Research articlesImpact of strontium substitution on the structural, magnetic, dielectric and ferroelectric properties of (x = 0.0–0.8) hexaferrites
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., (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., ) 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, 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 ; here ‘*’ represents a rotation of the block by180o along c-axis of hexagonal unit cell [3]. In total, unit cell of 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 (4), three octahedral sites (12, 2, 4) and one trigonal bipyramid site (2). Moreover, within these interstitial sites, Fe3+ ions occupying 2 (1 ion), 2 (1 ion) and 12 (6 ions) sites have spins directing upwards while in 4 (2 ions) and 4 (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, 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 by substituting barium (Ba2+) and/or iron (Fe3+) cations with different rare-earth metals (Eu, Ca, Sr, etc.) and/or 3d transition metals (Mn, Zn, Ti, etc.), respectively [8], [9], [10], [11], [12], [13], [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 (4) 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 hexaferrites decreases the saturation magnetization () with increasing Cr3+ content but the coercive field () increases up to a certain content of Cr3+ [15], [16]. Asiri et al. [17] have prepared (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 hexaferrites. Whereas, the AC susceptibility measurements of (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., ) are closely related to the distribution of Cr3+ ions on the five crystallographic sites. According to them, the magnetic parameters (, , ) increased for lower Cr3+ concentration (x ≤ 0.4) due to favorably occupancy of Cr3+ ions at 12, 2 and 4 sites and then these magnetic parameters decreased with increasing Cr3+ concentration on the basis of occupancy of Cr3+ ions at 12 and 2 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 (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 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., 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 hexaferrites (i.e., x = 0.0) is based on the improved dielectric [22] and microwave absorption [20] properties for than a pure hexaferrite sample. Subsequently, we investigated the effect of Sr2+ substitution on the crystallographic phase, morphologies, magnetic, dielectric and ferroelectric properties of prepared hexaferrites, in order to define an optimum composition which may be further useful for high frequency applications.
Section snippets
Experimental details
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, 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 .
Results and discussion
The XRD patterns of 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 [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
Conclusion
In summary, we have successfully synthesized 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
CRediT authorship contribution statement
M. Atif: Conceptualization, Supervision, Writing - review & editing. M. Hanif Alvi: Investigation, Visualization. S. Ullah: . Atta Ur Rehman: Formal analysis, Validation. M. Nadeem: Methodology, Resources. W. Khalid: Software, Validation. Z. Ali: Writing - review & editing. H. Guo: Resources.
Declaration of Competing Interest
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.
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 # TDF-044/2017.
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