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Effects of Bi2O3–CaCu3Ti4O12 composite additives on micromorphology, static magnetic properties, and FMR linewidth ΔH of NCZ ferrites
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Effects of Bi2O3–CaCu3Ti4O12 composite additives on micromorphology, static magnetic properties, and FMR linewidth ΔH of NCZ ferrites Rongdi Guoa,∗, Xiaofeng Zhanga, Zhong Yua,b, Ke Suna,b, Xiaona Jianga,b, Chuanjian Wua, Zhongwen Lana,b a b
School of Materials and Energy, Chengdu, 610054, China State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, 610054, China
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Keywords: NiCuZn ferrites Bi2O3–CaCu3Ti4O12 composite additives Micromorphology Ferromagnetic resonance FMR linewidth Static magnetic properties
NiCuZn (NCZ) ferrites have been widely used in non-reciprocal microwave/millimeter ferrite devices, such as circulators. With the development of microwave/millimeter devices and components to high frequency, miniaturization, and lightweight applications, NCZ ferrites have needed to satisfy the essential requirements of high saturation magnetization 4πMs, low coercivity Hc, and low ferromagnetic resonance (FMR) linewidth ΔH. Herein, 0.1 wt% Bi2O3 and 0.0–3.5 wt% CaCu3Ti4O12 (CCTO) composite additives were introduced to NCZ ferrites, the influence of Bi2O3–CCTO composite additives on micromorphology, static magnetic properties, and FMR linewidth ΔH of NCZ ferrites have been demonstrated in detail. The results show that increasing the CCTO amounts in NCZ ferrites, saturation magnetization 4πMs monotonically decreases from 5484Gs to 4819Gs, and both coercive force Hc and FMR linewidth ΔH first decreases and then increases with minimums of 31A/m and 147Oe, respectively. In addition, the theory of spin-wave narrowing and the law of approach to saturation have been adopted for the separation calculation of FMR linewidth ΔH to the crystalline anisotropy linewidth ΔHa and porosity induced linewidth ΔHp. The sample with 0.1 wt% Bi2O3 and 1.5 wt% CCTO possesses high saturation magnetization 4πMs (5321Gs), remanence Br (200 mT), low coercivity Hc (31A/m), and low FMR linewidth ΔH (147Oe). The state-of-the-art NCZ ferrites with outstanding performances manifest significant applied potency for microwave/millimeter devices and components in phase array radar systems.
1. Introduction The ferrites circulator that realizes the non-reciprocal transmission of electromagnetic wave signal is one of the important parts of T/R components in phase array radar systems [1,2]. To meet the operational requirements of modern radar, ferrites circulators are desired to have certain characteristics, such as high frequency, miniaturization, high performance, and high reliability. However, the properties of ferrites play significant roles in scattering parameters (S-parameters) of circulators. Remarkably, insertion losses S21 of ferrites circulators are closely related to the ferromagnetic resonance (FMR) linewidth ΔH of ferrites. NCZ ferrites are extensively applied in non-reciprocal microwave/millimeter devices and components with broadly adjustable saturation magnetization 4πMs, high density d, and high Curie temperature Tc; low coercive force Hc, low dielectric loss tangent tanδε, and low FMR linewidth ΔH. In the process of ferrite preparation, despite the main formula basically determining the intrinsic properties of the material, appropriate
∗
additives could control the solid-state reaction level, composition of the crystalline phase, grain size, density, porosity, saturation magnetization, resistivity, and other performances of the ferrites [3–22]. In this regard, according to the different compositions, additives could be divided into three categories: 1) the segregation of additives at the grain boundary, such as CaO, and SiO2; 2) the additives with low melting point, resulting in liquid phase sintering, and a solid solution in a spinel structure, such as Bi2O3, MoO3, and V2O5 [3,5,7]; 3) diffusion into crystal lattices, affecting the ions occupation and distribution of the ferrites, such as TiO2, Co2O3, MnO2, and Al2O3 [4,14]. Yan Yang et al. fabricated the Ni0.2Cu0.2Zn0.6Fe2O4 ferrite ceramics with enhanced gyromagnetic properties for LTCC applications at 900 °C, 925 °C, and 950 °C [7]. Composite additives of 0.5 wt% MnO2 and x wt% Bi2O3 (x = 0.0–3.0) have been applied for reducing sintering temperature via promoting grain growth and help reduce FMR linewidth ΔH. M. Ishaque et al. studied the FMR linewidth ΔH of NiY2xFe2-2xO4 (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) at 9.5 GHz [16]. The results showed that as the Y substitution increases, the specific saturation magnetization σs
Corresponding author. E-mail address: [email protected] (R. Guo).
https://doi.org/10.1016/j.ceramint.2019.12.133 Received 17 November 2019; Received in revised form 11 December 2019; Accepted 13 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Rongdi Guo, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.133
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Fig. 1. SEM micrographs of NCZ ferrites with CCTO doping concentrations from 0.0 wt% to 3.5 wt%.
gradually decreased, the coercive force Hc gradually increased, and the ΔH first decreased and then increased with a minimum of 281Oe at x = 0.06. Briefly, although many studies have been promoted, it is difficult for NCZ ferrites adopted for non-reciprocal devices and components to meet the requirements of high saturation magnetization 4πMs, low coercivity Hc, and low FMR linewidth ΔH. In consideration of CaCu3Ti4O12 (CCTO) is a type of giant dielectric material with large dielectric constant and high thermal stability, herein, CCTO additive is introduced in NCZ ferrites to explore the influence on micromorphology, static magnetic properties, and FMR linewidth ΔH.
