Investigation of structural and magnetic properties of nanocrystalline Mn-doped SrFe12O19 prepared by proteic sol–gel process

Journal of Magnetism and Magnetic Materials 395 (2015) 263–270

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Investigation of structural and magnetic properties of nanocrystalline Mn-doped SrFe12O19 prepared by proteic sol–gel process W.M.S. Silva a, N.S. Ferreira b, J.M. Soares c, R.B. da Silva c, M.A. Macêdo a,n a

Physics Department, Federal University of Sergipe, São Cristóvão 49100-000, Brazil Departamento de Física, Universidade Federal do Amapá, Macapá 68902-280, Brazil c Departamento de Física, Universidade do Estado do Rio Grande do Norte, Mossoró, RN 59610-210, Brazil b

art ic l e i nf o

a b s t r a c t

Article history: Received 22 May 2015 Received in revised form 21 July 2015 Accepted 25 July 2015 Available online 28 July 2015

Nanoparticles of SrFe12-xMnxO19 (x ¼ 0.0 and 0.10) were synthesized by a proteic sol–gel process. Thermogravimetric and differential thermal analyses (TG–DTA) indicated the formation of nanocrystalline strontium ferrite phase at a calcination temperature of 1000 °C. Structural and microstructural evolutions of the samples were studied by X-ray powder diffraction (XRD) and the Rietveld method. XRD patterns demonstrated that all samples consisted of single-phase M-type strontium hexaferrite. The crystal lattice constant did not change significantly with manganese substitution, ranging from 0.5877(3) nm (x ¼0.0) to 0.5876(3) nm (x ¼1.0). In addition, the average crystallite size, which was determined from the Williamson–Hall formula, was about 46.4–52.6 nm. Infrared spectroscopy (FT-IR) showed the presence of three principal absorption bands in the frequency ranges around 435–535 cm  1 and around 595 cm  1, indicating the formation of the hexaferrite. Scanning electron microscopy (SEM) revealed that particles consisted of irregular platelets with sizes from 68 to 204 nm. Room-temperature Mössbauer investigations revealed that manganese ions preferentially occupied the 12k, 4f1, 4f2, and 2a sites. Hysteresis loops (M–H) showed that the saturation magnetization, remanence, and coercivity decreased with manganese doping. This effect is discussed in terms of the distribution of metal cations in the tetrahedral and octahedral sites. & 2015 Elsevier B.V. All rights reserved.

Keywords: M-type hexaferrite Proteic sol–gel process Crystal structure Magnetic property

1. Introduction In recent years, M-type strontium hexaferrite (Sr-hexaferrite, SrFe12O19, SrM) has been the subject of extensive research due to its possible use in different technological applications. These materials are used in the magnetic recording industry and in magneto-optic devices, microwave devices, microelectromechanical systems, transformer cores, and antennas [1]. The physical properties of strontium ferrite depend intrinsically on the synthesis route. Strontium hexaferrite is a ferromagnetic material having a structure belonging to the hexagonal space group P63/mmc and a magnetocrystalline anisotropy parallel to the c-axis of the unit cell. The crystalline structure of Sr-hexaferrite is constructed from a periodic repetition of SRS*R*blocks, where the cubic S block has a spinel structure and the hexagonal R block contains a Sr2 þ ion, and the S* and R* blocks are axially symmetric around the hexagonal axis-c by 180°. Twenty-four Fe3 þ n

Corresponding author. E-mail addresses: [email protected] (W.M.S. Silva), [email protected] (M.A. Macêdo). http://dx.doi.org/10.1016/j.jmmm.2015.07.085 0304-8853/& 2015 Elsevier B.V. All rights reserved.

ions occupy interstitial positions in five different crystallographic sites and are distributed as follows. Three octahedral sites (12k, 4f2, and 2a) and site 4f1 have a geometric tetrahedron formed by oxygen atoms. Site 2b has a bipyramidal geometry with a hexahedral triangular base formed by five oxygen atoms around the Fe3 þ ion [2]. Numerous researchers [3,4] have been working on the effect of substitution of Fe3 þ ions with different compounds, where the partial replacement of these ions modifies various magnetic properties [5,6]. Herein, we present a study of the effect of substitution of Fe 3 þ by Mn3 þ on the structural and magnetic properties of strontium hexaferrite synthesized by the proteic sol–gel process.

