Tuning of physical properties of multifunctional Mg-Zn spinel ferrite nanocrystals: a comparative investigations manufactured via conventional ceramic versus green approach sol-gel combustion route

This work focused on the impact of synthesis routes on the structural, microstructural, magnetic, electrical and dielectric characteristics of Mg1−x Zn x Fe2O4 (x = 0.00, 0.25, 0.50, 0.75, and 1.00) nanocrystals manufactured via the ceramic and green approach sol–gel route. The powder X-ray diffraction (XRD) analysis reveals that the entire synthesized ferrite solids crystallize in single phase spinel structure. The XRD outcomes highlight the impact of the synthesis routes and Zn2+ replacement on the morphology, crystallite size and structural parameters of magnesium nano-ferrites. The transmission electron microscopy (TEM) images illustrate that the process of synthesis causes extensive lessening of grain and crystallite sizes. The magnetic study reveals that the magnetic properties of magnesium ferrite can be tuned by zinc substitution. The saturation magnetization (Ms), retentivity (Mr), coercive force (Hc) and magneton number diminutions meaningfully with the replacement of diamagnetic Zn2+ ions in Mg-ferrite for both the synthesized systems. The deterioration of magnetic parameters with Zn2+ substitution can be clarified on the base of the random spin canting model. Likewise, the magnetic parameters, enhanced meaningfully for sol-gel derived samples this can be attributed due to decline of crystallite size effect. The DC electric resistivity displays NTCR behaviour like ideal semiconducting materials for all the produced samples. The DC resistivity values of sol-gel produced samples were found to be little bit higher than that of ceramic derived samples. The experimental dielectric constant as a function of frequency behaviour can be elucidated with the support of the heterogeneous model of the polycrystalline structure of ferrites. The dielectric constant and loss tangent decreases with Zn2+ content for both the systems. The dielectric constant enhances for sol-gel derived samples; however, lower values of loss tangent were found. The obtained outcomes can be suitable for multifunctional applications in electronics devices and biomedical field.


Introduction
Whenever we talk about magnetic materials, the first solid that comes into our mind is spinel ferrite ceramic material. The spinel ferrite materials (M-Fe 2 O 4 , M-divalent metal cations) are widely investigated solid oxide materials across the globe because of versatile electro-magnetic assets [1]. Even after nearby 90 years of disclosure, spinel ferrite persisted as the most frequently utilized ferromagnetic material [2]. These materials have a wide spectrum of potential applications such as microwave absorbing materials [3], magnetic resonance

Experimental details
Mixed Mg-Zn nanoparticles with a chemical formula of Mg 1−x Zn x Fe 2 O 4 for x=0.00, 0.25, 0.50, 0.75, and 1.00 were produced by conventional ceramic and green approach sol-gel combustion techniques.
(a) Ceramic technique: Analytical grade (AR) oxides of respective ions, i.e. magnesium oxide (MgO), zinc oxide (ZnO) and ferric oxide (Fe 2 O 3 ) with high concentration were purchased from Merck. These oxides were accurately measured on an electronic balance and mixed together consistent with stoichiometric proportions. The mixed oxides were finely crushed for 3-4 h using agate mortar and pestle. The pulverized powder is then heat treated at 1020°C for 24 h in a controllable muffle furnace and cooled steadily to normal temperature. The sintered powder is now milled for 3-4 h and lastly sintered at 1100°C for 24 h followed by slow cooling. The sintered powders are hard-pressed into cylindrical pellet form using hydraulic press and by applying a pressure 6 ton cm −2 . Lastly, sintered powder and pellets were used for characterizations and for the measurement of electrical, dielectric and magnetic properties. After some time, the auto-combustion reaction takes place and quickly the gel converts into a fluffy powder. The loose fleecy powder was composed and pulverized with the aid of pestle mortar. The crushed powder was exposed to a thermal treatment at the temperature of 650°C for 5 h. The processed powder samples were utilized for the characterizations. Heat-treated powder samples were mixed with polyvinyl alcohol (PVA) agent as a binder and pressed into cylindrical tablet form (10 mm diameter and less than 3 mm thickness) by the isostatic pressing method under the pressure of 550 kg cm −2 . The tablets were heat treated in an oven at 400°C for 2 h for the eradication of PVA. The cylindrical tablets were utilized for additional electric and dielectrical estimations.

