Impact of strontium substitution on the structural, magnetic, dielectric and ferroelectric properties of Ba1-xSrxFe11Cr1O19 (x = 0.0–0.8) hexaferrites

Journal Pre-proofs Impact of strontium substitution on the structural, magnetic, dielectric and ferroelectric properties of Ba 1−x S r x F e 11Cr 1O19 (x = 0.0 – 0.8) hexaferrites M. Atif, M. Hanif Alvi, S. Ullah, Atta Ur Rehman, M. Nadeem, W. Khalid, Z. Ali, H. Guo PII: DOI: Reference:

S0304-8853(19)33901-0 https://doi.org/10.1016/j.jmmm.2020.166414 MAGMA 166414

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

14 November 2019 4 January 2020 6 January 2020

Please cite this article as: M. Atif, M. Hanif Alvi, S. Ullah, A. Ur Rehman, M. Nadeem, W. Khalid, Z. Ali, H. Guo, Impact of strontium substitution on the structural, magnetic, dielectric and ferroelectric properties of Ba 1−x S r x F e 11Cr 1O19 (x = 0.0 – 0.8) hexaferrites, Journal of Magnetism and Magnetic Materials (2020), doi:

https://doi.org/10.1016/j.jmmm.2020.166414

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.

© 2020 Published by Elsevier B.V.

Impact of strontium substitution on the structural, magnetic, dielectric and ferroelectric properties of 𝑩𝒂𝟏 ― 𝒙𝑺𝒓𝒙𝑭𝒆𝟏𝟏𝑪𝒓𝟏𝑶𝟏𝟗 (x = 0.0 – 0.8) hexaferrites M. Atifa,*, M. Hanif Alvia, S. Ullaha,c, Atta Ur Rehmana, M. Nadeemb, W. Khalida, Z. Alia, H. Guoc aFunctional bPolymer

cKey

Materials Lab, Department of Physics, Air University, PAF Complex E-9, Islamabad, Pakistan

Composite Group, Physics Division, Directorate of Science, PINSTECH, P.O. Nilore, Islamabad, Pakistan

Lab of Material Physics, Department of Physics and Engineering, Zhengzhou University, Zhengzhou, P.R. China

Dedicated to the memory of Prof. Roland Grössinger (1944 – 2018), Institut für Festkörperphysik, Technische Universität Wien, Austria

ABSTRACT Strontium substituted barium hexaferrites i.e., 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 with x = 0.0‒0.8 were prepared via co-precipitation technique. Single-phase magnetoplumbite structure was confirmed for all samples by X-ray diffraction, whereas scanning electron microscopy showed the variation in microstructure on Sr2+ substitution. Magnetic measurements revealed that the substitution causes prominent enhancement in saturation magnetization and coercive field for x = 0.4 sample. However, under same condition, decrease in real part of dielectric permittivity, dielectric loss tangent and ac conductivity were observed by impedance spectroscopy, while these parameters started to increase for x > 0.4. The dielectric permittivity exhibited frequencydependent dispersive behavior due to Maxwell-Wagner interfacial polarization whereas weak ferroelectric hysteresis loops were obtained for the prepared samples. Moreover, it was found that the Sr2+ substitution substantially enhanced the resistive properties which is elucidated in term of localization of charge carrier’s mobility due to reduction in 𝐹𝑒3 + ―𝑂 ― 𝐹𝑒2 + network and microstructural variations. Here, it is proposed that x = 0.4 sample exhibits maximum magnetization (64 emu/g), moderate real part of dielectric permittivity at low frequency (>103), diminished dielectric loss tangent (~0.05) and colossal value of resistance (~1010 Ω) which makes this material interesting for high frequency applications. Keywords: Hexaferrites; Co-precipitation; Porosity; Magnetic properties; Impedance *Corresponding author. E-mail address: [email protected] (M. Atif) 1

1.

Introduction Over the past few years, hexaferrites have been extensively used in the electronic industries

as permanent magnets, data storage, microwave absorbers, etc. due to their remarkable magnetic and dielectric properties [1,2]. However, among different types of hexaferrites, M-type hexaferrites i.e., 𝑀𝐹𝑒12𝑂19 (M = Ba, Sr, Pb) are the most widely investigated material on the basis of their excellent chemical stability, cost-effective and easy to synthesize [3]. Barium hexaferrite (i.e., 𝐵𝑎𝐹𝑒12𝑂19) has been gaining interest among researchers due to its tunable values of coercivity, higher values of saturation magnetization, high permittivity and permeability, high electrical resistivity and low losses [4,5]. With the appropriate cationic replacements at Ba2+ and/or Fe3+ site, the above-mentioned properties can be tailored according to the requirements. However, while determining effects of replacements, the crystal structure of these hexaferrites plays a decisive role. Generally, 𝐵𝑎𝐹𝑒12𝑂19 possesses hexagonal structure and has P63/mm space group. Whereas, its unit cell is made up of S (Fe6O8) and R (BaFe6O11) building blocks and is symbolically represented by 𝑅𝑆𝑅 ∗ 𝑆 ∗ ; here ‘*’ represents a rotation of the block by180o along c-axis of hexagonal unit cell [3]. In total, unit cell of 𝐵𝑎𝐹𝑒12𝑂19 contains 64 ions, out of which 2 are barium (Ba2+) ions, 38 are oxygen (O2-) ions and 24 iron (Fe3+) ions which are distributed in five different interstitial sites as: one tetrahedral site (4𝑓1), three octahedral sites (12𝑘, 2𝑎, 4𝑓2) and one trigonal bipyramid site (2𝑏). Moreover, within these interstitial sites, Fe3+ ions occupying 2𝑎 (1 ion), 2𝑏 (1 ion) and 12𝑘 (6 ions) sites have spins directing upwards while in 4𝑓1 (2 ions) and 4𝑓2 (2 ions), the ions spins are directed downwards [6]. So, the magnetic moment in total (μB/f.u) is defined by 4 Fe3+ ions in upward spin direction. Since, each Fe3+ ion has a net magnetic moment of 5 μB; therefore, 𝐵𝑎𝐹𝑒12𝑂19 has a net magnetic moment of 20 μB/f.u [7]. There are several reports on the enhancement of dielectric as well as magnetic properties of M-type 𝐵𝑎𝐹𝑒12𝑂19 by substituting barium (Ba2+) and/or iron (Fe3+) cations with different rareearth metals (Eu, Ca, Sr, etc.) and/or 3d transition metals (Mn, Zn, Ti, etc.), respectively [8-14]. From these reports, it has been found that the replacement of Fe3+ by some non-magnetic cation increases the net magnetic moment due to their preference to occupy at the anti-parallel tetrahedral (4𝑓1) site. Alternatively, if Fe3+ cation is substituted by some magnetic cation, the net magnetic moment is found to decrease due to weakening of crystal field strength and magnetic interaction between the magnetic cations leading to small coercive field. However, the partial replacement of Fe3+ ions by Cr3+ cations in 𝐵𝑎𝐹𝑒12 ― 𝑥𝐶𝑟𝑥𝑂19 hexaferrites decreases the saturation magnetization 2

