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Magnetic, dielectric and microwave properties of M–Ti substituted barium hexaferrites (M=Mn2+, Co2+, Cu2+, Ni2+, Zn2+)
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 8645–8657 www.elsevier.com/locate/ceramint
Magnetic, dielectric and microwave properties of M–Ti substituted barium hexaferrites (M¼ Mn2 þ , Co2 þ , Cu2 þ , Ni2 þ , Zn2 þ ) H. Sözeria,n, H. Deligözb, H. Kavasc, A. Baykald a
TUBITAK-UME, National Metrology Institute, PO Box 54, 41470 Gebze, Kocaeli, Turkey b Istanbul University, Chemical Engineering Department, 34320 Avcilar, Istanbul, Turkey c Department of Physics, Istanbul Medeniyet University, 34700 Göztepe, Istanbul, Turkey d Fatih University, Department of Chemistry, B.Çekmece 34500, Istanbul, Turkey
Received 10 January 2014; received in revised form 16 January 2014; accepted 16 January 2014 Available online 24 January 2014
Abstract Several divalent cations together with tetravalent Ti4 þ ion were replaced by two trivalent Fe3 þ ions of barium hexaferrite in the form of BaFe10M2 þ Ti4 þ O19. Samples were prepared by using solid state reaction route and 1 wt% B2O3 was added to inhibit the crystal growth at lower temperatures. Magnetic, dielectric and microwave properties of samples were investigated by X-ray crystallography, scanning electron microscopy, magnetization and near field microwave measurements. Magnetization measurements revealed that saturation magnetization of the cation substituted samples is less than that of the pure barium hexaferrite. Except Co2 þ substituted barium hexaferrite, coercivities of the samples are nearly 1 kOe. While measurement of dielectric constants of Zn2 þ , Mn2 þ , Co2 þ and Cu2 þ substituted samples yields a significant enhancement (E10–102 times) with respect to Ni2 þ substituted barium hexaferrite in permittivity through local polarization of Fe3 þ electronic charges activated with nearby divalent ions. It is suggested that Zn2 þ and Mn2 þ substitution acts to reduce the electron hopping probability between Fe2 þ and Fe3 þ . All samples have approximately the same microwave absorption properties in such a way that minimum reflection loss (RL) of 10 dB occurs at 15 GHz. Meanwhile, Zn2 þ and Mn2 þ substituted samples have quite wide absorption bandwidths of 4 GHz at 10 dB. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Powders: solid state reaction; C. Dielectric properties; D. Ferrites; E. Hard Magnets
1. Introduction The development of electronic devices used in wireless communications like TV and radio broadcasting systems, mobile phones, radars, Local Area Networks etc. working in frequency ranges from MHz up to GHz has created a problem of electromagnetic interference (EMI). Therefore, shielding of EMI has become unavoidable not only due to its interference with other electronical devices but also its harmful effects on health. Hence, microwave absorbers are needed to reduce the radiation pollution caused by increasing number electromagnetic sources. There are wide variety of absorber materials that can be used to suppress EMI depending on whether they are suitable for low or high frequency applications. Conventional spinel ferrites do not n
Corresponding author. Tel.: þ90 262 679 5000; fax: þ 90 262 679 5001. E-mail address: [email protected] (H. Sözeri).
function well in the GHz range due to a drop of the complex permeability as given by the Snoek's limit. Among the hexaferrites, barium hexaferrite (BaM) powders are ideal fillers for the development of electromagnetic attenuation materials, due to their low cost, low density, high stability, large electrical resistivity, and high microwave magnetic loss. The magnetic loss of this material is mainly due to the resonance absorption of moving magnetic domains in a low frequency and incoherent rotation of magnetization in a relatively higher frequency [1]. Both magnetic and microwave properties of hexaferrites like saturation magnetization, coercivity and loss tangent (tan δ), can be tailored by cationic substitution of Fe3 þ ions by different tetravelent (Ti4 þ , Sn4 þ , Zr4 þ , Ir4 þ ) and divalent (Co2 þ , Mn2 þ , Zn2 þ , Ni2 þ ) ions [2–5]. BaM has a ferromagnetic resonance frequency around 40 GHz due to high magnetic anisotropy field. The substitution of such cations is effective to decrease the resonance frequency down to the microwave range by reducing the magnetic anisotropy field. The magnetic properties of the substituted hexaferrites
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depend directly on the electronic configuration of the dopant cations and on their preference to occupy different Fe sublattices of the magnetoplumbite structure [6]. BaM crystallizes in a hexagonal structure with space group P63/mmc, where the unit cell contains two molecules of BaM and have five different Fe-sublattices, namely 12k, 4f2, 2a, 4f1 and 2b. The spins in the 12k-, 2a- and 2b-sites are parallel to each other and to the crystallographic c-axis, whereas those in 4f2 and 4f1 point in the opposite direction. Their mutual orientation is given by the Gorter collinear model as 12k (↑), 4f1 (↓), 4f2 (↓), 2a (↑) and 2b (↑). Ferro- and anti-ferromagnetically ordered sites are coupled by super exchange interactions through the O2 atoms. The net magnetic polarization J at a temperature T of BaM per formula unit (f.u.) can be expressed as JðTÞ ¼ 6m12k ðTÞ 2m4f1 ðTÞ 2m4f2 ðTÞþ 1m2a ðTÞþ 1m2b ðTÞ ð1Þ where mn describes the magnetic moment of Fe3 þ ion in the n-th sublattice. As a result, the substitution of divalent or tetravalent cations for Fe3 þ ions has a great effect not only on the magnetic but also microwave properties due to the change in complex permeability that is related to saturation magnetization as m″¼ Ms/2HAα. In this work, we have synthesized BaM powders in the form of BaFe10M2 þ Ti4 þ O19, where M¼ Mn2 þ , Zn2 þ , Co2 þ ,Cu2 þ , Ni2 þ , by the solid state reaction method. 1 wt% B2O3 was used as an inhibitor of the crystal growth and to reduce the phase formation temperature. The structural, electrical, magnetic and microwave absorption properties of the BaM powders have been studied by XRD, TEM micrography, FT-IR, dc conductivity, vibrating sample magnetometry (VSM) and vector network analyzer (VNA) techniques.
0.1 Hz to 1 MHz at 10 1C intervals. The temperature was controlled with a Novocontrol cryosystem, which is applicable between 100 and 250 1C. The microwave measurements, complex transmission (S21) and reflection (S11) coefficients, were performed with a ATM technology waveguide system and HP PNA E8364B Vector network analyzer in the frequency range of 8.2–18 GHz. The obtained coefficients were used in the NRW algorithm to calculate the effective relative permittivity and the permeability of each composite with thickness of 2 mm. 3. Results and discussion 3.1. XRD analysis Fig. 1 shows XRD powder patterns of BaFe10M2 þ Ti4 þ O19 (M: Mn þ 2, Zn þ 2, Co þ 2, Cu þ 2 and Ni2 þ ) having sharp and intense peaks. The peaks of the host material as well as substituted ferrites appear at the same positions but with different intensities and full width at half maximum (FWHM). All of the observed diffractions in Fig. 1 correspond to BaM (ICDD No. 74-1121) indicating that the powders are monophase [9,10]. The lattice parameter “a” of cation substituted compounds is larger than that of pure BaM, see Table 1. This expansion can be attributed to larger ionic radii for Ti that is 0.0605 nm, as compared to the ionic radius of Fe that is 0.055 nm [11]. In this
2. Experimental Appropriate amounts of high purity BaCO3, Fe2O3, M2 þ (ZnO, CuO, MnO, CoO, NiO) and TiO2 powders were weighed and mixed to prepare BaFe10M2 þ Ti4 þ O19 by solid state reaction route. B2O3 was added to this mixture with 1 wt % as an inhibitor for the crystal growth [7]. The initial Fe:Ba ratio was taken as 10. After mixing and grinding for 15 min in ethly alcohol, precursors were pelletized under the pressure of 200 MPa to improve the formation of the hard phase fraction [8]. Heat treatment was performed at 1000 1C for 2 h. The structural properties and phase fractions were investigated using an X-ray powder diffractometer (Shimadzu-XRD 6000, CuKα radiation). The surface morphology and microstructure of the samples were examined with a scanning electron microscope (JEOL 6335F, Field Emission Gun). The magnetic characterization of the samples, all in powder form, was performed at room temperature using a vibrating sample magnetometer (LDJ Electronics Inc., Model 9600) in an applied field of 15 kOe. The proton conductivity studies of the samples were performed using a Novocontrol impedance spectrometer. The films were sandwiched between gold blocking electrodes and the conductivities were measured in the frequency range
Fig. 1. XRD patterns of cation substituted samples.
