Enhanced structure and microwave magnetic properties of MgZn ferrite by Cd2+ ion substitution for LTCC applications

Journal Pre-proof 2+ Enhanced structure and microwave magnetic properties of MgZn ferrite by Cd ion substitution for LTCC applications Jie Li, Yan Yang, Gang Wang, Li Guo, Yiheng Rao, Gongwen Gan, Huaiwu Zhang PII:

S0272-8842(19)33336-X

DOI:

https://doi.org/10.1016/j.ceramint.2019.11.146

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CERI 23515

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Ceramics International

Received Date: 28 October 2019 Revised Date:

14 November 2019

Accepted Date: 17 November 2019

Please cite this article as: J. Li, Y. Yang, G. Wang, L. Guo, Y. Rao, G. Gan, H. Zhang, Enhanced 2+ structure and microwave magnetic properties of MgZn ferrite by Cd ion substitution for LTCC applications, Ceramics International (2019), doi: https://doi.org/10.1016/j.ceramint.2019.11.146. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier Ltd.

Enhanced structure and microwave magnetic properties of MgZn ferrite by Cd2+ ion substitution for LTCC applications Jie Li*, Yan Yang, Gang Wang, Li Guo, Yiheng Rao, Gongwen Gan, Huaiwu Zhang State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, China Correspondence author: [email protected] (Jie Li) Abstract: Due to its excellent magnetic properties, MgZn ferrite is a nearly optimal material for low temperature co-fired ceramic (LTCC) phase shifter applications. In this study, low temperature co-fired Mg0.8Zn0.2-xCdxFe2O4 ferrites (x=0.00-0.10, with increments of 0.02) were synthesized using a solid-state method with the aid of 2.5 wt.% BBSZ (33%mol Bi2O3-21%mol B2O3-11%mol ZnO-35%mol SiO2) glass sintering. The effects of the Cd2+ ions on phase formation, microstructure, magnetic permeability, and gyromagnetic properties were investigated. The results indicated that a suitable amount of Cd2+ ion substitution did not change the phase formation of MgZn ferrite. However, with further increases of the Cd2+ ion content, the superfluous Cd2+ ions and Fe3+ ions formed CdFe2O4, which affected the microstructure, density and magnetic properties of the samples. More specifically, when x=0.04, Mg0.8Zn0.16Cd0.04Fe2O4 ferrite, sintered at 920 °C, showed excellent magnetic permeability (µ'~56.6 @1 MHz-20 MHz) and had a high cut-off frequency (~100 MHz). Furthermore, the ferromagnetic resonance line widths (∆H) were measured as a function of the Cd2+ substitution at 9.56 GHz, and the relationships between ion occupancy and microstructure were also discussed. A narrow ∆H (~228.2 Oe) can be obtained by adding an optimal amount of Cd2+ ions. These observations indicate that MgZn ferrites with a suitable amount of Cd2+ ions are promising candidate materials for LTCC electronic device applications.

Keywords:

MgZn

ferrite;

Cd2+

ion 1

substitution;

Magnetic

performance;

microstructure; FMR linewidth

Introduction The rapid development of the electronic information field has led to more requirements for electronic materials. Furthermore, lower costs to achieve higher integration and enhanced geometries for higher performance are in high demand. Multilayer chip inductors (MLCI) and low temperature co-fired ceramics (LTCC) are widely components used in various electronic circuits [1-3]. Among the known electronic materials, ferrite plays an important role due to its high magnetic performance and wide range of applied frequencies. Ferrite materials have been applied for inductance and filtering and in isolators and phase shifters [4-6]. MgZn ferrite is an excellent magnetic material with tailored properties for various technological applications and a good history of application that has been studied for many years [7-11]. To improve the magnetic performance of MgZn ferrite, many methods have been developed in recent years. Researchers have used different synthesis methods, such as microwave refluxing, sol-gel synthesis, coprecipitation and citrate-gel synthesis [12-15]. Some researchers have used different ions as substitutes for Mg2+ or Fe3+ ions to increase the magnetic properties of MgZn ferrite, such as In3+ ions [16], Co3+ ions [17], Mn2+ ions [10], and Pr3+ ions [18]. The substituted ions can change the lattice size and occupy different lattice positions. Therefore, ion 2

