Influence of cobalt on structural and magnetic properties of nickel ferrite nanoparticles

Influence of cobalt on structural and magnetic properties of nickel ferrite nanoparticles

Journal of Molecular Structure 1052 (2013) 177–182 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.el...

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Journal of Molecular Structure 1052 (2013) 177–182

Contents lists available at ScienceDirect

Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

Influence of cobalt on structural and magnetic properties of nickel ferrite nanoparticles Ali A. Ati, Zulkafli Othaman ⇑, Alireza Samavati Ibnu Sina Institute for Fundamental Science Studies, Universiti Teknologi Malaysia, 81310 Skudai, Johor Baharu, Malaysia

h i g h l i g h t s  Nickel ferrite nanoparticles with different composition are synthesized using co-precipitation method.  The spinel ferrites are improved by the substitutions of Co + 2 ions.  These nanocrystallites are small enough to achieve suitable signal-to-noise ratio decisive for high density recording media.  The values of Ms, Hc and Mr are found to be strongly affected by the Co contents.

a r t i c l e

i n f o

Article history: Received 2 July 2013 Received in revised form 19 August 2013 Accepted 19 August 2013 Available online 27 August 2013 Keywords: Co-precipitation Nickel ferrites Spinel phase Nanocrystallites

a b s t r a c t Improving the magnetic response of nanocrystalline nickel ferrites is the key issue in high density recording media. A series of cobalt substituted nickel ferrite nanoparticles with composition Ni(1x)CoxFe2O4, where 0.0 6 x 6 1.0, are synthesized using co-precipitation method. The XRD spectra revealed the single phase spinel structure and the average sizes of nanoparticles are estimated to be 16–19 nm. These sizes are small enough to achieve the suitable signal to noise ratio in the high density recording media. The lattice parameter and coercivity shows monotonic increment with the increase of Co contents ascribed to the larger ionic radii of the cobalt ion. The specific saturation magnetization (Ms), remanent magnetization (Mr) and the coercivity (Hc) of the spinel ferrites are further improved by the substitutions of Co+2 ions. The values of Ms for NiFe2O4 and CoFe2O4 are found to be 43.92 and 78.59 emu/g, respectively and Hc are in the range of 51–778 Oe. The FTIR spectra of the spinel phase calcinated at 600 °C exhibit two prominent fundamental absorption bands in the range of 350–600 cm1 assigned to the intrinsic stretching vibrations of the metal at the tetrahedral and octahedral sites. The role played by the Co ions in improving the structural and magnetic properties are analyzed and understood. Our simple, economic and environmental friendly preparation method may contribute towards the controlled growth of high quality ferrite nanopowders, potential candidates for recording. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction The growth and characterizations of magnetic nanoparticles due to their widespread fundamental and technological importance has received much attention in recent years [1]. Spinel ferrites of M+2 Feþ3 2 O4 (M = Ni, Cu, Zn, Mn) are attractive for numerous applications such as microwave devices [2], recording media [3], magnetic fluids [4], gas sensors [5], high density information storage [6], ferro-fluids [7] and catalysts [8], to cite a few. Several techniques including solid-state reaction [9], high energy ball milling [10], sol–gel [11], hydrothermal synthesis [12], chemical co-precipitation [13], combustion [14], micro-emulsion [15] and microwave hydrothermal [16] are developed to make nickel ferrite nanoparticles.

⇑ Corresponding author. Tel.: +60 75534189. E-mail address: [email protected] (Z. Othaman). 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.08.040

The unit cell of spinel structured nanoparticles contain 32 oxygen atoms closely packed in the face center cubic lattice in which the metal ions are distributed in two different sub-lattices (interstices) of 8 tetrahedral (A-site) and 16 octahedral (B-site). Co-precipitation method has many advantages over other methods such as the effect of minimal contamination, processing simplicity, low cost, high level of reactivity, easy control of the particle size and the efficiency of more homogeneous mixing of the component materials that lead to the formation of nanocrystallites [17]. The process of conventional solid state reaction involves the mixing of oxides or carbonates with sporadic grinding followed by annealing at high temperatures in the range of 1300–1700 °C [18]. Despite its fabrication simplicity of spinel ferrites it suffers from many drawbacks such as long period of production, high energy consumption, large particle size and the presence of various impurities that lead to inhomogeneous ferrite structures. It is the inhomogeneity that results the formation of voids and thereby weakens the proper transfer of mechanical signal.

