Microstructure and magnetic properties of low-temperature sintered M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 for LTCC process

Accepted Manuscript Microstructure and magnetic properties of low-temperature sintered M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 for LTCC process Qian Liu, Chongsheng Wu, Yu Wang, Xin You, Jie Li, Yingli Liu, Huaiwu Zhang PII: DOI: Reference:

S0304-8853(18)31765-7 https://doi.org/10.1016/j.jmmm.2018.11.059 MAGMA 64618

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

7 June 2018 6 November 2018 8 November 2018

Please cite this article as: Q. Liu, C. Wu, Y. Wang, X. You, J. Li, Y. Liu, H. Zhang, Microstructure and magnetic properties of low-temperature sintered M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 for LTCC process, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.11.059

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Microstructure and magnetic properties of low-temperature sintered M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 for LTCC process QianLiua, ChongshengWua, Yu Wanga, Xin Youb, JieLia, YingliLiua, *, HuaiwuZhanga a. State Key Laboratory of Electronic Thin Films and Integrated Devices, University of electronic science and technology of China, Chengdu, Sichuan 610054, China b. Key Laboratory of Advanced Materials of Yunnan Province, Kunming University of Science and Technology. *Corresponding author: Yingli Liu Email address: [email protected] Abstract M-type barium hexaferrite (BaM) of composition successfully

synthesized

through

a

ceramic

BaZn0.6Sn0.6Fe10.8O19

was

process.

aids

Sintering

Bi2O3·B2O3·SiO2·ZnO (BBSZ) were added to lower the sintering temperature. The formation of pure M phase for samples sintered at 900oC with sufficient BBSZ was confirmed from XRD patterns analysis. Large saturation magnetization (69.02 emu/g) and relatively low magnetic anisotropy field (7.4 kOe) were obtained simultaneously when sample was synthesized with 2.5 wt% BBSZ dosage. These properties make the material suitable for applications in LTCC process. The ferromagnetic resonance (FMR) linewidth (ΔH) decreased efficiently by 719 Oe owing to the sintering aids. This M-type hexaferrite is also potential candidate for the design of microwave devices such as circulators and isolators operated at low frequencies. Meanwhile, the influences of sintering aids upon the microstructure, complex permeability and

magnetic properties of M-type hexaferrite BaZn0.6 Sn0.6Fe10.8O19 were investigated. Key words: M-type hexaferrite;BBSZ; LTCC process. 1. Introduction To keep up with the high pace of development on advanced communication system, Miniaturized and integrated communication devices are extremely proposed.[1, 2]. Low Temperature Co-fired Ceramics (LTCC) multilayer devices, composed of alternating ferrites and internal metallic electrode, have been extensively studied to reduce microwave device size [3]. Silver (Ag) is usually adopted as inner electrode metal on account of its low loss and electrical resistance at high frequencies. However, the melting point of Ag is 961oC. Correspondingly, LTCC materials with sintering temperature of around 900oC are required to avoid the diffusion of Ag in ferrites [4]. Regarded as a kind of popular microwave magnetic materials, M-type barium ferrite has high value of saturation magnetization, coercivity, magnetic crystalline anisotropy and Curie temperature. These advantages make it suitable for applications in electronic components, such as circulator, filter, isolator, inductor and phase shifter [5-7]. Unfortunately, high sintered temperature above 1000oC is common required for M-type barium ferrite to form single crystalline structure, which is conflict with LTCC technology [8]. Therefore, it's extremely urgent to lower the sintered temperature. For this target, several methods have been investigated such as adding glass or oxide with low melt point, chemical processing and application of ultra-fine raw materials [9-11]. Among these ways, adding glass with low melt point has been

considered to be convenient and effective. In this paper, BBSZ additives were adopted to lower the sintered temperature of M-type barium ferrite BaZn0.6Sn0.6 Fe10.8O19. Meanwhile,

