ABSTRACT

Thermoelectric Power studies of Ni-Mg ferrites having chemical formula Ni_1–xMg_xFeO₄ (x = 0.2, 0.4, 0.6 and 0.8) were investigated from room temperature to well beyond Curie temperature by the differential method. The Seebeck coefficient is negative for all the composition. It clearly speaks that all the considered ferrite compositions behave as n-type semiconductors. Plots of Seebeck coefficient (S) versus temperature shows maximum at Curie temperature. The values of the charge carrier concentration have been computed from the observed values of Seebeck coefficient. The electrical properties of the Ni-Mg mixed ferrites have been measured at room temperature by two-probe method. On the basis of these results an explanation for the conduction mechanism in Ni-Mg mixed ferrites is suggested.

1. Introduction

Hall effect and thermoelectric power studies are widely used in the interpretation of the conduction mechanism in semiconductors. The result of interpretation of Hall effect reveals more straight forwarding precise results. However, in the case of low mobility semiconductors such as ferrites, it is sometimes difficult to measure the Hall effect, in such cases the study of thermoelectric measurements is the only alternative. Moreover the measurement of thermo-e.m.f. is simple and its sign gives vital information about the type of conduction in semiconductor whether they are n-type or p-type. Magnesium substituted ferrites have been used in many electronic devices for high frequency because of their high electrical resistivity, high Curie temperature and low cost.

The dielectric properties of Ni-Mg ferrites have been studied in detail [1]. As per author’s knowledge no information is available on thermometric power studies of mixed Ni-Mg ferrites in the literature. Moreover, there is a need for a thorough study of thermoelectric power studies of magnesium substituted nickel ferrites as a function of composition and temperature. The results of such a study are presented in this paper.

2. Experimental Details

2.1. Sample Preparation

The polycrystalline magnesium substituted Nickel ferrites having the compositional formula Ni_1–xMg_xFeO₄ (x = 0.2, 0.4, 0.6 and 0.8) were prepared by double sintering ceramic method and the samples are annealed at 800˚C for 5 hr. Final sintering of the specimens was carried out at 1200˚C for 5 hr, the sintering atmosphere was being air. The details of the method of the preparation have been given in earlier publication [2].

2.2. Measurements

The Seebeck coefficients were measured by a differential method [3,4] from room temperature to well beyond Curie temperature. The temperature gradient across the sample was measured using two pairs of copper constantan thermocouples. The sample was mounted on top of two well-separated copper blocks with silver paint. The temperature difference between two ends of the sample was kept at 10˚C throughout the measured temperature range. To eliminate the effects of the reference leads, the absolute thermoelectric power of Cu was subtracted from the measured thermoelectric voltage. A Keithly 181 nano voltmeter was used for the voltage measurements.

The thermoelectric power or Seebeck coefficient (S) was calculated using the relation

where ∆E is the thermo e.m.f. produced across the sample due to the temperature difference ∆T. The electrical conductivity of the samples has been measured at room temperature by two-probe method [5] using a Kiethly electrometer model 6517A. The electrical conductivity (σ) of the Ni-Mg ferrites under investigation has been computed using the formula,

where I is the current passing through the specimen in amperes, V is the voltage applied to the specimen in volts, t is the thickness of the sample in centimeters and A denotes the area of sample in square centimeters. The Curie temperature T_c of the sample was determined by gravity method [6].

3. Results and Discussions

3.1. Composition Dependence of Lattice Parameter

X-ray diffraction patterns for all ferrites under investigation have been obtained using Cuk_α radiation of Rigaku DMAX II X-ray diffractomer. The lattice parameter of the mixed ferrites was calculated from d-spacing values. The variation in the lattice parameter with magnesium content is shown in Figure 1. It can be seen from the figure that the lattice parameter varies linearly with the magnesium ion content. A similar linear variation has also been observed in Zn-Mg-Cu [7] and Co-Zn [8] ferrites.

3.2. Composition Dependence of Seebeck Coefficient

The value of Seebeck coefficient (S) at 310 K computed from the measured values of thermo-e.m.f. is given in the Table 1. It can be seen from the table that the sign of Seebeck coefficient is negative for the Ni-Mg ferrites. On the basis of its negative sign, the magnesium substituted Nickel ferrites have been classified as n-type semiconductors. It can also be seen from the table that the value of Seebeck coefficient varies from −189 to −286 μV/K as the magnesium content increases from 0.2 to 0.8 moles.

3.3. Temperature Dependence of Thermoelectric Power

The variation of the thermoelectric power (S) with temperature is shown in the Figure 2. It can be seen from the figure that the value of thermoelectric power for all Ni-Mg ferrites increases with increasing temperature up to a certain temperature, which is designed as T_s (K). However, beyond this temperature the value of S starts decreasing with increasing temperature. The negative value of thermoelectric power found over the entire temperature range studied shows that the majority charge carriers are electrons. A similar variation of thermoelectric power with temperature was observed in Mg-Cr [9] and Lithium ferrites [10]. The values T_s(K) for each composition are given in the Table 1. The Curie temperature T_c(K) values are also included in table for comparison. It can be seen from the table that the values T_s (K) and T_c(K) are close, thereby indicating that the change in the behavior of the thermoelectric power with temperature may be due to magnetic transition, where the material becomes the paramagnetic. Thus, it is clear that in case of Ni-Mg ferrites the non-magnetic property under study (thermoelectric power) is exhibiting a clear-cut and well-defined transition at the Curie temperature, like the magnetic property such as susceptibility, permeability and spontaneous magnetization. The value of thermoelectric power is maximum at T_c(K). This clearly indicates

that the magnetic ordering has a marked influence on the thermoelectric property of these ferrite samples.

The variation of the Curie temperature T_c(K) with magnesium composition is shown in Figure 3. It can be noted from the figure that the value of T_c(K) decreases with increase of magnesium content. The decrease of Curie temperature with increase of magnesium content can be explained on the basis of the number of magnetic ions present in the two sub-lattices and their mutual interactions. As Fe³⁺ ions are gradually replaced by magnesium ions, the number magnetic ions being decrease at both sides, which also weakens the strength of AB exchange interactions of the type Fe_A³⁺-O^2–-Fe_B^3–. Thus the thermal energy required to offset the spin alignment decrease, thereby decreasing the Curie temperature. A similar decrease of the T_c(K) with the composition was also observed by Zaki [11] in case of Mg-Zn ferrites and Ravinder and Latha in Mn-Zn [12] ferrites.

3.4. Composition Dependence of Electrical Conductivity (σ), Carrier Concentration (n) and Mobility (μ)

Table 2 gives the experimental values of electrical conductivity and Seebeck coefficient values at room temperature. The carrier concentration for these ferrites was