Author’s Accepted Manuscript Structural and magnetic study of dysprosium substituted cobalt ferrite Nanoparticles Hemaunt Kumar, R.C. Srivastava, Jitendra Pal Singh, P. Negi, H.M. Agrawal, D. Das, Keun Hwa Chae www.elsevier.com/locate/jmmm
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S0304-8853(15)30626-0 http://dx.doi.org/10.1016/j.jmmm.2015.09.077 MAGMA60685
To appear in: Journal of Magnetism and Magnetic Materials Received date: 11 November 2014 Revised date: 17 August 2015 Accepted date: 24 September 2015 Cite this article as: Hemaunt Kumar, R.C. Srivastava, Jitendra Pal Singh, P. Negi, H.M. Agrawal, D. Das and Keun Hwa Chae, Structural and magnetic study of dysprosium substituted cobalt ferrite Nanoparticles, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2015.09.077 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Structural and Magnetic Study of Dysprosium Substituted Cobalt Ferrite Nanoparticles Hemaunt Kumar1*, R. C. Srivastava1, Jitendra Pal Singh2, P. Negi1, H. M. Agrawal2, D. Das3and Keun Hwa Chae2 1
Department of Physics, Govind Ballabh Pant University of Agr. &Technology, Pantnagar, Uttarakhand 263145, India 2 Advanced Analysis Centre, Korea Institute of Science and Technology, Seoul 790-731, Korea 3 UGC-DAE CSR Kolkata Centre, Kolkata-700098, India Abstract
The present work investigates the magnetic behaviour of Dy3+ substituted cobalt ferrite nanoparticles. X-ray diffraction studies reveal presence of cubic spinel phases in these nanoparticles. Raman spectra of these nanoparticles show change in intensity of Raman bands, which reflects cation redistribution in cubic spinel lattice. Saturation magnetization and coercivity decrease with increase of Dy3+concentration in these nanoparticles. Room temperature Mössbauer measurements show the cation redistribution in these nanoparticles and corroborates the results obtained from Raman Spectroscopic measurements. Decrease in magnetization of Dy3+ substituted cobalt ferrite is attributed to the reduction in the magnetic interaction and cation redistribution. Keywords: Nanoparticles, Dy3+ doping, Raman Spectra, VSM and Mössbauer.
Corresponding Author Dr. Hemaunt Kumar Department of Physics Govind Ballabh Pant University of Agr. & Technology, Uttarakhand-263145 (India) Email: [email protected]
1. Introduction: CoFe2O4 is a hard magnetic material with high coercivity value upto 5.4 kOe and moderate saturation magnetization of ~80emu/g [1, 2]. This makes CoFe2O4 an important magnetic material for various applications such as in electronic devices, ferro-fluids, drug delivery system, microwave devices, and high-density information storage [1, 2]. Bulk cobalt ferrite possesses inverse spinel structure. It has half of the Fe3+ ions on the A site (tetrahedral) and remaining half of the Fe3+ ions and all the divalent metal ions situate on the B-site (octahedral). Synthesis of this material in nanoregime induces cation redistribution over A and B sites in the spinel ferrite which leads to magnetic properties different from its bulk counterpart [1-3]. Thus nanosized CoFe2O4 is neither completely normal spinel nor completely inverse spinel, but has mixed spinel structure [4, 5]. Magnetic behavior of this material is not only affected by size of nanoparticles but also influenced by thermal treatment during synthesis and the nature of dopant ions [1, 6]. Saturation magnetization of La3+ doped cobalt ferrite decreases with increasing concentration of La3+ ions . Mössbauer study of Y3+ substituted cobalt ferrite nanoparticles  showed two sets of six- line hyperfine patterns, indicating the presence of Fe3+ ions at both A and B sites. On increasing the concentration of Y3+, the hyperfine field strength and isomer shift first increases and then decreases, whereas the quadrupole splitting continuously increases. Magnetization and blocking temperature decrease with substitution of Dy and Gd ions to CoFe2O4 . Three percent doping of Ce, Sm, Er, Gd, Dy and La into CoFe2O4 decreases the specific magnetization, however, Ho and Yb doping increases this parameter . An increase of Gd3+ concentration from 0 to 10% in the lattice, saturation magnetization of CoFe2O4 decreases from 63 emu/g to 27.26 emu/g . Ten percent doping of Nd, Eu and Gd decreases saturation magnetization of CoFe2O4, however, increase is observed in coercivity . Dy3+ions enhance the coercivity of CoFe2O4 by 150% . Thus various studies show that magnetic properties of rare earth
