Structural and magnetic properties of Ca-substituted barium W-type hexagonal hexaferrites

Structural and magnetic properties of Ca-substituted barium W-type hexagonal hexaferrites

Journal of Magnetism and Magnetic Materials 379 (2015) 16–21 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials j...

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Journal of Magnetism and Magnetic Materials 379 (2015) 16–21

Contents lists available at ScienceDirect

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Structural and magnetic properties of Ca-substituted barium W-type hexagonal hexaferrites Kai Huang a,b, Xiansong Liu a,n, Shuangjiu Feng a, Zhanjun Zhang a, Jiangying Yu b, Xiaofei Niu a, Farui Lv a, Xing Huang a a Engineering Technology Research Center of Magnetic Materials, Anhui Province, School of Physics & Materials Science, Anhui University, Hefei 230601, PR China b Department of Mathmatic and Physics, Anhui Jianzhu University, Hefei 230601, PR China

art ic l e i nf o

a b s t r a c t

Article history: Received 20 May 2014 Received in revised form 13 October 2014 Accepted 10 November 2014 Available online 27 November 2014

A series of W-type hexagonal ferrites with the composition Ba1  xCaxCo2Fe16O27 (x ¼0, 0.1, 0.3, 0.4 and 0.5) were synthesized using a sol–gel method. The effects of doping on structural and magnetic properties are studied by X-ray diffraction, thermal analyzer, scanning electron microscopy, vibrating sample magnetometer and vector network analyzer, respectively. The X-ray diffraction analysis shows that the samples belong to the W-type hexagonal ferrite. The lattice constants a and c decreases as Ca contents increases. The grains exhibit well defined hexagonal shape. The saturation magnetization and the intrinsic coercive force increases with the increase of the Ca substitution amount. The real part of complex permittivity (ε′) and imaginary part (ε″) increase with more addition of Ca2 þ amount. The imaginary part of complex permittivity (μ′) increases and the real part (μ′′) goes down after Ca2 þ is doped. Furthermore, the Ca2 þ ions doped in the ferrite improved microwave absorbency. & 2014 Published by Elsevier B.V.

Keywords: W-type hexaferrites Ca2 þ substitution Sol–gel Magnetic properties

1. Introduction Barium based hexagonal ferrite is a ferromagnetic material with high performance of permanent magnetic property, high uniaxial magnetocrystallilne anisotropy constant, saturation magnetization, excellent chemical stability and corrosion resistance [1,2]. Hexagonal ferrites have many applications, magnetic recording media, motor components, absorbers, microwave and mm-wave devices [3–5]. There are six possible different types of hexagonal ferrites which are designated as M, W, Y, Z, X and U depending upon their crystal structure. The crystal structure of W-type BaM2Fe16O27 hexagonal ferrite can be considered as a superposition of R and S blocks along the hexagonal c-axis with a structure of RSSRnSnSn, where the S and R blocks have the formulae Fe6O8 and BaFe6O11, respectively, and an asterisk (n) means that the respective block is turned 180° along the c-axis [6,7]. In W-type barium hexagonal structure the iron ions exist on seven different sites known as 4fvi, 2d, 12k, 6g, 4f, 4fiv, and 4e or five magnetic sites [8]. An improvement in the intrinsic magnetic properties of hexaferrites such as permeability, resistivity and saturation magnetization can be achieved by substituting the different divalent or trivalent ions at different sites into the hexagonal lattice [9]. n

Corresponding author. Fax: þ86 551 5107674. E-mail address: [email protected] (X. Liu). 0304-8853/& 2014 Published by Elsevier B.V.

