Magnetic properties and Mössbauer spectroscopy on Ga, Al, and Cr substituted hexaferrites

Journal of Alloys and Compounds 585 (2014) 465–473

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Magnetic properties and Mössbauer spectroscopy on Ga, Al, and Cr substituted hexaferrites M. Awawdeh a, I. Bsoul b, S.H. Mahmood c,⇑ a

Physics Department, Yarmouk University, Irbid, Jordan Physics Department, Al al-Bayt University, Mafraq, Jordan c Physics Department, The University of Jordan, Amman, Jordan b

a r t i c l e

i n f o

Article history: Received 19 June 2013 Received in revised form 25 September 2013 Accepted 26 September 2013 Available online 8 October 2013 Keywords: Barium hexaferrite Magnetic properties First anisotropy constant Mössbauer spectroscopy

a b s t r a c t M-type hexaferrites BaFe12xMxO19 (M = Ga, Al, Cr) were prepared by ball milling and sintering. X-ray diffraction patterns indicated the formation of the hexaferrite phase at a sintering temperature of 1100 °C, with small amounts of a-Fe2O3 secondary phase in the Al and Cr substituted hexaferrites. Scanning electron microscopy indicated that the particle size does not change with Ga substitution, and decreases with increasing Al and Cr substitutions. The atomic ratios determined by energy dispersive X-ray spectroscopy were close to stoichiometric ratios. Mössbauer spectra were used to determine the site preference for each type of cationic substitution. The results of the magnetic measurements indicated constancy of the anisotropy filed and similarity of the behaviors of the first anisotropy constant (K1) and the saturation magnetization with increasing substitution level for all types of substitutions. The coercivity was found to increase with increasing the concentrations of the substituents, and a maximum increase in the coercivity for the Al substituted hexaferrites was observed. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Six decades after the discovery of hexagonal ferrites, interest in the synthesis and characterization of various derivatives of these ferrites is still growing [1,2]. The increasing demand for high frequency devices, high density magnetic recording, and permanent magnets at low cost have generated a great drive for research relevant to the fabrication of new hexaferrites with favorable characteristics for different applications [1–5]. M-type hexaferrite MeFe12O19 (Me = Ba, Sr, Pb) is the earliest developed material of the class of hexagonal ferrites, and is still receiving considerable attention due to the possibility of tuning its magnetic properties to a specific application. The magnetic characteristics were modified by a suitable substitution of Fe3+ ions by a trivalent ion such as Cr, Al, Ga, In, Sc, As [6–11], or combinations of a tetravalent ion such as Ti4+, Ru4+, Zr4+, or Sn4+ with a divalent ion such as Ni2+, Co2+, Zn2+, Ti2+ or Sn2+ [12–16]. The properties hexaferrites were found to be affected by the preparation method, and the experimental conditions [13,17–23]. The unit cell of BaM contains two molecules and is described by the stacking sequence RSR⁄S⁄, where R⁄ and S⁄ are the hexagonal (R) and spinel (S) blocks rotated by 180° around the hexagonal c-axis. Ba2+ ions replace an oxygen ion substitutionally in the ⇑ Corresponding author. Tel.: +962 796709673. E-mail addresses: [email protected], [email protected] (S.H. Mahmood). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.09.174

middle layer of the R block, and the ferric ions occupy three octahedral (12k, 4f2 and 2a) sites, one tetrahedral (4f1) site and one trigonal bi-pyramidal (2b) site. In this structure there are three spinup (2a, 2b, and 12k) sublattices and two spin-down (4f1 and 4f2) sublattices for Fe3+ ions. Accordingly, the magnetic moment per formula can be expressed as follows:

m ¼ 6m12k þ m2b þ m2a  2m4f 1  2m4f 2

ð1Þ

This results in a net magnetic moment of 20 lB per formula unit, corresponding to a magnetization of about 100 emu/g at 0 K for the unsubstituted BaM hexaferrite. These ferrites are characterized by a high magnetocrystalline anisotropy resulting from the coupling of the spins of Fe3+ at different sites. According to the single-ion model it was shown that iron ions at 2b sites have the largest positive contribution to the magnetocrystalline anisotropy, where as those at 12k sites have a relatively weak negative contribution [24]. The remaining sites have relatively weak positive contributions. Accordingly, the substitution of Fe3+ ions by other metal ions at different sites is expected to modify the magnetic properties of the hexaferrites. The effects of substituting Fe3+ ions by Al3+, Ga3+, and Cr3+ ions was examined by several investigators who reported different results on the preferential site occupations and modifications of the properties of hexaferrites induced by the substitution [6,8,10,11,25–29]. Ga3+ ions were variously reported to occupy 12k [26,27] and 4f2 sites [10,29] resulting in noncollinear spin

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

(317)

(2014)

(228)

(220)

(2011)

(217) (209)

(300)

(1011)

(1110)

(205)

(206)

(114) (203)

(108) (200)

(101) (102)

Fe2O3

(116)

(107) (110)

x=0.6

(112) (008)

structure. Al3+ ions, on the other hand, were reported to prefer 12k sites [27], or to be distributed among spin-up and spin-down sites [10,25]. The significant decrease in saturation magnetization in Al substituted hexaferrites was attributed to the distruction of the correlation between spin directions in the different blocks of the unit cell [10]. However, there seems to be an agreement that Cr3+ ions substitute Fe3+ ions at octahedral sites [8,28,29] resulting in a non collinear spin structure or reversal of the Cr3+ spins [28]. The variations of the experimental results in the literature could be due to variations in the preparation conditions, and to the goodness of fits to the experimental data. This work is concerned with a comparative study of the preferential cationic distributions and the magnetic properties and hyperfine interactions for the systems BaFe12xMxO19 (M = Cr, Al, Ga) prepared by ball milling and sintering at 1100 °C. Mössbauer spectroscopy (MS) and the magnetic measurements will be used to explain the effects of the different substituents and their concentrations on the properties of the hexaferrites.

(006)

466

Intensity (a.u.)

x=0.4

x=0.2

X=0.0 2. Experimental procedures

100

Intensity (%)

75

043-0002

50 25 0 20

30

40

50

60

70

2θ (deg.)

(220)

(2011)

(217)

(300)

(209)

(1110)

(1011)

(206)

(205)

(114)

(203)

(112) (008)

(110)

x = 0.8

(200) (108)

(107)

Fig. 1. XRD patterns of BaFe12xCrxO19 with the standard pattern of BaFe12O19 (file no.: 043-0002) for comparison.

x = 0.6

Intensity (a.u.)

The precursors of BaFe12xMxO19 with M = Ga, Al, or Cr were prepared by ball milling stoichiometric ratios of high purity oxides (Fe2O3, Cr2O3, Al2O3, Ga2O3), and barium carbonate (BaCO3). Fritsch Pulverisette 7 planetary ball-mill was used with a ball to powder ratio of 8:1. The milling was carried out for 16 h with an angular frequency of 250 rpm. The resulting powders were then pressed at 5 tons/ sq inch into disks, 1 cm in diameter. Seven samples of the Cr substituted system and five samples of the Al substituted system were sintered at temperatures ranging from 800 to 1200 °C for 2 h and examined by XRD and SEM. Samples sintered at 800 °C consisted of hematite (a-Fe2O3) as a majority phase. Barium hexaferrite phase started developing at 900 °C, along with an intermediate BaFe2O4 phase. At 1100 ° C, the intermediate phase disappeared and XRD pattern showed almost single-phase of BaFe12O19. At temperatures higher than 1100 °C, SEM data showed that the grain size increases in uncontrollable manner and agglomerates in different masses. Accordingly, the samples of interest in this study were those sintered at 1100 °C. XRD analysis was carried out with Philips X’Pert PRO X-ray diffractometer (PW3040/60) with CuKa radiation. The grain structure and stoichiometry of the majority of the prepared samples were examined by scanning electron microscope (SEM FEI Quanta 600) equipped with energy dispersive X-ray spectroscopy (EDS) facility for elemental analysis. Room temperature magnetic measurements were carried out using a vibrating sample magnetometer (VSM MicroMag 3900, Princeton Measurements Corporation), with applied fields up to 10 kOe. Mössbauer spectra were collected over 512 channels using a conventional constant acceleration Mössbauer spectrometer operating in transmission geometry. 57 Co=Cr source was used, and room temperature Mössbauer spectrum of a-Fe foil was used for calibration. The spectra were fitted with the appropriate number of components using standard v2 minimization routines. Fitting results with v2 values appreciably higher than 1 were rejected, and theoretical fits with v2 < 1 were obtained for most spectra.