Characterizations
Several analytical techniques were used for the characterizations of manufactured spinel ferrite materials. The phase identification of the Mg 1−x Zn x Fe 2 O 4 samples was characterized by powder X-ray diffractometer (Ultima IV, Rigaku Corporation, Japan). The surface topography and internal structure were scrutinized utilizing transmission electron microscope (TEM, Philips CM200) instrument. The surface analysis was examined with 'Brunauer-Emmett-Teller (BET)' technique via recording the 'N 2 absorption-desorption isotherm'. The 'Quantachrome' instrument with v-5.2 model was utilized to trace the isotherm. The room temperature magnetic possessions of the produced ferrites were calculated from the M-H loops, measured with a pulse field hysteresis loop technique (Magnata Company) with an extreme applied field of 7 kOe. DC electrical conductivity mechanism in prepared ferrites was evaluated by studying their DC electrical resistivity. The typical and easy two-probe equipment was employed to examine electrical characteristics of the pelletized ferrite samples. The DC electrical resistivity was calculated in the temperature scales from~472 K-975 K. The room temperature dielectric characteristics were completed operating LCR-Q meter (Hioki 3532-50, Japan) as a component of frequency. It is also evident that all the reflection peaks are highly intense besides sharp-edged thus, the samples are single phase in nature. From figure 2 it is clearly seen that, the intensity of reflected planes of sol-gel synthesized sample is very much higher than that of ceramically derived solids, indicating growth in the proportions of crystallinity in the material [49].

Results and discussion
As well, from figure 2 it is evidenced that modulated shifting of 2θ values takes place for (311) plane with zinc substitution due to microstructural variation. Ceramic produced samples shows distinctive shift in 2θ values than that of sol-gel derived samples which may be due to change in the crystallinity and micro-strain of the material or incorporation of defects [50]. The shifting of peak towards lower 2θ values with Zn 2+ concentration indicates expansion of unit cell volume. As the peak shift for sol-gel derived samples is in a lesser amount of 2θ ensuring the improved stability of the crystals. The average crystallite size (t) of the Mg-Zn ferrite powders was estimated by using the most intense peak (311) and using the well-known Debye-Scherer's formulation [51,52]. Figure 3 shows the distinction of average crystallite size with Zn 2+ concentration x in comparison with two different synthesis routes. It was observed that we got crystals in the range of 54.82 nm to 59.77 nm for ceramic resultant samples, however, crystals of size 17.65 nm to 27.49 nm for sol-gel derived samples. Figure 4 illustrates the compositional reliance of lattice constant (a) concerning to the Zn 2+ replacement for respective synthesis route. It is noticed from figure 4 that, lattice constant (a) shows the increasing trend with upsurge in the Zn 2+ content x following the Vegard's regulation i.e. the replacement of Mg 2+ ions (0.66 Å) by higher ionic radius Zn 2+ (0.82 Å) ions [53]. The identical tendency was observed for the both systems. The ceramically synthesized samples tend to exhibit higher values of the lattice constant due to the existence of defects and impurities. Such defects are introduced through sample synthesis and differ according to the process of synthesis and reagents used [54]. Rendering to the reports, the lattice constant and crystallite size are directly    interconnected [55]. The lattice constant of pristine MgF 2 O 4 and ZnFe 2 O 4 solids and the observed increasing trend per Zn 2+ replacement well resembles with the stated data in the literature [56].
The X-ray density (dx) entities were calculated by means of unit cell volume and molecular weight [57]. The compositional discrepancy of X-ray density values is revealed in figure 5. The growing tendency was observed for the values of dx for both the systems. The increment of the dx values with increasing Zn 2+ ions replacement is may be caused due to the rise in the molecular weight of the Mg-Zn compositions. The equivalent inclination and slight difference in X-ray density were observed for both the systems.