Fig. 2. a) Average grain size D and standard deviations, and b) XRD patterns of NCZ ferrites with CCTO doping concentrations from 0.0 wt% to 3.5 wt%.
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189.7 ± 21.7 μm, respectively. It is a typical example that the inferior uniformity microstructure consists of giant grains and smaller grains, showing that the growth of large grains is at the expense of small grains [23,24]. However, as the CCTO concentrations surpass 2.0 wt%, the grains become more uniform and the average grain sizes are stabilized at 2.2 μm. The XRD patterns of NCZ ferrites with 0.0 wt% to 3.5 wt% CCTO doping are demonstrated in Fig. 2(b). Typical diffraction peaks of (220), (311), (222), (400), (422), (511), (440), (531), (620), and (533) are indexed for NCZ spinel crystal structure compared with the No. 08–0234 JCPDS card, confirming the formation of the pure spinel crystal phase. The relationships between the density d, static magnetic properties, and CCTO doping amounts of NCZ ferrites are presented in Fig. 3. As CCTO doping concentration in NCZ ferrites increases, the density d in Fig. 3(a) monotonically decreases because of the inhomogeneous microstructure. The variation of saturation magnetic induction Bs in Fig. 3(b) is similar to the density d, and the variation can be explained as Eq. (1),
Fig. 3. The variation of density d and static magnetic properties of NCZ ferrites versus CCTO doping concentrations from 0.0 wt% to 3.5 wt% for a) the density d, b) saturation magnetic induction Bs, c) remanence Br, and d) coercive force Hc.
r
Bs = Bs (0)⋅
dm ⎛ T ⋅ 1− ⎞ dx ⎝ Tc ⎠ ⎜
⎟
(1)
Thus, the Bs of the NCZ ferrite is related to the actual sintered density dm of the material in addition to its intrinsic parameters: saturation magnetic induction Bs(0) at absolute zero, theoretical density dx, and Tc of the material. The test temperature T is related to the fact that the Bs of the material is positively correlated with the actual sintered density. When the amount of CCTO added was increased from 0.0 wt% to 0.5 wt %, the density d and saturation magnetic induction Bs of the sample slightly increased. When the amount of CCTO added is greater than 0.5 wt%, both the density d and the saturation magnetic induction Bs decrease. Combined with SEM analysis, CCTO can effectively promote grain growth during sintering, while the excessive concentration of CCTO can inhibit grain growth and decrease ferrite density d and saturation magnetic induction Bs. Fig. 3(c) shows the relationship between the remanence Br of NCZ ferrites and the CCTO doping concentrations. As the CCTO concentration in NCZ ferrites increases, the remanence Br first decreases for CCTO concentrations less than 1.5 wt% and then increases. In polycrystals, it is assumed that the single easy magnetization axis of the grains is uniformly distributed. When polycrystals are magnetized and saturated in a certain direction, as the external magnetic field is reduced to zero, the magnetic moment in each crystal does not change from the saturation magnetization direction back to its original easy magnetization axis direction, but only to those easy magnetization axis directions closest to the external magnetic field direction because of the existence of irreversible magnetization [25]. However, stray stress, impurities, grain boundaries, and crystal defect result in the reduction of the remanence Br via the demagnetizing field. According to the SEM results in Figs. 1 and 2(a), for the NCZ ferrite with 1.5 wt% CCTO, there are many pores in giant grains, leading to the demagnetizing field resulting in the reduction of the remanence Br. The relationship of coercivity Hc versus CCTO doping concentrations from 0.0 wt% to 3.5 wt% is illustrated in Fig. 3(d). As the CCTO concentrations increase, the coercive force Hc first decreases and then increases. In addition, the coercivity Hc is the maximum critical field of the irreversible domain wall displacement caused by the reverse domain in the magnetization process, which is inversely proportional to the grain size D. When the CCTO content is 1.5 wt%, the grain size reaches a maximum of 189.7 ± 21.7 μm. However, the single-domain critical size is about 2.9 μm for NiZn ferrites according to the report by P. J. Vanderzagg et al. [26], and the grains are multi-domain states for NCZ ferrites with CCTO concentrations less than but including 1.5 wt%, and are single-domain states for NCZ ferrites with CCTO concentration greater than 1.5 wt%. Thus, the NCZ ferrite with 1.5 wt% CCTO has the
2. Materials and methods NCZ ferrites with the main formula of Ni0.52Cu0.1Zn0.40Fe1.98O4 were fabricated by oxide ceramic process. The high purity Fe2O3, ZnO, NiO and CuO powders were mixed with the presented stoichiometric ratio for 6 h using a planetary ball mill. After drying at 80 °C, the mixture was calcined at 900 °C for 2.5 h. Then, 0.1 wt% Bi2O3 and 0.0–3.5 wt% CaCu3Ti4O12 additives were added to the powders. After grinding again, 12 wt% PVA was applied for the powders to prepare particles for molding samples. Then, the particles having a uniform particle size were pressed at 8 MPa into ring-shaped green-pressings (outer diameter 18 mm × inner diameter 8 mm × height 5 mm). Lastly, the green-pressings body was sintered at 1040 °C for 3 h. Small spheres (diameter 0.8 mm) were shaped from partial sintered toroidal samples for testing of specific saturation magnetization σs and FMR linewidth ΔH. The MAXIMA XRD-7000 (XRD, Cu target, Ka radiation, 40 kV, and 30 mA) was adopted to characterize the crystalline phase with a diffraction angle from 25° to 75°. The cross-section micromorphology was obtained by a JEOL JSM-6490L scanning electron microscope and average grain sizes D were measured with the interception method. The static magnetic performance at 1 kHz and 1600A/m were measured by the B–H analyzer of IWATSU SY-8232. The FMR linewidth ΔH was obtained by the waveguide perturbation method that adopted the N5227A Agilent network analyzer (VNA-FMR), electromagnet, and Gauss meter combination test. Lake Shore 8604 vibrating sample magnetometer VSM has been applied for the magnetic hysteresis loop and the magnetization curve with an external magnetic field H from −5000Oe to 5000Oe and from 0Oe to 5000Oe, respectively.
