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Crystal structure and physical properties of microwave sintered Sr1−xLaxFe12−xCuxO19 (x=0–0.5) ferrites for LTCC applications
Journal of Magnetism and Magnetic Materials 411 (2016) 62–67
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Crystal structure and physical properties of microwave sintered Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites for LTCC applications Long Peng n, Lezhong Li, Xiaoxi Zhong, Rui Wang, Xiaoqiang Tu Sichuan Province Key Laboratory of Information Materials and Devices Application, College of Optoelectronic Technology, Chengdu University of Information Technology, Chengdu 610225, Sichuan, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 November 2015 Received in revised form 7 March 2016 Accepted 11 March 2016 Available online 12 March 2016
The Sr1 xLaxFe12 xCuxO19 (x ¼ 0–0.5) ferrites for use in low temperature co-fired ceramics (LTCC) technology were prepared by improved solid phase method. Dependence of their crystal structure, magnetic, electrical, and dielectric properties on La–Cu substitution amount were investigated. Pure M-type phase is obtained for the ferrites with La–Cu substitution amount x r0.3. With x further increasing to 0.5, the multiphase structure is formed, where the M-type phase coexists with the LaFeO3 phase and Sr2FeO4 x phase. In single M-type phase region with x r0.3, the varied magnetic properties can be well explained considering the occupancy effects of La3 þ and Cu2 þ ions in magnetoplumbite structure. Their electrical transport behavior is found to be correlated with La–Cu substitution amount. Single metal–semiconductor (M–S) transition is clearly observed in the ferrites with a high doping amount as x ¼0.3. The polarization behavior from 1 kHz to 10 MHz follows the charge polarization mechanism, and the temperature dependence of real permittivity (ε′–T curves) and dielectric loss (tgδ–T curves) strongly suggests the complicated multiparticle polarization and relaxation. & 2016 Elsevier B.V. All rights reserved.
Keywords: M-type hexaferrites LTCC Intrinsic magnetic properties Resistivity Complex permittivity
1. Introduction On account of the potential applications in microwave low temperature co-fired ceramics (LTCC) ferrite devices especially for the nonreciprocal circulators and isolators, the M-type Ba/Srhexaferrites with magnetoplumbite structure have attracted intensive attention in recent years [1–6]. For the M-type hexaferrites, the Fe3 þ ions are believed to be located at five crystallographic sites, including the 12k↑, 2a↑, and 2b↑sites with up-spin configuration, and the 4f1↓and 4f2↓sites with down-spin configuration. Magnetically ordered state and hyperfine field of sublattice in the M-type hexaferrites can be adjusted by the replacement of Fe3 þ ions using ferromagnetic or non-magnetic metal ions, and their magnetic properties vary more or less as results [4– 11]. Electrical transport and dielectric behaviors of the M-type hexaferrites are strongly correlated with the hopping of electrons between Fe2 þ to Fe3 þ at octahedral sites (12k, 2a, and 4f2). Recent investigations indicated that the magnetic, electrical, and dielectric properties of the Sr-hexaferrite were improved by the combined substitution of Al–Ga, Zr–Cu, and Zr–Zn for Fe3 þ ions at octahedral sites [12–14]. As we know, the La–Co combined substitution can enhance the n
Corresponding author. E-mail address: [email protected] (L. Peng).
http://dx.doi.org/10.1016/j.jmmm.2016.03.041 0304-8853/& 2016 Elsevier B.V. All rights reserved.
magnetic properties of M-type hexaferrites even prepared at low sintering temperatures compatible with the LTCC technology [4–6]. Owing to the Cu2 þ has similar ion radius and replacement ability as Co2 þ but low cost, the La–Cu substituted Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites with appropriate amount of Bi2O3 B2O3 SiO2 ZnO (BBSZ) glass were fabricated by microwave sintering method at low sintering temperatures in this work, and their crystal structure, magnetic, electrical, and dielectric properties were systematically investigated.
2. Experimental Improved solid phase method was employed to fabricate the La–Cu substituted strontium ferrites with chemical composition of Sr1–xLaxFe12 xCuxO19 (x¼0–0.5). The starting materials of SrCO3 (98 wt%), Fe2O3 (99 wt%), La2O3 (99.99 wt%), and CuO (99.99 wt%) powders were weighed and mixed by planetary ball milling (PBM) at 300 r/min for 2 h with ethanol as medium. After drying and sifting, the mixed powders were calcinated at 1250 °C for 2 h and then crushed by PBM at 400 r/min for 4 h with 3–5 wt% BBSZ glass to get low temperature sintering powders with average particle size of 1.1–1.2 μm. These powders with 6 wt% polyvinyl alcohol (PVA) binder were pressed into disks with a diameter of 12 mm at 45 MPa, and then sintered at 850–870 °C for 30 min using microwave sintering furnace (GER-M2) with working frequency of
L. Peng et al. / Journal of Magnetism and Magnetic Materials 411 (2016) 62–67
2.45 GHz and output power of 2 kW. Annealing at 750 °C for 12 h was carried out for the sintered samples subsequently. Particle size of the powders was tested by laser particle analyzer (JL-1178). Sintering density was measured by precision density balance (FA2004J) with resolution of 0.1 mg based on the Archimedes method. Crystal structure was detected by X-ray diffraction (XRD, DX-2700) with Cu Kα radiation. Magnetization curves and magnetic hysteresis loops were tested by vibrating sample magnetometer (VSM, Versalab) with applied magnetic field from 20 to þ20 kOe. Effective magnetic anisotropy constant was calculated by the law of approach to saturation (LATS) [4]. Direct-current (DC) resistivity from room temperature to 750 °C was measured using precision power supply (Agilent, B2912A) with resolution of 10 fA/100 nV from 200 to þ 200 V. Complex permittivity from room temperature to 750 °C was analyzed by high frequency inductance capacitance resistance meter bridge (LCR, Wayne Kerr 6500 P) in a frequency range of 1 kHz– 10 MHz. A high temperature test system (GWM-200, Whpusite Instruments) was employed to help these electrical measurements performed at high temperatures with a heating rate of 2 °C/min.