To inspect topographical information, the TEM images are observed for the selective produced samples. It is perceived from all the TEM images that the samples are agglomerated in spherical and cubical profile in nanometric scale. A clear difference is observed among the ceramic and sol-gel derived samples.   function (solid line). The TEM outcomes are in upright agreement with the size reduction detected using XRD analysis discussed earlier.
Brunauer-Emmett-Teller (BET) analysis was used to determine the surface related parameters of the typical samples. The N 2 adsorption-desorption isotherm curves of pure magnesium ferrite prepared by ceramic and sol-gel auto combustion route are shown in figure 8.
The different parameters such as specific surface area, Average Pore diameter and Pore volume were determined from the analysis. The values of these parameters are noted in the table 1.
The specific surface area of the magnesium ferrite prepared by ceramic and sol-gel auto combustion route was found to 1.357 m 2 gm −1 and 34.228 m 2 gm −1 respectively. It is well known that the surface area of the materials tremendously increases when it changes from bulk from to nano form. Thus, in the present case the major difference in the surface area was observed. In addition to the specific surface area, the average pore diameter and pore volume of the magnesium ferrite prepared by ceramic and sol-gel auto combustion route was also measured by Barett-Joyner-Halenda (BJH) analysis. The average pore diameter of the magnesium ferrite prepared by ceramic and sol-gel auto combustion route was found to 4.12 nm and 8.69 nm respectively. The pore volume of the magnesium ferrite prepared by ceramic and sol-gel auto combustion route was found to 0.0198 c.c./gm and 0.0755 c.c./gm respectively.  [58].
In few literatures, it was observed that the Ms values initially upsurges for low Zn 2+ content and then decreases, which was explained on the basis of Neel's two sublattice model [59]. In this case, we found that the Ms values decrease steadily with Zn 2+ concentration for all the samples. This observed trend could be understood on the basis the random spin canting model, substitution of diamagnetic cations in one sublattice of ferrimagnet originates spin canting in the further sublattice responsible for the reduction in the overall magnetization per formula unit. This can be accredited to the magnetization of A-sublattice are so diluted that the A-B exchange mechanism is no further stays dominant and thus B-B sublattice interaction becomes noteworthy. This B-B sublattice interaction interrupts the parallel alignment of spin magnetic moments on the octahedral [B]-site and hereafter canting of spin arises. Neel's three-sublattice non-collinear spin canting model is principal for the increasing Zn 2+ content samples and henceforth decrease in Ms values [60]. Analogous outcomes for Mg-Zn sol-gel derived solids were found in the literature manufactured by co-precipitation route [61]. Now in comparison with the process of synthesis route and crystallite size, it was found that the magnetic entities were significantly enhanced for sol-gel derived solids. Figure 10 shows the juxtapose of Ms values with different concentrations of Zn 2+ for ceramic and sol-gel routes.
It was seen that, as the crystallite size declines the saturation magnetization was seen to increment. Henceforth, sol-gel derived shows superior magnetic properties as compared to ceramic derived solids. The pure ZnFe 2 O 4 sample (x=1.00) shows completely paramagnetic nature which has crystallite size of 59 nm obtained by ceramic technique. For sol-gel derived pure ZnFe 2 O 4 solid, the crystallite size reduces up to 17 nm and therefore as expected the sample show typical ferromagnetic nature (Ms=21.22 emu g −1 and Hc=197.12 Oe). The smaller values of magnetization for ceramic derived samples can be explained on the base of growing grains and its size. In light of the grain development, grain traps the between granular pores. Since, during the grain development, the pores among the grains are trapped. This inter-granular sponginess is may be accountable for the diminution of magnetic entities [62,63].