3. Results and discussion The SEM micromorphologies of NCZ ferrites with 0.0 wt% to 3.5 wt % CCTO doping amounts are exhibited in Fig. 1. The average grain size D shown in Fig. 2(a) first increases and then decreases from 2.1 ± 0.446 μm to 189.7 ± 21.7 μm because of the promotion for grain growth via the solid solution of copper. As the CCTO concentration in NCZ ferrites increases, it is confirmed that the liquid phase mass transfer effect of Bi2O3 and solid solution of CuO at the grain boundary results in promoting or hindering the grain growth of NCZ ferrites. In particular, for NCZ ferrites with 1.0 wt% and 1.5 wt% CCTO, the average grain sizes and standard deviations are 159.8 ± 21.4 μm and 3
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Fig. 4. a) – h) FMR absorption fitted (Lorentz Fit and Gauss Fit) spectra of NCZ ferrites with CCTO doping concentrations from 0 wt% to 100 wt%. The blue color circles are experimental results. The magenta line and gray line are Lorentz fit and Gauss fit spectra, respectively. i) FMR linewidth of NCZ ferrites for Lorentz fit and Gauss fit spectra. j) R-squared of NCZ ferrites for Lorentz fit and Gauss fit spectra, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 5. The magnetic hysteresis loop and magnetization curve of NCZ ferrites versus CCTO doping concentrations from 0.0 wt% to 3.5 wt% for a) magnetic hysteresis loop (external magnetic field H from −5000Oe to 5000Oe), b) magnetic hysteresis loop (magnetic field H from 2000Oe to 4000Oe), c) magnetization curve (external magnetic field H from 0Oe to 5000Oe), and d) magnetization curve experimental and fitting results for NCZ with 1.5 wt% CCTO. e) the absolute value of first-order crystalline anisotropy constant |K1|, and f) the porosity P of NCZ ferrites versus CCTO doping concentrations from 0.0 wt% to 3.5 wt%.
where ΔHi, which could be ignored for only a few Oe, denotes the single-crystal intrinsic linewidth; ΔHa represents the crystalline anisotropy induced linewidth, and ΔHp indicates the porosity induced linewidth. Therefore, compared with the ΔHi, ΔHa and ΔHp should be considered for ΔH. According to the spin-wave narrowing theory of E. Schlömann, ΔHa could be proposed as Eq. (3) [28], (3)
Ha = 4 K1 /3μ0 Ms
(4)
where Ha is the crystalline anisotropy field, 4πMs denotes the saturation magnetization, G (ω, ωi) indicates the frequency function, and K1 represents the first-order crystalline anisotropy constant. Correspondingly, ΔHp can be expressed as Eq. (5),
Fig. 6. Separation calculation of FMR linewidth ΔH of NCZ ferrites with CCTO concentrations from 0.0 wt% to 3.5 wt%.
ΔHp = 0.17P 4πMs
minimum 31A/m of coercive force Hc. The absorption spectra of FMR are presented in Fig. 4(a) – (h). To analyze the FMR absorption spectra, nonlinear fitting calculation have been considered with Lorentz and Gauss models for the test results. Simultaneously, Fig. 4(i) and (j) exhibit the FMR linewidth ΔH and Rsquared in respect of Lorentz and Gauss fitting models. As the CCTO concentration in NCZ ferrites increases, the FMR linewidth ΔH first decreases and then increases from 147Oe to 307Oe. Obviously, the Rsquared values (> 0.960) of FMR absorption spectra fitted by the Lorentz model are better than that (> 0.934) fitted by the Gauss model, and this result aggress with that from other investigations [7]. On the basis of the composition of the FMR linewidth ΔH, ΔH would be described as the following [27],
ΔH = ΔHi + ΔHa + ΔHP
ΔHa = (2.07Ha2/4πMs ) G (ω, ωi )
2 ω (3 cos2 θ0 + 1.4) /cos θ0 ωi
(5)
where P represents the porosities of NCZ ferrites, θ0 represents the angle between wave vector k and the static internal resonance field, and generally, cosθ0 is approximately equal to 1/3 for spherical samples [29]. To obtain the crystalline anisotropy linewidth ΔHa and porosity induced linewidth ΔHp, which are related to crystalline anisotropy constant K1 and porosity P, respectively, the magnetic hysteresis loops, magnetization curves, and porosity have been tested. The magnetic hysteresis loops have been shown in Fig. 5(a) and (b) for magnetic fields from −5000Oe to 5000Oe and 2000Oe to 4000Oe, respectively. As the CCTO doping concentration in NCZ ferrites increases, the saturation magnetization 4πMs of NCZ ferrites monotonically decreases from 5484Gs to 4819Gs. The magnetization curves have been
(2) 5
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and components in array phase radars.
