Effect of scandium substitution on magnetic and transport properties of the M-type barium hexaferrites

Journal of Alloys and Compounds 815 (2020) 152467

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Effect of scandium substitution on magnetic and transport properties of the M-type barium hexaferrites Surbhi Gupta a, S.K. Deshpande a, V.G. Sathe b, V. Siruguri a, * a b

UGC-DAE Consortium for Scientific Research, Mumbai Centre, BARC Campus, Trombay, Mumbai, 400085, India UGC-DAE Consortium for Scientific Research, Indore Centre, University Campus Khandwa Road, Indore, 452001, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 June 2019 Received in revised form 24 September 2019 Accepted 25 September 2019 Available online 26 September 2019

Polycrystalline M-type barium hexaferrite compounds with scandium doping BaFe12-xScxO19 (0
x
2.5) were prepared by the solid state route. Room temperature neutron diffraction measurements show that the prepared samples are in single phase and exhibit a ferrimagnetic structure at room temperature. Temperature and field dependent magnetization study shows that due to doping of scandium, TC is lowered from 696 K in the undoped compound to 428 K for x ¼ 2.5. The transport properties were investigated by ac conductivity measurements. Our results indicate that the temperature dependence of dc conductivity for x ¼ 2.5 does not follow Arrhenius behaviour but rather closely follows the Mott variable range hopping mechanism (VRH) of polarons. For the parent compound and with for Sc doping x ¼ 1.5, the conductivity is found to be due to overlapping large polaron tunnelling (OLPT). © 2019 Elsevier B.V. All rights reserved.

Keywords: Barium hexaferrite Crystal structure Magnetic properties Transport properties

1. Introduction In the past few years, ceramic materials which possess combined magnetic and electrical order are being widely investigated [1e5]. Recently, hexaferrites have attracted attention due to the magnetoelectric characteristics present in the parent as well as doped compounds [6e10] which leads to possibility of their application in spintronics devices such magneto-dielectric capacitors and spin filtering tunnel junctions. Hexaferrites are classified into the different categories- M, W, Y, X, U, Z, depending on their crystalline structure [11]. Among these, M type Barium hexaferrites with magnetoplumbite structure have drawn tremendous attention due their properties like high saturation, coercivity, magnetic anisotropy, electrical resistivity and Curie temperature coupled with chemical stability and low cost [12e14]. All these properties of M-type hexaferrites make them suitable for a wide range of applications like permanent magnets, high density recording media electronic devices, communication equipment, microwave and radio frequency applications [15e24]. The M-type hexagonal structure is made up of two building blocks: spinel(S) block consists of oxygen ions and Fe ions on tetrahedral and octahedral sites, and the R block comprises the oxygen and barium ions and iron ions in the

* Corresponding author. E-mail address: [email protected] (V. Siruguri). https://doi.org/10.1016/j.jallcom.2019.152467 0925-8388/© 2019 Elsevier B.V. All rights reserved.

interstitial, tetrahedral, octahedral, and bipyramidal sites. Both these structural blocks are aligned in the c-direction with the stacking sequence of RSR*S* where * indicates the 180 rotation of the corresponding blocks with the c-axis [11] The unit cell of Mtype barium hexaferrrite consists of 64 ions (2 barium ions, 24 iron ions, 38 oxygen ions) at 11 different symmetrical sites. Barium ions occupy the 2d sites, iron ions occupy three octahedral sites (2a, 4f1 and 12k), one trigonal bipyramidal site (2b) and one tetrahedral sites (4f2). According to Gorter [25], the spins at each iron site are arranged ferrimagnetically in which those at 2a, 2b and 12k are arranged parallel (with spin up) and 4f1,4f2 spins arranged parallel with each other (spin down) but antiparallel to those at 2a, 2b and 12k. These spins are coupled with each other through the superexchange interaction mediated by oxygen ions Fe3þ-O2--Fe3þ. It is known that substitution of magnetic and non-magnetic ions leads to modification in the structural, magnetic and the transport properties of these materials. These properties are highly dependent upon the electronic configuration, atomic magnetism and site preference of the substitution ions. Some reports are available where the modification in the magnetic and electrical properties of the barium hexaferrites occurs due to doping. Most of the investigations have been carried out to study the changes in the magnetic properties with doping while the effects on the electrical properties were not studied much [26e30]. S. V Trukhanov et al. [31] and A. V Trukhanov et al. [32] have reported changes in the crystal and magnetic properties depending on the doping

2

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467

concentration of indium ions and aluminium ions, respectively. Ashima et al. [33] observed lower saturation magnetization and high coercivity in CaeSr substituted barium hexaferrites, and that the variation of dielectric constant and dielectric loss with frequency followed the Maxwell-Wagner and Koop theory. Auwal et al. [34] have reported the electrical and dielectric properties of yttrium doped barium hexaferrite and observed that conductivity in the system is due to both electronic and polaron hopping mechanisms. Pattanayak et al. [35] have reported the transport properties of parent compound and explained the role of different microstructures in conduction mechanism. Recently, magnetodielectricity has been reported in the scandium doped barium hexaferrites [36,37]. It has been reported that under the application of magnetic field, the magnetic structure gets modified from longitudinal conical to transverse conical at low temperature, and this induces polarization in the system. Nedkov et al. [38] have reported that microwave operating frequency of the material can be varied by varying the concentration of scandium doping in the barium hexaferrites. However, to the best of our knowledge, no reports are available on how the doping concentration of scandium affects the crystal and magnetic properties of the barium hexaferrite system. Also, there are not many reports on the mechanism of conduction or its dependence on doping concentration in the scandium doped compound. Considering the previous reports on scandium doped hexaferrites, we have studied the change in the crystal and magnetic structure in the parent compound and two doped compounds with different scandium doping concentrations. The transport properties were also investigated by studying the temperature and frequency dependent ac conductivity to elucidate the conduction mechanism involved.

