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Effect of Mg–Zr substitution and microwave processing on magnetic properties of barium hexaferrite
Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
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Effect of Mg–Zr substitution and microwave processing on magnetic properties of barium hexaferrite Manju Sharma n, Subhash C. Kashyap, H.C. Gupta Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India
art ic l e i nf o
Keywords: Hexaferrite Microwave processing Substitution Coercivity Microstructure
a b s t r a c t The effect of substitution of Mg–Zr for Fe in M-type barium hexaferrite (BHF) and of processing technique on the magnetic properties and microstructure has been reported in the present paper. Significant changes in magnetic properties have been observed on substituting Fe ions by Mg and Zr ions in M-type barium hexaferrite, i.e. BaFe12O19 as well as by single mode microwave processing. The single mode microwave processing of the undoped sample reduced the coercivity to nearly 25% of the value for the sintered sample along with the enhancement in magnetization, thereby making it suitable for memory devices. The improvement in magnetic properties is explained on the basis of microstructure. The addition of substituents, though assisted in the formation of single phase, it, however, degraded the magnetization besides decreasing the coercivity, possibly due to substitution at the octahedral sites. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Barium hexaferrite (BHF) is well established in the field of permanent magnets, ferrofluids, microwave devices and magnetic recording media because of its versatile properties like high anisotropy field, high coercivity, high resonance frequency, low eddy current losses, chemical stability, resistant to corrosion, etc. [1]. Barium hexaferrite (BaFe12O19), an M type ferrimagnetic material, has hexagonal symmetry with space group P63/mmc. A unit cell of BHF consists of two formula units of BaFe12O19, i.e. 64 ions. Oxygen forms the closed packed structure while Fe and Ba ions are distributed among five sublattice sites namely 2a, 12k, 4f2 (octahedral), 2b (trigonal bipyramidal), and 4f1 (tetrahedral) [2]. Cations at 2a, 2b and 12k sites (a total of 16) possess spin up while at 4f1 and 4f2 (a total of 8) possess spin down. The magnetic moment of BaFe12O19 is only because of Fe3 þ ions. At absolute zero, 16 Fe3 þ ions are with spin up (say) while 8 Fe3 þ ions have down spin. Thus the net Fe3 þ ions with spin up are 8 in number. Each of the Fe3 þ has a magnetic moment of 5 Bohr Magneton (mB). Thus overall magnetic moment of BHF unit cell is expected to be 40mB. Because of the persistent and increasing demand of perpendicular magnetic recording and microwave absorbing materials,
n
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Sharma).
attempts have been made to modify the magnetic properties of BHF by substitution of Ba or Fe ions with suitable magnetic and non-magnetic cations. Besides, the change in the properties depends on the method of preparation and processing conditions as well [3–5]. The substitution at any of the Fe3 þ ions sites will lead to change in the magnetic properties of BHF. It has been reported that doping of BHF with Zn–Zr, Co–Ti, La–Pr, Co, Ni–Zr, Co–Zr, Zn–Ti, and Mg–Ti ions leads to change their dielectric and magnetic properties [6–14]. Among the cation combinations added, some of which, occupy some preferred sublattice sites (e.g. Zn2 þ prefers to substitute iron ions at tetrahedral site) and lead to the change in magnetic properties including magnetization, coercivity and anisotropy. The magnetic properties can be tailored by modifying microstructure as well. Microwave (MW) processing technique has been effectively employed for modifying the microstructure of materials and hence improving their magnetic and mechanical properties. Microwave sintering has led to the decrystallization of hexaferrites and modified their magnetic properties. Polycrystalline ferromagnetic bulk Zn1 xCoxOy samples have also been successfully synthesized by employing MW processing in a single mode cylindrical MW resonant cavity (operating at 2.45 GHz) [15–18]. It is expected that the Mg–Zr being nonmagnetic cations will affect the magnetic properties of unit cell of BHF, by creating imbalance in the spins up and down. It is anticipated that the modified microstructure of the substituted samples obtained from H-field processing will render improvement in magnetic
http://dx.doi.org/10.1016/j.physb.2014.04.035 0921-4526/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: M. Sharma, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.035i
M. Sharma et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
properties. In the present paper, preliminary results of our successful effort to change the magnetic properties of BHF by substitution as well as by change in microstructure via microwave processing are reported. An attempt has been made to understand these magnetic properties.
2. Experimental A series of Mg–Zr substituted Ba(MgZr)xFe12 2xO19 (with x¼ 0.0, 0.3, 0.6 and 1.0) precursor pellets has been prepared by a conventional solid state reaction method. High purity BaCO3, Fe2O3, MgO, and ZrO2 chemicals were mixed in desired stoichiometric amount and ball milled for 24 h. The ball milled powder was then calcined at 1000 1C for 10 h. After adding 2 wt% aqueous solution of polyvinyl alcohol to the calcined powder, it was subjected to hand grinding for 1 h and then pelletized. The pellets were subjected to two different processing techniques: (1) conventional sintering in furnace at 1200 1C for 5 h, and (2) microwave processing in H-field in a single mode cylindrical cavity (in TE011 mode) either at 1100 1C for 10 min, in each case, or at 1200 1C for 5 min at microwave frequency of 2.45 GHz. The sample temperature was measured by an optical pyrometer (Raytek, RAYMA2SCSF). The structural characterization of the samples was carried out by employing X-ray diffraction (X-Pert PRO Philips) in θ–2θ geometry using Cu Kα radiation of wavelength 1.54060 Å with step size Δ(2θ) of 0.051. The micro-structural studies were carried out by employing a scanning electron microscope (Zeiss EVO 50). The magnetic characterization has been carried out at room temperature by using a vibrating sample magnetometer (EG&G 155 PAR) with sensitivity of 10 6 emu/g.