3.1. Theoretical calculation Lattice constants a and c of the ferrites are calculated from the interplanar spacing dhkl corresponding to the (107) and (114) peaks of M-type phase by
(1)
where h, k, and l are the Miller indices. Moreover, the porosity P of the ferrites is calculated by
P=1−
ds dx
(2)
2M NA V
(3)
V = 0.8666a2c
(4)
dx =
where ds is the sintering density, dx is the x-ray density, M is the molar mass, V is the lattice volume, and NA is the Avogadro's number. Relationship of the direct-current resistivity ρd and drift mobility μd of the ferrites can be expressed as [12]
μd =
1 neρd
Fig. 1. XRD patterns of the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites.
activation energy, and k is the Boltzmann constant. According to the linear relation of lnρd 103/T in a special region, the activation energy for the hopping of electrons between Fe2 þ to Fe3 þ at octahedral sites can be calculated.
3. Results and discussion
−1/2 ⎛ 4 h2 + hk + k2 l2 ⎞ dhkl = ⎜ + 2⎟ 2 ⎝3 a c ⎠
3.2. Crystal structure Fig. 1 presents the XRD patterns of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites. The undoped ferrites exhibit single M-type phase structure with strong (107), (114), and (008) diffraction peaks according to the standard PDF file (no. 33-1340) of hexagonal structure with space group of P63/mmc (194). The cooperation of microwave sintering process with suitable addition of BBSZ efficiently promotes the reaction of SrCO3 and Fe2O3 to produce M-type hexaferrites at low sintering temperatures. The La–Cu substitution with x r0.3 just depresses the diffraction intensity of (008) and (006) peaks, and the pure M-type hexaferrites are still obtained. However, the coexistence of M-type phase with LaFeO3 phase and Sr2FeO4 x phase is observed when the substitution amount reaches 0.4 and 0.5, thus the multiphase structure is inevitably formed. Fig. 2 shows the lattice constants of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites. It is found that the length of a-axis changes very little with increasing substitution amount in single M-type phase region (xr 0.3) and has an average value of 5.875 Å, but the c-axis length decreases gradually from 23.036 to 23.002 Å with increasing x. This is different from the previous study, where the c-
(5)
and
n=
NA ds B M
(6)
where n is the concentration of charge carrier, e is the charge on electron, and B is the number of iron atoms in chemical formula. In the intrinsic excitation region, the direct-current resistivity of the ferrites obeys the well-known Arrhenius equation as [15]
⎛E ⎞ ρd = ρ0 exp ⎜ a ⎟ ⎝ kT ⎠ where
63
(7)
ρ0 is the temperature independent constant, Ea is the
Fig. 2. Lattice constants of the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites.
64
L. Peng et al. / Journal of Magnetism and Magnetic Materials 411 (2016) 62–67
density dx, sintering density ds, and porosity with La–Cu substitution amount, as shown in Fig. 3. The shrink of lattice volume for the M-type benefits the enhancement of dx and ds in the region with xr 0.3. But the porosity increases when the substitution amount exceeds 0.3 in multiphase region. 3.3. Magnetic properties
Fig. 3. X-ray density, sintering density, and porosity of the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites.
axis length increases with increasing La–Cu substitution amount [10]. Ionic radius difference of the La3 þ (1.22 Å)-Sr2 þ (1.32 Å) pair and Cu2 þ (0.78 Å)-Fe3 þ (0.67 Å) pair can compensate well with each other [5,9]. Hence, the lattice deformation of M-type structure with decreased c-axis length is more affected by the varied binding energy with partial La3 þ –Cu2 þ substitution for Sr2 þ –Fe3 þ . It is notable that the Fe3 þ –O2 distance and Fe3 þ –O2 bond angle depend on the variation of c-axis length, which possess important influence on the hyperfine field of sublattice [8]. With further increasing x, the LaFeO3 phase and Sr2FeO4 x phase appear with elevated lattice constants, indicating that the formation of multiphase structure produces non-ignorable crystal defects such as the oxygen vacancies. It agrees well with the variation of their x-ray
It is well known that the suitable substitution of La3 þ for Sr2 þ can raise the hyperfine fields of 12k site and 2b site for the M-type hexaferrites, and then the intrinsic magnetic properties are improved usually [7–9]. In addition, the Cu2 þ probably prefers to replace the Fe3 þ at 4f2 site considering that the 4f2 site is closer to the Sr-layer than other octahedral sites (12k and 2a) for electrovalent balance associated with the substitution of Sr2 þ by La3 þ in the Sr-layer [10]. Magnetic properties of the Sr1 xLaxFe12–xCuxO19 (x ¼0–0.5) ferrites as functions of La–Cu substitution amount in single M-type phase region (xr0.3) can be clearly explained in term of the occupancy effects of La3 þ and Cu2 þ in magnetoplumbite structure, as shown in Fig. 4. Seen from Fig. 4(a), the saturation magnetization Ms of the ferrites increases from 58.5 to 63.1 emu/g rapidly when the substitution amount increases from 0 to 0.2, and then decreases to 62.6 emu/g slightly with x further increasing to 0.3. The occupancy of Cu2 þ ions at 4f2↓site with x r0.2 weakens the magnetic moment in antiparallel direction on account of the lower magnetic moment of Cu2 þ than Fe3 þ , raising the total magnetic moment of M-type phase. But the excessive substitution of La3 þ –Cu2 þ for Sr2 þ –Fe3 þ can lead to the formation of localized spin canting, and the magnetic moment orientation of Fe3 þ deviates from a collinear structure for the ferrites [7]. It is suggested that the slight decrease of Ms for the ferrite with x ¼0.3 is the reflection of
Fig. 4. Magnetic properties of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites as functions of La–Cu substitution amount.