(𝑀𝑠) with increasing Cr3+ content but the coercive field (𝐻𝑐) increases up to a certain content of Cr3+ [15,16]. Asiri et al. [17] have prepared 𝐵𝑎𝐶𝑟𝑦𝐹𝑒12 ― 𝑦𝑂19 (0.0 ≤ y ≤ 1.0) hexaferrites and found that the saturation magnetization is maximum for y = 0.3 but the coercive field decreases with increasing Cr3+ contents in 𝐵𝑎𝐶𝑟𝑦𝐹𝑒12 ― 𝑦𝑂19 hexaferrites. Whereas, the AC susceptibility measurements of 𝐵𝑎𝐹𝑒12 ― 𝑥𝐶𝑟𝑥𝑂19 (0.0 ≤ x ≤ 1.0) hexaferrites showed a frequency-dependent magnetic response [18]. In addition, the relative sensitivity of prepared samples is found to be strongly influenced by Cr3+ substitution and is highest for x = 0.3 sample. Slimani et al. [19] reported that the magnetic properties of Cr3+ substituted strontium hexaferrites (i.e., 𝑆𝑟𝐶𝑟𝑥𝐹𝑒12 ― 𝑥 𝑂19) are closely related to the distribution of Cr3+ ions on the five crystallographic sites. According to them, the magnetic parameters (𝑀𝑠, 𝑀𝑟, 𝐻𝑐) increased for lower Cr3+ concentration (x ≤ 0.4) due to favorably occupancy of Cr3+ ions at 12𝑘, 2𝑎 and 4𝑓2 sites and then these magnetic parameters decreased with increasing Cr3+ concentration on the basis of occupancy of Cr3+ ions at 12𝑘 and 2𝑎 sites for x > 0.4. Recently, Sözeri et al. [20] have investigated the effect of Cr3+ substitution on the magnetic and microwave absorption properties of 𝐵𝑎𝐹𝑒12 ― 𝑥𝐶𝑟𝑥𝑂19 (0.0 ≤ x ≤ 1.0) hexaferrites. They found that the magnetic properties decreased with increasing Cr3+ content while the Cr3+ substituted samples displayed better microwave properties than a pure 𝐵𝑎𝐹𝑒12𝑂19 sample. Moreover, Kumar et al. [21] explored the structural, magnetic and dielectric properties of substituted barium hexaferrites and found that the physical properties are not only influenced by the cationic distribution, but lattice strain is also responsible for altering the properties of prepared hexaferrites. They have established a correlation between the magnetic and dielectric properties with lattice strain developed within the substituted hexaferrites and then elucidated the observed variation in magnetic properties because of lattice strain mediated magnetism in the prepared barium hexaferrites. Here, the aforementioned reports are mostly concerned with the effects of morphology and cationic distribution on the magnetic properties of these hexaferrites; however, very limited discussion is found on their effects on the dielectric and resistive properties. Thus, it is considered useful to explore the microstructure, magnetic, dielectric and ferroelectric behavior, as this will improve the understanding of the magnetic and dielectric characteristics of hexaferrites. In the present work, strontium substituted barium hexaferrites i.e., 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 with x = 0.0, 0.2, 0.4, 0.6 and 0.8 have been prepared by co-precipitation technique. Apart from Fe3+ ions, the prepared hexaferrites have addition of Cr3+ ions as well. Here, the selection of that concentration level for Cr3+ in 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites (i.e., x = 0.0) is based on the improved 3

dielectric [22] and microwave absorption [20] properties for 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19 than a pure 𝐵𝑎𝐹𝑒12𝑂19 hexaferrite sample. Subsequently, we investigated the effect of Sr2+ substitution on the crystallographic phase, morphologies, magnetic, dielectric and ferroelectric properties of prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites, in order to define an optimum composition which may be

further useful for high frequency applications.

2.

Experimental details 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with different concentrations (x = 0.0, 0.2, 0.4, 0.6 and

0.8) were synthesized via co-precipitation technique. The precursors used for the preparation of hexaferrites were 𝐵𝑎(𝑁𝑂3)2, 𝐹𝑒(𝑁𝑂3)3.9𝐻2𝑂, 𝐶𝑟(𝑁𝑂3)3.6𝐻2𝑂, 𝑆𝑟(𝑁𝑂3)2.6𝐻2𝑂, 𝐾𝑂𝐻 and distilled water. Firstly, the stoichiometric amounts of nitrates were dissolved in 100 ml of distilled water. Then, the starting mixture containing nitrates were added drop wise in the 100 ml solution of precipitating catalyst 𝐾𝑂𝐻. Precipitates were formed by heating the solutions at 70 °𝐶 under stirring at a hotplate for 2 h. The obtained precipitates were then collected and washed several times before getting dried at 100 °𝐶 overnight in oven. Afterward, the obtained dried powders were annealed at 1000 °𝐶 for 12 h. Finally, pellets were formed from the annealed powders by applying a pressure of 5 tons/cm2 followed by 6 h sintering at 1200 °𝐶. X-ray diffraction (XRD) patterns of the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 samples were taken by using CuKα radiation (Siemens D-5000 diffractometer) from 20o to 90o with a scan step of 0.02o. The morphological studies of the prepared samples were done by the scanning electron microscope (Hitachi S4800, FE-SEM). The room temperature magnetic properties of all samples were measured by Magnetic measurement system (MPMS XL-7, Quantum design Corporation, USA). Sintered pellets of all samples were subjected to impedance spectroscopy at room temperature, within the frequency range of 0.05 Hz to 10 MHz, using an Alpha-N Analyser (Novocontrol, Germany). Ferroelectric measurements were also carried out at room temperature by using Precision Multiferroic II ferroelectric tester (Radiant Technology Inc., USA).

3.

Results and discussion The XRD patterns of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with different concentrations (x

= 0.0, 0.2, 0.4, 0.6 and 0.8) are presented in Fig. 1. The observed XRD patterns correspond to hexagonal structure with space-group P63/mmc in agreement with the JCPDS file no. 84-0757 4

[23]. No un-indexed peaks have been found in the XRD patterns which confirm the purity of prepared hexaferrites. It is also observed that the peak position of substituted samples moved slightly toward the higher angles with increasing Sr2+ contents (x). The shifting of peaks can be associated with the difference in ionic radii among Sr2+ (1.32 Å) and Ba2+ (1.49 Å) cations [24]. The relationships used to estimate lattice parameters (a and c) and volume of unit cell (V) for the 1