3. Results and discussion 3.1. Structural and microstructural analyses 3.1.1. Conventionally sintered Ba(MgZr)xFe12 2xO19 samples The X-ray diffractograms of the conventionally sintered samples of Ba(MgZr)xFe12 2xO19 are presented in Fig. 1. It is noted that the peak positions in all the diffractograms match well with the standard JCPDS#78-0133, thereby confirming the formation of hexagonal phase for M type BHF for all the compositions when
Fig. 1. X-ray diffractograms of conventionally sintered (at 1200 1C for 5 h) Ba (MgZr)xFe12 2xO19 samples.
conventionally sintered at 1200 1C for 5 h. All the peaks have been shifted to lower angles understandably because the substituents (Mg, and Zr) are of larger radii (0.71 Å and 0.72 Å respectively, in tetrahedral coordination) as compared to that of Fe3 þ ions (0.64 Å in tetrahedral coordination) and reflect the increase in the dimensions of the unit cell. The ‘a’ and ‘c’ for pure BHF have been obtained as 5.88 and 23.15 Å, respectively. The corresponding values of lattice parameters for the substituted samples, which show a consistent increase with the increase in cation (Mg, Zr) concentration are shown in Table 1. The crystallite size in these conventionally sintered samples has been calculated by using the Scherrer equation t¼
0:89λ βcosθB
where t is crystallite size, β is FWHM, and θ is diffraction angle. The values of crystallite size, listed in Table 1, show an increase from 26 to 32 nm with increase in the concentration of substituents. It appears that the substituents are grain growth promoters. The scanning electron micrographs of the conventionally sintered polycrystalline Ba(MgZr)xFe12 2xO19 samples (where x¼0.0, 0.3, 0.6 and 1.0) are shown in Fig. 2. It may be noted from the micrographs that there are numerous pores at the intergrain boundaries of the particles, and the sample is not adequately dense, and possibly because the time and temperature for sintering are not appropriate to yield a dense sample. Particle size as calculated from software “image J” varies from 282 nm (x ¼0.0) to 206 nm (x¼ 1.0). Thus particle size is decreasing with increase in the concentration of Mg and Zr. 3.1.2. Microwave sintered Ba(MgZr)xFe12 2xO19 samples Microwave (MW) processing can, in general, be employed in a mixed/multi-mode, in combination with other processing techniques (hybrid) or specifically in separated E- or H-fields of em radiation and is termed as a single mode processing. In multimode processing, energy is distributed over a number of modes covering almost entire volume, while in a single mode cavity the energy is by and large confined to E/H mode and in a smaller volume. This in turn leads to more interaction with certain materials. It is known that all materials are neither good absorber of microwaves nor interact equally with Eand H-fields of microwaves [19–20], and respond differently to these separated fields. For example E-field processing of polycrystalline SiGe alloy transformed it into an amorphous phase; and processing of the later in a single mode (H-field) yielded the crystalline phase again [21]. In the present case, the application of single mode (H-field) cavity is employed to have better MW-magnetic material interaction even in a shorter duration. The interaction, though not well understood so far, can be explained on the bases of dielectric and magnetic losses in general, possibly the later being dominant in the present case. Another possibility is of anisothermal heating of the calcined pellet [22]. Due to larger difference in absorptivity of the constituents (BaFe12O19 along with BaCO3, BaFe2O4, and Fe2O3) for the microwave field huge differences in temperatures are created, which accelerate the reaction and lead to the formation of doped single phase material. X-ray diffractograms of the microwave sintered (1100 1C for 10 min) samples of Ba(MgZr)xFe12 2xO19 are presented in Fig. 3(a). Three prominent peaks (21.61, 281 and 331), corresponding to the impurities (BaCO3, BaFe2O4, and Fe2O3, respectively), are appearing in all the MW processed (1100 1C for 10 min) compositions except for x ¼1.0. The presence of some undesirable phases in the processed pellet with xo1.0 suggests that the temperature is insufficient for the formation of single phase. However, with an increased concentration of Mg–Zr ions, i.e. for x¼1.0 single hexagonal phase appears to form at the same temperature. The XRD patterns of both pure BHF sintered at 1200 1C for 5 min in single mode cavity and
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Table 1 Lattice parameters (a, c), crystallite size, coercivity (Hc), saturation magnetization (Ms), retentivity (Mr), squareness ratio (Mr/Ms), and anisotropy field (Ha) of conventionally (1200 1C for 5 h) and microwave sintered (for x¼ 0.0 at 1200 1C, 5 min denoted as MP125) samples of Ba(MgZr)xFe(12 2x)O19, (x¼ 0.0– 1.0). x
a (Å)
c (Å)
Cell volume (Å3)
Crystallite size (nm)
Hc (Oe)
Ms (emu/g)
Mr (emu/g)
Mr/Ms
Ha (Oe)
0.0 0.3 0.6 1.0 0.0 (MP125)
5.882 5.893 5.898 5.902 5.885
23.157 23.209 23.221 23.225 23.261
693.82 697.99 699.53 700.60 697.65
26 31 28 32 46
3786 2853 2133 1333 956
47 45 46 38 59
21 20 22 14 16
0.45 0.44 0.48 0.37 0.27
22361 19258 17818 17366 10388
Fig. 2. Scanning electron micrographs of conventionally sintered (at 1200 1C for 5 h) samples of Ba(MgZr)xFe12 2xO19 (a) x ¼0.0 (b) x ¼ 0.3 (c) x ¼0.6 (d) x¼ 1.0 and microwave sintered samples (e) BaFe12O19 at 1200 1C for 5 min, and (f) Ba(MgZrFe10)O19 at 1100 1C for 10 min.
