Microstructure, magnetic and optical properties of Nb3+ and Y3+ ions co-substituted Sr hexaferrites

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Microstructure, magnetic and optical properties of Nb3+ and Y3+ ions cosubstituted Sr hexaferrites S. Günera,∗, M.A. Almessiereb,c, Y. Slimanib, A. Baykald, I. Ercanb a

Institute of Inorganic Chemistry, RWTH Aachen University, D-52074, Aachen, Germany Department of Biophysics, Institute for Research and Medical Consultations (IRMC), Imam Abdulrahman Bin Faisal University, P.O. Box 1982, 31441, Dammam, Saudi Arabia c Department of Physics, College of Science, Imam Abdulrahman Bin Faisal University, P.O. Box 1982, 31441, Dammam, Saudi Arabia d Department of Nano-Medicine, Institute for Research and Medical Consultations (IRMC), Imam Abdulrahman Bin Faisal University, P.O. Box 1982, 31441, Dammam, Saudi Arabia b

ARTICLE INFO

ABSTRACT

Keywords: Sr hexaferrite Rare earth Structure Morphology Magnetization Optical properties

Series of SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) hexaferrites (HFs) were fabricated via citrate sol-gel approach. Structural and magneto-optical properties of ensembles were investigated in detail. The structural and morphological analyses revealed the formation of M-type Sr hexaferrite nanoparticles. Diffuse reflectance data were registered to estimate the direct optical energy band gaps (Eg) in a range of 1.77 eV-1.87 eV. Room temperature (RT, 300 K) and low temperature (10 K) magnetic hysteresis curves were recorded by enforcing applied dc magnetic field up to ± 70 kOe. Magnetic parameters were significantly tuned due to coordination of Nb3+ and Y3+ rare earth ions. Specified magnetic data reveal the strong ferromagnetic characteristics of pristine SrFe12O19 and co-doped HFs with Nb3+ and Y3+ ions at both temperatures. RT squareness ratio (SQR) has an exception only for pristine sample as 0.506, which is in the margin of theoretical limit assigning the single-domain nature with uniaxial anisotropy. However, all co-doped samples have SQR = 0.288–0.485 values that are smaller than theoretical limit of 0.50, implying multi-domain nature at RT and at 10 K. Co-doped ions cause lowering in super-exchange interactions between different sites and resulting the decrements of intrinsic magneto-crystalline anisotropy and coercivity fields. The specified magnetic characteristics make our fabricated SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) HFs good candidates as permanent magnets applications and high-density recording media.

1. Introduction Exploration focuses on Sr hexaferrites (SHFs) to improve both macroscopic magnetic features and structure for many applications including, recording media, microwave devices, multi-ferroic and bioceramic applications [1–6]. The magneto-optical and structural properties of Sr hexaferrites can be amended by adding a small ratio of doping elements (co-doping) such as transition and rare earths metals, which lead to outstanding characteristics in comparison with nonsubstituted Sr hexaferrite material [7–9]. So, the enhancement in magnetic properties are affected by concentration, distribution and site occupancy of the substitution ions amongst sublattices. Variations at coercive field and magneto crystalline anisotropy, super-exchange interactions and defects in the crystal lattice are all detected [10–15]. Sr

∗

hexaferrites with varied substitution transition - rare earth metals such as La-Cu [16], La-Co [17], La-Zn [18], Pr-Zn [19] and Ti–Ru [20] have been investigated to clarify the impact of these substitutions on their magnetic properties for suitable potential application. The reports state that the microstructural characteristics and magnetic properties of the all compositions are remarkably enhanced [16–20]. These features make a Sr hexaferrite a good candidate for high density recording media and in microwave absorption application due to its great saturation magnetization, chemical stability and large coercivity [21]. Moreover, exploring the magnetic properties in higher and lower temperature are very important for investigating alteration in their behavior. Hence, the structural, optical and magnetic features of SrNbxYxFe12–2xO19 (0.0 ≤ x ≤ 0.05) HFs have been explored in the present study.