4ℎ2 + ℎ𝑘 + 𝑘2

prepared hexaferrites are given as: 𝑑2 = 3

𝑎2

and d being the inter-planner spacing) and 𝑉 =

𝑙2

+ 𝑐2 (h,k,l being the miller indices of the planes 3 2

𝑎2𝑐, respectively [25]. The values of lattice

parameters for the un-substituted hexaferrite (i.e., 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19) is found to be a = 5.883 Å and c = 23.381 Å. Here, the obtained values are similar to the respective values of parent compound 𝐵𝑎𝐹𝑒12𝑂19 (a = 5.889 Å and c = 23.172 Å) prepared under similar conditions, due to almost identical ionic radii of Cr3+ (0.63 Å) and Fe3+ (0.64 Å) cations [24]. However, with the substitution of Sr2+ in place of Ba2+ cations, the values of a and c are found to be gradually decreased, as shown in Table 1, due to the fact that ionic radii of Sr2+ is smaller than Ba2+ cations. As a result of this, the volume of unit cell (V) is also found to decrease with increasing Sr2+ substitution (x), which is in accordance with the literature [26]. Moreover, the strain factor (ε) and average crystallite size (D) of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites were estimated using the Williamson-Hall method, which is defined as [27]: 𝛽𝑐𝑜𝑠𝜃 =

𝑘𝜆 + 4𝜀𝑠𝑖𝑛𝜃 𝐷

(1)

where β, θ, k and λ are the full width at half maxima of diffraction peak, Bragg angle, Scherrer constant and X-ray’s wavelength. Fig. 2 shows the representative plot between βcosθ versus 4sinθ for x = 0.4 sample. Here, the slope of fitted line from the plot provides a value of lattice strain (ε) within the crystal and y-intercept yields the average crystallite size (D). The estimated values of D and ε for all prepared samples by using Eq. (1) are tabulated in Table 1. It is found that the values of D vary within the range of 51‒74 nm with increasing Sr2+ substitution. Whereas, the value of ε apparently increases with increasing Sr2+ substitution which might be because of lattice strain produced by the replacement of Ba2+ cations with comparatively smaller Sr2+ cations in the crystal lattice. The surface morphological analysis of prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites were studied using SEM. Fig. 3 demonstrates the SEM micrographs for the prepared hexaferrites with different concentrations sintered at 1200°𝐶 for 6 h. The SEM micrographs reveal that 5

heterogeneous distribution of grain sizes are observed in all of the samples and most of the grains are of hexagonal shape. The average grain size (G) was estimated using the ImageJ software and the obtained values are tabulated in Table 1. It is found that initially the average grain size decreases from 3.17 μm (x = 0.0) to 2.23 μm (x = 0.4). Therefore, the porosity of prepared hexaferrites increases due to observed decrease in the grain size. However, with further increase in Sr2+ concentration (i.e., x > 0.4), some of the grains are tended to aggregation formed by the Van der Waals interactions among the grains [28], resulting in the improvement of inter-granular connectivity along with an increase of grain size to 3.73 μm (x = 0.8). Fig. 4 displays the magnetic hysteresis loops of the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites measured at room temperature. These hysteresis loops indicate that all prepared hexaferrites display ferromagnetic behavior. Whereas, for all compositions, hysteresis loops are found to be non-saturated in the available range of applied magnetic field (±15 kOe). For the unsubstituted hexaferrite sample (i.e., 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19), the value of saturation magnetization (i.e., Ms at 15 kOe) is found to be 42 emu/g. Comparing with the parent compound 𝐵𝑎𝐹𝑒12𝑂19, the value of Ms obtained for our 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19 hexaferrite is smaller than 𝐵𝑎𝐹𝑒12𝑂19 (Ms = 52 emu/g) [29]. As discussed earlier, the magnetic moment of M-type hexaferrites (i.e., 𝐵𝑎𝐹𝑒12𝑂19) is 20 𝜇𝐵 which can be inferred from the net magnetic moments of Fe3+ cations at different sites: 𝑀 = 𝑀 (12𝑘 + 2𝑎 + 2𝑏)↑ ― 𝑀(4𝑓1 +4𝑓2)↓ [2]. However, a partial substitution of Cr3+ (3 𝜇𝐵) in place of Fe3+ (5 𝜇𝐵) cations in 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19 reduces the ferromagnetic order in comparison to 𝐵𝑎𝐹𝑒12𝑂19 hexaferrites. This is due to the fact that Cr3+ cations have great tendency to reside on 12k, 2a and 4𝑓2 sites when Fe3+ cations are replaced with Cr3+ cations, which will result in a breakdown of magnetic collinearity of the lattice [16]. Moreover, it can also be seen from the figure that with the substitution of Sr2+ in 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites, the magnetization (Ms) value is found to increase from 42 emu/g (x = 0.0) to 64 emu/g (x = 0.4). Further rise in Sr2+ substitution (i.e., after x = 0.4), the magnetization value begins to decrease, reaching 45 emu/g at x = 0.8 (Table 1). Here, the initial rise observed in the value of Ms is credited to competition among Ba2+ and Sr2+ cations which causes Sr2+ cations to occupy octahedral sites [29]. As a result, Fe3+ cations begin to migrate from octahedral to tetrahedral sites in order to accommodate substituted Sr2+ cations which results in the initial increase of Ms till x = 0.4 sample. Further rise in Sr2+ substitution (i.e., after x = 0.4), Sr2+ cations will disrupt the magnetic collinearity, resulting in a spin canted structure,

6

which causes a decrease in magnetization. Moreover, the magnetic moment (𝑛𝐵) per unit formula was also calculated by using the relation [30,31]: 𝑛𝐵 =

𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 × 𝑀𝑠

(2)

5585

where Ms is the saturation magnetization of the samples in emu/g. The obtained values of 𝑛𝐵 for the prepared hexaferrites are presented in Table 1. It is found that the initial increase in 𝑛𝐵 values is attributed to the enhancement of superexchange interactions between different sites [32,33]. Whereas, with further increase in Sr2+ substitution (i.e., x > 0.4), the value of 𝑛𝐵 begins to increase with x indicating a weakening of superexchange interactions, as discussed above. Law of Approach to Saturation (𝐿𝐴𝑆) was used to determine compositional dependence of magnetocrystalline anisotropy constant (𝐾1) for the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites that describes the magnetic field (𝐻) dependence of magnetization for 𝐻 >> 𝐻𝑐 and is expressed as [34]:

(

𝑀(𝐻) = 𝑀𝑠 1 ―

𝑏

)

(3)

𝐻2

where 𝑀𝑠, H and b are the saturation magnetization, applied magnetic field and magnetocrystalline anisotropy

contribution,

respectively.