temperature powder neutron diffraction (ND) patterns (below the paramagnetic to ferrimagnetic transition temperature), as shown in Fig. 1 (a), (b) and (c). Rietveld refinement was carried out using the Fullprof program [40]. All the samples crystallize in hexagonal structure conforming to the space group P63/mmc. From the refined data, we have observed that in the crystal structure, barium (Ba) ion occupies 2c Wyckoff position, the magnetic Fe3þ ions occupy the octahedral sites with the Wyckoff positions (Fe1 - 2a, Fe4 - 4f2 and Fe5 - 12k), Fe3 is tetrahedrally coordinated (at 4f1) and Fe2 is at trigonal bi-pyramidal site(at 2b), while oxygen is found at the Wyckoff positions, O1 (4e), O2 (4f), O3 (12j), O4 at 6h and O5 at 12k and scandium ions are distributed equally among all the iron sites in doped samples. A schematic view of the crystal structure of the barium hexaferrites is shown in the Fig. 2(a). As it is known that all the samples are ferrimagnetically ordered at room temperature, it is expected that there will be magnetic contribution to the Bragg peaks. Hence, the magnetic part is also added as a second phase in the refinement process, as can be seen in the Fig. 1 (a), (b) and (c). Since the measured data does not show any additional magnetic peaks other than the nuclear peaks with magnetic contribution, the magnetic phase can then be indexed using the k¼ (0 0 0) propagation vector. Symmetry elements and basis vector (BV) of the irreducible representation (G) corresponding to the k ¼ (0 0 0) propagation vector were obtained from the BasIreps program available in the Fullprof suite [40]. It generates 12 Gs of the propagation vector group Gk. All the obtained G values were sequentially tested. Out of these, it was found that the BV6 of G6 of 12k site gives the best fit to the data. Good agreement between the experimental and calculated patterns implies that the sample has formed in single phase without any impurities. Various parameters such as lattice parameters, cell volume, bond length and goodness of fit parameters

2. Experimental techniques Polycrystalline M-type barium hexaferrite BaFe12O19 (BFO) and scandium doped barium hexaferrites (BaFe12-xScxO19) with the doping concentrations x ¼ 1.5 and 2.5 (BaFe10.5Sc1.5O19 and BaFe9.5Sc2.5O19) were prepared by conventional solid state route. For synthesis, BaCO3, Fe2O3, Sc2O3 were taken as the starting materials. These powders were weighed according to their stoichiometric ratio and ground for 2 h in mortar pestle and after that, the powders introduced into the ball mill, which was run for 16 h with the 250 rpm speed. The final mixed powders were pressed to form circular pellets under 50 N of atmospheric pressure and finally, the pellets were sintered at 1150  C for 3 h. The synthesized samples were characterized for phase purity by the room temperature neutron diffraction in air, measured using a wavelength of 1.48 Å at Dhruva reactor, set up by UGC-DAE CSR Mumbai Centre [39]. The magnetization measurements were conducted using a Vibrating Sample Magnetometer (VSM) coupled to 90 kOe Quantum Design (Model: 6000, USA) Physical Property Measurement System (PPMS). The ac conductivity characteristics were evaluated from the dielectric measurements carried out on the samples in the form of circular pellets using the Alpha-AN impedance analyser (Novocontrol Technologies, Germany) equipped with Quatro liquid nitrogen cryosystem, in a range of frequency (500 Hz- 5 MHz) and temperature (280 Ke540 K). 3. Results and discussions 3.1. Crystal and magnetic structure studies The phase purity, crystal and magnetic structures of the prepared samples BaFe12O19 (BFO), BaFe10.5Sc1.5O19 (BFSc1.5O) and BaFe9.5Sc2.5O19 (BFSc2.5O) were determined by measuring room

Fig. 1. Refined room temperature neutron diffraction pattern of (a) BaFe12O19, (b) BaFe10.5Sc1.5O19 and (c) BaFe9.5Sc2.5O19.

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467

3

Fig. 2. Schematic view of the (a) crystal structure, (b) and (c) magnetic structure of BFO and Sc doped samples, respectively obtained from the refined neutron diffraction data. (Fe1: red, Fe2: blue, Fe3: green, Fe4: yellow and Fe5: orange). (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

of structural and magnetic refinement for all the prepared samples are listed in Table 1. It is observed that due to doping of scandium, the values of lattice parameters increase as compared to parent compound [41]. This is due to the replacement of Fe3þ (0.645 Å) ion by scandium ion Sc3þ (0.745 Å) of large ionic radius and hence, higher the doping, larger will be the lattice parameters. Due to partial replacement of the magnetic ion by the nonmagnetic scandium ion, the magnetic moment at the corresponding position also decreases. Magnetic moments (mB) for Fe ions at different positions for BFO are - 2a: 5.23, 2b: 5.55,4f1: 4.26,4f2: 5.55,12k: 5.23 and for the two scandium doped samples are - 2a: 1.39, 2b: 2.08,4f1: 3.59,4f2: 3.89,12k: 1.32 and 2a: 0.33, 2b: 3.27,4f1: 3.51, 4f2: 2.97, 12k: 0.79, respectively, which lead to decrease in total magnetic moment. This happens because of breaking of exchange interactions between the magnetic sublattices due to replacement of magnetic iron ions by the nonmagnetic scandium ions. The