(RE) doped cobalt ferrite cannot be explained on the basis of magnetic moment of RE ions only. This aspect is also observed by Myrtil and Zhang  where magnetic properties of cobalt ferrite nanoparticles doped by a number of lanthanide series ions cannot be well understood with respect to either the number of unpaired 4f electrons or the magnetic moment of the lanthanide series ions.In this context a deep study, focusing on the change of local structure and microstructure of CoFe2O4 induced by RE ion will open pathways to understand the underlying mechanism. Thus, present work is motivated to understand the effect of Dy3+doping in cobalt ferrite using Raman, Mössbauer, near edge X-ray absorption fine structure spectroscopy (NEXAFS). 2. Experimental Nanoparticles of CoFe2-xDyxO4 (x=0.0-0.15) were synthesized by using the chemical route . Aqueous solutions of ferric nitrate, cobalt nitrate and dysprosium nitrate were prepared separately in stoichiometric proportion. The solutions were mixed with citric acid solution by keeping cations to citric acid molar ratio of 1:2. The mixture was kept on a magnetic stirrer at 80oC until it became viscous. The viscous solution was heated overnight at 100oC in an oven to form the precursor. The precursor obtained for different compositions were annealed at 300oC to get the final product. X-ray diffraction (XRD) data were collected on Bruker D8 ADVANDCED X-ray diffractometer using the Cu Kα radiation (λ=1.54178Å). The microRaman system used in this study consisted of a 200 m W Ar ion laser (488 nm) with a focal spot size of 10-20 μm. The hyperfine interactions were studied by spectroscopy in transmission geometry at room temperature using
Co in Rh as the source.
The hysteresis curves of these nanoparticles were recorded using a 14T-6000 PPMS vibrating sample magnetometer (VSM). Dy M-edge spectra of these samples were recorded at 10D XAS-KIST beam line at Pohang Accelerator Lab, Pohang, South Korea.These spectra were
recorded using a grating (1800 lines/mm) and 100µm×100µm slit. The spectra were background subtracted and normalised with respect to height. 3. RESULT AND DISCUSSION 3.1 XRDstudy Fig. 1 shows XRD patterns for CoFe2-xDyxO4 (x=0.00, 0.05, 0.10,0.15) ferrite systems. XRD patterns were indexed using Joint Committee for Power Diffraction Standards (JCPDS) Card No. - 03-0864. It confirms formation of single phase of cubic spinel structure. In the present case no secondary phase is observed upto the Dy3+ concentration of x= 0.15. This concentration is higher compared to the methods reported earlier . The values of crystallite size of these nanoparticles were estimated from the broadening of the most intense XRD peak using the Scherrer’s formula . D
where, D is the crystallite size, λ is the X-ray wavelength (1.5405Å), β is the full width at half maximum (FWHM) for the most intense peak (in radian) and θ is the Bragg’s angle. The crystallite size is 9±1, 8±1, 8±1 and 8±1 nm for Dy3+ concentration of 0.00, 0.05, 0.10 and 0.15 respectively (Table 1).The crystallite size remains almost constant with Dy3+ doping which is attributed to the rare earth ions. These ions inhibit grain growth which results in the invariance of crystallite size in RE doped ferrites . 3.2 Raman Study According to the group theory, ferrite has a cubic spinel structure with O7h (Fd3m) space group, which gives rise to 39 normal modes.
A1g, 1Eg, 3F2g are Raman active modes in the cubic spinel phase [6, 17]. Bands corresponding to these modes generally appear in the range of 200 to 1200 cm-1 [17-19]. Hence, Raman
spectra of Dy3+ doped cobalt ferrite nanoparticles were recorded in the range of 200 to 1200 cm-1 and are shown in Fig. 2. Bands above 600 cm-1 in the spectra of all nanoparticles, appear due to A1g symmetry involving symmetric stretching of oxygen atom with respect to metal ion at the tetrahedral site.