the composition

BaZn0.6Sn0.6Fe10.8O19 was selected to reduce the high magnetic anisotropy of M-type hexaferrite. So it might be applied in the design of microwave circulators operated at low frequencies such as Ku band. The influences of sintering aids upon the microstructure and magnetic properties were investigated. The complex permeability was also studied to further understand the effects of sintering aids. Moreover, The ferromagnetic resonance (FMR) linewidth of the material was measured to explore the potential applications in microwave devices. 2. Experimental Polycrystalline M-type hexaferrites of composition BaZn0.6Sn0.6 Fe10.8O19 were fabricated through a ceramic process consisting of repeated steps of ball milling and sintering. Through ball milling for 12 h, the starting materials BaCO3, Fe2O3, ZnO, and SnO2 were homogeneously mixed (Raw materials supplier: Cologne Chemical Industry). After pre-sintered at 900C in air for 4 h, various contents of BBSZ additives (0.5-4.0 wt% in step of 0.5 wt%) were added to the powders. The powders were reduced to fine particles of 0.5–1.0 µm diameter by ball milling for a further 12 h. Using 8.0 wt% of polyvinyl alcohol (PVA) as binder, the resulting powders were granulated and shaped into plates and rings of 2-3 mm thickness. Finally, the compacted samples were sintered at 900oC in air for 4h. Partial sintered samples are crashed and grinded into small spheres with diameter about 1 mm for the

measurement of ferromagnetic resonance (FMR) linewidth (ΔH). The phase compositions of samples were obtained using X-ray diffractometer (XRD, DX-2700, Haoyuan Co.) with CuKα radiation. The morphologies of the samples were measured through scanning electron microscopy (SEM; JEOL, JSM-6490). The static magnetic properties were examined using a vibrating sample magnetometer (VSM; MODEL, BHL-525). The complex permeability spectra were measured over frequencies ranging from 0.01 to 3GHz using an Agilent 4991 RF impedance analyzer. The ferromagnetic resonance (FMR) linewidth (ΔH) was measured in TE106 cavity at 16.8 GHz using perturbation method. 3. Result and discussion

Fig.1 XRD patterns of pre-sintered sample and final-sintered BaZn0.6Sn0.6Fe10.8O19

with different BBSZ contents The XRD pattern of pre-sintered sample and samples with different BBSZ contents sintered at 900oC was shown in Fig.1. It illustrated that no peaks of BBSZ were observed although BBSZ was added. The phenomenon benefited from the fact that BBSZ existed as liquid phase and kept as amorphous phase during cooling [4, 12]. BBSZ played a significant role in crystallization of barium ferrite. The formation mechanisms of pure M-type barium ferrite had been proposed containing following two reactions [13]: 𝐵𝑎𝐶𝑂3 + 𝐹𝑒2 𝑂3 = 𝐵𝑎𝐹𝑒2 𝑂4 + 𝐶𝑂2 (1) 𝐵𝑎𝐹𝑒2 𝑂4 + 5𝐹𝑒2 𝑂3 = 𝐵𝑎𝐹𝑒12 𝑂19 (2) As seen in Fig.1, the phase of intermediate BaFe2 O4 was detected in the XRD patterns of pre-sintered sample. The impure phase was also detected with insufficient BBSZ dosage (0.5 wt% and 1.0 wt%), which was not enough to ensure the pure phase formation of M-type hexaferrite. Once BBSZ dosage reached to high level (≧ 1.5wt%), no secondary phase was detected. All diffraction features could be indexed to space group P63/mmc (JCPDS file number 39-1433), corresponding to M-type hexagonal crystal structure. It confirmed the phase purity for samples with high level of

BBSZ

dosage.