In the last few years various properties of barium hexaferrites have been studied by replacing Fe3 þ ions and Ba2 þ ions with divalent and trivalent cations such as Cr, Co, La, Cu, Mg, etc. and with various bivalent–tetravalent cation combinations such as Cu–Ti, and Zr–Mn [10–16]. For years, many researchers have studied Srdoped Ba hexaferrites [17,18]. However, Very few investigations are available on Ca-substituted W-type hexaferrites. Ca also belongs to the same group in the periodic table and has same electronic configuration as that of Sr and Ba, and Ca as a base is chosen for its intrinsic properties like the reduction in sintering temperature and enhancement in the properties. Therefore, it seemed interesting to investigate further these properties by substituting Ca2 þ for Ba2 þ as part of more general studies of W-type hexagonal ferrite. In this work the sol–gel method was used to fabricate calcium barium hexaferrites. The aim of the present study is to investigate the effect of Ca2 þ substitution on the structural, magnetic properties and absorbing performance of W-type hexaferrites.

2. Experimental procedure A series of W-type hexaferrites with the formula Ba1  xCax Co2Fe16O27 (x ¼0, 0.1, 0.3, 0.4 and 0.5) was prepared by using the sol–gel method. All starting materials were of high-purity compounds. A stoichiometric mixture solution was prepared by

K. Huang et al. / Journal of Magnetism and Magnetic Materials 379 (2015) 16–21


dissolving Ba(NO3)2, Zn(NO3)2,Co(NO3)2, Ca(NO3)3, Fe(NO3)3 and citric acid in deionized water. The mole ratio of total metal ions to citric acid was 1:1.5. NH3  H2O was then added to adjust pH value to around 7. The resultant sol was then stirred, using a magnetic stirrer, while being heated at 80 °C, to transform the sol into a gel. The gel was then dried at 120 °C for about 4 h to make the dry precursor powder, and then sintered at 1100 °C for 8 h in air. Finally, sintered at 1300 °C for 5 h in air. The thermal decomposition behavior of the precursor powder was examined simultaneously by means of differential thermal analysis (DTA, Leading 449F3) with a heating rate of 10 °C min. X-ray diffraction (XRD, MACM18XHF) analysis of powder samples were carried out using an X-ray diffractometer equipped with a CuKα radiation source (λ ¼ 1.5406 Å). The morphology was examined using a scanning electron microscope (SEM, Hitachi S-4800). The magnetic properties like saturation magnetization values (Ms) and coercivity (Hc) were measured by a vibrating sample magnetometer (VSM, Quantum Design PPMS EC-II) at an applied field of 15 kOe at room temperature. A network analyzer (Agilent PNA 8363B) was employed to measure the electromagnetic parameters (ε′, ε″, μ′, μ″) in the frequency range of 2– 18 GHz using a reflection/transmission technique. The samples were prepared as follows: the particles and the paraffin were mixed together (7:3, mass ratio) and pressed to be cyclic samples with internal diameter 3.04 mm, external diameter 7.00 mm and height 3.00–4.00 mm.

3. Results and discussion 3.1. Structural properties Fig. 1 shows XRD spectra of the ferrite powders samples Ba1  xCaxCo2Fe16O27 (x¼ 0, 0.1, 0.3, 0.4 and 0.5) synthesized in the sol–gel process and after high temperature sintering at 1300 °C in air. The analysis of XRD results confirms that all powders are W-type phase with a space group of p63/mmc and no any extra peak were observed in the powder XRD patterns. Hence we can draw a conclusion that Ca2 þ has entered the lattice of hexaferrites successfully. The lattice parameters for all the powders samples are calclated with Ca2 þ (0.99 Å) substitution by the computer program (Jade5) from the XRD data and the calculated values are summarized in Table 1. Fig. 2 shows the dependence of the lattice constants on the amount of Ca. It is seen that the lattice parameters a and c decrease from 5.868 to 5.860 Å, and from 33.142 to 32.681 Å, respectively, as BaCo2Fe16O27 is changed to Ba1.5Ca0.5Co2Fe16O27. This can be attributed to the fact that ionic radius of Ca2 þ (0.99 Å) is smaller than that of Ba2 þ (1.49 Å). Since Ba2 þ is found in R-blocks [17], the substitution of Ba2 þ by Ca2 þ decreases the distance amongst the layers, which leads to the decrease in the lattice parameter a and c. V is the volume of the unitcell, 2π (V = a2c sin 3 ). Hence crystal structure becomes more compact with substitutions. Similar type of variation in cell parameters on substitutions has also been reported [19].

