Structural, Electrical and Magnetic Properties of ...

Journal of Nano Research Vol. 14 (2011) pp 1-9 Online available since 2011/Apr/14 at www.scientific.net © (2011) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/JNanoR.14.1

Structural, Electrical and Magnetic Properties of Nanocrystalline Mg-Co Ferrites Prepared by Co-Precipitation M. Anis-ur-Rehman,1,a Muhammad Ali Malik,1,b Kishwar Khan,2,c and Asghari Maqsood2,d 1

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Applied Thermal Physics Laboratory, Department of Physics, COMSATS Institute of Information Technology, Islamabad, Pakistan

Thermal Transport Laboratory, School of Chemical and Materials Engineering, NUST, Islamabad, Pakistan Corresponding author: [email protected] Submitted: September 18, 2010; revised: December 05, 2010; accepted: December 13, 2010

Key words: Nanoparticles, ferrites, wet chemical, magnesium, dielectric constant

Abstract. Mg-Co nano crystalline ferrites having general formula Mg1-xCoxFe2O4 (x=0, 0.05, 0.1, 0.15, 0.2, 0.25) were prepared by co-precipitation method. X-ray powder diffraction (XRD) patterns of the prepared samples show the formation single spinel phase. The crystallite size, lattice parameters and porosity of samples were calculated by XRD data analysis as function of cobalt concentration. The crystallite size for each sample was calculated using the Scherrer formula considering the most intense (3 1 1) peak lies in the range 27-35 nm. The lattice parameters increased with increase of cobalt concentration. It is because of the fact that cobalt has greater ionic radius then magnesium. The dielectric constant, dielectric loss tangent and ac electrical conductivity of the prepared samples is also measure. The observed variation in electrical and dielectric properties is explained on the basis of cations distribution among tetrahedral (A) and octahedral (B) sites. The variance in saturation magnetization, remanence magnetization and magnetic moment was also measured from BH curve of samples. Introduction Nanocrystalline materials are offering a variety of novel features [1-2]. The surprising properties of nanomaterials have generated ever-increasing interest for scientific insight of materials. The nanosize magnetic particles have special properties as compared to bulk because of the large volume fraction that atoms occupy at surface [3] with decrease of particle size. Conventional antennas for the frequency band of 3–30 MHz (HF) and 30–300 MHz (VHF) are physically large, and therefore not suitable for portable applications. A pertinent challenge is to reduce the physical dimensions without affecting its electrical performances [4]. Mg-Co nanomaterials are potential candidates to miniaturize the size of antennas. The excellent combination of magnetic and dielectric properties of these materials can be used to fulfill the future demands of high frequency applications. As the technology advances, the miniaturization of electronic devices and the increase of operational frequency are in demand. To improve the performance of these materials and size miniaturization, the current focus is on nanotechnology. The size miniaturization of high frequency antenna can be achieved by using the materials of higher refractive index n = √εrµ r where n is the refractive index, εr and µr are the relative permittivity and permeability of the material, respectively). The dielectric constant of our prepared nanocrystalline Mg ferrite samples is 14, which is larger than the already reported value of 7 for their bulk counterpart [4]. In the present work, the effect of cobalt concentration on structural, dielectric, electrical and magnetic properties of magnesium ferrites is reported. The investigation of dielectric properties as the function of frequency has revealed the usefulness of these materials in HF, VHF and microwave region. The co-precipitated Mg-Co ferrites have showed the enhanced dielectric properties, which is due to the impact of nanometer size regime.

