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Authors requiring further information regarding Elsevier’s archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright Author's personal copy Journal of Magnetism and Magnetic Materials 322 (2010) 3688–3691 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm Finite size effect on Gd3 + doped CoGdxFe2  xO4 (0.0 rx r0.5) particles R.P. Pant n, Manju Arora, Balwinder Kaur, Vinod Kumar, Ashok Kumar EPR Spectroscopy Section, National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India a r t i c l e in f o a b s t r a c t Article history: Received 20 April 2010 Available online 24 July 2010 Nanoparticles of CoGdxFe2  xO4 (where x ¼ 0.0, 0.1, 0.3, 0.5) series have been prepared by chemical co-precipitation. The effect of Gd3 + ion concentration on crystalline phase, crystallinity, crystallite size, molecular vibrations and magnetic resonance has been investigated in detail. The crystallinity decreases with an increase in Gd3 + ion concentration and changes the structural parameters. The spin lattice relaxation has been correlated with the doping ion concentration. Similarly, the superparamagnetic behavior of these particles has been observed with EPR spectroscopy. & 2010 Elsevier B.V. All rights reserved. Keywords: Co-precipitation Spinel ferrite XRD EPR FTIR 1. Introduction Magnetic nanoparticles have drawn considerable attention of the researchers due to their potential applications in the field of high-density magnetic recording, magnetic fluid, microwave devices [1–4], etc. The physical and chemical properties of these materials are influenced by their chemical composition, synthesis and environmental conditions. In literature various techniques viz. sol–gel, modified oxidation process, hydrothermal process, forced hydrolysis method, ball milling and micro-emulsion are reported for nanosized particle synthesis [5–9]. In earlier studies, the substitution of Li + , Al3 + , Zn2 + , Mn2 + and lanthanide metal ions in cobalt ferrite [10–15] was investigated. These metal ions substitution in cobalt ferrite lattice modifies the magnetic properties. In the present investigation, different concentrations of gadolinium ions (Gd3 + ) are substituted in the lattice of cobalt ferrite to understand the effect of Gd3 + concentration on structural and magnetic properties. The Gd3 + substituted cobalt ferrites (CoGdxFe2  xO4, where x¼0, 0.1, 0.3, 0.5) were synthesized by chemical route. This method has an advantage in preparing multicomponent materials easily without any contaminations with desired stoichiometry. In this, particle size, chemical homogeneity and degree of agglomeration can be easily controlled. The prepared samples were characterized using XRD, FTIR and EPR spectroscopy techniques to reveal a correlation between the structural and magnetic properties. The EPR parameters are closely connected with the immediate environment and anisotropy of the local crystal fields near impurity Gd3 + and the spectral properties of impurity Gd3 + in cobalt ferrite crystal n Corresponding author. Tel.: +91 11 45608309; fax: + 91 11 45609310. E-mail address: rppant@mail.nplindia.ernet.in (R.P. Pant). 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.07.026 lattice. Investigations on the EPR parameters and the local structure of this center may be helpful in understanding the magnetic properties of the material. 