Journal of Science: Advanced Materials and Devices (2018) 230e235 Contents lists available at ScienceDirect Journal of Science: Advanced Materials and Devices journal homepage: www.elsevier.com/locate/jsamd Original Article Dielectric, magnetic and magnetoelectric studies of lithium ferrite synthesized by solid state technique for wave propagation applications Ganapathi Rao Gajula a, *, Lakshmi Rekha Buddiga b, Chidambara Kumar K.N a, Arun Kumar Ch c, Madhavaprasad Dasari d a Department of Physics, GEBH, Sree Vidyanikethan Engineering College, Tirupati 517102, A.P., India Department of Chemistry, GEBH, Sree Vidyanikethan Engineering college, Tirupati 517102, A.P., India Department of Physics, Andhra University, Visakhapatnam 530 003, A.P., India d Department of Physics, GIT, GITAM University, Visakhapatnam 530 045, A.P., India b c a r t i c l e i n f o a b s t r a c t Article history: Received 14 March 2018 Received in revised form 21 April 2018 Accepted 22 April 2018 Available online 28 April 2018 The electric and magnetic properties of a lithium ferrite (LF) synthesized using the solid state reaction technique have been reported The XRD studies reveal the cubic nature, from which the crystallite size and the lattice constant are determined to be 16.84 nm and 2.847 Å repectively The FESEM confirms the coarseness in the samples with a low porosity Variations of the dielectric constant and dielectric loss with temperature at different frequencies have been studied The dielectric constant increases more steeply in the negative direction with increasing temperature beyond 500 C at 100 kHz The impedance plot exhibits almost complete semicircles at all temperatures, whose centers situated on the real axis This suggests that the sample obeys the Debye behavior The magnetic studies reveal a soft magnetic characteristic of LF The ME voltage coefficient decreases with increasing magnetic field and attains a constant value in the high magnetic field © 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) Keywords: Li0.5Fe2.5O4 XRD Dielectric constant Impedance spectroscopy Magnetic transition temperature ME coefficient Introduction Magnetic spinel ferrites have encapsulated the global market and seized the awareness of many researchers due to their enchanting and tropical electromagnetic properties [1] Due to a wide range of applications, the lithium ferrite (LF: Li0.5Fe2.5O4) is considered to be one of the most flexible ferrites The LF belongs to the group of soft materials and has potential applications such as computer memory chips, microwave devices, magnetic recording media, transformer cores rod antennas, radio frequency coil fabrication, many branches of telecommunication, electronic engineering [2e6], in the information storage, switching devices and phase shifters because of their excellent rectangular hysteresis loop characteristics [7] At room temperature, the LF exhibites high permeability and high saturation magnetization Thus, it is * Corresponding author E-mail address: ganapathi.gajula@gmail.com (G.R Gajula) Peer review under responsibility of Vietnam National University, Hanoi considered a highly potential material for applications from low to microwave frequencies In microwave frequencies, the LF is a useful material due to its high resistivity semiconductor, and low eddy-current losses [8,9] The electrical properties of LFs depend on the method of the preparation, its grain size, chemical composition and sintering temperature [10e12] Besides the high resistivity, the LF can work in high frequency devices thanks to its mechanical hardness, square loop properties and high Curie temperature [13,14] Indeed, the LF material has the highest Curie temperature of 670 C among all ferrimagnetic oxides [15] It is used in computer core industry [16] The high ratios of anisotropy to magnetostriction in the LF result in a lower stress-sensitivity of the remanence Depending on the distribution of lithium and iron ions in octahedral sites (B-sites) and the sintering temperature, the structure of the LF changes from the ordered phase (a) to the disordered phase (b) [17] Many research groups [18e21] have focused their attention on the synthesis of the LF because this material exhibited a positive dielectric constant A material exhibiting negative dielectric constant can be used in the application like wave propagation, electrolyte behavior, and biomembrane functions [22] Also, materials having very