Corresponding author. E-mail address: [email protected] (S. Güner).

https://doi.org/10.1016/j.ceramint.2019.10.191 Received 14 August 2019; Received in revised form 12 October 2019; Accepted 20 October 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: S. Güner, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.10.191

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by X-ray diffraction (XRD; Rigaku Miniflex). The morphology and the chemical compositions were inspected by SEM-EDX (FEI Teneo scanning electron microscope equipped with an EDX detector of EDAX). Fourier transform infrared spectrometry was executed via (FT-IR; Bruker alpha II). The optical response and band gap energy were calculated by applying the UV–Vis diffuse reflectance spectrophotometry. Magnetization analyses (Hysteresis loops) were done and recorded by vibrating sample magnetometry (VSM). 3. Results and discussion 3.1. Phase analysis The hexagonal structure of SrNbxYxFe12-2xO19 (0.01 ≤ x ≤ 0.05) HFs was verified via powder XRD patterns as shown in Fig. 1. All of them were matched very well with the JCPDS Card 00-043-0002. The c/a ratio calculations are found in the range 3.91–3.92 that proves also the formation of M-type hexaferrite [22] as registered in Table 1. It is obvious that all products were synthesized without any impurity. According to the powder XRD patterns, the most intense peaks shifted towards the lower angles due to the internal stress caused by strain increment due the variant in ionic radii of substitution ions. The XRD experimental data are used as a basis for calculation of the lattice constants “a” and “c” through full proof as recorded in Table 1. It was found that although the “a” is almost constant, the “c” shows remarkable increase due to the extension of the c-axis by the substitution of Nb3+ and Y3+ ions [23]. The average of crystallites size for all ratios were assessed by Scherrer's formula and were found in the range of 29–47 nm.

Fig. 1. XRD powder patterns of SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) HFs.

Table 1 The refined structural parameters for SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.0 5) HFs. x

a = b (Å)

c (Å)

V (Å)3

c/a

Crystallite Size (nm) ± 0.13

χ2(chi2)

RBragg

0.00 0.01 0.02 0.03 0.04 0.05

5.882(1) 5.883(7) 5.883(3) 5.882(6) 5.884(0) 5.884(1)

23.042(7) 23.075(7) 23.082(6) 23.083(6) 23.086(0) 23.091(9)

689.46 69.57 694.16 694.33 695.02 695.50

3.92 3.92 3.92 3.92 3.92 3.92

46.91 29.05 41.51 43.04 37.71 43.35

1.61 1.61 1.98 2.23 2.21 2.02

18.24 9.72 14.01 13.59 13.81 11.89

3.2. Morphology analysis Fig. 2 represents the SEM images of SrNbxYxFe12–2xO19 (0.01 ≤ x ≤ 0.05) HFs. The samples are formed by smaller crystals of few tens of nm randomly, which are oriented, packed and aggregated irregularly. The hexagonal–plate shape observed by high magnification imaging reveals a regular morphology, as displayed in Fig. 2. The SrNbxYxFe12–2xO19 (x = 0.03 and 0.05) HFs displayed a preferential segregation of the elements Sr, Y, Nb, Fe and O as confirmed by the EDX and elemental mapping with no existence of other element, in fact the crystal structure showed a homogeneous chemical composition as seen in Fig. 3. Fig. 4 indicates the TEM and HR-TEM images of SrNbxYxFe12–2xO19 (x = 0.05) HFs. The sample displays an agglomeration of the hexagonal platelet morphology with grain size around 50 nm. The HR-TEM revealed the hexagonal symmetry corresponding with the SrFe12O19 phase with lattice planes {201}, {110} and {111} at 0.25, 0.29 and 0.49, respectively.

2. Experimental 2.1. Preparation and characterization The composition of SrNbxYxFe12–2xO19 (0.00 ≤ x ≤ 0.05) HFs have been synthesized via citrate sol-gel technique using Sr(NO3)2·4H2O (99%), Fe(NO3)3·9H2O (99%), Nb(NO3)3 (99%), C6H8O7 (99%) and Y2O3 (99%) as initial materials, which were obtained from Sigma Aldrich without further purification. The stoichiometric amount of metal nitrates and citric acid were thawed in de-ionized water under stirring at 90 °C. The Y2O3 was disbanded in 10 ml of conc. HCl. Then, two solutions were mixed with each other with continuous stirring for 45 min and the pH at 7 was adjusted by NH3 solution. Afterward, the solution temperature has been raised up to 190 °C for 35 min then increased to 350 °C for igniting the viscous gel. The final powder was calcined at 1000 °C for 6 h to get solid product. The structural characterization of the final products was examined

3.3. Fourier transform infrared spectrum (FT-IR) FT-IR analysis of SrNbxYxFe12–2xO19 (0.00 ≤ x ≤ 0.05) HFs was implemented and discussed in Fig. 5 which indicated to marker absorption bands of M-type hexaferrite at 422 (due to absorption of Fe–O band in octahedral site [24]), 543 and 585 cm−1 (due to absorption of Fe–O band in tetrahedral site). It was also found that all FT-IR bands exhibited shift (were observed at a higher wave number) to the higher wave numbers as a result of the difference in the length of metal oxygen band for both tetrahedral and octahedral site by the substitution [25]. It is clear that there are no extra peaks, which provide an evidence of purity of all products.