Table 1 Parameters obtained by Rietveld refinement of the neutron diffraction pattern: Lattice parameters, volume, bond lengths, R-factors and goodness of fit for BFO, BFSc1.5O and BFSc2.5O samples. Refined parameters

BFO

BFSc1.5O

BFSc2.5O

a (Å) c (Å) V (Å3) Fe1eO4 Fe2eO1 Fe2eO3 Fe3eO2 Fe3eO4 Fe4eO3 Fe4eO5 Fe5eO1 Fe5eO2 Fe5eO4 Fe5eO5 Rwp,% RB,% Rexp, % RMag, %

5.884(1) 23.2136(9) 695.90(3) 2.008(7) 2.210(1) 1.810(9) 1.952(2) 1.929(7) 2.134(6) 1.931(2) 2.023(7) 2.080(1) 2.081(8) 2.010(1) 3.94 6.90 1.53 7.48 6.66

5.9203(1) 23.5597(9) 715.16(3) 1.9841(2) 2.4300(2) 1.7458(3) 1.9406(4) 1.9725(1) 2.2809(2) 1.9832(3) 1.9921(2) 2.0335(1) 2.0863(1) 1.9204(2) 8.21 6.40 5.17 7.23 2.52

5.9329(1) 23.6018(8) 719.47(3) 2.0355(3) 2.4081(1) 1.7948(2) 1.9126(1) 1.9191(3) 2.2232(2) 1.9978(1) 1.9857(1) 2.0937(2) 2.0995(2) 1.9333(1) 5.77 8.288 5.69 11.2 1.03

c2

(Å) (Å) (Å) (Å) (Å) (Å) (Å) (Å) (Å) (Å) (Å)

schematic view of magnetic structure of BFO and Sc doped samples obtained from the refined data of neutron diffraction is shown in Fig. 2 (b) and (c), respectively. We have observed that in case of BFO sample, the spin arrangement follows the Gorter model [25], i.e., Fe moments at the 2a, 2b and 12k sites are parallel to each other along the positive c-axis while the moments of the Fe ions at the 4f1 and 4f2 sites are along the negative c direction, therefore forming ferrimagnetic structure as shown in Fig. 2 (b). But in case of doped (Sc) samples, the spin arrangements of iron ions do not follow the Gorter model [25], though the structure remains ferromagnetic. In this case, the Fe moments at the 2a, 4f1, 4f2 and 12k sites are parallel to each other along the positive c-axis while the moment of the Fe ion at the 2b site is along the negative c direction, therefore antiparallel to other sites. 3.2. Magnetic properties To examine the magnetic properties, dc magnetization has been carried out. Fig. 3 (a) shows the temperature dependence of specific magnetization measured in the zero field cooled (ZFC) condition in the temperature range of 300 Ke800 K for BFO and 300 Ke650 K for Sc doped samples. It is observed that the BFO shows the change in magnetization ~700 K as shown in a lower inset of Fig. 3 (a) and in doped samples, value of magnetization is almost constant above ~500 K temperature but as the temperature decreases, the doped samples show an increase in the value of magnetization. These changes in magnetization are an indication of the change in the magnetic state from paramagnetic to ferrimagnetic. To obtain the transition temperature (TC) values, we have plotted the derivative (dc/dT), as shown in the inset of Fig. 3 (a). The derivative curves indicate the TC for BFO is at 696 K, BFSc1.5O is at 452 K and for BFSc2.5O, Tc ~428 K. It is known that the parent compound BaFe12O19 undergoes transition from paramagnetic to ferrimagnetic state around 700 K which is close to the observed Tc in our BFO sample [12]. Therefore, our studies indicate that doping of nonmagnetic Sc ion leads to a decrease in TC, higher the doping higher is the shift in TC as observed in case of BFSc2.5O. We have also observed that the value of specific magnetization is lower in case of BFSc2.5O as compared to the BFSc1.5O sample. This is due to

4

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467

Fig. 3. (a) Temperature dependence of specific magnetization of BFO, BFSc1.5O, and BFSc2.5O, (b) the inverse of the ZFC susceptibility c1 (T) as a function of temperature with the Curie-Weiss fit, (c) room temperature hysteresis loop of BFO, BFSc1.5O and BFSc2.5O.

replacement of larger number of Fe3þ magnetic ions in case of BFSc2.5O sample as compared to BFSc1.5O. The decrease in the TC confirms that the substitution of non-magnetic scandium ions at the magnetic Fe sites leads to the reduction of the number of neighbouring magnetic irons and therefore, the magnetic order shifts to lower temperature. Hence, we can conclude that the doping with scandium has a discernable influence on the magnetic properties of the hexaferrites. It was also observed that the ZFC curve of c(T) shows CurieWeiss behaviour for all the prepared samples. Fig. 3(b) shows the temperature dependence of the inverse of c (c-1 vs. T) for BFSc2.5O and BFSc1.5O and an inset shows the inverse of c (c-1 vs. T) for BFO. The high temperature region (T > 700 for BFO, T > 550 K for BFSc1.5O and T > 500 K for BFSc2.5O) of the curve obeys the CurieWeiss law, c ¼ C/(T-q) where C is the Curie constant and q is the paramagnetic Curie temperature. A linear fit shown in red in Fig. 3 (b) gives the value of q ¼ 695 K for BFO, q ¼ 505 K for BFSc1.5O and for BFSc2.5O, q ¼ 474 K. The positive value in all the cases indicates positive exchange correlation between the Fe3þ moments. We have also calculated the degree of frustration factor f ¼ q/TC which turns out to be 0.99 for BFO, 1.12 for BFSc1.5O and 1.11 for BFSc2.5O, respectively. Using the value of Curie constant C, the value of effective magnetic moment perpformula unit has been derived by ffiffiffiffiffiffi using theqformula meff ¼ 2.828 CA per formula unit (f.u), where ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2.828 ¼ 3kB =NA m2B , kB is the Boltzmann constant, NA is the Avogadro number and mB is the Bohr magneton and A is the molecular weight. The effective magnetic moments obtained for BFO,