The coercive force (Hc) overall increases with the Zn 2+ content (as depicted in figure 11). Likewise, the Hc values of sol-gel derived samples are superior than ceramic derived solids. The upsurge in Hc values may be attributed to the conversion from the unblocked to the blocked state, i.e. due to withdrawal from the super-paramagnetic state of the nanocrystals [64]. This conversion happens in lesser nanoparticles when the anisotropy energy governs the thermal energy in the system. Generally, the coercivity primarily depends upon two chief parameters, such as, crystallite size and anisotropy constant [65]. The crystallite size decreases for sol-gel synthesis and it results to increase in the coercivity value. Consuming the values of saturation magnetization, the magnetic moment per formula unit, n B (in Bohr magneton) was estimated [66].   Figure 12 shows the discrepancy of magneton number for the two different synthesis routes with Zn 2+ content x. The magneton number values were found to be higher for sol-gel derived samples. The magneton number (n B ) values decrease with Zn 2+ content for both the systems.  The anisotropy constant (K ) also decreases with Zn 2+ concentration x for two different synthesis techniques as depicted in figure 13. The Mr values don't show linearity behavior, since the large ionic radii of zinc favor a normal spinel geometry over the mixed spinel geometry of magnesium ferrite nanocrystals which reasons a loss of magnetic energy that results in random remanent values. Also, the microstructural defect influences the coercivity and retentivity of the material [67].
The DC electrical resistivity investigations of the produced Mg 1−x Zn x Fe 2 O 4 samples were estimated as a component of temperature. Figures 14(a) and (b) shows the discrepancy of the logarithm of resistivity (log ρ) as a component of the reciprocal of temperature (1000 / T) for ceramic and sol-gel route individually.
It is evident from figures 14(a) and (b) that the dc resistivity fall off with rising thermal treatment conforming Arrhenius relation for both the systems [68]. It was observed that, as Zn 2+ content increases from x=0.0 to 0.75 the dc resistivity values increase. However, the dc resistivity values of x=1.00 i.e. pristine ZnFe 2 O 4 solid was found pointedly lower. The same trend is observed for both the systems.
The observed values of resistivity with rising temperature can be accredited to the to the consequence of ferromagnetic ordering on conduction phenomenon. This is attributed to the destruction of spontaneous magnetization and the variation in additional charge carrier concentration in the region of Curie transition temperature [69]. In ferrites, being magnetic semiconductors, resistivity in the low temperature called as region I is attributed to impurities and is extrinsic in nature. At high temperatures, the resistivity called as region II is intrinsic in nature and is due to the electron hopping mechanism among Fe 3+ to Fe 2+ ions. This linear reduction in resistivity values with temperature was accredited to the thermally activated mobility of the charge transporters. It implies that the discontinuity at Curie temperature is accredited to the magnetic transformation  from well-ordered ferrimagnetic state to disordered paramagnetic state, which contains dissimilar activation energies. The resistivity of ferrites, in over-all, relies upon the density, porosity, grain size, grain boundary area, defects and chemical composition of the material [70,71].  The dc resistivity values of sol-gel prepared solids were found to be a little bit higher than that of ceramic derived solids. The detected behaviour can be understood on the foundation of size of the nanocrystals. The crystallites of lower nano-grained assembly were attained through the sol-gel route. It was well acknowledged fact that the smaller crystallite size indicates a growth of the insulating surface on the grain boundaries, which normally accounts for huge electrical resistance of the polycrystalline materials. The other fact is that the porous structure of these samples hinders the motion of charge carriers leading to an increase in the resistivity [72].
This observed negative temperature coefficient resistance (NTCR) behaviour of these synthesized ferrites makes them useful for high frequency device applications wherever eddy current losses become vital factor [73].