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Fig. 2. SEM images of SrNbxYxFe12-2xO19 (0.01 ≤ x ≤ 0.05) HFs.

3.4. Optical properties

power n denotes the type of electronic transition and it equals 2 for a direct one, A is a proportionality constant and Eg is the energy band gap, Eg values can be obtained by plotting ( h ) 2 vs (h ) graphs. An elongated linear fit line to the straight part of plot could intercept the energy axis for ( h ) n = 0 , so the corresponding value at energy axis is assigned as optical Eg. Calculation of optical band gaps using DR % graphs is based on the Kubelka-Munk theory [29–31]. Fig. 7 shows the Tauc plots, which include both elongated fit lines and corresponding direct band gap energies for all NPs. Pristine SrFe12O19 HF (x = 0.00)

Fig. 6 displays the UV–Vis percent diffuse reflectance (DR %) versus applied wavelength (λ) graphs of SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) HFs from which the optical band gap energies could be calculated using the expression (also known as Tauc formula) [26–28] as below:

( h ) n = A (h where,

Eg )

is absorption coefficient, h

(1) is the incident photon energy,

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Fig. 3. EDX and elemental mapping of SrNbxYxFe12-2xO19 (x = 0.03 and 0.05) HFs.

has minimum band gap magnitude of 1.77 eV. A remarkable bounce to 1.87 eV is extrapolated for the SrNb0.01Y0.01Fe11.98O19 HF. Increment at doped ion's content slightly decreases the Eg values until 1.84 eV due to the reduction in the space between conduction and valance bands by

creating a sub-level, which is relevant to lattice disorder and crystallite size [32,33]. All estimated band gap values could be accepted comparable with respect to band gap range of semiconductors. In the literature, there are

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Fig. 4. TEM (left) and HR-TEM images (right) of SrNb0.05Y0.05Fe11.90O19 HFs.

combustion and for doped samples with different ions like Cr3+, Zr3+, Tm3+, Bi3+, La3+, Y3+ [34–41]. According to detected results at our previous investigations, we mainly observed decrements at Eg values until 1.34 eV for most doped samples except the one that was doped by Tm3+ ions. We observed Eg value increased until 2.18 from 1.75 eV [41]. 3.5. Magnetic properties Fig. 8 (a) and (b) show the magnetic hysteresis curves of SrNbxYxFe12–2xO19 (0.0 ≤ x ≤ 0.05) HFs obtained from VSM measurements at 300 K and at 10 K respectively. The hysteresis data were recorded enforcing dc magnetic field up to ± 70 kOe. The analyses carried on hysteresis curves revealed that the coordination of Nb3+ and Y3+ ions (with contents of x = 0.00–0.05) causes significant changes at magnetic properties of strontium hexaferrite (HFs) samples. Although a strong magnetic field was applied, saturation magnetizations (Ms) were deduced from the linear fits of M vs. 1/H2 data by using Stoner-Wohlfarth (S–W) theory [33,42,43]. Two of linearly fitted data belonging to the SrNb0.04Y0.04Fe11.92O19 and SrNb0.03Y0.03Fe11.94O19 HFs are presented in Fig. 9(a) and (b), respectively. Ms values decreased from 68.82 emu/g to 45.56 emu/g for 300 K and from 102.73 emu/g to 64.59 emu/g for 10 K magnetization data. Similarly, remnant magnetizations (Mr) were detected to decrease from 34.80 emu/g to 14.77 emu/g at 300 K and from 49.83 emu/g to 18.60 emu/g at 10 K measurements. Replacement of some Fe3+ ions by rare earth Nb3+ and Y3+ ions might cause decrement at super exchange interactions between A and B sites. The observed raises in the Ms and Mr values at 10 K are linked to the reduction in thermal changes of surface magnetic moments [44,45]. The magnetic moments of the hexaferrite nanoparticles were affected by extra quantitative ordering of spins in high magnetic fields and at low temperatures. The increments of both Ms and Mr values were due to the drop of temperature, as a result, Hc decreases as well [44,45]. The ratio of Mr/Ms which is known as squareness ratio (SQR), for undoped SrFe12O19 SQR equals 0.506 at 300 K. A theoretical limit of 0.50 determines single-domain nature with uniaxial anisotropy according to S–W theory [33]. However, all samples have smaller SQR values compared to the theoretical limit at 10 K. This case confirms the multi-domain nature especially for Nb3+ and Y3+ doped samples [46,47]. Coercive field (Hc) value of loops carried at 300 K decreases almost linearly from 4334 Oe belonging to SrFe12O19 to 2116 Oe belonging to doped sample with x = 0.05. This refers to the change of microstructural anisotropies (particle sizes and shapes) causing a

Fig. 5. FT-IR spectra of SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) HFs.