2. Experimental 2.1. Synthesis M-type SrFe12  xMnxO19 (x ¼0 and 0.10) samples were synthesized by the proteic sol–gel process. Appropriate amounts of Fe (NO3)3  9H2O (99% Sigma-Aldrich), Sr(NO3)3 (99% Sigma-Aldrich),

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Fig. 2. Thermogravimetric (TGA) curve and differential thermal analysis (DTA) curve of the sol–gel precursor, SrFe12  xMnxO19 (x¼ 0.0). Fig. 1. Flow chart for sample preparation of Sr-hexaferrite nanopowders by the proteic sol–gel process.

was calibrated with a 25-mm-thick

and MnCl2  4H2O (99% Sigma-Aldrich) were dissolved in filtered coconut water and mixed for several seconds to form the sol. The sol was dried at 100 °C for 24 h for gel formation and dehydration. Afterwards, it was transformed into xerogel. For the decomposition of organic materials and salts, the xerogel was calcined first at 500 °C for 1 h at a heating rate of 1 °C per minute in an open atmosphere. Then, the temperature was increased at a rate of 1 °C per minute up to a temperature of 1000 °C, where it remained for 1 h, and then the xerogel was removed from the oven at room temperature. The systematic procedure for the synthesis of hexaferrite is illustrated in Fig. 1.

3. Results and discussion

2.2. Characterization The thermal behavior of the samples was studied by thermogravimetric and differential thermal analyses (TG–DTA) using a simultaneous TG–DTA (TA Instruments) at a heating rate of 10 °C per minute in air synthetic with a flux of 100 ml/min. Fourier transform infrared (FT-IR) spectroscopy measurements of the samples were performed on a Virian Resolutions Pro in the range of 400–4000 cm  1 with KBr pellets. X-ray diffraction (XRD) analysis of the calcined SrFe12  xMnxO19 (with x ¼0.0 to 0.10) samples was performed in a Rigaku X-ray diffractometer using a Cu Kα radiation tube operated at 40 kV and 40 mA. XRD data were taken in step scan mode in the range of 20–80° (2θ) with step sizes of 0.02° and a scan speed of 0.02°/min. The structural and microstructural parameters were extracted using Rietveld refinement with the program FullProf [7]. The Bragg peaks were modeled with the pseudo-Voigt function, and the background was estimated by linear interpolation between selected background points. The determination of the average crystallite sizes (D) and the lattice microstrain (ɛ) was estimated by line-broadening analysis using Williamson–Hall (W–H) [8] methods. The morphology, microstructure, and particle size of the SrFe12O19 and SrFe11.9Mn0.10O19 nanopowders were examined by scanning electron microscopy (SEM) using a MIRAN 3 TESCAN with an accelerating voltage of 13.0 kV. The SEM images were analyzed with the software ImageJ [9]. The magnetic properties of SrM samples were measured at room temperature using a maximum external field of 70 kOe and a Magnetic Properties Measurements System (MPMS; Quantum Design). 57Fe Mössbauer effect measurements were taken at room temperature in a transmission geometry using a conventional constant-acceleration spectrometer operating in triangular wave mode with a 57Co source in a rhodium matrix. The spectrometer

α-Fe foil.