The higher values of the DC resistivity are because of compositional stoichiometry, better crystal structures and the enhanced microstructures attained by the sol-gel technique. A microstructure with smaller grain contains a greater number of grain boundaries. The grain boundaries are regions of discrepancy among the energy states of the in-line grains and hence, acts as blockades to the movement of electrons. The high resistivity observed in the sol-gel derived samples is thus credited to the small grain size [74]. Figures 15(a) and (b) represents the disparity of dielectric constant as a component of frequency (log f ) manufactured by two different synthesis techniques. It can be observed in both cases that the dielectric constant was higher at low frequency spectrum and it decreases exponentially with increasing frequency. The frequency dependence dielectric constant can be elucidated with the support of the heterogeneous model of the polycrystalline structure of ferrites suggested by Koops [75,76]. The values of dielectric constant decreases pointedly with Zn 2+ concentration for both the systems. The cause for this observation is the diminution in the number of Fe 2+ ions owing to the substitution of the Zn 2+ dopants possibly at the octahedral sites. Henceforth, because of interruption of electron transfer between Fe 2+ and Fe 3+ by the dopants, a reduced space charge polarization is projected to decrease the dielectric constant. In view of synthesis techniques, it can be observed from figures 15(a) and (b) that the dielectric constant of sol-gel derived samples is greater than that of the ceramically synthesizes samples. The XRD and TEM examinations suggested that the crystallite size of the sol-gel samples reduced than that of ceramic samples. The dielectric constant increases with decrease in crystallite size as reported in the literature [77]. Figures 16(a) and (b) shows the discrepancy in the dielectric loss factor of Mg-Zn ferrites manufactured via the two different routes. The dielectric loss tangent (tan δ) demonstrates same tendency as that of dielectric constant. The tan δ values decrease as the frequency increases. This deterioration designates the usual behavior of spinel ferrite solids. It occurs when the hopping frequency of electric charge carriers could not act in accordance with the modification of the exterior applied alternating electric field beyond a certain critical frequency. Also, the tan δ values of sol-gel derived samples were lower than that of ceramic derived samples due to decrease in crystallite size. The mixed Mg-Zn ferrite exhibit a lower value of dielectric constant and dielectric loss as compared to other spinel ferrites due to the presence of Zn 2+ ions in the samples [78]. The obtained dielectric behaviour can be much beneficial for the fabrication of electronic appliances that can be operated in a wide frequency range.

Conclusions
A sequence of nano-sized crystals of Mg 1−x Zn x Fe 2 O 4 (x=0.00, 0.25, 0.50, 0.75, and 1.00) spinel ferrites have been manufactured by ceramic and the facile ecofriendly sol-gel route. XRD diffractograms are in accordance with the cubic spinel structure with space group Fd-3m without any impurity traces. The average sizes of the crystals were estimated to be in the range of 54-59 nm for ceramic and 17-27 nm for sol-gel derived solids. The impact of the synthesis procedure on the internal structure of the produced ferrites inspected by TEM observations, which well supported the XRD outcomes. The process of synthesis meaningfully enhances the magnetic, electrical and dielectric characteristics of synthesized Mg-Zn ferrite solids. The magnetic study revealed that all the magnetic entities decreases with zinc replacement. The sol-gel derived solids showed superior magnetic properties than that of ceramic derived solids. For pure magnesium ferrite, the highest value of Ms=89.69 emu g −1 for sol-gel and Ms=68.38 emu g −1 for ceramic route were found. Magnetic data showed that by reducing crystallite size, Ms and Hc values significantly enhances. The dc electrical resistivity investigations confirmed the ideal semiconducting nature of the produced samples. The dc resistivity values overall increase with the zinc substitution for both the systems, however, the pure ZnFe 2 O 4 solid shows highly resistive nature. The dc resistivity values of sol-gel derived solids were higher than that of ceramic derived solids which can be attributed to lower grain sizes. The dielectric constant and loss tangent both decrease with growing frequency and zinc concentration in accordance with Koops model. The sol-gel samples show improved dielectric constant values which can be elucidated on the base of reduced crystallite size effect. These investigations evidenced that the physical properties of spinel ferrites can be easily tuned by the process of synthesis by controlling the crystallite size. The obtained magnetic properties of this mixed Mg-Zn spinel ferrite nanoparticles make them beneficial for potential applications in biomedical sciences. The obtained electrical and dielectric properties can be valuable for the fabrication of electronic appliances.