Preparations, optical, structural, conductive and magnetic evaluations of RE's (Pr, Y, Gd, Ho, Yb) doped spinel nanoferrites

Journal Pre-proof Preparations, optical, structural, conductive and magnetic evaluations of RE's (Pr, Y, Gd, Ho, Yb) doped spinel nanoferrites M. Yousaf, Majid Niaz Akhtar, Baoyuan Wang, Asma Noor PII:

S0272-8842(19)33012-3

DOI:

https://doi.org/10.1016/j.ceramint.2019.10.149

Reference:

CERI 23217

To appear in:

Ceramics International

Received Date: 5 September 2019 Revised Date:

10 October 2019

Accepted Date: 16 October 2019

Please cite this article as: M. Yousaf, M.N. Akhtar, B. Wang, A. Noor, Preparations, optical, structural, conductive and magnetic evaluations of RE's (Pr, Y, Gd, Ho, Yb) doped spinel nanoferrites, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.10.149. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Preparations, optical, structural, conductive and magnetic evaluations of RE’s (Pr, Y, Gd, Ho, Yb) doped spinel nanoferrites M. Yousafa, Majid Niaz Akhtarb,*, Baoyuan Wanga, Asma Noorc a

Hubei Collaborative innovation center for advanced Organic Chemical Materials, faculty of Physics and Electronic Science, Hubei University Wuhan, Hubei 430062 P.R China b Department of Physics, Muhammad Nawaz Sharif University of Engineering and Technology (MNSUET), Multan, 60000, Pakistan c Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab of Ferro & Piezoelectric Materials and Devices, Ministry-of-Education Key Laboratory of Green Preparation and Application for Functional Materials, School of materials Science & Engineering, Hubei University, Wuhan 430062, China Corresponding Author: [email protected], [email protected]

Abstract Rare earths RE’s (Pr, Y, Gd, Ho, Yb) substituted MnZn spinel ferrites with composition of Mn0.5Zn0.5M0.02Fe1.98O4 (M= Pr, Y, Gd, Ho, Yb) are prepared by sol gel combustion approach. Low sintering temperature (500˚C) is used to sinter the RE’s doped MnZn samples. MnZn samples are further characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM) to measure the cubic crystalline structure, particle size, morphology, porosity and grain size. Cubic crystalline phase of prepared RE’s doped MnZn ferrites is confirmed by x-ray diffraction (XRD). The morphology, porosity and grain size are observed using FESEM. The magnetic properties of RE’s doped MnZn nanoferrites are analyzed by vibrating sample magnetometer (VSM). Coercivity (Hc), remanence (Mr) and saturation magnetization (Ms) are calculated from the magnetic loops. The saturation and remanence of the nanoferrites are increased by the substitution of RE’s metal ions and varies from 14.76 to 26.36 emu/g and 9.98 to 22.48 emu/g respectively. Bohr magneton and anisotropy constant are calculated from the recorded magnetic data. The conductive analysis of the prepared samples is studied at 40˚C -300˚C temperature, leading to the conductivity measurements from 1.12x10-2 Ω-1-cm-1 to 9.52x10-2 Ω-1-cm-1. UV-Vis spectroscopy is used to determine the semi conducting nature of RE’s doped MnZn spinel ferrite samples. The magnetic, conductive and optical study of the RE’s doped MnZn nanoferrites sintered at low temperature suggests the use of these materials for microwave absorption, supercapacitor, lithium ion batteries and nanoelectronics industrial applications. Keywords: RE doped MnZn nanoferrites; X-ray diffraction; UV-Vis spectroscopy; Conductive properties; Magnetic properties;

1. Introduction In present years, ferrites have been used for industrial applications because of their higher thermal stability, reasonable magnetic permeability and higher electrical properties [1, 2]. Generally, spinel ferrites have cubic crystalline structure having general formula M. Fe2O3, where ‘M’ is divalent metal cations (Ni2+, Zn2+, Fe2+ and Mn2+) [3]. The occupancy of oxygen ions at the lattice sites in spinel structure is distributed by 32 and 16 ions at the lattice sites respectively. The metal ions distributions at the respective lattice sites, grain and grain boundaries effect may improve the electrical properties of the spinel ferrites [4]. The detailed study about the metal cations distribution into the spinel structure has been discussed by Akhtar et. al. [5]. Mn-Zn ferrites are used in microelectronics and electronics resonator because of the better combined electrical and magnetic properties.