2. Experimental Ferrites having chemical composition Ni(1x)Co(x) Fe2O4, where x = 0, 0.2, 0.4, 0.6, 0.8, 1.0 are prepared by the chemical coprecipitation method using stoichiometric amount of CO(CH3COO)24H2O (99%, Merck), Ni(NO3)26H2O (98.5%, Merck), Fe(NO3)39H2O (98.5%, Merck) and NaOH (99%, Merck) as raw materials. All chemicals of analytical grades are used without further treatment. The solutions of the desired concentrations are prepared in de-ionized water and heated under constant stirring. NaOH solution of 2 M is added drop-wise at 70 °C to form the precipitate. The pH of the aqueous solution is maintained in the range of 12.5–13 during the co-precipitation process. The co-precipitated products are then washed several times with de-ionized water until the pH of the filtered water reached about 7–8. The precipitates are then filtered and dried over-night in the oven at 150 °C to remove the water content. Finally, the pure single phase spinel structures are obtained after annealing the precipitates for 8 h at the rate of heating 5 °C/min. The structural characterization is performed at room temperature using powder X-ray diffractometer (XRD, D8 Advanced) with Cu-Ka radiations (1.54178 Å) at 40 kV and 10 mA. The scanning range of 2h from 20° to 80° and a slow speed of scanning 2°/ min with a resolution of 0.011 is employed. The Scherrer equation is used to determine the sizes of ferrite nanoparticles. Fourier transformed infrared (FTIR) spectra are recorded using Perkin Elmer 5DX FTIR after mixing 1 mg of ferrite sample with 100 mg of potassium bromide (KBr). The contents are crushed well in the mortar with a pestle for 5 min until a fine mixture is resulted, which is further used to make pellets in a die of diameter 10 mm. A pressure of about 4–5 ton is applied for 2 min to make the pellets via a hand press machine. The room magnetic properties are measured employing vibrating sample magnetometer (VSM, Lake Shore 7303-9309 VSM). Each sample is calcined for 8 h at 600 °C prior to the measurement. 3. Results and discussion Fig. 1 shows the XRD spectra of all the synthesized cobalt substituted nickel ferrites with concentrations ranging from 0.0– 1.0. The structure and phase purity of the as prepared product were confirmed by analyzing the powder X-ray diffraction patterns. The diffraction patterns of all the compositions confirm the formation of single phased cubic spinel structure of NiFe2O4 (x = 0.0) (JCPDS No. 10.0325) and for pure CoFe2O4 (x = 1.0) (JCPDS No. 22.1086).

Intensity (a.u.)

Experimental plot Gaussian fit peak position: 35.7° FWHM: 0.42°

2400 2000 1600 1200

[440]

[511]

[400]

[422]

35.0 35.5 36.0 36.5 2θ degree

[222]

[ 220]

The magnetization in Co doped nickel ferrites strongly depend on the cation distribution of the magnetic Fe3+, Ni+2 and Co2+ ions among the A and B sites. Bulk cobalt ferrite (CoFe2O4) is a wellknown hard magnetic material that possesses high coercivity (5400 Oe), high chemical stability, good electrical insulation, significant mechanical hardness and moderate saturation magnetization (80.0 emu/g) at room temperature. However, nanosized CoFe2O4 particles acquire much higher values of coercivity and saturation magnetization [19,20], in which the magnetic properties are particle size and the preparation method dependent. The production of high quality Co substituted Ni ferrites remain challenging. The effect of Co as dopants on the structural properties Ni ferrite nanoparticles and the mechanism behind the enhancement of magnetic response is far from being understood. Here, we report the effect of cobalt concentration on the structural, magnetic and morphological properties of Co substituted Ni ferrites synthesized using co-precipitation method sintered at 600 °C for 8 h. The mechanism responsible for the improvement of the magnetic and structural properties is analyzed in detail.

