Synthesis, structural, magnetic and dielectric properties of zirconium copper doped M-type calcium strontium hexaferrites

Journal of Alloys and Compounds 617 (2014) 437–443

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Synthesis, structural, magnetic and dielectric properties of zirconium copper doped M-type calcium strontium hexaferrites Muhammad Naeem Ashiq a, Raheela Beenish Qureshi a, Muhammad Aslam Malana a,⇑, Muhammad Fahad Ehsan b a b

Institute of Chemical Sciences, Bahauddin Zakariya University, Multan 60800, Pakistan National Center for Nanoscience & Technology (NCNST), 11 Beiyitiao, Zhongguawn, Beijing, China

a r t i c l e

i n f o

Article history: Received 1 July 2014 Received in revised form 31 July 2014 Accepted 1 August 2014 Available online 10 August 2014 Keywords: Nanostructured materials Chemical synthesis Dielectric response Magnetization X-ray diffraction Scanning electron microscopy

a b s t r a c t Zirconium copper substituted calcium strontium hexagonal ferrites with composition Ca0.5Sr0.5Fe122xZrx CuxO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8) have been synthesized by the chemical co precipitation procedure. These compounds were characterized by X-ray diffraction, thermogravimetry and scanning electron microscopy. Dielectric and magnetic properties of these hexaferrites were also explored. XRD analysis confirmed the single hexagonal phase of all the compounds and the average crystallite size was between 37 and 47 nm. The dielectric parameters show relaxation behaviour at higher frequencies. The values of dielectric parameters increase with dopants. In the range of magnetic field studied, the saturation magnetization decreases as the dopant contents increase which may be due to the nonmagnetic character of the substituents. The coercivity of the Zr–Cu doped derivatives of strontium calcium hexaferrites is increased up to x = 0.2 and then decreased. The values of coercivity are above 600 Oe which make them suitable materials for use in industries in longitudinal magnetic recording media. Ó 2014 Elsevier B.V. All rights reserved.

1. Introduction Hexagonal ferrites have gained lots of attention due to their remarkable physical and chemical properties since last few decades. Exhaustive research work on these ferrites has been initiated due to their captivating applications in microwave devices, computer memory chip, transformer, radio frequency coils, micro strip antennas and high density recording media [1,2]. Ferrites with hexagonal structure can be classified into six different classes: M-type (AFe12O19), W-type (AB2Fe16O27), X-type (A2B2Fe20O46), Y-type (A2B2Fe12O22), Z-type (A3B2Fe24O41) and U-type (A4B2Fe36O60) where A represents Ba, Sr, Pb, Ca and B is bivalent cations of transition metal like Ni, Co, Zn [3], etc. The M-type nanohexagonal ferrite structure is based on hexagonal crystal lattice with 64 ions per unit cell on 11 different symmetry sites. In this structure smaller Fe3+ cations are arranged over five distinct interstitial sites namely three octahedral sites (12k, 2a, 4f2), one tetrahedral (4f1) and one trigonal bipyramidal site (2b) [4] in which 5 oxygen atoms surround the Fe3+cation. Excellent magnetic behaviour of these ferrites can be described by the ordering of magnetic moments of the Fe3+cations and the super exchange interaction, in which three ⇑ Corresponding author. Tel.: +92 61 9210092; fax: +92 61 9210085. E-mail address: [email protected] (M.A. Malana). http://dx.doi.org/10.1016/j.jallcom.2014.08.015 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.

parallel (12k, 2a, 2b) and two antiparallel sites (4f1, 4f2) are coupled via O2 ions [5]. These hexaferrites (M-type) have been widely studied because of their magnificent chemical stability, greater microwave magnetic loss, high Curie temperature and relatively large magnetization [6,7]. At higher frequency region, these nanomaterials are considered more useful as compared to other magnetic materials attributable to their good electrical properties such as low eddy current losses and high resistivity [8]. The electrical and magnetic properties of hexaferrites are influenced by the method of synthesis, average particle size, chemical composition and dopants [9–11]. In general, the typical method to obtain the metal oxide materials is solid state reaction at high temperature i.e. 1200 °C [12]. However, it is very difficult to synthesize pure, strain free, smaller particle size and homogenous nanohexagonal ferrites by this typical method. Hence, other methods such as aerosol pyrolysis [13], micro emulsion [14], dehydration and rotary evaporation, sol–gel combustion and chemical co-precipitation methods have also been employed [9]. In the present work, it was decided to use chemical co-precipitation method to prepare the nanomaterials having pure phase and small average crystallite size. The electrical and magnetic properties of the nanohexagonal ferrite materials can be modified by the addition of different cations at M (Ca, Sr, Ba, Pb) as well as Fe3+ sites. Various cations and

