Electrical, Dielectric and Magnetic Properties of Mg 2+ Doped Zn-Co-La Spinel Ferrites for High Microwave Frequency (5.7-13.4 GHz) Applications

In the present research work, Mg 2+ is inserted into Zn 0.4 Co 0.6-x Mg x Fe 1.9 La 0.1 O 4 (X = 0.00, 0.15, 0.30, 0.45, 0.60) soft ferrites prepared by the co-precipitation method. Structural, optical, vibrational, dielectric, electrical and magnetic properties were explored by XRD, SEM, UV, FTIR, SMU two probe, Raman, LCR and VSM methods respectively. XRD pattern revealed the formation of cubic spinel structure in all fabricated nanoferrites by the replacement of Co 2+ ions with Mg 2+ ions. UV-Vis also verified the substitution. The declining nature of optical band gap in the range of 2.65 – 1.85 eV were observed by UV-Vis proving the Mg 2+ ions replacement. Uniform spherical shaped nanoparticles were inspected by SEM investigation in form of microimages confirming the XRD outcomes. DC Electrical resistivity showed remarkable decreasing trend with the enhancement of Mg 2+ concentration from 4.61 x 10 9 to 5.39 x 10 6 Ω-cm as Mg 2+ is more conductive than Co 2+ . Five active phonon modes of Raman were examined by Raman spectroscopy. Dielectric parameters including dielectric losses and impedance demonstrated diminishing trends with the enrichment of Mg 2+ contents. Various magnetic fabricated spinel ferrite materials are highly acceptable in microwave devices and in electrical transformers to remove eddy current losses.


Introduction
In the modern era spinel ferrites are investigated for wide range of applications in the fields of electronics, electrical, magnetic, optoelectronics and electrochemical sciences [1]. The broad functions of these spinel ferrite materials have convinced the researchers. Size, shape, purity and magnetic stability of the ultrafine powder consisting of ferrite nanomaterials depends upon the processing conditions and synthesis method [2][3]. These soft ferrites can be used in different applications like magneto optical recording media and pulse power by varying the different parameters like low temperature annealing, doping, sintering at high temperature [4][5][6][7][8][9]. Ferrites retain the properties like lower dielectric fatalities, higher resistivity, larger Curie temperature, abatement cost and high macerating magnetization. Such characteristics may be improved with the addition of rare earth metals. Researchers have examined effects on structures and magnetic outcomes by accumulation of Sm, Gd, La, Ce and Y [10][11][12]. For different electrical and microwave properties lanthanum doped spinel ferrites have gained attention of researchers [12]. Electrical and magnetic properties are transformed by adding such rare earth metals in soft ferrites materials. Insertion of rare earth element (Re = La 3+ ) in spinel ferrites is potential candidate to bring modifications in electrical and structural behaviors of nanoferrites [11,13]. In zinc, cobalt and magnesium ferrites rare earth metals are inserted to develop characteristics of materials and to bring improvements in resistivity and permeability [1]. Substitution of magnesium with cobalt shows typical dielectric behavior and low dielectric losses [14]. Different synthesis techniques like spraypyrolsis [15], citrate gel [16], hydro thermal [17], sol-gel [18], microwave refluxing [19] and co-precipitation [20,21] can be implemented to synthesize the nanoferrite particles. Co-precipitation method is one of the best techniques to prepare nanoparticles as compared to others because of its' simplicity and low economic cost [22]. In current research work, we are studying Mg -Co nanoferrites Zn0.4Co0.6-xMgxFe1.9La0.1O4 (X = 0.00, 0.15, 0.30, 0.45, 0.60) fabricated by co-precipitation technique.
Structural, optical, electrical, vibrational, dielectric and magnetic properties of synthesized nanomaterials have been investigated. and La(NO3)3.6H2O4) were liquefied in de-ionized water distinctly according to the stochiometric ratios. The solutions were blended together with constant stirring at 60 ˚C. To keep pH 12 sodium hydro-oxide solution was augmented as a precipitous factor. Formed solution was kept in 80 ˚C pre-heated water for 12 hours. In order to eradicate impurities of precipitates the solutions were washed severally with de-ionized water and ethanol. Obtained crystals were placed in oven at 80 ˚C to make water free. With the help of granite mortar pestle precipitates were crushed into powder. Powder was positioned in muffle furnace at 800 ˚C for 4 hours and 10 minutes. The specimens were again grinded until to accomplish fine powder.

Synthesis Process
Schematic drawing of synthesis process is publicized in Fig. 1.