L. Peng et al. / Journal of Magnetism and Magnetic Materials 411 (2016) 62–67
considerable localized spin canting effect. Effect of La–Cu substitution amount on the effective magnetic anisotropy constant Keff is shown in Fig. 4(b). The coexistence of opposite effects of La3 þ substituting for Sr2 þ and Cu2 þ substituting for Fe3 þ on the magnetocrystalline anisotropy for the M-type hexaferrites is observed. It can be seen that the low doping with x ¼0.1 raises the Keff from 3.21 106 to 3.25 106 erg/cm3. But it decreases to 3.22 106 erg/cm3 gradually when the substitution amount increases from 0.1 to 0.3. In the low doping region with xr0.1, the function of La3 þ substituting for Sr2 þ is more significant than Cu2 þ substituting for Fe3 þ . Contrarily in the doping region of 0.1 rx r0.3, the influence of Cu2 þ substituting for Fe3 þ on the magnetocrystalline anisotropy is evidently strengthened. The occupancy of Cu2 þ at 4f2 site deteriorates the magnetocrystalline anisotropy of M-type structure due to the different contributions of a single Fe3 þ ion at each site to the anisotropy constant: 2b 44f2 42a 44f1 412k [16]. Magnetic anisotropy field Ha (2Keff/Ms) of the ferrites is obtained subsequently, as shown in Fig. 4(c). It is obvious that the Ha decreases from 18.38 to 16.95 kOe when La–Cu substitution amount increases from 0 to 0.3, mainly determining the fall of intrinsic coercivity Hci from 3.69 to 3.54 kOe presented in Fig. 4(d). Competition between the Ms and Keff dominates the reduction of Ha, which goes through three stages in single M-type phase region considering the variation of Ms and Keff with x carefully. In the doing region with x r0.1, the Ms increases more rapid than the Keff with increasing x (Ms↑↑, Keff↑), but it just inverses in the doping region of 0.2 rx r0.3 where the Keff decreases more rapid than the Ms with increasing x (Ms↓, Keff↓↓). In the middle doping region of 0.1 rx r0.2, the Ms increases but the Keff decreases with increasing x (Ms↑, Keff↓). Besides, the Ha has an obvious increment with increasing x in multiphase region (Ms↓↓, Keff↓), but the Hci still
65
declines swiftly. It indicates the enhanced effects of microstructure on the Hci owing to the relation of Hci ¼ αHa NMs/μ0, where α is the microstructure factor which increases with decreasing grain size, N is the demagnetization factor determined by many parameters one of which is the aspect ratio, and μ0 is the permeability of free space. This is high in accordance with the previous study [10], where the grain size of the La–Cu substituted Sr-hexaferrites starts to increase when the substitution amount exceeds 0.3. 3.4. Electrical properties Temperature dependence of direct-current resistivity (ρd–T curves) and drift mobility (μd-T curves) for the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.3) ferrites is shown in Fig. 5. It is found that the undoped and doped ferrites with La–Cu substitution amount xr 0.2 exhibit typical semiconducting behavior. However, the single metal–semiconductor (M–S) transition behavior with a resistivity peak at a specific temperature called transition temperature (Tp ¼63.8 °C) is observed in the ferrites with x¼ 0.3. The ρd is dominated by the metallic-conduction mechanism and increases with temperature in the T oTp region. Instead, the ρd in a wide temperature region (T4Tp) relies on the intrinsic semi-conduction mechanism and follows an exponential decay with temperature. The metallic-conduction region (m region) and semi-conduction region (s region) are associated with the scattering effect with lattice vibration and the hopping of electrons between Fe2 þ to Fe3 þ at octahedral sites, respectively. Hopping of electrons between Fe2 þ to Fe3 þ plays a role of intrinsic excitation for the ferrites actually. The intrinsic excitation is expected to raise the concentration of charge carrier n and drift mobility μd, and reduce the ρd in the “s region”. But the enhanced lattice vibration scattering effect with temperature impedes the electron
Fig. 5. Temperature dependence of direct-current resistivity and drift mobility for the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrites.