Furthermore, MnZn ferrites have been used in inductors, choke coils, recording heads and magnetic amplifiers [6]. MnZn spinel nanoparticles have been attracted by many researchers from the last few decades due to its cubic crystalline structure, higher magnetization and opto-electric properties [7-9]. High electrical conductivity and low eddy current loses make MnZn spinel ferrites valuable for industrial applications [10-12]. The electrical properties are strongly depend on the doping elements, environmental factors and synthesis techniques [13]. The conductivity of semiconducting nature spinel ferrites increases at higher temperature because of the oxygen ion movement at their lattice sites [14, 15]. The temperature dependent dc conductivity of spinel ferrites usually depend on grain size, morphology, distribution of cations and anions at lattice sites [16, 17]. The doping of RE’s metal ions enhances electromagnetic properties due to cation distributions at their interstitial sites as reported by many researchers [1820]. The smaller concentration of RE metal ions in spinel ferrites is promising due to their magneto-optical applications [21]. The doping of RE metal ions (RE+3) in ferrimagnetic oxide compounds may cause to change the structure of spinel ferrite due to the trivalent Fe-Fe interactions ions on lattice site [22]. The RE+3-Fe+3 interactions in spinel ferrites may leads the changes in magnetizations and curie temperature. The rare earth doped spinel ferrite suitable for electrical properties. Many researchers have been worked on RE doped spinel ferrites as electrode material for lithium ion batteries and supercapacitor applications [23, 24]. Xiaoqin Tang et al. [25] has been worked on Mn doped zinc spinel ferrites and successfully synthesized Zn1-xMnxFe2o4 electrode by showing highest specific capacity of 1547 mAhg-1 and 1157 mAh g-1 in the initial discharge/charge process, with a coulombic efficiency of 74.8%. Moreover, the smaller concentration of RE metal ions can enhance electrical conductivity and magnetooptical behavior of spinel ferrites as explained by Author links open overlay panel Zaheer Abbas Gilani et al. [26]. Tholkappiyan et al. [27] reported spinel ferrites (Zn1−xBaxFe2O4) could be used for solar cell applications because of their measured optical bandgap (2.42eV to 2.50eV) exist in semiconductor region. Many researchers have been worked on MnZn spinel ferrites nanoparticles and explained its magnetic paremeters and dielectric behavior, but the conduction mechanism and semiconducting nature of MnZn spinel ferrites have been rarely discussed; therefore, an attempt has been made to prepare and characterize the RE doped MnZn ferrite to analyze conductivity and bandgap of the material. In this work, sol gel combusted rare earth RE’s MnZn with composition of Mn0.5Zn0.5M0.02Fe1.98O4 (M= Pr, Y, Gd, Ho, Yb) were prepared. The prepared samples were sintered at low sintering temperature (500˚C). Cubic crystalline structure, grain size, porosity, morphology and magnetic parameters were studied using XRD, SEM, and VSM respectively. The temperature dependent DC conductivity and UV-Vis spectroscopy were also investigated to measure the conductive behavior as well bandgap of RE’s doped MnZn spinel ferrites.

2. Experimental 2.1 Materials and Methods Manganese Zinc (MnZn) spinel ferrites substituted with RE metal ions were synthesized using sol gel combustion approach. Manganese nitrate hexahydrate [Mn(NO3)3.6H2O] (aladdin, 99.0%], iron nitrate nonahydrate [Fe(NO3)3.9H2O] (aladdin,99.9%), zinc nitrate hexahydrate [Zn(NO3)3.6H2O] (aladdin,99.5%], praseodymium nitrate hexahydrate [Pr(NO3)2.6H2O sigma aldrich ,99.5%], yttrium nitrate hexanitrate [Y(NO3)2.6H2O sigma

aldrich, 99.5%], gadolinium nitrate hexahydrate Gd(NO3)2.6H2O sigma aldrich, 99.5%], holmium nitrate hexahydrate [Ho(NO3)2.6H2O sigma aldrich, 99.5%], ytterbium nitrate hexahydrate [Yb (NO3)2.6H2O sigma aldrich, 99.5%] were used as a precursors. Citric acid (C6H8O7) was mixed into the solution as a chelating agent. The dropwise added ammonia solution was used to control and maintain its pH ~7. The stoichiometric amounts of the precursors were weighed and dissolved in 100 ml deionized water to make a solution of each individual material. Further, the solutions were mixed together into one beaker to prepare a homogenous solution. In addition, the solution was stirred and heated at 85˚C until the solution was changed into gel. The gel was auto combusted at 110˚C temperature and then kept in oven for 12 hours to obtain the material in the form of ashes. The ashes were grinded for 30 minutes. Finally, RE’s doped MnZn spinel ferrites were sintered at 500˚C with 10o/min range for 5 hours.