Fig. 3. X-ray diffractograms of (a) Ba(MgZr)xFe12 2xO19 samples microwave sintered at 1100 1C for 10 min, and of (b) undoped BHF samples microwave sintered at 1200 1C for 5 min and conventionally sintered at 1200 1C for 5 h.
conventionally sintered (1200 1C for 5 hr) samples are shown in Fig. 3(b). Formation of pure phase has been established in both the samples. The comparable values of lattice parameters, a (¼ 5.885 Å)
and c (¼23.261 Å) of the MW sintered sample establish the utility of MW processing for preparing the desired samples in a short time.
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The scanning electron micrographs of MW sintered samples Ba (MgZr)xFe12 2xO19with x ¼0.0 (at 1200 1C for 5 min) and x¼1.0 (for 1100 1C for 10 min) are presented in Fig. 2(e) and (f). These micrographs are seen to be quite different from the conventionally sintered samples. The MW processed samples apparently have smaller number of pores, thereby resulting in denser samples than obtained by the conventional solid state sintering. Besides the increase in particle size (0.650 mm for MW at 1200 1C for 5 min and 1.0 mm for MW at 1100 1C for 10 min samples), well faceted hexagonal crystallites are visible in the micrograph of substituted sample with x ¼1.0 and sintered at lower temperature of 1100 1C. Thus Mg–Zr ions are growth promoters, is indeed confirmed. Microwave processed more compact samples will understandably result in higher magnetization and lower coercivity. 3.2. Magnetic measurements It is known that the variation of magnetization (M) in high applied field (H) follows the law of approach to saturation [6,23]: the zero field saturation magnetization (Ms) and anisotropy field (Ha) are related to magnetization, M(H), as A B C MðHÞ ¼ M s 1 þ þ 2 þ 3 þ X d H H H H
Fig. 4. M–H loops of conventionally sintered (at 1200 1C for 5 h) Ba (MgZr)xFe12 2xO19 samples, with x ¼ (a) 0.0 (b) 0.3 (c) 0.6 (d) 1.0 (e) x ¼0.0 microwave sintered at 1200 1C for 5 min and (f) x ¼1.0 microwave sintered at 1100 1C for 10 min.
where Xd is high field differential susceptibility, A/H pertains to the existence of inhomogeneities (vanishes at high fields), and B/H2 and C/H3 refer to the magnetocrystalline anisotropy. For a uniaxial hexagonal compound, with anisotropy constants K 2 5 K 1 , the constants B, C and Ha can be written as B¼
1 2 H 15 a
C¼
2 3 H 105 a
Ha ¼
2K 1 Ms
The M–H loops of conventionally sintered Ba(MgZr)xFe12 2xO19 (with x ¼0.0, 0.3, 0.6 and 1.0) and of MW sintered pure and substituted samples (with x ¼1) are shown in Fig. 4. A typical plot of the observed data along with its linear fit is shown in Fig. 5. The Hc,, Ms, and Ha values obtained from the loops, using the above equations and Fig. 5 are listed in Table 1. It is noted that Ha decreases with increase in the concentration of Mg–Zr in the samples. A monotonic decrease in coercivity with an increase in the concentration of Mg–Zr ions has been observed to range from 3786 Oe to 1333 Oe. The maximum coercivity of 3786 Oe of pure M type BHF is due to the strong uniaxial anisotropy. The observed decrease in Hc values with an increase in the concentration of Mg–Zr ions may be explained on the basis of decrease in magnetocrystalline anisotropy with increase in Mg–Zr concentration. As evident from the Table 2, the largest positive contribution to anisotropy constant comes from 2b and 4f2 sites. That means the Mg and Zr ions may be substituting iron ions at these sites thus decreasing the coercivity. The maximum value of Ms for pure BHF is reported as 72 emu/g [24]. The Ms for pure conventionally sintered BHF is obtained as 47 emu/g. The value is quite close to that (44.9 emu/g) obtained by using a coprecipitation method with Fe3 þ /Ba2 þ ratio 10.9 and calcined at 1200 1C [25]. The lower values obtained in both the above cases could be due to the spin canting, sample inhomogeneity or superparamagnetic fractions present in the samples. Pure BHF does not seem to saturate at the applied field. It is because of the high value of anisotropy field (Table 1). The Ms value is also observed to decrease from 47 emu/g for x ¼0.0 to 38 emu/g for x¼ 1.0. This decrease in Ms can be explained
Fig. 5. A 1/H2 vs. magnetization plot of the observed data of conventionally sintered undoped BHF sample depicting its linear fitting.