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Intensity (a.u.)

178

x=0.0 x=0.2 x=0.4 x=0.6 x=0.8 x=1.0

20

30

40

50

60

70

80

Angle (2θ) Fig. 1. XRD spectra for Ni(1x)Co(x)Fe2O4 with the Gaussian fit of the peak (3 1 1) (inset).

The lattice constant ‘a’ is determined from the XRD data by using powder-x-software [21]. All the samples exhibit a poly-oriented structure with several peaks characteristic of the crystalline planes 220, 311, 222, 400, 422, 511, and 440 corresponding to the single cubic phase of Ni(1x)Co(x)Fe2O4. The broadening of the X-ray diffraction peaks for as-prepared sample is attributed to the nanocrystalline particle size [21]. The absence of any additional peaks related to impurities indicates the high purity of our nickel–cobalt ferrite samples. The sizes of the nanocrystallies are estimated from the XRD spectra using Debye–Scherrer’s equation [22,23].

D ¼ Kk=b cos h

ð1Þ

where k is the wavelength of the X-ray radiation, K is a constant taken as 0.89, b is full width at half maximum (FWHM) of line broadening and h is the angle of diffraction. The most intense peak (3 1 1) are used to estimate the sizes of Ni(1x)Co(x)Fe2O4 nanocrystallites and are found the range of 17–19 nm (Table 1). Our nanoparticles are much smaller compared to those reported earlier (25–30 nm and 18–23 nm) [24,25]. These nanocrystallites are small enough to achieve suitable signal-to-noise ratio detrimental for high density recording media. The lattice constant (a), cell volume (V) and the density are calculated from the XRD spectra using the following relations: 2

2

2

2

1=2

a ¼ ½d ðh þ k þ l Þ

ð2Þ

V ¼ a3

ð3Þ

dx ¼

8M Na3

ð4Þ

where M is the molecular weight and N is the Avogadro number. Fig. 2 represents the influence of cationic stoichiometry on the lattice parameters which shows monotonic increment obeying Vegard’s law for increasing Co content [26]. This is due the smaller ionic radius of Ni+2 (0.69 Å) compared to that of Co2+ (0.74 Å). The lattice parameter is calculated as a = 8.345, which is found to be in agreement with JCPDS value. The density of X-rays calculated from the XRD pattern is summarized in Table 1. The density of X-ray decreases with the increase of lattice constant as expected. The room temperature FTIR spectra recorded in the wave-number range of 350–4000 cm1 are shown in Fig. 3. The assignments for the absorption bands are summarized in Table 1. Following Waldron [27], we describe ferrites as continuously bonded crystals in which the atoms are bonded to all nearest neighbors by equivalent strength of ionic, covalent or van der Waals interactions. In ferrites the metal ions occupy two different

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A.A. Ati et al. / Journal of Molecular Structure 1052 (2013) 177–182 Table 1 The characteristic parameters for each composition at room temperature. Composition

m1 (cm1)

m2 (cm1)

DXRD (nm)

Lattice const. (a)(Å)

NiFe2O4 Ni0.8Co0.2Fe2O4 Ni0.6Co0.4Fe2O4 Ni0.4Co0.6Fe2O4 Ni0.2Co0.8Fe2O4 CoFe2O4

601.46 599.13 603.84 602.22 605.95 592.62

402.51 398.17 406.85 403.38 405.11 395.01

17.34 19.26 18.82 19.43 16.13 17.38

8.330 ± 0.002 8.335 ± 0.002 8.337 ± 0.002 8.352 ± 0.002 8.358 ± 0.002 8.361 ± 0.002

Lattice Constant (A°)

8.365 8.360 8.355 8.350 8.345 8.340 8.335 8.330 8.325 0.0

0.2

0.4

0.6

0.8

1.0

Co+2 Content (x) Fig. 2. Variation of lattice parameter (a) with cobalt concentration.

x=0.0

Intensity (a. u.)

x=0.8 x=0.6 x=0.4 x=0.2 x=1.0

4000

3000

2000

1000

Wavenumber (cm-1) Fig. 3. FTIR spectra of Ni(1x)Co(x)Fe2O4.