Low frequency phonon modes, Eg and 3F2g in the spectra
correspond to the symmetric and anti-symmetric bending of metal-oxygen bond at octahedral sites [6, 17-19]. Thus presence of bands at ~302, 467, 593 and 681 cm-1 reflect Eg, F2g(2), F2g(1), A1g(1) modes of spinel lattice. Presence of these bands in Raman spectra of all the samples envisages that all modes are present in Dy3+ doped nanoparticles (Fig. 2). To further elucidate the role of Dy3+ ions in spinel lattice, band-position, band width and Raman intensity were estimated using Lorentzian fitting (Fig. 2). Fig. 3 shows variations of band positions of Eg, F2g(2), F2g(1) and A1g(1) mode as a function of Dy3+ concentration. Band positions corresponding to mode F2g(2) and F2g(1) shifts towards higher wave number with increase of Dy3+ concentration. The variations of band positions are different for the modes Eg and A1g(1). Band positions shifts towards higher values upto the Dy3+ concentration of 0.10 for these modes. For Dy3+ concentration of x=0.15, band position shifts towards lower wave numbers for these modes.
Values of band –width for modes Eg, F2g(2), F2g(1) and
A1g(1) show lower value at the lowest concentration of x=0.00 (Fig. 4) and increase the value for all Raman modes with Dy3+ ion concentration except x=0.15. Change in the values of band-width corresponding to various modes is attributed to crystalline disorder. In Fig. 5 variations of band intensities for various modes are shown. Modes A1g(1) and F2g(1) exhibit opposite variation with Dy3+ concentration indicating exchange of cations among A and Bsite. Intensity of band corresponding to the mode F2g(2) decreases with increase of Dy3+ concentration. Intensity of the band corresponding to the mode Eg increases for Dy3+ concentration of x=0.05 and then decreases.
3.3 VSM Study Hysteresis curves of Dy3+ doped cobalt ferrite nanoparticles at room temperature are shown in Fig.6. These curves exhibit coercive behaviour and do not saturate upto 30 kOe. The observed behaviour of these samples is similar to the previously reported cobalt ferrite [1-6]. The saturation magnetization (σs) were estimated by extrapolating to higher magnetic field data using the relation [
⁄ ) ] and plotting a graph between
intercept along σ- axis (for 1/H=0) give the saturation magnetization. The estimated values of saturation magnetization (σs) are collated in Table 1. The value of saturation magnetization of cobalt ferrite nanoparticles at room temperature is 51 emu/g, which is lower than the value of bulk cobalt ferrite (80 emu/g) [1, 3, 20]. The reduction of magnetization for small size cobalt ferrite is generally observed and is in with previous studies [21, 22]. This effect in small sized cobalt ferrite nanoparticle is explained on the basis of large surface to volume ratio of nanoparticles, canting of surface spins. The values of saturation magnetization are 28, 29 and 30 emu/g for Dy3+concentration of x=0.05, 0.10 and 0.15 respectively. Slight increase of saturation magnetization with Dy3+ concentration is attributed to incorporation of higher magnetization entity in form of Dy3+ ions inside cobalt ferrite lattice. It is clear from Table 1 that saturation magnetization of Dy3+ doped cobalt ferrite nanoparticles is almost 60% of saturation magnetization of pure cobalt ferrite [9-10]. This effect may be attributed to the strong tendency of RE ions to occupy which result redistribution of cations among different sites [9-10]. Thus, observed lowering of magnetization of the CoFe2-xDyxO4 nanoparticles can be understood taking into account changes in the cation composition and distribution in the cubic spinel structure as well as surface and finite size effects in nanometer sized particles. Dy3+ (0.91Å) ions have larger ionic radii than Fe3+(0.64Å) ions, so that Dy3+ substitution distorts the lattice and destroys the homogeneous composition, thus causing the deterioration in magnetic moment. The larger
