Nanosize dependent electrical and magnetic properties of NiFe 2 O 4 ferrite

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1 Indian Journal of Pure & Applied Physics Vol. 50, October 2012, pp Nanosize dependent electrical and magnetic properties of NiFe 2 O 4 ferrite Sukhdeep Singh 1, N K Ralhan 1, R K Kotnala 2 & Kuldeep Chand Verma 3 * 1 Department of Chemistry, Eternal University, Baru Sahib, (H.P.) , India 2 National Physical Laboratory, New Delhi, India 3 Department of Physics, Eternal University, Baru Sahib, (H.P.) , India * kuldeep0309@yahoo.co.in; deepsukh.singh@gmail.com Received 29 December 2011; accepted 16 July 2012 The effect of nanosize particles on dielectric and magnetic properties of NiFe 2 O 4 (NF) ferrite has been studied. NF nanoferrites were prepared by a chemical combustion route followed by annealing temperatures from 400 to 700 C. The X-ray diffraction (XRD) shows minor amorphous behaviour of NF at 400 C. However, spinel ferrite structure has been observed with higher annealing in the temperature range C. The average particles size is measured using Debye- Scherer s relation. The particle's size is also measured by transmission electron microscopy and the average value is at nm scale. Room temperature dielectric properties viz.; dielectric constant ( ) and loss (tan ) for all the specimens have been studied as a function of applied frequency in the range 100 Hz-20 MHz. These studies indicate that the values of and tan depend on the size of particles. The cole-cole plot of impedance spectroscopy has been used to find the grain size and boundary effect on electrical properties of NF nanograins. The magnetic measurements show the value of saturation magnetization (M s ) is 0.003, 44.95, 38.83, emu/g, respectively, observed at 400, 500, 600 and 700 C. Keywords: Nanostructures, Chemical synthesis, Electron microscopy (TEM), Electrical properties, Magnetic properties 1 Introduction The synthesis of nanosized magnetic oxide particles, such as spinel nanoferrites of the type MFe 2 O 4 (M is a divalent metal cation), is intensively investigated in terms of their applications in highdensity magnetic recording media and magnetic fluids. These materials are also largely used in electric and electronic devices and in catalysis. In order to improve sinterability and magnetic properties, the investigation of alternative, non-conventional synthesis methods to obtain ferrites in the form of nanostructured powders is the current subject 1,2. It is well known that the method of preparation plays a very vital role in determining the chemical, structural and magnetic properties of spinel ferrites 3-6. Nickel ferrite, NiFe 2 O 4 (NF) is an important member of the spinel family and it is found to be the most versatile technological materials suited for high-frequency applications due to its high resistivity 7. In the bulk state, this material possesses an inverse spinel structure, in which tetrahedral (A) sites are occupied by Fe 3+ ions and octahedral [B] sites by Fe 3+ and Ni 2+ ions. It exhibits ferrimagnetism that originates from the antiparallel orientation of spins on [A] and [B] sites. Spinel ferrites are good dielectric materials and they have wide applications ranging from microwave frequency to radio frequency. The dielectric properties of these ferrites are very sensitive to the method of preparation and sintering condition. NF ferrite is the most interesting one because of its high resistivity and low eddy current losses. Several investigators have explored the ferrites in the nano domain extensively as they exhibit very interesting magnetic properties at such low grain sizes 8. The dielectric behaviour of NF ferrites in the bulk form has been extensively studied 9,10. Verma et al 9. have prepared NF ferrite using citrate precursor method and observed that the permittivity loss is of the order of , which is lower by one to two orders of magnitude compared with the values normally reported for ferrites produced by conventional ceramic method 11. Recently, spinel ferrites have been shown to exhibit interesting electrical conductivity and dielectric properties in the nano crystalline form compared with that of the micrometer size grains 12,13. Nanostructure processing provides a new opportunity to fabricate novel magnetic materials with new functionalities. Among various methods of fabricating ferrite nanoparticles, the chemical combustion technique appears as an attractive route which