low https://doi.org/10.1016/j.jsamd.2018.04.007 2468-2179/© 2018 The Authors Publishing services by Elsevier B.V on behalf of Vietnam National University, Hanoi This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/) G.R Gajula et al / Journal of Science: Advanced Materials and Devices (2018) 230e235 231 magnetoelectric (ME) voltage coefficient and high dielectric loss shall be used in sensor based applications [23] We have already reported the X-Ray diffraction (XRD) and field emission scanning electron microscope (FESEM) analysis of the LF [24] In this paper, we discuss the variation of the dielectric constant and loss with temperature (at different frequencies of kHz, 10 kHz, 100 kHz, MHz and 10 MHz), the variation of dielectric constant and loss with frequency (at various temperatures of 30 C, 120 C, 220 C, 320 C and 420 C), the impedance spectroscopy, magnetization as well as the magnetoelectric voltage coefficient for this material Experimental LF was prepared using conventional solid state reaction method The compounds used for the preparation of LF are Li2CO3 (99%, Loba Chemie), Fe2O3 (98%, Loba Chemie) The Li2CO3 and Fe2O3 powders were ground together using an agate mortor The grinding process was carried out for 10 h to obtain a homogeneous mixture and distribution of the ingredients The sample was calcinated at 800 C for h After completion of the calcination process, the powder was again ground for h and then added Poly Vinyl Alcohol to the calcinated powder and grinded up to become a fine powder A die set of 10 mm diameter was used to transform powder into pellets The powder was placed on a die set and pressed by applying a pressure of tonnes for using a hydraulic press The strength of these pellets was increased by a sintering process at a temperature of 900 C for h The XRD measurements were carried out using Bruker D8 Advance X-Ray Diffractometer, FESEM with energy dispersive analysis of X-ray (EDAX) for morphology and quantitative elemental analysis of a sample was analyzed by Carlzeiss ultra55 The dielectric measurements were obtained from LCR Meter, Wayne Kerr Electronics Pvt Ltd., Model: 1J43100 The variation of magnetization with temperature (MT) of the samples was measured by Vibrating Sample Magnetometer (VSM) Quantum Design PPMS, Model 6000 and the magneto-electric voltage coefficient (aME) was measured with respect to the DC magnetic field (Hdc) by superimposing Oe AC magnetic field generated by Helmholtz coils at a frequency of kHz The output voltage of the composite was measured using SR 830 DSP lock-in amplifier We have characterized these sintered pellets, which will be discussed in the forthcoming sections Results and discussion 3.1 X-ray diffraction The XRD pattern of the LF is shown in Fig We have indexed the diffraction peaks of LF using JCPDS no 89-7832 & 88-06711 The XRD pattern of LF reveals the formation of a spinel cubic structure [20] The extra peaks pertaining to the impurities are not observed, which strongly confirm the high crystalline nature of the sample The lattice parameter of the LF has been calculated using a ¼ dhkl pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi h2 ỵ k2 ỵ l2 (1) The lattice parameter of the LF is 8.247 Å, which is equal to the earlier reported value [25] The average crystallites size of the LF has been calculated using Scherrer's relation D¼ kl bhkl cos q (2) It turns out that the average crystallite size of the LF is 16.84 nm Fig X-ray diffraction pattern of the synthesized LF 3.2 Morphological studies The EDAX spectrum and SEM micrograph of the LF are shown in Fig 2(a) and (b), respectively Fig 2(a) confirms the presence of elements Fe and O but Li element cannot be identified by the instrument due to its low atomic number Fig 2(b) shows that micrograph exhibits clear grains and clear grain boundaries with small pores The average grain size of the LF calculated using ImageJ software is 6.2491 mm The SEM confirms the coarseness in the sample with low porosity The larger grains observed here are due to an agglomeration of smaller grains These larger uneven grains can show important differences in calculating the polarization or the ME effect [26] 3.3 Dielectric studies 3.3.1 Temperature dependence of the dielectric constant The temperature dependence of the dielectric constant (30 ) of the LF at different frequencies (1 kHz, 10 kHz, 100 kHz, MHz and 10 MHz) is shown in Fig From Fig 3, it is clearly seen that the