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Journal of Nano Research Vol. 14

Experimental Procedure The chemical co-precipitation method was used to synthesize the polycrystalline powders of Mg spinel ferrites. The co-precipitating aqueous solutions of Fe(NO3)3·9H2O, Mg(NO3)2·6H2O and Co(NO3)2·6H2O mixtures were prepared in deionized water. The co-precipitation reaction occurred in two steps: the co-precipitation step and ferritisation step [5]. In co-precipitation step, the precipitating agent NaOH was mixed in the solution and the metal hydroxides were obtained in the form of colloidal particles. The precipitating reagent (NaOH) was mixed quickly into the metal solutions to obtain ferrites of a smaller, less dispersed in size and more chemically homogeneous with constant stirring, until co-precipitation occurred. In ferritization step, the colloidal particles of metal hydroxides were heated at 75˚C for 40 min, which result in transformation of metal hydroxides to the Mg ferrite. The product was washed with deionized water several times to remove sodium and nitrate ion impurities from Mg-Co ferrites precipitates. The precipitates were dried at 100˚C for several hours. Polycrystalline powder was then pelletized in circular disks, having diameter of 13 mm, by applying uniaxial pressure of 1000 lb/cm2 for 2 minutes using hydraulic press. The pellets of Mg1-xCoxFe2O4 (x=0, 0.05, 0.1, 0.15, 0.2, 0.25) powder were made and sintered at temperatures of 950±5˚C for 3 h, and were named accordingly M-100, M-95, M-90, M85, M-80, M-75. Characterization The crystallite size of a polycrystalline material was calculated using Scherrer formula [6]: ,

(1)

where t is the crystallite size, λ is the wave length of incident X-ray, θB is the diffraction angle and β is the full width at half maximum (FWHM). The porosity (P) of sample was determined using the formula [7] ,

(2)

where dm and dx are the measured density and the theoretical density respectively. The measured density was calculated using the relation ,

(3)

where h the height, r the radius and m is the mass of cylindrical pellet of sample. The theoretical density was calculated by using the formula .

(4)

In spinel structure, 8 subcells form a unit cell. Here 8 is the number of formula units in a unit cell, N is the Avogadro’s number, M is the molecular weight of the one formula unit and V is the volume of the unit cell. The complex relative permittivity of the prepared samples was measured in the frequency range of 1 MHz to 1 GHz using Agilent 4991A by the capacitance method. The dielectric constant (εr) was calculated by the relation [8] ,

(5)

where d is the thickness, C is capacitance, εo is the permittivity of free space, and A the crosssectional area of the pellet. of the prepared samples was measured using relation The dielectric loss tangent ( , where ε” and ε’ are the imaginary and real parts of the dielectric constant, respectively.

(6)


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The ac conductivity in the frequency range of 300 Hz to 3 MHz was determined using the values of dielectric constant (εr) and dielectric loss tangent (tanδ) in the relation [9]: ,

(7)

where σac is the ac conductivity, ω is the angular frequency and εo is the permittivity of free space. Results and Discussion Structural Properties

(440)

(511)

(400)

(311)

(220)

The X-ray diffraction patterns of the samples shown in Figure 1 indicated the formation of single spinel phase. The crystallite size was calculated from the X-ray diffraction most intense peak (311) lies in the range of 27 to 35 nm. All the peaks of cubic crystal system corresponding to space group Fd-3m were indexed with the standard pattern for MgFe2O4 reported in ICDD PDF card # 00-0011120. The calculated values of crystallite size, lattice parameter, mass density, theoretical density and porosity of Mg-Co ferrites samples as a function of cobalt concentration are given in table 1. The lattice parameters are almost constant within the experimental errors for the prepared samples in our case; a slightly different behavior is reported by Ahmed and EL-Khawlani [10], this may be due to the variation in range of Co concentration.

M-100

Intensity (a.u.)

M-95 M-90 M-85

M-80

M-75

20

30

40

50

60

2θ degrees

Fig. 1 XRD patterns of Mg1-xCoxFe2O4 samples sintered at 950˚C.