M.N. Ashiq et al. / Journal of Alloys and Compounds 617 (2014) 437–443

combination of cations (bivalent-tetravalent) have been reported to substitute in M-type hexaferrites by many researchers [9,15– 17]. Literature gives a lot of evidences of research on the SrFe12O19, BaFe12O19, Ba0.5Sr0.5Fe12O19 and their doped derivatives [9,15–18]. However, Ca0.5Sr0.5Fe12O19 and its tetravalent-divalent cations substituted derivatives have received least attention. Therefore, the main focus of this work is to study the effect of substitution of Fe3+ by Zr4+ and Cu2+ on the electrical and magnetic properties of Ca0.5Sr0.5Fe12O19. Thermal and structural (SEM and XRD) analyses of the synthesized nanomaterials are also discussed in this manuscript.

100

3.27min 89.95°C 92.53%

95

Weight (%)

438

9.19min 207.25°C 89.02%

90

36.49min 751.10°C 80.99%

15.34min 330.47°C 81.38%

85

2. Experimental 80 0

2.1. Chemicals

200

400

600

Temperature (°C) The chemicals used in this work are Fe(NO3)39H2O (97% Riedel Dehaen), CaCl22H2O (99% Merck), Sr(NO3)3 (99%Fluka), ZrOCl28H2O(98% Merck), CuCl22H2O(99% Merck) and ammonia solution (33% Merck).

800

1000

Universal V4.2E TA Instruments

Fig. 1. Thermogram of Sr0.5Ca0.5Fe10.4Zr0.8Cu0.8O19.

2.2. Synthesis of Sr0.5Ca0.5Fe122xZrxCuxO19 (x = 0.0–0.8)

x=0.0 114 202

107

3. Results and discussion 3.1. Thermal analysis TG curve of the unannealed sample Ca0.5Sr0.5Fe10.4Zr0.8Cu0.8O19 (Fig. 1) shows total weight loss of 19.01% in four steps. The first weight loss of 7.47% in the region 50–89 °C can be attributed to the removal of free water. The second step of 3.51% weight loss in the region 90–207 °C can be ascribed to the exclusion of hydrated water. A weight loss of 7.64% occurring in the range from 208 to 330 °C corresponds to the conversion of metal hydroxides to their oxides. The last weight loss of 0.39% in the region 331–751 °C may be related to the beginning of formation of hexagonal phase in the metal oxides. 3.2. XRD analysis X-ray diffraction analysis was used to examine the crystalline structure and phase purity of the Ca0.5Sr0.5Fe122xZrxCuxO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8). The indexed powder XRD patterns of the prepared samples are shown in Fig. 2. All the diffraction peaks

300

2110 2013

x=0.4

214 300

2110 2013

x=0.6

205

214 300

2110 2013

40

50

107 201 205 205 107 201 006

10

20

30

x=0.2

2110 2013

205

006

0016

214 300

107 201 006

2.3. Characterization Thermogravimetric analysis (TGA) of the synthesized sample (Ca0.5Sr0.5Fe10.4Zr0.8Cu0.8O19) was performed to observe structural changes during heating on Universal 4.2E TA Instruments. The nanomaterial was run from 303 K to 1273 K at heating rate of 10 °C/min. The crystalline structure of the synthesized hexaferrites was identified by PAnalytical X-ray Diffractometer using Cu Ka as radiation source. The synthesized materials were characterized by Scanning electron microscope (JEOL-JSM-6700F) to visualize their microstructure i.e. shape, grain size and morphology. The magnetic properties of the synthesized materials at ambient temperature were studied by Lake Shore-74071 vibrating sample magnetometer (VSM). The dielectric properties of these samples were recorded as a function of frequency in the range 1 MHz–3 GHz at room temperature by using a RF Impedance/Material Analyzer, Agilent E4991A.

201

006

Intensity

Zirconium and copper doped M-type Sr–Ca hexaferrites with composition Sr0.5Ca0.5Fe122xZrxCuxO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8) have been synthesized by chemical co-precipitation method. The required amounts of the metal salts were dissolved in deionized water in each 250 ml measuring flask to prepare the solutions. All these solutions were then homogenously mixed in a 2000 ml beaker by using hot plate with magnetic stirrer. After half an hour, 4.0 M ammonia solution (precipitating agent) was started to add drop wise in the reaction mixture and the pH of the reaction mixture was maintained between 10 and 11 to achieve the maximum precipitation. Then the solution was stirred at 343 K for 3 h. The resulted precipitates were washed by deionized water to take out their water soluble impurities i.e. nitrates and chlorides, etc. and dried in an electric oven at 373 K for 48 h. The dried precursors were annealed at 1273 K for 6 h in a furnace (VULCAN™ A-550) to obtain the single phase M-type hexagonal structure of the compounds.