Fig. 1. XRD patterns of substituted Ba1  xCaxCo2Fe16O27 hexaferrite samples.

Table 1 Effects of Ca-substitution on lattice parameters (a and c), c/a and volume (V) for all Ba1  xCaxCo2Fe16O27 hexaferrites. Ca content

Chemical formula

a (Å)

c (Å)


V (Å3)

X¼0 X ¼ 0.1 X ¼ 0.3 X ¼ 0.4 X ¼ 0.5

BaCo2Fe16O27 Ba0.9Ca0.1Co2Fe16O27 Ba0.7Ca03Co2Fe16O27 Ba0.6Ca0.4Co2Fe16O27 Ba0.5Ca0.5Co2Fe16O27

5.868 5.864 5.861 5.860 5.860

33.142 33.011 32.882 32.723 32.681

5.648 5.629 5.611 5.583 5.576

988.27 983.02 977.85 973.45 972.20

3.2. Thermal analysis To have more understanding of the Ba1.7Ca0.3Co2Fe16O27 forming mechanism from gel to the solid precursor obtained by the sol–gel route, the dried gel was thermally analyzed and the resultant TG/DSC curves in the region of 40–1000 °C are shown in Fig. 3. During heat treatment of the as-prepared powder, several processes such as dehydration, oxidation of the residual organic groups, decomposition and sintering took place.

Fig. 2. Lattice constants (a) and (c) of the hexagonal ferrite Ba1  xCaxCo2Fe16O27 magnetic powder.


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as-prepared sample. The second step in the TG curve shows a huge mass weight loss about 65%, which is accompanied with the largest sharp exothermic peak around 216 °C in the DSC curve, which is associated with the autocatalytic oxidation reduction reaction between the citric acid and metal nitrates. The third step occurs with the weight loss of 8% by the two border endothermic peaks at 587 and 616 °C in the DSC curve. These weight losses may be attributed to the decomposition of the remaining citric acid [20]. It can be seen from the figure that above 800 °C there is less weight loss for the normal sample, which confirms the high thermal stability of the sample synthesized by the sol–gel method. 3.3. Scanning electron microscopy

Fig. 3. TG/DSC curve of as-prepared Ba1.7Ca0.3Co2Fe16O27 dried gel.

The TG curve exhibits three weight loss steps corresponding to one sharp exothermic peak and three small endothermic peaks in the DSC curve. The first step occurs at about 100 °C with the weight loss of 4%, which corresponds to the endothermic peak in DSC curve. This weight loss can be due to evaporation of water for

The morphology and the grain size of the samples were determined using scanning electron microscopy. SEM micrographs of Ba1  xCaxCo2Fe16O27 (x ¼0, 0.1, 0.3, 0.4 and 0.5) samples are shown in Fig. 4. It is observed that the grains are like platelets that have well defined hexagonal shape and orientation of grains is random. The values of particle sizes calculated from SEM are about 17, 16, 15, 16 and 14 μm for BaCo2Fe16O27, Ba1.9Ca0.1Co2Fe16O27, Ba1.7Ca0.3Co2Fe16O27, Ba1.6Ca0.4Co2Fe16O27 and Ba1.5Ca0.5Co2Fe16O27 samples, respectively. It is evident that the shape and diameters of most the grains remain almost independent of Ca2 þ substitution.

Fig. 4. SEM micrographs for all hexaferrite samples.

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Fig. 5. M–H loops for all Ba1  xCaxCo2Fe16O27 hexaferrite samples.

It can be seen that the grains are uniform hexagonal platelets in shape and contrast which also confirms that the single-phase ferrites agreement with XRD analysis. The platelet shaped hexaferrite can be used as potential material for application in microwave absorbing coatings [21].

