1. Introduction

There is an increasing interest in magnetic ferrite materials because of their broad applications in numerous technological fields including ferrofluids, drug delivery and high-density information storage^[1]. To meet the requirements for their use in high-density recording media, the particles must not only have suitable saturation magnetization, remanence, coercivity, high blocking temperature but also reduced size and uniform shape. Among other materials, the CoFe$_{2}$O$_{4}$ ferrite system has received increased awareness for its potential use in high-density recording media. This is due to its significant properties such as strong anisotropy, high saturation magnetization and coercivity along with good mechanical hardness and chemical stability^[2]. Paulsen et al.^[3] reported that the Curie temperature and magnetostriction of substituted Co ferrites are tunable by adjusting the suitable substitution level. Such substituted Co ferrites with optimized material properties are used in magneto–mechanical stress sensors. It has been shown that Mn substituted cobalt ferrites are excellent candidates for stress sensors due to a large magnetomechanical effect and high sensitivity to stress. Palamaru et al.^[4] reported the preparation and characterization of CoMn$_{x}$Fe$_{2-x}$O$_{4}$ by the low temperature combustion method. They demonstrated an increase in the saturation magnetization with the increase in Mn content; also thermal treatment induces a decrease in the coercive field. Kim et al.^[5] investigated the structural and magnetic properties of Mn substituted cobalt ferrite synthesized by the sol gel method. They found that Co–Mn powder fired at 673 and 723 K had a spinal cubic structure and a mixed nature of paramagnetic and ferromagnetic phases. A Mossbauer spectrum shows that the Fe ions are on both A (tetrahedral) and B (octahedral) sites in ferric high-spin states. Ji et al.^[6] studied Ni–Mn–Co ferrite with equal permittivity and permeability of 7–9, as well as a sufficiently low dielectric and magnetic loss tangent less than 0.001 and 0.01 below 200 MHz, respectively.

Various methods have been developed to prepare the CoFe$_{2}$O$_{4}$ ferrite system such as sol–gel^[5], micro-emulsion with oil in water micelles^[7] or reverse micelles^[8], aqueous co-precipitation and calcination^[9] and combustion^[10]. Despite these different routes, the preparation of Co-ferrite systems suitable for high-density recording is still a challenge. Indeed, the nanomaterials often show a superparamagnetic behavior at RT and a very low saturation magnetization. At present enormously magnetostrictive and resistive ferrites are becoming a significant issue for magnetoelectric (ME) composites which illustrate the ME effect. Substitution of different metals such as Zn, Mn, etc is useful to enhance ME sensitivity.

Therefore an effort has been made to synthesize the nanoparticles of a Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$ (0 $\leqslant$ $x$ $\leqslant$ 0.5) system by using a resourceful self-propagating combustion route. In this article we report the synthesis of a Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$ (0 $\leqslant$ $x$ $\leqslant$ 0.5) system by an efficient self-propagating auto combustion route. At the outset, the objective of the present study is to study the influence of Mn substitution on structural, morphological, electrical, dielectrical and magnetic properties. Also, the thermomagnetic behavior and Curie temperature of the developed system is studied.

2. Experimental

The Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$ (0 $\leqslant$ $x$ $\leqslant$ 0.5) ferrite system was prepared by the combustion of a redox mixture containing stoichiometric amounts of corresponding metal nitrates and glycin in a Pyrex dish. The equivalence ratio, ${\mathit{\Phi }_{\rm{e}}}$ (O/F) of the redox mixture for the combustion is calculated by using total oxidizing and reducing valencies of the oxidizer (O) and of the fuel (F), which serve as numerical coefficients so that ${\mathit{\Phi }_{\rm{e}}}$ (O/F), becomes unity and the heat released is at its maximum^[11]. According to the principles used in propellant chemistry, the oxidizing and reducing valencies of various elements are considered as follows: C $=$ 4, H $=$ 1, O $=$ $-2$, N $=$ 0, M $=$ 2, 3, etc. Thus, typically for ferrites i.e. M$^{2+}$Fe$^{2+}_3$O$_{4}$ the oxidizing valency of a divalent metal nitrate, M(NO$_{3})$$_{2}$ becomes $-10$; for trivalent Fe(NO$_{3})$$_{3}$, it is $-15$, which should be balanced by the total valences in the fuel; glycine (H$_{2}$NCH$_{2}$COOH), which add up to $+9$. Hence the stoichiometric composition of the redox mixture, in order to release the maximum energy for the reaction, requires $-40$ $+$ 9$n$ $=$ 0 or $n$ $=$ 4.44 mol of glycine. A mixture of cobalt nitrate, manganese nitrate and ferric nitrate in stoichiometric molar proportion was melted in a Pyrex dish by heating at 80 ℃ . Glycine was added to the melt and the slurry was introduced into a furnace preheated to 400 ℃ . After evaporation of the water content, the mixture frothed and ignited to combust with a flame, giving a voluminous and foamy Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$. After formation of as synthesized powder, the pellets were prepared by pressing in a die of 1 cm diameter and 0.1 mm thickness with the help of a hydraulic press by applying a pressure of 5 ton/in to form the pellets using PVA as a binder. The pellets were finally sintered at 400 ℃ in the presence of air.