x = 0.4

x = 0.2

3. Results and discussion 3.1. Structural analysis XRD patterns for the Ga substituted samples indicated the presence of a single hexaferrite phase consistent with standard pattern (JCPDS: 043-0002) and very small fluctuations of the lattice parameters about their average values (a = (5.889 ± 0.003) Å, c = (23.201 ± 0.007) Å) [7]. However, the Al and Cr substituted samples show the presence of small amounts of hematite (a-Fe2O3) phase demonstrated by the small peak between the two main structural peaks of the hexaferrite phase (Figs. 1 and 2). The amounts of the secondary hematite phase were evaluated from the relative intensities of the main structural peaks and were found to be 2.8 and 3.7 wt.% for Al and Cr substituted hexaferrites, respectively. The lattice parameters generally decreased with increasing Al or Cr substitution recording a maximum decrease of about 0.2%. X-ray density (qx) for each sample was determined from the molecular weight and the cell volume evaluated from the cell parameters. The density for the Al substituted system was found

Intensity (%)

x = 0.0

100 75 50 25 0

043-0002

25

30

35

40

45

50

55

60

65

2θ (deg.) Fig. 2. XRD patterns of BaFe12xAlxO19 with the standard pattern of BaFe12O19 (file no.: 043-0002) for comparison.

to decrease linearly from 5.30 g/cm3 for the un-substituted sample down to 5.20 g/cm3 for the sample with x = 0.8. On the other hand, the X-ray density for the Ga substituted system was found to

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

increase slightly from 5.30 g/cm3 for the un-substituted sample up to 5.36 g/cm3 for the sample with x = 1.0. The density for the Cr substituted samples, however, fluctuates between 5.30 and 5.32 g/cm3. The behavior of the X-ray density is associated with the significant reduction in molecular weight with Al substitution, the small increase in molecular weight with Ga substitution, and the insignificant variation of the molecular weight with Cr substitution. The bulk density (qb) of each prepared disk was calculated from the mass of the disk and its volume. The bulk density was found to be almost constant (2.6 ± 0.1 g/cm3) for the measured disks as a consequence of fabricating the disks under the same experimental conditions. The porosities (P = 1 – qb/qx) of the samples were then calculated from these densities, and found to be 0.51 ± 0.02. The similarity of the porosities of the samples is essential for comparing the magnetic properties of the samples. 3.2. Electron microscopy Scanning electron microscopy imaging indicated that the particle size tends to decrease with Al concentration as indicated by Fig. 3. On the other hand, SEM images indicated that the change in particle size with increasing Cr or Ga concentrations is less appreciable as shown by Figs. 4 and 5. Further, the atomic ratios derived from EDS spectra were close to stoichiometry as indicated by the data for representative samples in Table 1. However, the observed atomic ratios for Al in the substituted hexaferrites seem to deviate toward values lower than the expected values at high Al concentrations. 3.3. Mössbauer spectroscopy Mössbauer spectrum for the un-substituted sample was best fitted with five components corresponding to the five sites of Fe3+ ions in the hexaferrite lattice. The spectra for the Ga