For

the

fitting

process,

the

high-fields

part

(8 𝑘𝑂𝑒 ≤ 𝐻 ≤ 15 𝑘𝑂𝑒) of the hysteresis curves were checked by Eq. (3) and the values of 𝑀𝑠 and b were obtained for all samples. Using these fitted values, the magnetocrystalline anisotropy constant (𝐾1) was evaluated using the relation [35]: 12

( )

15𝑏 𝐾 1 = 𝑀𝑠 4

(4)

The estimated values of 𝐾1 are tabulated in Table 1. It is found that initially the value of 𝐾1 increases up to x = 0.4 and then its value decreases with further increase in Sr2+ substitution (x > 0.4). Here, the observed behavior in 𝐾1 with x is attributed to the migration of Fe3+ cations between different sites to accommodate substituted Sr2+ cations (as discussed above). The inset in Fig. 4 elaborates the low field region of hysteresis loops, from where we have calculated the coercivity (Hc) values for all prepared samples. The Hc value is found to increase as Sr2+ substitution increases from 1.8 kOe (x = 0.0) to 3.5 kOe (x = 0.4), but with further increase in Sr2+ substitution, Hc value 7

begins to decrease up to 1.2 kOe (x = 0.8). Here, the observed variation in Hc value (see Table 1) depends on the microstructural changes (like grain sizes and defects) and magnetic anisotropy constant [10]. It can be seen from Fig. 3 that with increasing Sr2+ substitution, there is an increase in the intergranular pores due to reduction in the grain size which increases Hc value till x = 0.4 composition. Further enhancement in Sr2+ concentration (i.e., after x = 0.4), the decrease in strength of intergranular pores because of growth in the size of grain causing a reduction of the Hc value. The room temperature frequency-dependence of real part of dielectric permittivity (𝜀′) for the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites is shown in Fig. 5(a). The dielectric characteristics depicted by the prepared hexaferrites may be elucidated on the basis of Maxwell and Wagner model [36]. According to this model, conducting ferrites of dielectric structure comprise of layered structure in which conducting grains (Gs) are surrounded by feebly conducting grain boundaries (GBs). In the low frequencies, when an electric field is applied, the charge carriers begins to migrate through Gs and then accumulate at GBs causing high interfacial polarization and thereby 𝜀′ possessing a very high value. Whereas, in high frequency range, the interfacial polarization significantly decreases due to lagging of electric dipoles along with the applied field, which results in substantial decrease in the values of 𝜀′ [37,38]. Generally, the physical interpretation of dielectric dispersion can be done on the basis of electric dipoles formed by Sr2+, Ba2+ and Fe3+ cations with their surrounding O2- anions. However, the main source of polarization in hexaferrites is the conversion of Fe3+ into Fe2+ ions on the octahedral sites. Therefore, the electron hopping that occurs between adjacent Fe3+ and Fe2+ cations leads to local displacement of charge carriers, thereby contributing to dielectric polarization and relaxation [39,40]. From the figure, it can be seen that the maximum value of 𝜀′ is obtained for the un-substituted x = 0.0 sample (i.e., 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19). Comparing with the parent compound 𝐵𝑎𝐹𝑒12𝑂19, the value of 𝜀′ obtained for our 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19 hexaferrite is higher than 𝐵𝑎𝐹𝑒12𝑂19 prepared under similar conditions [22]. A possible reason for this enhanced behavior is that the exchange reaction between 𝐹𝑒3 + ―𝑂 ― 𝐶𝑟3 + is weaker than 𝐹𝑒3 + ―𝑂 ― 𝐹𝑒3 + making these 𝐵𝑎𝐹𝑒11𝐶𝑟1𝑂19 sample more active in the available frequency range [41]. However, with the substitution of Sr2+ in 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1 𝑂19 hexaferrites, it is found that the value of 𝜀′ initially decreases till x = 0.4 sample and then it begins to increase with further increase in Sr2+ substitution (x). Here, the observed decrease in the value of 𝜀′ is attributed to the reduction in electron transfer between Fe3+ and Fe2+ cations at octahedral sites due to migration of Fe3+ cations from octahedral to tetrahedral sites in order to 8

accommodate substituted Sr2+ cations at octahedral sites. However, with further rise in Sr2+ substitution (i.e., after x = 0.4), the lattice strain become more evident (as discussed earlier) causing the formation of defects. As a result, there is an enhancement in the space charge polarization which leads to localization in the carrier’s mobility and hence increase in the 𝜀′ value. Fig. 5(b) shows the variation of dielectric loss tangent (tanδ) with frequency for the prepared hexaferrites at room temperature. It is evident from the figure that tanδ decreases, as the frequency increases for all samples, similar to 𝜀′ behavior, followed by a peak behavior. These peaks appear due to resonance phenomenon when the frequency of electron jumping during electron exchange between Fe3+ and Fe2+ cations synchronizes with the applied electric field’s frequency [42]. Moreover, with increasing Sr2+ substitution in 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites, the value of tanδ is observed to decrease up to x = 0.4 along with a shifting of resonance peak towards the lower frequencies. The reason may be a reduction in the electron transfer among Fe3+ and Fe2+ cations. However, after the Sr2+ substitution of x > 0.4, the value of tanδ begins to increase as the resonance peak shifts to higher frequencies because of increase in the electron transport, as discussed above. Here, the achieved values of tanδ for the substituted samples lie between the range of 0.05 ― 0.21 at a frequency of 107 Hz. The diminished values of tanδ for the substituted samples, in comparison to literature values [23], are accredited to the respective reduction of electron transport due to Sr2+ substitution in comparison to un-substituted sample. Fig. 6 shows the Nyquist plots (Z/ vs. Z//) for the synthesized 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites measured at room temperature. Typically, the Nyquist plots are characterized by the presence of one or more semicircles, corresponding to different relaxation processes present in the system. For our hexaferrites samples, two poorly resolved semicircles are clearly visible which are usually attributed to the contributions related to grains (Gs) and grain boundaries (GBs) [43,44]. It is evidently observed from the figure that the semicircle’s radius at a lower frequency region is considerable higher than the radius of semicircle at higher frequency region, indicating huge difference between the magnitude of Gs and GBs resistance. Moreover, the shape and size of both semicircles is found to be changed with increasing Sr2+ substitution due to associated changes of the impedances in the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites. To further investigate the Nyquist plots, the model of equivalent circuit was used to fit the experimental data. Initially, we have fitted the Nyquist plots by using two parallel circuits (𝑅1𝐶1) and (𝑅2𝑄) connected in series. Here (𝑅1, 𝑅2) are the resistances, 𝐶1 correspond to capacitance and 𝑄 is a constant phase element 9

indicating the departure from ideal dielectric response. Mathematically, the capacitance (C) of Q is given by: 𝐶=𝑄

1 𝑛 (1 ― 𝑛) 𝑛

𝑅

(5)

