Susceptibility of Magnetic Graphite-CoCl 2 Intercalation Compounds
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Cambridge,
MA 02139,
USA
ABSTRACT Precise measurements of the temperature dependence of the magnetic susceptibility for stage 2 and 5 graphite-CoCl 2 intercalation compounds are reported. Comparison of the experimental results with theoretical calculations based on a two-dimensional planar model show agreement with theory, suggesting that graphite intercalation compounds represent ideal two-dimensional magnetic systems.
INTRODUCTION Recent theoretical and experimental activity in the study of two-dimensional magnetism has focused on the effect of dimensionality on the magnetic behavior near phase transitions. However, the two-dimensional systems have a particular interest of their own insofar as they exhibit many similarities to the actual three-dimensional systems, but it is much simpler to perform theoretical calculations in two- than in three-dimensional space. It was first pointed out by Peierls [1] and later proven mathematically by Mermin et al. [21 that long range order can not occur in one- and twodimensional magnetic solids. Yet magnetic phase transitions at finite temperatures do occur in two-dimensional systems [3]. In recent years, the studies of two-dimensional magnetism have been focused on the understanding of the nature of these magnetic phase transitions through various theoretical models and by their observation in actual experimental systems. Intercalation of magnetic species in graphite provides a very useful system for the study of two-dimensional magnetism and for the investigation of dimensionality in magnetic [4] and structural [5] behavior generally. In these systems the interplanar interaction of magnetic moments can be systematically and continuously reduced by inserting a controlled number, n, of diamagnetic graphite layers between each adjacent pair of magnetic intercalate layers. The number n is called the stage of the graphite intercalation compound (GIC). In the case of the transition metal chloride compounds, the distance between the neighboring magnetic layers can be varied from -9 A (stage 1) to more than 30 A (stage 7). Many of the transition metal chloride intercalation compounds have been the subject of magnetic investigations 14,6-8] and in almost all cases the existence of magnetic phase transitions have been reported. However, in most cases the nature of the phase transition is not well established. Recent susceptibility measurements on FeCl 3 -GIC, CoC1 2 -GIC and NiCl 2 -GIC have shown similarities in both the temperature and the magnetic field dependence of the magnetic behavior of these intercalation compounds [7]. In the present work we have chosen the CoCl 2 -GIC system for a more critical investigation because of our better understanding of the pristine CoC1 2 magnetic properties [9-12]; pristine CoC1 2 already has a very weak interplanar interaction [12]. In addition, the CoC1 2 -GIC is air stable and available in a number of different well staged *Center for Materials Science and Engineering, iFrancis Bitter National Magnet Laboratory,
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