Structural and magnetic properties of praseodymium substituted barium-based spinel ferrites

Materials Research Bulletin 98 (2018) 77–82

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Structural and magnetic properties of praseodymium substituted bariumbased spinel ferrites

T

Muhammad Shakil Shaha, Khuram Alib, Irshad Alic, Asif Mahmoodd, Shahid M. Ramaye, ⁎ Muhammad Tahir Faridc, a

Division of Analytical chemistry, Institute of Chemical Sciences, Bahauddin Zakariya University, 60800, Multan, Pakistan Nano-optoelectronics Research Laboratory, Department of Physics, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan c Department of Physics, Bahauddin Zakariya University Multan, 60800, Pakistan d Chemical Engineering Department, College of Engineering, King Saud University Riyadh, Saudi Arabia e College of Science, Physics and Astronomy Department, King Saud University, P.O. Box 2455, 11451 Riyadh, Saudi Arabia b

A R T I C L E I N F O

A B S T R A C T

Keywords: Spinel ferrites X-Ray diffraction Sem Susceptibility Y-K angles

In this research work BaPrxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.10) spinel ferrites have been synthesized successfully by sol-gel technique. X-ray diffraction analysis confirmed the fcc spinel phase of the synthesized samples. All the samples showed inhomogeneous grain size distribution observed through scanning electron microscopy (SEM).Temperature dependence normalized AC susceptibility and Curie temperature studies revealed that BaFe2O4 exhibited multidomain (MD) structure with high Curie temperature. On the other hand, multidomain to single domain (SD) transitions occurred when the praseodymium is substituted into barium spinel ferrites. Narrow loops showed the soft nature of ferrites and it was confirmed from magnetic properties of the prepared samples. Decreasing trend of saturation magnetization and remanence was observed with the substitution of praseodymium contents. Coercivity and anisotropy constant both enhanced with the praseodymium concentration. The above-mentioned parameters revealed that the synthesized spinel ferrites might be useful for the of high-density magnetic recording applications.

1. Introduction Based on the structural properties, ferrites or ferrimagnetic oxides are mainly divided into spinel, garnet and hexagonal ferrites. Spinel ferrite is the most widely studied structure of the ferrites family. Spinel ferrites have many potential applications, including power conditioning, electronics, bio processing, and magnetic resonance imaging enhancement [1]. High values of electrical resistivity and low eddy current losses make spinel ferrites usable for microwave frequency application [2]. Generally, the chemical formula of a cubic spinel ferrite is written as MFe2O4, where M represents the divalent metal ion, such as Ba2+, Zn2+, Fe2+, Mg2+, Ni2+, Cd2+, and Cu2+ [3–6]. Spinel ferrite has face centered cubic structure (fcc) with eight formula units for each unit cell. Oxygen ions or anion arrangement in the lattice helps to determine the spinel crystal structure of ferrite. Barium spinel ferrites have extensive applications in a variety of fields, including microwave and switching devices [7]. These properties can be enhanced by substituting specific rare-earth (RE) ions into the interstitial sites of lattices. Many researchers reported the change in

⁎

structural and magnetic properties with the substitution of these RE elements.RE materials normally change the strains and structural spins of the ferrite material [8]. Therefore, a minor addition of RE cations, alters the structural as well as electromagnetic properties [9]. Previous studies have also revealed the change in ferrites properties with the substitution of Pr, Tb, Dy ions [10–12]. It is evident that Fe–Fe interaction (spin coupling effect of 3d electrons) can affect the resistivity and magnetic properties of ferromagnetic oxides. Therefore by incorporating RE ions into the spinel lattice or RE–Fe interactions (3d–4fcoupling) would also leads to change in both the electrical and magnetic behavior of ferrites. Consequently, it is established that different RE metals have different effects on ferrites. In the present work, Sol-gel method has been used to determine the effect of minor substitution of Pr3+ on structural and magnetic properties of BaFe2O4 spinel ferrites.