where ‘n’ represents degree of deviation of C from ideal behavior. Generally, the value of n varies from 0 (for ideal resistor) to 1 (for ideal capacitor) [45]. The software ZView was employed to fit experimental data by using the model (equivalent circuit) discussed above. From this fitting, it is found that the experimental data is well fitted in the high frequency region; however, a disparity is found among the fitted and experimental data in the Nyquist plots at a lower frequency region which might be due to inhomogeneity as well as defect chemistry in the prepared samples. To address these issues, Vendrell et al. [46] recently proposed different combinations of new equivalent circuits to separate out the contribution of different electro-active regions from the impedance measurements of electroceramic materials. In the similar way, we have also tried to modify our above mentioned equivalent circuit by the incorporation of another capacitance (𝐶2) parallel to the second circuit i.e., (𝑅1𝐶1)(𝑅2𝑄𝐶2). From this fitting, we found a good agreement among the fitted and experimental data in the Nyquist plots at both frequency regions. The values of the fitting parameters are listed in Table 2. On the basis of these obtained parameters, we found that the first circuit (i.e., 𝑅1𝐶1) has parameters comparable to the geometrical capacitance of Gs ( ≈ 10 ―12 ― 10 ―13 Fcm-1) [47]. Therefore, the semicircles on the higher frequency region is due to the contribution of Gs conduction. However, the fitting results from the second circuit (i.e., 𝑅2𝑄 𝐶2) exhibit that 𝑅2 is higher than 𝑅1, while Q is of the order of ≈ 10 ―9 ― 10 ―10 and the value of n are in the range of 0.45–0.53. By using Eq. (5), we have calculated the values of capacitance (C) for all of the samples and found that the obtained values of C correspond to the geometrical capacitance of GBs ( ≈ 10 ―8 ― 10 ―10 Fcm-1) [47]. Whereas, the value of C2 is found to be of the order of ≈ 10 ―11 Fcm-1 which might be attributed to the contribution of space charge or interface [47]. Here, the presence of space charge present around Gs may create an interface which hinders the carrier mobility. This localization of space charge can potentially contribute towards the high values of dielectric constant of these type of hexaferrites. Moreover, it is found that the value of total resistance (𝑅𝑇 = 𝑅1 + 𝑅2) increases about two orders of magnitude with increasing Sr2+ substitution till x = 0.4. However, with further increase in Sr2+ substitution (i.e., x > 0.4), the value of total resistance starts to decrease. Here, the initial significant rise in the values of 𝑅𝑇 and 𝑅2 with Sr2+ substitution can be understood due to following reasons. Firstly, as the increase in Sr2+ 10

substitution decreases the grain size (as discussed earlier) which in turn increases the porosity of samples; hence, the GBs region increases with highly resistive nature. Secondly, it is well established that the conduction mechanism within hexaferrites is governed by electron hopping between Fe3+ and Fe2+ ions at the octahedral site. However, as the number of Sr2+ cations increase at octahedral sites with x (as discussed above) suppresses Fe3+ turning into Fe2+ in the prepared samples. Thus, the hoping mobility through 𝐹𝑒3 + ―𝑂 ― 𝐹𝑒2 + network decreases which may result in the overall decrease of conductivity; hence, resistance is expected to increase till x = 0.4 sample. Whereas, the decrease observed in the values of resistance after x > 0.4 is ascribed on the basis of minimizing the GBs region due to increase in the strain induced defects (as discussed earlier) which lead to decrease resistance contribution from GBs resulting in the enhancement of conductivity. The frequency-dependence of ac conductivity (𝜎𝑎𝑐) for the synthesized 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1 𝑂19 hexaferrites with x = 0.0, 0.2, 0.4, 0.6 and 0.8 are depicted in the Fig. 7. With increasing frequency, 𝜎𝑎𝑐 gradually increases and then shows dispersion for higher frequency for all the samples which is in accordance with Jonscher power law [48]: 𝜎𝑎𝑐(𝑤) = 𝜎0 + 𝐴𝑤𝑠

(6)

where w, 𝜎0, A and s are angular frequency, frequency-independent conductivity, characteristic parameter that determine strength of polarizability and slope of the frequency-dependent region having values in the range from 0 to 1, respectively. By using Eq. (6), the experimental data was fitted, and the results are in good agreement with the experimental data. The values of fitted parameters are presented in Table 1. Generally, the conduction mechanism in hexaferrites is governed between Fe3+ and Fe2+ ions at the octahedral site by electron hopping. In the lower frequency region, GBs are more effective thus frequency-independent behavior is attained. Whereas, at higher frequency region, the increase in 𝜎𝑎𝑐 is ascribed to increased electron hopping between Fe3+ and Fe2+ ions at the octahedral sites and also due to Gs effect [49]. With the increase in Sr2+ substitution in the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites, one can see that initially 𝜎𝑎𝑐 decreases because the number of Fe3+ ― Fe2+ ions decreases, limiting the mobility of electron transfer at octahedral sites. Above x = 0.4, the 𝜎𝑎𝑐 increases because of increase in the presence of conducting channels at octahedral sites which leads to increase in the electron transport, as discussed above. Furthermore, the inset of figure displays the behavior of A and s versus Sr2+ substitution in the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites. It can be seen that the behavior of 11

A and s are contrary to one another. Initially, the value of A decreases and s increases sharply with increasing Sr2+ substitution (x). This behavior is attributed to the reduced interaction among the charge carriers which further decreases the polarization [50]. Above x = 0.4, the interaction among the charge carriers begins to increase, thus the value of A increases and s decreases, slightly. This behavior indicates that the electron hopping enhances the polarization along with increased conduction. Fig. 8(a-e) display the polarization against applied electric field (i.e., P-E) loops of 𝐵𝑎1 ― 𝑥 𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites. Since, the ferroelectric behavior in M-type of hexaferrites is elucidated by considering 𝐹𝑒𝑂6 octahedron in a unit cell of M-type hexaferrites wherein Fe cation is positioned at the middle of an octahedron of 𝑂2 ― anions [51]. When an external electric field is applied, Fe cation display the off-center shift which leads to the distortion of 𝐹𝑒 ― 𝑂 bond, thereby induces electric polarization in M-type hexaferrites. However, it is evident from the figure that all samples display unsaturated hysteresis loop behavior which might be due to the formation of oxygen vacancies on the surface of grains producing the leakage current instead of polarization [52,53]. Thus, the weak response of electric dipoles for the applied field is observed for the prepared samples. Table 3 lists the ferroelectric parameters of all samples calculated from P-E loops. It can be seen that the value of remnant polarization (Pr) reduces with x due to decrease in the off-center polarization developed in the 𝐹𝑒𝑂6 octahedron on the basis of difference in the ionic radii of Sr2+and Ba2+cations. However, for x = 0.4 sample, slightly higher values of remnant polarization (Pr) and coercivity (Hc) is obtained in comparison to other substituted samples which is probably due to better microstructural features having reduced distribution of grain sizes. The variation in leakage current density (J) with Sr2+ substitution (x) is presented in Fig. 9(a) for the prepared 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites measured under an applied voltage (V) of ±600 V. It is observed that, with increasing x, the value of leakage current density gradually reduces from 1.27 x 10-4 A/cm2 (x = 0.0) to 2.57 x 10-7 A/cm2 (x = 0.8). This behavior is attributed to the reduction of oxygen vacancies due to decrease in the Fe2+/Fe3+ ratio at octahedral sites (as discussed above). However, the order of reduction in the value of leakage current density for x = 0.4 composition is more in comparison to other compositions which may be accredited to comparatively reduced grain size [54], as seen from SEM analysis. Moreover, to understand the leakage current mechanism, we have plotted the leakage current density (J) against electric field (E) in a logarithmic scale, as presented in Fig. 9(b). It can be seen that a linear behavior is observed 12

between the graph of log(J) and log(E) which is in accordance with the power law: J α 𝐸𝑚, where m is the logarithmic plot’s slope which suggests the conduction nature [55]. Generally, the slope ‘m’ has a value lying between 1 (Ohmic conduction) and 2 (space charge limited conduction). From the linear fitting, ‘m’ decreases from 1.54 (x = 0.0) to 1.14 (x = 0.8) with increasing Sr2+ substitution (see Table 3) which indicates that Ohmic conduction is the dominating conduction mechanism in the prepared samples.