2 740 INDIAN J PURE & APPL PHYS, VOL 50, OCTOBER 2012 provides mono disperse particles showing surprisingly good crystallinity and enhanced magnetic characteristics. In the present study, the structural, microstructural, dielectric and magnetic behaviour to the effect of nano size particles with change in annealing temperature, have been reported. The nano size effect is studied by grains and grains boundaries using colecole model of impedance spectroscopy. 2 Experimental Details NF was prepared by the chemical combustion technique. All the reagents used were of analytical grade. Nickel nitrate, ferric nitrate, urea and ethylene glycol were used as the starting materials. These materials were mixed in desired stoichiometric ratio with heating rate increasing from room temperature to 70 C. A self ignition process has been taken place about min. The resulting material was heated at 400, 500, 600 and 700 C for 5h. The phase structure and nano-behaviour of the specimens were carried out by X-ray diffraction (XRD) using X-Pert PRO system. The microstructures were analyzed by transmission electron microscopy (TEM) using Hitachi H The magnetic measurements were carried out by using vibrating sample magnetometer (VSM-735). The dielectric properties and impedance spectroscopy in the frequency range 20 Hz-20 MHz have been studied using an impedance analyzer (Wayne Kerr 6500B). The electrical measurements were carried out on pellet specimen (sintered at 1000 C/5h). 3 Results and Discussion Figure 1 shows the XRD pattern of NF samples taken after their annealing at various temperatures ranging from 400 to 700 C. As seen, NF with the spinel structure has been formed at 400 C with minor impurity peaks. However, a polycrystalline spinel phase is formed with 500 C. It is noted that a small amount of -Fe 2 O 3 phase had also been formed with 600 and 700 C. The peaks at 2 = 30.48, 34.99, 37.48, 42.58, 48.02, and attributed to (220), (311), (222), (400), (331), (422) and (511) reflections of the cubic structure of the spinel with Fd3m space group. The lattice parameter (a) was calculated, i.e, a(å) ~ 8.152, 8.154, and 8.161, respectively, for NF with 400, 500, 600 and 700 C. It was found that there is no significant change in the lattice parameter values of the samples thermally treated at various temperatures. Thus, independent of Fig. 1 XRD pattern of NF nanoferrites with annealing temperature the annealing temperature, the a values of NF samples with various crystallite sizes are close to the lattice parameter of bulk NF (8.170Å). The broad XRD line indicates that the ferrite particles are nano-size. The average crystallite sizes for the most intense peak [(311) plane] were calculated from the XRD data using Scherer formula are 16, 17, 21 and 25 nm, respectively, for NF with 400, 500, 600 and 700 C. It is observed that these patterns reveal a cubic spinel ferrites phase with good crystallinity and broad peaks due to smaller nano particle size. Figure 2 shows the micrograph for NF samples. The micrographs show uniform grains with similar shapes and sizes in each specimen except with 400 C of little amorphous behaviour. It is seen that as annealing temperature increases, the grain size increases. The values of particles size is 15, 17, 22 and 27 nm, respectively, with 400, 500, 600 and 700 C. The magnetization hysteresis loops recorded at room temperature for the NF with different annealing temperatures are shown in Fig. 3. The measured values of the saturation magnetization (M s ), 0.003, 44.95, and emu/g, remanent magnetization (M r ) 0.001, 9.8, 7.2 and 5.2 emu/g, and coercive field (H c ) 284, 130, 99, 131 Oe, respectively,

3 SINGH et al.: ELECTRICAL AND MAGNETIC PROPERTIES OF NiFe 2 O Fig. 2 TEM images (a) 400 C, (b) 500 C, (c) 600 C and (d) 700 C for NF ferrites Fig. 3 M-H hysteresis for NF ferrites for NF with 400, 500, 600 and 700 C. Magnetic moment per unit formula in Bohr magnetron ( B ) was calculated from saturation magnetization of hysteresis loops by using relation 14 : B = M M s /5585, where M is the molecular weight, M s is saturation magnetization (emu/g) and 5585 is magnetic factor. The values of B for NF are , 1.88, 1.62 and 1.89, respectively, with 400, 500, 600 and 700 C. It is observed that NF with 500 C have similar values of M s and H c and B than with 600 and 700 C but large M r. Generally, the grains size increases