2. Experimental Polycrystalline samples of gadolinium substituted nanosized cobalt ferrite particles having the chemical formula CoGdxFe2  xO4 (where x ¼0, 0.1, 0.3, 0.5) were prepared by co-precipitation method, using Aldrich high purity salts of CoCl2, Fe(NO3)3  9H2O and GdCl3  6H2O as initial reactants. One molar aqueous solutions were prepared and mixed in respective stoichiometry and heated at 60 1C. Then CoGdxFe2  xO4 was precipitated by adding ammonia drop by drop in the above solution with constant stirring. Precipitation and formation of nanoferrite took place by the conversion of metal salts into hydroxides, which occurred immediately, followed by transformations of hydroxides into ferrites. Fine particles were collected by filtering the solution and washed several times with double distilled water to remove unreacted salts. The precipitated nanoparticles were dried at 85 1C for 1 h. Oleic acid was used as the surfactant for coating the obtained nanoparticles. These samples were annealed at 500 1C for 2 h to improve the crystallinity of the material. The crystalline phase and the structural parameters are analyzed by a Bruker D-8 Advance Powder X-ray diffractometer (XRD) at 40 kV and 40 mA, using the CuKa radiation as an X-ray source with maintaining step size rate 0.021/s. IR transmittance spectra of these gadolinium doped cobalt ferrite powder samples are measured on Perkin Elmer GX 2000 Optica Fourier Transform Infrared (FTIR) spectrophotometer in 4000–400 cm  1 region at ambient temperature. Each spectrum is an average of 100 scans at 4 cm  1 resolution. Samples are taken in the KBr pellet form. EPR Author's personal copy 3689 R.P. Pant et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3688–3691 3.1. X-ray diffraction analysis X-ray diffraction (XRD) patterns of all the as obtained samples at 373 K show very broad peaks, which indicate the poor crystallinity and ultra-fine nature of the particles. To improve the crystallinity, samples annealed at 773 K for 1 h are shown in Fig. 1(a)–(d), which represent CoGdxFe2  xO4 for x¼0.0, 0.1, 0.3, and 0.5, respectively. All the samples perfectly match with the cubic spinel structure of cobalt ferrite and no extra peak has been observed (JCPD card no. 022-1086). To calculate the crystallite size, a slow scan of selected diffraction peaks (3 1 1), (4 0 0), (5 1 1) and (4 0 0) was recorded. From the full width at half maximum (FWHM) of peaks, the crystallite size is calculated using the Scherrer formula (D ¼Kl/b cos y). All the structural parameters calculated from the peaks are listed in Table 1 and presented in Fig. 2. An appreciable increase in the unit cell parameters of Gd3 + doped cobalt ferrite is attributed to the larger ionic radii of Gd3 + (0.938 Å) ions as compared to Fe3 + (0.6459 Å) ions at octahedral sites. A slight variation in the diffraction peak 22 8.45 20 8.44 18 8.43 8.42 16 8.41 14 0.0 0.1 0.2 0.3 0.4 Crystallite size (10-9m) 3. Result and discussion 24 8.46 Lattice parameter (10-10m) spectrum of the samples of different Gd3 + ion doped nanocrystalline cobalt ferrite is recorded on reflection-type X-band CW E-line Century EPR spectrometer (Varian Make, Model E-112) at ambient temperature to study the behavior of magnetic dipolar and superexchange interactions. Magnetic field is modulated at 100 kHz and 10 mW microwave power is used to avoid saturation effect. DPPH was used as a standard reference material for the determination of g-value. 0.5 Gd3+ ion concentration Fig. 2. Variation in lattice parameters with Gd3 + ion concentration, where (a) x ¼0, (b) x ¼0.1, (c) x¼ 0.3 and (d) x¼ 0.5. positions with concentration indicates the strain induced in lattice. A decrease in the diffraction peak intensity with doping concentration has been observed due to less crystallization. In order to see the doping ion concentration, the X-ray density dx was calculated using the formula [16–18] dx ¼8M/Na3, where M, N and ‘a’ are the molecular weight, Avogadro’s number and lattice parameter, respectively, and tabulated in Table 1. X-ray density increases linearly with Gd3 + concentration since gadolinium ion has larger ionic radii than iron atom. 3.2. FTIR spectroscopic characterization 311 440 220 511 400 (d) 422 Intensity (A.U.) (c) (b) (a) 20 30 50 40 70 60 2 Theta (degree) Fig. 1. (a, b, c, d): XRD patterns of as dried and calcined CoGdxFe2  xO4 (x¼ 0.0, 0.1, 0.3, 0.5). Table 1 Effect of Gd3 + ion concentration on structural parameters. Samples annealed at 773 K temperature Crystallite size (nm) Lattice parameter ‘a’ (Å) X-ray density (gm/cm3) CoGd0.0Fe2O4 CoGd0.1Fe1.9O4 CoGd0.3Fe1.7O4 CoGd0.5Fe1.5O4 23 17 15 16 8.412 8.417 8.426 8.461 5.437 5.453 5.886 6.362 Theoretically, all AB2O4 transition metals are normal and inverse spinel oxides have four infrared active modes. These vibrations occur in the n1 (650–550 cm  1), n2 (525–390 cm  1), n3 (380–335 cm  1) and n4 (300–200 cm  1) regions [19]. The n1 and n2 bands are observed due to intrinsic vibrations of tetrahedral (Td) and octahedral (Oh) coordination compounds. Both of these high frequency modes are attributed to the intrinsic vibrations of E-symmetry. Absorption of n1 is caused by the stretching of tetrahedral metal ion and oxygen bonding (Table 2), while n2 vibration is observed by the vibration of oxygen in the direction perpendicular to the axis joining the tetrahedral ion and oxygen; n3 mode is obtained from the Fe3 + /Gd3 + –O2  complexes at octahedral site [20]. The frequency of n4 vibration depends on the mass of tetrahedral metal ion complexes, which gives information about the vibration of ions at tetrahedral site. IR spectra recorded in 4000–400 cm  1 showed characteristic peak of tetrahedral and octahedral Fe  O stretching band at 582 and 416 cm  1 in all the samples as shown in Fig. 2. The intensity and peak positions of these modes vary with gadolinium ion concentration due to change in crystalline field effect and strain in lattice by gadolinium ion substitution. In addition to these vibrational modes, a broad hump due to water symmetric stretching and antisymmetric stretching with maxima at about 3400 cm  1 and bending mode at 1629 cm  1 is observed in these spectra. The broadness of stretching mode is attributed to the existence of hydrogen bonding. C H symmetric stretching and antisymmetric stretching of  CH2  group are observed as a double band at 2864 and 2902 cm  1 in all the three samples (Fig. 3). The symmetric and antisymmetric stretching modes of carboxylate ions are obtained at 1416 and 1551 cm  1, respectively. The appearance of these peaks in the spectra confirmed the presence of adsorbed oleic acid on the surface of nanoparticles. Author's personal copy 3690 R.P. Pant et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3688–3691 Table 2 IR transmittance peak positions of 0.1%, 0.3% and 0.5% Gd3 + substituted cobalt ferrite nanosize powders along with their tentative assignments. Peak position (cm  1) Sl. no. 1 2 3 4 5 6 7 8 Assignment 0.1% Gd3 + 0.3% Gd3 + 0.5% Gd3 + 722.43 670.07 618.26 — — 460.70 418.72 407.94 721.13 670.07 618.43 — — 459.57 420.99 410.78 720.56 669.50 620.67 569.64 549.78 460.14 420.42 409.64 u1 Fe  O Td symmetry rocking mode u2 Fe  O octahedral twisting mode u3 Fe  O octahedral wagging mode u Gd O antisymmetric stretching mode u Gd O octahedral symmetric stretching mode u Fe  O Td stretching mode u Fe  O octahedral stretching mode u O Gd  O bending mode 140 (d) 6000 (d) g = 2.1265 (c) Intensity (a.u.) Transmittance (a.u.) 5000 120 (b) 100 (c) 4000 3000 2000 80 (a) g = 2.1796 (b) (a) g = 2.3778 1000 0 4000 g = 2.2665 3500 3000 2500 2000 1500 1000 Wavenumber (cm-1) 3+ Fig. 3. IR transmittance spectrum of all Gd substituted cobalt ferrite in 4000–400 cm  1 region, where (a) x ¼0, (b) x¼ 0.1, (c) x ¼0.3 and (d) x ¼0.5. 3.3. EPR spectroscopic characterization EPR spectroscopy is a very sensitive technique for determining the paramagnetic species, role of magnetic dipolar interactions, anisotropy and superparamagnetic behavior during nanocrystalline ferrites formation. Magnetic dipole interactions among nanoparticles and superexchange interactions between the magnetic ions through oxygen ions are the two predominant factors that determine the g-value of EPR parameters and resonance linewidth DH. Strong dipole interactions give a large resonance linewidth and g-value, while strong superexchange interactions produce a small linewidth and g-value. The superexchange interactions generally increase when the distance between the