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Fig. 6. Effects of La–Cu substitution amount on the electrical properties of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrite.
conduction and the μd decreases correspondingly in the “m region”. Obviously, the M–S transition temperature used to separate the “m region” and “s region” depends on the competition between lattice vibration scattering and intrinsic excitation. Effects of La–Cu substitution amount on the electrical properties of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrite is given in Fig. 6. For the undoped and doped ferrites with La–Cu substitution amount xr0.2, the maximum value of direct-current resistivity occurs at room temperature (ρmax ¼ ρRT), which is obtained at the transition temperature Tp for the ferrites with x ¼0.3. The replacement of Cu2 þ for Fe3 þ at octahedral 4f2 site with x r0.2 decrease the concentration of Fe3 þ and correlative intrinsic excitation, leading to the increased direct-current resistivity ρRT/ρmax and activation energy Ea. Nevertheless, the spin canting effect is enhanced high enough to weaken the magnetic confinement to the electronic conduction in magnetoplumbite structure when the substitution amount further increases to 0.3, thus the ρRT/ρmax and Ea decreases accordingly. Typically, the ferrites with x ¼0.2 can provide high ρRT/ρmax of 16.8 109 Ω cm for microwave applications, as well as the μRT/μmin of 0.12 10 13 cm2 V 1 s-1 and Ea of 1.016 eV. 3.5. Dielectric properties It is observed that the complex permittivity of the Sr1–xLaxFe12 xCuxO19 (x¼0–0.3) ferrites varies with frequency and temperature strongly. Fig. 7 shows the temperature dependence of real permittivity (ε′–T curves) and dielectric loss (tgδ–T curves) for the undoped and doped ferrites with x¼ 0.2 from 1 kHz to 10 MHz.
In whole frequency range, their dielectric behavior obeys the traditional charge polarization mechanism for ferrite materials which happens since the frequency of the hopping of electron exchange between Fe2 þ to Fe3 þ is far from the frequency of alternatingcurrent field. Multiparticle polarization appears in these ferrites from the ε′–T and tgδ–T curves, especially for the low frequency measurement at 1 kHz and 10 kHz. Besides, the consistent motion of maximum ε′ and tgδ to the high temperature region with increasing frequency indicates the polarization relaxation phenomenon [17–19]. Table 1 lists the room temperature dielectric parameters of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrites at different frequency. Generally, the real permittivity ε′ decreases with increasing La–Cu substitution amount from 0 to 0.2 but increases with x further increasing to 0.3 again. There is a strong relationship between the electrical conduction mechanism and polarization mechanism in ferrite materials [20]. The formation of space charge polarization correlated with the electron displacement is impeded because the ionic replacement of Sr2 þ –Fe3 þ by La3 þ –Cu2 þ increases the resistivity and activation energy of the ferrites with appropriate La– Cu substitution. This is suggested to be responsible for the reduction of ε′ with increasing x and vice versa. The imaginary permittivity ε″ and dielectric loss tgδ of these ferrites are not very good, which have not been improved significantly by the La–Cu substitution especially for testing in high frequency above 100 kHz. Frequency dependence of ε′ and ε″ in microwave bands remains to be investigated further for understanding their electromagnetic loss mechanism.
4. Conclusions The Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) hexaferrites for use in LTCC circulators/isolators are successfully fabricated by microwave sintering with appropriate amount of BBSZ glass at 850–870 °C for 30 min. Analysis of the crystal structure, magnetic, electrical, and dielectric properties strongly denotes that the La3 þ –Cu2 þ ions can partially substitute the Sr2 þ –Fe3 þ ions in a doping region with xr0.3. The Cu2 þ ions are suggested to preferentially replace the Fe3 þ ions at octahedral 4f2 site according to their dependence of saturation magnetization Ms and effective magnetic anisotropy constant Keff on La–Cu substitution amount, and the reduction of magnetic anisotropy field Ha and intrinsic coercivity Hci is obtained as results. Their electrical transport and polarization behaviors in a frequency range of 1 kHz–10 MHz from room temperature to 750 °C are found to follow the conduction mechanism of semiconductors and charge polarization mechanism of ferrite
Fig. 7. Temperature dependence of real permittivity and dielectric loss for the undoped and doped ferrites with x¼ 0.2.
L. Peng et al. / Journal of Magnetism and Magnetic Materials 411 (2016) 62–67
Table 1 Room temperature dielectric parameters of the Sr1–xLaxFe12–xCuxO19 (x¼ 0–0.3) ferrites at different frequency. Test frequency
1 kHz
10 kHz
100 kHz
1 MHz
10 MHz
Real permittivity ε′, x ¼ 0.0 Real permittivity ε′, x ¼ 0.1 Real permittivity ε′, x ¼ 0.2 Real permittivity ε′, x ¼ 0.3 Imaginary permittivity ε″, x ¼0.0 Imaginary permittivity ε″, x ¼0.1 Imaginary permittivity ε″, x ¼0.2 Imaginary permittivity ε″, x ¼0.3 Dielectric loss tgδ, x ¼0.0 Dielectric loss tgδ, x ¼0.1 Dielectric loss tgδ, x ¼0.2 Dielectric loss tgδ, x ¼0.3
4.136 3.708 3.208 3.565 0.236 0.033 0.032 0.110 0.057 0.009 0.010 0.031
3.983 3.711 3.211 3.575 0.084 0.059 0.058 0.004 0.021 0.016 0.018 0.001
3.903 3.812 3.312 3.566 0.043 0.030 0.030 0.021 0.011 0.008 0.009 0.006
3.839 3.783 3.283 3.672 0.065 0.034 0.036 0.022 0.017 0.009 0.011 0.006
3.741 3.742 3.242 3.638 0.041 0.041 0.039 0.040 0.011 0.011 0.012 0.011
materials, respectively. The obtained ferrites with x ¼0.2 can provide optimal Ms of 63.1 emu/g, Ha of 17.03 kOe, and Hci of 3.59 kOe, and suitable room temperature direct-current resistivity ρd of 16.8 109 Ω cm for microwave applications.