Table 2 Anisotropy constant (K1) for Fe3 þ ion at each site in barium hexaferrite [8]. Site K1(cm
1
/ion)
2a
2b
12k
4f1
4f2
0.23
1.40
0.18
0.18
0.51
on the basis of Fe3 þ ions occupying different sublattice positions (2a, 2b, 12k, 4f1, 4f2). Fe3 þ ions on 2a, 12k and 2b sites are with up spin and at 4f1 and 4f2 sites with down spin. The outer electronic configurations of Mg–Zr are d0. Thus according to ligand field theory, there are no preferred sites for these cations. However, electronegativity could determine the preferential sites. Electronegativity values for Mg and Zr are 1.31 and 1.33, respectively. Elements with higher electronegativity are known to substitute octahedral site preferentially over the elements with lower electronegativity [10]. It is, however, difficult to specify the ion (Mg2 þ and Zr4 þ ), which will occupy the octahedral site (because of small difference in their electronegativity values). Mg2 þ and Zr4 þ , the nonmagnetic ions, if occupying a site with up spin, the net magnetization should decrease because the total number of ions with up spin decreases. Similar type of variation in Ms value of Mg–Ti substituted BHF has been reported [11,14]. Also the presence of large amount of nonmagnetic cations results in the decrease in the superexchange interactions between 4f1–12k sites
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and between 4f1–2a sites, thereby resulting the decrease in Ms. Also the abrupt fall in magnetic properties may be attributed to the loss in magnetic collinearity, i.e. canting of spins [26]. It is clearly evident from the loops in Fig. 4 that Hc is drastically reduced for the MW sintered BaFe12O19 sample (956 Oe) as compared to that obtained for the conventionally sintered sample (3786 Oe). This difference is due to the drastic change in the particle size of the two samples, as the coercivity is known to vary almost inversely with the grain size. On the other hand the Ms has increased (59 emu/g) for the MW processed sample, thus bringing this value more close to the maximum reported value. It is noted from the hysteresis loops of MW and conventionally sintered BaMgZrFe10O19 samples that the magnetization values are nearly the same (38 emu/gm) for both the samples, and lower than the values for pure samples. The coercivity has dropped to almost negligible value in case of MW sintered sample, possibly due to further increase in particle size in the presence of Mg–Zr.
4. Conclusions Single phase Mg–Zr substituted M-type barium hexaferrite polycrystalline samples have been synthesized by conventional as well as microwave sintering of the precursor powder formed by solid state sintering. While the conventional sintering requires long time (5 h) and high temperature of 1200 1C, the MW processing takes only 5 min to yield the single phase. The conventionally sintered undoped samples have high coercivity values nearly 4000 Oe, and the MW sintered ones have drastically lower values o1000 Oe. The Ms value of the corresponding samples is recorded to be 47 and 59 emu/g. The improved magnetic properties can be useful for magnetic recording application. The Mg–Zr substitution though induces single phase formation at lower temperature; it however results in a decrease in both, Ms as well as coercivity. The observed deterioration in magnetic properties has been attributed to the changes in microstructure and substitution of Mg–Zr possibly at octahedral sites.
5
References [1] Umit Ozgur, Yahya Alinov, Hadis Morkoc, J. Mater. Sci.: Mater. Electron. 20 (2009) 789. [2] W.D. Townes, J.H. Fang, A.J. Perrotta, Zeitschrift fur Kristallographie, Bd 125 (1967) 437. [3] M.M. Rashad, I.A. Ibrahim, J. Magn. Magn. Mater. 323 (2011) 2158. [4] Geok Bee Teh, Yat Choy Wong, Richard D. Tilley, J. Magn. Magn. Mater. 323 (2011) 2318. [5] M. Javed Iqbal, M. Naeem Ashiq, Pablo Hernandez Gomez, J. Alloys Compd. 478 (2009) 736. [6] Z.W. Li, C.K. Ong, Z. Yang, F.L. Wei, X.Z. Zhou, J.H. Zhao, A.H. Morrish, Phy. Rev. B 62 (2000) 6530. [7] J. Ibrahim Bsoul, J. Phys. 2 (2009) 95. [8] Ounnunkad Suriya, Solid State Commun. 138 (2006) 472. [9] Kajal K. Mallick, Philip Shepherd, Roger J. Green, J. Magn. Magn. Mater. 312 (2007) 418. [10] Manisha V. Rane, D. Bahadur, S.D. Kulkarni, S.K. Date, J. Magn. Magn. Mater. 195 (1999) L256. [11] Mohammad Hossein Shams, Seyed Mohammad Ali Salehi, Ali Ghasemi, Mater. Lett. 62 (2008) 1731. [12] Mukesh C. Dimri, R. Stern, Subhash C. Kashyap, K.P. Bhatti, D.C. Dube, Phys. Status Solidi A 206 (2009) 270. [13] Charu Lata Dube, Subhash C. Kashyap, D.K. Pandya, D.C. Dube, Phys. Status Solidi A 206 (2009) 2627. [14] U. Meisen, A. Eiling, IEEE Trans. Magn. 26 (1990) 21. [15] Subhash C. Kashyap, Microwave processing – a new dimension in synthesis of materials, in: Proceedings of the Applied Electromagnetic Conference (AEMC), Kolkata IEEE Xplore , 14–16 December 2009 http://dx.doi.org/10.1109/AEMC. 2009.5430602. [16] Geetanjali, Charu Lata Dube, Subhash C. Kashyap, J. Supercond. Novel Magn. 24 (2011) 567. [17] Purushotam Yodili, Ramesh Peelamedu, Dinesh Agrawal, Rustom Roy, Mater. Sci. Eng. B 98 (2003) 269. [18] Charu Lata Dube, Subhash C. Kashyap, D.C. Dube, and D.K. Agarwal, Microwave processing of ZnO based dilute magnetic semiconductors, in: Proceedings of the IEEE Conference on Microwaves-08, December 2008 126. [19] K.J. Rao, P.D. Ramesh, Bull. Mater. Sci. 18 (4) (1995) 447. [20] J. Cheng, R. Roy, D. Agrawal, Mater. Res. Innov. 5 (2002) 170. [21] J. Cheng, D. Agrawal, Y. Zhang, R. Roy, A.K. Santra, J. Alloys Compd. 491 (2010) 517. [22] R.D. Peelamedu, R. Roy, D. Agrawal, Mater. Res. Bull. 36 (2001) 2723. [23] H.C. Fang, C.K. Ong, X.Y. Zhang, Y. Li, X.Z. Wang, Z. Yang, J. Magn. Magn. Mater. 191 (1999) 277. [24] B.D. Cullity, C.D. Graham, Introduction to Magnetic Materials, Wiley, United States of America, 2009. [25] M.M. Rashad, M. Radwan, M.M. Hessien, J. Alloys Compd. 453 (2008) 304. [26] Muhammad Javed Iqbal, M. Naeem Ashiq, Pablo Hernandez Gomez, J. Alloys Compd. 478 (2009) 736.