illustrated in Fig. 5(c), and the variation trend of saturation magnetization 4πMs are exactly the same as that of magnetic hysteresis loops. The law of approach to saturation is applied for nonlinear fitting calculation of the first-order crystalline anisotropy constant K1, as shown in Eq. (6) and Eq. (7) [27],
a b MH = MS ⎛1 − − 2 − ⋅⋅⋅⋅⋅⋅⎞ + χP H H H ⎝ ⎠ b=
8 K12 105 MS2
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.
(6)
Acknowledgments (7)
The authors are grateful for the financial support from the National Natural Science Foundation of China under Grant No.51472045 and No.51772046, and the Fundamental Research Funds for the Central Universities ZYGX2018J038, ZYGX2018J040, and ZYGX2018J042.
where MH represents magnetization corresponding to external magnetic field H, a and b denote the magnitude of magnetization resistance in the process of technical magnetization, χP stands for the paramagnetic susceptibility. As typical of NCZ ferrite with 1.5 wt% CCTO, the tested magnetization curves exhibited in Fig. 5(d) were fitted by the law of approach to saturation, and the fitted result coincides well with the tested results. As described, the absolute value of the first-order crystalline anisotropy constant |K1| of NCZ ferrites calculated is shown in Fig. 5(e), and as the CCTO doping concentration in NCZ ferrites increases, |K1| monotonically increases from 2.816 × 103 J/m3 to 4.62 × 103 J/m3, which are similar to those in previous investigations [15,21]. As known that the magnetocrystalline anisotropy constant K1 of ferrites depends on the properties and symmetry of magnetic ions. With regard to the occupation of Fe3+ ions at A and B sites, the magnetocrystalline anisotropy constant KFeA and KFeB is a positive value and a negative value, respectively. It is worth noting that |KFeB| is greater than |KFeA| [25]. In addition, superposition of magnetocrystalline anisotropy of individual magnetic ions comprises macroscopic magnetocrystalline anisotropy. The absolute value of crystalline anisotropy constant |K1| of Ni0.3Zn0.45Fe2.25O4 and CuFe2O4 are 1.7 × 103 J/m3 and 6.0 × 103 J/m3, respectively [25]. Thus, as the CCTO doping concentration in NCZ ferrites increases, the formation of CuFe2O4 on account of Cu2+ originated from CaCu3Ti4O12 would result in the increasing of |K1| for NCZ ferrites. Besides, for the sake of calculation of porosity induced linewidth ΔHp, the porosity P of NCZ ferrites are shown in Fig. 5(f), as the CCTO concentration in NCZ ferrites increases, the porosity P monotonically increases, which is opposite to the various tendencies of density d in Fig. 3(a). Consequently, by combining the crystalline anisotropy constant |K1| and porosity P, the crystalline anisotropy linewidth ΔHa and porosity induced linewidth ΔHp could be calculated by Eq. (2) – (4). Fig. 6 shows that as the CCTO concentration in NCZ ferrites increasing, ΔHa has a monotonically increasing trend from 52Oe to 85Oe; however, ΔHp first decreases and then increases with a minimum of 147Oe. For a certain CCTO doping content, although both the crystalline anisotropy linewidth ΔHa affected by |K1| and porosity induced linewidth ΔHp affected by P contribute to ΔH, ΔH mainly originates from the porosity induced line broadening contribution ΔHp.