substitution is considered one of the most effective ways to improve the magnetic properties of MgZn ferrite [19]. However, superfluous ion substitution can produce secondary phases, which would affect the microstructure and properties of MgZn ferrite. This phenomenon cannot be ignored in ion substitution research. In this study, we chose the Cd2+ ion as a substitute for the Zn2+ ion in MgZn ferrite with the aim of obtaining higher magnetic permeability, low magnetic loss and a narrow ferromagnetic resonance line width. Additionally, to meet the requirements of the fabrication technology used to produce electronic devices using LTCC, sintering glass was added to the materials to achieve sintering at low temperatures.

Experimental Mg0.8Zn0.2-xCdxFe2O4 ferrite materials (x=0.00, 0.02, 0.04, 0.06, 0.08 and 0.10) were prepared via traditional solid-state methods at low temperature with 2.5 wt.% BBZS glass (33%mol Bi2O3-21%mol B2O3-11%mol ZnO-35%mol SiO2) as a sintering aid. MgO, ZnO, CdO and Fe2O3, as raw materials, were added in the form of a powder. The powder was mixed and milled for 18 h using a ball mill. The milled powder was dried and then pre-sintered at 1060 ºC for 4 h. After pre-sintering, 2.5 wt.% BBSZ glass was added to the powder, and it was milled again for 16 h. After drying, the powder was pressed into thick rings with 8 wt.% of 3

polyvinyl alcohol (PVA) as a binder. Finally, the materials were sintered at 920 ºC for 6 h. An X-ray diffractometer (XRD, DX-2700, Haoyuan Co.) was used to determine the phase composition of the samples using Cu Kα radiation, and a scanning electron microscope (SEM, JSM-6490, JEOL) was used to characterize the microstructure of the samples. The bulk density was measured using an auto density tester (GF-300D, AND Co.). An Agilent 4291B impedance analyzer was used to characterize the complex magnetic permeability of the samples over a frequency range of 1 MHz to 1 GHz. The ferromagnetic resonance linewidth (FMR, ∆H) was tested using a network analyzer at 9.56 GHz. All the measurements were carried out at room temperature.

Results and Discussions

Fig. 1 X-ray diffraction patterns of samples with different Cd2+ ion contents

Figure 1 shows the XRD patterns of samples with various CdO contents. 4

For x values from 0.00 to 0.10, all the samples produced peaks belonging to the MgZn ferrite phase. This result indicated that Cd2+ ion substituted MgZn ferrites were successful synthesized at 920 ºC with 2.5 wt.% BBSZ glass. Furthermore, the XRD results showed that there were no other phases appearing for x values from 0.00 to 0.04. When the Cd2+ ion content was increased further, Cd2+ and Fe3+ produced a second phase, CdFe2O4, for x values from 0.06 to 0.10. Based on the XRD data, the lattice constants, cell volumes and X-ray theoretical densities were calculated and are shown in Table 1. For increasing x values, the lattice constant of the samples increased from 8.362 Å to 8.426 Å, and the cell volume also increased. This change was due to the different ionic radius of the Cd2+ and Zn2+ ions. The ionic radius of the Cd2+ ion is 0.97 Å, and the ionic radius of the Zn2+ ion is 0.74 Å. When Cd2+ ions substitute the Zn2+ ions in MgZn ferrite, Cd2+ ions enter the lattice position of Zn2+ and increase the lattice constant to achieve a low substitution content. With further increases of the Cd2+ content, some of the Cd2+ ions enter the lattice and some of the Cd2+ ions produce CdFe2O4 with Fe3+ ions. CdFe2O4 is a dielectric material, which diminishes the magnetic properties of the samples. Table 1 Summary of lattice constants (a), cell volumes (V), X-ray theoretical densities (ρx) and grain sizes (D) of the samples Samples