3.1. Thermal analysis Fig. 2 shows the TG–DTA curve of the synthesized SrFe12  xMnxO19 (x¼ 0.0). Thermal analysis of the sample was carried out up to 1000 °C at a heating rate of 10 °C/min in an argon gas with platinum crucibles to observe the structural variations, such as the weight loss and transformation of different phases during heat treatment. The TGA curve shows that the first mass loss occurred between 45 °C and 100 °C along with an exothermic peak at 53 °C and a slightly weak endothermic trough at 76 °C in the DTA curve, which are respectively associated with the decomposition of nitrates and the evaporation of both water molecules still present in the sample and water adsorbed from the atmosphere after the preparation of the xerogel before measurement. The second mass elimination between 150 and 258 °C is associated with the melting of proteins [10]. At temperatures of 272 and 334 °C, it was possible to observe a third mass loss associated with an exothermic peak at 293 °C due to the probable formation of hydroxides. The fourth mass loss between at 370 and 783 °C is correlated with the decomposition of hydroxides and organic matter from the precursor agents in coconut water as well as the formation of several metal oxides and mono-ferrite. The fifth exothermic peak at 754 °C suggests the formation of the hexaferrite phase (inset in Fig. 2) [11]. 3.2. FT-IR spectroscopy The spectra were recorded by mixing 0.05% in KBr powder to obtain better resolution of the bands. Fig. 3 shows the absorption bands at 435, 535, and 595 cm  1, which are characteristic of hexaferrite [12,13]. The peaks at 1200, 1470, and 1560 cm  1 are related to the M–O–M (metal–oxygen–metal) bands, such as Fe– O–Mn and Fe–O–Fe. The presence of these bands suggests that Fe– O and Sr–O bonds could interact with Mn dopant atoms. The variation in the band positions resulted from differences in the distances between tetrahedral and octahedral sites at which they occurred in the intervals of 400–500 cm  1 and 500–700 cm  1. This directly resulted in a shift of the absorption peaks due to variations in the distances of Fe3 þ –O2  caused by the replacement of Mn3 þ [14]. The relatively broad peaks at 3440 cm  1 for both samples became narrow and are associated with the deformation vibration of hydroxyl groups (OH) obtained in a wet atmosphere.

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265

Fig. 3. FT-IR spectra of SrFe12  xMnxO19 (x ¼ 0.0 and 0.10) synthesized at 1000 °C for 1 h.

3.3. XRD studies The observed, calculated, and difference XRD profiles for SrFe12O19 and SrFe11.9Mn0.10O19 after the final cycle of Rietveld refinement are shown in Fig. 4. The peaks of the XRD patterns presented in Fig. 4(a) and (b) were successfully indexed with the hexagonal structure of M-type SrFe12O19 hexaferrite (ICDD file no. 202518) belonging to the P63/mmc space group. However, a small broad peak at a 2θ value of about 25°, marked as “*”, was observed in all samples, which is attributed to traces of strontium carbonate (SrCO3) [15,16]. We expect that SrCO3 was formed during the first decomposition of the xerogels at 500 °C for 1 h. Although the TG– DTA results suggest the complete decomposition of Sr(NO3)3 and organic materials from the coconut water between 370 and 783 °C, the calcination for 1 h at 1000 °C was not sufficient for the complete decomposition of the organic materials. In fact, it is clear from the TG–DTA analysis that organic materials must have decomposed at temperatures up to 1000 °C; however, times longer than 1 h are required to avoid the formation of carbonates. The structural parameters, reliability parameters, unit cell volume, microstrain, and average size of the crystallites are given in Table 1. The refinement lattice parameters for pure (x ¼0.0) samples were a¼ 5.8767(3) Å and c ¼23.0328(2) Å. For x ¼0.10 samples, the lattice parameters were almost those of pure SrFe12O19, and the low doping level did not alter the unit cell volume because of the comparable ionic radius of the rV(Mn3 þ )¼0.72 Å and rV(Fe3 þ ) ¼0.72 Å ions. The crystallite size and lattice strain of the SrFe12O19 and SrFe11.9Mn0.10O19 nanoparticles were calculated from the broadening of the X-ray diffraction peaks using the Williamson–Hall approach [17]. The observed XRD broadening is associated with both size and lattice strain effects and can be appropriately characterized by the Williamson–Hall equation, Eq. (1)

β cos θ =

kλ + 4εsinθ D

(1)

where ε , D , λ , β , k , and θ are the effective strain, particle size, wavelength, shape coefficient for the reciprocal lattice point (k¼ 1 was chosen, considering that the shape of this point is spherical), and Bragg's angle, respectively. According to Eq. (1), a plot of β cos θ versus sinθ (Williamson–Hall plot) yielded a straight line, where the microstrain is given by the slope of the line, ε , and the crystallite size, D , from the intersection kλ /D with the vertical axis. Williamson–Hall (WH) plots for SrFe12  xMnxO19 (x ¼0.0 and

Fig. 4. Rietveld refined XRD pattern of nanoparticles of SrFe12  xMnxO19 with (a) x¼ 0 and (b) x ¼0.10.