ions of Dy3+(0.91Å) replace Fe3+(0.64Å) ions on B sites [23, 24], this causes decrease in the magnetic moment on the B sublattice as well as decreasing the strong negative Fe3+-Fe3+ interaction in the doped samples. The magnetic moment (ηB=MB-MA) is decreased by antiferromagnetic coupling. The change in antiferromagnetic exchange interaction of cubic spinel ferrite nanoparticles is due to substitution of Fe3+ ions by Dy3+ ions . Coercivity decreases almost linearly from 1.30 kOe to 0.77 kOe as Dy3+ concentration increases from 0.00 to 0.15. This result is different from that is observed by previous workers [10, 13]. Moreover, remanent magnetization also reduces after Dy3+ doping (Table 1). 3.4 Mössbauer Study: The Mössbauer spectra recorded at room temperature for dysprosium doped cobalt ferrite are shown in Fig.7. Spectra of all the samples were fitted with two six line sub patterns that are assigned to tetrahedral sites (A) and octahedral sites (B) of a typical spinel crystal structure. The area of the subspectra and recoil free fractions of Fe3+ at A and B site were used to estimate the relative concentration of iron at each site according to the relation: nB/nA=IBfB/IAfA, where IA and IB are the areas of A and B sites and nA and nB are the concentrations of iron corresponding to A and B sites respectively. The ratio of the recoilless fractions were taken as fB/fA =0.94 at room temperature . Thus assuming the structural formula as (Co12 Fe3 ) A [Co2 Fe 23 ] B O4 the inversion parameter (λ) was calculated by using the relation 0.94×IB/IA= (2-λ)/λ. The structural formula for all the synthesized samples is shown in Table 2. It shows exchange of cations between A and B-sites with increase of Dy3+ concentration. This effect is also evidenced by Raman spectroscopy. The Mössbauer spectra of these nanoparticles show a doublet, which become more significant when the concentration of Dy3+ is increased. Area of this doublet increases from 16% to 38% as Dy3+concentration increases from 0 to 0.15. Presence of this doublet in these nanoparticles may be attributed to the superparamagnetic natures of nanoparticles . The hyperfine
parameters like isomer shift (I.S.), Quadrpole splitting (Q. S.), line-width (LWD) and hyperfine field (HF) were estimated by fitting these spectra and are collated in Table 2. In the Dy3+ ion doped cobalt ferrite nanoparticles the I.S. of Fe3+ ions at A and B sites change with Dy3+ ion concentration. This indicates that the s-electron density at the Fe3+ nucleus is affected by Dy3+ doping concentration as I.S. values depends on the s-electron charge density of the absorber . Values of I.S. on A-sites are less than that of the B-sites for pure CoFe2O4 nanoparticles. This is attributed to the smaller overlapping of orbitals of Fe3+ and O2- ions at B-site due to the larger bond separation of Fe3+-O2- compared to A-site. The change of I.S. at B-site (Table 2) is ascribed to change in Fe3+-O2- bond separation at B-site due to insertion of larger ionic radii of Dy3+ ions at this site. Small values of Q.S. for pure cobalt ferrite nanoparticles at A and B-site shows presence of cubic symmetry (Table 2). This symmetry is preserved for doped CoFe2O4 as indicated by the values of Q.S. for these nanoparticles . Bhf at A-site is larger than Bhf at B-site for concentration x=0.00 than that for the other concentrations x=0.05, 0.10 and 0.15.The crystal grain size decreases with increase of the concentration of the Dy3+ ions in the samples. The collective magnetic excitations reduce the hyperfine field of the sample besides superparamagnetism . As the concentration of doped Dy3+ ions increases, the size of the crystal grain decreases, whereas lattice distortion and concentration of deficiency both increases, resulting in a decrease in the hyperfine field. 3.5 Dy M-edge spectra study: To elucidate role of Dy3+ ions in CoFe2O4 further Dy M-edge spectra of samples are recorded in (Fig. 8). These spectra exhibit spectral features corresponding to M5 and M4 for these samples. Presence of these spectral feature ascribed to the dipole selection rule [31, 32]. Dy M-edge spectra reflect presence of spectral features corresponding to M5 and M4 edge. These