Lattice constant (Å)

Cell volume (Å3)

X-ray density (g/cm3)

Grain size (%)

x=0.00

8.362

584.67

5.19

2.31±0.14

x=0.02

8.384

589.33

5.22

2.44±0.20

5

x=0.04

8.399

592.52

5.29

2.96±0.23

x=0.06

8.411

594.08

5.38

3.12±0.21

x=0.08

8.417

596.41

5.41

2.17±0.16

x=0.10

8.426

598.22

5.47

1.92±0.11

Density plays a key role in controlling the magnetic properties of ferrites. The bulk densities and X-ray densities of the Mg-Zn ferrite samples with Cd2+ ions are shown in Figure 2. The bulk density was lower than the X-ray theoretical density, which may be due to the existence of pores that formed and developed during the sample preparation or sintering processes. The density linearly increased with the increase in x since the Zn2+ ions at the tetrahedral sites were replaced with Cd2+ ions. The increase of density contributes to the enhancement of magnetic properties.

Fig. 2 X-ray theoretical density and the bulk density of samples with different Cd2+ ion contents

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Figure 3 shows SEM images of the section surface of samples with different Cd2+ ion contents. For low temperature sintering, Cd2+ substituted MgZn ferrite had a compact grain morphology, which resulted in high densification. For x=0.00 and x=0.02, the grains had regular shapes and the grain size was approximately 1-2 µm. As x increased further, the grain size increased and some larger particles appeared due to the substitution of Cd2+ ion causing a lattice distortion that promoted grain growth. As the x value increased further (0.08 and 0.10), the grain size decreased and some pores appeared between the grains. Superfluous Cd2+ ions produced the secondary phase CdFe2O4, which prevented energy transfer among ferrite grains. Therefore, the grain size was reduced and pores appeared, which negatively affected the magnetic properties of the ferrite materials. In addition, the variations in the grain size trends shown in Table 1 also coincide with the above discussion.

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Fig. 3 SEM images of samples with different Cd2+ ion contents

Figure 4 shows the complex magnetic permeability of the samples. As shown in figure 4, as the content of substituted Cd2+ ions increases, the real part of the magnetic permeability (µ') first increases and then decreases. The maximum value was approximately 56.6 when x=0.04. After x=0.06, the value of µ' rapidly decreased. The value of µ' was only approximately 28.3 when x=0.10. This result shows that substitution of Cd2+ ions can improve magnetic performance for suitable amounts of substitution. In addition, the imaginary part of the magnetic permeability (µ") remained at a low value and showed the samples had low magnetic loss.

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(a) (b) Fig. 4 Complex permeability (µ′ and µ″) of MgZn ferrite with different Cd2+ ions

For ferrite materials, the chemical composition is the dominant factor that influences the magnetic properties. In this MgZn ferrite, the changes in the complex magnetic permeability were mainly attributed to the substitution of Cd2+ ions, where Zn2+ ions were replaced with Cd2+ ions. In Mg0.8Zn0.2Fe2O4 ferrite, Mg2+ and Zn2+ ions occupy all the octahedral sites and Fe3+ ions occupy the tetrahedral sites. Because of the differences in the ionic radius of Mg, Zn and Fe ions in ferrite, the Mg-O, Zn-O and Fe-O bond distances are redistributed between both sites. Therefore, when Cd2+ ions replace Zn2+ ions, the balance is disturbed, which leads to changes in the magnetic properties. Furthermore, Mg2+ ions and Zn2+ ions are nonmagnetic elements, and their magnetic moments are 0 µB. The magnetic moment of Cd2+ ion is also 0 µB. Hence the total magnetic moment does not change when Cd2+ replaces Zn2+. However, Cd2+ ion substitution changes the bond distance of the M-O bonds (M=Mg, Zn, Cd and Fe), which is the main reason behind the increase of the real part of 9

the magnetic permeability when the substituted Cd2+ content increases from 0.00 to 0.04. Other reasons are the larger grain size, the reduced pores and the higher density. When x was increased to 0.06, the complex magnetic permeability decreased. This decrease was attributed to three reasons. First, the addition of superfluous Cd2+ ions resulted in the production of CdFe2O4, which reduced the magnetic properties and decreased the real part of the magnetic permeability of the sample. Second, the uneven grain size and shape played roles in the decrease of the magnetic properties. Third, the pores among the grain boundaries reduced the real part of the magnetic permeability and increased magnetic loss.

