Crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites with narrow FMR linewidth

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Crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites with narrow FMR linewidth Chuanjian Wua, Jinpeng Lia, Zhongwen Lana,b,∗∗, Zhong Yua,b, Xiaona Jianga,b, Rongdi Guoa, Quanbang Luoa, Ke Suna,b,∗ a b

School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 610054, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, China

A R T I C LE I N FO

A B S T R A C T

Keywords: Barium hexaferrites Iron deficiency Anisotropy field FMR linewidth

M-type hexaferrites which exhibit strong uniaxial anisotropy field, are promising for the applications in selfbiased microwave and millimeter wave (MMW) devices. Here we demonstrate crystallographically textured hexaferrites with narrow ferromagnetic resonance (FMR) linewidth synthesized by the conventional ceramic method. Meanwhile, the relationship between iron-deficiency content and microstructure, crystallographic, and magnetic properties are researched. These hexaferrites have strong remanence ratio of above 0.9 along c-axis. By fitting of Lorentzian function to absorption derivative, we probe detailedly into the zero-field FMR linewidth of Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites. When x equals 1.0, it indicates the narrowest FMR linewidth of 324Oe, as compared with previous reports for crystallographically textured hexaferrites.

1. Introduction Driven by the technique of monolithic microwave integrated circuits (MMIC), miniaturization and planarization are needed for the next generation of microwave and millimeter wave (MMW) devices, such as circulators or isolators, to meet the ever increasingly high-frequency demands [1,2]. Conventional MMW devices, developed based on the spinel and garnet ferrites, require an external permanent magnet to provide an indispensable biasing field. However, this design would result in the increment of longitudinal dimension [3,4]. The elimination of permanent magnet is beneficial to make these devices smaller and planar. In order to achieve this goals, magnetic materials must possess the large magnetocrystalline anisotropy field Ha and high remanence ratio Mr/Ms to establish their built-in magnetic fields. Hence crystallographically textured hexaferrites have gained considerable attentions in self-biased MMW devices [5–7]. On the other hands, large ferromagnetic resonance (FMR) linewidth △H of bulk hexaferrites would inevitably block the realization of practical devices [8]. Various cation substitution had been attempted to optimize the △H in recent years, as well as Ha and Mr/Ms, such as Sc [9], La-Al [10], and La-Co [11]. Nevertheless, doping Ba and Fe sublattices make it difficult to enable the simultaneous improvements of △H, Ha, and Mr/Ms. Additionally, bulk hexaferrrites have still rather

∗

large △H of 600~1000Oe [12–14], in contrast to the spinel and garnet ferrites. Challenge remains as finding an effective way to regulate the textured hexaferrites equipped with narrower △H, large Ha, and high Mr/Ms. It is known that the △H is related closely to porosity within the grain and grain boundary [15,16]. Moderate iron deficiency allows to reinforce the driving force of solid-state sintering, leading to the low porosity and narrow △H [17]. Especially in the spinel and garnet ferrites, no significate deterioration in the anisotropy are observed due to iron deficiency weakening the symmetry of crystal structure [18]. But with respect to iron deficiency of bulk hexaferrites, more attentions had been devoted to the microstructure and static magnetic properties for the demands of multiferroic and permanent magnetic materials [19,20]. The relationship between iron-deficiency content and Mr/Ms, Ha and △H in close relation to the self-biased applications was scarcely reported. Previous first-principle calculation revealed that Cu2+ ions would occupy mainly the 2b(↑) and 4f2(↓) sites, and thus the inclusion of La3+ and Cu2+ ions allows to tailor the magnetic properties of magnetic materials [21]. In this work, we utilize La-Cu substitution to optimize the Ha and 4πMs of crystallographically textured hexaferrites, and lowmelting additives to promote the uniformity and densification of grain growth. The effect of iron deficiency on the magnetic and microwave

Corresponding author. School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 610054, China. Corresponding author. School of Materials and Energy, University of Electronic Science and Technology of China, Chengdu, 610054, China. E-mail addresses: [email protected] (Z. Lan), [email protected] (K. Sun).