sub-lattices designated as tetrahedral (A-site) and octahedral (Bsite) positions with respect to the geometrical configuration of the oxygen nearest neighbors. The band with the higher wave number (m1) observed in the range of 584–610 cm1 corresponds to the intrinsic stretching vibrations of the metal at the tetrahedral site (Mtetra M O). The other band (m2) appeared around 385– 450 cm1 is attributed to the octahedral-metal stretching (Mocta M O) [28,29]. The appearance of much weaker absorption bands around 1100–1300 cm1, 1400–1700 cm1, 2850–3000 cm1 and 3400 cm1 are assigned to the vibrations of ions NO 3 , carboxyl group (COO), the stretching of the C–H bands and hydrogen bonded O–H groups, respectively [30,31]. The noticeable change in the band positions are resulted from the difference in the distance of Fe3+–O2 ions associated with the octahedral and tetrahedral complexes. Interestingly, the characteristic band m1 shows a shift towards lower frequency region with the increase of co-substitution contents (Table 1). The observed increase in the metal–

Mean a (Å)

8.345 ± 0.002

Cell vol. (V) (Å3)

Jump length L (Å)

X-ray density (qx-ray)/g cm3

578.0 579.05 579.46 576.96 583.85 584.48

2.9450 2.9468 2.9475 2.9528 2.9549 2.9560

5.386 5.377 5.375 5.347 5.336 5.332

oxygen stretching frequency is related to the higher atomic mass of Co compare to Ni. The occurrence of weaker intensity corresponding to the carboxyl, hydroxyl and nitrate groups is related to the high temperature generated during combustion process. The room temperature isothermal hysteresis loops of Ni(1x) Co(x)Fe2O4 nanoparticles are illustrated in Fig. 4. The values of saturation magnetization (Ms), remanent magnetization (Mr) and coercivity (Hc) determined from hysteresis loops are summarized in Table 2. The saturation magnetizations in all cases are found to be lower than their bulk counterparts with 55 emu/g for NiFe2O4 and 93.3 emu/g for CoFe2O4. The magnetic properties of the inverse spinel (NiFe2O4) structure with formula (NiFe)B(Fe)A can be explained in terms of the cations distribution and the magnetization of Ni2+ ions that occupy octahedral sites while Co2+ and Fe3+ ions occupy both octahedral (B) as well as tetrahedral (A) sites [32]. The s-shaped nature of the M–H curve at lower fields and a linear variation at higher fields indicate the presence of small magnetic particles exhibiting super-paramagnetic behaviors [33]. The non-retainment of any magnetization after the removal of magnetic field in super-paramagnetic materials is an important property for carriers in magnetic target [34]. In fact, the manifestation of ferromagnetism and super-paramagnetism are highly particle size dependent. Particles having mean sizes below 30 nm manifest super-paramagnetism as reported [33,35]. The values of Ms obtained by us are 43.92 and 78.59 emu/g for NiFe2O4 and CoFe2O4, respectively. Our results for Ms are in consistent with the observation of Lingyun et al. (43.25 emu/g) and Young et al. (78.0 emu/g) [36,37]. Furthermore, the present values of Ms for the as synthesized samples are slightly greater than those obtained by Nguyen et al. [38] and Shah et al. [39]. Interestingly, the magnetic behavior of nickel ferrite nanoparticles is very much sensitive to the nature of crystallinity and particle morphology. The increase in saturation magnetization may be due to the change in exchange interactions between tetrahedral and octahedral sub-lattices, increased crystallinity and narrow particle size distribution [40]. In case of nickel ferrite, any configuration of Ni2+, Co+2 and Fe3+ ions in both octahedral and tetrahedral sites tends to increase the net magnetization per formula unit [41]. The tiny coercivity associated with the hysteresis loops essentially suggests the enhanced coalescence of the crystallites in the nanostructures that results increased magnetic coupling and higher magnetization. The values of Ms, Hc and Mr are found to be strongly affected by the Co contents. The ferromagnetic behavior of all the samples with different values of Hc is clearly revealed by the M–H curves. The higher anisotropic nature of Co than Ni ions allows Hc to increase with the increased doping of Co ions. This increase in Hc with Co content as evidenced from the M–H curve is attributed to the increase in anisotropy field, which in turn increase the energy of the domain wall. The increase in the values of Ms and Mr can be explained on the basis of Neel’s theory and distribution of cations at tetrahedral (A) and octahedral (B) sites. The spins of electrons of Fe ions at A and B sites are antiparallel to each other that cancel to produce a net