Corresponding author. E-mail address: [email protected] (M.T. Farid).

http://dx.doi.org/10.1016/j.materresbull.2017.09.063 Received 22 February 2017; Received in revised form 20 August 2017; Accepted 28 September 2017 Available online 29 September 2017 0025-5408/ © 2017 Elsevier Ltd. All rights reserved.

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2. Experimental procedure

Table 1 Summary of results of various properties of BaPrxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.10): lattice constant (a), volume (V), measured density (ρm), X-ray density (ρx), porosity (P), average ionic radii of A-site and B-site rA and rB, Bond Lengths A-O, B-O. Jump Lengths (LA and LB) and Crystallite size D.

2.1. Samples preparation Spinel ferrites with composition BaPrxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.10) were successfully synthesized by Sol-gel process followed by auto-combustion technique. Initially, measured quantities of analytical grade Ba (NO3)2 and Fe3Cl2 were dissolved in 100 ml of de-ionized water. The Pr2O3 (99.99% pure) was first dissolved in HCl to get praseodymium chloride and then mixed with the solution. In this process citric acid was used as a chelating agent. Homogeneity of the precursor’s solution was achieved by stirring and heating at ≈ 80 °C. One molar NH3 solution was added drop wise into the main solution so the pH is maintained at 7.After that a viscous gel is achieved by 8 h continuous stirring of the solution. The product was then subjected to selfcombustion at 370 °C for 3 h. This leads towards a fluffy product that was grounded and then sintered in a furnace at 700 °C for 5 h. A load of 30 kn of Paul–Otto Weber Hydraulic Press was used to obtain fine pellets of 0.13 cm diameter and 0.14 cm to 0.26 cm thickness. Polyvinyl alcohol (3–5 wt.%) was used for the binding purpose of powdered ferrite material. The binding material was evaporated when the pellet samples were annealed at 250 °C for 1 h, followed by 7 h sintering at 950 °C.

Composition

x = 0.000

x = 0.025

x = 0.050

x = 0.075

x = 0.100

Lattice constant ‘a’ (Å) Volume (Å3) Dm (gm/cm3) Dx(gm/cm3) P (%) rA (Å) rB (Å) A-O (Å) B-O (Å) Jump Length (LA) (Å) Jump Length (LB) (Å) D (311) nm

8.351

8.373

8.417

8.436

8.449

582.392 5.37 4.28 20.29 0.45804 0.73775 1.80804 2.0878 3.61609

587.007 5.415 4.3 20.59 0.46281 0.74325 1.81280 2.09325 3.62562

596.309 5.48 4.33 20.98 0.47233 0.75425 1.82233 2.10425 3.64467

600.357 5.52 4.35 21.19 0.47644 0.759 1.82644 2.109 3.65289

603.137 5.58 4.37 21.68 0.47926 0.76225 1.82926 2.11225 3.65852

2.95252

2.96029

2.97585

2.98257

2.98716

50.43

47.24

43.9

38.98

37.86

substitution of praseodymium ions. The diffraction peaks become lower and broader with the substitution of Pr3+ ions. It may be due to the poor crystallization accompanied the slower growth of crystallite. The appearance of the second phase (PrFeO3) is observed for x ≥ 0.075. The peaks of secondary phase correspond to 2θ = 32.347° and 46.384° with hkl values (112) and (220). These peaks were indexed by JCPDS card no. ICCD# 00-019-1012for PrFeO3. The appearance of the secondary phase is mainly due to the high reactivity of Fe3+ ions with Pr3+ ions on the grain boundary. The second phase is orthorhombic and having insulating behavior [11,12]. The XRD data has also been used to calculate the crystallite size (D), bulk density (ρb), X-ray density (ρx), lattice constant (a) of the spinel ferrite and values are listed in Table 1. By putting the lattice constant ‘a’ in Standley’s equations, ionic radii of A-site and B-site (rA, rB), as well as bond lengths of tetrahedral (A-O) and octahedral (B-O) sites, were calculated.