70

4


Fig. 2 SEM micrographs of (a) M-100, (b) M-95, (c) M-90, (d) M-85, (e) M-80 and (f) M-75. Scanning electron micrographs were carried out for all the samples are shown in Figure 2. It is clear that the small particles of the prepared Mg-Co ferrites agglomerated with each other due to the magnetic interaction. The morphology of the samples M-95, M-90, M-85 and M-75 has indicated the formation of smooth external faces. The dielectric constant (εr) of the samples was calculated using capacitive method, in the frequency range of 1 MHz to 1 GHz, are shown in the Figure 3. The calculations of εr in the frequency range of 1 MHz-1 GHz were carried out using Agilent 4991A Impedance analyzer. The dielectric constant of the prepared sample remains constant in the in the frequency range of 30 MHz to 1 GHz, which makes these materials suitable for the use in VHF devices. The size miniaturization of high frequency devices is proportional to the square root of dielectric constant of matrix material. The dielectric constant of the prepared nanocrystalline Mg-Co ferrites lies in the range of 12-15 in VHF and microwave frequency region, which makes possible the size miniaturization of devices [4]. Figure 3 indicated that the dielectric constant also depend upon the increase of cobalt concentration in magnesium ferrite. The composition dependence of dielectric constant of prepared samples can be described on the basis of cation distribution. The magnesium ferrites in bulk form have an inverse spinel. But Pradeep et al. reported that the nano crystalline MgFe2O4 prepared by wet chemical method tend to exist as mixed spinel structure [11]. Moreover, the dopant cobalt ions tend to occupy the octahedral site.

Dielectric Constant (ε )


M-100 M-95 M-90 M-85 M-80

18

5

Fig. 3 Dielectric constant (εr) of Mg1-xCoxFe2O4 spinel ferrites in frequency range of 1 MHz to 1 GHz.

16

14

12

10 0

200

400

600

800

1000

Frequency (MHz)

Dielectric Properties The dielectric constant of M-100 sample is maximum, which then decreases with increase in cobalt concentration. The increase of cobalt concentration resulted in replacement of magnesium ions as well as migration of Fe ions from B site to A site. This resulted in decrease in electron hopping between Fe2+ and Fe3+ ions. Moreover, the nearest distance pairs of Fe2+–Fe3+ decrease in B-sites with the increase of cobalt ions which tend to occupy octahedral sites. It resulted in the electron hopping between Fe2+ and Fe3+ ions and hole hopping Co3+ and Co2+ ions happen to longer distance in B-sites [12]. The dielectric loss tangent (tan δ) of prepared samples were measured in the frequency range of 1 MHz-1 GHz. The figure 4 indicated that the dielectric loss tangent also depend upon composition of prepared samples. The dielectric loss tangent of the prepared sample is very small and is of the order of 10-2, which makes these materials suitable for the use in HF, VHF and microwave frequency region. For the present case, the decrease in conductivity of the samples with increase in cobalt concentration resulted in decrease in dielectric losses. M-100 M-95 M-90 M-85 M-80

0.40 0.35 0.30

Fig. 4 Dielectric constant tangent (tanδ) of Mg1-xCoxF2O4 spinel ferrites in frequency range of 1 MHz to 1 GHz.

tanδ

0.25 0.20 0.15 0.10 0.05 0.00 0

200

400

600

800

1000

Frequency (MHz)

AC Electrical Properties AC electrical conductivity (σac) of nanocrystalline Mg-Co ferrite figure 5. At lower frequency, the dc resistivity potion of electrical conductivity is dominant. As the frequency increased, the σac increased with the increase in frequency. The ac electrical conductivity of M-100 sample is maximum, which then decreases with increase in cobalt concentration. The increase of cobalt concentration on octahedral site has resulted in migration of Fe ion from octahedral site to tetrahedral site, as elaborated above.

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M-100 M-95 M-90 M-85 M-80 M-75

-10

ln σ ac (S / m)

-12

-14

-16

4

6

8

10

12

14

16

ln f (Hz)

Fig. 5 AC conductivity (

) as a function of frequency of Mg-Co spinel ferrites.