Characterization Techniques
Numerous characterizations techniques are employed to study structural, morphological, optical, electrical, dielectric, vibrational and magnetic properties.
Values of corporeal factors like crystalline size 'D', lattice constants 'a', inter planar spacing 'd' and volume of unit cell 'V' achieved commencing XRD data are given in Table 1.
Replacement of cobalt with magnesium signifies that cobalt contents have been successfully entered spinel structure as disclosed by crystallite size and lattice constant. The heights of prominent peaks are increasing with decreasing contents of Co 2+ . This indicates that cobalt atoms are entirely incorporated into zinc magnesium ferrites. Same varieties of results were found by A. C. Druc et al. [14]. Also, as the Mg 2+ contents increase the crystallite size decreases for X = 0.00 to X = 0.45. But when the cobalt content diminishes the crystallite size amplifies to maximum at (X = 0.60). This is because of the relocation of Fe 3+ ions as Fe 3+ ions shifts from octahedral site to tetrahedral site to keep the charge neutral [23]. The lattice constant is observed in the range of 8.433 -8.428 for X = 0.00 -0.60. Variations in lattice parameters are due to distinctions in ionic radii of both magnesium (0.72 A˚) and cobalt (0.74 A˚) [24]. Such variations in lattice constant were also discovered in Mg 2+ doped Co0.6-xZn0.4Fe2O4 soft ferrites synthesized by co-precipitation technique observed by Manpreet Kaur et al., [25]. Additionally, the highest peak (311) is moving to the right with the enhancement of Mg 2+ concentration as shown in the inset of Fig. 2. Table 1 also signifies that inter planar d-spacing and lattice constant are representing the same trends with the addition of magnesium contents. It means lattice parameter 'a' is lessening slightly with the substitution of Mg 2+ contents and magnesium has been successfully replaced with Co +2 ions. Such conduction is due to incorporation of lower ionic radius of Mg 2+ than the Co 2+ . Calculated values of X-ray density (dx) measured from XRD analysis and Bulk density (db) measured from nanoferrites pallets are illustrated in Table   1. dx is higher than db because of the appearance of a few empty spaces during synthesis and annealing stage in nanocomposites [26][27]. Furthermore, dx is decreasing gradually because lattice constant is decreasing. Porosity (P %) [28] is increasing steadily for X = 0.00 -0.60 as clear from Table 1. H. M. Zaki et al. [29]. have explained the same kinds of porosity percentages. Also bulk density is decreasing because atomic weight of Mg 2+ is lower than the Co 2+ results the increase in porosity as clear from Table 1.

Scanning Electron Microscopy
Obtained micro images from Scanning electron microscopy (SEM) of nanoferrites sintered at 800 ͦ C for all compositions are illustrated in Fig. 3 (ae). From the micrographs it can be observed the formation of dispersed spherical nanoferrites particles. It can also be revealed that particles size is decreasing with amplify in concentration of Mg 2+ contents for X = 0.00 to X = 0.45. While the particle size is detected higher for X = 0.60 specimen. Hence, the majority of nanoparticles are in agglomerated contour and homogeneous allocation due to growth of magnesium doping. Grains expansion occurs as of grain boundary distribution.
Activation energy for Mg 2+ is lower (grain boundary diffusion) than cobalt (lattice diffusion) [31][32]. The illustrated outcomes are well supported with the XRD results. Also declination in sharpening of peaks with doping verified the decline in particle size [25].

Ultraviolet Visible Spectroscopy Analysis
Various methods are employed to study optical properties, UV-Vis is one of the accepted methods as compared to others. Synthesized intermingles of Zn0.4Co0.6-xMgxFe1.9La0.1O4 (X = 0.00, 0.15, 0.30, 0.45, 0.60) spinel ferrites through de-ionized water was prepared. By plotting relation between (αhν)² and (hν) the optical band gap energy (Eg) is calculated for linear portion. Eg for direct band gap is computed as of direct transition. UV-vis optical spectrum is plotted in Fig. 4 along with band gap. Absorption spectrum of the given ferrite samples lie in the visible region. With the boost of Mg 2+ contents form X = 0.0 to 0.60 band gap energy (Eg) decreases as was expected. Eg decreases from 2.65 to 1.85 eV with the increasing concentration of Mg 2+ or decreasing concentration of Co 2+ . These band gaps ranges are strongly agreed to the results demonstrated by A. Godlyn Abraham et al. [33]. The reduction in Eg value is because of reduction in contents of Mg 2+ as Mg 2+ is more conductive than Co 2+ . The decreasing distances amongst valance band and conduction band means that nanoferrites are showing stronger semiconducting nature. Optical band gap energies (Eg) of soft nanoferrites are exposed in Table 3.   [35]. For cobalt ferrites these modes are well adjusted with reported literature [36]. Maximum position 692 cm -1 is allotted to Zn0.4Mg0.6Fe1.9La0.1O4 type of ferrites and is strongly agreed with present situation [37]. In Raman spectroscopy due to distinctions in ionic radii of magnesium, cobalt and iron significant distributions are observed for band distances. Results obtained from bond distances distribution illustrate similar double peak arrangement related to A1g and Eg.

Raman Spectroscopy
Magnesium has lower atomic mass than cobalt resulting the Raman band to shift higher frequency. Relation between positions of Raman active modes against Mg 2+ concentration is given in Fig. 6.