with increasing annealing temperature and shows higher magnetization. But the occurrence of magnetic behaviour in our specimens of NF may be due to two reasons: Firstly, related with drastic nickel loss at higher temperatures (extra peaks seen at 600 and 700 C annealing in XRD). Secondly, due to the occurrence of superparamagnetic state when the particles size lies in nano scale 15. The superparamagnetic behaviour is dominant when the size of nanograins is large. The superexchange interactions between the magnetic cations in spinel ferrites are antiferromagnetic. But, the ferrimagnetic order arises since the intersublattice exchange is stronger than the intrasublattice exchange. As the particle size decreases, the coercivity increases to reach a maximum at a threshold particle size, which could be characteristically described as transformation from multidomain nature to single domain nature, and then decreases. Figure 4 shows the frequency dependence of dielectric constant ( ) and loss factor (tan ) [inset] for all the temperatures of different grains sizes. The dielectric constant decreases with the grain size reduction. Usually the electron exchange between Fe 2+ and Fe 3+ ions in octahedral sites, which results in local displacement of electrons in the direction of electric field, determines electric polarization in spinel ferrites. The variations in the dielectric constant of ferrites are mainly attributed to the variations in the concentration of Fe 2+ ions Hence, polarization and dielectric constant are expected to increase with the concentration of Fe 2+. Moreover, there are also reports 19,20 of a decrease in dielectric constant with a decrease in particle size. In the present studies, for the lower grain size (16 nm), there is a migration of some of the Fe 3+ ions from octahedral [B] to tetrahedral (A) sites, which is clearly evident from the in-field Mössbauer studies 20. Therefore, the dielectric constant is lower for the 400 C annealed (16 nm) sample compared to that of the as-prepared sample 700 C (25 nm). The values of are 16, 27, 38 and 42 with frequency of 1 MHz, and dispersionless dielectric response up to 5, 17, 10 and 3 MHz, respectively, for NF with 400, 500, 600 and 700 C. The dielectric

4 742 INDIAN J PURE & APPL PHYS, VOL 50, OCTOBER 2012 constant normally should decrease monotonically with the frequency in ferrites, because the electronic exchange between Fe 2+ Fe 3+ ions cannot follow the alternating electric field beyond a certain critical frequency. In the present sample, shows initially decrease with increase in the frequency. The presence of Ni 3+ and Ni 2+ ions in B sites gives rise to p-type carriers which also contribute to the net polarization in addition to the n-type carriers. However, the contribution of the p-type carriers should be smaller than that from the n-type carriers with an opposite sign. Since the p-type carriers have a lower mobility than the n-type carriers, the contribution to polarization from the former will decrease more rapidly even at low frequencies than the latter. As a consequence, the net contribution will increase initially and then decrease with frequency as observed in the present samples. Rezlescu 16 has also reported a similar behaviour in the case of Cu-Ni ferrites. Fig. 4 (inset) shows the plot between dielectric loss (tan ) and frequency. The value of tan decreases with frequency for all the grain sizes. The decrease in tan takes place when the jumping rate of charge carriers lags behind the alternating electric field beyond a certain critical frequency. High temperature annealing leads to the escape of Fe 2+ ions from the lattice, which results in greater structural imperfections and high dielectric losses. In the present study, tan is only of the order of 10 3 at lower annealing of 500 and 600 C and 10 1 at 700 C. However, tan shows large loss at 400 C due to some amorphous behaviour as seen in the XRD and TEM. In this nanostructured NF it is found that the conductivity increases with grain size reduction due to oxygen vacancy conduction 21 and this explains the observed increase in tan with grain size reduction. The impedance spectroscopy (IS) is a very convenient and powerful experimental technique that enables us to correlate the properties of a material with its microstructure and helps to analyze and separate the contributions from various components (i.e., through grains, grain