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Fig. 5 FMR spectra of MgZn ferrites with different Cd2+ ion contents

Figure 5 shows the FMR spectra and Gauss fitting of the sample spectra for different Cd2+ contents. As shown in figure 5, when x increases from 0.00 to 0.04, the value of the FMR linewidths are 257.04 Oe, 236.04 Oe and 228.20 Oe, respectively. For further increases in x from 0.06 to 0.10, the values are 259.20 Oe, 278.10 Oe and 295.70 Oe, respectively. The results show that the FMR linewidth of MgZn ferrite can be reduced by optimizing the content of substituted Cd2+ ions. The FMR linewidth is related to the magnetocrystalline anisotropy constant [20]. Therefore, the grain size, pores, microstructure and magnetic permeability play key roles 11

in reducing the FMR linewidth (∆H). First, the changes of the magnetic permeability is consistent with the results for the FMR linewidth reduction [2]. In general, the total linewidth is attributed to three major contributions: ∆H= ∆Hi + ∆Ha + ∆Hp, where ∆Hi, ∆Ha, and ∆Hp are the intrinsic contribution, the random anisotropy field contribution, and the porosity contribution, respectively. The first part of the formula (∆Hi) is small compared with the other two parts (∆Ha and ∆Hp) and can be ignored [21]. When Cd2+ ion are substituted into MgZn ferrite, moderate substitution contents increased the magnetic permeability, which led to a decrease of ∆Ha. Hence, the samples presented a relatively uniform size distribution and their pores significantly decreased, which caused the reduction of the ∆Hp value, resulting in the decrease of ∆H [20]. Second, the uniform and compact MgZn ferrite had a high density at low substitution content (x=0.04), which is beneficial for reducing the FMR linewidth. On the other hand, replacing Zn2+ ions with Cd2+ ions produced a secondary phase (CdFe2O4) at a high substitution content, which explains why the FMR linewidth gradually increased after x=0.06 [20]. In summary, MgZn ferrite with a narrow FMR linewidth (∆H=228.2 Oe) was obtained by adjusting the content of substituted Cd2+ ions using a low temperature cofired technique with a BBSZ additive. Conclusion In this study, Cd2+ ion substituted MgZn ferrite materials were 12

synthesized via a solid-state method at low temperature with a BBSZ additive. The results showed that the Cd2+ ions could enter the lattice sites of Zn2+ ions at low substitution contents and that a secondary phase CdFe2O4 was produced at high substitution contents. Furthermore, excellent magnetic permeability was obtained when the Cd2+ ion substitution content was 0.04, and the sample had a narrow FMR linewidth (∆H=228.2 Oe). The phase formation, ion occupancy, density and pores were the factors that influenced the magnetic properties, which was discussed in detail. Overall, the magnetic properties of MgZn ferrites were enhanced by Cd2+ ion substitution. The material synthesized in this work has great potential for application in LTCC microwave devices (inductance, filtering, isolators and phase shifters).

Acknowledgments This work was supported by National Key Scientific Instrument and Equipment Development Project No.51827802, and by Major Science and Technology projects in Sichuan Province No. 2019ZDZX0026, and by the National Natural Science Foundation of China No. 51872041, and by Foundation for University Teacher of Education of China No. ZYGX2019J011. References [1] P. Yang, Z.Q. Liu, H.B. Qi, Z.J. Peng, X.L. Fu, High-performance inductive couplers based on novel Ce3+ and Co2+ ions co-doped Ni-Zn ferrites, Ceramics International, 45 (2019) 13685-13691. 13

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Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

no.