467

substituted samples (Fig. 6) demonstrate the splitting of the 12k component by developing a new component with the hyperfine field of about 370 kOe and center shift and quadrupole shift almost equal to that for the 12k component (Table 2). This component is assigned to 12k sites perturbed by the presence of Ga3+ ions in neighboring sites, and named 12k1. The spectral intensities of the various components in Table 1 indicate that the most affected site by the substitution of Ga3+ ions is the 4f2 site. The observed splitting of the 12k component in our work is consistent with that reported by others [16,30–32], and is induced by the substitution of Fe3+ ions by Ga3+ ions at 4f2 sites resulting in local cancellation of the 12k–4f2 superexchange interactions. The decrease in hyperfine field and broadening of the spectral lines for the various components could be signature of the development of non-collinear spin structure as a result of the Ga substitution [10,32]. The fluctuations in the hyperfine parameters of the 2a and 4f1 components could arise from the mixing of these components due to similarity of their hyperfine parameters. Further, the small observed decrease in ionic population of these sites as determined from the spectral intensities leads to the conclusion that partial substitution of Ga3+ ions at these sites should not be excluded. The spectral intensity of the 12k + 12k1 sites, however, leads to the conclusion that substitution of Ga3+ ions at 12k sites is excluded, contrary to previously reported results [26,27]. In addition, the component associated with the 2b site can be distinguished due to its high quadrupole shift. However, small variations of its relative intensity should not be conclusive in determining the level of substitution at this site due to strong overlap with other components, specifically, the 12k1 component. The number of cations in the 12k + 2b sites for the un-substituted sample is 6.7, which is similar to that for the sample with x = 1 (6.8), indicating that the observed drop in intensity for the 2b component and the increase in intensity for the 12k components are due to their overlap rather than to preferential substitution at these sites.

Fig. 3. SEM images of BaFe12xAlxO19.

468

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

Fig. 4. SEM images of BaFe12xCrxO19.

Fig. 7 shows the spectra for the Al substituted hexaferrites. The spectra were fitted with six components; the sixth arising from the residual a-Fe2O3 phase observed in XRD patterns (Table 3). The rather low v2 values indicate reliable fitting results. In this system, the 12k component does not split into two distinctive components as observe in the case of Ga substitution. Moreover, the smaller effect of Al substitution on the hyperfine parameters supports the reported conclusion that the two systems assume different equilibrium in superexchange interactions [32]. The spectral intensities indicate that the preferential site for the Al3+ ions is the 4f2 site, contrary to previous results which reported preference for 12k sites [25,27]. Also, the fluctuations in the intensities of the 4f2, 2a, and 4f1 components arise from the overlap of these components, which leads to the conclusion that in analyzing the spectral intensities, one should consider the cumulative intensity of the three components. The number of Fe3+ cations at these three sites (normalized to 100% for the hexaferrite components) is 5.5 for x = 0.1, while it is 4.8 for x = 0.8, the difference being equal to the difference in x. This result indicates that the substitution of Al3+ occurs mainly at these three sites, and excludes the possibility of occupying 12k sites. Mössbauer spectra for BaFe12xCrxO19 hexaferrites are shown in Fig. 8, and the fitting parameters are listed in Table 4. The hyperfine fields do not change appreciably with Cr concentration, which could be due to the fact that Cr3+ ions have spin

Fig. 5. SEM images of BaFe12xGaxO19.

Table 1 The number of atoms per formula in the substituted hexaferrites as derived from EDS spectra. Sample

Ba

Fe

Al

Cr

Ga

0.3 Al 0.6 Al 0.8 Al

0.88 0.95 1.07

11.65 11.50 11.43

0.35 0.50 0.57

– – –

– – –

0.3 Cr 0.6 Cr

1.00 0.85

11.60 11.29

– –

0.40 0.71

– –

0.2 Ga 0.6 Ga 1.0 Ga

1.10 0.98 1.02

11.72 11.40 11.08

– – –

– – –

0.28 0.60 0.92

magnetic moment, contrary to the Al3+ and Ga3+ ions. Analysis of the normalized spectral intensities for the various components indicates that the reduction of the number of Fe3+ at 12k sites is 0.24, and the reduction of the number in the 4f1 + 4f2 + 2a sites is 0.30 as the concentration of Cr increases from 0.1 to 0.6. Since the total reduction of the intensities of these sites is almost equal to the difference in x, one can safely assume substitution of Cr3+ ions at these sites. The substitution of Cr3+ ions at the 12k sites lowers the hyperfine field for the 4f2 component leading to overlap with the 2a and 4f1 components, and a consequent rise in the relative intensities of the latter components.