A.A. Ati et al. / Journal of Molecular Structure 1052 (2013) 177–182

Magnetization (emu/g)

180

100 80 60 40 20 0 -20 -40 -60 -80 -100

100 80 60 40 20 0 -20 -40 -60 -80 -100

x=0.0 -10000

-5000

0

5000

100 80 60 40 20 0 -20 -40 -60 -80 -100

10000

-10000

-5000

0

5000

100 80 60 40 20 0 -20 -40 -60 -80 -100

10000

-5000

0

5000

0

5000

-5000

0

5000

100 80 60 40 20 0 -20 -40 -60 -80 -100

10000

10000

x=0.8 -10000

x=0.4 -10000

-5000

100 80 60 40 20 0 -20 -40 -60 -80 -100

x=0.2 -10000

x=0.6

10000

x=1.0 -10000

-5000

0

5000

10000

Applied Magnetic Field (Oe) Fig. 4. The room temperature M–H curves of Ni

(1x)Co(x)Fe2O4.

Table 2 The room temperature characteristics magnetic parameters for each composition. Composition

Hc (Oe)

Mr (emu/g)

Mr/Ms

Ms (emu/g)

NiFe2O4 Ni0.8Co0.2Fe2O4 Ni0.6Co0.4Fe2O4 Ni0.4Co0.6Fe2O4 Ni0.2Co0.8Fe2O4 CoFe2O4

51 ± 0.328 228 ± 1.71 520 ± 3.9 742 ± 5.565 743 ± 5.647 778 ± 5.835

16.63 ± 0.332 17.548 ± 0.350 20.931 ± 0.418 19.672 ± 0.393 40.137 ± 0.802 15.808 ± 0.316

0.378 ± 0.001 0.427 ± 0.002 0.417 ± 0.002 0.503 ± 0.002 0.505 ± 0.002 0.51 ± 0.002

43.92 ± 0.4392 41.04 ± 0.4104 49.36 ± 0.4936 39.10 ± 0.391 31.29 ± 0.3129 78.59 ± 0.7859

magnetic moment of 2lB due to Ni2+ ions at B sites. The substituents (Co2+) with magnetic moment of 3lB preferably occupy B sites. The increase in Ms and Mr originates from the replacement of Ni ion (two unpaired electrons) with Co ion (three unpaired electrons) that increases the number of unpaired electrons at octahedral sites. The value of Hc varies from 51 to 778 Oe (Table 2) with the variation of Co concentration. The higher values of Ms and Hc (as much as 600 Oe) are preferable for the high density recording media [42]. Our results demonstrate that we are able to achieve improved values of Ms, Mr and Hc (below 600 Oe). We affirm that the samples with composition Ni0.8Co0.2Fe2O4 and Ni0.6Co0.4Fe2O4 are suitable candidates for high density recording media. Fig. 5a and b show the variation of Hc and Mr and as a function of Co contents. The steady increase in Hc (a measure of magnetocrystalline anisotropy) with the increase of Co content is primarily ascribed to the large coercivity associated with the hard ferrite structure of CoFe2O4 and the magneto-crystalline anisotropic property of Co ions. The increase in coercivity may also originate from the exchange anisotropy due to spin disorder at the particle surface, effect of spin-canting, spin-glass-like behavior in surface layers of nanoparticles due to local chemical disorder and broken exchange interactions [32,43]. In addition, the enhancement of the surface barrier potential due to the distortion of crystal lattice caused by the deviation of atoms from the normal positions in the