Table 1 Lattice constant of pure BaM and BaFe10M2 þ Ti4 þ O19 (M: Mn2 þ , Zn2 þ , Co2 þ , Ni2 þ , Cu2 þ ) compounds.
Pure BaM [17] Mn2 þ Co2 þ Ni2 þ Cu2 þ Zn2 þ
a (Å)
c (Å)
5.894 5.901 5.905 5.908 5.910 5.911
23.248 23.227 23.226 23.227 23.229 23.230
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expansion, the contribution of M ions is small but its effect is increasing from Mn to Zn also (Table 1). Contrary to the lattice parameter “a”, “c” remains almost constant. As it was indicated by Li et al. [11], this may imply that the M (Zn2 þ , Mn2 þ , Co2 þ , Ni2 þ , and Cu2 þ ) and Ti ions may preferentially occupy some sites in the five different crystallographic sites of the M-type ferrite. 3.2. SEM analysis SEM micrographs of BaFe10M2 þ Ti4 þ O19 (M2 þ : Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ and Ni2 þ ) are shown in Fig. 2. Well defined hexagonal grains are seen with prominent grain boundaries. The number of grains, morphology, grain boundaries changes with the composition (type of the substituted transition metals).
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In general, the grain size varies from 0.35 mm to 0.55 mm for all samples. The smallest grain size is observed for Zn–Ti substituted sample. The grain size of all samples are homogeneous. 3.3. Magnetic characterization The magnetic properties of M-type hexaferrites can be changed with partial substitution of di-, tri- and tetravalent ions to occupy the spin down states at 4f1 and 4f2 sites. Thus, net magnetization of the BaM can be increased to a maximum of 40 mB. In case of Zn–Ti substitutions, a weak increase of Ms was reported for small amount of substitutions (BaFe12 2xZnxTix, xo0.3) in the literature [2,12–16]. This increase was explained by the site preferences of Zn and Ti ions at 4f1 and 4f2 with spin down Fe3 þ ions. Therefore, substitution with Zn2 þ reduces this negative
Fig. 2. SEM micrographs of (a) Mn2 þ , (b) Zn2 þ , (c) Co2 þ , (d) Cu2 þ , and (e) Ni2 þ substituted BaM samples.
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contribution and increases the net magnetic moment [2,14]. For higher Zn–Ti amounts, Ms decreases due to the presence of too many non-magnetic ions in the crystal structure, which decreases the exchange interactions between the Fe3 þ ions. It was shown in Fig. 3 that Ms of our sample is 51 emu/g which is slightly less than that of the pure sample (reported as in between 55 and 60 emu/g depending on preparation conditions in Refs. [8,17]), which is in accordance with the literature for high Zn–Ti amounts. It was reported that Co2 þ ions occupy both the tetrahedral 4f1 and octahedral 4f2 positions [18,19] by assuming (in agreement with neutron diffraction experiments [19]) that cobalt ions hardly enter 2a positions and only small amounts of Ti4 þ enter into 2b and 4f1 positions. As a result of high Co–Ti substitution, like Zn–Ti substituted sample, saturation magnetization is slightly less than that of the pure BaM. However, coercivity decreases substantially to 566 Oe, which is the smallest value among the other samples. This decrease can be explained by large grain size and decrease in magnetocrystalline anisotropy. Our observations in Ms and Hc are in parallel with the results presented in Ref. [20]. Mn2 þ ions occupy the spin down position of Fe3 þ (3d0) in tetrahedral (4f1) site, while Ti4 þ replaces Fe3 þ ions in octahedral lattice [21]. In the series of BaFe10MnTiO19 compounds, Fe3 þ ions are partially replaced by the equal amount of Mn2 þ and Ti4 þ ions. Mn2 þ (3d5) ions prefer to replace the spin down of Fe3 þ (3d5) ions in tetrahedral sublattices (4f1) and Ti4 þ (3d0) ions substitute either for spin up or spin down of Fe3 þ ions in octahedral sublattices [21,22]. In case of high amount of Mn2 þ 60
Mn Ni Cu Co Zn
40
M, emu/g
20
3.4. MW measurements
0
According to the transmission line theory, reflection loss (RL) is related to the input impedence (Zin) of a microwave absorbing material, terminated by conductor, with the Eqs. (2 and 3) below. Z 1 in RLðdBÞ ¼ 20 log ð2Þ Z in þ 1
-20
-40
-60
and Ti4 þ ions substitution, Ti4 þ ions substitute for the spindown sublattice (4f2) and other octahedral spin-up sublattices (12k, 2a, 2b). Therefore, one may expect a decrease in saturation magnetization [23] as we observed in Fig. 3. It should be noticed that Ms decreases to 45 emu/g which is smaller than that of Zn2 þ and Co2 þ substituted samples. Ni2 þ ions replace Fe3 þ ions at 4f2 and 12k sites for small values of substitution and prefers 12k site for larger amounts. According to the ligand field theory, ions with d1, d2, d3 and d4 prefer tetrahedral coordination, while ions with d6, d7, d8 and d9 prefer octahedral coordination. Ions with d0, d5 and d10 have no preference [24]. Fig. 3 implies that some of Ni2 þ ions with magnetic moment of 2 mB are replaced with Fe3 þ ions having magnetization of 5mB at 12k in high spin state, which results in a sharp decrease in Ms. It has been reported that, like Co2 þ ions, Cu2 þ also occupies the 4f2 (octahedral site) which is a low spin state contributing negatively to the total saturation magnetization [25]. The radius of Fe3 þ ions at octahedral site is 0.67 Å and that of Cu2 þ is 0.78 Å. After the substitution, the c-axis length increases slightly which decrease the effect of low spin state contributions (4f1 and 4f2) to magnetization. As a result, net magnetization was enhanced in Zr–Cu and La–Cu substitutions to Sr-hexaferrite as reported in Refs. [14] and [26] respectively. In our case, Cu–Ti substitution to BaM, no considerable change was observed in Ms values, despite a small increase in the c-axis length. In M-type hexaferrites the 12k, 4f2 (octahedral) and 2b (trigonal bipyramidal) sites are known as major contributors to the magnetocrystalline anisotropy [26]. As mentioned above, Cu2 þ occupies the 4f2 and has a negative impact on the magnetocrystalline anisotropy, consequently the coercivity decreases. The results of magnetic characterization of all samples are shown in Table 2.
and -15000
-10000
-5000
0
5000
10000
15000
H, Oe
Fig. 3. M–H hysterisis curves of the cation substituted BaM samples.
sffiffiffiffiffi qffiffiffiffiffiffiffiffiffi μ00r 2π tanh j f d ε00r μ00r Z in ¼ c ε00r
ð3Þ
Table 2 Magnetic parameters of the BaFe10M2 þ Ti4 þ O19 samples, magnetic moments and ionic radiuses of the substituted cations. Divalent cations 2þ
Mn Co2 þ Ni2 þ Cu2 þ Zn2 þ
Magnetic moment, mB
Ionic Radii, Å
Ms, emu/g
Hc, Oe
Mr, emu/g
5 3 2 1 0
0.71 0.74 0.69 0.78 0.74
45 51 38 49 51
1173 566 1280 1045 900
20.2 14.6 15.8 21.3 20.7
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3.5.1.1. ac Conductivity. The ac conductivities of metal doped (Mn2 þ , Zn2 þ , Co2 þ Cu2 þ and Ni2 þ ) BaM were measured from 20 to 120 1C using impedance spectroscopy as a function of frequency. The frequency-dependent ac conductivity plots of divalent metal substituted BaM samples are shown in Fig. 5(a)–(e). The ac conductivities of samples were obtained using the following standard equation [28]; s0 ðωÞ ¼ sac ðωÞ ¼ ε00 ðωÞωε0
ð4Þ
0
Fig. 4. Reflection loss values of cation substituted BaM.