469

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

100 98

100 Pure

98 0.2 Ga

Intensity (%)

96 94

96

92 90

94

88 92

86 84

90

100

100

Intensity (%)

0.6 Ga

1.0 Ga

98

98

96

96

94

94 -15

-10

-5

0

5

10

Velocity (mm/s)

-15 15

-10

-5

0

5

10

15

Velocity (mm/s)

Fig. 6. Mössbauer spectra for BaFe12xGaxO19.

Table 2 The hyperfine fields (Bhf) in kOe, center shifts (CS) in mm/s, quadrupole splittings (QQ) in mm/s, and percentage relative intensities (I) of the components of the spectra of BaFe12xGaxO19 hexaferrites. Parameter

Site

0.0

0.2

0.6

1.0

Bhf1 Bhf2 Bhf3 Bhf4 Bhf5 Bhf6

4f2 2a 4f1 12k 2b 12k1

517 511 493 417 403 –

518 509 493 420 406 371

515 494 490 419 404 371

508 486 483 415 383 368

CS1 CS2 CS3 CS4 CS5 CS6

0.38 0.37 0.27 0.37 0.32 –

0.38 0.42 0.27 0.36 0.33 0.36

0.40 0.45 0.23 0.36 0.28 0.34

0.39 0.49 0.23 0.36 0.44 0.37

QQ(1) QQ(2) QQ(3) QQ(4) QQ(5) QQ(6)

0.23 0.07 0.22 0.42 2.20 –

0.20 0.04 0.21 0.41 2.23 0.42

0.15 0.18 0.16 0.42 2.21 0.38

0.17 0.13 0.14 0.42 2.33 0.47

I1 I2 I3 I4 I5 I6

16.9 10.7 16.8 50.4 5.1 –

14.5 7.2 20.0 44.8 5.7 7.8

13.7 9.2 17.4 36.9 5.2 17.5

11.4 9.6 17.0 32.4 4.2 25.5

v2

0.98

1.38

0.97

1.08

3.4. Magnetic measurements Hysteresis loops were measured for all samples at room temperature and the coercive fields were determined from the loops. The magnetization does not saturate up to applied fields of 10 kOe, and is dominated by domain rotation in the high-field region, where the magnetization (in emu/cm3) is determined by the law of approach to saturation [33]:

M ¼ MS ð1  A=H  B=H2 Þ þ vH

ð2Þ

Here vH is the field-induced forced magnetization term, Ms is the spontaneous saturation magnetization of the domains, A is a constant associated with the effects of inclusions and micro-stress, and B is associated with the contribution of magnetocrystalline anisotropy. For hexagonal crystals the magnetocrystalline term is given by:

B ¼ 4K 21 =15M 2S

ð3Þ

Ha ¼ 2K 1 =MS

ð4Þ

Here Ha is the anisotropy field and K1 is the first anisotropy constant. A plot of the magnetization vs. 1/H2 in the field range 8.5 kOe < H < 10 kOe for each sample gave a perfect straight line, indicating that the contributions of the inclusions/micro-stress and the forced magnetization terms are negligible. The saturation magnetization was determined from the intercept of the straight line in accordance with Eq. (2). Further, the anisotropy field and the first anisotropy constant were determined from the slope of the straight line according to Eqs. (3) and (4). Fig. 9 shows the saturation magnetizations (rs) for all samples of the hexaferrites substituted by different cations. The saturation magnetization generally decreases with increasing the concentration of the substituent, recording a maximum decrease of about 15% for the Ga substituted hexaferrites and of about 31% for Al substituted hexaferrites. Since substitution of Ga at 4f2 spin-down sites should result in a net increase in the magnetic moment per unit cell, the observed decrease in magnetization can only be explained by the effect of Ga on weakening the 4f2–12k superexchange interaction, and the consequent onset of deviation from axial collinear spindirection in the 12k sublattice [10,32]. Also, the large drop in magnetization of the Al substituted hexaferrites cannot be accounted for by Al substituting Fe at spin-up sites, which is evidence for the onset of non-collinear spin structure or weakening the directional correlation between spins in the different sublattices [10]. In addition, assuming a magnetic moment of 3 lB per Cr3+ ion, we conclude that substitution of Cr at spin-down sites should result in an increase in saturation magnetization, while substitution at spin-up sites should result in a decrease below 10%, which does