1110300

214

60

317

x=0.8 317

70

80

2 theta Fig. 2. XRD patterns of Sr0.5Ca0.5Fe12–2xZrxCuxO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8) samples.

were matched with ICSD pattern with reference code 01-080-1197. The XRD patterns confirm the formation of single phase magnetoplumbite structure (M-Type) in all the synthesized materials. The slight alteration in peak position may be attributed to the substitution of zirconium and copper. The separate peak of these substituted cations is not appeared which confirmed their successful substitution in the Ca0.5Sr0.5Fe12O19 [19]. The average crystallite sizes of all the samples were calculated by Scherrer’s formula (Eq. (1)) [20].

D ¼ kk=b cos h

ð1Þ

Here k is wavelength of X-rays used i.e. 1.541 Å, b is full width at half maximum, h is Bragg’s angle and K is Scherer’s constant i.e. 0.9. The average crystallite size was in the range of 37–47 nm that is much smaller than many of those already reported [21,22]. In order to obtain the suitable signal to noise ratio, the particle size less than 50 nm is required in the high density recording media [23]. In the current studies, the synthesized compounds have particle size small enough (<50 nm) to get the suitable signal to noise ratio in high density recording media. 3.3. SEM analysis Scanning electron micrographs of the Sr0.5Ca0.5Fe12O19 and Sr0.5Ca0.5Fe1210.8Zr0.6Cu0.6O19 (Fig. 3(a) and (b)) were obtained to

M.N. Ashiq et al. / Journal of Alloys and Compounds 617 (2014) 437–443

439

Fig. 3. SEM micrographs of (A) Sr0.5Ca0.5Fe12O19 and (B) Sr0.5Ca0.5Fe12–10.8Zr0.6Cu0.6O19.

investigate their microstructure and grain size distribution. It can be seen that the undoped sample (Ca0.5Sr0.5Fe12O19) has a plate like structure having grain size between 0.263 and 1.05 lm. The orientation of the grains is regular and the particles have narrow range of grain size. The Zr–Cu substituted derivative of M-type Sr–Ca hexaferrite (Fig. 3b) also shows the platelet like shape of the particles having grain size in the range of 0.263–4.208 lm. It is clear from the figure that the particles indicate the strong inter grain connectivity by the doping of Zr4+ and Cu2+.

3.4. Magnetic properties The hysteresis loops of the synthesized Ca0.5Sr0.5Fe122xZrxCuxO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8) are illustrated in Fig. 4 and the values of different magnetic parameters i.e. saturation magnetization (Ms), remanence (Mr) and coercivity (Hc) are given in Table 1. In M-type hexaferrites, the Fe3+ ions inhabit three different symmetry sites i.e. octahedral, tetrahedral and trigonal bipyramidal. These Fe3+ ions are coupled via O2 anions by super exchange interaction ensuing in the formation of ferromagnetic structure [24–26]. In M-type hexaferrites, there are two formula units per unit cell and net magnetic moment of 20 lB per formula unit [27]. The beginning of magnetic behaviour in ferrites is due to net magnetic moment of ions with up and down spin in various symmetry sites, as indicated by ferromagnetic theory. It is obvious from Table 1 that the saturation magnetization and remanence fall as the concentration of dopants (Zr4+ and Cu2+) increases. This decrease in Ms firstly, may be ascribed to the magnetic dilution

40

Magnetization (emu/g)