Acknowledgments This work is supported by the National Natural Science Foundation of China under Grant no. 51502025, the Project of Science and Technology Supporting Plan in Sichuan Province of China under Grant no. 2016GZ0108, and the Open Fund of Sichuan Province Key Laboratory of Information Materials and Devices Application (Grant no. 2015Z001).
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Crystal structure and physical properties of microwave sintered Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites for LTCC applications Long Peng n, Lezhong Li, Xiaoxi Zhong, Rui Wang, Xiaoqiang Tu Sichuan Province Key Laboratory of Information Materials and Devices Application, College of Optoelectronic Technology, Chengdu University of Information Technology, Chengdu 610225, Sichuan, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 16 November 2015 Received in revised form 7 March 2016 Accepted 11 March 2016 Available online 12 March 2016
The Sr1 xLaxFe12 xCuxO19 (x ¼ 0–0.5) ferrites for use in low temperature co-fired ceramics (LTCC) technology were prepared by improved solid phase method. Dependence of their crystal structure, magnetic, electrical, and dielectric properties on La–Cu substitution amount were investigated. Pure M-type phase is obtained for the ferrites with La–Cu substitution amount x r0.3. With x further increasing to 0.5, the multiphase structure is formed, where the M-type phase coexists with the LaFeO3 phase and Sr2FeO4 x phase. In single M-type phase region with x r0.3, the varied magnetic properties can be well explained considering the occupancy effects of La3 þ and Cu2 þ ions in magnetoplumbite structure. Their electrical transport behavior is found to be correlated with La–Cu substitution amount. Single metal–semiconductor (M–S) transition is clearly observed in the ferrites with a high doping amount as x ¼0.3. The polarization behavior from 1 kHz to 10 MHz follows the charge polarization mechanism, and the temperature dependence of real permittivity (ε′–T curves) and dielectric loss (tgδ–T curves) strongly suggests the complicated multiparticle polarization and relaxation. & 2016 Elsevier B.V. All rights reserved.
Keywords: M-type hexaferrites LTCC Intrinsic magnetic properties Resistivity Complex permittivity
1. Introduction On account of the potential applications in microwave low temperature co-fired ceramics (LTCC) ferrite devices especially for the nonreciprocal circulators and isolators, the M-type Ba/Srhexaferrites with magnetoplumbite structure have attracted intensive attention in recent years [1–6]. For the M-type hexaferrites, the Fe3 þ ions are believed to be located at five crystallographic sites, including the 12k↑, 2a↑, and 2b↑sites with up-spin configuration, and the 4f1↓and 4f2↓sites with down-spin configuration. Magnetically ordered state and hyperfine field of sublattice in the M-type hexaferrites can be adjusted by the replacement of Fe3 þ ions using ferromagnetic or non-magnetic metal ions, and their magnetic properties vary more or less as results [4– 11]. Electrical transport and dielectric behaviors of the M-type hexaferrites are strongly correlated with the hopping of electrons between Fe2 þ to Fe3 þ at octahedral sites (12k, 2a, and 4f2). Recent investigations indicated that the magnetic, electrical, and dielectric properties of the Sr-hexaferrite were improved by the combined substitution of Al–Ga, Zr–Cu, and Zr–Zn for Fe3 þ ions at octahedral sites [12–14]. As we know, the La–Co combined substitution can enhance the n
Corresponding author. E-mail address: [email protected] (L. Peng).
http://dx.doi.org/10.1016/j.jmmm.2016.03.041 0304-8853/& 2016 Elsevier B.V. All rights reserved.
magnetic properties of M-type hexaferrites even prepared at low sintering temperatures compatible with the LTCC technology [4–6]. Owing to the Cu2 þ has similar ion radius and replacement ability as Co2 þ but low cost, the La–Cu substituted Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites with appropriate amount of Bi2O3 B2O3 SiO2 ZnO (BBSZ) glass were fabricated by microwave sintering method at low sintering temperatures in this work, and their crystal structure, magnetic, electrical, and dielectric properties were systematically investigated.
2. Experimental Improved solid phase method was employed to fabricate the La–Cu substituted strontium ferrites with chemical composition of Sr1–xLaxFe12 xCuxO19 (x¼0–0.5). The starting materials of SrCO3 (98 wt%), Fe2O3 (99 wt%), La2O3 (99.99 wt%), and CuO (99.99 wt%) powders were weighed and mixed by planetary ball milling (PBM) at 300 r/min for 2 h with ethanol as medium. After drying and sifting, the mixed powders were calcinated at 1250 °C for 2 h and then crushed by PBM at 400 r/min for 4 h with 3–5 wt% BBSZ glass to get low temperature sintering powders with average particle size of 1.1–1.2 μm. These powders with 6 wt% polyvinyl alcohol (PVA) binder were pressed into disks with a diameter of 12 mm at 45 MPa, and then sintered at 850–870 °C for 30 min using microwave sintering furnace (GER-M2) with working frequency of
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2.45 GHz and output power of 2 kW. Annealing at 750 °C for 12 h was carried out for the sintered samples subsequently. Particle size of the powders was tested by laser particle analyzer (JL-1178). Sintering density was measured by precision density balance (FA2004J) with resolution of 0.1 mg based on the Archimedes method. Crystal structure was detected by X-ray diffraction (XRD, DX-2700) with Cu Kα radiation. Magnetization curves and magnetic hysteresis loops were tested by vibrating sample magnetometer (VSM, Versalab) with applied magnetic field from 20 to þ20 kOe. Effective magnetic anisotropy constant was calculated by the law of approach to saturation (LATS) [4]. Direct-current (DC) resistivity from room temperature to 750 °C was measured using precision power supply (Agilent, B2912A) with resolution of 10 fA/100 nV from 200 to þ 200 V. Complex permittivity from room temperature to 750 °C was analyzed by high frequency inductance capacitance resistance meter bridge (LCR, Wayne Kerr 6500 P) in a frequency range of 1 kHz– 10 MHz. A high temperature test system (GWM-200, Whpusite Instruments) was employed to help these electrical measurements performed at high temperatures with a heating rate of 2 °C/min.