2.2

Materials characterizations of RE’s doped MnZn spinel nanoferrite

The synthesized RE’s doped MnZn spinel nanoferrites were characterized by the techniques such as XRD, FESEM and VSM. Room temperature XRD patterns were recorded in the range of 20o to 80o to find the crystallite size and cubic crystalline spinel phase of MnZn spinel ferrites. The morphology, grain size and porosity of the RE’s doped MnZn spinel ferrites were observed by FESEM. The magnetic properties MnZn spinel ferrites were analyzed using VSM (PPMS-DynaCool-9T, Quantum Design, USA). In addition, magneto crystalline anisotropy, Bohr magneton, Y-K angles and initial permeability were also determined by magnetic data. The optical properties of RE’s doped MnZn spinel nanoparticles were studied by UV-Vis spectrophotometer. Further, the conductivity measurements were done using the Keithlay (IT8511, China) instrument at the higher temperature of 40˚C to 300˚C respectively.

3. Results and Discussions 3.1 Structural Analysis XRD patterns of RE doped MnZn samples depicted in Figure 1 shows the pure crystalline structure with cubic phase at low sintering temperatur. RE’s doped MnZn spinel ferrites cubic phase was confirmed by the standard ICSD card (PDF#40-1119) using Jade software. The calculated average crystallite size, d-spacing, cell volume and lattice parameter of MnZn spinel ferrites doped with RE metal ions are tabulated in Table 1. The variations in the calculated values of the crystallite size and lattice parameters of MnZn spinel ferrites values are different due to the substitution of different RE’s metal cations [28]. The cell volume and micro strains of the RE’s doped spinel ferrite are also listed in Table 1. The average crystallite size of RE’s doped MnZn spinel ferrites is determined by Debye Scherrer formula as given in the equation [29],

=

.

(1)

Where, ‘D’ is the crystallite size, λ is CuKα radiation with 1.5416 Å, β~ 0.9, and ‘θ’ is the diffraction angle. Williamson-Hall leads to determine both crystallites size and strain [30]. The lattice parameter and micro strain (ε) are calculated [31]

= β

ℎ +

+

(2)

θ = ! % + 4 ɛ sinθ

(3)

"# $

a is the lattice parameters of the samples, β is the FWHM and d is interplanar distance of RE’s doped MnZn spinel ferrite. The observed average lattice parameters are varied from 8.36Å to 8.56Å due to the different ionic radius of RE’s Pr3+ (1.013Å), Y3+ (0.90Å), Gd3+ (0.938Å), Ho3+ (0.901Å), Yb3+ (0.858Å) metal ions. It is clearly observed that the lattice parameters and crystallite size increases with the doping of RE metal ions. The values obtained for these parameters are greater than pure MnZn ferrite as depicted in Table 1. The values of the lattice parameter and crystallite size increased which is due to the generation of stresses and the expansions in the spinel lattice. The variations may be because of the substitution of RE’s metal ions. Moreover, the average crystallite size and lattice parameters have different values with the doping of RE’s ions which is due to the larger ionic radii of substituted metal ions than Fe ions (0.64 Å) [32]. The structural properties of spinel ferrites are strongly dependent on the distribution of RE’s metal ions over the interstitial sites. RE’s metal ions overlapped with Fe3+ ions at the octahedral sites, which may cause to distort the lattice at the interstitial sites. Therefore, the structural parameters such as average crystallite size and lattice parameters may also change with the substitution of RE’s metal ions. The variations in FWHM, d-spacing, cell volume and macrostrain is due to the substitution of different RE’s metal ions in MnZn spinel ferrites. The cell volume and micro strain values are decreased with the substitution of RE’s metal ions, which is the confirmation that actual spinel structure still exist after the substitution of RE’s metal ions. Graphical representations of the crystallite size and lattice parameter are depicted in Figure 2. The cations distributions such as ionic radii (rA, rB), bond length (A-O, B-O) , shared edges and unshared edges of RE’s doped MnZn samples are calculated [33].

+, = -. − 0.253 √3 − 67

(4)

+8 = -0.625 − .3 − 67

(5)

6: = 0.25 √3

(6)

6; = 0.25 √2

(7)



:<

= √2 2. − . 5

(8)



;<

= √2 1 − 2.

(9)



;<>

=

4. − 3. + 11/16

(10)

It is observed that the calculated parameters are decreased with the doping of RE’s contents in MnZn ferrites. It is also noticed that the ionic radii and lattice constant of the RE’s doped MnZn spinel ferrites have strong co-relation with each other. Table 2 shows the calculated average ionic radius per molecule at the octahedral and tetrahedral sites (rA and rB), bond length with its shared octahedral and tetrahedral and unshared octahedral and tetrahedral

edges of RE’s doped MnZn spinel ferrites. These important parameters explain the structural properties of the ferrites and used as a link between the conductivity and magnetic parameters. The site radii at A sites have lower values than those on B sites as depicted as presented in Table 2. The decreasing trend may occur due to the moment of Fe3+ ion from A-sites to B-sites in spinel ferrites interstitial sites. However, the bond length at A-sites are less than that of the B-sites which may be due to smaller ionic radius of Fe3+ replaced by larger ionic radius of RE’s metal ions as reported by A.B. Gadkari et al [34].