The structural analysis of powders was made by using an X-ray diffractometer (Siemens D5000) using Cu-K$\alpha$ radiation ($\lambda$ $=$ 1.5406 Å). The surface morphology was observed using a JEOL JSM-6360 scanning electron microscope (SEM), Japan. The DC resistivity with respect to temperature was measured using a Keithly electrometer-6514. The frequency dependent dielectric permittivity ($\varepsilon')$ and loss tangent (tan $\delta$) in the range from 20 Hz to 1 MHz were studied using a precision LCR meter bridge (HP 4284 A). The hysteresis loops were obtained by using high field hysteresis loop tracer (Magneta B–H loop tracer).

3. Results and discussion

3.1 X-ray diffraction

Figure 1 shows XRD patterns of Co-ferrite samples deposited at various Mn substitutions ($x =$ 0, 0.1, 0.2, 0.3, 0.4 and 0.5). All the samples are polycrystalline and show a single phase spinel cubic crystal structure. It shows characteristic reflections of a ferrite phase with strongest (311) orientation^{[12, 13]}. The XRD patterns are indexed using JCPDS card Nos. 22-1086 and 74-2403. The X-ray density ($\rho_{\rm x})$ and measured density ($\rho_{\rm m})$ decreases while porosity increases with Mn content. The linear increase in lattice constant with Mn content may be attributed to the replacement of smaller Co$^{2+}$ (0.78 Å) by the larger Mn$^{2+}$ (0.83 Å) ions in the Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$ system obeying Vegards's law^[14] as shown in Table 1. The average crystallite size is estimated using Scherrer's formula as shown in Table 1. The average crystallite size decreases with Mn content up to 0.3 and then increases.

3.2 Scanning electron microscopy (SEM)

Figure 2 shows scanning electron micrographs of the Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$ with $x =$ 0.2, 0.3, and 0.4 respectively. The Mn substitution induces randomly organized agglomerated grains with different shapes and sizes with slight inter-granular pores. It is observed that the average grain size goes on decreasing with increase in Mn content. It shows significant enhancement in crystallinity. Average grain size varies are in the range of 50–100 nm.

3.3 Electrical properties

Figure 3 shows variation of DC electrical resistivity with temperature. It is seen that the electrical resistivity decreases with an increase in the temperature semiconducting nature of samples. This is due to the increase in the thermally activated drift mobility of charge carriers according to the hopping conduction mechanism. The electrical resistivity decreases with Mn substitution up to 0.3 and then increases. It might be due to a large number of Mn ions occupying A-sites and forcing the Fe ions at A-sites to migrate to B-sites. The hopping probabilities between Mn$^{3+}$ and Mn$^{2+}$ ions might also become appreciably high at higher Mn concentration^[15]. The activation energy has been calculated by using the Arrhenius relation,

$\rho =\rho _0 \exp {\frac{\Delta E}{kT}},$

(1)

where $\Delta E$ is the activation energy, $k$ the Boltzmann constant and $T$ the absolute temperature. The activation energies are calculated from the slopes of the paramagnetic and ferromagnetic region and presented in Table 2. The change in slope is observed in ferrite samples due to variation in the Curie temperature^[16]. It is evident that the lower activation energy in the ferromagnetic region is attributed to the phase transition or impurity phases, while the change in activation energy is attributed to the change in conduction mechanism^[17]. We know that from thermodynamics the magnetic transition is a second-order transition and is accompanied by an expansive change in volume^[18]. It is observed that the Curie temperature increases from 410 to 510 K as Mn composition increases from $x =$ 0 to 0.5. The observed variations in Curie temperature have been explained on the basis of the super exchange interactions among the tetrahedral and octahedral cations in the spinel lattice. The net magnetic moment of the spinel lattice is the difference between the magnetic moments of the B and A sub-lattices. Hence, it is therefore expected that a greater amount of thermal energy will be required to off-set the influence of exchange interactions^[19].