4.

Conclusion In summary, we have successfully synthesized 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with x

= 0.0‒0.8 by co-precipitation technique. XRD analysis confirmed the formation of single phase M-type hexaferrites. SEM images revealed that grains are of hexagonal like shape and porosity of samples changes with Sr2+ substitution. Magnetic hysteresis loops showed that the magnetic parameters (Ms and Hc) first increases and afterward decreases with increasing Sr2+ substitution with a maximum value of Ms (64 emu/g) and Hc (3.4 kOe) for x = 0.4 sample. Whereas, the ferroelectric hysteresis loops indicated slightly higher values of coercivity (Ec) and remnant polarization (Pr) for x = 0.4 sample in comparison to other substituted samples. Impedance spectroscopy analysis displayed the presence of two relaxation processes attributed to Gs and GBs contributions; however, in determining the resistive properties of prepared samples GBs plays a dominant role. Significant increase in the total resistance is obtained for x = 0.4 sample; while real part of dielectric permittivity, dielectric loss tangent and ac conductivity exhibited their minimum values. The observed variation in dielectric/resistive properties with substitution is explained on the basis of reduced polarizability and microstructural effects (i.e., grain size and porosity). Here, our findings of maximum magnetization, diminished dielectric loss tangent, moderate dielectric permittivity and colossal value of resistance for x = 0.4 sample makes this material interesting for high frequency applications. Acknowledgement The Authors are grateful to the Higher Education Commission (HEC) of Pakistan for providing financial support through the research grants # 5232/Federal/NRPU/R&D/HEC/2016 and # TDF044/2017.

13

References [1] K. Tanwar, D.S. Gyan, P. Gupta, S. Pandey, O. Parkash, D. Kumar, Investigation of crystal structure, microstructure and low temperature magnetic behavior of Ce4+ and Zn2+ co-doped barium hexaferrites (BaFe12O19), RSC Adv. 8 (2018) 19600–19609. [2] L. Deng, Y. Zhao, Z. Xie, Z. Liu, C. Tao, R. Deng, Magnetic and microwave absorbing properties of low-temperature sintered BaZrxFe12-xO19, RSC Adv. 8 (2018) 42009–42016. [3] R.C. Pullar, Hexagonal ferrites: A review of the synthesis, properties and applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (2012) 1191–1334. [4] S. Castro, M. Gayoso, J. Rivas, J.M. Greneche, J. Mira, C. Rodríguez, J. Magn. Magn. Mater. 152 (1996) 61–69. [5] P. Campbell, Permanent Magnet Materials and their Application, Cambridge University Press, Cambridge, 1994. [6] B.J. Evans, F. Grandjean, A.P. Lilot, R.H. Vogel, A. Gérard, 57Fe hyperfine interaction parameters and selected magnetic properties of high purity MFe12O19 (M=Sr, Ba), J. Magn. Magn. Mater. 67 (1987) 123– 129. [7] R.A. McCurrie, Ferromagnetic Materials: Structure and Properties, Academic Press Limited, London, 1994. [8] Z.W. Li, C.K. Ong, Z. Yang, F.L. Wei, X.Z. Zhou, J.H. Zhao, A.H. Morrish, Site preference and magnetic properties for a perpendicular recording material: BaFe12-xZnx/2Zrx/2O19 nanoparticles, Phys. Rev. B 62 (2000) 6530–6537. [9] A. Zafar, Atta ur Rahman, S. Shahzad, S. Anwar, M. Khan, A. Nisar, M. Ahmad, S. Karim, Electrical and magnetic properties of nano-sized Eu doped barium hexaferrites, J. Alloys Compd. 727 (2017) 683– 690. [10] N. Tran, H.S. Kim, T.L. Phan, D.S. Yang, B.W. Lee, Electronic structure and magnetic properties of Ba1-xSrxCoFe11O19 hexaferrites, Ceram. Int. 44 (2018) 12132–12136. [11] N. Raghuram, T. Subba Rao, K. Chandra Babu Naidu, Electrical and impedance spectroscopy properties of hydrothermally synthesized Ba0.2Sr0.8-yLayFe12O19 (y = 0.2–0.8) nanorods, Ceram. Int. 19. https://doi.org/10.1016/j.ceramint.2019.11.042 [12] C. Singh, S.B. Narang, S. Hudiara, Y. Bai, K. Marina, Hysteresis analysis of Co–Ti substituted Mtype Ba–Sr hexagonal ferrite, Mater. Lett. 63 (2009) 1921–1924. [13] H. Sözeri, H. Deligöz, H. Kavas, A. Baykal, Magnetic, dielectric and microwave properties of M–Ti substituted barium hexaferrites (M = Mn2+, Co2+, Cu2+, Ni2+, Zn2+), Ceram. Int. 40 (2014) 8645–8657. [14] N. Raghuram, T.S. Rao, K.C.B. Naidu, Magnetic properties of hydrothermally synthesized Ba1xSrxFe12O19 (x = 0.0–0.8) nanomaterials, Appl. Phys. A 125 (2019) 839. [15] S. Kumar, S. Supriya, R. Pandey, L.K. Pradhan, M. Kar, Crystal structure and magnetic properties of Cr doped barium hexaferrites, AIP Conf. Proc. 1942 (2018) 130040. [16] S. Ounnunkad, P. Winotai, Properties of Cr-substituted M-type barium ferrites prepared by nitrate– citrate gel-autocombustion process, J. Magn. Magn. Mater. 301 (2006) 292–300. [17] S. Asiri, S. Güner, A.D. Korkmaz, Md. Amir, K.M. Batoo, M.A. Almessiere, H. Gungunes, H. Sözeri, A. Baykal, Magneto-optical properties of BaCryFe12-yO19 (0.0 ≤ y ≤ 1.0) hexaferrites, J. Magn. Magn. Mater. 451 (2018) 463–472. [18] Y. Slimani, A. Baykal, A. Manikandan, Effect of Cr3+ substitution on AC susceptibility of Ba hexaferrite nanoparticles, J. Magn. Magn. Mater. 458 (2018) 204–212. 14

[19] Y. Slimani, A. Baykal, Md. Amir, N. Tashkandi, H. Güngüneş, S. Guner, H.S. El Sayed, F. Aldakheel, T.A. Saleh, A. Manikandan, Substitution effect of Cr3+ on hyperfine interactions, magnetic and optical properties of Sr-hexaferrites, Ceram. Int. 44 (2018) 15995–16004. [20] H. Sözeri, F. Genç, M.A. Almessiere, I.S. Ünver, A.D. Korkmaz, A. Baykal, Cr3+-substituted Ba nanohexaferrites as high-quality microwave absorber in X band, J. Alloys Compd. 779 (2019) 420–426. [21] S. Kumar, M.K. Manglam, S. Supriya, H.K. Satpal, R.K. Singh, M. Kar, Lattice Strain Mediated Dielectric and Magnetic Properties in La Doped Barium Hexaferrite, J. Magn. Magn. Mater. 473 (2019) 312–319.