x¼ 0.10) nanoparticles are shown in the insets of Fig. 4. In all cases, the WH plot shows a linear dependence and negative slope, which correlate with the presence of a homogeneous crystallite size distribution and an effective compressive strain in the crystal lattice, respectively. The average crystallite size and lattice strain determined from the WH plots (Fig. 4) for the investigated samples with x¼ 0.0–0.10 are listed in Table 1. The average crystallite size tended to decrease (from approximately 52.6 70.1 nm to approximately 46.4 70.1 nm) with an increase in Mn content. Moreover, the WH plot showed that the lattice strain decreased from  9.1  10  4 to  11.2  10  4 when x increased from 0.0 to 0.10. This slight decrease in average crystallite size may not be attributed to contraction of the SrFe12O19 crystalline lattice since the replacement of Mn3 þ by Fe3 þ did not significantly change the lattice parameters of SrFe12O19, as shown in Table 1. However, it seems that the presence of Mn obstructed the crystal growth of SrFe12O19, as reported for other spinel ferrites [18]. Hence, this behavior can be expected because of the presence of Mn in the system, which liberated more heat during the growth process, and hence the particle growth was obstructed because of the decrease in the molecular concentration at the crystal surface [19]. Although there is a structural isomorphism for M-type SrFe12  xMnxO19 prepared by the proteic sol–gel process when

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Table 1 Lattice parameters, a, b, and c (in Å); volume (V) (in Å3); average crystalline size obtained from the XRD results 〈DXRD〉 (in nm); and microstrain (ε) of SrFe12  xMnxO19 (x¼ 0.0 and 0.10). Sample (x)

a ¼b

c

(c/a) ratio

V

〈DXRD〉

ε (  10  4)

χ2

0.0 0.10

5.8767 (3) 5.8765 (3)

23.0328 (2) 23.0330 (2)

3.919342 3.919509

688.88 (8) 688.84 (8)

52.6 (1) 46.4 (1)

 9.1  11.2

1.33 1.19

Table 2 Bond lengths of the SrFe12  xMnxO19 (x¼ 0 and 0.10) samples, as obtained from Rietveld analysis after the last refinement cycle. Bonds

x ¼0.0

x¼ 0.10

SrO12 polyhedron Sr–O(3)  6 Sr–O(5)  6 〈Sr–O〉

2.9481 2.7647 2.8564

2.9413 2.6922 2.8167

Fe(1)O6, octahedron Fe(1)–O(4)  6

2.1076

2.1545

Fe(2)O5 trigonal bipyramidal Fe(2)–O(1)  1 Fe(2)–O(3)  3 O(1)–O(3)  1 O(3)–O(3)  1

2.2711 1.8983 2.9599 3.2879

2.4386 1.8714 2.9301 3.2361

Fe(3)O4, tetrahedron Fe(3)–O(2)  1 Fe(3)–O(4)  3

1.9555 1.7935

2.0026 1.6896

Fe(4)O6, octahedron Fe(4)–O(3)  3 Fe(4)–O(5)  3 〈Fe(4)–O〉 O(3)–O(5)

2.0278 2.0382 2.033 2.8814

2.0132 2.1535 2.0833 2.8146

Fe(5)O6, octahedron Fe(5)–O(1)  1 Fe(5)–O(2)  1 Fe(5)–O(4)  2 Fe(5)–O(5)  2 〈Fe(5)–O〉

2.0265 2.0165 2.1347 1.9692 2.0418

1.9859 1.9694 2.2332 2.0557 2.0888

trigonal site was higher for the doped sample. This behavior is attributed mainly to a decrease in the O(1)–O(3) interatomic distance, followed by an increase in the O(3)–O(3) bond length due to electrostatic repulsion. This interatomic separation between O(3) and O (3) acted as a potential barrier with a minimum opening of 1.895 Å (compatible with the ionic radius of Fe3 þ ). As a result, the Fe(2) ions exhibited a rapid occupation diffusion dynamic among the two pseudo-tetragonal positions at room temperature [22]. 3.4. SEM analysis