Fig.2 Rietveld refinement XRD pattern of BaZn0.6 Sn0.6Fe10.8O19 with 2.5 wt% BBSZ. The Rietveld refinement XRD pattern of BaZn0.6 Sn0.6Fe10.8O19 with 2.5 wt% BBSZ was presented in Fig.2. The structure refinement data for all samples were shown in Table.1. The values of χ2 for all samples were smaller than 2 except for the sample with 0.5 wt% BBSZ. It could be assumed that the refinement results were credible with the investigated parameters. The lattice parameters a and c didn't have a regular change tendency along with the increase of BBSZ contents. However, both a and c of samples with insufficient BBSZ dosage (0.5 wt% and 1.0 wt%) are smaller than that of the others, which might be attributed to the impure phase. It was considered that the phase formation of M-type barium ferrite could be enhanced by appropriate BBSZ liquid phase.

a (Å)

c (Å)

χ2

ρ (g/cm3)

0.5

5.9032

23.3116

2.059

3.864

1.0

5.9104

23.3133

1.644

3.921

1.5

5.9189

23.3271

1.641

3.971

2.0

5.9125

23.3258

1.261

4.034

2.5

5.9127

23.3243

1.478

4.129

3.0

5.9135

23.3283

1.487

4.212

3.5

5.9121

23.3221

1.491

4.217

4.0

5.9139

23.3302

1.541

4.242

BBSZ contents (wt%)

Table.1 Rietveld refinement results and the bulk density (ρ) for BaZn0.6Sn0.6Fe10.8O19 with different BBSZ contents.

Fig.3 SEM micrographs of sintered BaZn0.6Sn0.6Fe10.8O19 samples with different BBSZ contents: (a) 0.5 wt%, (b) 1.0wt%, (c) 1.5 wt%, (d) 2.0wt%, (e) 2.5 wt%, (f) 3.0wt%, (g) 3.5 wt% and (h) 4.0wt%. The morphology of samples with different BBSZ contents was presented in Fig.3.

At low level of BBSZ contents (0.5 wt% and 1.0 wt%), porous microstructure was observed in Fig.3(a), (b), which resulted from the fact that the amount of liquid phase was insufficient for the densification of M-type barium ferrite. As the BBSZ addition was further enhanced, morphology changed from irregular plate-like shapes to hexagonal platelet. Meanwhile, the structure became more and more compact, which was in accordance with the increasing tendency of bulk densities (ρ) presented in Table.1. Apparently, the grain size also increased along with the enhancement of BBSZ dosage. However, the grain size didn't change a lot with sufficient BBSZ as seen in Fig.3(e)-(h). once excessive BBSZ dosage was added, the transportation and diffusion of crystalline grain were inhibited, which in turn leaded to limitation of domain wall and spin rotation . This could explain the stagnation of grain growth.

Fig.4 Hysteresis loops of BaZn0.6Sn0.6Fe10.8O19 with different BBSZ contents

Fig.5 The variations of saturation magnetization (Ms) and coercivity (Hc) as a function of BBSZ contents Fig.4 presented the hysteresis loops of Ba(ZnSn)0.6Fe10.8O19 with different BBSZ contents. Moreover, the variations of saturation magnetization (Ms) and coercivity (Hc) as a function of BBSZ contents were plotted in Fig.5. The Ms exhibited an increasing trend along with increase of BBSZ contents first and obtained a maximum when 2.5 wt% BBSZ was added, and then it decreased dramatically. The crystal structure of M-type hexaferrite could be understood as an alternating stacking of spinel and R blocks in the direction of hexagonal c-axis [14]. The stacking structure was symbolically described as RSR*S*, where R represents rhombohedral block containing one BaO3 layer and two O4 layers composited as Ba2+Fe3+O2-11 , and S is 2spinel block including two O4 layers in form of Fe3+ 6 O 8 . The blocks marked with an

asterisk are corresponding blocks rotated 180o around the hexagonal c-axis [15, 16].

