Temperature Dependence Of D. C. Electrical Resistivity

3. Results and Discussion
3.1. Temperature dependence of D.C. Electrical Resistivity
The temperature dependence of the D.C. electrical resistivity of all the samples of Co-Cr ferrite system was studied using two probe method in the temperature range of 200-7000C.
The resistivity of the prepared samples was calculated using the relation 1. From fig. 1 and 4, it is clear that the resistivity decreases with increasing temperature ensuring the semiconducting nature of the prepared spinel ferrites [20]. This could be ascribed to the increase in the drift mobility of electric charge carriers that are thermally activated on increasing the temperature. Fig.1. Variation of Electrical Resistivity (ρ) of CoCrxFe2-xO4 (x=0.0 to 1.0) with temperature
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Compositional dependence of D.C. Resistivity
DC electrical resistivity of the Co-Cr nano ferrite system was found to increase from 1.4 x 106 to 46.2 x 106 Ω-cm at room temperature i.e. 300K with the increase in Cr concentration from 0.0 to 1.0 as summarized in Table 1 and is evident from fig.2. These values clearly signify that the resistivity in Co-Cr nano ferrites is increased by nearly 40 times to that of pure Cobalt ferrite with increase in dopant (Cr) concentration from 0.0 to 1.0. Co-Cr nano ferrites with such high resistivity that leads to very low eddy current losses are desirable in electronic inductors, transformers and electromagnets.
The variation of resistivity with Cr concentration X at some specific temperatures 570K, 670K and 770K were shown in fig.3 and the values are summarized in Table 1. The increase in resistivity with corresponding increase in composition is evident from the fig.3. Fig.2 Variation of Resistivity ρ with Cr composition at 300K Fig. 3 Variation of resistivity ρ with Cr composition of CoCrxFe2-xo4 (x=0 to 1.0) system of CoCrxFe2-xo4 at three different
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When chromium is doped in cobalt ferrites, a partial replacement of Fe3+ ions by Cr3+ ions take place in octahedral (B) site because, Cr ions have strong octahedral site preference. As a consequence, the substitution of Cr in Cobalt Ferrites would decrease the number of Fe3+/Fe2+ ion pairs present at the octahedral sites resulting in a decrease in hopping of ions and increase in resistivity.
The increase in resistivity may also be due to the increase in activation energy (Table 2) with increase in Cr concentration. This increase in activation energy indicates that the Cr ions entering into octahedral sites impedes the electron hopping between Fe2+ and Fe3+ ions for electrical conduction by occlusion of Fe2+ and Fe3+ transformation. This led to an increase in resistivity and a decrease in the conduction. The value of activation energy from table 2 indicates that the conduction process in the present Co-Cr nano ferrite system is due to hopping of polarons.
The substitution of Cr ion cobalt ferrites results in the formation of stable electric bonds between the Cr3+ and Fe2+ ions. This localizes Fe2+ charge carriers and hampers the Verwey mechanism thus increasing the resistivity. A similar behavior was reported in case of Ni-Al ferrite system [23] and Bi2O3 doped CoFe2O4

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