Study on the electromagnetic behavior evaluation of Y3+ doped cobalt nanocrystals synthesized via co-precipitation route

Journal of Magnetism and Magnetic Materials 372 (2014) 68–73

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Study on the electromagnetic behavior evaluation of Y3 þ doped cobalt nanocrystals synthesized via co-precipitation route M. Ishaque a,n, Muhammad Azhar Khan b,n, Irshad Ali a, Hasan M. Khan a, M. Asif Iqbal a,c, M.U. Islam a, Muhammad Farooq Warsi d a

Department of Physics, Bahauddin Zakariya University, Multan 60800, Pakistan Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan c College of E & ME, National University of Science and Technology, Islamabad, Pakistan d Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan b

art ic l e i nf o

a b s t r a c t

Article history: Received 24 May 2014 Received in revised form 29 June 2014 Available online 30 July 2014

A series of nanocrystalline cobalt ferrites doped with yttrium ions were synthesized by chemical coprecipitation technique. The X-ray diffraction analysis reveals that all the samples exhibit cubic spinel phase as main phase along with few traces of orthorhombic phase (YFeO3). The crystallite size calculated by Scherrer’s formula is found in the range of 48–34 nm. This crystallite size is small enough to obtain the suitable signal to noise ratio in the high density recording media applications. The lattice constant was found to decrease from 8.385 Å to 8.348 Å with the increase of yttrium contents which may be attributed to the solubility limit of yttrium ions. The dc electrical resistivity was found to increases from 4.95  106 Ω-cm to 8.39  107 Ω-cm with the increase of yttrium contents. Yttrium doped samples exhibit lower dielectric constant and dielectric loss tangent as compared to pure CoFe2O4 nanocrystals. An appreciable increase in coercivity has been observed by the Y3 þ addition. The enhanced dc electrical resistivity and coercivity (Hc ¼ 1273 Oe) of cobalt nanoparticles (5 wt% doped Y3 þ ) are favorable for their potential use in microwave devices and high density recording media applications. & 2014 Elsevier B.V. All rights reserved.

Keywords: Nanocrystals Co-precipitation XRD Electrical properties Dielectric properties Magnetic properties

1. Introduction Spinel ferrites nanoparticles have been considered as highly important electronic material over the last few decades due to their remarkable electrical and magnetic properties [1]. Nanosized ferrite material demonstrated novel properties that largely differ from their bulk counterparts due to their quantum size effect [2].This fact makes the nanocrystalline spinel ferrites the most promising in many applications like microwave devices, information storage system, recording media, ferrofluid technology and medical diagnostics. Among soft ferrites, cobalt ferrite is the most attractive material due to its high resistivity, smaller dielectric parameters, high coercivity, moderate saturation magnetization, high cubic magnetocrystalline anisotropy, high chemical stability and reasonable mechanical hardness [3]. It is known that additives play an important role in improving the electrical and magnetic properties of spinel ferrites [4]. For example, the role of addition of small amount of Si in Co–Zn ferrites [5] drastically increases the electrical resistivity. The addition of small amount of Er in Mn–Zn ferrites lowered the power losses due to n

Corresponding authors. Tel.: þ 92 3316807179/ þ92 3335121491. E-mail addresses: [email protected] (M. Ishaque), [email protected] (M. Azhar Khan). http://dx.doi.org/10.1016/j.jmmm.2014.07.043 0304-8853/& 2014 Elsevier B.V. All rights reserved.

increase of electrical resistivity [6]. The doping of small amount of Nb in nickel ferrites decreases the particle size from 67 to 30 nm and improves coercivity from 67.98 to 111 Oe [7]. The present work reports on the synthesis of yttrium doped cobalt nanocrystals via co-precipitation technique. Co-precipitation technique was selected mainly due to economic perspectives. This method is cheap and versatile. By this method, the doping of the metal ions of choice can be done easily and the dopants levels/contents can also be controlled readily. Several reports in the literature have been found that had been used the co-precipitation route for synthesis of substituted nanocrystalline particles [8–10]. The main focus of the present work is to enhance the electrical resistivity and coercivity in order to make useful these nanocrystals in microwave and recording media devices fabrications. We are able to optimize electrical and magnetic properties such as dc electrical resistivity and coercive field as well.