In this basic structure of M-type barium ferrite, Fe3+ ions distribute on five different crystallographic sites: three octahedral sites (12k, 2a, 4f2), one tetrahedral site (4f1) and one trigonal bipyramidal site (2b). Among these sites, the 12k, 2a and 2b sites have upward (↑) spin configurations while the other two (4f1 and 4f2) have down (↓) spin configurations [17]. Magnetism in M-type ferrite originates from the net magnetic moment of ions with spin upward and downward configurations in sublattice sites. On account of the same composition of all samples, it was acceptable to deduce that the variations of Ms mainly result from different mounts of BBSZ liquid phase. The sintering of M-type barium ferrite was significantly enhanced by appropriate BBSZ liquid phase, which contributed to the increase of magnetic moment in turn. However, excessive nonmagnetic BBSZ liquid phase resulted in degradation of Ms. The remanence ratio was below of 0.5. It might be attributed to Zn-Sn substitution and BBSZ additive, which brought impurities and pores in some extent. On the one hand, there was a demagnetization field around impurities and pores, resulting in uneven magnetization inside the material, which leads to reduced remanence in turn. On the other hand, the inhomogeneity of impurities and pores resulted in the nucleation growth of anti magnetization domain, which also reduced remanence. The value of coercivity (Hc) decreased at low level of BBSZ contents (0.5 wt% and 1.0 wt%). It could be explained that BBSZ liquid phase promoted grain growth, in turn, enhanced grain size leaded to degradation of Hc. When sufficient BBSZ dosage was added, grain growth slew down. Hc kept almost a constant. Moreover, based on the fact that both saturation magnetization and magnetic

anisotropy are intrinsic parameters, the magnetic anisotropy constant (K1) and the magnetic

anisotropy

field

(Hk)

were

estimated

according

to

the

law of approach to saturation (LATS)，which were expressed as following formulas [18]: 𝑎

𝑏

𝐻

𝐻2

𝑀𝐻 = 𝑀𝑆 (1 − −

− ⋯ ) + 𝜒𝑃 𝐻

(3) In terms of BaM hexaferrite, Parameter b could be expressed as following equation: 𝑏=

4

𝐾1

15 µ20𝑀𝑠2

(4) Once parameter b is confirmed as the coefficient of 1/𝐻^2 in formula (4), the magnetic anisotropy constant (K1) is also obtained. Then the magnetic anisotropy field (Hk) is estimated according to the formula as follow: 2𝐾1

𝐻𝑘 = µ

(5)

0 𝑀𝑆

Calculated K1 and Hk versus increased BBSZ contents were present in Table.2. It was reasonable that K1 changed little as an intrinsic parameter [19]. Sample sintered with 2.5 wt% BBSZ contents possessed excellent value of Ms (69.02 emu/g) and relatively low Hk (7.4 kOe) simultaneously, which were highly suitable for the design of circulator operated in Ku-band. Ms(emu/g)

Hc (Oe)

K1 (×105J/m3)

Hk (kOe)

0.5

55.72

822

0.98

9.10

1.0

56.32

652

0.96

8.67

BBSZ contents (wt%)

1.5

56.38

495

0.95

8.49

2.0

63.18

478

1.01

7.96

2.5

69.02

489

1.05

7.40

3.0

66.55

486

1.01

7.22

3.5

61.06

485

0.98

7.62

4.0

54.89

486

0.94

8.04

Table.2 The Variations of the magnetic anisotropy constant (K1) and the magnetic anisotropy field (Hk) versus different BBSZ contents.

Fig.6 Variations of real part of permeability (μ') as a function of frequency for different BBSZ dosage

Fig.7 Variations of imaginary part of permeability (μ'') as a function of frequency for different BBSZ dosage The real part (μ') and imaginary part (μ'') of permeability at the frequencies ranging from 1 MHz to 3 GHz were presented in Fig.6 and Fig.7 respectively. In terms of pure M-type barium ferrite, it possesses high saturation magnetization, coercivity and low magnetic permeability. Real part of permeability characterizes the storage capability of magnetic energy, while the imaginary part of permeability represents the loss of magnetic energy [20]. As observed in Fig.6, the value of μ' maintains near 1 until a sharp peak occurs. The center frequency is about 2.7 GHz. The value of μ'' keeps increasing along with frequency and reaches a maximum in magnetic loss corresponding to the peak of μ'. The center frequency (fr) decreases clearly with increase of BBSZ content from 0.5 wt% to 2.0 wt%. When BBSZ contents exceed 2.0 wt%, the center frequency remains nearly constant. The relation between center