Fig. 6. DR % vs λ spectra of SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) HFs in the sweep range of 200 nm–800 nm.

reported direct Eg data by some groups including our group for pristine SrFe12O19 NPs (as 1.75, 1.80, 1.83 and 2.10 eV) which were synthesized by different methods like hydrothermal route or sol-gel auto-

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Fig. 7. Tauc plots and estimated energy band gaps of SrNbxYxFe12-2xO19 (0.00 ≤ x ≤ 0.05) HFs.

diverse magnetocrystalline. It is well-known that Fe3+ ions on 4f2 and 2b sites is the main origin of uniaxial magneto-crystalline anisotropy of hexagonal ferrites [48]. Dopant ions occupy the 4f2 and cause a decrease in super-exchange interactions between different sites resulting in a decrease of magnetocrystalline anisotropy field (Ha) [49]. As result, coercivity decreases too. We calculated experimental magnetic moments (nB, Magneton number) in units of μB by using the formula nB = Molecular weight x Ms /5585 [50–52]. Magnetic moments decreased from 13.08 μB to 8.95 μB for 300 K data and from 19.53 μB to 12.68 μB for 10 K data. Fig. 10 shows the doped ion content dependence of Mr, Ms and Hc magnetic parameters that registered from both 300 K and 10 K VSM measurements. Sustained and significant decrements at all magnetic parameters of pristine SrFe12O19 HF sample are observed. Among the plotted data, just 10 K coercivity of 2824 Oe of sample SrFe12O19 is much smaller than its 300 K value. This sharp decrease in

Hc is due to lowering of Ha and a subsequent growth in Ms [53]. Further, doped samples with ion content of x = 0.01–0.04 have higher coercivity magnitudes except the sample with x = 0.05, too. Tables 2 and 3 contain the evolution of all measured or calculated magnetic parameters as fraction of co-doped ions at 300 K and at 10 K, respectively. Eq. (2) correlates the magnetic parameters Hc, Ms and effective magneto-crystalline anisotropy constant (Keff) [54,55]:

Hc = 0.64

K eff Ms

(2)

We used Eq. (2) to calculate the Keff magnitudes for all samples using the 300 K and 10 K magnetization data. All values are in the order of 105 Erg/g and in accordance with the reported order of magnitudes in literature [56–58]. The 2Keff/Ms ratio is directly proportional to the magnetocrystalline anisotropy field [55–59]:

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Fig. 8. M − H hysteresis loops registered from (0.00 ≤ x ≤ 0.05) NHFs (a) at 300 K and (b) at 10 K.

Ha =

SrNbxYxFe12–2xO19

2K eff Ms

(3)

Fig. 9. M vs 1/H2 graphs of (a) SrNb0.04Y0.04Fe11.92O19 HFs at 300 K and (b) SrNb0.03Y0.03Fe11.94O19 HFs at 10 K.

Ha values decrease from 13.54 kOe to 6.61 kOe due to effect of dopant ions for 300 K data. On the other hand, a sudden decrease in anisotropy field of pristine SrFe12O19 HF from 13.54 kOe (300 K value) to 8.83 kOe at 10 K is also remarkable. The reason for this decrement had been explained while mentioning about the similar decrement of Hc above. Similar decrement trend at 10 K data for Ha values is again observed for co-doped samples from 11.49 kOe to 6.02 kOe with increasing ion content from x = 0.00 to x = 0.05. If the NPs had perfect spherical symmetry, then single-domain particles would have intrinsic magnetocrystalline anisotropy field in the order of 17 kOe. We evaluated approximately 3.5–6 kOe smaller values for the samples with x = 0.00–0.04. Decrement for sample with x = 0.05 from 17 kOe of Ha is much more drastic. This is due to the strong impact of shape anisotropy commonly observed in HFs [56]. Our research group previously completed and published a study on SrNbxFe12-xO19 hexaferrites synthesized by the same sol-gel method [60]. We observed marginal changes at magnetization characteristics of just Nb3+ doped samples with ion contents, x = 0.02–0.08. However, there are significant tuning effects on magnetic properties of co-substitution of both Nb3+ and Y3+ ions with the equal contents, x = 0.01–0.05.

