Magnetic Properties

The magnetic properties of fullerenes are interesting and surprising. Materials with closed-shell configurations are diamagnetic. Thus C60 itself is expected to be diamagnetic, since by Hund's rule the ground state for the C60 molecule is a nondegenerate / — 0 state [18.1]. The diamagnetic behavior of C60 is, however, unique, distinctly different from that of graphite, and interesting in its own right, because of an unusual cancellation of ring currents in the molecule. This is discussed in §18.1.

There are several methods for introducing paramagnetic behavior into fullerenes, including methods for producing magnetically ordered phases. For example, the introduction of a magnetic dopant ion with an unfilled d ox f shell (either endohedrally of exohedrally) is expected to lead to Curie paramagnetism. Evidence for such effects is presented in §18.5. The introduction of conduction electrons delocalized on a fullerene shell would be expected to lead to Pauli paramagnetism (see §18.4). Furthermore, the addition (or subtraction) of electrons to (or from) fullerene molecules could lead to unfilled valence shells and thus give rise to magnetic states (see §18.3). In fact, paramagnetic behavior due to unfilled p-bands has also been reported for this exceptional system (see §18.5), arising because of the electron localization on the molecular sites. At low-temperatures, phase transitions to magnetically ordered phases could occur (see §18.5). Many of these effects have already been observed in fullerenes and fullerene-derived compounds. In this chapter we review present knowledge about the various types of magnetic behavior and magnetic phases that have been observed in fullerene-based systems.

18.1. Diamagnetic Behavior

The large number of aromatic rings in fullerenes suggests that they might be highly diamagnetic, setting up shielding screening currents upon application of a magnetic field [18.2,3]. More detailed analysis shows that the hexagonal rings contribute a diamagnetic term to the total susceptibility while the pentagonal rings contribute a paramagnetic term of almost equal magnitude, thereby leading to an unusually small diamagnetic susceptibility for C60 in comparison with other ring compounds [18.4-6] and with graphite [18.7] itself, which has only hexagonal rings. This approach to the analysis of the magnetic susceptibility is in good agreement with \ measurements on both C60 and C70, as discussed below [18.8-10].

The points in Fig. 18.1 show the measured temperature dependence of the magnetic susceptibility for a C60 powder sample [18.8]. The results show a temperature-independent contribution to the susceptibility at high T of Xg — —0.35 x 10~6 emu/g, which is identified with the intrinsic \ for where the subscript g refers to the susceptibility per gram of sample. In addition, a temperature-dependent paramagnetic contribution is observed at low-temperature (see Fig. 18.1) which is identified with 1.5 x 10 4 unpaired electron spins per C atom, in agreement with typical electron spin resonance (ESR) measurements on C^ (see §16.2.1) [18.8]. Measurements of x(T) on a C60 single crystal, carefully prepared and handled to avoid oxygen contamination, show no low-temperature Curie term [18.11], from which it is concluded that the low-temperature Curie contribution is likely due to oxygen contamination. Detailed measurements of x(T) near the phase transition temperature Tm (see §7.1.3) show a 1.2% change in x(T) at T01, with x{Tm) more negative than xiT^) for the susceptibility at either side of the phase transition [18.11].

The results of x(T) for C70 are similar to those for C60 in functional form, except that the temperature-independent magnitude of x f°r C70 is about twice as large, xg - -0.59 x 10 6 emu/g [18.8], consistent with the larger number (5 additional) of hexagons in C70 as compared with Q,,. These values for x are compared in Table 18.1 with the corresponding diamagnetic susceptibility for graphite, diamond, and carbon nanotubes (see § 19.6).

The remarkably small value of the temperature-independent diamagnetic susceptibility for C60 has been explained by calculating the ring current in pentagonal and hexagonal rings in the CgQ structure [18.6], following the London theory [18.17] where the current J,7 from site R, to the nearest-neighbor site R• is expressed in terms of eigenvectors C" of the Hamiltonian matrix as lj =

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