[22] Atta Ur Rehman, M. Atif, M. Nadeem, Structural and impedance spectroscopy properties of Cr3+ substituted BaFe12-xCrxO19 (0.0 ≤ x ≤ 1.0) hexaferrites, submitted (2019). [23] F.M.M. Pereira, C.A.R. Junior, M.R.P. Santos, R.S.T.M. Sohn, F.N.A. Freire, J.M. Sasaki, J.A.C. de Paiva, A.S.B. Sombra, Structural and dielectric spectroscopy studies of the M-type barium strontium hexaferrite alloys (BaxSr1-xFe12O19). J. Mater. Sci.: Mater. Electron. 19 (2008) 627–638. [24] R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic distances in halides and chalcogenides, Acta Cryst. A 32 (1976) 751–767. [25] B. D. Cullity, “Plane Spacings ” in lattice geometry, Elements of X-Ray Diffraction, edited by M. Cohen. Addison-Wesley publishing company Inc. MI, U.S.A, 1978. [26] N. Raghuram, T.S. Rao, K.C.B. Naidu, Investigations on functional properties of hydrothermally synthesized Ba1-xSrxFe12O19 (x = 0.0−0.8) nanoparticles, Mat. Sci. Semicon. Proc. 94 (2019) 136–150. [27] G.K. Williamson, W.H. Hall, X-ray line broadening from filed aluminium and wolfram, Acta Met. 1 (1953) 22–31. [28] M.A. Almessiere, B. Unal, Y. Slimani, A.D. Korkmaz, N.A. Algaroua, A. Baykal, Electrical and dielectric properties of Nb3+ ions substituted Ba-hexaferrites, Results in Physics 14 (2019) 102468. [29] S.B. Narang, C. Singh, Y. Bai, I.S. Hudiara, Microstructure, hysteresis and microwave absorption analysis of Ba1-xSrxFe12O19 ferrite, Mater. Chem. Phys. 111 (2008) 225–231. [30] M.A. Almessiere, Y. Slimani, M. Sertkol, M. Nawaz, A. Baykal, I. Ercan, The impact of Zr substituted Sr hexaferrite: Investigation on structure, optic and magnetic properties, Results in Physics 13 (2019) 102244. [31] Nehru Boda, Gopal Boda, K. Chandra Babu Naidu, M. Srinivas, Khalid Mujasam Batoo, D. Ravinder, A. Panasa Reddy, Effect of rare earth elements on low temperature magnetic properties of Ni and Co-ferrite nanoparticles, J. Magn. Magn. Mater. 473 (2019) 228-235. [32] M.A. Almessiere, Y. Slimani, H.S. El Sayed, A. Baykal, S. Ali, I. Ercan, Investigation of Microstructural and Magnetic Properties of BaVxFe12-xO19 Nanohexaferrites, J. Supercond. Nov. Magn. 32 (2019) 1437. [33] M.A. Almessiere, Y. Slimani, H.S. El Sayed, A. Baykal, Ca2+ and Mg2+ incorporated barium hexaferrites: structural and magnetic properties. J. Sol-Gel Sci. Technol. 88 (2018) 628–638. [34] M.A. Almessiere, Y. Slimani, H.S. El Sayed, A. Baykal, I. Ercan, Microstructural and magnetic investigation of vanadium-substituted Sr-nanohexaferrite, J. Magn. Magn. Mater. 471 (2019) 124–132. [35] M.A. Almessiere, Y. Slimani, A. Baykal, Impact of Nd-Zn co-substitution on microstructure and magnetic properties of SrFe12O19 nanohexaferrite, Ceram. Int. 45 (2019) 963–969. [36] K.W. Wagner, The Distribution of Relaxation Times in Typical Dielectrics, Ann. Phys. 40 (1993) 817– 819. [37] M. Atif, M. Idrees, M. Nadeem, M. Siddique, M.W. Ashraf, Investigation on the structural, dielectric and impedance analysis of manganese substituted cobalt ferrite i.e., Co1−xMnxFe2O4 (0.0 ≤ x ≤ 0.4), RSC Adv. 6 (2016) 20876–20885. [38] S. Dastagiri, M.V. Lakshmaiah, K.C. Babu Naidu, N.S. Kumar, A. Khan, Induced dielectric behavior in high dense AlxLa1-xTiO3 (x = 0.2–0.8) nanospheres, Mater Sci: Mater Electron 30 (2019) 20253–20264. [39] N. Yasmin, M. Zahid, H.M. Khan, M. Hashim, Misbah Ul Islam, S. Yasmin, M. Altaf, B. Nazar, M. Safdar, M. Mirza, Structural and dielectric properties of Gd-Zn substituted Ca0.5Ba0.5Fe12O19 M-type hexaferrites synthesized via auto- combustion method, J. Alloys Compd. 774 (2019) 962–968. 15

[40] N.S. Kumar, R.P. Suvarna, K.C. Babu Naidu, Microwave heated lead cobalt titanate nanoparticles synthesized by sol-gel technique: Structural, morphological, dielectric, impedance and ferroelectric properties, Mater. Sci. Eng. B 242 (2019) 23–30. [41] S. Kumar, S. Supriya, M. Kar, Correlation between temperature dependent dielectric and DC resistivity of Cr substituted barium hexaferrites, Mater. Res. Express 4 (2017) 126302. [42] M. Atif, M. Nadeem, R. Grossinger, R. Sato Turtelli, Studies on the magnetic, magnetostrictive and electrical properties of sol–gel synthesized Zn doped nickel ferrite, J. Alloys Compd. 509 (2011) 5720– 5724. [43] M. Atif, M. Nadeem, W. Khalid, Z. Ali, Structural, magnetic and impedance spectroscopy analysis of (0.7)CoFe2O4+(0.3)BaTiO3 magnetoelectric composite, Mater. Res. Bull. 107 (2018) 171–179. [44] Y. Slimani, B. Unal, E. Hannachi, A. Selmi, M.A. Almessiere, M. Nawaz, A. Baykal, I. Ercan, M. Yildiz, Frequency and dc bias voltage dependent dielectric properties and electrical conductivity of BaTiO3–SrTiO3/(SiO2)x nanocomposites, Ceram. Int. 45 (2019) 11989–12000. [45] M. Idrees, M. Nadeem, M. Atif, M. Siddique, M. Mehmood, M.M. Hassan, Origin of colossal dielectric response in LaFeO3, Acta Mater. 59 (2011) 1338–1345. [46] X. Vendrell, A.R. West, Electrical Properties of Yttria-Stabilized Zirconia, YSZ Single Crystal: Local AC and Long Range DC Conduction, J. Electrochem. Soc., 165 (2018) F966-F975. [47] J.T.S. Irvine, D.C. Sinclair, A.R. West, Electroceramics: Characterization by Impedance Spectroscopy, Adv. Mater. 2 (1990) 132. [48] A.K. Jonscher, Dielectric Relaxations in Solids, Chelsea Dielectrics, London, 1983. [49] B. Unal, M. Almessiere, Y. Slimani, A. Baykal, A.V. Trukhanov, I. Ercan, The Conductivity and Dielectric Properties of Neobium Substituted Sr-Hexaferrites. Nanomaterials 9 (2019) 1168. [50] M. Atif, M.W. Asghar, , W. Khalid, Z. Ali, S. Badshah, Synthesis and investigation of structural, magnetic and dielectric properties of zinc substituted cobalt ferrites, J. Phys. Chem. Solids 123 (2018) 36– 42. [51] P. Kumar, A. Gaur, R.K. Kotnala, Magneto-electric response in Pb substituted M-type bariumhexaferrite, Ceram. Int. 47 (2017) 1180–1185. [52] N.S. Kumar, R.P. Suvarna, K.C. Babu Naidu, Multiferroic Nature of Microwave‐Processed and Sol‐Gel Synthesized Nano Pb1‐xCoxTiO3 (x = 0.2–0.8) Ceramics, Cryst. Res. Technol. 53 (2018) 1800139. [53] U. Naresh, R.J. Kumar, K.C. Babu Naidu, Optical, magnetic and ferroelectric properties of Ba0.2Cu0.8xLaxFe2O4 (x = 0.2–0.6) nanoparticles, Ceram. Int. 45 (2019) 7515–7523. [54] B.B. Yang, D.P. Song, R.H. Wei, X.W. Tang, L. Hu, J. Yang, W.H. Song, J.M. Dai, X.B. Zhu, Y.P. Sun, Ni doping dependent dielectric, leakage, ferroelectric and magnetic properties in Bi7Fe3xNixTi3O21 thin films, Appl. Surf. Sci. 440 (2018) 484–4904. [55] S. Godara, B. Kumar, Effect of Ba–Nb co-doping on the structural, dielectric, magnetic and ferroelectric properties of BiFeO3 nanoparticles, Ceram. Int. 41 (2015) 6912–6919.

