Co-Doped Rare-Earth (La, Pr) and Co-Al Substituted M-Type Strontium Hexaferrite: Structural, Magnetic, and Mossbauer Spectroscopy Study

The present study investigates the influence of La and Pr doping on the structural, magnetic properties, and hyperfine fields of Sr0.7RE0.3Fe12−2x CoxAlxO19, (RE: La and Pr, x = 0.0 0.8) hexaferrite compounds prepared via auto-combustion technique. The XRD analysis shows a linear decrease in a and c lattice and unit cell volume contraction with the content x. The room temperature magnetic study shows that for the Pr doped Sr0.7Pr0.3Fe12−2x CoxAlxO19 (Pr-SrM), the magnetization value monotonically decreases while for La doped Sr0.7La0.3Fe12−2xCoxAlxO19 (La-SrM) magnetization value shows a noticeable increase in magnetization value with x. The coercivity of the Pr-SrM compound was observed to decrease while that of the La-SrM compound showed a marked 40% increase at x = 0.2 (~5829 Oe) in comparison to undoped SrFe12O19 (~3918 Oe). A difference in Curie temperature was also observed, with Tc ~ 525 ̊C at x = 0.4 for Pr-SrM and Tc = 505 ̊C for x = 0.4 for La-SrM compound. The observed differences in magnetic properties have been explained on the basis of the site occupancy of Co and Al in the presence of rare-earth ions. The presence of non-magnetic rare-earth ion, La, improved saturation magnetization, and coercivity and deemed suitable replacement for Sr. The hyperfine parameters namely quadrupole shift showed a decrease with the La or Pr doping independent of (Co-Al) ions doping. Overall, the Mossbauer analysis suggests that the (Co-Al) impurities prefer occupancy at 2a site. How to cite this paper: Ghimire, M.L., Kunwar, D.L., Dahal, J.N., Neupane, D., Yoon, S. and Mishra, S.R. (2020) Co-Doped Rare-Earth (La, Pr) and Co-Al Substituted M-Type Strontium Hexaferrite: Structural, Magnetic, and Mossbauer Spectroscopy Study. Materials Sciences and Applications, 11, 474-493. https://doi.org/10.4236/msa.2020.117033 Received: June 5, 2020 Accepted: July 17, 2020 Published: July 20, 2020 Copyright © 2020 by author(s) and Scientific Research Publishing Inc. This work is licensed under the Creative Commons Attribution International License (CC BY 4.0). http://creativecommons.org/licenses/by/4.0/ Open Access M. L. Ghimire et al. DOI: 10.4236/msa.2020.117033 475 Materials Sciences and Applications


Introduction
The M-type hexaferrite, Strontium hexaferrite, is an excellent candidate for technological applications because of its high uniaxial magneto-crystalline anisotropy, large magnetization, high permeability, low conductive losses, excellent chemical stability, corrosion resistance and excellent high-frequency response [1] [2] [3]. Strontium hexaferrite has been widely used as materials for industrial applications, such as in microwave devices, small motors, electromagnetic wave absorber, and ferroxdures [4] [5]. Besides this, the popularity of strontium hexaferrite is also due to its economic success, which is its low price per unit available magnetic energy and its wide availability. So far, efforts have been made to further improve electrical, dielectric and magnetic properties of strontium hexaferrite by means of doping, heat treatment, ion substitution, and processing conditions [6] [7] [8] [9] [10].
In M-type hexaferrite, the structure is comprised of 64 ions per hexagonal unit cell on 11 distinct basis sites. The 24 Fe 3+ iron atoms of a unit cell occupy five different interstitial sites: three octahedral sites (2a, 12k, and 4f 2 ), one tetrahedral site (4f 1 ) and one trigonal bi-pyramidal site (2b). The coupling of these sites by superexchange interaction via O − 2 gives rise to a ferrimagnetic structure. The sites 12k, 2a, and 2b are sites with spin up while sites 4f 2 and 4f 1 are sites with the downs-spin [10]. This provides an ample opportunity to tune the magnetic properties of M-type hexaferrite compound by carefully engineering site occupancy in favor of increasing net magnetization of the compound. In view of this, attempts are made with either partial substitution of Sr 2+ or Fe 3+ sites. For example, substitution with non-magnetic ions such as Al 3+ [11] [12], Zn 3+ [13], Ga 3+ [14] and Cd 3+ [15] [16] and magnetic ions such as Co 2+ [17] and Cr 3+ [18] at Fe sites, or with the partial substitution of Sr 2+ site by RE 3+ such as La 3+ [19] [20], Nd 3+ [21], Sm 3+ [22] [23] and Pr 3+ [24] ions, and substitution of Sr 2+ /Fe 3+ together with Pr-Zn [25], La-Cu [26], and La-Zn [27]. The majority of these studies have attributed changes in the magnetic behavior of doped SrM either to the site-occupancy, which perturbs exchange interaction between Fe 3+ -O 2− Fe 3+ , anisotropy changes occurring at 2b sites due to perturbation in electric field gradient or extrinsic features such as particle size.
Doping of non-magnetic Al 3+ in SrFe 12−x Al x O 19 has been reported to bring a considerable enhancement in the coercivity along with a reduction in saturation magnetization [11] [28] [29]. Thus with the Al 3+ substitution, in spite of a large increase in the coercivity, the compound attains abysmal magnetization value. A strategy could be designed to keep both coercivity and magnetization value high.