2.2. Characterization of the samples The structural characterization of the synthesized materials were done by using XRD diffractometer (Bruker axis D8 operating at 40 kV and at 30 mA) in the 2θ range of 15° to 80°. Cukα as a radiation source was used. Scanning Electron Microscopy (SEM) (Hitachi S4160) was used to observe morphology and microstructure of the spinel ferrites. Room temperature magnetic properties like the saturation magnetization Ms, remanence magnetization Mr and the coercivity Hc were measured by using the Vibrating Sample Magnetometer (VSM) (Model BHV-50 of Riken Danish Company Limited Japan). The applied magnetic field H ranged from 0.0 kOe to 2.0 kOe. Other parameters like magnetic anisotropy constant K1, squareness ratio Mr/Ms and Y-K angles are calculated on the basis of the observed data from hysteresis loop. 3. Results and discussions 3.1. X-Ray diffraction analysis The X-ray diffraction patterns of BaPrxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.10) spinel ferrites are shown in Fig. 1. It can be observed from Fig. 1, that all the diffraction peaks for samples (x ≤ 0.05) correspond to cubic spinel structure. Peaks become lower and broader with the

1 rA=⎛⎜μ − ⎟⎞ a 3 − r 4⎠ ⎝

(1-a)

5 rB = ⎜⎛ − μ⎟⎞ a − r ⎝8 ⎠

(1-b)

1 A − 0 = ⎛⎜μ − ⎟⎞ a 3 4⎠ ⎝

(2-a)

5 B − 0 = ⎛⎜ − μ⎞⎟ ⎠ ⎝8

(2-b)

Where ‘a’ is the lattice constant; r (O ) is the radius of oxygen ion (1.35 Å); μ is the oxygen ion parameter, for ideal spinel ferrite μ = 3 8 [13]. Hopping lengths in tetrahedral sites (LA) and in octahedral sites (LB) which are nothing but the distance between the magnetic ions have been calculated using the following relations [13] 2−

LA = a

3 4

(3-a)

LB = a

2 4

(3-b)

All the above mentioned calculated quantities are also given in Table 1. A gradual increase in the lattice constant “a” with the increment of Pr3+ ion is shown in Fig. 2. This increase in the lattice constant can be credited to the larger ionic radius of Pr3+ ions as compared to that of Fe3+ ions. The larger Pr3+ions (ionic radius ≈1.013 Å) partially

Fig. 1. X-Ray Diffraction patterns of BaPrxFe2-xO4 ferrites.

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RE ions occupy either the Fe positions or go to the grain boundaries. However the probability of occupancy of A-sites is less for RE ions than Fe3+ ions, it is due to the fact that the tetrahedral sites are too small to be occupied by the large RE ions which have the large ionic radius [19]. The probability of occupancy of the octahedral B-sites by the RE ions (in our case Pr3+) is greater. Fig. 3 supports this idea as the average grain size of the samples decreases with the substitution ofPr3+ ions in the spinel lattice. Grain size was calculated from SEM images by line intercept method. The smaller grain size with the substitution of Pr3+ ions is in good agreement with XRD results [19,20].

3.3. AC magnetic susceptibility It is known that the magnetic properties of ferromagnetic oxides are governed by the Fe–Fe interaction (spin coupling of the 3d electrons) [21]. By introducing a small amount of RE ions (R) into the spinel lattice, the R–Fe interactions appear in a form of 3d–4f coupling leading to change in the magnetic behavior in the ferrite sample. Generally, an evaluation of this interaction can be obtained by analyzing the Curie temperature or magnetization [22]. To find the Curi temperature, the magnetic susceptibility of praseodymium substituted Ba-based spinel ferrites was measured in the temperature range of 300 K to 600 K. The mutual inductance technique at 20 Hz frequency was employed for this purpose. The magnetic susceptibility is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field. The graph between normalized susceptibility ( χ ) and temperature (K) is shown in Fig. 4. It can be

Fig. 2. Lattice Constant vs Pr-concentration for BaPrxFe2-xO4ferrites.