∗∗

https://doi.org/10.1016/j.ceramint.2019.12.110 Received 19 November 2019; Received in revised form 8 December 2019; Accepted 10 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Please cite this article as: Chuanjian Wu, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.110

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properties is investigated in detail. Especially, we present the narrowest FMR linewidth of 324Oe, high remanence ratio of above 0.9, and strong anisotropy field of 18.1kOe for crystallographically textured hexaferrites.

Table 1 Lattice constants a and c, ratio of c/a, and average grain size D of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites.

2. Experimental procedures M-type barium hexaferrites (BaM) with a composition of Ba0.8La0.2Fe11.8-xCu0.2O19-δ (0 ≤ x ≤ 1.4) were synthesized by a conventional ceramic method. The starting powders, BaCO3, Fe2O3, La2O3, and CuO in analytical grade were weighed according to the above stoichiometric proportions, and mixed in zirconium ball mills for 12 h. The mixed powders were dried, and calcined at 1100~1250 °C for 2 h in air. For the improved densification sintering, 2.5 wt% Bi2O3 and 2.0 wt% CuO were added as sintering aid during milling. In the following the powders were pressed in a strong longitudinal magnetic field (~10kOe) to produce crystallographically textured BaM cylindrical green flans. The specimens were then sintered at 900~1000 °C for 2 h inside a bell furnace. The phase composition of hexaferrite were determined on Shimadzu Maxima-X XRD-7000 with filtered Cu-Kα radiation. The cross-section morphology was identified with field emission scanning electron microscopy (FESEM, JEOL JSM-7800F). Archimedes method was employed for bulk densities. Porosity was deduced from the difference between the experimental and theoretical density. Magnetic properties at room temperature were carried out using vibrating sample magnetometer (VSM, Lake Shore 8604) with an applied field 2T. Ferromagnetic resonance (FMR) linewidth was characterized by ground co-planar waveguide (GCPW) within the transmission mode at zero field using a Keysight Performance Network Analyzer.

x

a (Å)

c (Å)

c/a

D (μm)

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4

5.89 5.89 5.89 5.90 5.90 5.90 5.90 5.90

23.31 23.28 23.25 23.24 23.23 23.23 23.21 23.20

3.96 3.95 3.95 3.94 3.94 3.94 3.93 3.93

1.39 1.51 1.56 1.59 1.59 1.55 1.53 1.53

± ± ± ± ± ± ± ±

0.68 0.67 0.65 0.55 0.61 0.58 0.67 0.78

that crystallographically textured barium hexaferrites exhibit strong caxis preferred orientation. According to Eq. (1) [22], the lattice constants a and c of hexaferrites are listed in Table 1.

1 4 h2 + hk + k 2 ⎞ l2 = ⎛ + 2 2 2 d 3⎝ a c ⎠ ⎜

⎟

(1)

where h, k, and l is the Miller indices, and d is the interplanar spacing calculated from the Bragg's law. We notice that the lattice constant a remains almost at a certain level, but the c decreases due to the increase of iron-deficiency content. Moreover, the value of c/a is well within the ratio range of 3.93~3.96 [23], which also implies that the as-sintered compacts correspond completely to the magnetoplumbite hexagonal structure. Fig. 2 presents the representative cross-section morphology of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexferrites. The specimens have the microstructure consisting of hexagonal platelet-like shape grains aligned parallel to c-axis. The average grain size D (see Table 1) ranges from 1.39 to 1.59μm, which lies prevailingly within the estimated single domain critical size range of 1.5μm. It is crucial to allow for high coercivity of texture bulk hexaferrites. Furthermore, density and porosity of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites are displayed in Fig. 3. For the reason that iron deficiency is beneficial to promote the ion and vacancy transformation, the grain would be more compact with the increase of iron-deficiency content. Nevertheless, an excessive iron deficiency also gives rise to abnormal grain growth and pores on the grain boundary [24]. Hence it is noted that the sintering density would reduce on the contrary at x > 1.0. Typical magnetic hysteresis loops of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites are depicted in Fig. 4. The