16

Figure Captions Fig. 1. X-ray diffraction patterns for 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with x = 0.0‒0.8 sintered at 1200oC for 6 h. Fig. 2. Representative Williamson-Hall plot between βcosθ versus 4sinθ for x = 0.4 sample. Fig. 3. Scanning electron microscopy images of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites for (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6 and (e) x = 0.8. Fig. 4. Magnetic hysteresis loops of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with x = 0.0‒0.8 measured at room temperature. Insets show (a) low field region of hysteresis loops, and (b) variation of magnetization (Ms) and coercivity (Hc) with Sr2+ substitution. Fig. 5. Frequency dependence of (a) dielectric permittivity (𝜀′), and (b) tangent loss (tanδ) of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with x = 0.0‒0.8 measured at room temperature. Fig. 6. Nyquist plots of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with x = 0.0‒0.8 measured at room temperature. Arrow indicates the increasing direction of frequency. Inset shows an equivalent circuit model used for fitting experimental data. Fig. 7. Frequency dependence of ac conductivity (𝜎𝑎𝑐) of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites with x = 0.0‒0.8 measured at room temperature. Inset shows the variation of A and n with Sr2+ substitution. Fig. 8. Ferroelectric hysteresis loops of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 hexaferrites measured at room temperature for (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6 and (e) x = 0.8. Fig. 9. (a) Variation of leakage current density (J) with applied voltage (V) for the 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11 𝐶𝑟1𝑂19 hexaferrites at room temperature. (b) Plot of Log(J) vs. Log(E) along with linear fits. Table 1 Estimated values of lattice parameters (a and c), volume of unit cell (V), average crystallite size (D), lattice strain (ε), average grain size (G), magnetic parameters (Ms, Mr and Hc), magnetic moment (𝑛𝐵) and anisotropy constant (K1) of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟𝑥𝑂19 hexaferrites with different concentrations (x). 17

Sample

a

c

V

D

ε

G

(x)

(Å)

(Å)

(Å3)

(nm)

(x10-3)

(μm)

𝑴𝒔

𝑴𝒓

𝑯𝒄

(emu/g) (emu/g) (kOe)

𝒏𝑩

𝑲𝟏

(𝝁𝑩)

(105 Erg/g)

0.0

5.883 23.381 698.77

74

1.52

3.17

42

21

1.8

8.33

3.09

0.2

5.868 23.276 694.07

63

1.78

3.06

55

24

2.2

10.81

4.11

0.4

5.861 23.340 694.32

51

2.09

2.23

62

36

3.5

12.07

4.72

0.6

5.855 23.242 689.99

56

2.13

2.97

51

27

2.4

9.84

3.85

0.8

5.849 23.169 686.41

67

2.21

3.73

45

24

1.2

8.60

3.21

Table 2 Fitted parameters of the Nyquist plots (R1, C1, R2, C2, Q, n and RT) and Jonscher power law (σo, A and s). Sample

Equivalent Circuit model R1

C1

R2

(Ω)

(F)

(Ω)

0.0

7.24E6

3.35E-13

9.57E7

1.41E-9

0.2

2.06E6

2.02E-12

1.48E8

0.4

5.01E7

1.64E-12

0.6

2.74E7

0.8

1.02E7

(x)

Q

n

Power Law C2

RT = R1+R2

σo

A

s

(F)

(Ω)

0.47

1.48E-11

1.02E8

2.74E-9

5.31E-9

0.45

2.52E-9

0.53

2.04E-11

1.50E8

7.32E-9

2.49E-12

0.93

7.26E10

2.01E-10

0.45

1.68E-11

7.26E10

5.16E-9

1.46E-12

0.97

7.55E-13

3.18E10

1.11E-10

0.47

1.71E-11

3.18E10

5.07E-9

1.52E-12

0.94

3.64E-12

2.44E8

6.42E-9

0.48

3.13E-11

2.54E8

2.54E-8

1.24E-11

0.88

Table 3 Ferroelectric parameters (Pr and Ec) and slope parameter of 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟𝑥𝑂19 hexaferrites with different concentrations (x). Sample

Pr

𝑬𝒄 18

m

(x)

(μC/cm2)

(kV/cm)

0.0

2.35

2.08

1.58

0.2

0.91

2.32

1.34

0.4

1.14

2.46

1.32

0.6

0.11

2.14

1.24

0.8

0.18

2.35

1.14

Author Statement

M. Atif: Conceptualization, Supervision, Writing - Review & Editing. M. Hanif Alvi: Investigation, Visualization. S. Ullah: Investigation, Writing- Original draft preparation. Atta Ur Rehman: Formal analysis, Validation. M. Nadeem: Methodology, Resources. W. Khalid: Software, Validation. Z. Ali: Writing - Review & Editing. H. Guo: Resources.

Highlights  Successfully synthesized 𝐵𝑎1 ― 𝑥𝑆𝑟𝑥𝐹𝑒11𝐶𝑟1𝑂19 (x = 0.0‒0.8) hexaferrites by coprecipitation technique.  Sr2+ substitution strongly affects on the structural, magnetic and dielectric properties of prepared hexaferrites.  40% of Sr2+ substituted sample (x = 0.4) shows a significant improvement in the magnetic and resistive properties along with a diminished tangent loss.

Declaration of interests

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. 19

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

20