compared to other strontium hexaferrites prepared by other routes, such as that reported in [19], small differences were observed in the bond length between the Fe3 þ , O2  and Sr2 þ ions in the samples of SrFe12  xMnxO19 with x¼ 0.0 and 0.10. It was also observed that both samples showed an increase in the majority of the average bond lengths (see Table 2) when compared to results published by Obradors et al. [20]. According to data presented in Table 2, the bond length values of the bonds between Sr–O(3) and Sr–O(5), which belong to a polyhedron located in the R-block of the hexagonal structure of the SrFe12  xMnxO19 samples (x ¼0.0 and 0.10), varied. This change can be attributed to axial compression in the strontium polyhedron [20,21]. The compression induced variations in the bonds between O(3) and O(5), which naturally affected the Fe(2) and Fe (4) neighbor bond, which belongs to the bipyramidal trigonal and octahedral sites. It was also observed that the doped sample with x ¼0.10 of Mn presented a decrease in the main interatomic distance of the Sr–O bond. This reduction resulted in a decrease of the Fe(2)–O(3), Fe(4)–O(3), and O(3)–O(5) bond lengths due to axial compression exerted by the polyhedron formed by strontium atoms. However, the structural distortion at the Fe(2) bipyramidal

Fig. 5 shows the SEM images of the samples calcined at 1000 °C for 1 h in air at a heating rate of 1 °C per minute. The conditions used during heat treatment and doping had a significant effect on the morphology of the magnetic particles of strontium hexaferrite. It was possible to verify the presence of various particles of irregularly shaped plates with diameters (d) and lengths (l) of an average of 68 nm and 138 nm, respectively, as shown in Fig. 5a for SrFe12 xMnxO19 (x¼0.0). However, the sample doped with manganese (x¼ 0.10) had particles with diameters and average lengths of 101 nm and 204 nm, respectively, presenting an irregular morphology of platelets and nanorods [23–25] (Fig. 5b). The origin of the formation of nanorods was associated with the preferential growth of certain crystallographic surfaces of single crystals of SrFe12 xMnxO19 due to the diffusion process within the doped sample. 3.5. Mössbauer spectroscopy analysis Fig. 6 shows the Mössbauer spectra at room temperature for the SrFe12  xMnxO19 (x ¼0.0 and 0.10) samples. The spectra were fitted with five Lorentzian sextets corresponding to the presence of Fe3 þ ions in the tetrahedral (4f1), octahedral (12k, 2a and 4f2), and bi-pyramidal trigonal (2b) sites of Sr-hexaferrite. According to the Gorder model [26], the positions and spin orientations of magnetic ions in the crystalline structure resulted from the superexchange interaction of O2  ions. These interactions were responsible for the magnetic moments of the Fe ions in the 12k, 12a, and 2b sites, which had spins aligned parallel to the crystallographic c-axis in a “spin up” configuration, while the spins of the Fe ions in the 4f1 and 4f2 sites were aligned in the opposite direction with a “spin down” configuration. Table 3 presents the hyperfine parameters and Fe ion occupations for the five sites obtained from the relative Mössbauer areas [27]. However, the values of the resonant sextets of the magnetic interactions for the SrFe12 xMnxO19 samples (x¼ 0.0 and 0.10) obtained from the spectra for the sum of values of the 2aþ 2be 4f1 þ 4f2 sites and for the 12k site were different than the theoretical relative populations. This difference is attributed to inequalities present among the Lamb–Mössbauer factors for the different sites [28]. For both samples, the absorption from sites with a spin down configuration increased when Mn3 þ ions were present in the hexaferrite lattice, favoring a decrease in the liquid magnetic momentum in the ferrimagnetic structure. Comparing the values of the respective Fe3 þ ion occupations on the hexagonal structure sites for the undoped sample, it is possible to infer that Fe3 þ ions preferentially occupied the 12k, 4f1, and 4f2 sites with some occupation of the 2a sites. Moreover, a lower preference in

W.M.S. Silva et al. / Journal of Magnetism and Magnetic Materials 395 (2015) 263–270

267

Fig. 5. SEM images of the samples calcined for 1 h: SrFe12  xMnxO19 (a,b) x¼ 0.0 and (c,d) x ¼0.10.