3.4 Dielectric properties

Figure 4 shows the variation of dielectric constant with frequency at room temperature for the prepared Co-ferrite system. From the figure, it is clear that the dielectric constant ($\varepsilon'$) decreases abruptly at lower frequencies and remains constant at higher frequencies showing the dispersion behavior of the dielectric constant for lower frequencies. This dielectric dispersion is attributed to Maxwell^[20] and Wagner^[21] types of interfacial polarization, which is in agreement with Koop's phenomenological theory^[22]. The large value of dielectric constant is associated with space charge polarization and an inhomogeneous dielectric structure. These inhomogeneites are impurities, grain structures and pores. The dielectric constant varies with applied frequency due to charge transport relaxation time. In ferrites at low frequency, rotation of Fe$^{2+}$ $\leftrightarrow$ Fe$^{3+}$ dipoles may be visualized as the exchange of electrons between the ions so that the dipoles align themselves in response to the alternating field giving the maximum polarization. The polarization decreases with increase in frequency and reaches a constant value due to the fact that beyond a certain frequency of external field the electron exchange between Fe$^{3+}$ $\leftrightarrow$ Fe$^{2+}$ ions cannot follow the alternating field^[23].

The variation of dielectric loss with frequency is shown in Fig. 5. At lower frequencies tan $\delta$ is large and it decreases with increasing frequency. The tan $\delta$ is the energy dissipation in the dielectric system, which is proportional to the imaginary part of dielectric constant. At higher frequencies the losses are reduced and the dipoles contribute to the polarization^[24]. The loss factor curve is attributed to domain wall resonance. At higher frequencies, losses are found to be low if domain wall motion is inhibited and magnetization is forced to change by rotation. It is clear from Figs. 4 & 5 that the dielectric constant and loss tangent decrease with an increase in Mn substitution.

To confirm the AC conduction mechanism, the variation of lg $\sigma_{\rm ac}$ with lg $\omega^{2}$ is studied (Fig. 6). The plots are observed to be almost linear indicating that the conduction increases with increase in frequency. The linearity of the plots confirm a small polaron mechanism of conduction. It is known that there are two types of polarons viz. small polarons and large polarons. In the small polaron model, the conductivity increases linearly with increase in frequency and in the case of large polarons, the conductivity decreases with an increase in frequency^[25]. The slight decrease in conductivity is attributed to conduction by mixed polarons.

3.5 Magnetic properties

Figure 7 shows the room temperature B–H hysteresis loop for a Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$ system. The variation of coercivity ($H_{\rm c}$), saturation magnetization ($M_{\rm s}$) and remnant magnetization ($M_{\rm r}$) with Mn content is shown in Table 3. It is found that saturation magnetization ($M_{\rm s}$) and remnant magnetization ($M_{\rm r}$) increase with Mn content up to $x =$ 0.3. The change in the magnetization due to Mn substitution might be due to a difference in the magnetic moment of the substituted ion on the A-site and B-site of the spinel Co-ferrite and a decrement in the magneto-crystalline anisotropy. A maximum magnetization of about 68.94 emu/gm is observed for the $x =$ 0.3 Mn composition and which might be the most suitable constituent phase for magnetoelectric (ME) composites. The coercivity (coercive force) decreases linearly with increasing Mn content from 1591 to 603 Oe. An initial increase in the saturation magnetization of Co$_{1-x}$Mn$_{x}$Fe$_{2}$O$_{4}$ at lower concentrations of Mn up to $x =$ 0.3, indicates that initially the Mn$^{2+}$ ions are substituted in the octahedral (B-site) and for higher concentration it occupies the tetrahedral (A-site) of the spinel lattice. The magnetic moment per formula unit in Bohr magneton is calculated using saturation magnetization (emu/gm)^[19]. It is seen that magnetic moment increases with Mn content up to $x =$ 0.3 attains maximum value 2.88 $\mu$B and then decreases for higher content (Table 3).

4. Conclusions

Nanoparticles of a Co–Mn ferrite system are successfully synthesized by following the combustion route. The XRD confirms the formation of a face centered cubic crystal structure. The dielectric behavior shows the electronic polarizability at higher frequencies due to space charge polarization. AC conductivity increases with an increase in the frequency due to the hopping mechanism of conduction. The magnetic study confirms that the saturation magnetization of cobalt ferrite increases with Mn content up to $x =$ 0.3 and decreases for higher Mn content. From this, it is concluded that the composition $x =$ 0.3 is the best suitable constituent phase for the magnetoelectric (ME) composite.

Acknowledgement: The authors are very grateful to the UGC-DSA-I, DST-PURSE and DST-FIST-Ⅱ programs for financial support.