3.1. Theoretical calculation Lattice constants a and c of the ferrites are calculated from the interplanar spacing dhkl corresponding to the (107) and (114) peaks of M-type phase by
(1)
where h, k, and l are the Miller indices. Moreover, the porosity P of the ferrites is calculated by
P=1−
ds dx
(2)
2M NA V
(3)
V = 0.8666a2c
(4)
dx =
where ds is the sintering density, dx is the x-ray density, M is the molar mass, V is the lattice volume, and NA is the Avogadro's number. Relationship of the direct-current resistivity ρd and drift mobility μd of the ferrites can be expressed as [12]
μd =
1 neρd
Fig. 1. XRD patterns of the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites.
activation energy, and k is the Boltzmann constant. According to the linear relation of lnρd 103/T in a special region, the activation energy for the hopping of electrons between Fe2 þ to Fe3 þ at octahedral sites can be calculated.
3. Results and discussion
−1/2 ⎛ 4 h2 + hk + k2 l2 ⎞ dhkl = ⎜ + 2⎟ 2 ⎝3 a c ⎠
3.2. Crystal structure Fig. 1 presents the XRD patterns of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites. The undoped ferrites exhibit single M-type phase structure with strong (107), (114), and (008) diffraction peaks according to the standard PDF file (no. 33-1340) of hexagonal structure with space group of P63/mmc (194). The cooperation of microwave sintering process with suitable addition of BBSZ efficiently promotes the reaction of SrCO3 and Fe2O3 to produce M-type hexaferrites at low sintering temperatures. The La–Cu substitution with x r0.3 just depresses the diffraction intensity of (008) and (006) peaks, and the pure M-type hexaferrites are still obtained. However, the coexistence of M-type phase with LaFeO3 phase and Sr2FeO4 x phase is observed when the substitution amount reaches 0.4 and 0.5, thus the multiphase structure is inevitably formed. Fig. 2 shows the lattice constants of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites. It is found that the length of a-axis changes very little with increasing substitution amount in single M-type phase region (xr 0.3) and has an average value of 5.875 Å, but the c-axis length decreases gradually from 23.036 to 23.002 Å with increasing x. This is different from the previous study, where the c-
(5)
and
n=
NA ds B M
(6)
where n is the concentration of charge carrier, e is the charge on electron, and B is the number of iron atoms in chemical formula. In the intrinsic excitation region, the direct-current resistivity of the ferrites obeys the well-known Arrhenius equation as [15]
⎛E ⎞ ρd = ρ0 exp ⎜ a ⎟ ⎝ kT ⎠ where
63
(7)
ρ0 is the temperature independent constant, Ea is the
Fig. 2. Lattice constants of the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites.
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density dx, sintering density ds, and porosity with La–Cu substitution amount, as shown in Fig. 3. The shrink of lattice volume for the M-type benefits the enhancement of dx and ds in the region with xr 0.3. But the porosity increases when the substitution amount exceeds 0.3 in multiphase region. 3.3. Magnetic properties
Fig. 3. X-ray density, sintering density, and porosity of the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.5) ferrites.
axis length increases with increasing La–Cu substitution amount [10]. Ionic radius difference of the La3 þ (1.22 Å)-Sr2 þ (1.32 Å) pair and Cu2 þ (0.78 Å)-Fe3 þ (0.67 Å) pair can compensate well with each other [5,9]. Hence, the lattice deformation of M-type structure with decreased c-axis length is more affected by the varied binding energy with partial La3 þ –Cu2 þ substitution for Sr2 þ –Fe3 þ . It is notable that the Fe3 þ –O2 distance and Fe3 þ –O2 bond angle depend on the variation of c-axis length, which possess important influence on the hyperfine field of sublattice [8]. With further increasing x, the LaFeO3 phase and Sr2FeO4 x phase appear with elevated lattice constants, indicating that the formation of multiphase structure produces non-ignorable crystal defects such as the oxygen vacancies. It agrees well with the variation of their x-ray
It is well known that the suitable substitution of La3 þ for Sr2 þ can raise the hyperfine fields of 12k site and 2b site for the M-type hexaferrites, and then the intrinsic magnetic properties are improved usually [7–9]. In addition, the Cu2 þ probably prefers to replace the Fe3 þ at 4f2 site considering that the 4f2 site is closer to the Sr-layer than other octahedral sites (12k and 2a) for electrovalent balance associated with the substitution of Sr2 þ by La3 þ in the Sr-layer [10]. Magnetic properties of the Sr1 xLaxFe12–xCuxO19 (x ¼0–0.5) ferrites as functions of La–Cu substitution amount in single M-type phase region (xr0.3) can be clearly explained in term of the occupancy effects of La3 þ and Cu2 þ in magnetoplumbite structure, as shown in Fig. 4. Seen from Fig. 4(a), the saturation magnetization Ms of the ferrites increases from 58.5 to 63.1 emu/g rapidly when the substitution amount increases from 0 to 0.2, and then decreases to 62.6 emu/g slightly with x further increasing to 0.3. The occupancy of Cu2 þ ions at 4f2↓site with x r0.2 weakens the magnetic moment in antiparallel direction on account of the lower magnetic moment of Cu2 þ than Fe3 þ , raising the total magnetic moment of M-type phase. But the excessive substitution of La3 þ –Cu2 þ for Sr2 þ –Fe3 þ can lead to the formation of localized spin canting, and the magnetic moment orientation of Fe3 þ deviates from a collinear structure for the ferrites [7]. It is suggested that the slight decrease of Ms for the ferrite with x ¼0.3 is the reflection of
Fig. 4. Magnetic properties of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) ferrites as functions of La–Cu substitution amount.