2. Materials and methods 2.1. Materials/chemicals used The co-precipitation method was used to synthesize CoFe2O4 þxY2O3 nanocrystalline ferrites. The chemical used in the synthesis of samples were FeCl3, CoO4C4H6  4H2O, Y2O3, NaOH and Na2CO3of

M. Ishaque et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 68–73

analytical grade having 99.99% purity. All the chemicals were soluble in deionized water except Y2O3. It was made soluble by using small quantity of HCl and heated up to 75 1C. 2.2. Synthesis of yttrium doped nanocrystalline cobalt ferrites The solution was prepared by dissolving FeCl3, Y2O3 and CoO4C4H6  4H2O in deionized water. The solution was mixed in a beaker. The prepared solution was mechanically stirred for 3 h using magnetic stirrer. Solution of NaOH and Na2CO3 were dropped slowly into the former solution. The brown precipitates formed by pouring the solution of NaOH and Na2CO3. The pH was found to be around 10. These precipitates were thoroughly washed with deionized water until the precipitates were free from sodium and chloride ions. The product was dried in furnace at 90 1C for 10 h to remove water contents. The dried product was mixed homogeneously in an agate mortar and pestle for 40 min. For pellet formation, a load of 25 kN was applied on each pellet. The pellets were pre sintered in digital furnace at temperature 500 1C for 5 h followed by furnace cooling. Final sintering was carried out at 1150 1C for 8 h. 2.3. Methods/techniques used The phase formation of the samples was confirmed by X-ray diffraction studies using X-ray diffractometer JDX-3532 JEOL Japan. The average crystallite size of each sample was determined from the full width at half maximum (FWHM) of the (3 1 1) peak using Debye Scherrer’s formula [11]. D¼

0:94λ β cos θ

ð1Þ

where λ is the wavelength, β is the full width at half maximum 2 2 (FWHM), θ is the Bragg’s diffraction angle. Here β ¼ ðβ M  β S Þ1=2 , βM is the full width at half maximum (FWHM) of the most intense peak (3 1 1) and β S is the standard instrumental broadening [12]. The structural morphology is studied using JEOL–JAPAN MODAL JSM 5910 scanning electron microscope. The physical density (Dp) and porosity percentage was computed using the formula reported in Ref. [13]. The room temperature dc electrical resistivity measurements were carried out by two probe method. A Source meter model 2400 (Keithley) was used. The resistivity of the sintered samples was determined by using the relation: ρ ¼RA/t where ‘R’ is resistance, ‘A’ is the area of electrode and 't' is thickness of the sample. The temperature dependent dc electrical resistivity has been measured in the temperature range 25–200 1C. Dielectric properties were studied in the frequency range 100 Hz– 100 kHz at room temperature using 1689 M digibridge. The dielectric constant was determined using the formula reported in Ref. [14].

3. Results and discussion 3.1. X-ray diffraction analysis X-ray diffraction patterns were taken by using X-ray diffractometer JDX-3532 JEOL Japan using CuKα radiation. The operating voltage and current were kept at 40 kV and 300 mA. The samples were scanned through 15–701 to identify the phases developed and to confirm the completion of solid state reaction. Fig. 1 shows the XRD patterns of CoFe2O4 þx Y2O3 (x ¼0 wt%, 1 wt%, 3 wt%, 5 wt %). The peaks of all the XRD patterns were indexed and analyzed. FCC cubic spinel phase was identified as main phase along with few traces of second phase. The presence of allowed fcc peaks corresponding to the planes (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0),