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4. Conclusions It is an effective approach to introduce Bi2O3 and CCTO composite additives to NCZ ferrites to regulate the microstructure and magnetic performances. For 0.1 wt% Bi2O3, as the CCTO concentration in NCZ ferrites increase from 0.0 wt% to 3.5 wt%, saturation magnetization 4πMs monotonically decreases from 5484Gs to 4819Gs, and both coercive force Hc and FMR linewidth ΔH first decrease and then increase with minimums of 31A/m and 147Oe, respectively. The separation calculation of FMR linewidth ΔH illustrates the contribution of ΔH that mainly originates from the porosity induced line broadening contribution ΔHp. The sample with 0.1 wt% Bi2O3 and 1.5 wt% CCTO has optimum comprehensive performances with high saturation magnetization 4πMs (5321Gs), remanence Br (200 mT), low coercivity Hc (31A/m), and low FMR linewidth ΔH (147Oe). This investigation presents a referential experience for microwave ferrites adopted for devices 6
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Effects of Bi2O3–CaCu3Ti4O12 composite additives on micromorphology, static magnetic properties, and FMR linewidth ΔH of NCZ ferrites Rongdi Guoa,∗, Xiaofeng Zhanga, Zhong Yua,b, Ke Suna,b, Xiaona Jianga,b, Chuanjian Wua, Zhongwen Lana,b a b
School of Materials and Energy, 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: NiCuZn ferrites Bi2O3–CaCu3Ti4O12 composite additives Micromorphology Ferromagnetic resonance FMR linewidth Static magnetic properties
NiCuZn (NCZ) ferrites have been widely used in non-reciprocal microwave/millimeter ferrite devices, such as circulators. With the development of microwave/millimeter devices and components to high frequency, miniaturization, and lightweight applications, NCZ ferrites have needed to satisfy the essential requirements of high saturation magnetization 4πMs, low coercivity Hc, and low ferromagnetic resonance (FMR) linewidth ΔH. Herein, 0.1 wt% Bi2O3 and 0.0–3.5 wt% CaCu3Ti4O12 (CCTO) composite additives were introduced to NCZ ferrites, the influence of Bi2O3–CCTO composite additives on micromorphology, static magnetic properties, and FMR linewidth ΔH of NCZ ferrites have been demonstrated in detail. The results show that increasing the CCTO amounts in NCZ ferrites, saturation magnetization 4πMs monotonically decreases from 5484Gs to 4819Gs, and both coercive force Hc and FMR linewidth ΔH first decreases and then increases with minimums of 31A/m and 147Oe, respectively. In addition, the theory of spin-wave narrowing and the law of approach to saturation have been adopted for the separation calculation of FMR linewidth ΔH to the crystalline anisotropy linewidth ΔHa and porosity induced linewidth ΔHp. The sample with 0.1 wt% Bi2O3 and 1.5 wt% CCTO possesses high saturation magnetization 4πMs (5321Gs), remanence Br (200 mT), low coercivity Hc (31A/m), and low FMR linewidth ΔH (147Oe). The state-of-the-art NCZ ferrites with outstanding performances manifest significant applied potency for microwave/millimeter devices and components in phase array radar systems.
1. Introduction The ferrites circulator that realizes the non-reciprocal transmission of electromagnetic wave signal is one of the important parts of T/R components in phase array radar systems [1,2]. To meet the operational requirements of modern radar, ferrites circulators are desired to have certain characteristics, such as high frequency, miniaturization, high performance, and high reliability. However, the properties of ferrites play significant roles in scattering parameters (S-parameters) of circulators. Remarkably, insertion losses S21 of ferrites circulators are closely related to the ferromagnetic resonance (FMR) linewidth ΔH of ferrites. NCZ ferrites are extensively applied in non-reciprocal microwave/millimeter devices and components with broadly adjustable saturation magnetization 4πMs, high density d, and high Curie temperature Tc; low coercive force Hc, low dielectric loss tangent tanδε, and low FMR linewidth ΔH. In the process of ferrite preparation, despite the main formula basically determining the intrinsic properties of the material, appropriate
∗
additives could control the solid-state reaction level, composition of the crystalline phase, grain size, density, porosity, saturation magnetization, resistivity, and other performances of the ferrites [3–22]. In this regard, according to the different compositions, additives could be divided into three categories: 1) the segregation of additives at the grain boundary, such as CaO, and SiO2; 2) the additives with low melting point, resulting in liquid phase sintering, and a solid solution in a spinel structure, such as Bi2O3, MoO3, and V2O5 [3,5,7]; 3) diffusion into crystal lattices, affecting the ions occupation and distribution of the ferrites, such as TiO2, Co2O3, MnO2, and Al2O3 [4,14]. Yan Yang et al. fabricated the Ni0.2Cu0.2Zn0.6Fe2O4 ferrite ceramics with enhanced gyromagnetic properties for LTCC applications at 900 °C, 925 °C, and 950 °C [7]. Composite additives of 0.5 wt% MnO2 and x wt% Bi2O3 (x = 0.0–3.0) have been applied for reducing sintering temperature via promoting grain growth and help reduce FMR linewidth ΔH. M. Ishaque et al. studied the FMR linewidth ΔH of NiY2xFe2-2xO4 (x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10, 0.12) at 9.5 GHz [16]. The results showed that as the Y substitution increases, the specific saturation magnetization σs
Corresponding author. E-mail address: [email protected] (R. Guo).
https://doi.org/10.1016/j.ceramint.2019.12.133 Received 17 November 2019; Received in revised form 11 December 2019; Accepted 13 December 2019 0272-8842/ © 2019 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Rongdi Guo, et al., Ceramics International, https://doi.org/10.1016/j.ceramint.2019.12.133
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Fig. 1. SEM micrographs of NCZ ferrites with CCTO doping concentrations from 0.0 wt% to 3.5 wt%.
gradually decreased, the coercive force Hc gradually increased, and the ΔH first decreased and then increased with a minimum of 281Oe at x = 0.06. Briefly, although many studies have been promoted, it is difficult for NCZ ferrites adopted for non-reciprocal devices and components to meet the requirements of high saturation magnetization 4πMs, low coercivity Hc, and low FMR linewidth ΔH. In consideration of CaCu3Ti4O12 (CCTO) is a type of giant dielectric material with large dielectric constant and high thermal stability, herein, CCTO additive is introduced in NCZ ferrites to explore the influence on micromorphology, static magnetic properties, and FMR linewidth ΔH.