where, μr'' and εr'' are complex permeability and permittivity, f is frequency and c is speed of light. It should be noted that, perfect absorption occurs when Z in ¼ 1 implying that frequency matching criteria is satisfied, that is impedence of free space is equal to that of the absorber. Fig. 4 shows the RL values of the divalent cation substituted BaM samples. It was observed that RL decreases with frequency for all samples and makes a wide minimum at around 15 GHz. This minimum corresponds to losses due to the domain wall motion. At the end of Ku band (i.e. 18 GHz) RL starts to decrease again, which probably gives rise to another minimum corresponding to resonance due to coherent spin rotation. The minimum RL values are slightly less than 10 dB for samples having Mn–Ti and Zn–Ti substitutions, and corresponds to more than 90% absorption of the incident EM radiation. Despite the smallness of the RL values, resonance bandwidths of these two samples, around 4 GHz, are quite wide. It is known from the ferromagnetic resonance theory that resonance frequency (f) is proportional with the anisotropy field (Ha) as f ¼ 2πγ H a , where γ is the gyromagnetic K1 ratio. Then, H a ¼ H c ¼ 0:64 M with the assumpition that s partciles are spherical to omit the effect of shape anisotropy on coercivity. As a result, one can conclude that increasing Ms, decreases the ferromagnetic resonance frequency of the magnetic particles [4,27]. 3.5. Conductivity and dielectric permittivity of BaFe10M2 þ Ti4 þ O19, M: Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ and Ni2 þ 3.5.1. Temperature and frequency dependency The frequency and temperature dependent ac conductivity, dc conductivity and dielectric permittivity (real and imaginary part of dielectric permittivity and dissipation factor) of BaFe10M2 þ Ti4 þ O19 (M: Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ and Ni2 þ ), were studied over a broad frequency (1 Hz to 3 MHz) and temperature range (20–120 1C). In the substitution reaction, divalent metals and titanium were doped while two ferrite ions were removed from the structure.
where s ðω) is the real part of conductivity, ω (2πf) is the angular frequency of the signal applied to the sample, ε00 is the imaginary part of complex dielectric permittivity and ε0 (8.852 10 14 F cm 1) is the vacuum permittivity. One can also see from Fig. 5(a) that the ac conductivities of Ni2 þ substituted BaM were 7.09 10 8 and 1.59 10 7 S cm 1 at 20 and 120 1C (1 MHz), respectively, while the conductivities of other samples were found to be at the range of 10 7 and 10 6 S cm 1 at the same frequency and temperature ranges. Thus, the conductivity of BaFe10M2 þ Ti4 þ O19 (M: Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ ) was about one order of magnitude higher than that of Ni2 þ substituted BaM. Apart from divalent metal doped samples, Cu2 þ substituted BaM has exhibited a conductivity value of 10 6 S cm 1 at the corresponding frequency and temperature ranges (Fig. 5(e)). The conductivities of metal substituted samples have significantly increased with temperature and did not obviously changed with temperature beyond a certain frequency. When Ni2 þ substituted BaM has exhibited a temperature independent behavior beyond 50 kHz, Co2 þ and Cu2 þ doped BaM showed the similar trend starting from 1 and 5 kHz, respectively. Regarding the temperature dependency of Mn2 þ substituted BaM, interestingly, it was found that the conductivity was high at low temperatures up to 1 kHz. After that frequency, the temperature dependency of Mn2 þ substituted BaM has reverted and a similar trend to those of all metal doped has been observed (Fig.5(b)). This significant improvement in conductivity (one or two orders of higher than that of Ni2 þ substituted sample) of BaFe10(Co, Mn, Zn, Cu) Ti4 þ O19 samples with temperature can corresponded to increasing the mobility of divalent ions leading to high electrical conductivity. In general, the parameters affecting the conductivity of the samples are defined in the following formula. s ¼ nFC ion þ υion þ
ð5Þ
where n is an electric charge of carrier ions, F is Faraday constant þ is the concentration of the carrier (9.6485 104C mol 1), Cion þ ions and υion is the mobility of carrier ions [29]. Regarding the frequency dependence of ac conductivity of BaFe10M2 þ Ti4 þ O19, a frequency-dependent behavior was observed at relatively higher frequencies depending on the type of an ion substituted. The conductivities of metal substituted BaM have exhibited temperature independency while exhibiting a temperature-dependent behavior at low frequencies. This behavior can be considered as a strong evidence for ionic conductivity [30]. In addition, BaFe10M2 þ Ti4 þ O19 over a certain frequency (50 kHz for Ni2 þ and 1 kHz, 150 kHz, 1 kHz and 5 kHz for Mn2 þ , Zn2 þ , Co2 þ and Cu2 þ doped BaM, respectively) obeyed the rule of the temperature independent expression for
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Fig. 5. The variation of ac conductivity of (a) Ni2 þ and (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
several low mobility polymers and even for crystalline materials, non-crystalline and liquid semiconductors as sac ðωÞ ¼ Aωn
ð6Þ
where ω is the angular frequency, n is the frequency exponent and A is a temperature independent constant [31]. Frequency exponent (n) values were calculated from the slopes of log sac log ω graph and shown in Fig. 6. The variations of n based on temperature were also given in the inset of Fig. 6. By fitting our data to Eq. (5), we found n values in the range of 0.55–0.8 for BaFe10(Mn,Co,Cu,Zn)TiO19, between 0.77 and 1.01 for Ni2 þ substituted BaM. Frequency exponent (n) values
of metal doped samples considerably decreased with temperature as shown in Fig. 6. It is consistent with the results reported for amorphous materials [32]. The variation of n values with temperature is a strong evidence for thermally activated polarization mechanism. On the other hand, if we assume that the conduction mechanism is based on ion migration in the applied electric field, the lower n values from the ac measurements can be explained by strong electrode polarization. 3.5.1.2. dc Conductivity. The direct current (dc) conductivities of all samples are found by extrapolation of frequency
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Fig. 6. Plots of log sac vs. log ωmax and n vs. inverse absolute temperature (inset) of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
independent part of the curves (i.e., lower frequency regimes) to the zero frequency. The dc conductivity vs. reciprocal temperature plots are depicted in Fig. 7. One can see from Fig. 7 that the dc conductivity of all samples strongly depends on temperature and it was found that Co2 þ and Cu2 þ substituted BaM samples have nearly 10 to 100 times higher dc conductivities than that of Ni2 þ substituted one. On the other hand, it can be emphasized that all samples exhibits Arrhenius behavior in the temperature range of 20–120 1C while, there are two different regimes, occurring in the range of 20–80 1C and 80–120 1C for Ni2 þ and Mn2 þ doped samples, respectively. Similarly, two different regimes for Co2 þ
and Cu2 þ substituted BaM samples were observed in the range of 20–40 1C and 40–120 1C. As indicated above, the conductivity of BaFe10M2 þ Ti4 þ O19, is thermally activated and its Arrhenius behavior can be formulated as shown in Eq. (7): log sdc ¼ log s0 –E a =k B T
ð7Þ
where sdc is the dc conductivity, Ea is the activation energy, k is the Boltzmann constant (8.617343 10 5 eV K 1) and T is the temperature in K. By using Eq. 7, Ea values of Ni2 þ substituted BaM were found to be 0.112 and 0.307 eV for the temperature ranges of 20–80 1C and 80–120 1C, respectively. In addition, the activation energies of Co2 þ and Cu2 þ substituted samples were
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Fig. 7. dc Conductivity of (a) Ni2 þ , (b) Zn2 þ , (c) Co2 þ and (d) Cu2 þ substituted BaM vs. reciprocal temperature.