BFSc1.5O and BFSc2.5O are 19.28 mB/f.u, 7.82 mB/f.u and 7.86 mB/f.u, respectively. Since iron is the only magnetic ion in this system, free ion moment of Fe3þions for BFO is 5.56 mB, BFSc1.5O is 2.41 mB and for BFSc2.5O is 2.55 mB, These observed values are lower than the theoretical effective magnetic moment of the free ion Fe3þ in the higher spin state (5.92 mB) which indicate a decrease in the average magnetic moment per Fe3þ ion site due to substitution of scandium ions. We may conclude that in these systems, the Fe sites are arranged ferrimagnetically with lower magnetic moments due to breaking of the positive Fe3þ-O-Fe3þ interaction in the separate magnetic sub lattices due to substitution of scandium ions at the Fe sites. Fig. 3 (c) shows the room temperature M-H hysteresis loop of the BFO, BFSc1.5O and BFSc2.5O samples. It was observed that for BFO sample, the magnetization reaches saturation in the applied magnetic field of ~20 kOe while for doped samples, saturation reaches in the applied magnetic field of 4 kOe and also the value of saturation magnetization decreases with the increase of doping concentration, which is consistent with the c-T data. We have also noticed that the value of coercive field is ~3.5 kOe in case of parent compound BFO, but due to doping of Sc, coercive field become almost negligible (~0.019 kOe) The observed decrease in the saturation magnetization can be explained on the basis of the fact that in case of parent compound BFO, Fe3þ ion occupies five different sites and because of super exchange interactions at these sites, the resultant total magnetic moment would be 20 mB/f.u. As Sc ion is equally distributed at all Fe sites (observed from refined ND data),

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467

number of Fe3þ-O-Fe3þ bonds decrease which leads to decrease in super exchange interaction between different Fe ions and hence, the saturation magnetization is lesser in higher scandium doped compositions. 3.3. Transport properties by ac conductivity studies Frequency-dependent conductivity measurement is the best tool to investigate the conduction mechanism involved in the transport process. The frequency dependence of ac conductivity is shown in Fig. 4 (a), (b) and (c) for BFO, BFSc1.5O and BFSc2.5O, respectively. The ac conductivity shows a frequency-independent “plateau” at low frequencies, and increases with frequency at higher frequencies in all the samples. Small deviations from this

5

“plateau” are observed at lower temperatures due to the contribution from grain-boundaries, and this contribution has been excluded from the analysis that follows in order to obtain only the bulk conductivity. In the low frequency region, increase of conductivity with the increase of temperature indicates the conduction process is due to thermally activated charge carriers. In case of ferrites, it is known that electrical conductivity can be explained by the Verwey’s hopping mechanism [42], according to which electronic conduction in ferrites is due to hopping of electrons between the same elements available in different valence states and distributed randomly at different crystallographic sites. In case of hexaferrites [43,44], conduction is generally due to the hopping of electrons between the Fe3þ and Fe2þ, and also if doping element is available with different valence states, then hopping between them will also participate in conduction. It is observed from Fig. 4 (a), (b) and (c) that the conductivity in the plateau regions is comparable in BFO and BFSc1.5O whereas conductivity in BFSc2.5O is lower than the other two, indicating higher dc conductivity in both BFO and BFSc1.5O as compared to BFSc2.5O. When the scandium ions replace the Fe ions at different crystallographic sites, the number of electrons available for hopping may decrease, which would lead to a decrease in the dc conductivity. In the low frequency region, due to availability of longer period of time to respond to applied electric field, the charge carriers successfully hop to the neighbouring vacant sites, resulting in a long range translation motion of ions, which contributes to the dc conductivity. In higher frequency region, two competing processes can occur, (i) the hopping charge carriers jump back to the initial position (unsuccessful hopping) and (ii) the hopping charge carriers relax in the new position i.e. successful hopping. As the ratio of the successful hopping to unsuccessful hopping increases, there is greater dispersive conductivity at higher frequency region. The ac conductivity spectra can be fitted with the Jonscher power law [45].

sðf Þ ¼ sdc þ s0 f s

(1)

where sdc is the dc conductivity, s0 is a constant, f is the experimental frequency and s is the exponent. The values of sdc, s0 and s at different temperatures were obtained from the best fit of Eq. (1) to the measured ac conductivity (see Fig. 4 (a), (b) and (c)). The inverse temperature dependence of the dc conductivity is depicted in Fig. 5 (a), (b) and (c) for BFO, BFSc1.5O and BFSc2.5O, respectively. For thermally activated hopping conduction, the dc conductivity follows the Arrhenius behaviour:



sdc ¼ s1 exp

Ea kB T

 (2)

where Ea is the activation energy, kB is the Boltzmann constant, T is the temperature, and s1 is a constant. It was observed that the dc conductivity shows significant deviation from the Arrhenius relation (Eq. (2)) in all the three compounds, indicating that the conduction may not be due to simple thermally activated hopping of charge carriers. Such deviations from Arrhenius behaviour have been observed in compounds showing conduction due to polarons [46,47]. Further analysis of the dc conductivity data was carried out by fitting experimental values to the relation,

"

1=4 #  sdc ¼ sp exp  T1=T

Fig. 4. Frequency dependence of the ac conductivity at selected temperatures of (a) BFO, (b) BFSc1.5O and (c) BFSc2.5O, symbols are the experimental data and the solid black line are the fits to the Jonscher power law given at Eq. (1).