30 20

x=0.0 x=0.2 x=0.4 x=0.6 x=0.8

due to the substitution of Fe3+ ions by nonmagnetic (Zr4+) and less magnetic (Cu2+) ions. Secondly, these dopants also lessen the super exchange interaction which may also cause the reduction of the saturation magnetization. The squareness ratio (Mrs) of all the compounds, calculated as the ratio of remanence and saturation magnetization, is given in Table 1. The compounds having squareness ratio P0.5 are in single magnetic domain while those having squareness ratio 60.5 are in multi magnetic domain [28]. The squareness ratio of all the synthesized compounds is less than 0.5 thus making them multi magnetic domain. It is clear from Table 1 that the coercivity first increases when x = 0.0–0.2 then drops in the remaining concentration range (x = 0.4–0.8) due to substitution of Zr4+ and Cu2+ ions. The initial increase may be due to the reduced magnetic exchange coupling [29] by the doping of nonmagnetic Zr4+ and less magnetic Cu2+ ions. The abrupt fall in coercivity may be interpreted in terms of reduced magnetocrystalline anisotropy. It has been reported that the main contributors of magnetocrystalline anisotropy are 12k, 4f2 (octahedral) and 2b (trigonal bipyramidal) [30]. It is known that Cu2+ions prefer to reside in 4f2 site while Zr4+ ions select to occupy the 2b sites [31,32]. These site preferences of dopant cations cause negative influence on magnetocrystalline anisotropy resulting in decreased coercivity of the compounds. The synthesized nanomaterials have coercivity value above 600 Oe, making them valuable candidates in industries for longitudinal magnetic recording media [33]. Additionally, it was also pointed out [34] that if the value of coercivity is greater than Mr/2 the nanomaterials are hard magnets and these are considered valuable for high frequency applications. If the coercivity is less than Mr/2, then the nanomaterials are semi hard magnets and are useful in information storage technology [35]. In the current studies, all the synthesized zirconium and copper doped calcium strontium hexaferrites have Hc > Mr/2 thus these materials are hard ferrites and are beneficial in high frequency application.

10

3.5. Dielectric measurements 0 -10 -20 -30 -40 -100000 -80000 -60000 -40000 -20000

0

20000 40000 60000 80000 100000

Magnetic field (Oe) Fig. 4. Hysteresis loops of the Sr0.5Ca0.5Fe12–2xZrxCuxO19 (x = 0.0, 0.2, 0.4, 0.6, 0.8) samples.

3.5.1. Frequency dependence The dielectric properties of the Ca0.5Sr0.5Fe122xZrxCuxO19 (x = 0.0, 0.4, 0.6, 0.8) were investigated in 1.0 MHz to 3.0 GHz frequency range at ambient temperature. The frequency dependence of dielectric constant (e0 ), dielectric loss (e00 ) and dielectric tangent loss (tan d) of the prepared ferrites is shown in Figs. 5–7. It is apparent from the figures that the e0 , e00 and tan d are nearly constant up to a definite frequency i.e. 1.79–1.94 GHz, 1.91–1.98 GHz and 1.92–1.98 GHz, respectively, beyond which resonance type behaviour is observed. In general, the dielectric dispersion is greater in low frequency regime (<500 Hz) and it comes to be independent of frequency

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Table 1 Magnetic parameters calculated from the hysteresis loops of the synthesized compounds. Ca0.5Sr0.5Fe122xZrxCuxO19

x = 0.0

x = 0.2

x = 0.4

x = 0.6

x = 0.8

Saturation magnetization (emu/g) Remanence (emu/g) Coercivity (Oe) Squareness ratio Mr/2

34.477 11.593 681.81 0.33 5.79

29.627 10.136 738.80 0.34 5.06

27.132 9.894 716.67 0.36 4.94

25.133 9.721 679.09 0.38 4.86

23.835 8.352 623.97 0.35 4.17

30

x=0.0 x=0.4 x=0.6 x=0.8

40

25 0.000

35

x=0.6

Dielectric Loss

20

Dielectric constant

30

25

-0.005

15

10

-0.010

20

0.00E+000

1.00E+009

2.00E+009

3.00E+009

5 15 0 10 0.00E+000

1.00E+009

2.00E+009

0.00E+000

3.00E+009

2.00E+009

Fig. 6b. Frequency dependence of dielectric loss (e00 ) of the synthesized samples Ca0.5Sr0.5Fe10.8Zr0.6Cu0.6O19 at room temperature and inset represents the frequency dependence of dielectric loss (e00 ) of the Ca0.5Sr0.5Fe10.4Zr0.8Cu0.8O19 at room temperature.

x=0.0 x=0.4

2.0

3.00E+009

Frequency (Hz)

Frequency (Hz) Fig. 5. Frequency dependence of dielectric constant of the synthesized samples Ca0.5Sr0.5Fe12–2xZrxCuxO19 (x = 0.0–0.8) at room temperature.

1.00E+009

x=0.0 x=0.4

0.15

1.5 0.10

0.05

0.5

Dielectric loss

Dielectric Loss

1.0

0.0 -0.5

0.00

-0.05

-1.0 -0.10

-1.5 0.00E+000

1.00E+009

2.00E+009

3.00E+009

Frequency (Hz) Fig. 6a. Frequency dependence of dielectric loss (e00 ) of the synthesized samples Ca0.5Sr0.5Fe12–2xZrxCuxO19 (x = 0.0 and 0.4) at room temperature.