determined in the ranges of 0.1–0.128 and 0.148–0.24 corresponding to the temperature ranges of 20–80 1C and 80–120 1C, respectively. 3.5.1.3. Frequency and temperature dependence of dielectric permittivity (ε0 and ε″). One of the goals of this study was to describe the effects of metal substitution on the BaFe10M2 þ Ti4 þ O19 dielectric properties. Therefore the complex permittivity parameters of real (ε0 ) and imaginary (ε″) parts of BaFe10M2 þ Ti4 þ O19 were studied. The real part of permittivity, commonly called as dielectric constant (ε0 ), as a function of frequency and temperature is shown in Fig. 8(a)– (e). The real part of permittivity of Ni2 þ and Zn2 þ substituted BaM has decreased sharply with frequency up to 1 kHz while ε0 of the Mn2 þ , Co2 þ and Cu2 þ substituted BaM dropped dramatically with frequency up to 100 kHz. This decline in real part of permittivity is relatively significant at higher temperatures. Besides, the real part of dielectric permittivity of all samples decreased with increasing frequency when temperature was kept constant. Furthermore, these curves keep their shapes but slide up at higher temperatures. This behavior can be attributed to the frequency dependence of the polarization mechanisms. When frequency increases the orientational polarization decreases, since alignment of dipole moments needs longer time than that of electronic and ionic polarizations. This causes a significant reduction in the real part of dielectric permittivity. Furthermore,
variation of ε0 with frequency also showed the presence of material electrode interface polarization processes which takes place at low frequencies [33–35]. For example, at 20 1C, ε0 values of Ni2 þ and Cu2 þ substituted BaM were found to be 15.1 and 108.9 at 1 Hz, respectively, while the particle had a dielectric constants of 12.6 and 17.7 at 100 KHz. Concerning the temperature dependency of real part of permittivity (ε0 ), one can see from Fig. 4 that ε0 of all samples increased markedly with temperature. This was clearly observed at low frequency region. The increment in ε0 of all samples with temperature showed the thermally activated process as explained in ac conductivity section. As it is known from literature, the real part of dielectric permittivity can be described by the mobility of the charge carriers which rises by increasing the temperature [29]. For example, at 20 1C, ε0 values of Ni2 þ and Cu2 þ substituted BaM were found to be 15.1 and 108.9 at 1 Hz, respectively, while they are equal to 56.5 and 140.7 at 120 1C at the same frequencies. Fig. 9(a)–(e) shows frequency dependence of an imaginary part of the permittivity, which is usually called as dielectric loss (ε″), at different temperatures for all samples. It was observed that ε″ dramatically decreases with frequency and reaches a minimum. At low frequencies, the interactions between the dipoles are high and weakens with increasing frequency leading to a remarkable reduction in dielectric loss [36]. Regarding the polarization mechanism, it can be emphasized that the interface
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Fig. 8. The variations of real part of dielectric permittivity (ε0 ) of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
polarization is dominant at lower frequencies while other mechanisms such as electronic and ionic exist at higher frequencies [37]. Oxygen vacancies, grain boundary defects and local valence variations lead to interfacial polarization, which in turn contributes to the increase of both real and imaginary part of permittivity at lower frequencies [38]. At high frequency (100 kHz–3MHz), the imaginary part of permittivity becomes less sensitive to both frequency and temperature and tended to be stabilized as reported earlier [39]. On the other hand, the imaginary part of dielectric permittivity of the samples was found to be strongly temperature dependent
at lower frequencies. However, temperature dependence reduces remarkably and finally becomes constant at higher frequencies. There is no peak in dielectric loss of all samples, except Mn2 þ substituted one, due to high dc conductivity and Maxwell–Wagner–Sillar (MWS) relaxation (interfacial polarization) effects [40]. A physical interpretation of dielectric dispersion can be done on the basis of electric dipoles formed by cations Ba2 þ , Fe3 þ and divalent atoms of the structure with their surrounding O2 ions. The main source of polarization in ferrites is the change of Fe3 þ ions to Fe2 þ ions at octahedral 2a
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Fig. 9. The variations of imaginary part of dielectric permittivity (ε″) of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
crystallographic sites. The electron hopping that occurs between adjacent Fe3 þ and Fe2 þ ions leads to a local displacement of electric charge carriers, thereby contributing to dielectric polarization and relaxation [41,42]. Regarding Co2 þ and Cu2 þ substitution, increase in ε0 and ε″ values (Fig. 8(d)–(e) and Fig. 9(d)–(e)) can be attributed to enhanced electron transfer between Fe3 þ and Fe2 þ ions as discussed above. Compared to the literature, Mn2 þ and Zn2 þ substitution has slightly improved dielectric properties of BaM similar
to Cr doped analogs [43]. Contrarily, Co2 þ and Cu2 þ dopings have yielded a significant enhancement ( E10–102 times) in permittivity through local polarization of Fe3 þ electronic charges activated with nearby divalent ions. Similar result was also reported for Bi3 þ doped BaFe12O19 [44]. In addition, the dissipation factor (tan δ), which is the ratio of ε0 and ε″, of samples was studied as a function of temperature and frequency, shown in Fig. 10(a)–(d). The peak in dissipation factor was not observed for Ni2 þ , Zn2 þ and Co2 þ
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Fig. 10. Tan δ values of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ and (d) Co2 þ substituted BaM depending on temperature and frequency.
doped BaM samples. This is probably due to the being out of temperature and frequency range applied in the study. Actually, the imaginary part of Ni2 þ and Zn2 þ substituted sample has reverted and start to increase suddenly beyond 100 kHz. In the case of BaFe10Mn2 þ Ti4 þ O19, the peak was clearly observed and shifted to higher frequencies with temperature. 4. Conclusion The effects of divalent metal substitution on the magnetic, electrical and microwave properties of BaM were investigated. Magnetic characterization reveals that saturation magnetization values of Co2 þ , Cu2 þ and Zn2 þ substituted BaM are around 50 emu/g while that of Mn2 þ and Ni2 þ substituted samples are less than this value. Coercivity of Co2 þ substituted sample ( 0.6 kOe) appears low compared to that of others, which are nearly 1 kOe. The conductivitiy of BaFe10M2 þ Ti4 þ O19 samples exhibits temperature-independent and -dependent behavior at high and low frequencies, respectively. This tendency can be considered as a strong evidence for ionic conductivity. The variation of n values with temperature is a strong evidence for thermally activated polarization mechanism. Activation energies (Ea) of Ni2 þ substituted BaM are found to be 0.112 and 0.307 eV for the temperature range of 20–80 1C and 80–120 1C, respectively.
In addition, the activation energy values of Co2 þ and Cu2 þ substituted BaM samples are determined in the range of 0.1–0.128 and 0.148–0.24 eV for the temperature ranges of 20–80 1C and 80–120 1C, respectively. The electrical resistivity, dielectric constant and dielectric losses of samples are slightly enhanced by the substitution of Zn2 þ , Mn2 þ , Co2 þ and Cu2 þ ions and yield a significant enhancement (E10–102 times) in permittivity through local polarization of Fe3 þ electronic charges activated with nearby divalent ions. In other words, it is suggested that Zn2 þ and Mn2 þ substitution acts to reduce the electron hopping probability between Fe2 þ and Fe3 þ . It can be clearly emphasized that the interface polarization is dominant at lower frequencies while other mechanisms such as electronic and ionic exist at higher frequencies. According to the microwave measurements, all samples have nearly same RL values of 10 dB at 15 GHz. This resonance absorption is due to domain wall motion which generally occurs at low frequencies. At 10 dB, resonance bandwidths are quite large and more than 4 GHz for Zn2 þ and Mn2 þ substituted samples. Acknowledgment This work is supported by TUBITAK (the Scientific and Technological Research Council of Turkey) with Project number 213M174.