Please cite this article as: M. Sharma, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.035i
Contents lists available at ScienceDirect
Physica B journal homepage: www.elsevier.com/locate/physb
Effect of Mg–Zr substitution and microwave processing on magnetic properties of barium hexaferrite Manju Sharma n, Subhash C. Kashyap, H.C. Gupta Department of Physics, Indian Institute of Technology Delhi, New Delhi 110016, India
art ic l e i nf o
Keywords: Hexaferrite Microwave processing Substitution Coercivity Microstructure
a b s t r a c t The effect of substitution of Mg–Zr for Fe in M-type barium hexaferrite (BHF) and of processing technique on the magnetic properties and microstructure has been reported in the present paper. Significant changes in magnetic properties have been observed on substituting Fe ions by Mg and Zr ions in M-type barium hexaferrite, i.e. BaFe12O19 as well as by single mode microwave processing. The single mode microwave processing of the undoped sample reduced the coercivity to nearly 25% of the value for the sintered sample along with the enhancement in magnetization, thereby making it suitable for memory devices. The improvement in magnetic properties is explained on the basis of microstructure. The addition of substituents, though assisted in the formation of single phase, it, however, degraded the magnetization besides decreasing the coercivity, possibly due to substitution at the octahedral sites. & 2014 Elsevier B.V. All rights reserved.
1. Introduction Barium hexaferrite (BHF) is well established in the field of permanent magnets, ferrofluids, microwave devices and magnetic recording media because of its versatile properties like high anisotropy field, high coercivity, high resonance frequency, low eddy current losses, chemical stability, resistant to corrosion, etc. [1]. Barium hexaferrite (BaFe12O19), an M type ferrimagnetic material, has hexagonal symmetry with space group P63/mmc. A unit cell of BHF consists of two formula units of BaFe12O19, i.e. 64 ions. Oxygen forms the closed packed structure while Fe and Ba ions are distributed among five sublattice sites namely 2a, 12k, 4f2 (octahedral), 2b (trigonal bipyramidal), and 4f1 (tetrahedral) [2]. Cations at 2a, 2b and 12k sites (a total of 16) possess spin up while at 4f1 and 4f2 (a total of 8) possess spin down. The magnetic moment of BaFe12O19 is only because of Fe3 þ ions. At absolute zero, 16 Fe3 þ ions are with spin up (say) while 8 Fe3 þ ions have down spin. Thus the net Fe3 þ ions with spin up are 8 in number. Each of the Fe3 þ has a magnetic moment of 5 Bohr Magneton (mB). Thus overall magnetic moment of BHF unit cell is expected to be 40mB. Because of the persistent and increasing demand of perpendicular magnetic recording and microwave absorbing materials,
n
Corresponding author. E-mail addresses: [email protected], [email protected] (M. Sharma).
attempts have been made to modify the magnetic properties of BHF by substitution of Ba or Fe ions with suitable magnetic and non-magnetic cations. Besides, the change in the properties depends on the method of preparation and processing conditions as well [3–5]. The substitution at any of the Fe3 þ ions sites will lead to change in the magnetic properties of BHF. It has been reported that doping of BHF with Zn–Zr, Co–Ti, La–Pr, Co, Ni–Zr, Co–Zr, Zn–Ti, and Mg–Ti ions leads to change their dielectric and magnetic properties [6–14]. Among the cation combinations added, some of which, occupy some preferred sublattice sites (e.g. Zn2 þ prefers to substitute iron ions at tetrahedral site) and lead to the change in magnetic properties including magnetization, coercivity and anisotropy. The magnetic properties can be tailored by modifying microstructure as well. Microwave (MW) processing technique has been effectively employed for modifying the microstructure of materials and hence improving their magnetic and mechanical properties. Microwave sintering has led to the decrystallization of hexaferrites and modified their magnetic properties. Polycrystalline ferromagnetic bulk Zn1 xCoxOy samples have also been successfully synthesized by employing MW processing in a single mode cylindrical MW resonant cavity (operating at 2.45 GHz) [15–18]. It is expected that the Mg–Zr being nonmagnetic cations will affect the magnetic properties of unit cell of BHF, by creating imbalance in the spins up and down. It is anticipated that the modified microstructure of the substituted samples obtained from H-field processing will render improvement in magnetic
http://dx.doi.org/10.1016/j.physb.2014.04.035 0921-4526/& 2014 Elsevier B.V. All rights reserved.