0.00E+000

1.00E+009

2.00E+009

3.00E+009

Frequency (Hz) Fig. 7a. Frequency dependence of dielectric tan loss of the synthesized samples Ca0.5Sr0.5Fe12–2xZrxCuxO19 (x = 0.0 and 0.4) at room temperature.

when the intensity of the electric field increases. It may be explained on the basis of induced electric moment in the material in the presence of external electric field. When frequency of the electric field rises, the polarization of the induced electric moment cannot synchronize the applied frequency. Hence, the dielectric material attains a constant value in high frequency regime [36]. In the current studies, the first constant trend of dielectric constant and dielectric loss may be related to the lack of coordination of induced electric moment with the electric field as a result of which dielectric dispersion is nearly independent of applied frequency. After a certain frequency of the external field, the

electronic exchange between Fe2+ and Fe3+ becomes equal to the frequency of applied field therefore resonance type behaviour appears in the synthesized hexaferrites [37,38]. First resonance peak appeared at lower frequency i.e. 1.79–1.98 GHz and second at higher frequency i.e. 2.52–2.58 GHz, in all synthesized samples. Former peak is ascribed to the interfacial polarization and later may be due to ionic relaxation. The high ionic relaxation in all the samples may be attributed to the variable valence state of ions [39]. It is also a probability that the synthesized nanomaterials may have higher magnitude of dielectric dispersion in low fre-

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M.N. Ashiq et al. / Journal of Alloys and Compounds 617 (2014) 437–443 0.04

x=0.0 x=0.4

0.30 0.25

x=0.6

0.00

0.20 0.15

-0.02 -0.04

1.0

0.10

Conductivity

Dielectric Loss Factor

0.02

0.8

x=0.8

0.6

-0.06 0.4

-0.08

0.05 0.00 -0.05

0.2

-0.10 0.0

-0.10

0.00E+000

1.00E+009

2.00E+009

-0.15

3.00E+009

-0.20

-0.12 0.00E+000

1.00E+009

2.00E+009

3.00E+009

-0.25 0.00E+000

Frequency (Hz)

1.00E+009

2.00E+009

3.00E+009

Frequency (Hz) Fig. 7b. Frequency dependence of dielectric tan loss of the Ca0.5Sr0.5Fe10.8Zr0.6Cu0.6O19 and inset represents the frequency dependence of dielectric tan loss of the Ca0.5Sr0.5Fe10.4Zr0.8Cu0.8O19 at room temperature.

quencies than the observed range because of space charge polarization at the grain boundaries as reported in literature for M-type hexaferrites [20].

(4f2) site thus, dielectric constant shows decline. Hence, this material (x = 0.8) is best among all the synthesized Zr–Cu doped derivatives of calcium strontium hexaferrites for high frequency applications [20]. Dielectric tan loss also exhibits the similar trend as that of the dielectric constant and dielectric loss (Fig. 10). 3.5.3. Room temperature AC conductivity Eq. (3) is used to calculate the room temperature AC conductivity of the synthesized materials which is as follows:

rac ¼ xeo e0 tan d

ð2Þ

x ¼ 2pf

ð3Þ

The room temperature AC conductivity calculated for all compositions are plotted as a function of frequency in Fig. 9. It is seen from the figure that the ac conductivity slightly increases with frequency up to 1.95–1.98 GHz above which resonance type behaviour appears. This may be interpreted in a way that at low range of x=0.6 0.0004

Dielectric Loss

Dielectric Constant

x=0.8

0.0002

4

0.0000 -0.0002 -0.0004 -0.0006

Conductivity

3.5.2. Composition dependence It can be seen from the Fig. 8 that the dielectric constant (e0 ) and dielectric loss (e00 ) increase by the substitution of Zr4+ and Cu2+ in place of two Fe3+ ions. The dielectric dispersion can be explained in terms of creation of electric dipoles of positive ions with their neighbouring O2 ions but the principal source of polarization in ferrites is the hopping of electron between Fe3+ and Fe2+on octahedral symmetry sites. The jumping of electron (Fe2+–Fe3+) produces local displacement of charge carriers, thus initiating the dielectric polarization and relaxation [40–42]. The intensity of the hopping of electrons depends upon the number of Fe3+ – Fe2+ ion pair at the octahedral site in the unit cell of hexaferrites. It has been reported that the Zr4+ions substitute the Fe3+ ions at trigonal bipyramidal 2b site and Cu2+ ions prefer to occupy octahedral (4f2) site. When the Zr4+ ions arrive into the 2b site then some Fe3+ ions of this site may transfer to the octahedral site thus enhancing the Fe3+ – Fe2+ ion pair at this site. This raises the interfacial polarization as well as the e0 and e00 in the synthesized hexaferrites. It is observable from the Fig. 8 that at the highest level of doping (x = 0.8) the dielectric constant decreases. This drop may be due to the excess and dominancy of Cu2+at this doping level which lessens the polarization owing to the reduction in Fe3+at octahedral

Fig. 9a. Frequency dependence of conductivity of the samples Ca0.5Sr0.5Fe12– (x = 0.0 and 0.4) at room temperature.