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CERAMICS INTERNATIONAL
Ceramics International 40 (2014) 8645–8657 www.elsevier.com/locate/ceramint
Magnetic, dielectric and microwave properties of M–Ti substituted barium hexaferrites (M¼ Mn2 þ , Co2 þ , Cu2 þ , Ni2 þ , Zn2 þ ) H. Sözeria,n, H. Deligözb, H. Kavasc, A. Baykald a
TUBITAK-UME, National Metrology Institute, PO Box 54, 41470 Gebze, Kocaeli, Turkey b Istanbul University, Chemical Engineering Department, 34320 Avcilar, Istanbul, Turkey c Department of Physics, Istanbul Medeniyet University, 34700 Göztepe, Istanbul, Turkey d Fatih University, Department of Chemistry, B.Çekmece 34500, Istanbul, Turkey
Received 10 January 2014; received in revised form 16 January 2014; accepted 16 January 2014 Available online 24 January 2014
Abstract Several divalent cations together with tetravalent Ti4 þ ion were replaced by two trivalent Fe3 þ ions of barium hexaferrite in the form of BaFe10M2 þ Ti4 þ O19. Samples were prepared by using solid state reaction route and 1 wt% B2O3 was added to inhibit the crystal growth at lower temperatures. Magnetic, dielectric and microwave properties of samples were investigated by X-ray crystallography, scanning electron microscopy, magnetization and near field microwave measurements. Magnetization measurements revealed that saturation magnetization of the cation substituted samples is less than that of the pure barium hexaferrite. Except Co2 þ substituted barium hexaferrite, coercivities of the samples are nearly 1 kOe. While measurement of dielectric constants of Zn2 þ , Mn2 þ , Co2 þ and Cu2 þ substituted samples yields a significant enhancement (E10–102 times) with respect to Ni2 þ substituted barium hexaferrite in permittivity through local polarization of Fe3 þ electronic charges activated with nearby divalent ions. It is suggested that Zn2 þ and Mn2 þ substitution acts to reduce the electron hopping probability between Fe2 þ and Fe3 þ . All samples have approximately the same microwave absorption properties in such a way that minimum reflection loss (RL) of 10 dB occurs at 15 GHz. Meanwhile, Zn2 þ and Mn2 þ substituted samples have quite wide absorption bandwidths of 4 GHz at 10 dB. & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. Keywords: A. Powders: solid state reaction; C. Dielectric properties; D. Ferrites; E. Hard Magnets
1. Introduction The development of electronic devices used in wireless communications like TV and radio broadcasting systems, mobile phones, radars, Local Area Networks etc. working in frequency ranges from MHz up to GHz has created a problem of electromagnetic interference (EMI). Therefore, shielding of EMI has become unavoidable not only due to its interference with other electronical devices but also its harmful effects on health. Hence, microwave absorbers are needed to reduce the radiation pollution caused by increasing number electromagnetic sources. There are wide variety of absorber materials that can be used to suppress EMI depending on whether they are suitable for low or high frequency applications. Conventional spinel ferrites do not n
Corresponding author. Tel.: þ90 262 679 5000; fax: þ 90 262 679 5001. E-mail address: [email protected] (H. Sözeri).
function well in the GHz range due to a drop of the complex permeability as given by the Snoek's limit. Among the hexaferrites, barium hexaferrite (BaM) powders are ideal fillers for the development of electromagnetic attenuation materials, due to their low cost, low density, high stability, large electrical resistivity, and high microwave magnetic loss. The magnetic loss of this material is mainly due to the resonance absorption of moving magnetic domains in a low frequency and incoherent rotation of magnetization in a relatively higher frequency [1]. Both magnetic and microwave properties of hexaferrites like saturation magnetization, coercivity and loss tangent (tan δ), can be tailored by cationic substitution of Fe3 þ ions by different tetravelent (Ti4 þ , Sn4 þ , Zr4 þ , Ir4 þ ) and divalent (Co2 þ , Mn2 þ , Zn2 þ , Ni2 þ ) ions [2–5]. BaM has a ferromagnetic resonance frequency around 40 GHz due to high magnetic anisotropy field. The substitution of such cations is effective to decrease the resonance frequency down to the microwave range by reducing the magnetic anisotropy field. The magnetic properties of the substituted hexaferrites
0272-8842/$ - see front matter & 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved. http://dx.doi.org/10.1016/j.ceramint.2014.01.082
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depend directly on the electronic configuration of the dopant cations and on their preference to occupy different Fe sublattices of the magnetoplumbite structure [6]. BaM crystallizes in a hexagonal structure with space group P63/mmc, where the unit cell contains two molecules of BaM and have five different Fe-sublattices, namely 12k, 4f2, 2a, 4f1 and 2b. The spins in the 12k-, 2a- and 2b-sites are parallel to each other and to the crystallographic c-axis, whereas those in 4f2 and 4f1 point in the opposite direction. Their mutual orientation is given by the Gorter collinear model as 12k (↑), 4f1 (↓), 4f2 (↓), 2a (↑) and 2b (↑). Ferro- and anti-ferromagnetically ordered sites are coupled by super exchange interactions through the O2 atoms. The net magnetic polarization J at a temperature T of BaM per formula unit (f.u.) can be expressed as JðTÞ ¼ 6m12k ðTÞ 2m4f1 ðTÞ 2m4f2 ðTÞþ 1m2a ðTÞþ 1m2b ðTÞ ð1Þ where mn describes the magnetic moment of Fe3 þ ion in the n-th sublattice. As a result, the substitution of divalent or tetravalent cations for Fe3 þ ions has a great effect not only on the magnetic but also microwave properties due to the change in complex permeability that is related to saturation magnetization as m″¼ Ms/2HAα. In this work, we have synthesized BaM powders in the form of BaFe10M2 þ Ti4 þ O19, where M¼ Mn2 þ , Zn2 þ , Co2 þ ,Cu2 þ , Ni2 þ , by the solid state reaction method. 1 wt% B2O3 was used as an inhibitor of the crystal growth and to reduce the phase formation temperature. The structural, electrical, magnetic and microwave absorption properties of the BaM powders have been studied by XRD, TEM micrography, FT-IR, dc conductivity, vibrating sample magnetometry (VSM) and vector network analyzer (VNA) techniques.
0.1 Hz to 1 MHz at 10 1C intervals. The temperature was controlled with a Novocontrol cryosystem, which is applicable between 100 and 250 1C. The microwave measurements, complex transmission (S21) and reflection (S11) coefficients, were performed with a ATM technology waveguide system and HP PNA E8364B Vector network analyzer in the frequency range of 8.2–18 GHz. The obtained coefficients were used in the NRW algorithm to calculate the effective relative permittivity and the permeability of each composite with thickness of 2 mm. 3. Results and discussion 3.1. XRD analysis Fig. 1 shows XRD powder patterns of BaFe10M2 þ Ti4 þ O19 (M: Mn þ 2, Zn þ 2, Co þ 2, Cu þ 2 and Ni2 þ ) having sharp and intense peaks. The peaks of the host material as well as substituted ferrites appear at the same positions but with different intensities and full width at half maximum (FWHM). All of the observed diffractions in Fig. 1 correspond to BaM (ICDD No. 74-1121) indicating that the powders are monophase [9,10]. The lattice parameter “a” of cation substituted compounds is larger than that of pure BaM, see Table 1. This expansion can be attributed to larger ionic radii for Ti that is 0.0605 nm, as compared to the ionic radius of Fe that is 0.055 nm [11]. In this
2. Experimental Appropriate amounts of high purity BaCO3, Fe2O3, M2 þ (ZnO, CuO, MnO, CoO, NiO) and TiO2 powders were weighed and mixed to prepare BaFe10M2 þ Ti4 þ O19 by solid state reaction route. B2O3 was added to this mixture with 1 wt % as an inhibitor for the crystal growth [7]. The initial Fe:Ba ratio was taken as 10. After mixing and grinding for 15 min in ethly alcohol, precursors were pelletized under the pressure of 200 MPa to improve the formation of the hard phase fraction [8]. Heat treatment was performed at 1000 1C for 2 h. The structural properties and phase fractions were investigated using an X-ray powder diffractometer (Shimadzu-XRD 6000, CuKα radiation). The surface morphology and microstructure of the samples were examined with a scanning electron microscope (JEOL 6335F, Field Emission Gun). The magnetic characterization of the samples, all in powder form, was performed at room temperature using a vibrating sample magnetometer (LDJ Electronics Inc., Model 9600) in an applied field of 15 kOe. The proton conductivity studies of the samples were performed using a Novocontrol impedance spectrometer. The films were sandwiched between gold blocking electrodes and the conductivities were measured in the frequency range
Fig. 1. XRD patterns of cation substituted samples.