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

Intensity (%)

Intensity (%)

Intensity (%)

Intensity (%)

470

100 98 0.2 Al 96 94 92 90 100 98 0.4 Al 96 94 92 90 100 98 0.6 Al 96 94 92 90 100

100 0.1 Al

98 96 94 100 98

0.3 Al

96 94 92 100 98 96 94 92 90 100 98

0.5 Al

0.7 Al

98

0.8 Al

96 96

94

94

92

-15

-10

-5

0

5

10

-15 15

-10

Velocity (mm/s)

-5

0

5

10

15

Velocity (mm/s)

Fig. 7. Mössbauer spectra for BaFe12xAlxO19.

Table 3 The hyperfine fields (Bhf) in kOe, center shifts (CS) in mm/s, quadrupole splittings (QQ) in mm/s, and percentage relative intensities (I) of the components of the spectra for BaFe12xAlxO19 hexaferrites. Parameter

Site

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

Bhf1 Bhf2 Bhf3 Bhf4 Bhf5 Bhf6

4f2 2a 4f1 12k 2b a-Fe2O3

523 513 494 418 401 527

518 516 492 418 406 520

520 515 492 419 405 526

517 512 490 418 406 520

516 512 489 416 406 523

517 507 484 413 403 521

513 510 483 412 403 520

514 511 485 415 403 522

CS1 CS2 CS3 CS4 CS5 CS6

0.37 0.36 0.28 0.36 0.29 0.40

0.38 0.40 0.27 0.36 0.30 32

0.39 0.36 0.26 0.36 0.30 0.40

0.41 0.29 0.27 0.36 0.29 0.37

0.40 0.35 0.27 0.36 0.30 0.40

0.36 0.38 0.27 0.36 0.29 0.40

0.39 0.36 0.28 0.36 0.32 0.39

0.39 0.36 0.27 0.36 0.32 0.39

QQ(1) QQ(2) QQ(3) QQ(4) QQ(5) QQ(6)

0.21 0.01 0.21 0.43 2.29 0.16

0.21 0.17 0.23 0.42 2.20 0.17

0.25 0.02 0.23 0.41 2.19 0.20

0.15 0.03 0.24 0.42 2.24 0.26

0.28 0.03 0.22 0.43 2.20 0.20

0.23 0.05 0.27 0.43 2.20 0.17

0.22 0.08 0.24 0.43 2.22 0.24

0.28 0.01 0.21 0.44 2.17 0.15

I1 I2 I3 I4 I5 I6

8.8 13.8 20.0 47.1 3.3 7.0

15.0 12.0 19.1 46.8 4.6 2.6

6.5 15.0 21.5 46.3 5.7 5.1

18.4 5.0 20.4 46.5 4.5 5.2

7.5 12.7 21.0 47.1 5.5 6.2

5.5 15.2 19.5 47.0 4.8 8.0

10.9 9.6 21.2 48.0 5.2 5.0

7.3 10.9 21.7 48.4 4.9 6.8

v2

0.55

0.78

0.67

0.68

0.91

0.82

0.72

0.75

not account for the observed 13–16% drop. This is also evidence of the disturbance of spin-collinear structure or spin reversal of Cr3+ ions as previously reported [28]. The dependence of the first anisotropy constant on the type and concentration of the substituent for the hexaferrites under investigation is shown in Fig. 10. This figure shows a remarkable similarity of the behavior of the first anisotropy constant to that of the saturation magnetization. In order to understand this result we analyze the relation between the first anisotropy constant and the slope (S = MsB) of the Ms vs. 1/H2 straight lines (Eq. (2)). From Eq. (3) we have:

K1 ¼

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 15 j S j M S 4

ð5Þ

Accordingly, the proportionality of K1 and Ms implies that (S) should be proportional to the saturation magnetization. A plot of the (S) vs. rs for all samples is shown in Fig. 11 (Ms = qrs, where q is the density). This figure shows a linear relation between (S) and the saturation magnetization for each type of cationic substitution, which implies that the anisotropy field for each type of substitution is independent of the concentration. In a previous investigation, a small decrease in resonance frequency of the

471

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

Intensity (%)

100 98

100

0.1 Cr

0.2 Cr

98

96 94

96

92 90

94

Intensity (%)

Intensity (%)

100 98

100 0.3 Cr

0.4 Cr

98 96 96

94 92

94

100

100 98

0.5 Cr

98

0.6 Cr

96

96

94

94

92 90

92 -15

-10

-5

0

5

10

-15 15

-10

Velocity (mm/s)

-5

0

5

10

15

Velocity (mm/s)

Fig. 8. Mössbauer spectra for BaFe12xCrxO19.

BaM-Ga BaM-Al BaM-Cr

65 Table 4 The hyperfine fields (Bhf) in kOe, center shifts (CS) in mm/s, quadrupole splittings (QQ) in mm/s, and percentage relative intensities (I) of the components of the spectra for BaFe12xCrxO19 hexaferrites. 0.3

0.4

0.5

0.6

520 514 494 418 408 524

519 513 492 418 412 522

519 512 490 417 412 520

517 515 490 416 408 518

516 515 490 417 411 519

CS1 CS2 CS3 CS4 CS5 CS6

0.40 0.35 0.27 0.36 0.30 41

0.40 0.34 0.27 0.36 0.28 0.40

0.42 0.33 0.26 0.36 0.32 0.40

0.42 0.33 0.25 0.36 0.25 0.38

0.40 0.38 0.29 0.37 0.33 0.39

0.37 0.39 0.26 0.37 0.27 0.37

QQ(1) QQ(2) QQ(3) QQ(4) QQ(5) QQ(6)

0.24 0.03 0.22 0.42 2.24 0.15

0.22 0.02 0.21 0.42 2.25 0.20

0.19 0.06 0.24 0.42 2.23 0.19

0.17 0.13 0.23 0.42 2.07 0.26

0.32 0.08 0.22 0.44 2.14 0.24

0.26 0.03 0.23 0.43 2.25 0.26

I1 I2 I3 I4 I5 I6

7.0 17.2 17.2 47.2 4.3 7.0

6.7 14.1 20.4 46.2 5.0 7.6

7.5 16.7 17.6 46.2 5.3 6.7

10.4 15.3 15.3 45.5 5.9 7.7

6.3 9.8 20.3 46.1 5.6 11.9

11.3 6.8 21.6 46.0 4.8 9.3

0.73

0.63

0.78

0.57

0.97

0.92

2

v

ferromagnetic resonance peak in Cr substituted hexaferrites was reported, and associated with the decrease in the anisotropy field [6]. Such a decrease was not observed in our samples due to the observed reduction in the first anisotropy constant with increasing Cr concentration. This difference in behavior could be due to the difference in the method of preparation of the hexaferrites, resulting in different particle size and shape. The slope of each line in Fig. 11 is

σs (emu/g)

0.2

520 513 494 418 408 524

55

50

45 0.0

0.2

0.4

0.6

0.8

1.0

x Fig. 9. Saturation magnetization of Ga, Al, and Cr substituted BaM hexaferrites as a function of centration (x).

2.1

BaM-Ga BaM-Al BaM-Cr

2.0 3

0.1

4f2 2a 4f1 12k 2b a-Fe2O3

6

Site

Bhf1 Bhf2 Bhf3 Bhf4 Bhf5 Bhf6

K1 (10 erg/cm )

Parameter

60

1.9 1.8 1.7 1.6 1.5 1.4 0.0

0.2

0.4

0.6

0.8

1.0

x Fig. 10. First anisotropy constant of Ga, Al, and Cr substituted BaM hexaferrites as a function of centration (x).