boundary, interfaces, etc.) of polycrystalline nanoferrites over wide frequency range. The high-frequency semicircular arc can be attributed to the contributions from the bulk materials (grains), arising due to a parallel combination of bulk resistance and bulk capacitance. The low frequency semicircular arc can be attributed to the contributions of grain boundary, arising due to a parallel combination of grain boundary resistance and capacitance 22. Therefore, for grains size effect on electrical and magnetic properties of NF ferites, Fig. 5 is the plot of Z versus Z (Nyquist or Cole-Cole plots) taken over a wide frequency range (20 Hz to 20 MHz) at temperature of 500 K. This temperature and frequency were selected as semicircles are formed in the Nyquist plots. All the semicircles exhibit some depression instead of a semicircle centered on the X-axis. Such behaviour is indicative of non-debye type relaxation and it also manifests that there is a distribution of relaxation time instead of a single relaxation time in the material 23. The values of resistance of grains (R g ) and boundaries (R gb ) have been obtained from the intercept of the semicircular arcs on the real axis (Z ). The values of resistance of grain boundaries, R gb is higher than grains, R g indicates that the effect of grain boundaries is Fig. 4 Frequency dependent dielectric constant ( ), tan (inset) of NF specimens Fig. 5 Cole-Cole plots

5 SINGH et al.: ELECTRICAL AND MAGNETIC PROPERTIES OF NiFe 2 O dominant on electrical and magnetic properties when the size of nanoparticles is quite small. 4 Conclusions NF nanoparticles were prepared by a chemical combustion route using ethylene glycol as solvent. These ferrites were annealed at different temperatures and shows different size of nano particles. XRD diffraction shows the spinel phase with Fd3m space group and the average values of particles size using Scherer formula are 16, 17, 21 and 25 nm, respectively, with 400, 500, 600 and 700 C. These values of particles size are consistent with those measured by the TEM. The values of saturation magnetization depend upon size of nanoparticles and show better results at low annealing of 500 C. The values of are 16, 27, 38 and 42 with frequency of 1 MHz, and dispersionless dielectric response up to 5, 17, 10 and 3 MHz, respectively, for NF with 400, 500, 600 and 700 C. The results of magnetic and electrical measurements are explained on the basis of nanosized by grains and boundary effect by cole-cole model of impedance spectroscopy. Acknowledgement The authors would like to acknowledge the Sophisticated Analytical Instrumentation facility (SAIF) of Punjab University, Chandigarh, India, for XRD and TEM measurements. The authors are also grateful to Dr R K Kotnala, National Physical Laboratory, New Delhi, India, for electrical and magnetic measurements. References 1 Calvin S, Shultz M D, Glowzenski L & Carpenter E E, J Appl Phys, 107 (2010) Venkataraju C, Sathishkumar G & Sivakumar K, J Alloys Comp, 498 (2010) Azadmanjiri J, Ebrahimi S A S & Salehani H K, Ceram Int, 33 (2007) Cheng Y, Zheng Y, Wang Y, Bao F & Qin Y, J Solid State Chem, 178 (2005) Yang J M, Tsuo W J & Yen F S, J Solid State Chem, 145 (1999) Aphesteguy J C, Bercoff P G & Jacobo S E, Physica B, 398 (2007) Albuquerque A S, Ardisson J D, Macedo W A A, Lopez J L, Paniago R & Persiano A I C, J Magn Magn Mater, 226 (2001) Sepelak V, Baabe D, Litterst F J & Becker K D, J Appl Phys, 88 (2000) Verma A & Dube D C, J Am Ceram Soc, 88 (2005) Mohan G R, Ravinder D, Reddy A V R & Boyanov B S, Mater Lett, 40 (1999) Okamura T, Fujimura T & Date M, Phys Rev, 85 (1952) Dias A & Moreira R L, Mater Lett, 39 (1999) Ponpandian N, Balaya P & Narayanasamy A, J Phys Condens Matter, 14 (2002) Smith J & Wijn H P J, Ferrites, John Wiley and Sons, New York, Verma K C, Ram M, Kotnala R K, Bhatt SS & Negi N S, Indian J Pure & Appl Phys,48 (2010) Rezlescu N & Rezlescu E, Phys Status Solidi A, 23 (1974) Irvine J T S, Huanosta A, Velenzuela R & West A R, J Am Ceram Soc, 73 (1990) Koops C G, Phys Rev 83 (1951) Kaur J, Gupta V, Kotnala R K & Verma K C, Indian J Pure & Appl Phys, 50 (2011) (in press). 20 Dube D C, J Phys D, 3 (1970) Sivakumar N, Narayanasamy A, Ponpandian N, Greneche J M, Shinoda K, Jeyadevan B, Tohji K, J Phys D, 39 (2006) Li Y M, Liao R H, Jiang X P & Zhang Y P, J Alloys Compd, 484 (2009) Verma K C, Kotnala R K, Verma V & Negi N S, Thin Solid Films, 518 (2010) 3320.