Table 1 Lattice constant of pure BaM and BaFe10M2 þ Ti4 þ O19 (M: Mn2 þ , Zn2 þ , Co2 þ , Ni2 þ , Cu2 þ ) compounds.
Pure BaM [17] Mn2 þ Co2 þ Ni2 þ Cu2 þ Zn2 þ
a (Å)
c (Å)
5.894 5.901 5.905 5.908 5.910 5.911
23.248 23.227 23.226 23.227 23.229 23.230
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expansion, the contribution of M ions is small but its effect is increasing from Mn to Zn also (Table 1). Contrary to the lattice parameter “a”, “c” remains almost constant. As it was indicated by Li et al. [11], this may imply that the M (Zn2 þ , Mn2 þ , Co2 þ , Ni2 þ , and Cu2 þ ) and Ti ions may preferentially occupy some sites in the five different crystallographic sites of the M-type ferrite. 3.2. SEM analysis SEM micrographs of BaFe10M2 þ Ti4 þ O19 (M2 þ : Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ and Ni2 þ ) are shown in Fig. 2. Well defined hexagonal grains are seen with prominent grain boundaries. The number of grains, morphology, grain boundaries changes with the composition (type of the substituted transition metals).
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In general, the grain size varies from 0.35 mm to 0.55 mm for all samples. The smallest grain size is observed for Zn–Ti substituted sample. The grain size of all samples are homogeneous. 3.3. Magnetic characterization The magnetic properties of M-type hexaferrites can be changed with partial substitution of di-, tri- and tetravalent ions to occupy the spin down states at 4f1 and 4f2 sites. Thus, net magnetization of the BaM can be increased to a maximum of 40 mB. In case of Zn–Ti substitutions, a weak increase of Ms was reported for small amount of substitutions (BaFe12 2xZnxTix, xo0.3) in the literature [2,12–16]. This increase was explained by the site preferences of Zn and Ti ions at 4f1 and 4f2 with spin down Fe3 þ ions. Therefore, substitution with Zn2 þ reduces this negative
Fig. 2. SEM micrographs of (a) Mn2 þ , (b) Zn2 þ , (c) Co2 þ , (d) Cu2 þ , and (e) Ni2 þ substituted BaM samples.
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contribution and increases the net magnetic moment [2,14]. For higher Zn–Ti amounts, Ms decreases due to the presence of too many non-magnetic ions in the crystal structure, which decreases the exchange interactions between the Fe3 þ ions. It was shown in Fig. 3 that Ms of our sample is 51 emu/g which is slightly less than that of the pure sample (reported as in between 55 and 60 emu/g depending on preparation conditions in Refs. [8,17]), which is in accordance with the literature for high Zn–Ti amounts. It was reported that Co2 þ ions occupy both the tetrahedral 4f1 and octahedral 4f2 positions [18,19] by assuming (in agreement with neutron diffraction experiments [19]) that cobalt ions hardly enter 2a positions and only small amounts of Ti4 þ enter into 2b and 4f1 positions. As a result of high Co–Ti substitution, like Zn–Ti substituted sample, saturation magnetization is slightly less than that of the pure BaM. However, coercivity decreases substantially to 566 Oe, which is the smallest value among the other samples. This decrease can be explained by large grain size and decrease in magnetocrystalline anisotropy. Our observations in Ms and Hc are in parallel with the results presented in Ref. [20]. Mn2 þ ions occupy the spin down position of Fe3 þ (3d0) in tetrahedral (4f1) site, while Ti4 þ replaces Fe3 þ ions in octahedral lattice [21]. In the series of BaFe10MnTiO19 compounds, Fe3 þ ions are partially replaced by the equal amount of Mn2 þ and Ti4 þ ions. Mn2 þ (3d5) ions prefer to replace the spin down of Fe3 þ (3d5) ions in tetrahedral sublattices (4f1) and Ti4 þ (3d0) ions substitute either for spin up or spin down of Fe3 þ ions in octahedral sublattices [21,22]. In case of high amount of Mn2 þ 60
Mn Ni Cu Co Zn
40
M, emu/g
20
3.4. MW measurements
0
According to the transmission line theory, reflection loss (RL) is related to the input impedence (Zin) of a microwave absorbing material, terminated by conductor, with the Eqs. (2 and 3) below. Z 1 in RLðdBÞ ¼ 20 log ð2Þ Z in þ 1
-20
-40
-60
and Ti4 þ ions substitution, Ti4 þ ions substitute for the spindown sublattice (4f2) and other octahedral spin-up sublattices (12k, 2a, 2b). Therefore, one may expect a decrease in saturation magnetization [23] as we observed in Fig. 3. It should be noticed that Ms decreases to 45 emu/g which is smaller than that of Zn2 þ and Co2 þ substituted samples. Ni2 þ ions replace Fe3 þ ions at 4f2 and 12k sites for small values of substitution and prefers 12k site for larger amounts. According to the ligand field theory, ions with d1, d2, d3 and d4 prefer tetrahedral coordination, while ions with d6, d7, d8 and d9 prefer octahedral coordination. Ions with d0, d5 and d10 have no preference [24]. Fig. 3 implies that some of Ni2 þ ions with magnetic moment of 2 mB are replaced with Fe3 þ ions having magnetization of 5mB at 12k in high spin state, which results in a sharp decrease in Ms. It has been reported that, like Co2 þ ions, Cu2 þ also occupies the 4f2 (octahedral site) which is a low spin state contributing negatively to the total saturation magnetization [25]. The radius of Fe3 þ ions at octahedral site is 0.67 Å and that of Cu2 þ is 0.78 Å. After the substitution, the c-axis length increases slightly which decrease the effect of low spin state contributions (4f1 and 4f2) to magnetization. As a result, net magnetization was enhanced in Zr–Cu and La–Cu substitutions to Sr-hexaferrite as reported in Refs. [14] and [26] respectively. In our case, Cu–Ti substitution to BaM, no considerable change was observed in Ms values, despite a small increase in the c-axis length. In M-type hexaferrites the 12k, 4f2 (octahedral) and 2b (trigonal bipyramidal) sites are known as major contributors to the magnetocrystalline anisotropy [26]. As mentioned above, Cu2 þ occupies the 4f2 and has a negative impact on the magnetocrystalline anisotropy, consequently the coercivity decreases. The results of magnetic characterization of all samples are shown in Table 2.
and -15000
-10000
-5000
0
5000
10000
15000
H, Oe
Fig. 3. M–H hysterisis curves of the cation substituted BaM samples.
sffiffiffiffiffi qffiffiffiffiffiffiffiffiffi μ00r 2π tanh j f d ε00r μ00r Z in ¼ c ε00r
ð3Þ
Table 2 Magnetic parameters of the BaFe10M2 þ Ti4 þ O19 samples, magnetic moments and ionic radiuses of the substituted cations. Divalent cations 2þ
Mn Co2 þ Ni2 þ Cu2 þ Zn2 þ
Magnetic moment, mB
Ionic Radii, Å
Ms, emu/g
Hc, Oe
Mr, emu/g
5 3 2 1 0
0.71 0.74 0.69 0.78 0.74
45 51 38 49 51
1173 566 1280 1045 900
20.2 14.6 15.8 21.3 20.7
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3.5.1.1. ac Conductivity. The ac conductivities of metal doped (Mn2 þ , Zn2 þ , Co2 þ Cu2 þ and Ni2 þ ) BaM were measured from 20 to 120 1C using impedance spectroscopy as a function of frequency. The frequency-dependent ac conductivity plots of divalent metal substituted BaM samples are shown in Fig. 5(a)–(e). The ac conductivities of samples were obtained using the following standard equation [28]; s0 ðωÞ ¼ sac ðωÞ ¼ ε00 ðωÞωε0
ð4Þ
0
Fig. 4. Reflection loss values of cation substituted BaM.