3.2 Microstructural Analysis Figure 3 shows the FESEM images of RE’s doped MnZn spinel ferrite samples. Figure 3 (a-f) depicted the FESEM images of MnZn ferrite, Pr doped MnZn, Y doped MnZn, Gd doped MnZn, Ho doped MnZn and Yb doped MnZn nanoferrites respectively. The grain size of RE’s doped MnZn spinel ferrites was evaluated by line intercept technique. The measured grain size range for large grains was found from 10 µm to 13 µm of RE’s doped MnZn spinel ferrites. However, the average grain size which was found in the range of 48 nm to 89 nm for MnZn and RE’s doped MnZn ferrite. The variations in the grains size was also depends on the substitution of different RE’s metal cations in spinel ferrite. Table 3 depicted the calculated grain size values of the MnZn and RE’s doped MnZn nanoferrites. The calculated grain size usually depend on agglomerations, porosity and low sintering temperature of RE’s doped MnZn spinel ferrites [35]. The agglomerations, porosity and existence of grain growth of MnZn ferrites are also observed. The larger grain size and agglomerations in prepared samples may be identified due to the low sintering temperature (500˚C). The magneto-optical and electrical properties of MnZn substituted with RE’s metal ions can be restrained by grain size effect, homogeneity and shape of the particles. Figure 4(a-f) illustrate the energy-dispersive spectrometer (EDS) and elemental mapping images of MnZn ferrites doped with RE’s metal ions. The elemental mapping images show the occurrence of Mn, Zn, Fe, O as well as RE’s metal ions over the surface. The quantitative analysis proves the homogeneous distribution and formation of desired composition.

3.3 Magnetic Properties The magnetic behavior of RE’s doped MnZn spinel ferrites was analyzed using VSM. Magnetic anisotropy constant, magnetic moments, remanence (Mr), saturation (MS) and coercivity (Hc) are calculated from observed hysteresis loops as depicted in Figure 5. However, initial permeability and Y-K angles also determined using the recorded magnetic data. MS of spinel ferrites samples is different due to the higher ionic radius of different RE’s metal and interactions of RE’s metal cations with Fe ions at tetrahedral site and octahedral site [36].

The saturation

magnetization also depends on the substitution values in spinel ferrites samples [37]. For the smaller dopant concentration of RE’s metal ions, most of metal cation in spinel structure enter to the lattice and uniformly distribute at tetrahedral site and octahedral site. In this study, RE’s metal ions have magnetic moment and the total magnetic moment will be changed due to the small concentration of rare earth metal ions. In addition, Pr+3, Y+3, Gd+3, Ho+3 and Yb+3 have magnetic moment to the lattice site in spinel structure, that’s why MS of prepared samples is different than previous reported literature [38-40]. The magnetic remanence of prepared samples is calculated from the magnetic recorded data in the range of 9.98emu/g to 22.48emu/g respectively. In addition, Y doped MnZn spinel

ferrites shows the higher magnetic remanence values as compared to other substituted RE’s (Pr, Gd, Ho, Yb) metal ions. Magnetic squareness values of prepared samples are in the range of 0.67-0.85, which show the soft magnetic behavior of MnZn spinel ferrite. The narrow loop shows low coercivity values of the RE’s doped MnZn ranging from 5.62-28.80 Oe as presented in table 3. The different trend in coercivity values based on Stoner-Wolforth model and important indication of paramagnetism [41, 42]. The change in coercivity of MnZn samples due to the resistance of domain wall motion, which strongly depends on grain size effect in microstructural properties as explained by Zhong et al. [43].The different ionic radii and smaller concentration of RE’s ions play a vital role to observe the variations in magnetic properties of MnZn spinel ferrites. MnZn ferrite has lower magnetic moment (0.12) whereas RE’s doped MnZn ferrites have higher magnetic moments as depicted in Table 3. Ms of the materials is strongly dependent on the magnetic moments of the doped metal cations. It can be observed that lower values of the magnetic moments resulted the lower saturation magnetization whereas higher values of Ms resulted larger values accordingly. The theoretical effective magnetic moments values have used for calculations and elaborated by Almessiere et al. [44]. Further, the the magnetic properties of spinel structure are affected because of the variations in the magnetic moments and distribution of the ions at the A and B sites. Ms and Hc are important parameter’s which show strong magnetic relation for the variations in magnetic properties. Moreover, the substitution of rare earth metal ions also caused to change the magnetic properties in spinel ferrites. The variation in magnetic properties of RE’s doped spinel ferrite sample can be explained by the magnetic interactions and the replacement of Fe3+ ions at their lattice sites [45]. Bohr magneton (µ B), magneto crystalline, initial permeability anisotropy constant (K) and Y-K angles are evaluated using the relations given below [46-47]. Bohr magneton @; = K; = 6 + L