dielectric constant increases up to 550 C for all samples Beyond this temperature, the dielectric constant decreases with increasing the temperature A kink is observed at 550 C The nature of variation of dielectric constant with temperature at all frequencies except kHz is same till 500 C Also, in this temperature range, the dielectric constant of the LF decreases with an increase in the frequency The dielectric constant becomes negative and increases in the negative direction beyond 600 C and 500 C temperatures for the frequencies of 10 kHz and 100 kHz, respectively For the frequencies of MHz, 10 MHz, the material exhibits small negative dielectric constant beyond the temperature 600 C This may be due to the dimensional resonance, domain resonance, irregular grain boundaries In this case, the domain walls reduce dipole charges and then produce negative charges at high frequencies beyond a certain high temperature At the frequency of kHz, the dielectric constant shows a jumping behavior around 650 C As can be seen below, this may relate to the steep decrease of the dielectric loss Fig 4, however, it may also be connected to instrumental errors Note that, the negative dielectric constant was successfully explained by Jones et al [27] According to Jones et al., during relaxation process, holes are added to the material which recombine with free electrons in the dipole This recombination of holes with free electrons reduces the charge of the dipole and hence yields negative values of the dielectric constant Champness and Clark [28] claim that the negative values of dielectric constant arise from the inductive behavior of materials [29] and depend on 232 G.R Gajula et al / Journal of Science: Advanced Materials and Devices (2018) 230e235 Fig (a) EDAX spectrum, (b) FESEM micrograph of the LF Fig Temperature dependence of the dielectric constant at frequencies of kHz, 10 kHz, 100 kHz, MHz and 10 MHz for the LF the penetration of minority carriers Moreover, Martens et al [30] proposed a model based on the space charge carrier and the Poisson equation to explain the negative dielectric constant effect, caused by the distribution of relaxation times The materials with negative dielectric constant have attracted many research groups due to their applications in wave propagation, electrolyte behavior, and bio-membrane functions [22] 3.3.2 Temperature dependence of the dielectric loss The variation of the dielectric loss (tan d) with temperature at different frequencies is shown in Fig It is clearly seen from this figure that, initially, the dielectric loss of the LF increases slightly with increasing the temperature at all frequencies At kHz, the dielectric loss increases with increase in the temperature till 600 C Beyond 600 C, the dielectric loss reaches the maximum and then decreases steeply with increasing the temperature At the frequency of 10 kHz, a similar behaviour is observed beyond 510 C Here, however, the dielectric loss suddenly decreases to zero and then shows a negative dielectric loss with a minimum at 540 C It is also seen from Fig that the behavior of dielectric loss at frequencies of 100 kHz, MHz, 10 MHz is similar to that observed at 10 kHz, but with the much lower magnitudes As the frequency increases from kHz to MHz, the dielectric loss of the LF Fig Temperature dependence of the dielectric loss at different frequencies of the LF decreases which reaches a minimum value at 10 MHz This is considered to be caused by the domain wall resonance 3.3.3 Frequency dependence of the dielectric constant The variation of dielectric constant of the LF with frequency at different temperatures is shown in Fig We see from Fig that at 30 C, the dielectric constant decreases with increase in frequency up to 12.76 kHz, beyond that frequency, the dielectric constant reaches constant Also, we see from Fig that the dielectric constant of the LF exhibits a tendency to increase with increasing temperature Hence, at low frequency region, the dielectric constant of the LF shows the dispersion at all temperatures The observed behaviour of the dielectric constant can be explained on the basis of Koops theory [31] According to Koops theory, a high conducting grain is surrounded by non-conducting grain boundaries The grain boundaries are more effective at low frequencies than grains and grains are more effective at higher frequencies than grain boundaries Due to the large resistance of grain boundaries, the charge