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considerable localized spin canting effect. Effect of La–Cu substitution amount on the effective magnetic anisotropy constant Keff is shown in Fig. 4(b). The coexistence of opposite effects of La3 þ substituting for Sr2 þ and Cu2 þ substituting for Fe3 þ on the magnetocrystalline anisotropy for the M-type hexaferrites is observed. It can be seen that the low doping with x ¼0.1 raises the Keff from 3.21 106 to 3.25 106 erg/cm3. But it decreases to 3.22 106 erg/cm3 gradually when the substitution amount increases from 0.1 to 0.3. In the low doping region with xr0.1, the function of La3 þ substituting for Sr2 þ is more significant than Cu2 þ substituting for Fe3 þ . Contrarily in the doping region of 0.1 rx r0.3, the influence of Cu2 þ substituting for Fe3 þ on the magnetocrystalline anisotropy is evidently strengthened. The occupancy of Cu2 þ at 4f2 site deteriorates the magnetocrystalline anisotropy of M-type structure due to the different contributions of a single Fe3 þ ion at each site to the anisotropy constant: 2b 44f2 42a 44f1 412k [16]. Magnetic anisotropy field Ha (2Keff/Ms) of the ferrites is obtained subsequently, as shown in Fig. 4(c). It is obvious that the Ha decreases from 18.38 to 16.95 kOe when La–Cu substitution amount increases from 0 to 0.3, mainly determining the fall of intrinsic coercivity Hci from 3.69 to 3.54 kOe presented in Fig. 4(d). Competition between the Ms and Keff dominates the reduction of Ha, which goes through three stages in single M-type phase region considering the variation of Ms and Keff with x carefully. In the doing region with x r0.1, the Ms increases more rapid than the Keff with increasing x (Ms↑↑, Keff↑), but it just inverses in the doping region of 0.2 rx r0.3 where the Keff decreases more rapid than the Ms with increasing x (Ms↓, Keff↓↓). In the middle doping region of 0.1 rx r0.2, the Ms increases but the Keff decreases with increasing x (Ms↑, Keff↓). Besides, the Ha has an obvious increment with increasing x in multiphase region (Ms↓↓, Keff↓), but the Hci still
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declines swiftly. It indicates the enhanced effects of microstructure on the Hci owing to the relation of Hci ¼ αHa NMs/μ0, where α is the microstructure factor which increases with decreasing grain size, N is the demagnetization factor determined by many parameters one of which is the aspect ratio, and μ0 is the permeability of free space. This is high in accordance with the previous study [10], where the grain size of the La–Cu substituted Sr-hexaferrites starts to increase when the substitution amount exceeds 0.3. 3.4. Electrical properties Temperature dependence of direct-current resistivity (ρd–T curves) and drift mobility (μd-T curves) for the Sr1 xLaxFe12 xCuxO19 (x¼ 0–0.3) ferrites is shown in Fig. 5. It is found that the undoped and doped ferrites with La–Cu substitution amount xr 0.2 exhibit typical semiconducting behavior. However, the single metal–semiconductor (M–S) transition behavior with a resistivity peak at a specific temperature called transition temperature (Tp ¼63.8 °C) is observed in the ferrites with x¼ 0.3. The ρd is dominated by the metallic-conduction mechanism and increases with temperature in the T oTp region. Instead, the ρd in a wide temperature region (T4Tp) relies on the intrinsic semi-conduction mechanism and follows an exponential decay with temperature. The metallic-conduction region (m region) and semi-conduction region (s region) are associated with the scattering effect with lattice vibration and the hopping of electrons between Fe2 þ to Fe3 þ at octahedral sites, respectively. Hopping of electrons between Fe2 þ to Fe3 þ plays a role of intrinsic excitation for the ferrites actually. The intrinsic excitation is expected to raise the concentration of charge carrier n and drift mobility μd, and reduce the ρd in the “s region”. But the enhanced lattice vibration scattering effect with temperature impedes the electron
Fig. 5. Temperature dependence of direct-current resistivity and drift mobility for the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrites.
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Fig. 6. Effects of La–Cu substitution amount on the electrical properties of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrite.
conduction and the μd decreases correspondingly in the “m region”. Obviously, the M–S transition temperature used to separate the “m region” and “s region” depends on the competition between lattice vibration scattering and intrinsic excitation. Effects of La–Cu substitution amount on the electrical properties of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrite is given in Fig. 6. For the undoped and doped ferrites with La–Cu substitution amount xr0.2, the maximum value of direct-current resistivity occurs at room temperature (ρmax ¼ ρRT), which is obtained at the transition temperature Tp for the ferrites with x ¼0.3. The replacement of Cu2 þ for Fe3 þ at octahedral 4f2 site with x r0.2 decrease the concentration of Fe3 þ and correlative intrinsic excitation, leading to the increased direct-current resistivity ρRT/ρmax and activation energy Ea. Nevertheless, the spin canting effect is enhanced high enough to weaken the magnetic confinement to the electronic conduction in magnetoplumbite structure when the substitution amount further increases to 0.3, thus the ρRT/ρmax and Ea decreases accordingly. Typically, the ferrites with x ¼0.2 can provide high ρRT/ρmax of 16.8 109 Ω cm for microwave applications, as well as the μRT/μmin of 0.12 10 13 cm2 V 1 s-1 and Ea of 1.016 eV. 3.5. Dielectric properties It is observed that the complex permittivity of the Sr1–xLaxFe12 xCuxO19 (x¼0–0.3) ferrites varies with frequency and temperature strongly. Fig. 7 shows the temperature dependence of real permittivity (ε′–T curves) and dielectric loss (tgδ–T curves) for the undoped and doped ferrites with x¼ 0.2 from 1 kHz to 10 MHz.