frequency and magnetic anisotropy field (Hk) of M-type hexaferrite can be expressed as: 2𝜋𝑓𝑟 = 𝛾𝐻𝑘 , where γ is the absolute gyromagnetic ratio [21]. According to the estimated value of Hk in Table.2, Hk decreased clearly with the increase of BBSZ contents until it exceeded 2.0 wt%, which could explain the variation tendency of fr.

Fig.8 The ferromagnetic resonance (FMR) linewidth (ΔH) for M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 with different BBSZ dosage. The ferromagnetic resonance (FMR) linewidth (ΔH) measured at 16.8 GHz for M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 with different BBSZ dosage were shown in Fig.8. Samples with 0.5 wt% and 1.0 wt% BBSZ dosage didn't detect obvious ferromagnetic resonance, which might be attributed to the porosity and impure phase. It was observed that the value of ΔH decreased by 719Oe due to BBSZ additive. The minimum ΔH still had a high value of 1596 Oe, but it is much less than the large

ΔH (~2000 Oe) of commercially available BaM [22]. The total ΔH primarily originated from three contributions, which were the intrinsic linewidth, random anisotropy field, and porosity. Further work should be done to narrow the linewidth, such as reducing porosity through optimizing the synthetic process and modifying grain distributions. 1.4 Conclusion In this study, BBSZ was added to lowering the sintering temperature to 900 oC during the synthesis process of M-type barium ferrite. Single phase M-type barium ferrite BaZn0.6Sn0.6 Fe10.8O19 had been successfully synthesized when sufficient BBSZ dosage (≧1.5wt%) was added. The grain size increased and the densification of samples was enhanced owe to BBSZ liquid phase. Meanwhile, the saturation magnetization first increased with BBSZ contents, and then a dramatical decrease occurred because of excessive nonmagnetic BBSZ liquid phase. The coercivity first decreased along with the growth of BBSZ contents then kept almost a constant. Moreover, samples with 2.5 wt% BBSZ additive exhibited excellent magnetic properties that appropriate values of saturation magnetization (69.02 emu/g), FMR linewidth (1658 Oe) and magnetocrystalline anisotropy field (7.40kOe) were obtained simultaneously. These characters made the material suitable for LTCC process and design of microwave devices such as circulator operated at Ku band. Acknowledgment Funding: This work was supported by the National Natural Science Foundation of China [grant numbers 61671118] and the National Natural Science Foundation of

China under [grant numbers 61371053]. Reference [1] Y. Wang, Y. Liu, J. Li, Q. Liu, H. Zhang, V.G. Harris, LTCC processed CoTi substituted M-type barium ferrite composite with BBSZ glass powder additives for microwave device applications, AIP Advances, 6 (2016) 056410. [2] Y. Liu, Q. Liu, C. Wu, Y. Wang, J. Li, L. Gao, H. Zhang, Y. Liu, Q. Liu, C. Wu, Investigation on Zn-Sn co-substituted M-type hexaferrite for microwave applications, Journal of Magnetism & Magnetic Materials, 444 (2017) 421-425. [3] D. Kim, D.H. Kim, G.H. Baek, J.I. Ryu, C.D. Park, C.S. Kim, I.S. Song, J.C. Kim, An FBAR‐and LTCC‐based RF front‐end module for Wi‐Fi and WiMAX dual ‐mode systems, Microwave and Optical Technology Letters, 52 (2010) 753-757. [4] D. Chen, Y. Liu, Y. Li, W. Zhong, H. Zhang, Microstructure and magnetic properties of low-temperature sintered CoTi-substituted barium ferrite for LTCC application, Journal of Magnetism and Magnetic Materials, 323 (2011) 2837-2840. [5] Y. Chen, A.L. Geiler, T. Chen, T. Sakai, C. Vittoria, V. Harris, Low-loss barium ferrite quasi-single-crystals for microwave application, Journal of applied physics, 101 (2007) 09M501. [6] Y. Chen, A.L. Geiler, T. Sakai, S.D. Yoon, C. Vittoria, V.G. Harris, Microwave and magnetic properties of self-biased barium hexaferrite screen printed thick films, Journal of Applied Physics, 99 (2006) 721. [7] V.G. Harris, A. Geiler, Y. Chen, S.D. Yoon, M. Wu, A. Yang, Z. Chen, P. He, P.V. Parimi, X. Zuo, Recent advances in processing and applications of microwave ferrites,