2xZrxCuxO19

-0.0008 -0.0010 -0.0012

2

-0.0014 -0.0016 0.00E+000

1.00E+009

2.00E+009

3.00E+009

0

0.00E+000

1.00E+009

2.00E+009

3.00E+009

Frequency (Hz) Conc. of dopants Fig. 8. Composition dependence of (e0 and e00 ) of the synthesized compounds Ca0.5Sr0.5Fe12–2xZrxCuxO19 (x = 0.0–0.8) at room temperature.

Fig. 9b. Frequency dependence of conductivity of Ca0.5Sr0.5Fe10.8Zr0.6Cu0.6O19 and inset represents the frequency dependence of conductivity of the Ca0.5Sr0.5Fe10.4Zr0.8Cu0.8O19 at room temperature.

M.N. Ashiq et al. / Journal of Alloys and Compounds 617 (2014) 437–443

Conductivity

Dielectric Tan Loss

442

Conc. of dopants Fig. 10. Composition dependence of dielectric tan loss and conductivity of the synthesized compounds Ca0.5Sr0.5Fe12–2xZrxCuxO19 (x = 0.0–0.8) at room temperature.

frequency, the jumping of electrons between Fe2+ and Fe3+ is less but as frequency goes to the higher values, the hopping of electrons is increased and conductivity is also increased [19]. It may be attributable that as compared to the activation of wide-ranging diffusive conduction, only a fraction of energy is required for the hopping (backward and forward) of the electrons [43]. As pointed out by Murthy and Sobhanadri, [44] there is a strong correlation concerning dielectric and conductivity mechanism in ferrites. They supposed that conduction mechanism in ferrites is similar to the polarization process which is due to electronic exchange between Fe3+ and Fe2+. It is also true in the present studies because the dielectric constant, dielectric loss, dielectric tan loss and conductivity (Figs. 8 and 10) show same trend by the inclusion of Zr4+ and Cu2+ at the Fe3+ site in these synthesized compounds. 4. Conclusions Coprecipitatively synthesized Zr–Cu doped strontium calcium hexaferrites have the average crystallite size in the range of 37– 47 nm which is small enough to obtain appropriate signal to noise ratio for high density recording media. Dielectric constant increases with increase in dopant concentration up to x = 0.6 and then decrease (x = 0.8). Due to small value of dielectric constant, the last sample (Ca0.5Sr0.5Fe10.4Zr0.8Cu0.8O19) may be suitable candidate in high frequency applications. The fall in saturation magnetization in the synthesized samples is ascribed to the magnetic dilution and less super exchange interaction. Squareness ratio obtained for these compounds is less than 0.5 indicating their multi magnetic domain nature. The values of coercivity of all the synthesized samples are greater than Mr/2 reflecting their hard nature, thus, these samples are applicable in high frequency applications. References [1] H. Kojimain, Fundamental properties of hexagonal ferrites with magnetoplumbite structure, in: E.P. Wohlfarth (Ed.), Ferromagnetic materials, North Holland, Amsterdam, 1982. [2] M. H. Abdullah, S. H. Ahmel, Sains Malaysians 22 P I, 1993.; S.A. Mazen, M.H. Abdallah, R.I. Nakhla, H.M. Zaki, F. Metawe, X-ray analysis and IR absorption spectra of Li–Ge ferrite, Mater. Chem. Phys. 34 (1) (1993) 35–40. [3] J. Smith, H.P.J. Wijn, Ferrites, Philips Technical Library, Eindhoven, 1959. [4] J. Qiu, Y. Wang, M. Gu, Effect of Cr substitution on microwave absorption of BaFe12O19, Mater. Lett. 60 (2006) 2728–2732. [5] L. Lechevallier, J.M. Le Breton, A. Morel, J. Teillet, Structural and magnetic properties of Sr1xSmxFe12O19 hexagonal ferrites synthesised by a ceramic process, J. Alloys Comp. 359 (2003) 310–314. [6] N. Dishovske, A. Petkov, I. Nedkov, I. Razkazov, Hexaferrite contribution to microwave absorbers characteristics, IEEE Trans. Magn. 30 (1994) 969–971. [7] S.R. Janasi, M. Emura, F.J.G. Landgraf, D. Rodrigues, The effects of synthesis variables on the magnetic properties of coprecipitated barium ferrite powder, J. Magn. Magn. Mater. 238 (2002) 168–172.