3.4. Magnetic properties The magnetic hysteresis loop measurements of the W-type ferrite with different Ca substitution amounts are shown in Fig. 5 and the magnetic properties are summarized in Table 2. The hysteresis loops of the substituted hexaferrites are affected combined by many factors such as chemical composition of ferrites sintering conditions, magneto-crystalline anisotropy, grain size, etc. [22]. It can be observed from the table that the substituted hexaferrites show the soft character of magnets. The saturation magnetization (Ms) was very high, while coercive field (Hc) was comparatively low. It is observed that the samples of Ca substituted hexaferrites exhibit higher saturation magnetization and coercivity than that of unsubstituted sample. As shown in Table 2, Ms increases from 61.8 to 63.6 emu g  1 (x¼ 0.50) with the increasing of Ca content. The composition of ferrite can be one of the important factors in saturation magnetization. Due to the substitution of Ba2 þ by Ca2 þ , the lattice parameters shrink as Table 1 indicates. Correspondingly, the distance of Fe–O decreases and the Fe3 þ –O–Fe3 þ superexchange interaction for 12k and 2b sublattice was enhanced [2]. Therefore, the saturation magnetization of Ca substituted hexaferrites is higher than that of unsubstituted hexaferrite. The value of coercivity Hc increases with the increase of the Ca substitution amount. According to ferromagnetic theory, the coercivity in W-type hexaferrite can be explained by the equation as follows [23]: Table 2 Magnetic properties of Ba1  xCaxCo2Fe16O27 ferrites at room temperature. Ca content

Chemical formula

Ms (emu/g)

Hc (Oe)

X¼0 X ¼ 0.1 X ¼0.3 X ¼ 0.4 X ¼0.5

BaCo2Fe16O27 Ba0.9Ca0.1Co2Fe16O27 Ba0.7Ca03Co2Fe16O27 Ba0.6Ca0.4Co2Fe16O27 Ba0.5Ca0.5Co2Fe16O27

61.8 62.2 63.3 62.4 63.6

98.8 120.7 185.9 246.5 363.8

Fig. 6. Frequency dependences of complex permittivities (ε′ and ε″) of Ba1  xCax Co2Fe16O27 (x¼ 0–0.5).

Hc = 0.64

K1 Ms

where K1 is the magneto-crystalline anisotropy constant and Ms is the saturation magnetization. Since Ms slightly increases, it can be concluded that reasons for the above-mentioned variation of Hc may be contributed to the difference in the magneto-crystalline anisotropy of the Ca2 þ and Ba2 þ ions by substituting Ca2 þ (0.99 Å ) ions for Ba2 þ (1.49 Å). The increased magneto-crystalline anisotropy causes an enhancement in coercivity. In other words, the magneto-crystalline field increases when Ba2 þ is replaced by the smaller Ca2 þ . Similar results were reported earlier for substituted Ba-hexaferrites [24]. The magnetic properties of the present Ca substituted hexaferrite particles with high saturation magnetization and low coercivity are favorable for their applications such as microwave devices. 3.5. Dielectric and magnetic parameters Fig. 6a and b shows the frequency dependence of the complex permittivity for different Ca2 þ doped samples at the frequency range from 2.0 GHz to 18.0 GHz. It is shown that both the real part (ε′) and imaginary part (ε″) increase with increasing Ca2 þ substitution content. The difference of complex permittivity among the ferrites may be attributed to the significant contribution of Ca2 þ ions. The dielectric properties of polycrystalline ferrite arise


K. Huang et al. / Journal of Magnetism and Magnetic Materials 379 (2015) 16–21

Fig. 8. Microwave absorbing curves of Ba1  xCaxCo2Fe16O27 (x ¼0–0.5).

respectively. Table 2 presents the Ms values of Ba1  xCaxCo2Fe16O27. It shows that the Ms values of ferrites doped with Ca2 þ are higher than that of the undoped ones; therefore the μ′′ value would also be higher. Furthermore, when Ca2 þ is doped, the lattice distortion generates an increase in internal stress, which also leads to a higher value of μ′′. 3.6. Microwave absorbing properties

Fig. 7. Frequency dependences of complex permeabilities (μ' and μ′′) of Ba1  xCax Co2Fe16O27 (x ¼0–0.5).