replaces relatively smaller Fe3+ ions (ionic radius ≈0.64 Å) causing the expansion of unit cell, consequently affecting lattice constant [14,15]. The lattice constant of bi-phasic samples also increases with the substitution of Pr3+ ions. So it can be concluded that spinel lattice is not compressed by the secondary phase. There is no solubility limit for Pr3+ ions in Ba ferrites in the given range (0 ≤ x ≤ 0.10). It has been observed from Table 1 that ionic radii (rA, rB), jump lengths (LA and LB) and bond lengths (O-A and O-B) become larger as the concentration of Pr3+ions increases due to the replacement of smaller ionic radii (Fe3+) with larger ionic radii (Pr3+), and their distribution among the tetrahedral and octahedral sites [16]. The values of bulk density and X-ray density of the prepared samples are shown in Table 1. It can be noted from Table 1 that the both densities i.e., bulk density (ρb), X-ray density (ρx) of the samples enhance with increasing Pr3+ concentration [17]. The increase in densities might be due to the substitution of Pr3+ ions and also due to the formation of the small grains size of ferrites [18]. There is a slight increase in porosity with an increase of Pr3+substitution, as the grain grow dense together they leave pores behind thereby increasing porosity as can be seen in SEM Fig. 3 (a-e). The Debye Scherer’s formula was used to calculate the average crystallite size (D) of the spinel ferrite from the most intense peak (311) by using the equation [17].

?? =

0.94λ βCosθ

χRT

observed from Fig. 4 that normalized susceptibility of two samples (x = 0.00, x = 0.025) slowly increases as the temperature rises from 300 K. A peak has been observed in all two samples at a certain temperature. That temperature is called “Blocking temperature”. After Blocking temperature, an abrupt fall up to zero in normalized susceptibility has been observed. A relatively sharp fall of the magnetic susceptibility close to Curie temperature Tc due to the loss of the magnetic behavior is apparent from Fig. 4. The increase in susceptibility with peak values suggests there is the existence of multi-domain (MD) particles in the spinel ferrites [23]. The peak is found to suppress with the substitution of Pr3+ in BaFe2O4 and also Curie temperature (Tc) decreases with Pr3+ content. The three samples (x = 0.05, x = 0.075 and x = 0.10) show the exponential decrease in normalized susceptibility with the increase of temperature. These spinel ferrites are presenting SD to SP transition. Fig. 5 shows the variation of Tc as a function of Pr3+. The sample which contained more contents has low-value of Curie temperature and vice versa. The accepted fact is that the Fe3+-O-Fe3+ as well as Fe3+Fe3+ (interactions between two ferric ions) are the basic interactions, which have been observed in ferromagnetic substances [24]. The actual cause of a linear decrease in Curie temperature with praseodymium concentration is change in Fe3+-O-Fe3+ and Fe3+-Fe3+angles. And it ultimately causes a decrease of the magnetic moment interactions. The samples having Rare earth substitution show lower values of Curie temperature as compared to those who do not possess Rare earth doping [25] and our experimental results are completely agreed with these findings. The magnetic moments of ferric ions reside in a collinear way owing to the occurrence of superexchange interactions. A partial disorder and weakness are caused by Fe3+-O-Fe3+superexchange interactions when Rare Earth ions substitute Fe3+ ions. In this process, the valence of Fe changes from a very high spin state to a very low spin state, i.e. from 3d5 with 5 μB to 3d6 with 4 μB [26,27]. Such changes cause the arrangement to deviate from collinear to the non-collinear way and this leads to falling the Curie temperature [28]. Apart from this, the decrease in Curie temperature might be due to the lesser Pr-Fe interactions at octahedral sites as compared to the Fe–Fe interactions [29,30].

(4)

Here θ is the Bragg’s angle for diffraction peak, β is the full width at half maxima for diffraction peak and λ is the wavelength of X-ray radiations. The crystallite size of the sample lies within the range of 37 nm to 50.40 nm. The increasing concentration of Pr3+ ions reduces the grain growth probably due to segregation on or near the grain boundaries, which hampers its movement. As the magnetic recording performance of the magnetic material is improved for well-crystallized materials with nano dimension, the effect of praseodymium substitution seems to be extremely value-able in this regard. 3.2. SEM analysis The morphology and microstructure of BaPrxFe2-xO4 compositions were investigated by SEM micrographs given in Fig. 3(a–c). It can be observed that all the particles are agglomerated in irregular shapes, grain boundaries are less visible. Moreover, the samples exhibit inhomogeneous grain size distribution. The lighter grains in Fig. 3c seem to represent the secondary phase, which is encircled and enlarged. The 79

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Fig. 3. SEM image BaPrxFe2-xO4 ferrites.