carriers produce space charge polarization As a result, the dielectric constant is larger at low frequency region [32] Furthermore, increasing the frequency, the charge carriers change their direction of motion due to the fact that this accumulation of charge at the grain boundary decreases which results in the decrease of dielectric constant The dielectric constant increases with increasing temperature in the low frequency region due to an G.R Gajula et al / Journal of Science: Advanced Materials and Devices (2018) 230e235 Fig Frequency dependence of the dielectric constant at different temperatures of the LF exchange of electron between Fe2ỵ and Fe3ỵ ions at octahedral sites is thermally activated [32] 3.3.4 Frequency dependence of the dielectric loss The variation of dielectric loss (tan d) of the LF with frequency at different temperatures is shown in Fig We see from Fig that the dielectric loss of the LF decreases with increase in frequency at all temperatures up to 12.76 kHz, beyond that frequency the dielectric loss reaches constant at all temperatures And also we see from Fig that the dielectric loss of LF is high in the low frequency region at all temperatures and at low frequency region the dielectric constant increases with increasing temperature At low frequency region, the dielectric loss of the LF exhibits dispersion between temperatures, the dielectric loss trend follows as a dielectric constant curve The dielectric loss is almost constant for the frequency at low temperatures, which might be due to the inability of the electric dipoles to respond to an applied electric field The LF ceramic could present an increased conductivity as well as an increased dielectric loss at low frequencies [33,34] 3.4 Impedance studies 3.4.1 Temperature dependence of the impedance The variation of impedance (Z0 ) of the LF with temperature from 30 C to 700 C at different frequencies of kHz, 10 kHz, 100 kHz, MHz and 10 MHz is shown in Fig At kHz frequency, the Fig Frequency dependence of the dielectric loss at different temperatures for the LF 233 Fig Variation of the impedance with temperature at different frequencies of the LF impedance increases with increasing temperature, reaches the maximum at 132 C Beyond this temperature, the impedance sharply decreases with increasing temperature till 300 C After that, the impedance (Z0 ) attains a constant value We also see from Fig that the impedance of the LF material decreases with increase in frequency and attains minimum value for frequencies of 10 kHz and 100 kHz The decrease in impedance at higher frequencies may be attributed to the hopping of electrons between localized ions [35] and the accumulation of charges at grain boundaries [36] 3.4.2 Frequency dependence of the impedance The variation of impedance (Z0 ) with frequency at different temperatures of the LF is shown in Fig The figure clearly shows that the impedance is rather high at low frequencies at 30 C temperature The impedance decreases monotonically with increasing frequency up to 25 Hz, beyond that frequency, the impedance reaches constant At higher temperatures, the amplitude of Z0 is strongly supressed However, a similar frequency dependence is still observed This behaviour suggests the presence of space charges which leads to a slow dynamic relaxation process [37] The constant value of the impedance, however, is a consequence of the creation of space charges as a result of which the barrier in the ceramic sample is lowered [38,39] 3.5 Nyquist plots Fig shows the plot between the real and imaginary parts of complex impedance Z0 and Z00 at various temperatures of the LF Fig Variation of the impedance with frequency in different temperatures for the LF 234 G.R Gajula et al / Journal of Science: Advanced Materials and Devices (2018) 230e235 Fig Variation of the real and imaginary parts of the impedance at different temperatures for the LF Fig 11 Variation of the magnetoelectric voltage coefficient with magnetic field for the LF magnetic switching circuits and magnetic amplifiers, etc Hence, the LF can be used in the above-mentioned applications 3.6.2 Magnetoelectric voltage coefficient The variation of ME voltage coefficient with the magnetic field of LF is shown in Fig 11 We see from Fig 11 that the value of ME voltage coefficient decreases with increasing low magnetic fields and attains a constant value in high magnetic fields, which might be due to the presence of strain in magnetic