Please cite this article as: M. Sharma, et al., Physica B (2014), http://dx.doi.org/10.1016/j.physb.2014.04.035i
M. Sharma et al. / Physica B ∎ (∎∎∎∎) ∎∎∎–∎∎∎
2
properties. In the present paper, preliminary results of our successful effort to change the magnetic properties of BHF by substitution as well as by change in microstructure via microwave processing are reported. An attempt has been made to understand these magnetic properties.
2. Experimental A series of Mg–Zr substituted Ba(MgZr)xFe12 2xO19 (with x¼ 0.0, 0.3, 0.6 and 1.0) precursor pellets has been prepared by a conventional solid state reaction method. High purity BaCO3, Fe2O3, MgO, and ZrO2 chemicals were mixed in desired stoichiometric amount and ball milled for 24 h. The ball milled powder was then calcined at 1000 1C for 10 h. After adding 2 wt% aqueous solution of polyvinyl alcohol to the calcined powder, it was subjected to hand grinding for 1 h and then pelletized. The pellets were subjected to two different processing techniques: (1) conventional sintering in furnace at 1200 1C for 5 h, and (2) microwave processing in H-field in a single mode cylindrical cavity (in TE011 mode) either at 1100 1C for 10 min, in each case, or at 1200 1C for 5 min at microwave frequency of 2.45 GHz. The sample temperature was measured by an optical pyrometer (Raytek, RAYMA2SCSF). The structural characterization of the samples was carried out by employing X-ray diffraction (X-Pert PRO Philips) in θ–2θ geometry using Cu Kα radiation of wavelength 1.54060 Å with step size Δ(2θ) of 0.051. The micro-structural studies were carried out by employing a scanning electron microscope (Zeiss EVO 50). The magnetic characterization has been carried out at room temperature by using a vibrating sample magnetometer (EG&G 155 PAR) with sensitivity of 10 6 emu/g.
3. Results and discussion 3.1. Structural and microstructural analyses 3.1.1. Conventionally sintered Ba(MgZr)xFe12 2xO19 samples The X-ray diffractograms of the conventionally sintered samples of Ba(MgZr)xFe12 2xO19 are presented in Fig. 1. It is noted that the peak positions in all the diffractograms match well with the standard JCPDS#78-0133, thereby confirming the formation of hexagonal phase for M type BHF for all the compositions when
Fig. 1. X-ray diffractograms of conventionally sintered (at 1200 1C for 5 h) Ba (MgZr)xFe12 2xO19 samples.
conventionally sintered at 1200 1C for 5 h. All the peaks have been shifted to lower angles understandably because the substituents (Mg, and Zr) are of larger radii (0.71 Å and 0.72 Å respectively, in tetrahedral coordination) as compared to that of Fe3 þ ions (0.64 Å in tetrahedral coordination) and reflect the increase in the dimensions of the unit cell. The ‘a’ and ‘c’ for pure BHF have been obtained as 5.88 and 23.15 Å, respectively. The corresponding values of lattice parameters for the substituted samples, which show a consistent increase with the increase in cation (Mg, Zr) concentration are shown in Table 1. The crystallite size in these conventionally sintered samples has been calculated by using the Scherrer equation t¼
0:89λ βcosθB
where t is crystallite size, β is FWHM, and θ is diffraction angle. The values of crystallite size, listed in Table 1, show an increase from 26 to 32 nm with increase in the concentration of substituents. It appears that the substituents are grain growth promoters. The scanning electron micrographs of the conventionally sintered polycrystalline Ba(MgZr)xFe12 2xO19 samples (where x¼0.0, 0.3, 0.6 and 1.0) are shown in Fig. 2. It may be noted from the micrographs that there are numerous pores at the intergrain boundaries of the particles, and the sample is not adequately dense, and possibly because the time and temperature for sintering are not appropriate to yield a dense sample. Particle size as calculated from software “image J” varies from 282 nm (x ¼0.0) to 206 nm (x¼ 1.0). Thus particle size is decreasing with increase in the concentration of Mg and Zr. 3.1.2. Microwave sintered Ba(MgZr)xFe12 2xO19 samples Microwave (MW) processing can, in general, be employed in a mixed/multi-mode, in combination with other processing techniques (hybrid) or specifically in separated E- or H-fields of em radiation and is termed as a single mode processing. In multimode processing, energy is distributed over a number of modes covering almost entire volume, while in a single mode cavity the energy is by and large confined to E/H mode and in a smaller volume. This in turn leads to more interaction with certain materials. It is known that all materials are neither good absorber of microwaves nor interact equally with Eand H-fields of microwaves [19–20], and