[8] M.N. Ashiq, M.J. Iqbal, I.H. Gul, Structural, magnetic and dielectric properties of Zr–Cd substituted strontium hexaferrites (SrFe12O19) nanoparticles, J. Alloys Comp. 487 (2009) 341–345. [9] A. Davoodi, B. Hashemi, Magnetic properties of Sn–Mg substituted strontium hexaferrite nanoparticles synthesized via co precipitation method, J. Alloys Comp. 509 (2011) 5893–5896. [10] M.N. Ashiq, S. Saleem, M.A. Malana, A. Rehman, Physical, electrical and magnetic properties of nanocrystalline Zr–Ni doped Mn ferrite synthesized by co-precipitation method, J. Alloys Comp. 486 (2009) 640–644. [11] M. Hashim, Alimuddin, S. Kumar, B.H. Koo, S.E. Shirsath, E.M. Mohammed, J. Shah, R.K. Kotnala, H.K. Choi, H. Chung, R. Kumar, Structural, electrical and magnetic properties of Co–Cu ferrite nanoparticles, J. Alloys Comp. 518 (2012) 11–18. [12] M. Radwan, M.M. Rashad, M.M. Hessien, Synthesis and characterization of hexaferrite nanoparticles, J. Mater. Process. Technol. 181 (2007) 106–109. [13] T.G. Carreno, M.P. Morales, C.J. Serna, Barium ferrite nanoparticles prepared directly by aerosol pyrolysis, Mater. Lett. 43 (2000) 97–101. [14] J. Fang, J. Wang, L. Gan, S. Ng, J. Ding, X. Liu, Fine strontium ferrite powders from an ethanol-based microemulsion, J. Amer. Ceram. Soc. 83 (2000) 1049– 1055. [15] Y.S. Hong, C.M. Ho, H.Y. Hsu, C.T. Liu, Synthesis of nanocrystalline Ba(MnTi)xFe122xO19 powders by the sol–gel combustion method in citrate acid–metal nitrates system (x = 0, 0.5, 1.0, 1.5, 2.0), J. Magn. Magn. Mater. 279 (2004) 401–410. [16] Ashima, S. Sanghi, A. Agarwal, Reetu, Rietveld refinement, electrical properties and magnetic characteristics of Ca–Sr substituted barium hexaferrites, J. Alloys Comp. 513 (2012) 436–444. [17] Q. Fang, H. Cheng, K. Huang, J. Wang, R. Li, Y. Jiao, Doping effect on crystal structure and magnetic properties of chromium-substituted strontium hexaferrite nanoparticles, J. Magn. Magn. Mater. 294 (2005) 281–286. [18] C. Singh, S.B. Narang, I.S. Hudiara, Y. Bai, F. Tabatabaei, Static magnetic properties of Co and Ru substituted Ba–Sr ferrite, Mater. Res. Bull. 43 (2008) 176–184. [19] G. Asghar, M. Anis-ur-Rehman, Structural, dielectric and magnetic properties of Cr–Zn doped strontium hexa-ferrites for high frequency applications, J. Alloys Comp. 526 (2012) 85–90. [20] M.J. Iqbal, M.N. Ashiq, Physical and electrical properties of Zr–Cu substituted strontium hexaferrite nanoparticles synthesized by co-precipitation method, Chem. Eng. J. 136 (2008) 383. [21] M.J. Iqbal, M.N. Ashiq, I.H. Gul, Physical, electrical and dielectric properties of Ca-substituted strontium hexaferrite (SrFe12O19) nanoparticles synthesized by co-precipitation method, J. Magn. Magn. Mater. 322 (2010) 1720–1726. [22] C. Mu, N. Chen, X. Pan, X. Shen, X. Gu, Preparation and microwave absorption properties of barium ferrite nanorods, Mater. Lett. 62 (2008) 840–842. [23] S. Che, J. Wang, Q. Chen, Soft magnetic nanoparticles of BaFe12O19 fabricated under mild conditions, J. Phys. Condens. Matter 15 (2003) L335–L339. [24] L. Lechevallier, J.M. Le Breton, J.F. Wang, I.R. Harris, Structural analysis of hydrothermally synthesized Sr1xSmxFe12O19 hexagonal ferrites, J. Magn. Magn. Mater. 269 (2004) 192–196. [25] M. Pieper, A. Morel, F. Kools, NMR analysis of La + Co doped M-type ferrites, J. Magn. Magn. Mater. 242 (2002) 1408–1410. [26] A. Morel, J.M. Le Breton, J. Kreisel, G. Wiesinger, F. Kools, P. Tenaud, Sublattice occupation in Sr1xLaxFe12xCoxO19 hexagonal ferrite analyzed by Mössbauer spectrometry and Raman spectroscopy, J. Magn. Magn. Mater. 242 (2002) 1405–1407. [27] A. Goldman, Modern Ferrite Technology, second ed., Springer, Pittsburgh, PA, USA, 2006. [28] C. Sudakar, G.N. Subbanna, T.R.N. Kutty, Wet chemical synthesis of multicomponent hexaferrites by gel-to-crystallite conversion and their magnetic properties, J. Magn. Magn. Mater. 263 (2003) 253–268. [29] I. Bsoula, S.H. Mahmood, Magnetic and structural properties of BaFe12xGaxO19 nanoparticles, J. Alloys Comp. 489 (2010) 110–114. [30] Z. Yang, C.S. Wang, X.H. Li, H.X. Zang, (Zn, Ni, Ti) substituted barium ferrite particles with improved temperature coefficient of coercivity, Mater. Sci. Eng. B 90 (2002) 142–145. [31] H. Zhang, J. Zhou, Y. Wang, L. Li, Z. Yue, Z. Gui, Dielectric characteristics of novel Z-type planar hexaferrite with Cu modification, Mater. Lett. 55 (2002) 351–355. [32] A. Gruskova, J. Lipka, M. Papanova, D. Kevicka, A. Gonzales, G. Mendoza, I. Toth, J. Slama, Mössbauer study of microstructure and magnetic properties (Co, Ni)– Zr substituted Ba ferrite particles, Hyperfin Interact. 156–157 (2004) 187–194. [33] Y. Li, R. Liu, Z. Zhang, C. Xiong, Synthesis and characterization of nanocrystalline BaFe9.6Co0.8Ti0.8M0.8O19 particles, C. Mater. Chem. Phys. 64 (2000) 256–259. [34] R. Skomki, J.M.D. Coey, Permanent magnetism, British Library Cataloguing-inPublication Data. ISBN: 0750304782. [35] I. Ali, M.U. Islam, M.S. Awan, M. Ahmad, Effects of Ga–Cr substitution on structural and magnetic properties of hexaferrite (BaFe12O19) synthesized by sol–gel auto-combustion route, J. Alloys Comp. 547 (2013) 118–125. [36] H.V. Keer, Principles of the Solid State, New Age Int. Pub. Ltd., Mumbai, 2000. [37] M.B. Reddy, P.V. Reddy, Low-frequency dielectric behaviour of mixed Li–Ti ferrites, Appl. Phys. 24 (1991) 975–981. [38] S.C. Watawe, B.D. Sarwede, S.S. Bellad, B.D. Sutar, B.K. Chougule, Frequency and temperature-dependent dielectric properties of cobalt-substituted lithium ferrites, J. Magn. Magn. Mater. 214 (2000) 55–60.