Fig. 2. a) Average grain size D and standard deviations, and b) XRD patterns of NCZ ferrites with CCTO doping concentrations from 0.0 wt% to 3.5 wt%.
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189.7 ± 21.7 μm, respectively. It is a typical example that the inferior uniformity microstructure consists of giant grains and smaller grains, showing that the growth of large grains is at the expense of small grains [23,24]. However, as the CCTO concentrations surpass 2.0 wt%, the grains become more uniform and the average grain sizes are stabilized at 2.2 μm. The XRD patterns of NCZ ferrites with 0.0 wt% to 3.5 wt% CCTO doping are demonstrated in Fig. 2(b). Typical diffraction peaks of (220), (311), (222), (400), (422), (511), (440), (531), (620), and (533) are indexed for NCZ spinel crystal structure compared with the No. 08–0234 JCPDS card, confirming the formation of the pure spinel crystal phase. The relationships between the density d, static magnetic properties, and CCTO doping amounts of NCZ ferrites are presented in Fig. 3. As CCTO doping concentration in NCZ ferrites increases, the density d in Fig. 3(a) monotonically decreases because of the inhomogeneous microstructure. The variation of saturation magnetic induction Bs in Fig. 3(b) is similar to the density d, and the variation can be explained as Eq. (1),
Fig. 3. The variation of density d and static magnetic properties of NCZ ferrites versus CCTO doping concentrations from 0.0 wt% to 3.5 wt% for a) the density d, b) saturation magnetic induction Bs, c) remanence Br, and d) coercive force Hc.
r
Bs = Bs (0)⋅
dm ⎛ T ⋅ 1− ⎞ dx ⎝ Tc ⎠ ⎜
⎟
(1)
Thus, the Bs of the NCZ ferrite is related to the actual sintered density dm of the material in addition to its intrinsic parameters: saturation magnetic induction Bs(0) at absolute zero, theoretical density dx, and Tc of the material. The test temperature T is related to the fact that the Bs of the material is positively correlated with the actual sintered density. When the amount of CCTO added was increased from 0.0 wt% to 0.5 wt %, the density d and saturation magnetic induction Bs of the sample slightly increased. When the amount of CCTO added is greater than 0.5 wt%, both the density d and the saturation magnetic induction Bs decrease. Combined with SEM analysis, CCTO can effectively promote grain growth during sintering, while the excessive concentration of CCTO can inhibit grain growth and decrease ferrite density d and saturation magnetic induction Bs. Fig. 3(c) shows the relationship between the remanence Br of NCZ ferrites and the CCTO doping concentrations. As the CCTO concentration in NCZ ferrites increases, the remanence Br first decreases for CCTO concentrations less than 1.5 wt% and then increases. In polycrystals, it is assumed that the single easy magnetization axis of the grains is uniformly distributed. When polycrystals are magnetized and saturated in a certain direction, as the external magnetic field is reduced to zero, the magnetic moment in each crystal does not change from the saturation magnetization direction back to its original easy magnetization axis direction, but only to those easy magnetization axis directions closest to the external magnetic field direction because of the existence of irreversible magnetization [25]. However, stray stress, impurities, grain boundaries, and crystal defect result in the reduction of the remanence Br via the demagnetizing field. According to the SEM results in Figs. 1 and 2(a), for the NCZ ferrite with 1.5 wt% CCTO, there are many pores in giant grains, leading to the demagnetizing field resulting in the reduction of the remanence Br. The relationship of coercivity Hc versus CCTO doping concentrations from 0.0 wt% to 3.5 wt% is illustrated in Fig. 3(d). As the CCTO concentrations increase, the coercive force Hc first decreases and then increases. In addition, the coercivity Hc is the maximum critical field of the irreversible domain wall displacement caused by the reverse domain in the magnetization process, which is inversely proportional to the grain size D. When the CCTO content is 1.5 wt%, the grain size reaches a maximum of 189.7 ± 21.7 μm. However, the single-domain critical size is about 2.9 μm for NiZn ferrites according to the report by P. J. Vanderzagg et al. [26], and the grains are multi-domain states for NCZ ferrites with CCTO concentrations less than but including 1.5 wt%, and are single-domain states for NCZ ferrites with CCTO concentration greater than 1.5 wt%. Thus, the NCZ ferrite with 1.5 wt% CCTO has the
2. Materials and methods NCZ ferrites with the main formula of Ni0.52Cu0.1Zn0.40Fe1.98O4 were fabricated by oxide ceramic process. The high purity Fe2O3, ZnO, NiO and CuO powders were mixed with the presented stoichiometric ratio for 6 h using a planetary ball mill. After drying at 80 °C, the mixture was calcined at 900 °C for 2.5 h. Then, 0.1 wt% Bi2O3 and 0.0–3.5 wt% CaCu3Ti4O12 additives were added to the powders. After grinding again, 12 wt% PVA was applied for the powders to prepare particles for molding samples. Then, the particles having a uniform particle size were pressed at 8 MPa into ring-shaped green-pressings (outer diameter 18 mm × inner diameter 8 mm × height 5 mm). Lastly, the green-pressings body was sintered at 1040 °C for 3 h. Small spheres (diameter 0.8 mm) were shaped from partial sintered toroidal samples for testing of specific saturation magnetization σs and FMR linewidth ΔH. The MAXIMA XRD-7000 (XRD, Cu target, Ka radiation, 40 kV, and 30 mA) was adopted to characterize the crystalline phase with a diffraction angle from 25° to 75°. The cross-section micromorphology was obtained by a JEOL JSM-6490L scanning electron microscope and average grain sizes D were measured with the interception method. The static magnetic performance at 1 kHz and 1600A/m were measured by the B–H analyzer of IWATSU SY-8232. The FMR linewidth ΔH was obtained by the waveguide perturbation method that adopted the N5227A Agilent network analyzer (VNA-FMR), electromagnet, and Gauss meter combination test. Lake Shore 8604 vibrating sample magnetometer VSM has been applied for the magnetic hysteresis loop and the magnetization curve with an external magnetic field H from −5000Oe to 5000Oe and from 0Oe to 5000Oe, respectively.