69

(4 2 2), (5 1 1/3 3 3) and (4 4 0) confirms the formation of cubic spinel structure. It was observed that all the samples are biphasic except the sample with x ¼0 (CoFe2O4). The diffraction pattern of CoFe2O4 (x ¼0) matched well with JCPDS card 22-1086 for Coferrite. The sample CoFe2O4 showed a single phase spinel structure with no impurities in the XRD pattern. There are three extra peaks at diffraction angles 2θ ¼23.11, 39.61, 45.511 (indicated by the n in Fig. 1) which are identified as YFeO3 phase. These peaks can be identified as (1 1 0), (2 1 1), (1 2 2) reflection of YFeO3 (JCPDS # 80150). The average value of the lattice constant for all the samples was calculated using the Nelson–Riley function [11].
 1 cos 2 θ cos 2 θ ð2Þ þ FðθÞ ¼ 2 sin θ θ The variation of lattice constant as a function of yttrium concentration is listed in Table 1. The lattice constant is found to decrease from 8.385 Å to 8.348 Å with increasing yttrium contents. This means that the addition of yttrium in the samples cannot enter into the spinel lattice and it may diffuse to the grain boundaries and react with Fe to form YFeO3 (secondary phase). It is possible that the spinel lattice is compressed by the intergranular secondary phase due to the differences in the thermal expansion coefficients [15]. The lattice constant of bulk CoFe2O4 is reported as 8.3957 0.005 Å [13]. In the present work, the value of lattice constant for CoFe2O4 comes out to be 8.385 Å. The difference in the value of lattice constant of CoFe2O4 may be due to different sintering atmosphere and the method of preparation. The doping of Y2O3 in CoFe2O4 affects not only the phase composition but also the size of the spinel lattice. Such a reduction in the lattice constant has also been reported in the literature [12]. The average crystallite size of each sample under investigation was determined from the full width at half maximum (FWHM) of the most intense peak (3 1 1) using Debye Scherrer’s formula [11]: D¼

0:94λ β cos θ

ð3Þ

where λ is the wavelength, β is the full width at half maximum 2 2 (FWHM), θ is the Bragg’s diffraction angle. Here β ¼ ðβ M  βS Þ1=2 , βM is the full width at half maximum (FWHM) of the most intense peak (3 1 1) and β S is the standard instrumental broadening [12]. The values of average crystal size are listed in Table 1. The average crystal size was found to decreases from 48.51 nm to 34.11 nm with the increase of yttrium contents. Fig. 2 shows the SEM micrographs of samples with doping level x¼ 0 wt%, 1 wt%, 5 wt% and the grain size calculated from these micrographs are listed in Table 1. These micrographs exhibit the non-uniform grain size distribution. It is seen from the table that grain size decreases with the increase of yttrium addition. It is known that grain growth is a function of grain boundary mobility. The addition of yttrium reduces the grain growth which may be due to the segregation of yttrium at or near the grain boundary which in turns block the movements of the particles [12,16]. The grain size of the samples decreases from 0.689 to 0.65 mm. The average crystallite size estimated from SEM spectra is larger than the average particle size estimated from XRD data, suggesting the size of many grains. It may be the aggregation of more than one particle due to strong inter-particle interactions [17]. The physical density (Dp) of the sintered samples have been measured using Archimedes principle. The values of physical density are listed in Table 1. It can be seen from the table that the physical density of all the samples increases with the increase of yttrium addition. The addition of yttrium induces second phase (YFeO3) as confirmed by XRD patterns. The formation of second phase fills intergranular voids and thereby enhances the physical density with the addition of yttrium. The results of density in the

3500

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M. Ishaque et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 68–73

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Fig. 1. X-ray diffraction patterns for CoFe2O4 þx Y2O3 (a) x¼ 0, (b) x¼ 1 wt%, (c) x ¼3 wt%, (d) x ¼5 wt%). (*) indicates secondary phase.