magnetic ions and oxygen ions decreases and the corresponding bonding angles are close to 1801. EPR spectra of Fe3 + ions exhibit two resonance signals at g-values around 2.00 and 4.3. The signal at g-value of about 4.3 is due to the isolated Fe3 + ions in the host lattice, while the signal at g-value around 2.00 has been attributed to the pairs or small clusters of Fe3 + and Gd3 + ions. In gadolinium substituted cobalt ferrite (CoGdxFe2 xO4, where x¼0, 0.1, 0.3, 0.5) nanoparticles as shown in Fig. 4, a broad asymmetric signal superimposed with a narrow signal was observed with g-values as 2.3778, 2.2665, 2.1796 and 2.1265, respectively. The appearance of this signal reveals the formation of CoGdxFe2 xO4 and superparamagnetic behavior of these nanoparticles. While in Gd3 + (x¼0.1) substituted cobalt ferrite nanoparticles, EPR spectrum consists of a broad resonance peak with linewidth DHPP ¼904 G and higher g-value 2.2665 in Gd3 + series 2000 4000 6000 Magnetic Field (Gauss) 500 Fig. 4. EPR spectrum of Gd3 + substituted cobalt ferrite, where (a) x ¼0, (b) x¼ 0.1, (c) x ¼0.3 and (d) x¼ 0.5. indicates the formation of CoGd0.1Fe0.9O4 and strong magnetic dipole interaction among these particles. This can be explained as some of FeO6 octahedrons were distorted and some of them were transformed into FeO4 tetrahedra. This means that Fe3 + O Fe3 + and Fe3 + O Gd3 + pairs have probably long bond lengths and large deviations of bonding angles from 1801, which produce weak and superexchange interactions among Fe3 +  O Fe3 + Fe3 + O Gd3 + pairs. The weak superexchange interactions result in the broadening of resonance linewidth and large g-value. In CoGdxFe2 xO4 for x¼0.3 and 0.5, with an increase in substitution of Gd3 + ions concentration, the distortion of polyhedra in the particles causes a reduction in crystallinity. The bond lengths of Fe O and Gd O bonds decrease and the bonding angles of ionic pairs increased towards 1801. This causes the strong superexchange interactions among cations through oxygen ions and a decrease in EPR linewidth and g-value. The superparamagnetism is observed in these samples due to extremely fine nanoparticles, which make it easier for them to be thermally activated to overcome magnetic anisotropy. As the size of magnetic particles is less than the superparamagnetic critical dimensions, above the blocking temperature, its thermal fluctuations can overcome the magnetic anisotropy, so that magnetic moments can rotate in different easy directions and superparamagnetism is exhibited [21–24]. 4. Conclusion XRD studies of cobalt ferrite and different gadolinium ion concentration substituted cobalt ferrite nanoparticles reveal that Author's personal copy R.P. Pant et al. / Journal of Magnetism and Magnetic Materials 322 (2010) 3688–3691 there is a minor shift in the peak position on Gd3 + ion substitution, which results in an increase in lattice parameters and change of lattice. Infrared studies confirmed the ferrite formation and gadolinium ions acquired octahedral site in the cobalt ferrite lattice. The shift in peak positions of ferrite pertaining peaks to higher frequency due to gadolinium substitution and their intensities are interpreted in terms of the change in crystal field on Gd3 + ion substitution in the crystal lattice. Such superparamagnetic particles can be used in the preparation of ferrofluids, which are widely used in many practical applications. Acknowledgment We are extremely thankful to the Director, National Physical Laboratory, for his continuous encouragement and permission to carry out this work. References [1] K. Dong-Hyun, E.N. David, T.J. Duane, S.B. Christopher, J. Magn. Magn. Mater. 320 (2008) 2390. [2] A.T. Ngo, M.P. Pileni, J. Phys. Chem. B 105 (2001) 53. 3691 [3] M.H. Sousa, F.A. Tourinbo, J. Phys. Chem. B 105 (2001) 168. [4] D.H. Han, H.L. Luo, Z. Yang, J. Magn. Magn. Mater. 161 (1996) 376. [5] V. Kumar, A. Rana, M.S. 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