Fig. 4. AC magnetic susceptibility vs Temperature of BaPrxFe2-xO4ferrites. Fig. 5. Curie temperature vs Pr-concentration for BaPrxFe2-xO4ferrites.

3.4. Magnetic properties magnetization (Ms), remanence magnetization (Mr) and coercivity (Hc) were measured and shown in Fig. 7 as a function of praseodymium composition. It can be observed that all the samples have the lower values of Hc (few hundred Oe), thereby indicating the soft magnetic nature of these samples, however, the coercivity increases with the

The Magnetization at room temperature for BaPrxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.10) compositions as a function of applied magnetic field H are shown in Fig. 6. From these loops, saturation 80

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Fig. 8. Squareness Ratio and Anisotropy Constant vs Pr-concentration for BaPrxFe2-xO4 spinel ferrites.

observed. As the squareness ratio is less than 0.5, then prepared samples tobehave as single domain particles [34]. The squareness (SQR) ratiois found between 0.31 and 0.35. So compositions discussed in the present case may lie in the range of single domain. Coercivity ‘Hc’ depends upon the several factors like magnetic domain size, particle morphology, porosity, grain size and magnetocrystalline anisotropy etc. The coercivity has an inverse relation with the grain size. Larger grains tend to contain a greater number of domain walls. So when these domain walls get magnetized or demagnetized, the domain wall movement requires less energy compared to domain rotation. In contrast with the contribution to magnetization or demagnetization due to domain rotation, the wall movement increases as the number of walls increases with increasing grain sizes [35]. Therefore, samples having larger grains are expected to have lower coercivity, and vice versa. This may be the reason for the decrease in coercivity of the compositions being discussed. The other factor of decrease in coercivity can be the partial solubility of Pr+3 ions into the Ba spinel ferrites, the secondary phase (PrFeO3) appeared at (x ≥ 0.075),this phase opposes the movement of domain walls as well as disturbs the orientation of grains. It also gives rise to internal stresses. The Pr3+ion substitution into the spinel lattice decreases the magnetization and enhances coercivity following Brown’s relation [36,37]. Moreover, the low coercivity values make these materials suitable for their use in core materials [38,39]. The anisotropic constant K1 is calculated by using the relation [40].

Fig. 6. M–H loops for BaPrxFe2-xO4 spinel ferrites.

Fig. 7. Saturation magnetization, remanence, Coercivity vs Pr-concentration for BaPrxFe2-xO4 spinel ferrites.

substitution of Pr3+ions, while MS and Mr decreases. There are three types of possible interactions in spinel ferrites, A-B, A-A, and B-B. Out of three interactions, A-B interaction predominates. The subsequent reduction in both MS and Mr may be ascribed to the weakening of AB-exchange interactions [31]. In BaPrxFe2-xO4, the Fe ions reside on tetrahedral as well as on the octahedral sites, while Pr3+ions (1.013°A) occupy the octahedral site due to the larger ionic radius as compared to Fe3+ ions (0.64°A). So there is a lesser chance of Pr3+ to reside on tetrahedral site. As Pr3+is non-magnetic, therefore magnetic moments on the octahedral site is less than magnetic moments on tetrahedral site and it has not much contribution in magnetization of spinel ferrites. This can be one of the reasons for the decrease in magnetization. The other factor of decrease in saturation magnetization may be due to the transfer of ferric ions from B-sites to A-sites. It has been also observed that decrease in grain size may lead to decrease in saturation magnetization due to enhancement in the disarrangement of magnetic moments on the surface of the particles. Hence net magnetization of synthesized samples reduced. Such type of results with RE ion substitution is reported by other researchers as well [32,33]. Squareness ratio Mr/Msas a function of Pr3+concentration ‘x’ is shown in Fig. 8. The increasing behavior of squareness ratio has been