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Fig.1 XRD patterns of pre-sintered sample and final-sintered BaZn0.6 Sn0.6Fe10.8O19

with different BBSZ contents

Fig.2 Rietveld refined XRD pattern of BaZn0.6Sn0.6Fe10.8O19 with 2.5 wt% BBSZ

Fig.3 SEM micrographs of sintered BaZn0.6Sn0.6Fe10.8O19 samples with different BBSZ contents: (a) 0.5 wt%, (b) 1.0wt%, (c) 1.5 wt%, (d) 2.0wt%, (e) 2.5 wt%, (f) 3.0wt%, (g) 3.5 wt% and (h) 4.0wt%.

Fig.4 Hysteresis loops of BaZn0.6Sn0.6Fe10.8O19 with different BBSZ contents

Fig.5 The variations of saturation magnetization (Ms) and coercivity (Hc) as a function of BBSZ contents

Fig.6 Variations of real part of permeability (μ') as a function of frequency for different BBSZ dosage

Fig.7 Variations of imaginary part of permeability (μ'') as a function of frequency for different BBSZ dosage

Fig.8 The ferromagnetic resonance (FMR) linewidth (ΔH) for M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 with different BBSZ dosage.

a (Å)

c (Å)

χ2

ρ (g/cm3)

0.5

5.9032

23.3116

2.059

3.864

1.0

5.9104

23.3133

1.644

3.921

1.5

5.9189

23.3271

1.641

3.971

2.0

5.9125

23.3258

1.261

4.034

2.5

5.9127

23.3243

1.478

4.129

3.0

5.9135

23.3283

1.487

4.212

3.5

5.9121

23.3221

1.491

4.217

4.0

5.9139

23.3302

1.541

4.242

BBSZ contents (wt%)

Table.1 Rietveld refinement results and the bulk density (ρ) for BaZn0.6Sn0.6Fe10.8O19 with different BBSZ contents.

Ms(emu/g)

Hc (Oe)

K1 (×105J/m3)

Hk (kOe)

0.5

55.72

822

0.98

9.10

1.0

56.32

652

0.96

8.67

1.5

56.38

495

0.95

8.49

2.0

63.18

478

1.01

7.96

2.5

69.02

489

1.05

7.40

3.0

66.55

486

1.01

7.22

3.5

61.06

485

0.98

7.62

4.0

54.89

486

0.94

8.04

BBSZ contents (wt%)

Table.2 The Variations of the magnetocrystalline anisotropy constant (K1) and the magnetocrystalline anisotropy field (Hk) versus different BBSZ contents.

1. M-type hexaferrite BaZn0.6Sn0.6Fe10.8O19 was synthesized at low sintering temperature of 900oC. 2. Rietveld refinement were adapted to make investigated parameters more standard. 3. Appropriate values of saturation magnetization (69.02 emu/g), FMR linewidth (1658 Oe) and magnetocrystalline anisotropy field (7.40kOe) were obtained simultaneously for sample with 2.5 wt% BBSZ additive. 4. This M-type hexaferrite is potential candidate for the design of microwave devices such as circulator operated at Ku band.