A×AC

DDED×FGHIJ

∝NOP − 5 1 − L

Anisotropy constant (K) = Initial permeability @X =

(11) (12)

QR ×ST

(13)

AC Y ×$

(14)

U.VW

P

Where, ‘M’ is the molecular weight, Hc is the coercivity and Ms is the saturation magnetization of the RE’s doped MnZn spinel ferrites respectively. However, 'x’ is the concentration and ‘K; ’ is the experimentally calculated Bohr magnetons. Table 3 depicted the measured data recorded from magnetic loops of RE’s doped MnZn spinel ferrites. Generally, in spinel ferrites, the Fe ions (Fe+3) replaced by RE metal ions and magnetic ions moments may cause to change the magnetization. The coercivity (Hc), remanence (Hr) and saturation magnetization (Ms) depend on surface effect, strain and non-magnetic atoms of the prepared samples as explained by Y. Raghvendra et al [48]. Orbitalspin (L-S) and spin-spin (S-S) interactions of the metal ions are important because of their contribution in the magnetic parameters. In ferrimagnetic materials, super exchange interactions of rare earth metal ions at their interstitial sites may also cause to change the magnetic parameters. Neel's theory elaborated, AB interactions in spinel ferrites are desirable due to the strong exchange interconnection as compared to AA and BB as reported in literature [49]. Ferrimagnetic domains, internal structure, motion of ions and size of grains are also created

variations in magnetic properties [46]. In addition, Bohr magneton or magnetic moment calculated experimentally depicted the close relationship of Pr and Ho doped MnZn ferrites. However, theoretical magnetic moments depicted large difference in values of Pr and Ho doped MnZn ferrites. The recorded MH loops data depicted that Pr doped MnZn sample is almost saturated at the same applied field whereas Ho doped MnZn samples is not fully saturated. This may be the reason for lower magnetization of Ho doped MnZn samples as compared to Pr doped MnZn samples. Moreover, the magnetic moments of RE’s metals such as Pr3+ (3.4 - 3.6 µB), Yb3+ (4.3 - 4.9 µB), Gd3+ (7.9 - 8.0 µB), Ho3+ (10.4 - 10.7 µB) and Y3+ (3.1-3.5µB) are also responsible for the variations in the magnetic properties of spinel ferrites [44-45]. Table 3 depicted the higher values of magnetization of Ho and lower values of Y which indicated the influence of magnetic moments of the RE’s metals in MnZn ferrite. It can be seen that the calculated magnetic moments are also in consistent with the theoretical magnetic moments of all RE’s doped MnZn ferrite as shown in the Table 3. Further, rare earths have better super exchange interactions at respective lattice sites as compared to Fe3+. Therefore, the resulting magnetization for all the RE’s doped MnZn samples are higher as compared to undoped MnZn ferrite. The magnetic behavior of the prepared samples is superparamagnetic. As the coercivity of all the prepared MnZn and RE’s doped MnZn ferrites are very low and in the range of the soft ferrites. The soft character of the samples depicted the superparamagnetic behavior. Furthermore, Y doped MnZn ferrite shows highest values of the initial permeability as compared to other rare earth substituted samples due to the lower coercivity of the Y doped MnZn spinel ferrites as listed in Table 3. Y-K angles values are in the range of 34 to 35 which show strong bonding relations of rare earth cations at their respective sites in the MnZn spinel structure. The overall magnetic study based on our results, the RE’s metal ions are useful to improve the soft magnetic characteristics of MnZn samples. Pr+3, Gd+3, Y+3, Ho+3 and Yb+3 metal ions, as the small doping content are beneficial to improve the permeability, coercivity and magnetic losses. 3.4 UV-Vis

Spectroscopic Studies

Figure 6 shows the UV-Vis graphs of RE’s substituted MnZn spinel ferrites. All prepared samples have intensive absorption in abroad wavelength range from UV to visible light. The graph is plotted between the (αԧѵ)2 and photon energy (ԧѵ) to determine the band-gap (Eg) of RE’s doped MnZn spinel ferrites. The band gap (Eg) values of RE’s doped MnZn spinel ferrites are evaluated by Tauc’s relation as given below [50-51]. αԧѵ = A(ԧѵ-Eg) n

