (GIC), formed by the insertion of magnetic intercalants (e.g., FeC1 3 , COC1 2 , and NiC1 2 ) between the graphite layers, provide a system in which the layers of the magnetic material can be separated by a controlled number (equal to stage n) of diamagnetic graphite layers. Previous studies of the temperature dependence of the magnetic susceptibility and heat capacity at constant pressure Cp [1-6] of these compounds have shown that these compounds undergo magnetic phase transitions at low temperatures. Although the nature of this magnetic ordering is not fully understood yet, the experimental results have established that the magnetic ordering temperatures in these GIC are nearly stage-independent and that they are lower than those of the parent (intercalant) material. These magnetic states have thus been attributed to two-dimensional magnetic interactions in the GIC. The temperature dependence of the heat capacities of the graphite compounds intercalated with NiC1 2 [1,5], CoC1 2 [21, and FeC1 3 15] have been previously reported. In most of these studies, Cp(T) was measured in zero applied magnetic field, and the method of corresponding states [7,8] was used to separate the magnetic contribution (CM) to C from the electronic (CE) and the lattice (CL) contributions. In the method of corresponding states, CM for a magnetic compound (e.g. COC1 2 ) is estimated by subtracting the Cp of a non-magnetic compound with similar structure (such as MnC1 2 ) from C for the magnetic compound (81. The assumption of this method is that the CL + CE contributions to Cp for the two compounds are the same. In determining Cp for a magnetic GIC, however, one should be cautious about using this method, because it is rather difficult to grow two GIC samples with identical structures. The presence of secondary (admixed) stages, differences in in-plane densities, and the presence of intercalate islands and vacancies are among the considerations encountered in growing GIC with identical structures. In our study, therefore, we employed a method providing a more definitive measure of CM using the same sample without t
Department of Electrical Engineering and Computer Science. *Department of Physics. lFrancis Bitter National Magnet Laboratory sponsored by NSF. #Universite Catholique de Louvain, Belgium. $Center for Materials Science and Engineering.
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