(3)

which is obeyed by small polarons following the Mott variable range hopping (VRH) behaviour [48]. Here, sp and T1 are constants, and T is the experimental temperature value. It was found that the

6

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467

Fig. 5. Temperature dependence of the dc conductivity of (a) BFO, (b) BFSc1.5O and (c) BFSc2.5O. Solid red line is the fit of Arrhenius model and solid blue line is the fit of VRH model. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

dc conductivity in the BFO and BFS1.5O sample does not give a very good fit to Eq. (3) as shown in Fig. 5 (a), (b), but for the BFS2.5O sample, the fit was better, as can be seen in Fig. 5 (c). This indicates that the conductivity mechanism in the reported compounds is different and depends upon the doping concentration. At this point we can say that the conductivity BFS2.5O is due to the variable range hopping of small polarons, but this is not the case in BFO and BFS1.5O compounds. It is well-known that for the Mott’s VRH model, the hopping activation energy at a particular temperature is given by Ref. [48]. 1=4

W ¼ 0:25kB T 1 T 3=4

24

pkB NðEF Þd3

(5)

where d is the decay length of the localized wave function. From the neutron diffraction study we have obtained the average cationeoxygen distance in BFSc2.5O as 2.0366 Å. By taking the average cation-cation distance as twice this value, and taking this as the decay length, we have obtained the value of N(EF), as listed in Table 2. The hopping range (R) is also calculated by using the following relation

(4)

The values of W and T1 obtained from the curve fit of Fig. 5(c) are tabulated in Table 2. The value of T1 is related to the density of localized states N(EF) at the Fermi level according to Table 2 Parameters obtained from the fitting of experimental data of ac conductivity using the VRH model for BFS2.5O Parameters

BFSc2.5O

s0 (S/cm)

1.461018 3.81109 0.34e0.56 5.711018 32.93e27.94

T1 (K) W (eV) N(EF) (eV/cm3) R (Å)

T1 ¼

R¼

d1=4 ½8pkB NðEF ÞT1=4

(6)

The range of values of hopping range R obtained using the above formula is also given in Table 2 for BFS2.5O, and it was observed that hopping range is decreasing with the increase of temperature. It was observed that both the values of activation energy and the hopping range are satisfying the necessary conditions W » kBT and R  d, respectively, for Mott variable range hopping [49]. In order to gain further insight into the conduction mechanism in the reported samples, especially in case of BFO and BFSc1.5O, we have plotted the exponent parameter (s) obtained from the fitting of Jonscher law (Eq. (1)) for all the three samples, as a function of temperature. The results are shown in Fig. 6 (a), (b) and (c) for BFO, BFSc1.5O and

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467

. WHO ¼ e2 4ε r P P

7

(8)

where εp is the effective dielectric constant. The ac conductivity according to this model is given by

sac ¼ sdc þ

uR4 .   u e2 ðkB TÞ2 ½NðEF Þ2  12 2akB T þ WHO rp R2u p4

(9)

where kB is the Boltzmann constant and T is the temperature, N(EF) is the density of states at the Fermi level, a is the inverse localization length and Ru is the tunnelling distance at frequency u, which can be determined by the quadratic equation:

 ’ 2 Ru ¼ ½bWHO þ lnðut0 ÞR’u  bWHO r ’P ¼ 0

(10)

where. R’u ¼ 2aRu ; r ’P ¼ 2arP and b ¼ 1=kB T The frequency exponent according to OLPT model is given by the following relation

0 1 s@T A ¼ 1 

Fig. 6. Variation of frequency dependent exponent term ‘s’ of Josncher power law with temperature in (a) BFO, (b) BFSc1.5O and (c) BFSc2.5O. solid black line is guide to the eye.

8aRu þ ð6WHO rP =kB TRu Þ .  2aRu þ WHO rP kB TRu ÞÞ2

(11)

The OLPT mechanism has been reported in other ferrite compounds as well [55e60]. It is also reported in the parent compound BFO that both VRH and OLPT conduction mechanisms are involved in the system [35]. However, the observed minimum in s vs. T in our study suggests that the OLPT mechanism is responsible for the conduction in BFO and BFS1.5O. The weak temperature dependence of s in BFS2.5O sample suggests conduction due to hopping of charge carriers, and OLPT behaviour is not seen in this composition.

