Fig. 18.6. Plot of magnetization us. magnetic field H for (a) TDAE-C«, and (b) TDAE-Cjo at ~4.5 K (closed circles) and ~20 K (open circles) showing evidence for a magnetic phase transition in TDAE-Qo and no phase transition in TDAE-C70 down to 4.5 K [18.49].

plicated history-dependent x(T) behavior below Tm ~ 170 K [18.36]. The external field is believed to order the C60 molecules preferentially into one of the potential wells of the double-well potential associated with the mero-hedral disorder, leading to paramagnetic behavior in magnetic field-cooled samples corresponding to a single paramagnetic center [18.36],

Although early magnetization studies showed the TDAE-C60 material to have no magnetic hysteresis on cooling and heating through Tc [18.27], thereby indicating a vanishingly small remanent field, later studies [18.51] showed both hysteresis behavior and a small remanent magnetization of ~ 3 x 10"4 emuK/g (see Table 18.4). Furthermore, the Tc value of TDAE-C60 can be increased by cooling the sample in an external magnetic field (e.g., Tc is increased to 24.3 K at 0.5 T and to 28 K at 9 T [18.36,46]), while the application of pressure reduces Tc remarkably [18.52], so that by an applied pressure of 1.6 kbar, Tc has been reduced below 2 K. Under pressure, the magnetic moment decreases even more rapidly than the transition temperature. On the basis of this rapid pressure dependence of Tc, it has been assumed that the observed magnetic state is associated with weak itinerant ferromagnetism [18.46], which is supported by the magnitudes of the observed conductivity (~10~2 S/cm) and saturation magnetization (0.1 fiB/C60). Since many papers were published before the hysteresis and remanent magnetization were found [18.50], early magnetic studies of TDAE-C 60 identified the magnetic phase with superparamagnetism [18.33]. The correlation between orientational order of the C60 molecules and the spin ordering in TDAE-C60 at low-temperatures offers insight into the large pressure dependence observed for TDAE-C60. Since the application of pressure strongly affects the molecular orientation, it shifts the energy of the minima of the orientational double-well potential, thereby changing the overlap of the v-electron orbitals on neighboring C60 molecules [18.36], which would be expected to strongly affect the magnetic properties of TDAE-C60.

The magnetic contribution to the specific heat of TDAE-C^ shows a broad maximum at about 12 K [18.34], consistent with magnetic ordering of a finite number of spins, as would, for example, be characteristic of a spin glass [18.36], A number of other experiments also support a spin glass model for TDAE-C60 at low-temperature. Three special properties define the characteristic behavior of a spin glass: (i) frozen-in magnetic moments below a certain freezing temperature Tf accompanied by a peak in the susceptibility x{T), (ii) lack of periodic long-range magnetic order, and (iii) remanence and nonexponential time dependence of the magnetic relaxation below Tf [18.53]. Peaks in the ac susceptibility x'(J) are indeed observed [18.40,41] with characteristic behavior that depends on the ac frequency (75 < / < 1000 Hz). The large increase in intensity (by a factor of ~ 103) of the low field ESR line below Tc (rather than a divergent susceptibility) also supports spin-glass behavior [18.43]. The lack of long-range order follows from the absence of a large internal field in zero field ESR measurements below Tc [18.41], from the strong magnetic depolarization effects observed in the ¡xSR spectra below 10 K [18.42], and from the stretched exponential relaxation time dependence of one of the proton NMR lines [18.42,43], Evidence for magnetic relaxation comes from the strong frequency dependence of x' and x" [18-42], and the stretched exponential decay of the ESR signal after the field is switched off [18.43], This is also consistent with the pulsed ESR spin echo measurements on TDAE—Cgg which show strong evidence for inhomogeneous line broadening effects, also suggestive of spin-glass behavior [18.43].

As discussed in §16.2.2, the anisotropy of the g-factor of TDAE-C60 has not yet been measured, although an average g-value of 2.0003-2.0008 has been detemined by the electron spin resonance (ESR) technique at high temperature in the paramagnetic phase. A sharp increase in g-value is observed at low T for TDAE-C60, while the g-factors for the TDAE-C„c (nc = 70,84,90,94) are all approximately independent of temperature and close to the free electron g-value of 2.0023.

The magnetic susceptibility of the related compounds TDAE-C70, TDAE-C84, TDAE-Gk), and TDAE-C94 has been investigated from room temperature down to 5 K [18.54], using ESR techniques, but all of these compounds show strictly paramagnetic behavior with no indication of a phase transition to a magnetically ordered state to the lowest measured temperature (5 K).


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