(15)

However, ‘α’ is the absorption coefficient, ‘ԧѵ’ is the energy of incident photons, ‘A’ is probability transition constant factor and ‘n’ is an index (2,1/2), (3,3/2) for direct and indirect transition probability respectively. The direct bandgap of RE’s doped MnZn spinel ferrites observed in the range of 1.652 to 1.692 eV as indexed in Table 4. The obtained bandgap values are in good agreement and behave like a semiconducting nature as reported in literature [52-53]. It is also observed that bandgap of MnZn ferrites have different nature by the substitution of different RE’s ions due to the smaller crystallite size, lower concentration of RE’s metal ions and movement of ions from valance band to conduction band. The obtained band gap of RE’s doped MnZn spinel ferrites is also depends on sintering temperature, crystallite size, stochiometric deviations, carrier concentrations and lattice strain of the

material [54]. The crystallite size play a vital key role to analyze the bandgap phenomena in spinel ferrite samples due to overlapping of the bands with the change in crystallite size.

3.5 Conductivity

Measurements

Figures 7 shows conductivity (ln σdc) of RE’s doped MnZn ferrites. DC conductivity of MnZn samples gradually increase with the rise of temperature and show semi conducting behavior. The relation used to calculate the dc conductivity of the RE doped MnZn spinel ferrites [55] σdc = σo (- Ea/KT)

(16)

where ‘Ea’ is the thermal activation energy, ‘σo’ is the exponential parameter and ‘K’ is the Boltzmann’s constant. The specific change in conductivity by the substitution of RE’s ions can be observed with different slope of conductive curves as presented in Fig. 7. Fig. 7 depicted the graphical representation of ln (σ.T) of the RE’s doped MnZn spinel ferrites. The thermally activated dc conductivity was found in the range of 1.12x10-2 to 9.52x10-2 S.cm1

as shown in Table 5. The change of slope at specific temperature in RE’s doped MnZn spinel ferrites can be

observed in Fig. 7. This may be because of the electron and polaron hopping between divalent and trivalent Fe ions at the octahedral site [56-58]. Saafan et. al. [59] have explained conduction mechanism by the interaction of Fe ions at their interstitial sites. The specific change in the slope of RE’s doped MnZn spinel ferrites was observed by the change in ordered ferromagnetic state to disordered paramagnetic states at specific temperature, which is usually known as curie temperature. The conductivity of RE’s doped MnZn spinel ferrites was observed from 1.12x10-2 to 9.52x10-2 S.cm1. Table 5 listed the curie temperature, resistivity, conductivity, Ef and Ep of the MnZn ferrite and RE’s doped MnZn ferrites respectively. It can be seen that Yb doped MnZn spinel ferrites shows higher conductivity at operating temperature than other substituted RE’s metal ions. Yb doped MnZn spinel ferrite has smaller ionic radii (0.851 Å), good crystalline structure and higher saturation magnetization. The conduction mechanism depends on the activation energy and separation of ions in the spinel ferrite materials. In spinel ferrites, band structure nearly linked with each other, therefore slow movement of electrons occur in the prepared samples and causes to polarize or distort its lattice sites. Moreover, when the external fields apply, then polaron hopping in spinel ferrites arises by the deformation of surrounding lattice at their sites. Some oxygen vacancies are produced due to the lower concentration of rare earth metal ion which leads to increase its polarization at the surface of the RE’s doped spinel ferrites [60]. Conductivity of MnZn spinel ferrites also depends on the availability of charger carriers and their moments along octahedral sites. However, the conductivity of RE’s doped spinel ferrite is not too much higher as many researchers have been discussed due to the low sintering temperature of the prepared spinel ferrites. E.V Gopalan et al. has also explained the conducting mechanism of ferrite materials at low sintering temperature. Moreover, the unavailability of charge carriers and mobility of charges at spinel lattice sites are also responsible for the low conductivity in ferrites [61]. The activation energy of RE’s doped MnZn spinel ferrites are calculated from the Arrhenius plots as presented in Figures 7. It can be observed from the presented conductivity figures, that the

activation energy below curie temperature is much higher than the activation energy above curie temperature as shown in Table 5. Moreover, the activation energy in magnetic materials decreases as the conductivity increases reported by safaan S A et al. [44]. In addition, small number of oxygen vacancies transferred in spinel ferrites may also cause to decrease the activation energy in RE’s doped MnZn spinel ferrites.