4. Conclusion BFSc2.5O, respectively. For BFO and BFS1.5O, the exponent s shows a clear minimum at certain temperature region, whereas for BFS2.5O, the value of s is almost independent of temperature. It is known that the variation of s with respect to temperature can suggest the possible conduction mechanism involved depending on the different models [50e53]. In particular, if s decreases with temperature to reach a minimum value and then increases with further increase of temperature, the conduction is due to the overlapping large polaron tunnelling (OLPT) mechanism [54]. According to the OLPT model, the charge carriers are assumed to be the large polarons whose radius is large as compared to the interatomic distance, and the lattice distortion cloud around this large polaron overlaps with the neighbouring distorted polaronic cloud due to the long-range Coulomb interaction. This overlapping leads to the reduction in the polaron hopping energy, given by the following relation [54].

 r  WH ¼ WHO 1  P R

(7)

where rp is the polaron radius, R is the interatomic distance and WHO is the energy associated with the charge carriers transfer between the overlapped sites

The polycrystalline samples BFO, BFSc1.5O and BFSc2.5O were prepared using the solid state route. The Rietveld analysis of the room temperature neutron diffraction patterns indicates that prepared samples are single phasic and scandium ion is distributed equally at all iron sites. The magnetic refinement indicates the sample possesses ferrimagnetic structure at room temperature. The ac conductivity of all the prepared samples has been investigated in the frequency range of 500 Hz to 5 M Hz in the temperature range of 280 Ke540 K. It was observed that ac conductivity in the higher frequency region follows the Jonscher power law behaviour but the dc bulk conductivity does not follow Arrhenius behaviour in all three compounds. For BFS2.5O, the dc conductivity follows the VRH mechanism for small polarons, but there is significant deviation in the temperature dependence of the dc conductivity from VRH in the BFO and BFS1.5O compound. It was found that the overlapping large polaron tunnelling (OLPT) is the dominant mechanism of conduction in the case of BFO and BFS1.5O in the temperature range studied. Hence, the contrast in the physical properties observed as a function of scandium concentration in the present compounds is clear enough to support our conclusions. Thus, we can conclude that the doping of scandium in BaFe12O19 has a large effect on the magnetic as well as transport properties, and the mechanism of conduction is also significantly affected by changes in the dopant concentration.

8

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467

References [1] N.A. Spaldin, S.W. Cheong, R. Ramesh, Multiferroics: past, present, and future, Phys. Today 63 (2010) 38. [2] J. Wang, J.B. Neaton, H. Zheng, V. Nagarajan, S.B. Ogale, B. Liu, D. Viehland, V. Vaithyanathan, D.G. Schlom, U.V. Waghmare, N.A. Spaldin, K.M. Rabe, M. Wuttig, R. Ramesh, Epitaxial BiFeO3 multiferroic thin film heterostructures, Science 299 (2003) 1719e1722. [3] T. Goto, T. Kimura, G. Lawes, A.P. Ramirez, Y. Tokura, Ferroelectricity and giant magnetocapacitance in perovskite rare-earth manganites, Phys. Rev. Lett. 92 (2004) 257201. [4] Y. Noda, H. Kimura, M. Fukunaga, S. Kobayashi, I. Kagomiya, K. Kohn, Magnetic and ferroelectric properties of multiferroic RMn2O5, J. Phys. Condens. Matter 20 (2008) 434206. [5] S.K. Upadhyay, P.L. Paulose, E.V. Sampathkumaran, Extraordinarily large intrinsic magnetodielectric coupling of the Tb member within the Haldane spin-chain family R2BaNiO5, Phys. Rev. B 96 (2017) 014418. [6] A.S. Mikheykin, E.S. Zhukova, V.I. Torgashev, A.G. Razumnaya, Y.I. Yuzyuk, B.P. Gorshunov, A.S. Prokhorov, A.E. Sashin, A.A. Bush, M. Dressel, Lattice anharmonicity and polar soft mode in ferrimagnetic M-type hexaferrite BaFe12O19 single crystal, Eur. Phys. J. B 87 (2014) 232. [7] P.S. Wang, H.J. Xiang, Room-temperature ferrimagnet with frustrated antiferro electricity: promising candidate toward multiple -state memory, PRX 4 (2014) 011035. [8] S.P. Shen, Y.S. Chai, J.Z. Cong, P.J. Sun, J. Lu, L.Q. Yan, S.G. Wang, Young Sun, Magnetic-ion-induced displacive electric polarization in FeO5 bipyramidal units of (Ba,Sr)Fe12O19 hexaferrites, Phys. Rev. B 90 (2014) 180404. [9] G. Tan, X. Chen, Structure and multiferroic properties of barium hexaferrite ceramics, J. Magn. Magn. Mater. 