magnetic properties for SrNbxYxFe12–2xO19 (0.00 ≤ x ≤ 0.05) synthesized by citrate sol-gel approach. XRD pattern exhibited the formation of single phase of Sr hexaferrite with crystal size in the range of 29–47 nm. Also, FT-IR, SEM and TEM are reinforced the creation of Sr hexaferrite. DR % investigations revealed that all estimated direct Eg data are in a narrow band and can be accepted as in bandgap ranges of semiconducting materials. Co-doping process of Nb3+ and Y3+ ions lead to only marginal increments at band gap value of pristine SrFe12O19 HF. On contrary, co-doping of Nb3+ and Y3+ ions caused significant changes on magnetization characteristics of pristine sample. Replacement of dopant ions with Fe3+ causes to decrease in intrinsic anisotropy field and coercivity magnitudes. However, all pristine and co-doped samples maintain characteristic behavior of hexaferrites to be hard magnetic material. Most SQRs, which were calculated from 300 K and 10 K magnetization data, are smaller than the theoretical limit of 0.50 pointing to multi-domain structure for ensembles. Only pristine SrFe12O19 has 0.506 of SQR at 300 K assigning the single domain structure. According to the magnetic parameters, our compositions are potentially suitable for high-density recording media and permanent magnets applications.

4. Conclusion This paper explored the structure, morphology, optical and

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Acknowledgments The authors appreciate the support of the Institute for Research & Medical Consultations (Projects Application No. 2018-IRMC-S-1, No. 2018-IRMC-S-2) of Imam Abdulrahman Bin Faisal University (IAUSaudi Arabia). References [1] R.C. Pullar, Hexagonal ferrites: a review of the synthesis, properties and applications of hexaferrite ceramics, Prog. Mater. Sci. 57 (2012) 1191–1334. [2] Ü. Özgür, Y. Alivov, H. Morkoç, Microwave ferrites, Mater. Sci. Mater. Electron 20 (2009) 789. [3] T. Kimura, Magnetoelectric hexaferrites, Annu. Rev. Condens. Matter Phys. 3 (2012) 93–110. [4] P. Novak, J. Rusz, Exchange interactions in barium hexaferrite, Phys. Rev. B 71 (2005) 184433. [5] G. Tan, X. Chen, Structure and multiferroic properties of barium hexaferrite ceramics, J. Magn. Magn. Mater. 327 (2013) 87–90. [6] N. Koga, T. Tsutaoka, Preparation of substituted barium ferrite BaFe12−x(Ti0.5Co0.5)xO19 by citrate precursor method and compositional dependence of their magnetic properties, J. 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Fig. 10. Magnetization data of SrNbxYxFe12–2xO19 (0.0 ≤ x ≤ 0.05) as function of Nb3+ and Y3+ ions content. Table 2 Magnetic parameters of SrNbxYxFe12–2xO19 (0.00 ≤ x ≤ 0.05) HFs at 300 K. x

Hc (Oe)

Mr (emu/ g)

Ms (emu/ g)

SQR

nB (μB)

Keff (x105 Erg/g)

Ha (kOe)

0.00 0.01 0.02 0.03 0.04 0.05

4334 4119 3981 3678 3802 2116

34.80 32.65 28.86 22.79 22.70 14.77

68.82 71.12 63.21 49.77 48.26 45.56

0.506 0.459 0.456 0.458 0.470 0.324

13.08 13.50 12.01 9.46 9.18 8.67

4.66 4.58 3.93 2.86 2.87 1.50

13.54 12.87 12.24 11.49 11.88 6.61

Table 3 Magnetic parameters of SrNbxYxFe12–2xO19 (0.00 ≤ x ≤ 0.05) HFs at 10 K. x

Hc (Oe)

Mr (emu/g)

Ms (emu/g)

SQR

nB (μB)

Keff (x105 Erg/g)

Ha (k Oe)

0.00 0.01 0.02 0.03 0.04 0.05

2824 3678 3666 3390 3564 1928

49.83 46.90 41.63 32.79 35.60 18.60

102.73 98.21 88.24 70.77 73.93 64.59

0.485 0.477 0.472 0.463 0.481 0.288

19.53 18.64 16.76 13.45 14.06 12.30

4.53 5.64 5.05 3.75 4.12 1.95

8.83 11.49 11.45 10.59 11.14 6.02

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[28] [29]

[30]

[31]

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