occupancy for the trigonal bipyramidal sites (2b) was also observed. These results indicate that hexagonal particles formed and grew through columnar accommodation in the c-direction. Therefore, the sites with high crystallographic symmetry were preferentially filled with Fe3 þ ions. This increase in the lattice c-parameter could be due to incomplete occupation of the 2b sites. For the doped sample, it is evident that Fe3 þ ions preferentially occupied the 12k, 4f1, 4f2, and 2b sites with lower occupation of the 2a site. This result indicates that the presence of Mn affected iron redistribution in the lattice, forcing the Fe3 þ ions to avoid the 2b site, without causing any variation in the hexagonal c-parameter. The presence of a singlet in the Mössbauer spectrum of the doped sample could be the result of the paramagnetic behavior of some particles present in the matrix. The sequence of relative magnitudes of the hyperfine fields for the SrFe12  xMnxO19 (x ¼0) sample was in agreement with [28]: Hhf (2b) oHhf (12k) oHhf (4f1)o Hhf (4a) oHhf (4f2). However, for the SrFe12  xMnxO19 (x ¼0.10) sample, the sequence of relative values for the hyperfine fields Hhf was Hhf (2b) o Hhf (12k) oHhf (4f1)oHhf (4f2)oHhf (4a). The observed inversion in the hyperfine field values in relation to those of the undoped sample for the 4f2 and 2a sites indicates that the replacement of iron by manganese resulted in a small difference in the superexchange interactions and induced the existence of disordering in the local configuration in the inter and intra

sublattice, leading to a small perturbation in the magnetic interactions and resulting in a change in the collinear order of the blocks for the configuration of localized spin canting [29]. The isomer shift (δ) values for the samples were in the range of 0.20–0.60 mm/s, indicating that iron ions present in the hexaferrite were in the Fe3 þ oxidation state. As shown in Table 3, the 2a octahedral site presented a higher variation in δ when Mn3 þ ions were accommodated in the lattice of the hexaferrite, resulting in an increase in the Fe(1)–O(4) distance. This variation in the bonding length induced a decrease in the overlap between the 4s iron orbital and the oxygen ligand orbital and therefore led to an increase in the isomer shift [30]. According to Kuzmann et al. [31], the obtained values of δ of 4f1 for the strontium hexaferrite samples were lower than 0.3 mm/s. Thus, it is suggested that the Fe3 þ ion was in a high-spin configuration. The value of δ for the 2b site in the SrFe12  xMnxO19 (x ¼0) sample was 0.40 mm/s, which was higher than that (0.25 mm/s) for the same site in the SrFe12  xMnxO19 (x ¼0.10) sample. This difference may be related to the crystallographic and chemical environment of the 2b site because of the proximity of this site to Sr2 þ sites. In both samples, δ (2b) o δ (4f2) indicates that the proximity of these sites to Sr2 þ ions did not result in changes. There were some variations in the electric quadrupole splitting (Δ) as a function of the Mn concentration in the lattice (Table 3).

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Fig. 6. Mössbauer spectra of nanoparticles of SrFe12  xMnxO19 with (a) x¼ 0 and (b) x¼ 0.10.

Fig. 7. M–H hysteresis loops of SrFe12  xMnxO19 with (a) x¼ 0 and (b) x ¼0.10.

3.6. Magnetic measurements Table 3 Mössbauer parameters of the SrFe12  xMnxO19 (x ¼0 and 0.10) samples. x

Site

HF (T)

QS (mm/s)

IS (mm/s)

Area ratio (%)

0.0

4f2 2a 4f1 12k 2b

52.2367 51.2298 49.3845 41.1623 40.485

0.5895 0.0028 0.2534 0.4167 1.9697

0.4189 0.2782 0.2968 0.3551 0.4083

13.95 6.61 22.05 53.48 3.91

0.10

2a 4f2 4f1 12k 2b Singlet

51.1628 51.1383 49.6333 41.3183 40.5563

0.3978 0.3152 0.049 0.4042 2.0471

0.6093 0.3635 0.2097 0.3397 0.251 0.099

3.67 15.66 23.80 49.55 4.28 3.03

However, it was observed that the Δ values for the SrFe12  xMnxO19 (x ¼0.10) sample in the 2a and 2b sites were higher than those for the SrFe12  xMnxO19 (x ¼0) sample. The increase in Δ for the 2a site may be associated with distortion in the oxygen atoms that form the octahedron due to the substitution of Fe3 þ ions for Mn3 þ ions. The higher values of Δ for the 2b sites in both samples can be attributed to the strong symmetry breaking in the trigonal bipyramidal environments [32].