In whole frequency range, their dielectric behavior obeys the traditional charge polarization mechanism for ferrite materials which happens since the frequency of the hopping of electron exchange between Fe2 þ to Fe3 þ is far from the frequency of alternatingcurrent field. Multiparticle polarization appears in these ferrites from the ε′–T and tgδ–T curves, especially for the low frequency measurement at 1 kHz and 10 kHz. Besides, the consistent motion of maximum ε′ and tgδ to the high temperature region with increasing frequency indicates the polarization relaxation phenomenon [17–19]. Table 1 lists the room temperature dielectric parameters of the Sr1 xLaxFe12 xCuxO19 (x ¼0–0.3) ferrites at different frequency. Generally, the real permittivity ε′ decreases with increasing La–Cu substitution amount from 0 to 0.2 but increases with x further increasing to 0.3 again. There is a strong relationship between the electrical conduction mechanism and polarization mechanism in ferrite materials [20]. The formation of space charge polarization correlated with the electron displacement is impeded because the ionic replacement of Sr2 þ –Fe3 þ by La3 þ –Cu2 þ increases the resistivity and activation energy of the ferrites with appropriate La– Cu substitution. This is suggested to be responsible for the reduction of ε′ with increasing x and vice versa. The imaginary permittivity ε″ and dielectric loss tgδ of these ferrites are not very good, which have not been improved significantly by the La–Cu substitution especially for testing in high frequency above 100 kHz. Frequency dependence of ε′ and ε″ in microwave bands remains to be investigated further for understanding their electromagnetic loss mechanism.
4. Conclusions The Sr1 xLaxFe12 xCuxO19 (x ¼0–0.5) hexaferrites for use in LTCC circulators/isolators are successfully fabricated by microwave sintering with appropriate amount of BBSZ glass at 850–870 °C for 30 min. Analysis of the crystal structure, magnetic, electrical, and dielectric properties strongly denotes that the La3 þ –Cu2 þ ions can partially substitute the Sr2 þ –Fe3 þ ions in a doping region with xr0.3. The Cu2 þ ions are suggested to preferentially replace the Fe3 þ ions at octahedral 4f2 site according to their dependence of saturation magnetization Ms and effective magnetic anisotropy constant Keff on La–Cu substitution amount, and the reduction of magnetic anisotropy field Ha and intrinsic coercivity Hci is obtained as results. Their electrical transport and polarization behaviors in a frequency range of 1 kHz–10 MHz from room temperature to 750 °C are found to follow the conduction mechanism of semiconductors and charge polarization mechanism of ferrite
Fig. 7. Temperature dependence of real permittivity and dielectric loss for the undoped and doped ferrites with x¼ 0.2.
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Table 1 Room temperature dielectric parameters of the Sr1–xLaxFe12–xCuxO19 (x¼ 0–0.3) ferrites at different frequency. Test frequency
1 kHz
10 kHz
100 kHz
1 MHz
10 MHz
Real permittivity ε′, x ¼ 0.0 Real permittivity ε′, x ¼ 0.1 Real permittivity ε′, x ¼ 0.2 Real permittivity ε′, x ¼ 0.3 Imaginary permittivity ε″, x ¼0.0 Imaginary permittivity ε″, x ¼0.1 Imaginary permittivity ε″, x ¼0.2 Imaginary permittivity ε″, x ¼0.3 Dielectric loss tgδ, x ¼0.0 Dielectric loss tgδ, x ¼0.1 Dielectric loss tgδ, x ¼0.2 Dielectric loss tgδ, x ¼0.3
4.136 3.708 3.208 3.565 0.236 0.033 0.032 0.110 0.057 0.009 0.010 0.031
3.983 3.711 3.211 3.575 0.084 0.059 0.058 0.004 0.021 0.016 0.018 0.001
3.903 3.812 3.312 3.566 0.043 0.030 0.030 0.021 0.011 0.008 0.009 0.006
3.839 3.783 3.283 3.672 0.065 0.034 0.036 0.022 0.017 0.009 0.011 0.006
3.741 3.742 3.242 3.638 0.041 0.041 0.039 0.040 0.011 0.011 0.012 0.011
materials, respectively. The obtained ferrites with x ¼0.2 can provide optimal Ms of 63.1 emu/g, Ha of 17.03 kOe, and Hci of 3.59 kOe, and suitable room temperature direct-current resistivity ρd of 16.8 109 Ω cm for microwave applications.
Acknowledgments This work is supported by the National Natural Science Foundation of China under Grant no. 51502025, the Project of Science and Technology Supporting Plan in Sichuan Province of China under Grant no. 2016GZ0108, and the Open Fund of Sichuan Province Key Laboratory of Information Materials and Devices Application (Grant no. 2015Z001).
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