Verwey Transition of Nano Particles
6.2.5. Verwey Transition of Nano Particles
The DC conductivity data as log σ vs. 1/T plots in the TSPC mode from 77°K to room tempera- ture is shown in Figure 6.16 for the 700°C sample for two different heating rates of 0.35 K/min and 1°K/min respectively, and those for the 800°C sample are shown in Figure 6.17 for two different heat- ing rates of 1°K and 2°K respectively. It is seen from Figures 6.16 and 6.17 that there is a broad maximum around and below room temperature for the basalt glass heat-treated at 700°C and 800°C respectively.
Figure 6.16 : TSPC curves for the 700 samplee at the cooling rate of 0.35°K/min (filled triangle) and 1°K/min (filled circle)
The TSPC and TSDC data are similar to those reported in other glasses and dielectrics [26, 27], and they are normally related to the ‘trapping and release’ of charge carriers (i.e. ions or electrons). The trapping parameters may be derived from the TSPC and TSDC curves. However, in the present case, the situation may be quite complicated by the possible occurrence of Verwey transition in the nano-crystal- line magnetite phase within the glassy matrix.
It is worth mentioning that M S at ≈ 300°K increases by about 70% in the case of pure bulk magnetite in the particle size range of 20 nm-75 nm [28], whereas in the present case it increases by
249 about 50% between 650 - 900°C, i.e. within the nano particles size range of 4.5 nm - 7.0 nm. This
ELECTRICAL PROPERTIES
shows the ‘remarkable’ behaviour of ultra-fine nano particles of magnetite embedded within a glassy matrix. In the bulk crystalline magnetite phase, the Verwey transition temperature (T V ) is 119°K [29],
i.e. well below the observed peaks shown in Figures 6.16 and 6.17 respectively.
Figure 6.17 : TSPC curves for 800 sample at the cooling rate of 1°K/min (filled inverted triangle) and 2°K/min (inclined square), and TSDC curve at 1°K/min.
However, according to Krupyanski and Suzdalev [28], it should be noted that the Verwey transi- tion temperature is a function of crystallite size of the nano particles of magnetite, and T V is reported by these workers to be in the range 300°K to 350°K for nano crystals of magnetite of size 10 nm. Rogwiller and Kundig has also shown that the Verwey transition temperature is spread over a range of temperature from 100°K and 300°K in the nano crystals of magnetite with a mean size of about 14 nm [30]. The Verwey transition temperature in the present case (i.e. with a mean particle sizes between 5.5 nm and 6.4 nm) appears to be quite similar to the data of Krupyanski and Suzdalev [28] with a mean particle size of 10 nm. However, further experimentl work is needed to separate the possible contribu-
NANO MATERIALS
tions from the ‘non-steady-state’ thermally stimulated effects, and the Verwey transition temperature in the nano particles of magnetite. The tremendous effect of ‘superparamagnetism’ in this narrow range of sizes of nano particles in the basalt glass matrix may have some effect, which needs to be studied.
In summary, it can be said that the DC conductivity data indicates small polaron hopping be- tween the isolated Fe 2+ and Fe 3+ ions in the as-annealed glass. With the increase of heat-treatment temperature, associated with the formation of nano particles of magnetite, the hopping can take place between these sites in similar coordination within the small nano crystals of magnetite in a basalt glass matrix. The abrupt change in both the conductivity and the activation energy can be correlated with the magnetic data in terms of formation of the nano particles of magnetite and the possible structural change with increasing heat-treatment temperature. Preliminary AC conductivity data support the small polaron hopping transport mechanism. The TSPC and TSDC data cannot be used to derive the ‘trapping param-
eters’ due to the possible effect of Verwey transition. However, the Verwey transition temperatures appear to be almost close to that found for nano particles of magnetite with a mean size of 10 nm.