3. Results and discussion The SEM micromorphologies of NCZ ferrites with 0.0 wt% to 3.5 wt % CCTO doping amounts are exhibited in Fig. 1. The average grain size D shown in Fig. 2(a) first increases and then decreases from 2.1 ± 0.446 μm to 189.7 ± 21.7 μm because of the promotion for grain growth via the solid solution of copper. As the CCTO concentration in NCZ ferrites increases, it is confirmed that the liquid phase mass transfer effect of Bi2O3 and solid solution of CuO at the grain boundary results in promoting or hindering the grain growth of NCZ ferrites. In particular, for NCZ ferrites with 1.0 wt% and 1.5 wt% CCTO, the average grain sizes and standard deviations are 159.8 ± 21.4 μm and 3
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Fig. 4. a) – h) FMR absorption fitted (Lorentz Fit and Gauss Fit) spectra of NCZ ferrites with CCTO doping concentrations from 0 wt% to 100 wt%. The blue color circles are experimental results. The magenta line and gray line are Lorentz fit and Gauss fit spectra, respectively. i) FMR linewidth of NCZ ferrites for Lorentz fit and Gauss fit spectra. j) R-squared of NCZ ferrites for Lorentz fit and Gauss fit spectra, respectively. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
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Fig. 5. The magnetic hysteresis loop and magnetization curve of NCZ ferrites versus CCTO doping concentrations from 0.0 wt% to 3.5 wt% for a) magnetic hysteresis loop (external magnetic field H from −5000Oe to 5000Oe), b) magnetic hysteresis loop (magnetic field H from 2000Oe to 4000Oe), c) magnetization curve (external magnetic field H from 0Oe to 5000Oe), and d) magnetization curve experimental and fitting results for NCZ with 1.5 wt% CCTO. e) the absolute value of first-order crystalline anisotropy constant |K1|, and f) the porosity P of NCZ ferrites versus CCTO doping concentrations from 0.0 wt% to 3.5 wt%.
where ΔHi, which could be ignored for only a few Oe, denotes the single-crystal intrinsic linewidth; ΔHa represents the crystalline anisotropy induced linewidth, and ΔHp indicates the porosity induced linewidth. Therefore, compared with the ΔHi, ΔHa and ΔHp should be considered for ΔH. According to the spin-wave narrowing theory of E. Schlömann, ΔHa could be proposed as Eq. (3) [28], (3)
Ha = 4 K1 /3μ0 Ms
(4)
where Ha is the crystalline anisotropy field, 4πMs denotes the saturation magnetization, G (ω, ωi) indicates the frequency function, and K1 represents the first-order crystalline anisotropy constant. Correspondingly, ΔHp can be expressed as Eq. (5),
Fig. 6. Separation calculation of FMR linewidth ΔH of NCZ ferrites with CCTO concentrations from 0.0 wt% to 3.5 wt%.
ΔHp = 0.17P 4πMs
minimum 31A/m of coercive force Hc. The absorption spectra of FMR are presented in Fig. 4(a) – (h). To analyze the FMR absorption spectra, nonlinear fitting calculation have been considered with Lorentz and Gauss models for the test results. Simultaneously, Fig. 4(i) and (j) exhibit the FMR linewidth ΔH and Rsquared in respect of Lorentz and Gauss fitting models. As the CCTO concentration in NCZ ferrites increases, the FMR linewidth ΔH first decreases and then increases from 147Oe to 307Oe. Obviously, the Rsquared values (> 0.960) of FMR absorption spectra fitted by the Lorentz model are better than that (> 0.934) fitted by the Gauss model, and this result aggress with that from other investigations [7]. On the basis of the composition of the FMR linewidth ΔH, ΔH would be described as the following [27],
ΔH = ΔHi + ΔHa + ΔHP
ΔHa = (2.07Ha2/4πMs ) G (ω, ωi )
2 ω (3 cos2 θ0 + 1.4) /cos θ0 ωi
(5)
where P represents the porosities of NCZ ferrites, θ0 represents the angle between wave vector k and the static internal resonance field, and generally, cosθ0 is approximately equal to 1/3 for spherical samples [29]. To obtain the crystalline anisotropy linewidth ΔHa and porosity induced linewidth ΔHp, which are related to crystalline anisotropy constant K1 and porosity P, respectively, the magnetic hysteresis loops, magnetization curves, and porosity have been tested. The magnetic hysteresis loops have been shown in Fig. 5(a) and (b) for magnetic fields from −5000Oe to 5000Oe and 2000Oe to 4000Oe, respectively. As the CCTO doping concentration in NCZ ferrites increases, the saturation magnetization 4πMs of NCZ ferrites monotonically decreases from 5484Gs to 4819Gs. The magnetization curves have been
(2) 5
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and components in array phase radars.