respond differently to these separated fields. For example E-field processing of polycrystalline SiGe alloy transformed it into an amorphous phase; and processing of the later in a single mode (H-field) yielded the crystalline phase again [21]. In the present case, the application of single mode (H-field) cavity is employed to have better MW-magnetic material interaction even in a shorter duration. The interaction, though not well understood so far, can be explained on the bases of dielectric and magnetic losses in general, possibly the later being dominant in the present case. Another possibility is of anisothermal heating of the calcined pellet [22]. Due to larger difference in absorptivity of the constituents (BaFe12O19 along with BaCO3, BaFe2O4, and Fe2O3) for the microwave field huge differences in temperatures are created, which accelerate the reaction and lead to the formation of doped single phase material. X-ray diffractograms of the microwave sintered (1100 1C for 10 min) samples of Ba(MgZr)xFe12 2xO19 are presented in Fig. 3(a). Three prominent peaks (21.61, 281 and 331), corresponding to the impurities (BaCO3, BaFe2O4, and Fe2O3, respectively), are appearing in all the MW processed (1100 1C for 10 min) compositions except for x ¼1.0. The presence of some undesirable phases in the processed pellet with xo1.0 suggests that the temperature is insufficient for the formation of single phase. However, with an increased concentration of Mg–Zr ions, i.e. for x¼1.0 single hexagonal phase appears to form at the same temperature. The XRD patterns of both pure BHF sintered at 1200 1C for 5 min in single mode cavity and
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Table 1 Lattice parameters (a, c), crystallite size, coercivity (Hc), saturation magnetization (Ms), retentivity (Mr), squareness ratio (Mr/Ms), and anisotropy field (Ha) of conventionally (1200 1C for 5 h) and microwave sintered (for x¼ 0.0 at 1200 1C, 5 min denoted as MP125) samples of Ba(MgZr)xFe(12 2x)O19, (x¼ 0.0– 1.0). x
a (Å)
c (Å)
Cell volume (Å3)
Crystallite size (nm)
Hc (Oe)
Ms (emu/g)
Mr (emu/g)
Mr/Ms
Ha (Oe)
0.0 0.3 0.6 1.0 0.0 (MP125)
5.882 5.893 5.898 5.902 5.885
23.157 23.209 23.221 23.225 23.261
693.82 697.99 699.53 700.60 697.65
26 31 28 32 46
3786 2853 2133 1333 956
47 45 46 38 59
21 20 22 14 16
0.45 0.44 0.48 0.37 0.27
22361 19258 17818 17366 10388
Fig. 2. Scanning electron micrographs of conventionally sintered (at 1200 1C for 5 h) samples of Ba(MgZr)xFe12 2xO19 (a) x ¼0.0 (b) x ¼ 0.3 (c) x ¼0.6 (d) x¼ 1.0 and microwave sintered samples (e) BaFe12O19 at 1200 1C for 5 min, and (f) Ba(MgZrFe10)O19 at 1100 1C for 10 min.
Fig. 3. X-ray diffractograms of (a) Ba(MgZr)xFe12 2xO19 samples microwave sintered at 1100 1C for 10 min, and of (b) undoped BHF samples microwave sintered at 1200 1C for 5 min and conventionally sintered at 1200 1C for 5 h.
conventionally sintered (1200 1C for 5 hr) samples are shown in Fig. 3(b). Formation of pure phase has been established in both the samples. The comparable values of lattice parameters, a (¼ 5.885 Å)
and c (¼23.261 Å) of the MW sintered sample establish the utility of MW processing for preparing the desired samples in a short time.
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The scanning electron micrographs of MW sintered samples Ba (MgZr)xFe12 2xO19with x ¼0.0 (at 1200 1C for 5 min) and x¼1.0 (for 1100 1C for 10 min) are presented in Fig. 2(e) and (f). These micrographs are seen to be quite different from the conventionally sintered samples. The MW processed samples apparently have smaller number of pores, thereby resulting in denser samples than obtained by the conventional solid state sintering. Besides the increase in particle size (0.650 mm for MW at 1200 1C for 5 min and 1.0 mm for MW at 1100 1C for 10 min samples), well faceted hexagonal crystallites are visible in the micrograph of substituted sample with x ¼1.0 and sintered at lower temperature of 1100 1C. Thus Mg–Zr ions are growth promoters, is indeed confirmed. Microwave processed more compact samples will understandably result in higher magnetization and lower coercivity. 3.2. Magnetic measurements It is known that the variation of magnetization (M) in high applied field (H) follows the law of approach to saturation [6,23]: the zero field saturation magnetization (Ms) and anisotropy field (Ha) are related to magnetization, M(H), as A B C MðHÞ ¼ M s 1 þ þ 2 þ 3 þ X d H H H H
Fig. 4. M–H loops of conventionally sintered (at 1200 1C for 5 h) Ba (MgZr)xFe12 2xO19 samples, with x ¼ (a) 0.0 (b) 0.3 (c) 0.6 (d) 1.0 (e) x ¼0.0 microwave sintered at 1200 1C for 5 min and (f) x ¼1.0 microwave sintered at 1100 1C for 10 min.