Magnetic Properties Neel described that the structure of magnetic spinel consist of two sublattices, called the tetrahedral and octahedral sublattices respectively. The magnetization of sublattices acts in opposite direction to each other. The net magnetization of materials is the difference of the magnetizations of two sublattices (M = MB – MA). The plots of MH-curve measure by pulsed field magnetometer for MgCo ferrites as shown in the figure 6. The black curves in figure 6 state the response of magnetization as the function of applied magnetic field, while red curves correspond to the flux The saturation magnetization, remnant magnetization and coercivity of Mg-Co ferrite materials as the function of cobalt concentration are given in table 1. The saturation magnetization and remnant magnetization initially increases, attains the maximum value for M-85 sample and then decreases for the rest of samples. The behavior of saturation magnetization of Mg-Co ferrites as function of cobalt concentration can be explained on the basis of cations distribution on two sublattices. The initial increase in saturation magnetization is due to replacement of diamagnetic Mg2+ ion by Co2+ ions, which have the magnetic moment of 3 µB. Therefore, the saturation magnetization of the prepared samples initially increased and attained the maximum value of for M-85 sample. The decrease of saturation magnetization of M-80 and M-75 samples is the result of migration of Fe3+ ions from B site to A site, which have the magnetic dipole moment of 5 µB. The coercivity of all prepared sample increased monotonically with increase in cobalt concentration. The similar behavior was noted by the Kambally et al [13]. It is because of the reason that the cobalt ferrite have greater anisotropic constant as compared to that of magnesium ferrites.


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Fig. 6 Hysteresis loops of nanocrystalline Mg-Co spinel ferrites. Conclusions The doping of the cobalt has caused appreciable changes in the structural and electrical transport properties of magnesium ferrites. The Mg-Co ferrites were successfully prepared by chemical coprecipitation method. The X-ray diffraction patterns exhibited the formation of single phase spinel structure. The lattice parameters are almost constant within the experimental errors for the prepared samples. Dielectric loss tangents of prepared Mg-Co ferrites are very small and are of the order of 10-2.The dielectric constant of our prepared Mg-Co samples lies in the range of 12-15, which is larger than previously reported value bulk magnesium ferrites. This makes nanocrystalline materials

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Table 1. Crystallite size (D(311)), lattice constant (a), lattice volume (V), measured density (dm), X-ray density (dx), porosity (P), dielectric constant (εr) and dielectric loss tangent (tanδ), saturation magnetization (MS), remnant magnetization (Mr) and coercivity (Hc) of Mg1xCoxFe2O4 samples as the function of Co concentration. Parameter

M-100

M-95

M-90

M-85

M-80

M-75

D(311) (nm)

32±3

27±2

28±2

30±2

31± 2

35±3

8.36(1) 584

8.35(1) 582

8.36(1) 584

8.36(1) 584

8.37(1) 586

8.37(1) 586

3.62

3.60

3.62

3.78

3.79

3.88

4.54 20 13.96 11.85 0.08 0.06 47 11 21

4.48 22 12.83 10.49 0.05 0.05 48 17 146

4.62 22 12.63 10.75 0.04 0.04 50 24 229

4..67 19 12.14 10.25 0.03 0.02 95 58 312

4.69 19 11.99 10.18 0.01 0.01 71 42 343

4.73 18 64 36 375

a (Å ) V (Å3) (g/cm3) (g /cm3) P (%) εr at 300 MHz εr at 1 GHz tan δ at 100 MHz tan δ at 1 GHz

MS (emu/cm3) Mr (emu/cm3) Hc (Oe)

suitable for size miniaturization of high frequency antenna. Moreover, these materials retain their dielectric properties up to 1 GHz, which proves their usefulness in microwave region. The decrease in the values dielectric constant and dielectric loss tangent of nanocrystalline Mg-Co ferrites with increase in cobalt concentration makes tuning of impedance of prepared materials for a specific application. Acknowledgements The authors would like to acknowledge Higher Education Commission (HEC), Islamabad, Pakistan for providing financial support for this work through NRPU # 893.

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Journal of Nano Research Vol. 14 doi:10.4028/www.scientific.net/JNanoR.14 Structural, Electrical and Magnetic Properties of Nanocrystalline Mg-Co Ferrites Prepared by Co-Precipitation doi:10.4028/www.scientific.net/JNanoR.14.1