4. Conclusions RE’s doped MnZn spinel ferrites are properly synthesized at low sintering temperature by sol gel combustion approach. The crystal structure, morphology, porosity, optical bandgap, magnetic properties and conductive behavior of RE’s doped MnZn spinel ferrites were analyzed by XRD, FESEM, UV-Vis, VSM and conductivity measurements. Pure single phase of the RE’s doped MnZn ferrites is confirmed using x-ray diffraction. FESEM shows the porous structure by the substitution of RE’s metal ions and agglomerations due to the magnetic interaction between the nanoparticles. Y doped MnZn spinel ferrites shows higher magnetization, large initial permeability and low coercivity than other substituted RE’s metal ions. The temperature dependent dc conductivity of spinel ferrites samples shows semiconducting behavior of the prepared samples due to the moment of electron and polaron hopping at grain and grain boundaries. The variations in conductive properties at operating temperature is evaluated. The present study reveals the use of these RE’s doped MnZn nanoferrites for their applications in microwave absorption, electronics and microelectronic devices.

5. Acknowledgements The work was supported by the National Natural Science Foundation of China (Grant no. 51872080 and 51302033).

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List of Figures Figure 1 XRD patterns of Mn0.5Zn0.5M0.02Fe1.98O4 (M= Pr, Y, Gd, Ho, Yb) nano-ferrites Figure 2 Graphical representations of structural parameters of RE’s doped MnZn nanoferrites Figure 3 SEM images of (a) MnZn ferrite, (b) Pr doped MnZn, (c) Y doped MnZn, (d) Gd doped MnZn, (e) Ho doped MnZn and (f) Yb doped MnZn nanoferrites Figure 4 Energy-dispersive spectrometer (EDS) and elemental mapping images of (a) MnZn ferrite, (b) Pr doped MnZn, (c) Y doped MnZn, (d) Gd doped MnZn, (e) Ho doped MnZn and (f) Yb doped MnZn nanoferrites Figure 5 (a) Magnetic Hysteresis loops of (a) MnZn ferrite, (b) Pr doped MnZn, (c) Y doped MnZn, (d) Gd doped MnZn, (e) Ho doped MnZn and (f) Yb doped MnZn nanoferrites (b) The M-H enlarge View of prepares samples. Figure 6 (a) Optical absorption spectrum and (b) Tuac’s plot results in an optical Eg for the MnZn ferrite, Pr doped MnZn, Y doped MnZn, Gd doped MnZn, Ho doped MnZn and Yb doped MnZn nano-ferrites Figure 7 Variations in DC conductivity of (a) MnZn ferrite, (b) Pr doped MnZn, (c) Y doped MnZn, (d) Gd doped MnZn, (e) Ho doped MnZn and (f) Yb doped MnZn nanoferrites

List of Tables Table 1 XRD Parameters of MnZn ferrite, Pr doped MnZn, Y doped MnZn, Gd doped MnZn, Ho doped MnZn and Yb doped MnZn nano-ferrites Table 2 Site radii (rA, rB), bond length (RA, RB), shared edges (dAE, dBE) and unshared edges (dBEU) of MnZn ferrite, Pr doped MnZn, Y doped MnZn, Gd doped MnZn, Ho doped MnZn and Yb doped MnZn nano-ferrites Table 3 Magnetic parameters of MnZn ferrite, Pr doped MnZn, Y doped MnZn, Gd doped MnZn, Ho doped MnZn and Yb doped MnZn nano-ferrites Table 4 Band gap energy values of MnZn ferrite, Pr doped MnZn, Y doped MnZn, Gd doped MnZn, Ho doped MnZn and Yb doped MnZn nano-ferrites Table 5 Curie temperature, resistivity, conductivity and activation energies of MnZn ferrite, Pr doped MnZn, Y doped MnZn, Gd doped MnZn, Ho doped MnZn and Yb doped MnZn nano-ferrites

Sample (X)

Intensity (counts)