3. Results and discussion Fig. 1 shows the respective X-ray diffraction patterns of crystallographically textured barium hexaferrites with different iron-deficiency contents. All diffraction peaks have been indexed to the standard powder diffraction pattern of barium hexaferrites with JCPDF#430002, corresponding to the space group P63/mmc. We could conclude that the combined La3+ and Cu2+ are incorporated into the lattice of magnetoplumbite hexagonal structure within the range of 0.0~1.4 for Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites. The peaks parallel to c-axis of (006) and (008) have the comparatively high intensity, which indicates

Fig. 1. X-ray diffraction patterns of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites: (a) x = 0.0, (b) x = 0.2, (c) x = 0.4, (d) x = 0.6, (e) x = 0.8, (f) x = 1.0, (g) x = 1.2, (h) x = 1.4.

Fig. 2. Typical cross-section morphology of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites: (a) x = 0.0, (b) x = 0.4, (c) x = 1.0, (d) x = 1.4. 2

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magnetization 4πMs and remanence 4πMr increase first and then start to drop at x = 1.0. It is consistent well with the various trend of the density d. Besides, the remanence ratio Mr/Ms (//) maintains a relatively high value of above 0.90 throughout the tunable iron-deficiency content, which is a vital prerequisite for self-biased devices. Derived from the law of approach to saturation (see Fig. 4), we could obtain the anisotropy field Ha and anisotropy constant K1. It is found that crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites have strong uniaxial anisotropy. As indicated in Ref. [25], only the shift of 12k and 4f2 sublattices were observed in Mössbauer spectra due to the iron deficiency of the hexaferrites. Furthermore, the anisotropy of hexaferrites results mainly from 2b sublattice [26]. Hence we notice that the K1 has almost no changes with the increase of iron-deficiency content, and the Ha would be inversely proportional to 4πMs according to the basic equation Ha = 2K1/4πMs. In case of hexaferrites, Kools et al. [27] provided one model based on nucleation of domains to describe the coercivity Hc as following:

Hc = nHa − N (4πMr + 4πMs )/ μ0

Fig. 3. Density and porosity of crystallographically textured barium hexaferrites with different iron-deficiency contents.

(2)

where n, N, and μ0 are the grain size factor, demagnetization factor, and permeability of vacuum, respectively. The factor n increases with decreasing grain size, and N is governed by the specimen shape. Given that the varying trend of grain size, anisotropy field, and magnetization as mentioned previously, the Hc is deduced to decrease first and then increase, which is in accordance with the experimental results (see Table 2). The frequency response of the absorbed power at U-band (40~60GHz) was measured with the zero external biasing fields,

obvious discrepancy between the in-plane and out-of-plane loops suggests strong anisotropic magnetic characteristics in the hexaferrites, due to the preferred orientation along the c-axis. The relevant magnetic properties of crystallographically textured barium hexaferrites with different iron-deficiency contents are summarized in Table 2. Note that with the increase of iron-deficiency content, both the saturation

Fig. 4. Typical magnetic hysteresis loops of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites with an applied magnetic field parallel and perpendicular to c-axis: (a) x = 0.0, (b) x = 1.0. And the corresponding experimental and fitting magnetization curves: (c) x = 0.0, (d) x = 1.0. 3

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Table 2 Magnetic properties and zero-field ferromagnetic resonance (FMR) frequency of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites://and⊥denote an applied magnetic field parallel and perpendicular to c-axis. x

4πMs (Gs)

4πMr(Gs)

Mr/Ms

Hc(Oe)

Ha(kOe)

K1( × 105)J/m3

fr(GHz)