mainly due to the interfacial polarization and intrinsic electric dipole polarization [25]. Dielectric properties of polycrystalline ferrite arise mainly due to separated by higher resistivity grain boundaries as proposed by Koops, so it increases with grain boundaries enlarging [26]. This agrees well with Fig. 2, which shows the crystal sizes decrease and grain boundaries enlarge. Smaller grain size offers less hindrance to the applied field; thus polarization is enhanced leading to increase of both ε′ and ε″. Fig. 7a and b displays the frequency dependence of the real (μ′) and imaginary (μ′′) parts of the complex permeability of Ba1  xCax Co2Fe16O27 (x ¼0, 0.1, 0.3, 0.4 and 0.5). Fig. 7a shows that the μ′ values of almost all the ferrites decrease with the frequency increasing, which can be ascribed to relaxation phenomena of domain magnetization rotation or domain wall displacement occurring in the GHz region [25]. As shown in Fig. 7(a), μ′ increases with the increase in Ca2 þ content in the frequency range of 2–18 GHz. Low μ′ in substituted samples can be ascribed to high porosity. Pores act as an impediment to domain wall motion and induce local demagnetizing fields, resulting in reduction of μ′ in the doped samples [26]. Fig.7b shows that, with the increase of Ca2 þ content, the μ′′ curve increases at first, reaches the maximum when x¼ 0.40, and then decreases in the frequency range of 2–18.0 GHz. According to the electromagnetic theory, μ′′ is connected with Ms and HA, according to the equation μ" = MS /(2HAα) [27], where Ms, HA and α are magnetization, anisotropy field and extinction coefficient,

The microwave absorbing property of the microwave absorbing materials can be described by tanδ (δ is dielectric loss angle): tan δ = tan δe + tan δm = ε"/ε′ + μ"/μ′, where tanδe and tanδm are the dielectric loss and magnetic loss, respectively. The larger the tanδ , the better the microwave absorbing properties of the material. Fig. 8 shows tanδ , which can stand for microwave absorbing curves of samples Ba1  xCaxCo2Fe16O27. The curves show that all the samples have good effective absorbing properties at the frequencies of 2.0–18.0 GHz. It is also observed that the microwave absorbency increases with more addition of Ca2 þ ions when xr0.40, while it decreases a little when x¼0.5 at the frequency range of 2–15.0 GHz. Furthermore, the microwave absorbency reaches the best when x ¼0.40 in the frequency range of 4– 16.0 GHz. The result originated from the change of complex permittivity and permeability when Ca2 þ ions were doped. So we can get better microwave absorbing properties by these changes.

4. Conclusions The barium hexaferrites doped with Ca2 þ , Ba1  xCaxCo2Fe16O27 (x ¼0, 0.1, 0.3, 0.4 and 0.5) ferrites were investigated. XRD analysis reveals that all the samples are all hexagonal platelet-like W-type barium ferrite. The lattice parameters a, c and the c/a ratio decrease in the range of W-type hexagonal with increasing Ca2 þ substitution, which shows that the Ca2 þ substitutions have an effect on cation occupation in the ferrites. The synthesized samples exhibited paramagnetism and strong magnetism. The substitution of Ca2 þ for Ba2 þ leads to the increase of ε′ and ε″. The Ca2 þ ions doped in the barium hexaferrite system can be utilized for improvement of Ms in order to improve the μ′′. Furthermore, the microwave absorbency increases with more addition of Ca2 þ ions when x r0.40, while it slightly decreases when x ¼0.50 in the frequency range of 2–18.0 GHz.

K. Huang et al. / Journal of Magnetism and Magnetic Materials 379 (2015) 16–21

Acknowledgements The authors acknowledge the financial support from the National Natural Science Foundation of China under Grant nos. 51272003, 51472004, 11375011 and 21271007, the Natural Science Foundation of Anhui province under Grant no.1408085MA12, the Key Program of the Education Department of Anhui Province (Grant nos. KJ2013B057 and KJ2012A027), and from the Research Fund for the Doctoral Program of Higher Education of China under project 20123401110008.

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