spectral features reflect presence of Dy ions in octahedral environment [31, 32]. Calculated Dy M-edge in oxides exhibit three spectral features corresponding to M5 edge, however, spectra for these samples exhibit one peak followed by a shoulder S1 corresponding to M5 edge. This may be due to low concentration of Dy ions in the samples. Spectral features in Dy M-edge spectra do not change with concentration of Dy ions reflecting presence of same valence state of Dy ions in all the samples. It is well known that an assembly of nanoparticles exhibit particle size distribution rather than uniform size for all nanoparticles. In present case, even the particle size of nanoparticle is 9 nm (Table 1), it contains a portion of particles which has smaller size than this favouring superparamagnetism in the system. Thus magnetic behaviour of synthesized cobalt ferrite nanoparticle assembly can be better described by a combined effect of superparamagnetic and ferrimagnetic region. Presence of two sextet and one doublet supports this (Fig. 7). With Dy3+ doping area of doublet increases which shows increase of superparamagnetic region. The increase of this region with Dy3+ doping is attributed to reduction of crystallite size along with reduction of magnetic anisotropy. A huge reduction in remnant magnetization and coercivity (Table 1) favours the lowering of magnetic anisotropy with Dy3+ doping. 4.Conclusion Dy3+ doped cobalt ferrite nanoparticles were synthesised by the nitrate route at low sintering temperature (300oC). The XRD patterns show formation of the pure cubic spinel phase and mixed spinel structure. The Dy3+ ions can be incorporated in the cobalt ferrite lattice upto x=0.15, without any impurity phase. The crystallite size is almost constant with increase in Dy3+ ion concentration in the sample. The doping of Dy3+ ions in the samples reduced the saturation magnetization from 45 to 26emu/g. In the present case the magnetic moment measured by VSM technique at room temperature is different from that estimated from
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Table 1: Crystallite size (D), saturation magnetization (σs), remanent magnetization (σr) and coercivity (Hc) of the samples CoFe2-xDyxO4 for different concentration (x). x
0.00 0.05 0.10 0.15
9 8 8 8
52 28 29 30
) 13 8 4 5
Hc (kOe) 1.30 1.08 0.81 0.77
Table 2: Mössbauer parameters of the samples CoFe2-xDyxO4 for different concentration (x). A and B represents tetrahedral and octahedral site respectively. x
Line width (mm/s)
A B Para. A B Para. A B Para A B Para
0.22 0.34 0.17 -0.17 -0.15 -0.25 -0.14 -0.14 -0.21 -0.19 -0.14 -0.20
-0.10 0.09 -1.10 0.07 0.04 0.87 0.08 0.07 -0.80 0.01 0.05 0.82
0.43 1.63 1.54 1.06 0.81 0.58 0.86 0.70 0.64 1.22 0.70 0.86
48.4 48.5 -42.72 48.76 -43.12 48.68 -42.23 48.44 --
13.81 70.16 16.03 14.62 47.90 37.48 15.73 47.17 37.10 21.48 40.46 38.04
(Co0.65Fe0.35) [ Co0.35Fe1.65]O4
0.05 0.10 0.15
(Co0.51Fe0.49) [ Co0.49Dy0.05Fe1.46]O4 (Co0.38Fe0.62) [ Co0.62Dy0.10Fe1.28]O4 (Co0.27Fe0.73) [ Co0.73Dy0.15Fe1.12]O4
Figure caption: Figure 1: XRD patterns of CoFe2-xDyxO4 (x=0.00, 0.05, 0.10 & 0.15). Figure 2: Raman spectra of CoFe2-xDyxO4 (x=0.00, 0.05, 0.10 & 0.15). Red and green lines show the Lorentzian fitting of the spectra. Figure 3: Band position of modes A1g(1), F2g(1), F2g(2) and Eg modes as a function of Dy3+ concentration. Figure 4: Band- width of modes A1g(1), F2g(1), F2g(2) and Eg modes as a function of Dy3+ concentration. Figure 5: Raman intensity of modes A1g(1), F2g(1), F2g(2) and Eg modes as a function of Dy3+ concentration. Figure6: M-H curves of CoFe2-xDyxO4 (x=0.00, 0.05, 0.10 & 0.15). Figure 7: Mössbauer spectra of CoFe2-xDyxO4. Figure 8: Dy M-edge spectra of CoFe2-xDyxO4 (x= 0.05, 0.10, and 0.15) nanoparticles.
Fig. 1: XRD patterns of CoFe2-xDyxO4 (x=0.00, 0.05, 0.10 & 0.15).
Fig. 2: Raman spectra of CoFe2-xDyxO4 nanoparticles. Red and green lines show the Lorentzian fitting of the spectra.
Fig. 3: Band positions of modes A1g(1), F2g(1), F2g(2) and Eg modes as a function of Dy3+ concentration.
Fig. 4: Band- widths of modes A1g(1), F2g(1), F2g(2) and Eg modes as a function of Dy3+ concentration.
Fig. 5: Raman intensity of modes A1g(1), F2g(1), F2g(2) and Eg modes as a function of Dy3+ concentration.
Fig. 6: M-H curves of CoFe2-xDyxO4 (x=0.00, 0.05, 0.10 & 0.15) nanoparticles.
Fig.7: Mössbauer spectra of CoFe2-xDyxO4(x=0.0, 0.05, 0.10, and 0.15) nanoparticles.
Fig. 8: Dy M-edge spectra of CoFe2-xDyxO4 (x= 0.05, 0.10, and 0.15) nanoparticles.
Highlights are (1) Slight decrease in crystallite size after Dy3+ doping. (2) Saturation magnetization and coercivity decrease after Dy3+ doping. (3) Mössbauer measurements show the cation redistribution in the samples.