where, μr'' and εr'' are complex permeability and permittivity, f is frequency and c is speed of light. It should be noted that, perfect absorption occurs when Z in ¼ 1 implying that frequency matching criteria is satisfied, that is impedence of free space is equal to that of the absorber. Fig. 4 shows the RL values of the divalent cation substituted BaM samples. It was observed that RL decreases with frequency for all samples and makes a wide minimum at around 15 GHz. This minimum corresponds to losses due to the domain wall motion. At the end of Ku band (i.e. 18 GHz) RL starts to decrease again, which probably gives rise to another minimum corresponding to resonance due to coherent spin rotation. The minimum RL values are slightly less than 10 dB for samples having Mn–Ti and Zn–Ti substitutions, and corresponds to more than 90% absorption of the incident EM radiation. Despite the smallness of the RL values, resonance bandwidths of these two samples, around 4 GHz, are quite wide. It is known from the ferromagnetic resonance theory that resonance frequency (f) is proportional with the anisotropy field (Ha) as f ¼ 2πγ H a , where γ is the gyromagnetic K1 ratio. Then, H a ¼ H c ¼ 0:64 M with the assumpition that s partciles are spherical to omit the effect of shape anisotropy on coercivity. As a result, one can conclude that increasing Ms, decreases the ferromagnetic resonance frequency of the magnetic particles [4,27]. 3.5. Conductivity and dielectric permittivity of BaFe10M2 þ Ti4 þ O19, M: Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ and Ni2 þ 3.5.1. Temperature and frequency dependency The frequency and temperature dependent ac conductivity, dc conductivity and dielectric permittivity (real and imaginary part of dielectric permittivity and dissipation factor) of BaFe10M2 þ Ti4 þ O19 (M: Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ and Ni2 þ ), were studied over a broad frequency (1 Hz to 3 MHz) and temperature range (20–120 1C). In the substitution reaction, divalent metals and titanium were doped while two ferrite ions were removed from the structure.
where s ðω) is the real part of conductivity, ω (2πf) is the angular frequency of the signal applied to the sample, ε00 is the imaginary part of complex dielectric permittivity and ε0 (8.852 10 14 F cm 1) is the vacuum permittivity. One can also see from Fig. 5(a) that the ac conductivities of Ni2 þ substituted BaM were 7.09 10 8 and 1.59 10 7 S cm 1 at 20 and 120 1C (1 MHz), respectively, while the conductivities of other samples were found to be at the range of 10 7 and 10 6 S cm 1 at the same frequency and temperature ranges. Thus, the conductivity of BaFe10M2 þ Ti4 þ O19 (M: Mn2 þ , Zn2 þ , Co2 þ , Cu2 þ ) was about one order of magnitude higher than that of Ni2 þ substituted BaM. Apart from divalent metal doped samples, Cu2 þ substituted BaM has exhibited a conductivity value of 10 6 S cm 1 at the corresponding frequency and temperature ranges (Fig. 5(e)). The conductivities of metal substituted samples have significantly increased with temperature and did not obviously changed with temperature beyond a certain frequency. When Ni2 þ substituted BaM has exhibited a temperature independent behavior beyond 50 kHz, Co2 þ and Cu2 þ doped BaM showed the similar trend starting from 1 and 5 kHz, respectively. Regarding the temperature dependency of Mn2 þ substituted BaM, interestingly, it was found that the conductivity was high at low temperatures up to 1 kHz. After that frequency, the temperature dependency of Mn2 þ substituted BaM has reverted and a similar trend to those of all metal doped has been observed (Fig.5(b)). This significant improvement in conductivity (one or two orders of higher than that of Ni2 þ substituted sample) of BaFe10(Co, Mn, Zn, Cu) Ti4 þ O19 samples with temperature can corresponded to increasing the mobility of divalent ions leading to high electrical conductivity. In general, the parameters affecting the conductivity of the samples are defined in the following formula. s ¼ nFC ion þ υion þ
ð5Þ
where n is an electric charge of carrier ions, F is Faraday constant þ is the concentration of the carrier (9.6485 104C mol 1), Cion þ ions and υion is the mobility of carrier ions [29]. Regarding the frequency dependence of ac conductivity of BaFe10M2 þ Ti4 þ O19, a frequency-dependent behavior was observed at relatively higher frequencies depending on the type of an ion substituted. The conductivities of metal substituted BaM have exhibited temperature independency while exhibiting a temperature-dependent behavior at low frequencies. This behavior can be considered as a strong evidence for ionic conductivity [30]. In addition, BaFe10M2 þ Ti4 þ O19 over a certain frequency (50 kHz for Ni2 þ and 1 kHz, 150 kHz, 1 kHz and 5 kHz for Mn2 þ , Zn2 þ , Co2 þ and Cu2 þ doped BaM, respectively) obeyed the rule of the temperature independent expression for
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Fig. 5. The variation of ac conductivity of (a) Ni2 þ and (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
several low mobility polymers and even for crystalline materials, non-crystalline and liquid semiconductors as sac ðωÞ ¼ Aωn
ð6Þ
where ω is the angular frequency, n is the frequency exponent and A is a temperature independent constant [31]. Frequency exponent (n) values were calculated from the slopes of log sac log ω graph and shown in Fig. 6. The variations of n based on temperature were also given in the inset of Fig. 6. By fitting our data to Eq. (5), we found n values in the range of 0.55–0.8 for BaFe10(Mn,Co,Cu,Zn)TiO19, between 0.77 and 1.01 for Ni2 þ substituted BaM. Frequency exponent (n) values
of metal doped samples considerably decreased with temperature as shown in Fig. 6. It is consistent with the results reported for amorphous materials [32]. The variation of n values with temperature is a strong evidence for thermally activated polarization mechanism. On the other hand, if we assume that the conduction mechanism is based on ion migration in the applied electric field, the lower n values from the ac measurements can be explained by strong electrode polarization. 3.5.1.2. dc Conductivity. The direct current (dc) conductivities of all samples are found by extrapolation of frequency
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Fig. 6. Plots of log sac vs. log ωmax and n vs. inverse absolute temperature (inset) of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
independent part of the curves (i.e., lower frequency regimes) to the zero frequency. The dc conductivity vs. reciprocal temperature plots are depicted in Fig. 7. One can see from Fig. 7 that the dc conductivity of all samples strongly depends on temperature and it was found that Co2 þ and Cu2 þ substituted BaM samples have nearly 10 to 100 times higher dc conductivities than that of Ni2 þ substituted one. On the other hand, it can be emphasized that all samples exhibits Arrhenius behavior in the temperature range of 20–120 1C while, there are two different regimes, occurring in the range of 20–80 1C and 80–120 1C for Ni2 þ and Mn2 þ doped samples, respectively. Similarly, two different regimes for Co2 þ
and Cu2 þ substituted BaM samples were observed in the range of 20–40 1C and 40–120 1C. As indicated above, the conductivity of BaFe10M2 þ Ti4 þ O19, is thermally activated and its Arrhenius behavior can be formulated as shown in Eq. (7): log sdc ¼ log s0 –E a =k B T
ð7Þ
where sdc is the dc conductivity, Ea is the activation energy, k is the Boltzmann constant (8.617343 10 5 eV K 1) and T is the temperature in K. By using Eq. 7, Ea values of Ni2 þ substituted BaM were found to be 0.112 and 0.307 eV for the temperature ranges of 20–80 1C and 80–120 1C, respectively. In addition, the activation energies of Co2 þ and Cu2 þ substituted samples were
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Fig. 7. dc Conductivity of (a) Ni2 þ , (b) Zn2 þ , (c) Co2 þ and (d) Cu2 þ substituted BaM vs. reciprocal temperature.