327 (2013) 87e90. [10] A.V. Trukhanov, S.V. Trukhanov, V.G. Kostishin, L.V. Panina, I.S. Kazakevich, V.A. Turchenko, V.V. Kochervinskii, M.M. Salem, D.A. Krivchenya, Multiferroic properties and structural features of M-type Al-substituted barium hexaferrites, Phys. Solid State 59 (2017) 737e745. [11] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, propertiesand applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (2012) 1191e1334. [12] A. Collomb, P. Wolfers X. Obradors, Neutron diffraction studies of some hexagonal ferrites: BaFe12O19, BaMg2-W, and BaCo2-W, J. Magn. Magn. Mater. 62 (1986) 57e67. [13] M. Imamura, Y. Ito, M. Fujiki, T. Hasegawa, H. Kubota, T. Fujiwara, Barium ferrite perpendicular recording flexible disk drive, IEEE Trans. Magn. 11 (1986) 1185e1187. [14] K. Yamamori, T. Suzuki, T. Fujiwara, High density recording characteristics for Ba-ferrite flexible disks, IEEE Trans. Magn. 22 (1986) 1188e1190. [15] T. Koutzarova, S. Kolev, Ch Ghelev, I. Nedkov, B. Vertruen, R. Cloots, C. Henrist, A. Zaleski, Differences in the structural and magnetic properties of nanosized barium hexaferrite powders prepared by single and double microemulsion techniques, J. Alloy. Comp. 579 (2013) 174e180. [16] V. Skumryev, H.J. Blythe, J. Cullen, J.M.D. Coey, AC susceptibility of a magnetite crystal, J. Magn. Magn. Mater. 196 (1999) 515e517. [17] S.D. Yoon, C. Vittoria, S.A. Oliver, Magnetic and microwave magnetic properties of barium hexaferrite permanent magnet films having the c-axis in the film plane, J. Appl. Phys. 93 (2003) 4023. [18] M.P. Sharrock, L.W. Carlson, The application of barium ferrite particles in advanced recording media, IEEE Trans. Magn. 31 (1995) 2871e2876. [19] M. Isshiki, T. Suzuki, T. Ito, T. Ido, T. Fujiwara, Relations between coercivity and recording performances for Ba-ferrite particulate perpendicular media, IEEE Trans. Magn. 21 (1985) 1486e1488. [20] R. Ardiaca, R. Ramos, A. Isalgue, J. Rodriguez, X. Obradors, M. Pernet, M. Vallet, Hexagonal ferrite particles for perpendicular recording prepared by the precursor method, IEEE Trans. Magn. 23 (1987) 22e24. [21] S. Sugimoto, S. Kondo, K. Okayama, D. Book, T. Kagotani, M. Homma, M-type ferrite composite as a microwave absorber with wide bandwidth in the GHz range, IEEE Trans. Magn. 35 (1999) 3154e3156. [22] R. Sharma, R.C. Agarwala, V. Agarwala, Development of radar absorbing nano crystals by microwave irradiation, Mater. Lett. 62 (2008) 2233e2236. [23] T.J. Fal, R.E. Camley, Hexagonal ferrites for use in microwave notch filters and phase shifters, J. Appl. Phys. 104 (2008) 023910. [24] V.G. Harris, A. Geiler, Y. Chen, S.D. Yoon, M. Wu, A. Yang, Z. Chen, P. He, P.V. Parimi, X. Zuo, C.E. Patton, M. Abe, O. Acher, C. Vittoria, Recent advances in processing and applications of microwave ferrites, J. Magn. Magn. Mater. 321 (2009) 2035e2047. [25] E.W. Gorter, Saturation magnetization of some ferrimagnetic oxides with hexagonal crystal structures, Proc. IEEE B 104 (1957) 255e260. [26] Keshav Kumar, D. Pandey, Quantum phase transitions in Ba(1x)CaxFe12O19 (0
x
0.10), Phys. Rev. B 96 (2017) 024102. [27] Vitalii Turchenko, Alexey Trukhanov, Sergey Trukhanov, Ivan Bobrikov, Anatoly M. Balagurov, Features of crystal and magnetic structures of solid solutions BaFe12-xDxO19 (D¼Al3þ, In3þ; x¼0.1) in a wide temperature range, Eur. Phys. J. Plus 131 (2016) 82. €ssbauer spectroscopic investi[28] T.M. Clark, B.J. Evans, G.K. Thompson, 57Fe Mo gation of complex magnetic structures in Ga, Sc, and in substituted M-type hexagonal ferrites, J. Appl. Phys. 85 (1999) 5229. [29] C.C. Chauhan, R.B. Jotania, K.R. Jotania, Conductivity and dielectric properties of M-type barium magnesium hexaferrites, Int. J. Adv. Eng. Res. Stud. 1 (2012)