The coercivity (Hc) values, saturation magnetization (Ms), and remanent magnetization (Mr) were extracted from the hysteresis curves. The hysteresis loop exhibited a constricted characteristic for an applied field approaching zero, and the magnetization value relaxed more steeply compared to that of a typical ferromagnetic when the direction of the applied field was reversed; see Fig. 7 and Table 4. The magnetization of the SrFe12  xMnxO19 (x ¼0) sample is characteristic of a hard magnetic material with a coercive field of 3.85 kOe. It is evident that the SrFe12  xMnxO19 (x ¼0.10) sample was a soft magnet with a coercive field of 0.25 kOe. The addition of Mn3 þ to the lattice led to strong magnetic dipole coupling, providing an additional demagnetizing field. As the external magnetic field approached zero, the coupling of the dipoles caused neighboring spins to align antiparallel. However, when the external field increased above a certain value, the moments rotated to align parallel to the field. This coupling of magnetic dipoles led to a reduction in coercivity [33,34]. The values of saturation Table 4 Magnetic parameters of the SrFe12  xMnxO19 (x ¼0 and 0.10) samples. Sample

Ms (emu/g)

Mr (emu/g)

Hc (kOe)

Mr/Ms

nB

x ¼0 x ¼0.10

63.90 62.60

30.20 28.80

3.850 0.250

0.472 0.450

12.14 8.490

W.M.S. Silva et al. / Journal of Magnetism and Magnetic Materials 395 (2015) 263–270

magnetization (Ms) for the SrFe12  xMnxO19 samples with x ¼0 and 0.10 were 63.90 and 62.60 emu/g, respectively. The reduction in Ms is associated with the substitution of Fe3 þ ions by Mn3 þ ions. According to Eq. (2) [32], the magnetic moment of the Sr-hexaferrite sample is determined by the sum of the magnetic moments of ions in different positions in different crystal interstitial sites. →

→

→

←

(2)

The decrease in Ms resulted from the reduction in the concentration of Fe3 þ ions in the 12k and 2a sites belonging to a ⎯⎯⎯→ → → positive component of the moment (Ms ( 12 k + 2 b + 2 a)) and a 3þ slight increase in the occupation of Fe ions in the 4f1 and 4f2 sites belonging to a negative component of the moment ←⎯⎯⎯ ←⎯⎯⎯ ( Ms( 4f2 + 4f1 )). The replacement of Mn3 þ by Fe3 þ also led to a decrease in Mr, which was related to two mechanisms: (a) increasing the lengths of bonds between the ions of the Fe(1)– O(4), Fe(4)–O(5), Fe(5)–O(4) (octahedral), Fe(2)–O(1) (trigonal bipyramidal), and Fe(3)–O(2) (tetrahedral) bonds and (b) reducing the overlap of atomic orbitals, which led to weakening of the superexchange interactions between the ions that compose the Fe– O–Fe [35]. Another possible explanation for the reduction in Ms and Mr is related to spin canting [36]. As the Mn ions entered the iron sites, there was a deviation from a collinear arrangement in favor of an arrangement that was not collinear in spin. The value of the ratio Mr/Ms of both samples was less than 0.5 due to dipolar interactions between particles, as given in Table 4. The Bohr magneton number (nB) was determined by Eq. (3) [37].

nB =

molecular weight × Ms 5585

[3]

[4]

←

Ms = Ms(12k + 2b + 2a) − Ms(4f2 + 4f1)

[2]

[5]

[6]

[7]

[8] [9] [10]

[11] [12]

[13]

[14]

[15]

(3)

The nB values decreased with the addition of Mn3 þ ions to the hexagonal matrix of SrFe12  xMnxO19, as shown in Table 4. This decrease is attributed to the magnetic moment of the manganese ion, μ (Mn3 þ ), of 3.5μB [38], which was smaller than the moment of the iron ion, μ (Fe3 þ ), of 5μB, implying a decrease in the magnetization of the ferrimagnetic sublattice.

4. Conclusion The effects of the substitution of Mn3 þ ions in M-type strontium ferrite prepared by the proteic sol–gel process were investigated. XRD analysis showed that the two samples presented the hexaferrite phase with the P63/mmc space group. The particle size measured in SEM images confirmed the nanocrystallite size. FT-IR measurements showed that Sr–O and Fe–O bonds formed in the nanoparticles. Mössbauer spectrometry showed corresponding changes in the spectra when Mn ions entered the lattice of M-type strontium ferrite and indicated their positions in the magnetic structure of Sr–hexaferrite. M–H analysis of the results showed that there was a reduction in the saturation magnetization and remanence values and a considerable decrease in the coercive field.

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