M.N. Ashiq et al. / Journal of Alloys and Compounds 617 (2014) 437–443 [39] Y.P. Wang, L. Zhou, M.F. Zhang, X.Y. Chen, J.-M. Liu, Z.G. Liu, Room-temperature saturated ferroelectric polarization in BiFeO3 ceramics synthesized by rapid liquid phase sintering, Appl. Phys. Lett. 84 (2004) 1731–1733. [40] P. Singh, V.K. Babbar, A. Razdan, S.L. Srivastava, V.K. Agrawal, T.C. Goel, Dielectric constant, magnetic permeability and microwave absorption studies of hot-pressed Ba–Co Ti hexaferrite composites in X-band, J. Mater. Sci. 41 (2006) 7190–7196. [41] W. Jing, Z. Hong, B. Shuxin, C. Ke, Z. Changrui, Microwave absorbing properties of rare-earth elements substituted W-type barium ferrite, J. Magn. Magn. Mater. 312 (2007) 310–313.

443

[42] S.B. Narang, I.S. Hudiara, Microwave dielectric properties of M-Type barium, calcium and strontium hexaferrites substituted with Co and Ti, J. Cer. Proc. Res. 7 (2006) 113–116. [43] M.P. Kumar, T. Sankarappa, B.V. Kumar, N. Nagaraja, Dielectric relaxation studies in transition metal ions doped tellurite glasses, Solid State Sci. 11 (2009) 214–218. [44] V.R.K. Murthy, J. Sobhanadri, Dielectric properties of some nickel–zinc ferrites at radio frequency, Phys. Status Solidi A 36 (1976) K133–K135.