K1 =

Ms × Hc 0.96

(5)

Fig. 8 shows that anisotropic constant K1 increases with the increasing Pr3+substitution. Anisotropy constant shows the same behavior as Hc [41]. As the magnetic particles were assumed to have single domain characteristics, they are isolated in terms of exchange interacting spins. The graph shows that the magneto-crystalline anisotropy is dominant in all the prepared samples. So the strong interaction between grains has been confirmed. In present work,Pr+3 ions replace the Fe3+ ions, as Fe3+ ions became less, so anisotropy constant increases [42]. Generally, magnetic moments arise from localized 4f electrons of rare-earth ions at room temperature; but their magnetic moment location does not remain ordered, contributing minutely to the magnetization of the RE substituted spinel ferrites due to paramagnetism. The nB of all the prepared spinel ferrites is dependent upon the magnetic moment of the corresponding ions. The magnetic moment expressed by Bohr magnetron constant (nB) is determined by the formula [43].

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Acknowledgement The authors would like to extend sincere appreciation to the Deanship of Scientific Research at King Saud University for funding the work through the research group project No. RGP-311. References [1] [2] [3] [4] [5] [6] [7] [8] [9]

Fig. 9. Magnetic moments and Y-K angles vs Pr-concentration for BaPrxFe2-xO4 spinel ferrites.

[10] [11]

nB (μB ) =

Mw × Ms ρb × 5585

[12]

(6)

[13]

Where ρb, Ms and Mw have measured density, saturation magnetization, and molecular weight respectively. Fig. 9 shows the nB as a function of varying values of ‘x’ for Pr3+ substitution. It is obvious that nB decreases as Pr3+ concentration increases, which may also contribute to the decrease of the saturation magnetization Ms. Using values of Bohr magnetron, Yafet-Kittel angles for differentPr3+ substitution values were calculated by the following relation [44,45].

CosαYK =

Ms (μB ) + 5x (μB ) (10 − 1.7x ) μB

[14] [15] [16] [17] [18] [19] [20] [21]

(7)

[22] [23]

Here nB represents the magnetic moments and ‘x’ is the concentration ofPr3+ ions. From Fig. 9, it can be seen that Y-K angles show the increasing trend with the Pr3+ substitution. This exhibits that more triangular spin is favored in the B-site leading to the weakening in A-B interactions. It has been suggested that increase in Y-K angles is due to the spin- canting. This is due to the decrease in grain size with the Pr3+substitution which leads to increase in canting angles [46,47].

[24] [25]

[26]

[27]

4. Conclusion

[28] [29] [30]

Praseodymium substituted spinel ferrites with chemical composition BaPrxFe2-xO4 (x = 0.0, 0.025, 0.05, 0.075, 0.10) have been successfully synthesized by sol-gel method. X-ray diffraction analysis shows that single phase spinel ferrites are obtained at low values of Pr3+ concentration (x ≤ 0.05). While at large concentration of Pr3+ ion (x ≥ 0.075), the second phase (PrFeO3) has been observed due to its low solubility in Barium spinel ferrites. The morphological analysis confirms the formation of inhomogeneously distributed, loosely packed grains of decreasing sizes with increasing Pr3+ contents. Susceptibility measurements revealx ≤0.025 exhibit MD particle and on the further substitution of Pr3+(x ≥ 0.05), domain structure changes from SD to SP. Curie temperature was found to decrease on the substitution of Pr3+, which is attributed to the dilution of A-B interaction. On substation of Pr3+, peak obtained in the graph of normalized susceptibility of BaFe2O4 is suppressed may be attributed to the decrease in grain size. Magnetic measurements show that Ms and Mr decrease with the increase of Praseodymium concentration. Anisotropic constant K1 has the same behavior as Hc. Owing to the above mentioned characterized features it is deduced that these spinel ferrites can be potential candidates for high-density magnetic recording.

[31] [32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42]

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