Experimental
The series of Sr 0. 7

Results and Discussions
The room temperature XRD pattern of Sr 0.7 RE 0.  114), and (008) using equation [30]: where h, k, and l are Miller indices. The calculated lattice parameters a and c of the hexaferrites are shown in Table 2. The plot for a and c as a function of doping content is shown in Figure 2. A linear decrease in the lattice parameters a and c with the doping content was observed. The linear decrease in the lattice parameter may occur due to the substitution of Al 3+ (ionic radii, r ~ 0.51 Å) and    [31]. According to Wagner [32], an examination of c/a parameter ratio may be used to quantify the structure type, as the M-type (magnetoplumbite) structure can be assumed if the ratio is observed to be in the range 3.917 and 3.963. As per Table 2, the c/a ratios of as-prepared samples are in the range of 3.908 to 3.919, assuring that the as-prepared samples have maintained the M-type structure.
The crystallite size of as-synthesized particles was calculated using Scherrer's equation [33]: where k denotes the Scherrer constant (k = 0.9), λ is the wavelength of x-ray source (λ = 0.154056 nm), β is the full-width-half-maximum of a diffraction peak and θ is the diffraction angle. As listed in Table 2, the crystallite size of as-synthesized samples is in the range of 54 to 77 nm. In the comparison of the pure SrFe 12 O 19 sample with a crystallite size of 80.7 nm, the doped compounds show reduced crystallite size. Grain refinement is usually observed in rare-earth and doped oxide compounds [34]. This grain refinement is reported to result from the 1) increased microstrain and defect density with substitution content and 2) diffusion of substituent element to the grain boundaries, the migration that restrains the grain growth by lowering down the grain growth mobility. If the retarding force generated is more than the driving force for the grain growth due to dopants, the movement of the grain boundary is impeded [35].   The variation of coercivity, Hc, of La 3+ -SrM and Pr 3+ -SrM compounds as a function of x content is shown in Figure 5. Consistently La 3+ -SrM samples display higher Hc values than Pr 3+ -SrM compounds for all x values. The Hc value of La 3+ -SrM displays a marked ~50% increase at x = 0.2 when compared to that of pure SrFe 12 O (Hc ~ 3918 Oe). On the other hand, Hc value decreases linearly with increasing the x content for the Pr 3+ -SrM compound. Being an extrinsic where α is the microstructure factor which has a reciprocal dependence on the grain size, N is the demagnetization factor, 2K1/(μ o M s ) is the Ha is the magnetocrystalline anisotropy field [39].  Figure 6. The maximum Tc value for La 3+ -SrM and Pr 3+ -SrM was observed to be 505 and 525 K, respectively at x = 0.40. After attaining maximum value, the Tc decreases gradually for x > 0.40. The overall decrease in Tc after attaining maximum can be explained on the basis of three combined effects [45] [46], 1) substitution of Co 2+ -Al 3+ ions for Fe 3+ ions reduce the Fe 3+ ions and hence leads to a reduction of Fe 3+ -O 2− -Fe 3+ number of superexchange interaction and strength and 2) lattice contraction with the substitution alters the bond length and angle of Fe 3+ -O 2− -Fe 3+ from its optimum interaction strength value. However, in the beginning, Tc drops down for x = 0.20, where presumably Fe 3+ might be substituted more with non-magnetic Al 3+ and above combined effect become prevalent. However, the increasing magnetic Co 2+ ion content helps regain the strength of the superexchange interaction and hence increases the Tc value for x = 0.40 [47].
In order to investigate the site occupancies of transition metal ions and resulting hyperfine parameters in the samples, Mossbauer spectra were collected at room temperature. Figure 7 shows the fitted Mossbauer spectra as a function of content x in the as-synthesized samples. All spectra consist of five Lorentzian sextets originating from 12k, 4f1, 4f2, 2a, and 2b crystallographic sites of Fe 3+ ions. The spectra were fitted with the constrain that all the linewidths of the absorption lines were the same. The extracted Mossbauer parameters are listed in Table 4(a) and Table 4(b). The observed sequences of the magnetic hyperfine magnetic fields, HF (4f2 > 2a > 4f1 > 12k > 2b) and isomer shift, δ (the isomer shifts follow the sequence of 4f2 > 12k > 2a > 2b > 4f1) for both samples are in agreement with the reported results [47].
It is well known that the magnitude of the hyperfine magnetic field at the Fe 3+ site depends on the distribution of neighboring magnetic cations. The hyperfine field, HF, the parameter of the most intense 12k line effectively shows no variation   The isomer shift plot as a function of x is shown in Figure 8. The isomer shift, δ, for both series of samples for all sites visibly remains invariant except for the 2b site. Also, the 2b site's isomer shift value clearly reflects the influence of the

Conclusion
The structural, magnetic, and Mossbauer spectrum study of the influence of