Table 1 Phases, lattice constant, X-ray density, physical density and grain size for CoFe2O4 þ x Y2O3 nanocrystalline ferrites. Composition (x)

Lattice parameter a (Å)

Secondary phase

Lattice parameter a (Å)

Average grain size (μm) SEM

Physical density (g/cm3)

Average crystallite size (nm) Scherrer formula

CoFe2O4 CoFe2O4 þ1 wt% of Y2O3 CoFe2O4 þ3 wt% of Y2O3 CoFe2O4 þ5 wt% of Y2O3

8.385 8.375 8.361 8.348

– YFeO3 YFeO3 YFeO3

8.385 8.375 8.361 8.348

0.689 0.680 – 0.65

4.129 4.311 4.391 4.463

48.51 46.31 39.28 34.11

present work are consistent with the results reported in the literature [12]. 3.2. Electrical resistivity Fig. 3 shows the influence of yttrium addition on the dc electrical resistivity at room temperature. It is seen from the figure that resistivity increases with the increase of yttrium addition. The increasing trend of dc resistivity may be due to the formation of insulating intergranular layers. High concentration of Y3 þ ions diffused to the grain boundaries and formed inhomogeneous solid solution. This may tend to segregate at the grain boundaries and formed highly resistive layer of yttrium iron oxide YFeO3

(secondary phase) which enhances the resistivity. Further, it is known that resistivity increases with the decrease in grain size [18]. In the present samples, grain size gradually decreases and thereby enhances the resistivity. Temperature dependent dc electrical resistivity for all the samples has been measured in the temperature range 25–200 1C. Fig. 4 shows the Arrhenius plots of all the samples. It is seen from the figure that the temperature dependent resistivity of all the samples is observed to decrease with the increases of temperature. This can be attributed to the increase in drift mobility of charge carriers. The values of activation energies corresponding to the slope of Arrhenius plots for all the compositions have been estimated. Fig. 5 shows that the activation energy increases with

M. Ishaque et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 68–73

71

8

Log ρ (ohm .cm)

x=0 x=1

7

x=3 x=5

6

5

4 2.1

2.5

2.9

3.3

103/T (K) Fig. 4. Variation of Log ρ vs. 1000/T (K) for CoFe2O4 þx Y2O3 (x ¼0 wt%, 1 wt%, 3 wt %, 5 wt%).

0.6

E (ev)

0.56 0.52 0.48 0.44 0.4 0

1

2

3

4

5

Y-concentration (x) Fig. 5. Activation energy vs. Y-Concentration for CoFe2O4 þ x Y2O3 ferrite system (x ¼0 wt%, 1 wt%, 3 wt%, 5 wt%).

the increase in yttrium addition. The increase in activation energy is expected because the resistivity has been found to increase for the whole range of yttrium addition. It is concluded that the samples having high activation energy have high resistivity and vice versa. The value of activation energy of the present Y-doped cobalt nanocrystalline ferrites lies in the range from 0.41 eV to 0.57 eV. In ferrites, the activation energy is a function of mobility of the charge carriers. Activation energy depends upon the grain size. A small grain size implies a decreased grain to grain contact area for the electrons to flow through, which lead to higher barrier height [3]. In present samples, grain size decreases with the addition of yttrium. Therefore, activation energy increases with the addition of yttrium contents. Fig. 2. SEM micrograph of CoFe2O4 þ x Y2O3 ferrite system (a) x ¼0 wt%, (b) x¼ 1 wt %, (c) x¼ 5 wt%).

3.3. Dielectric properties

Log ρ (ohm .cm)

8.1 7.8 7.5 7.2 6.9 6.6 0

1

2

3

4

5

Y-concentration (x) Fig. 3. Variation of room temperature resistivity vs. Y-concentration for CoFe2O4 þ x. Y2O3 (x ¼0 wt%, 1 wt%, 3 wt%, 5 wt%).