0

3714

18.8

2.78

55.08

3735

1438

18.6

2.77

54.42

0.4

3774

1308

18.4

2.76

54.04

0.6

3808

1230

18.2

2.76

53.82

0.8

3825

1220

17.9

2.73

53.11

1.0

3836

1424

18.1

2.74

53.23

1.2

3797

1710

18.4

2.78

53.59

1.4

3772

0.92 0.21 0.93 0.21 0.93 0.21 0.92 0.21 0.92 0.21 0.92 0.21 0.92 0.19 0.92 0.20

1635

0.2

3448(∥) 781(⊥) 3480(∥) 786(⊥) 3510(∥) 810(⊥) 3518(∥) 801(⊥) 3520(∥) 796(⊥) 3533(∥) 803(⊥) 3507(∥) 740(⊥) 3463(∥) 751(⊥)

1854

18.6

2.79

53.93

Fig. 5. Typical zero field FMR spectra of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites: (a) x = 0.0, (b) x = 0.4, (c) x = 1.0, (d) x = 1.4.

where γ is the gyromagnetic ratio, and Nδ with δ = x, y, and z represents the demagnetizing factor along δ axis. Here in Eq. (3), the 4πMr would substitute for 4πMs numerically without external field applied. The anisotropy field extrapolated from FMR measurement (see Fig. 5) is very close to the one derived from the law of approach to saturation. For anisotropy at a relatively high value, could make the FMR frequency change consistently with it. Utilizing fitting of the Lorentzian function, we could obtain the half width at maximum △f. The corresponding FMR linewidth △H of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites is

applied parallel to the c-axis. Fig. 5 demonstrates the absorption derivative of crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexferrites. A reasonable fitting of the absorption derivative was performed by Lorentzian function consisting of symmetric and asymmetric components [28]. The corresponding zero-field ferromagnetic resonance (FMR) frequency are recorded in Table 2. According to the Kittel equation [29], we could establish the relationship between FMR frequency fr and magnetic properties (Ha and 4πMs):

fr = γ (Ha + (Nx − Ny )4πMs )⋅(Ha + (Nx − Nz )4πMs )

(3) 4

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4. Conclusions In

summary, crystallographically textured Ba0.8La0.2Fe11.8hexaferrites were successfully fabricated according to the conventional ceramic process. Both the saturation magnetization 4πMs and remanence 4πMr, which is consistent well with the trend of the density d, increase first and then start to drop at x = 1.0. Based on the law of approach to saturation, we note that the anisotropy constant has almost no changes for tailoring iron-deficiency hexaferrites due to no shift of 2b sublattice, and thereby the anisotropy field Ha would demonstrate the similar varying changes of the saturation magnetization 4πMs. The FMR linewidth is dependent mainly on the porosity and anisotropy field. A decrease with subsequent increment could be observed in △H with the increase of iron-deficiency contents. It is noteworthy that crystallographically textured barium hexaferrites exhibit high remanence ratio of above 0.9, high coercivity of 1424Oe, strong anisotropy field of 18.1kOe, and the narrowest FMR linewidth of 324Oe at x = 1.0, which allows a great opportunity for developing the next generation of self-biased and low-loss MMW devices. xCu0.2O19-δ

Fig. 6. Ferromagnetic resonance linewidth of crystallographically textured barium hexaferrites with different iron-deficiency contents.

