[1826

18.5. /7-level Magnetism

[18.18,19], The temperature dependence of is shown in Fig. 18.4 for various values of x in Cs^C60. From the measurements in Fig. 18.4, the Curie-Weiss temperature can be obtained yielding 0CW = —5 K, independent of the Cs concentration for 1 < x < 4. The term, however, is strongly dependent on x, as discussed in §18.4, having a negative value for x = 0, a maximum positive value at x = 1, then decreasing with increasing x and becoming negative for x > 3 [see Fig. 18.3(b)]. The maximum at x = 1 is identified with a state of metallic conduction, with occupation of the Cs ions only on octahedral sites, and the positive Xo contribution is attributed to Pauli paramagnetism of itinerant electrons.

At Cs concentration x > 3 the Cs^CH) system is no longer metallic and the electrons become localized. Using a localized picture for the electrons on the fullerene anions, the magnetic moment Pcff per C60 anion is determined from the Curie-Weiss contribution to the susceptibility [see Fig. 18.3(a)], where we see that the maximum magnetic moment in CsxC60 is observed experimentally at x — 4. This experimental result is consistent with the Hund's rule prediction for the various molecular ion ground states in Table 18.3, showing that a maximum J value (J = 2) for the C£0" ions (0 < n < 6) occurs at C^0 [18.1]. For acceptor-doped C60, the maximum J value is predicted to occur for the +3 ion (see Table 18.3) [18.1], although this effect has not yet been confirmed experimentally. Although the maximum magnetic moment in Fig. 18.3(a) occurs for the C^ ion, the measured effective Bohr magneton is much smaller than the theoretically calculated effective Bohr magneton given by g[J(J +1 )]1/2Mb — 2>/6fiB. The large discrepancy between the observed and expected Bohr magneton values may arise from a thermal averaging of J (since the spin-orbit interaction for fullerenes is so small) and from a partly itinerant character of the electron wave functions between the molecules in the fee lattice. On the other hand, the magnetic moment near room temperature is almost independent of T, from which it has been concluded that Tc ~ 600 K, an extraordinarily high Tc for a molecular crystal with no unfilled d or / levels [18.18,19].

18.5.2. Observations in TDAE-C60

Another example of a magnetic system with only s- and p-orbitals is obtained by attaching the organic donor TDAE (tetrakis-dimethylamino-ethylene) complex to C60 [18.27-29]. The crystal structure for TDAE-C60 is described in §8.7.3. Since this compound exhibits the highest magnetic ordering temperature (ferromagnetic transition temperature Tc = 16.1 K) of any organic magnet, this material has been extensively studied for its magnetic properties [18.27,30-36], ESR spectra (see §16.2.4) [18.31-33,36-41], nuclear magnetic resonance (NMR) spectra (see §16.1.9) [18.39,42,43], optical properties [18.36,41,44], crystal structure (see §8.7.3) [18.45], and vibrational spectra [18.41] (see §11.10).

It is expected that the reduced intermolecular C60-C60 distance in TDAE-C go (see Fig. 8.16) results in greater overlap of the intermolecular wave functions. This increased wave function overlap and electron charge transfer from the TDAE to C60 both lead to a more conducting state. This is borne out by reports of conductivity values of ~ 10~2S/cm in TDAE-C 60 [18.27,45,46]. Microwave conductivity measurements [18.47] have confirmed a magnetic transition at 16 K, but it is found that the temperature dependence of the conductivity is not metallic, but instead shows activated behavior down to T ~ 80 K with an activation energy of ~60 meV [18.40,47]. The implied electron localization and hopping transport mechanism is consistent with the absence of a Drude absorption in the optical conductivity [18.41], The anisotropic crystal structure of TDAE-C60 is expected to lead to anisotropic magnetic properties, but since single crystals of this compound have not yet been prepared, the anisotropic magnetic properties have not yet been studied.

According to ESR and susceptibility studies (see §16.2.2), a charge transfer of one electron from the TDAE to C60 takes place, giving rise to an unpaired spin on each C60 anion in the lattice (see Fig. 8.16). This charge transfer is consistent with optical studies showing the tlu level as the lowest unoccupied state [18.36,41], Other experiments providing confirmation for charge transfer include a shift in the Raman pentagonal pinch mode frequency, and this frequency shift is almost as large as that for KC60 and RbQo [18.36] (although there are significant differences in their respective Raman spectra [18.41]). A similar softening of the low-frequency infrared-active modes is observed, and as with the Raman spectra, there are significant differences in the detailed spectra [18.41], A shift in the 13C NMR line from 142.7 ppm in C60 to 188 ppm has also been reported for TDAE-C60 [18.36], close to the values of 177 ppm in CsC60 and 173 ppm in RbC60 [18.48],

This unpaired spin gives rise to an effective magnetic moment of ¿ieff = 1.72/Ltfl/C60 as determined by high-temperature (T > 50 K) magnetic susceptibility x{T) measurements (see Table 18.4). The broad and intense ESR signal that is observed for TDAE-C60 in the magnetically ordered state (see §16.2.4) suggests that the spins are localized on each C60 molecule and are ferromagnetically correlated. Coordinated temperature-dependent susceptibility a°d x"(T)] and ESR studies have shown a correlation between spin ordering in the low-temperature magnetic phase and orien-tational ordering of the C60 molecules in TDAE-C60 [18.36], The orienta-tional ordering temperature T01 = 170 K was established by temperature-

Table 18.4

Parameters relevant to the magnetism for TDAE-Qq.

Table 18.4

Parameters relevant to the magnetism for TDAE-Qq.

Property

Value

Reference

Tc

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