472

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

BaM-Ga Linear Fit of Sheet1 B

-S (10 emu.Oe /g)

6.0

2

2

-S (10 emu.Oe /g)

6.2

5.8

8

8

5.6 5.4 5.2 5.0 56

58

60 62 σs (emu/g)

64

6.4 6.2 6.0 5.8 5.6 5.4 5.2 5.0 4.8 4.6 4.4 4.2

66

BaM-Al Linear Fit of Sheet1 B

45

50

55 60 σs (emu/g)

65

BaM-Cr Linear Fit of Sheet1 B

6.0

2

-S (10 emu.Oe /g)

6.2

5.8

8

5.6 5.4 5.2 5.0 54

56

58

60 62 σs (emu/g)

64

66

Fig. 11. A plot of the negative slope of rs vs. 1/H2 lines as a function of rs for Ga, Al, and Cr substituted BaM hexaferrites.

8

8

12

12

6

8

H c (kOe)

7 10

BaM-Ga

10 6 8

H a (kOe)

BaM-Cr

H a (kOe)

H c (kOe)

7

5

5

6

6

4

4

4 0.0

0.1

0.2

0.3

0.4

0.5

0.6

4 0.0

x

0.2

0.4

0.6

0.8

1.0

concentration (x)

8 12 BaM-Al

10 6

8

5

H a (kOe)

H c (kOe)

7

6

4

4 0.0

0.2

0.4

0.6

0.8

x Fig. 12. Coercive fields and anisotropy fields as a function of x for Ga, Al, and Cr substituted hexaferrites.

equal to B = Ha2/15. Consequently, the anisotropy fields were determined from these lines and found to be 12.1 kOe, 11.4 kOe, and 11.5 kOe for Ga, Al, and Cr substituted hexaferrites, respectively.

Fig. 12 shows the coercivity and the anisotropy field as a function of x for each type of substitution. This figure demonstrates the constancy of the anisotropy field with x, where the values range

M. Awawdeh et al. / Journal of Alloys and Compounds 585 (2014) 465–473

from 11.7–12.1 kOe, which are consistent with the values obtained from the above analysis. The coercivity for the Ga-substituted system rises by 13% per Ga ion substituting an Fe ion, while the coercivity for the Cr-substituted system rises by 42% per ion, and that for the Al-substituted system by 53% per ion, demonstrating that the Al substitution is the most effective in raising the coercivity. The coercive field for a random assembly of platelet-like particles with crystal anisotropy easy axis along the c-axis and shape anisotropy easy axis in plane is given by Stoner-Wohlfarth model:

Hc ¼ 0:48ðHa  Nd MS Þ

ð6Þ

Here Nd is the demagnetization factor which is a function of the aspect ratio of the particles, Ha is the anisotropy field, and Ms is the saturation magnetization. Accordingly, the constancy of the anisotropy field indicates that the rise in coercivity results mainly from the second term in Eq. (6), which depends on both the saturation magnetization and the particle shape. Therefore, the drop in saturation magnetization, which is the most significant for the Al substituted system, is expected to result in an increase in coercivity. Further, the drop in saturation magnetization and the increase in coercivity for the Al substituted system could be partially associated with the decrease in particle size as Al concentration increases. These systems with high coercivity could be of interest for permanent magnet production at low cost. 4. Conclusions Fine particles of high quality of BaFe12xMxO19 (M = Ga, Al, or Cr) were prepared ball milling and calcination at 1100 °C. Mössbauer spectroscopy indicated that Ga3+ ions substitute Fe3+ ions mainly at 4f2 sites, whereas Al3+ ions prefer 4f1, 2a, and 4f2 sites, and Cr3+ ions prefer 12k, 4f2 and 2a sites. The saturation magnetization was found to decrease and the coercive field to increase with increasing concentration for all types of substitutions, and Al substitution was found to have the largest influence on the magnetic properties. The first anisotropy constant was found to behave similar to the saturation magnetization with increasing x, resulting in similar anisotropy fields for all samples.

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