domains [18] The maximum value of the ME voltage coefficient is 25 mV cmÀ1 OeÀ1 at the lowest applied magnetic field, which is very much small as compared with Ni-based Metglas/PZT laminates and (Fe90Co10)78Si12B10-AlN thin film by Greve et al [45] and Huong Giang et al [46] Any material exhibing large ME voltage coefficients at low magnetic fields and high dielectric loss shall be used in sensor applications [45e47] Fig 10 Magnetic hysteresis loop (M-H) taken at room temperature for the LF From the nature of the plot we confirm that the behavior of the impedance plot obeys the ColeeCole formula [40] The impedance plot exhibits good semicircles All the semicircles are complete at all temperatures Moreover, all the semicircles have their centers situated on the real axis, which suggests that the sample obeys Debye behavior [41] We see from Fig that semicircular arc shifts towards the origin as temperature increases This implies that the resistivity of the sample decreases and the mobile ion has a surfaced electrode effect at higher temperatures Hence the conductivity of the LF increases with increasing temperature As temperature increases, the area under the curve decreases 3.6 Magnetic properties 3.6.1 MeH loop The variation of magnetization with a magnetic field (MeH loop) of the LF at room temperature is shown in Fig 10 It is clearly seen that the LF exhibits a ferromagnetic behavior The saturation magnetization, remanent magnetization and coercive field are 64.51 emu/g, 0.48 emu/g and 2.8 Oe, respectively The obtained saturation magnetization value is very close to the earlier reported values [42,43] The area of the hysteresis loop is very narrow Also, retentivity and coercive field are very small for the LF, suggesting that the LF is a soft magnetic material [44] so that it can be easily magnetized and demagnetized A soft magnetic material is used to make temporary magnets and also used in transformer cores, Conclusion The LF was successfully synthesized using the convention solid state technique The compound was formed in the spinel cubic structure with the lattice parameter of 8.247 Å The porosity in the sample was rather low At some frequencies, the negative dielectric constant increases with increasing temperature due to a distribution of relaxation times The dielectric constant of the LF gets a high value at low frequencies and at room temperature The dielectric constant decreases with increasing frequency The dielectric constant attains constant at the high frequency region The dielectric loss of the LF is high in the low frequency region at all temperatures and at the low frequency region the dielectric constant increases with increasing temperature At frequencies of 100 kHz, MHz and 10 MHz, the variation of the impedance with temperature is very small The impedance plot obeys the ColeeCole formula, and the semicircular arc shifts towards the origin as temperature increases The retentivity and the coercive field of the LF are very small, which suggest that the LF is a soft magnetic material The ME voltage coefficient decreases with increasing magnetic field and attains a constant value in a high magnetic field Thus, the LF material has rather low ME voltage coefficient and high dielectric loss This material exhibits high potential for applications like wave propagation, electrolyte behavior, bio-membrane functions and sensing devices Acknowledgments We thank Dr P.D Babu for extending his MeH measurements of UGC-DAE Consortium for Scientific Research, Mumbai center, R5- G.R Gajula et al / Journal of Science: Advanced Materials and Devices (2018) 230e235 shed, BARC, Mumbai e 400 085 The INUP, IITB, Bombay are gratefully acknowledged for extending FESEM and dielectric properties and impedance investigations References [1] G Aravind, 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H Lensoff, H.D Childress, Effects of lithium and oxygen losses on magnetic and crystallographic properties of spinel lithium ferrite, J Am Ceram Soc 53 (6) (1970) 304 [10] M.A El Hiti, Dielectric. .. Investigation of structural and magnetic properties of nanocrystalline manganese substituted lithium ferrites, J Solid State Chem 182 (2009) 3217e3221 [6] Vivek Verma, M Abdullah dar, Vibhav Pandey,... Journal of Science: Advanced Materials and Devices (2018) 230e235 Fig Variation of the real and imaginary parts of the impedance at different temperatures for the LF Fig 11 Variation of the magnetoelectric