where Xd is high field differential susceptibility, A/H pertains to the existence of inhomogeneities (vanishes at high fields), and B/H2 and C/H3 refer to the magnetocrystalline anisotropy. For a uniaxial hexagonal compound, with anisotropy constants K 2 5 K 1 , the constants B, C and Ha can be written as B¼
1 2 H 15 a
C¼
2 3 H 105 a
Ha ¼
2K 1 Ms
The M–H loops of conventionally sintered Ba(MgZr)xFe12 2xO19 (with x ¼0.0, 0.3, 0.6 and 1.0) and of MW sintered pure and substituted samples (with x ¼1) are shown in Fig. 4. A typical plot of the observed data along with its linear fit is shown in Fig. 5. The Hc,, Ms, and Ha values obtained from the loops, using the above equations and Fig. 5 are listed in Table 1. It is noted that Ha decreases with increase in the concentration of Mg–Zr in the samples. A monotonic decrease in coercivity with an increase in the concentration of Mg–Zr ions has been observed to range from 3786 Oe to 1333 Oe. The maximum coercivity of 3786 Oe of pure M type BHF is due to the strong uniaxial anisotropy. The observed decrease in Hc values with an increase in the concentration of Mg–Zr ions may be explained on the basis of decrease in magnetocrystalline anisotropy with increase in Mg–Zr concentration. As evident from the Table 2, the largest positive contribution to anisotropy constant comes from 2b and 4f2 sites. That means the Mg and Zr ions may be substituting iron ions at these sites thus decreasing the coercivity. The maximum value of Ms for pure BHF is reported as 72 emu/g [24]. The Ms for pure conventionally sintered BHF is obtained as 47 emu/g. The value is quite close to that (44.9 emu/g) obtained by using a coprecipitation method with Fe3 þ /Ba2 þ ratio 10.9 and calcined at 1200 1C [25]. The lower values obtained in both the above cases could be due to the spin canting, sample inhomogeneity or superparamagnetic fractions present in the samples. Pure BHF does not seem to saturate at the applied field. It is because of the high value of anisotropy field (Table 1). The Ms value is also observed to decrease from 47 emu/g for x ¼0.0 to 38 emu/g for x¼ 1.0. This decrease in Ms can be explained
Fig. 5. A 1/H2 vs. magnetization plot of the observed data of conventionally sintered undoped BHF sample depicting its linear fitting.
Table 2 Anisotropy constant (K1) for Fe3 þ ion at each site in barium hexaferrite [8]. Site K1(cm
1
/ion)
2a
2b
12k
4f1
4f2
0.23
1.40
0.18
0.18
0.51
on the basis of Fe3 þ ions occupying different sublattice positions (2a, 2b, 12k, 4f1, 4f2). Fe3 þ ions on 2a, 12k and 2b sites are with up spin and at 4f1 and 4f2 sites with down spin. The outer electronic configurations of Mg–Zr are d0. Thus according to ligand field theory, there are no preferred sites for these cations. However, electronegativity could determine the preferential sites. Electronegativity values for Mg and Zr are 1.31 and 1.33, respectively. Elements with higher electronegativity are known to substitute octahedral site preferentially over the elements with lower electronegativity [10]. It is, however, difficult to specify the ion (Mg2 þ and Zr4 þ ), which will occupy the octahedral site (because of small difference in their electronegativity values). Mg2 þ and Zr4 þ , the nonmagnetic ions, if occupying a site with up spin, the net magnetization should decrease because the total number of ions with up spin decreases. Similar type of variation in Ms value of Mg–Ti substituted BHF has been reported [11,14]. Also the presence of large amount of nonmagnetic cations results in the decrease in the superexchange interactions between 4f1–12k sites
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and between 4f1–2a sites, thereby resulting the decrease in Ms. Also the abrupt fall in magnetic properties may be attributed to the loss in magnetic collinearity, i.e. canting of spins [26]. It is clearly evident from the loops in Fig. 4 that Hc is drastically reduced for the MW sintered BaFe12O19 sample (956 Oe) as compared to that obtained for the conventionally sintered sample (3786 Oe). This difference is due to the drastic change in the particle size of the two samples, as the coercivity is known to vary almost inversely with the grain size. On the other hand the Ms has increased (59 emu/g) for the MW processed sample, thus bringing this value more close to the maximum reported value. It is noted from the hysteresis loops of MW and conventionally sintered BaMgZrFe10O19 samples that the magnetization values are nearly the same (38 emu/gm) for both the samples, and lower than the values for pure samples. The coercivity has dropped to almost negligible value in case of MW sintered sample, possibly due to further increase in particle size in the presence of Mg–Zr.
4. Conclusions Single phase Mg–Zr substituted M-type barium hexaferrite polycrystalline samples have been synthesized by conventional as well as microwave sintering of the precursor powder formed by solid state sintering. While the conventional sintering requires long time (5 h) and high temperature of 1200 1C, the MW processing takes only 5 min to yield the single phase. The conventionally sintered undoped samples have high coercivity values nearly 4000 Oe, and the MW sintered ones have drastically lower values o1000 Oe. The Ms value of the corresponding samples is recorded to be 47 and 59 emu/g. The improved magnetic properties can be useful for magnetic recording application. The Mg–Zr substitution though induces single phase formation at lower temperature; it however results in a decrease in both, Ms as well as coercivity. The observed deterioration in magnetic properties has been attributed to the changes in microstructure and substitution of Mg–Zr possibly at octahedral sites.
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