3.3.1. Compositional dependence of dielectric constant Fig. 6 shows the variation of dielectric constant as a function of frequency at room temperature in the range 0.1 kHz to 100 kHz. The figure shows that dielectric constant decreases with the addition of Y3 þ ions. The dielectric constant is affected by the increased inhomogeneous dielectric structure when an increased concentration of Y3 þ ions is successively incorporated in Coferrite. The addition of yttrium seems to obstruct the development of the microstructure, thus contributing to increase in resistivity and decrease of dielectric constant. Similar behavior was reported in the literature [19]. It has been reported that a composition with high dc electrical resistivity acquires low values of dielectric constant and vice versa [20]. In the present samples, dc electrical resistivity increases with the increase of yttrium addition.

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M. Ishaque et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 68–73

Therefore, the decrease of dielectric constant is expected with increasing contents of Y3 þ ions. 3.3.2. Frequency dependence of dielectric constant It can be observed from Fig. 6 that the values of dielectric constant decreases continuously with the increase in frequency. The values of dielectric constant are high at low frequency and attain almost constant values at high frequency. It can be explained on the basis of space charge polarization. At low frequency, the space charge polarization builds up due to localized accumulation of charges under the influence of applied electric field. Therefore, high value of dielectric constant is expected at low frequency. As frequency increases, the probability of charge accumulation at the grain boundaries is reduced, thereby reducing the polarization. As a result, dielectric constant decreases with the increase of frequency. Koops [21] presented a theory in order to explain the frequency dependence behavior of dielectric constant in ferrite materials. It was suggested that ferrites formed inhomogeneous dielectric structure. It consists of well conducting grains separated by low conducting grain boundaries. In inhomogeneous dielectric structure, charge always require some time to move their axes parallel

10000

Dielectric constant

x=0

to an applied alternating electric field. As the frequency of the field increases, a stage will come when space charge carriers just started to move before the field reverses and make no considerable contribution to the space charge polarization [22]. As a result, dielectric constant decreases sharply with rise in frequency and attain nearly constant values. 3.3.3. Variation of dielectric loss tangent (tan δ) with frequency Fig. 7 shows the plot of loss tangent with frequency for the present samples. It can be observed from the figure that the loss tangent decreases continuously with frequency for all the samples. The value of tan δ is high at low frequency region while it is low at high frequency region. At low frequency, thin grain boundaries are more effective while at high frequency well conducting grains are more effective. It is, therefore, expected that energy loss is high in low frequency region (insulating grain boundary) because more energy is needed for hopping of charge carriers in this region. Therefore, tan δ is high in this region. Energy loss is low in high frequency region (well conducting grains) because small energy is required for hopping of charge carriers in this region. Therefore, tan δ is low in this region. The figure also shows that tanδ is found to be composition dependent. Its value decreases with the addition of yttrium contents. In present samples, resistivity of all the samples increases with the increase of yttrium contents. The increase in resistivity may give rise to reduction in tan δ [23].

x=1

1000

3.3.4. Relationship between dielectric constant ðε0 Þ and resistivity (ρ) Table 2 shows the values of resistivity (ρ), dielectric constant pffiffiffi pffiffiffi ðε0 Þ, square root of resistivity ð ρÞ and the product ðε0 ρÞ. It is seen from Table 2 that the dielectric constant is found to be roughly inversely proportional to the square root of resistivity and the pffiffiffi product ε0 ρ remains nearly constant. Such relationship was reported by several researchers in the literature [24].

x=3 x=5

100

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Frequency (KHz)

Fig. 6. Variation of dielectric constant as a function of frequency for CoFe2O4 þx. Y2O3 ferrite system (x ¼0 wt%, 1 wt%, 3 wt%, 5 wt%).