surface layers may also contribute to improve the coercivity. These changes in the magnetic properties are mainly attributed to the lower magneto-crystalline anisotropy and the weaker magnetic moment of Ni2+ ions compared to Co2+ ions. The particle size dependence of coercivity can be interpreted in terms of domain structure, critical diameter, strains, magneto-crystalline and shape anisotropy of nanocrystal [44]. Furthermore, the surface is likely to behave as an inactive and dead layer with inconsiderable magnetization. The weak increase in the remanent magnetization with the increase of Co content (Fig. 5b) is attributed to the small remanence associated with the soft nickel ferrites. The increase in remanence is mainly due to the Co ions. Fig. 5c show the variation of Ms and as a function of Co contents can be understood by fact due to relatively high orbital contribution to the magnetic moment Co2+ ions are known to give large induced anisotropy. Fig. 6 represents the room temperature compositional variation of the ratio of Mr and Ms called squareness, an important characteristic parameter for applied ferromagnetic materials. The values of squareness increase continuously as the Co content is increased from 0 (NiFe2O4) to 1.0 (CoFe2O4). The squareness is strongly influenced by the magnetic anisotropy of the samples, particle size, shape and density, crystal defects and the synthesis methods

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900

45

(a)

Remanent Magnetization(emu/g)

800

Coercive filed (Oe)

700 600 500 400 300 200 100 0 0.0

0.2

0.4 +2

0.8

35 30 25 20 15 0.0

1.0

0.2

80

0.4 +2

Co

Content (x) Saturation Magnetization (emu/g)

Co

0.6

(b) 40

0.6

0.8

1.0

Content (x)

(c)

70 60 50 40 30 0.0

0.2

0.4

0.6

0.8

1.0

Co+2 Content (x) Fig. 5. Compositional variation of coercivity (a), remanent magnetization (b) of Ni(1x)Co(x)Fe2O4 nanoparticles and saturation magnetization of Ni(1x)Co(x)Fe2O4 nanoparticles (c).

0.52

Squareness (Mr/Ms)

0.50 0.48 0.46 0.44 0.42 0.40 0.38 0.36 0.0

0.2

0.4 +2

Co

0.6

0.8

1.0

Content (x)

Fig. 6. Variation of squareness parameter with Co content.

[45]. The achieved squareness ratio above 0.50 (Table 2) clearly indicates that the synthesized materials are in single magnetic domain, highly beneficial for the memory devices. 4. Conclusion Cobalt substituted nickel ferrite nanoparticles of the form Ni(1x)Co(x)Fe2O4 and x = 0.0, 0.2, 0.4, 0.6, 0.8 and 1.0 are prepared following co-precipitation route. The effect of Co ions on structural and magnetic properties is reported. The obtained spinel type of structures displayed excellent crystallinity and reproducibility. The structural characterizations (XRD) revealed single spinel structure of Ni–Co nanocrystallites with sizes 16–19 nm. These sizes are

small enough to achieve suitable signal-to-noise ratio recommended for high density recording media. The increase in lattice parameters, X-ray density and jump length due to the incorporation of Co are ascribed to the larger ionic radii of nickel ions. The FTIR spectra consisting of two absorption bands confirm the presence of A and B sublattices. The room temperature ferromagnetic behaviors of Ni–Co ferrite nanoparticles are performed by VSM measurements. The enhancement in the magnetic properties with the increase of Co contents is attributed to anisotropic nature of Co ions. The role played by Co ions in improving the magnetic response is explained using various mechanisms. We assert that our easy and low cost fabrication method constitute a basis for producing high quality Ni–Co ferrite nanostructures useful for memory devices. The results for the achieved modification and tunable magnetic properties of the synthesized nanoparticles may be nominated for potential applications. Acknowledgments The authors are thankful to Dr. S.K. Ghoshal for many valuable suggestions and critical reading of the manuscript. We are also grateful to Ibnu Sina Institute and Physics Department of UTM for technical supports. References [1] D. Chen, Y. Zhang, C. Tu, Materials Letters 82 (2012) 10–12. [2] M.N. Ashiq, M.J. Iqbal, I.H. Gul, Journal of Alloys and Compounds 487 (2009) 341–345. [3] D.S. Jung, Y.C. Kang, Journal of Magnetism and Magnetic Materials 321 (2009) 619–623. [4] J. Huo, M. Wei, Materials Letters 63 (2009) 1183–1184.

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