2θ˚

FWHM

d-spacing

Crystallite size (nm) 32.41

lattice parameters (Å) 8.369

Cell volume 10-27 5.861

MnZn

311

35.16

0.702

2.511

Pr doped MnZn Y doped MnZn Gd doped MnZn Ho doped MnZn Yb doped MnZn

321

35.20

0.965

2.541

29.02

8.5237

5.970

X-ray density (g/cm3) 3.773

Micro Strain [%] 0.009 0.013

3.986 402

35.36

0.720

2.530

22.10

8.4501

5.905

0.009 4.010

300

34.85

0.751

2.581

25.5

8.4651

6.272

0.014 3.764

354

35.13

0.873

2.552

26.97

8.4577

6.054

0.012 3.942

419

35.25

0.756

2.553

27.60

8.4119

6.033

0.010 3.906

Table 1

Table 2

Site radii and bond length calculation of

rA

rB

RA

RB

dAE

dBE

Unshared edges dBEU

MnZn

0.5489

0.6920

1.8989

2.0432

3.1009

2.8168

2.9605

Pr doped MnZn

0.5622

0.7063

1.9122

2.0576

3.1226

2.8366

2.9813

Y doped MnZn

0.5539

0.6974

1.9039

2.0486

3.1091

2.8243

2.9684

Gd doped MnZn

0.5923

0.7387

1.9423

2.0900

3.1718

2.8812

3.0282

Ho doped MnZn

0.5705

0.7152

1.9205

2.0665

3.1361

2.8488

2.9942

Yb doped MnZn

0.5712

0.7160

1.9212

2.0673

3.1373

2.8499

2.9954

Site radii

Bond length

Shared edges

Table 3

Samples

Ms (emu/g)

Mr

Mr/Ms

(emu/g)

Hc

Grain

Magnetic

Anisotropy

Y-K

Initial

(Oe)

Size

Moment

constant

angles

permeability

(nm) MnZn

14.76

2.83

0.191

28.80

48

0.12

164.67

34.071

11.206

Pr doped MnZn

18.58

3.84

0.206

21.45

67

0.15

415.15

35.037

4.099

Y doped MnZn

17.03

3.64

0.213

27.85

71

0.14

151.33

34.822

24.771

Gd doped MnZn

25.85

6.23

0.241

5.62

79

0.21

486.30

34.882

1.5148

Ho doped MnZn

26.36

7.04

0.267

10.71

81

0.21

494.05

35.068

4.2735

Yb doped MnZn

23.99

4.13

0.171

19.46

89

0.20

294.08

34.761

4.6310

Table 4

Samples

Bandgap energy Eg (eV)

MnZn

1.652

Pr doped MnZn

1.692

Ydoped MnZn

1.689

Gd doped MnZn

1.679

Ho doped MnZn

1.682

Yb doped MnZn

1.687

Table 5

Composition

Tc K

Resistivity Ω.cm

Conductivity S.cm-1

Ef eV

Ep eV

MnZn

362 K

0.89 x102

1.12x10-2

0.13

0.16

373 K

2

7.37x10

-2

0.08

0.10

6.44x10

-2

0.05

0.07

8.22x10

-2

0.11

0.15

9.52x10

-2

0.07

0.11

1.62x10

-2

0.06

0.11

Pr doped MnZn Ydoped MnZn Gd doped MnZn Ho doped MnZn Yb doped MnZn

363 K 376 K 352 K 375 K

0.14 x10

2

0.16 x10

2

0.12 x10

2

0.11 x10

2

0.62 x10

(440)

(511)

(422)

(400)

(311) (222)

(220)

Intensity(a.u)

(111)

ICSD# 40-1119

Yb-MnZn Ho-MnZn Gd-MnZn Y-MnZn Pr-MnZn MnZn

20

30

40

50

2-Theta Degrees

Figure 1

60

70

80

8.58

12.5

8.56

12.0 11.5

8.52

11.0

8.50 8.48

10.5

8.46

10.0

8.44 8.42

9.5

8.40 9.0

8.38 Pr

Y

Gd

Ho

RE's doped MnZn samples (x)

Figure 2

Yb

Lattice parameter (Ao)

Crystallite Size (nm)

8.54

Figure 3

Figure 4

20

M(emu/g)

10

M nZn Pr-MnZn Gd-M nZn Yb-M nZn Y-MnZn Ho-M nZn

30 20 10

M(emu/g)

30

0 -10

0 -10

-20 -20

-30 -24000 -12000

0

12000 24000

Applied Field(Oe)

Figure 5

-30 -400-200 0 200 400

Applied Field(Oe)

Figure 6

Figure 7

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All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version.

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The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript

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Author’s name and Affiliation M. Yousafa, Majid Niaz Akhtarb,*, Baoyuan Wanga, Asma Noorc a

Hubei Collaborative innovation center for advanced Organic Chemical Materials, faculty of Physics and Electronic Science, Hubei University Wuhan, Hubei 430062 P.R China b Department of Physics, Muhammad Nawaz Sharif University of Engineering and Technology (MNSUET), Multan, 60000, Pakistan c Hubei Collaborative Innovation Center for Advanced Organic Chemical Materials, Hubei Key Lab of Ferro & Piezoelectric Materials and Devices, Ministry-of-Education Key Laboratory of Green Preparation and Application for Functional Materials, School of materials Science & Engineering, Hubei University, Wuhan 430062, China Corresponding Author: [email protected], [email protected]