Declaration of competing interest 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. Acknowledgments This present work was financially supported by the National Natural Science Foundation of China [Grant No.51772046], and the Fundamental Research Funds for the Central Universities [ZYGX2018J038, ZYGX2018J040, and ZYGX2018J042]. References [1] Q. Liu, C. Wu, Y. Wang, L. Zhou, J. Li, Y. Liu, H. Zhang, Textured M-type barium hexaferrite Ba(ZnSn)xFe12-2xO19 with c-axis anisotropy and high squareness ratio, Ceram. Int. 45 (2019) 4535–4539. [2] Z. Su, H. Chang, X. Wang, A.S. Sokolov, B. Hu, Y. Chen, V.G. Harris, Low loss factor Co2Z ferrite composites with equivalent permittivity and permeability for ultra-high frequency applications, Appl. Phys. Lett. 105 (2014) 062402. [3] V.V.K. Thalakkatukalathil, A. Chevalier, V. Laur, G. Verissimo, P. Queffelec, L. Qassym, R. Lebourgeois, Electromagnetic modeling of anisotropic ferrites-application to microstrip Y-junction circulator design, J. Appl. Phys. 123 (2018) 234503. [4] A.M. Kroening, Advances in ferrite redundancy switching for Ka-band receiver applications, IEEE Trans. Microw. Theory 64 (2016) 1911–1917. [5] C. Yu, A.S. Sokolov, P. Kulik, V.G. Harris, Stoichiometry, phase, and texture evolution in PLD-grown hexagonal barium ferrite films as a function of laser process parameters, J. Alloy. Comp. 814 (2020) 152301. [6] M. Sharma, S.C. Kashyap, Improvement in magnetic parameters of polycrystalline barium hexaferrite by nonmagnetic cation substitution and microwave processing, Ceram. Int. 45 (2019) 11226–11232. [7] V. Annapureddy, J.H. Kang, H. Palneedi, J.W. Kim, C.W. Ahn, S.Y. Choi, S.D. Johnson, J. Ryu, Growth of self-textured barium hexaferrite ceramics by normal sintering process and their anisotropic magnetic properties, J. Eur. Ceram. Soc. 37 (2017) 4701–4706. [8] X. Jiang, S. Li, Z. Yu, V.G. Harris, Z. Su, K. Sun, C. Wu, R. Guo, Z. Lan, Effect of cobalt substitution on magnetic properties of Ba4Ni2-xCoxFe36O60 hexaferrite, AIP Adv. 8 (2018) 056218. [9] A. Hilczer, K. Pasińska, B. Andrzejewski, M. Matczak, A. Pietraszko, Magnetic properties of Sr0.95Nd0.05Fe12-xScxO19 hexaferrite nanocrystals: (Tcone, H, x) phase diagram, Ceram. Int. 45 (2019) 1189–1195. [10] Y. Dai, Z. Lan, C. Wu, C. Yang, Z. Yu, R. Guo, W. Wang, C. Chen, X. Liu, X. Jiang, K. Sun, Tailoring magnetic properties of Al-substituted M-type strontium hexaferrites, Appl. Phys. A-Mater. 124 (2018) 842. [11] C. Liu, X. Liu, S. Feng, K.M.U. Rehman, M. Li, C. Zhang, H. Li, X. Meng, Effect of YLa-Co substitution on microstructure and magnetic properties of M-type strontium hexagonal ferrites prepared by ceramic method, J. Magn. Magn. Mater. 445 (2018) 1–5. [12] R. Topkaya, Effect of Zn substitution on temperature dependent magnetic properties of BaFe12O19 hexaferrites, J. Alloy. Comp. 725 (2017) 1230–1237. [13] V. Laur, G. Vérissimo, P. Quéffélec, L.A. Farhat, H. Alaaeddine, E. Laroche, G. Martin, R. Lebourgeois, J.P. Ganne, Self-biased Y-junction circulators using lanthanum- and cobalt-substituted strontium hexaferrites, IEEE Trans. Microw.

Fig. 7. Published values of FMR linewidth △H in crystallographically textured hexaferrites [13,33–38].

evaluated according to Eq. (4) [30], and illustrated in Fig. 6.

ΔH =

3 × Δf / γ

(4)

In polycrystalline ferrites, the total △H is generally attributed to three major contributions [31]:

ΔH = ΔHi + ΔHa + ΔHp

(5)

Here △Hi is the intrinsic linewidth, and △Ha and △Hp relate positively to the crystalline anisotropy and porosity induced linewidth broadening contributions. Karim et al. [32] speculated that hexaferrites possess a low intrinsic linewidth of 0.3~0.4Oe/GHz, which reveals that the total △H depends crucially on △Ha and △Hp. As a consequence, we notice that the △H decreases and then increases with the increase of iron-deficiency content. Particularly, crystallographically textured Ba0.8La0.2Fe11.8-xCu0.2O19-δ hexaferrites present the narrowest FMR linewidth of 324Oe at x = 1.0, as compared with previous researches (see Fig. 7). It is ascribed to a relatively high density and homogeneous grain size. 5

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