Dielectric Loss Tangent

6 x=0

5

x=1

4

x=3

3

x=5

2 1 0 0

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40

50

60

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80

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100

Frequency (KHz) Fig. 7. Plot of loss tangent (tan δ) vs. frequency at room temperature for CoFe2O4 þ x. Y2O3 (x ¼0, 1 wt%, 3 wt%, 5 wt%).

3.4. Magnetic properties 3.4.1. Hysterisis loops The M–H loops were plotted at room temperature using computer controlled Lake Shore (model 7400) vibrating sample magnetometer (VSM) and are shown in Fig. 8. The maximum applied field was up to 10 kOe. The VSM was calibrated with Ni standard having magnetization 3.475 emu at 5000 G. Fig. 9 shows the variation of saturation magnetization (Ms) and coericivity (Hc) vs. Y-concentration for the present samples. It is seen from Fig. 9 that the addition of Y3 þ ions in CoFe2O4 nanocrystalline ferrites results in an increase in coercivity and decrease in saturation magnetization. The increase in coercivity may be attributed to the presence of ultra thin layer of yttrium at the grain boundaries which impedes the domain wall motion [25]. It is an established fact that coercivity is inversely proportional to the grain size [26]. In present samples, grain size decreases with the increase of yttrium concentration. The gradual increase of coercivity (Hc) with yttrium contents may be attributed to the

Table 2 Variation of dielectric constant (ε0 ) and resistivity (ρ) in the case of CoFe2O4 þ x Y2O3 nanocrystalline ferrites. Composition (xwt%)

ε0

at

100 Hz

ε0 at 100 kHz

ρ ðΩ cmÞ

pffiffiffi ρ ðΩ

0 1 3 5

1873.61 267.86 57.78 57.47

7.63 4.65 3.01 1.92

4.95  106 9.7  106 4.31  107 8.39  107

pffiffiffi ε0 ρ 1=2

2224.86 3108.05 6565.06 9159.69

cm

1=2

at

100 Hz

ε0

pffiffiffi ρ at 100 kHz

Þ

4.16  106 0.83  106 0.38  106 0.53  106

1.69  104 1.44  104 1.97  104 1.76  104

M. Ishaque et al. / Journal of Magnetism and Magnetic Materials 372 (2014) 68–73

80

40

M (emu/g)

show that the addition of Y3 þ ions can improve the electrical transport properties. With the addition of yttrium contents, the quantity of pores reduces and physical density increases. The dc electrical resistivity increases while crystallite size decreases with the addition of yttrium contents. An appreciable decrease in dielectric loss has been observed by the yttrium addition. The high values of resistivity and small dielectric loss make these materials best candidate for high frequency applications. The saturation magnetization (Ms) decreases while coercivity (Hc) increases with the increase in doping level of yttrium. The increase in coercivity makes these nanomaterials suitable for their applications in high density recording media applications.

x = 0 wt % x = 1 wt % x = 3 wt % x = 5 wt %

60

20 0 -20 -40 -60 -80 -10000

73

-5000

0

5000

10000

H (Oe)

References

Fig. 8. Hysteresis loops for CoFe2O4 þ xY2O3 (x ¼ 0, 1 wt%, 3 wt%, 5 wt%).

80 1300 70

1200 1100

50

1000

40

900

Hc (Oe)

Ms (emu/g)

60

800

30

700

20

600 10 0

2

4

Y- concentration (x) Fig. 9. Variation of saturation magnetization (Ms) and coericivity (Hc) vs. Y-concentration for CoFe2O4 þx Y2O3(x¼ 0 wt%, 1 wt%, 3 wt%, 5 wt%).

decrease in grain size [27]. It is observed that saturation magnetization (Ms) can be related to coercivity (Hc) through Brown’s relation [28]: Hc ¼

2K 1

μo Ms

ð4:2Þ

This relation shows that Hc is inversely proportional to Ms. It is also consistent with the experimental findings in the present investigations. 4. Conclusion Y3 þ doped nanocrystalline cobalt ferrites were successfully synthesized by co-precipitation technique. The experimental results

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