25. [30] D.A. Vinnik, A. Yu Tarasovad, D.A. Zherebtsov, L.S. Mashkovtseva, S.A. Gudkova, S. Nemrava, A.K. Yakushechkina, A.S. Semisalova, L.I. Isaenko, R. Niewa, Cu-substituted barium hexaferrite crystal growth and characterization, Ceram. Int. 41 (2015) 9172e9176. [31] S.V. Trukhanov, A.V. Trukhanov, V.A. Turchenko, V.G. Kostishin, L.V. Panina, I.S. Kazakevich, A.M. Balagurov, Crystal structure and magnetic properties of the BaFe12-xInxO19(x¼0.1e1.2) solid solutions, J. Magn. Magn. Mater. 417 (2016) 130e136. [32] A.V. Trukhanov, L.V. Panina, S.V. Trukhanov, V.O. Turchenko, I.S. Kazakevich, M.M. Salem, Features of crystal structure and magnetic properties of M-type Ba hexaferrites with diamagnetic substitution, Int. J. Mater. Chem. Phys. 3 (2015) 286e294. [33] Sujata Sanghi Ashima, Ashish Agarwal, Reetu, Rietveld refinement, electrical properties and magnetic characteristics of CaeSr substituted barium hexaferrites, J. Alloy. Comp. 513 (2012) 436e444. [34] B. Unal Auwal, A. Baykal, U. Kurtan, A. Yıldız, Electrical and dielectric characterization of BieLa ion-substituted barium hexaferrites, J. Supercond. Nov. Magnetism 30 (2017) 1499e1514. [35] R. Pattanayak, S. Panigrahi, T. Dash, R. Muduli, D. Behera, Electric transport properties study of bulk BaFe12O19 by complex impedance spectroscopy, Phys. B 474 (2015) 57. [36] Y. Tokunaga, Y. Kaneko, D. Okuyama, S. Ishiwata, T. Arima, S. Wakimoto, K. Kakurai, Y. Taguchi, Y. Tokura, Multiferroic M-type hexaferrites with a room-temperature conical state and magnetically controllable spin helicity, Phys. Rev. Lett. 105 (2010) 257201. [37] Surbhi Gupta, Sanjay Kumar Upadhyay, V. Siruguri, V.G. Sathe E, V. Sampathkumaran, Observation of magnetoelastic and magnetoelectric coupling in Sc doped BaFe12O19 due to spin-glass-like phase, J. Condens. Matter. 31 (2019) 295701. [38] I. Nedkov, A. Petkov, Microwave absorption in Sc- and CoTi-substituted Ba hexaferrite powders, IEEE Trans. Magn. 26 (1990) 1483. [39] V. Siruguri, P.D. Babu, M. Gupta, A.V. Pimpale, P.S. Goyal, Pramana, A high resolution powder diffractometer using focusing optics, J. Phys. 71 (2008) 1197. [40] H.M. Rietveld, A profile refinement method for nuclear and magnetic structures, J. Appl. Crystallogr. 2 (1969) 65e71. [41] Yazan Maswadeh, Sami H. Mahmood, Awadallah Ahmad, Aynour N. Aloqaily, Synthesis and structural characterization of non- stoichiometric barium hexaferrite materials with Fe:Ba ratio of 11.5 e 16.16, IOP Conf. Ser. Mater. Sci. Eng. 92 (2015) 012019. [42] E.J.W. Verwey, J.H. De Boer Ree, Cation arrangement in a few oxides with crystal structures of the spinel type, Trans. Chem. Des. Pays. Bas. 55 (1936) 531. [43] H.P. Pinto, S.D. Elliott, Mechanism of the Verwey transition in magnetite: jahneTeller distortion and charge ordering patterns, J. Phys. Condens. Matter 18 (2006) 10427. [44] Muhammad Javed Iqbal, Saima Farooq, Impact of PreNi substitution on the electrical and magnetic properties of chemically derived nanosized strontiumebarium hexaferrites, J. Alloy. Comp. 505 (2010) 560e567. [45] A.K. Jonscher, The ‘universal’ dielectric response, Nature 264 (1977) 673e679. [46] L. Zhang, Z.J. Tang, Polaron relaxation and variable-range-hopping conductivity in the giant-dielectric-constant material CaCu3Ti4O12, Phys. Rev. B 70 (2004) 174306. [47] O. Bidault, M. Maglione, M. Actis, M. Kchikech, B. Salce, Polaronic relaxation in perovskites, Phys. Rev. B 52 (1995) 4191. [48] N.F. Mott, E.A. Davis, Electronic Processes in Non-crystalline Materials, Clarendon, Oxford, 1979. [49] A. Yildiz, S.B. Lisesivdin, M. Kasap, D. Mardare, Non-adiabatic small polaron hopping conduction in Nb-doped TiO2 thin film, Physica B 404 (2009) 1423e1426. [50] P. Pandit, S. Satapathy, P.K. Gupta, Effect of La substitution on conductivity and dielectric properties of Bi1xLaxFeO3 ceramics: an impedance spectroscopy analysis, Physica B 406 (2011) 2669e2677. [51] T.M. Meaz, S.M. Attia, A.M. Abo El Ata, Effect of tetravalent titanium ions substitution on the dielectric properties of CoeZn ferrites, J. Magn. Magn. Mater. 257 (2003) 296e305. [52] A. Ghosh, AC conduction in iron bismuthate glassy semiconductors, Phys. Rev. B 42 (1990) 1388e1393. [53] M.K. Fayek, M.F. Mostafa, F. Sayedahmed, S.S. Ata-Allah, M. Kaiser, On the electrical behavior of nickel ferrite-gallates, J. Magn. Magn. Mater. 210 (2000) 189e195. [54] A.R. Long, Frequency-dependent loss in amorphous semiconductors, Adv. Phys. 31 (1982) 553e637. [55] Ranjit Pattanayak, Rakesh Muduli, Ranjit Kumar Panda, Tapan Dash, Priyanka Sahu, Subhajit Raut, Simanchala Panigrahi, Investigating the effect of multiple grainegrain interfaces on electricand magnetic properties of [50 wt% BaFe12O19e50 wt% Na0.5Bi0.5TiO3]composite system, Physica B 485 (2016) 67e77. [56] R.K. Panda, R. Muduli, S.K. Kar, D. Behera, Dielectric relaxation and conduction mechanism of cobalt ferrite nanoparticles, J. Alloy. Comp. 615 (2014) 899e905. [57] E.V. Gopalan, K.A. Malini, S. Saravanan, D.S. Kumar, Y. Yoshida, M.R. Anantharaman, Evidence for polaron conduction in nanostructured manganese ferrite, J. Phys. D Appl. Phys. 41 (2008) 185005.

S. Gupta et al. / Journal of Alloys and Compounds 815 (2020) 152467 [58] A.U. Rahman, M.A. Rafiq, S. Karim, K. Maaz, M. Siddique, M.M. Hasan, Semiconductor to metallic transition and polaron conduction in nanostructured cobalt ferrite, J. Phys. D Appl. Phys. 44 (2011) 165404. [59] Arifa Jamil, M.F. Afsar, F. Sher, M.A. Rafiq, Arifa Jamil, M.F. Afsar, F. Sher, M.A. Rafiq, Temperature and composition dependent density of states

9

extracted using overlapping large polaron tunnelling model in MnxCo1xFe2O4 (x¼0.25, 0.5, 0.75) nanoparticles, Physica B 509 (2017) 76e83. [60] Shanming Ke, Peng Lin, Huiqing Fan, Haitao Huang, Xierong Zeng, Variablerange-hopping conductivity in high-k Ba(Fe0.5Nb0.5)O3 ceramics, J. Appl. Phys. 114 (2013), 104106-1-104106-7.