Electrical Properties
Two probes technique was applied by utilizing Keithley Electrometer Model 2401 for I-V characterization of nano-ferrites. Pallets of nano-ferrites were fabricated with particular dimensions. I-V curves in slope forms for all samples were measured between 323 -873 K temperatures. Calculated Electrical resistivity resistivity (ρ) for X = 0.00 -0.60 is in the range of 4.61*10 9 -5.39*10 6 ohm-cm as shown in Table 3 at 373 K temperature. Electrical resistivity is linked with arrangements of nanoferrite specimens and crystal configuration. Reducing in electrical resistivity with enhancement of magnesium contents is illustrated in Fig. 7. This reveals that with the augmentation of Mg 2+ concentrations, resistivity decreases.
All nanoferrites specimens have shown semi-conducting nature. Trends of electrical resistivity depending on variations in temperatures are explained in Fig. 8. From Fig. 8 graph advocates that from 323 -373 K temperature, log of resistivity increases and then start decreasing till to 873 K. In Fig. 8 curves are cracked into two regions knows as ferrimagnetic and paramagnetic [38]. The insertion of Zn-Mg-Co contents in fabricated ferrites boost quantity of divalent iron ions to enhance hopping between Fe iron ions and lead the reduction in DC resistivity. At 373 K temperature called transition temperature the kink appears. This indicates the transformations in magnetic arrangement commencing ferrimagnetism to paramagnetism during conductivity in soft ferrites. Segments positioned earlier than the transition temperature is ferrimagnetism whereas after this temperature is paramagnetism. Activation energies for ferrimagnetism (Ef) and paramagnetism (Ep) are illustrated in Table 3. Relation between activation energy (ΔE = Ep-Ef ) with increase in Mg 2+ contents is illustrated in Fig. 9.
Activation energy declines with the enhancement of Mg 2+ contents and reduction of Co 2+ contents for X = 0.00 -0.60. The calculated values of activation energies for all nanoferrites signify that hopping of electrons was likely responsible mechanism of conduction in ferrites.
Such decreasing activation energy behavior is well agreed with literature [39]. The hopping of electrons in same element with respective lattice sites leads the conduction procedure in ferrites [40]. M. R. Anantharaman et al, described that DC resistivity arises due to Fe 2+ ions [41]. Also the resistivity range demonstrated by the samples of the materials are highly applicable for telecommunication (10 8 Ω.cm) and low resistive devices like transformers to control eddy current losses [38][39][40][41][42][43]

Dielectric Properties
Dielectric characteristics are interdependent on synthesis situations including annealing temperature, duration, nature and amount of essences [44]. Fig. 11 gives the plot of dielectric losses (tan δ) and frequency. Dielectric loss demonstrates same trends like έ with rise of frequency. In small frequency region loss factor has greater values for nanoferrites because hopping frequency is larger than the applied frequency hence, loss is extreme as electrons track the field. Loss is least at higher frequency because electrons delay behind the field [45]. Fig.   12 stretches the relation amongst impedance (Z) and frequency. From the Fig. it can be observed that at low frequency for X = 0.00 impedance is higher and starts decline for the rest of the specimens with the increase of Mg 2+ contents. Results also perceived that as frequency enhances impedance of all nanoferrites attribute stable trends.

Magnetic Properties
Vibrating Sample Magnetometer called VSM was employed to examine magnetic performances of Mg 2+ doped Zn-Co-La nanoferrites. 1.5 T maximum field was applied at room temperature as exposed in M-H loop of Fig. 13   there is clear decrease in magnetization and in the overall magnetic behavior of the material.
As a matter of fact Co 2+ is ferromagnetic in nature whereas Mg 2+ is paramagnetic and this is clear evidence that with the more number of cobalt contents the material is supposed to manifest unlike having more number of paramagnetic counter parts. The experimental outcomes for Ms is 60.82 emu/g for X = 0.00 and this decreased to a minimum value of 25.94 emu/g for X = 0.60.  Table 4. Difference in magnetic moment of Mg 2+ (0 μB) and Co 2+ (3.88 µB) is caused in lessening nB and K [46]. Super exchange interactions are feeble in magnetic parameters of Mg 2+ inserted ferrites. Ferromagnetic orders dissimilarity commencing collinear to non collinear or triangle spin are bases in declining of these factors [47]. Magnetic characteristics are mainly attracted by ionic radius of doped materials and rest of cations in spinel ferrites.
Hence bond lengths and ionic radii are responsible for magnetic performance of nanoparticles.
Cations existed in nanoferrites have Zn 2+ (0 μB), Mg 2+ (0 μB), Co 2+ (3.88 µB), La 3+ (0 μB) and Fe 3+ (5µB) magnetic moments [9]. Individually Co 2+ has higher magnetic moment than Zn 2+ and Mg 2+ , so replacement of Mg 2+ ions have capability to share large number of Fe 3+ ions at particular site. This manner leads the discrepancy in initial permeability as in Table 4. It can be observed that initially μi increases for X = 0.00 and 0.15 and then starts decreases quickly and gains maximum value for X = 0.60. The growing trend shown by the μi is due to the increasing concentration of non-magnetic ions. The decreasing nature is because of reducing in bulk density as in Table 1