illustrated in Fig. 5(c), and the variation trend of saturation magnetization 4πMs are exactly the same as that of magnetic hysteresis loops. The law of approach to saturation is applied for nonlinear fitting calculation of the first-order crystalline anisotropy constant K1, as shown in Eq. (6) and Eq. (7) [27],
a b MH = MS ⎛1 − − 2 − ⋅⋅⋅⋅⋅⋅⎞ + χP H H H ⎝ ⎠ b=
8 K12 105 MS2
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.
(6)
Acknowledgments (7)
The authors are grateful for the financial support from the National Natural Science Foundation of China under Grant No.51472045 and No.51772046, and the Fundamental Research Funds for the Central Universities ZYGX2018J038, ZYGX2018J040, and ZYGX2018J042.
where MH represents magnetization corresponding to external magnetic field H, a and b denote the magnitude of magnetization resistance in the process of technical magnetization, χP stands for the paramagnetic susceptibility. As typical of NCZ ferrite with 1.5 wt% CCTO, the tested magnetization curves exhibited in Fig. 5(d) were fitted by the law of approach to saturation, and the fitted result coincides well with the tested results. As described, the absolute value of the first-order crystalline anisotropy constant |K1| of NCZ ferrites calculated is shown in Fig. 5(e), and as the CCTO doping concentration in NCZ ferrites increases, |K1| monotonically increases from 2.816 × 103 J/m3 to 4.62 × 103 J/m3, which are similar to those in previous investigations [15,21]. As known that the magnetocrystalline anisotropy constant K1 of ferrites depends on the properties and symmetry of magnetic ions. With regard to the occupation of Fe3+ ions at A and B sites, the magnetocrystalline anisotropy constant KFeA and KFeB is a positive value and a negative value, respectively. It is worth noting that |KFeB| is greater than |KFeA| [25]. In addition, superposition of magnetocrystalline anisotropy of individual magnetic ions comprises macroscopic magnetocrystalline anisotropy. The absolute value of crystalline anisotropy constant |K1| of Ni0.3Zn0.45Fe2.25O4 and CuFe2O4 are 1.7 × 103 J/m3 and 6.0 × 103 J/m3, respectively [25]. Thus, as the CCTO doping concentration in NCZ ferrites increases, the formation of CuFe2O4 on account of Cu2+ originated from CaCu3Ti4O12 would result in the increasing of |K1| for NCZ ferrites. Besides, for the sake of calculation of porosity induced linewidth ΔHp, the porosity P of NCZ ferrites are shown in Fig. 5(f), as the CCTO concentration in NCZ ferrites increases, the porosity P monotonically increases, which is opposite to the various tendencies of density d in Fig. 3(a). Consequently, by combining the crystalline anisotropy constant |K1| and porosity P, the crystalline anisotropy linewidth ΔHa and porosity induced linewidth ΔHp could be calculated by Eq. (2) – (4). Fig. 6 shows that as the CCTO concentration in NCZ ferrites increasing, ΔHa has a monotonically increasing trend from 52Oe to 85Oe; however, ΔHp first decreases and then increases with a minimum of 147Oe. For a certain CCTO doping content, although both the crystalline anisotropy linewidth ΔHa affected by |K1| and porosity induced linewidth ΔHp affected by P contribute to ΔH, ΔH mainly originates from the porosity induced line broadening contribution ΔHp.
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4. Conclusions It is an effective approach to introduce Bi2O3 and CCTO composite additives to NCZ ferrites to regulate the microstructure and magnetic performances. For 0.1 wt% Bi2O3, as the CCTO concentration in NCZ ferrites increase from 0.0 wt% to 3.5 wt%, saturation magnetization 4πMs monotonically decreases from 5484Gs to 4819Gs, and both coercive force Hc and FMR linewidth ΔH first decrease and then increase with minimums of 31A/m and 147Oe, respectively. The separation calculation of FMR linewidth ΔH illustrates the contribution of ΔH that mainly originates from the porosity induced line broadening contribution ΔHp. The sample with 0.1 wt% Bi2O3 and 1.5 wt% CCTO has optimum comprehensive performances with high saturation magnetization 4πMs (5321Gs), remanence Br (200 mT), low coercivity Hc (31A/m), and low FMR linewidth ΔH (147Oe). This investigation presents a referential experience for microwave ferrites adopted for devices 6
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