Chemistry Reference
In-Depth Information
Fig. 8. (a) Temperature dependence of the magnetic susceptibility for ACF samples
heat-treated at various temperature up to 1500 °C. (b) The field cooling effect on the
susceptibility vs temperature plots at HTT1100 and 800, which are in the vicinity of the
insulator-metal transition and far from the transition, respectively. Open and full circles
represent data for the zero field cooling and field cooling (
H
=1T) processes, respectively
(From reference 35).
change in the conductivity is faithfully tracked by the susceptibility as
given in Fig. 8(a).
35
Namely, the Curie-Weiss behavior featured with the
localized spins of edge state in the Anderson insulator regime is
converted to a less temperature dependent susceptibility of conduction
electrons in the metallic regime, where the net negative susceptibility is a
combination of small positive Pauli paramagnetic and large negative
orbital contributions. Here it should be noted that an anomalous feature
appears in the vicinity of the I-M transition, that is; the susceptibility has
a cusp at ca.7 K with a negative Weiss temperature (antiferromagnetic)
of -2 to -3 K. This is reminiscent of the onset of an antiferromagnetic
ordering. However, this is not the case, as a large field cooling effect on
the susceptibility evidences in Fig. 8(b).
35
Figure 8(b) indicates that the
susceptibility for the sample in the vicinity of the insulator-metal
transition has a large field cooling effect, particularly around the
temperature range in which the cusp emerges. The presence of a cusp
and its large field cooling effect are a consequence of the development of
spin glass state. In general, a spin glass state develops when the strengths
of exchange interactions
J
vary randomly in space. From the
magnetization curve analysis, a large randomness in the strengths of
exchange interactions is evident, the width of the distribution being