determined in the ranges of 0.1–0.128 and 0.148–0.24 corresponding to the temperature ranges of 20–80 1C and 80–120 1C, respectively. 3.5.1.3. Frequency and temperature dependence of dielectric permittivity (ε0 and ε″). One of the goals of this study was to describe the effects of metal substitution on the BaFe10M2 þ Ti4 þ O19 dielectric properties. Therefore the complex permittivity parameters of real (ε0 ) and imaginary (ε″) parts of BaFe10M2 þ Ti4 þ O19 were studied. The real part of permittivity, commonly called as dielectric constant (ε0 ), as a function of frequency and temperature is shown in Fig. 8(a)– (e). The real part of permittivity of Ni2 þ and Zn2 þ substituted BaM has decreased sharply with frequency up to 1 kHz while ε0 of the Mn2 þ , Co2 þ and Cu2 þ substituted BaM dropped dramatically with frequency up to 100 kHz. This decline in real part of permittivity is relatively significant at higher temperatures. Besides, the real part of dielectric permittivity of all samples decreased with increasing frequency when temperature was kept constant. Furthermore, these curves keep their shapes but slide up at higher temperatures. This behavior can be attributed to the frequency dependence of the polarization mechanisms. When frequency increases the orientational polarization decreases, since alignment of dipole moments needs longer time than that of electronic and ionic polarizations. This causes a significant reduction in the real part of dielectric permittivity. Furthermore,
variation of ε0 with frequency also showed the presence of material electrode interface polarization processes which takes place at low frequencies [33–35]. For example, at 20 1C, ε0 values of Ni2 þ and Cu2 þ substituted BaM were found to be 15.1 and 108.9 at 1 Hz, respectively, while the particle had a dielectric constants of 12.6 and 17.7 at 100 KHz. Concerning the temperature dependency of real part of permittivity (ε0 ), one can see from Fig. 4 that ε0 of all samples increased markedly with temperature. This was clearly observed at low frequency region. The increment in ε0 of all samples with temperature showed the thermally activated process as explained in ac conductivity section. As it is known from literature, the real part of dielectric permittivity can be described by the mobility of the charge carriers which rises by increasing the temperature [29]. For example, at 20 1C, ε0 values of Ni2 þ and Cu2 þ substituted BaM were found to be 15.1 and 108.9 at 1 Hz, respectively, while they are equal to 56.5 and 140.7 at 120 1C at the same frequencies. Fig. 9(a)–(e) shows frequency dependence of an imaginary part of the permittivity, which is usually called as dielectric loss (ε″), at different temperatures for all samples. It was observed that ε″ dramatically decreases with frequency and reaches a minimum. At low frequencies, the interactions between the dipoles are high and weakens with increasing frequency leading to a remarkable reduction in dielectric loss [36]. Regarding the polarization mechanism, it can be emphasized that the interface
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Fig. 8. The variations of real part of dielectric permittivity (ε0 ) of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
polarization is dominant at lower frequencies while other mechanisms such as electronic and ionic exist at higher frequencies [37]. Oxygen vacancies, grain boundary defects and local valence variations lead to interfacial polarization, which in turn contributes to the increase of both real and imaginary part of permittivity at lower frequencies [38]. At high frequency (100 kHz–3MHz), the imaginary part of permittivity becomes less sensitive to both frequency and temperature and tended to be stabilized as reported earlier [39]. On the other hand, the imaginary part of dielectric permittivity of the samples was found to be strongly temperature dependent
at lower frequencies. However, temperature dependence reduces remarkably and finally becomes constant at higher frequencies. There is no peak in dielectric loss of all samples, except Mn2 þ substituted one, due to high dc conductivity and Maxwell–Wagner–Sillar (MWS) relaxation (interfacial polarization) effects [40]. A physical interpretation of dielectric dispersion can be done on the basis of electric dipoles formed by cations Ba2 þ , Fe3 þ and divalent atoms of the structure with their surrounding O2 ions. The main source of polarization in ferrites is the change of Fe3 þ ions to Fe2 þ ions at octahedral 2a
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Fig. 9. The variations of imaginary part of dielectric permittivity (ε″) of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ , (d) Co2 þ and (e) Cu2 þ substituted BaM as a function of frequency and temperature.
crystallographic sites. The electron hopping that occurs between adjacent Fe3 þ and Fe2 þ ions leads to a local displacement of electric charge carriers, thereby contributing to dielectric polarization and relaxation [41,42]. Regarding Co2 þ and Cu2 þ substitution, increase in ε0 and ε″ values (Fig. 8(d)–(e) and Fig. 9(d)–(e)) can be attributed to enhanced electron transfer between Fe3 þ and Fe2 þ ions as discussed above. Compared to the literature, Mn2 þ and Zn2 þ substitution has slightly improved dielectric properties of BaM similar
to Cr doped analogs [43]. Contrarily, Co2 þ and Cu2 þ dopings have yielded a significant enhancement ( E10–102 times) in permittivity through local polarization of Fe3 þ electronic charges activated with nearby divalent ions. Similar result was also reported for Bi3 þ doped BaFe12O19 [44]. In addition, the dissipation factor (tan δ), which is the ratio of ε0 and ε″, of samples was studied as a function of temperature and frequency, shown in Fig. 10(a)–(d). The peak in dissipation factor was not observed for Ni2 þ , Zn2 þ and Co2 þ
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Fig. 10. Tan δ values of (a) Ni2 þ , (b) Mn2 þ , (c) Zn2 þ and (d) Co2 þ substituted BaM depending on temperature and frequency.
doped BaM samples. This is probably due to the being out of temperature and frequency range applied in the study. Actually, the imaginary part of Ni2 þ and Zn2 þ substituted sample has reverted and start to increase suddenly beyond 100 kHz. In the case of BaFe10Mn2 þ Ti4 þ O19, the peak was clearly observed and shifted to higher frequencies with temperature. 4. Conclusion The effects of divalent metal substitution on the magnetic, electrical and microwave properties of BaM were investigated. Magnetic characterization reveals that saturation magnetization values of Co2 þ , Cu2 þ and Zn2 þ substituted BaM are around 50 emu/g while that of Mn2 þ and Ni2 þ substituted samples are less than this value. Coercivity of Co2 þ substituted sample ( 0.6 kOe) appears low compared to that of others, which are nearly 1 kOe. The conductivitiy of BaFe10M2 þ Ti4 þ O19 samples exhibits temperature-independent and -dependent behavior at high and low frequencies, respectively. This tendency can be considered as a strong evidence for ionic conductivity. The variation of n values with temperature is a strong evidence for thermally activated polarization mechanism. Activation energies (Ea) of Ni2 þ substituted BaM are found to be 0.112 and 0.307 eV for the temperature range of 20–80 1C and 80–120 1C, respectively.
In addition, the activation energy values of Co2 þ and Cu2 þ substituted BaM samples are determined in the range of 0.1–0.128 and 0.148–0.24 eV for the temperature ranges of 20–80 1C and 80–120 1C, respectively. The electrical resistivity, dielectric constant and dielectric losses of samples are slightly enhanced by the substitution of Zn2 þ , Mn2 þ , Co2 þ and Cu2 þ ions and yield a significant enhancement (E10–102 times) in permittivity through local polarization of Fe3 þ electronic charges activated with nearby divalent ions. In other words, it is suggested that Zn2 þ and Mn2 þ substitution acts to reduce the electron hopping probability between Fe2 þ and Fe3 þ . It can be clearly emphasized that the interface polarization is dominant at lower frequencies while other mechanisms such as electronic and ionic exist at higher frequencies. According to the microwave measurements, all samples have nearly same RL values of 10 dB at 15 GHz. This resonance absorption is due to domain wall motion which generally occurs at low frequencies. At 10 dB, resonance bandwidths are quite large and more than 4 GHz for Zn2 þ and Mn2 þ substituted samples. Acknowledgment This work is supported by TUBITAK (the Scientific and Technological Research Council of Turkey) with Project number 213M174.
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