Author’s Accepted Manuscript Effect of doping of chromium ions on the structural and magnetic properties of nickel ferrite Aakash, Anirban Roychowdhury, Dipankar Das, Samrat Mukherjee www.elsevier.com/locate/ceri
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To appear in: Ceramics International Received date: 25 November 2015 Revised date: 22 January 2016 Accepted date: 27 January 2016 Cite this article as: Aakash, Anirban Roychowdhury, Dipankar Das and Samrat Mukherjee, Effect of doping of chromium ions on the structural and magnetic properties of nickel ferrite, Ceramics International, http://dx.doi.org/10.1016/j.ceramint.2016.01.188 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.
Effect of doping of chromium ions on the structural and magnetic properties of nickel ferrite
Aakasha, Anirban Roychowdhuryb, Dipankar Dasb, and Samrat Mukherjeec,* a
Department of Electronics and Communication Engg., Birla Institute of Technology, Mesra, Ranchi 835215 , Jharkhand, India
UGC-DAE, Consortium for Scientific Research, Kolkata Centre, III/LB 8, Bidhannagar, Kolkata 700098, West Bengal, India c
Department of Physics, National Institute of Technology, Patna 800005, Bihar, India
Corresponding author’s e-mail: [email protected]
Abstract In this paper we report the synthesis of Cr doped nickel ferrite denoted by the generic formula Ni1xCrxFe2O4
(x = 0.05, 0.10 and 0.15) through standard chemical co-precipitation method. An effort has
been made to study the effect of doping of trivalent ions on the formation of cationic vacancies in the crystal. The structural characterization of the samples was carried out using X-ray diffraction and Raman spectroscopy. X-ray diffraction confirmed the formation of single spinel phase. The refinement of the pattern using GSAS showed the presence of cationic vacancies in all the three samples which give rise to compressive strain. Raman spectroscopy and Mössbauer measurements indicated distortion in the crystal structure as doping of Cr3+ was increased. The magnetic measurements of all the samples exhibited negligible coercivity coupled with ferrimagnetic nature indicating very low magnetic anisotropy. Additionally the samples showed a steep magnetization curve making it an attractive choice for magnetic switching applications. Keywords:
spectroscopy. 1. Introduction
In the present era ferrites are one of the most widely investigated materials; interest shown by both industrial and the scientific community. They exhibit high electrical resistivity and low dielectric losses thereby showing minimal eddy current losses; ideal for the fabrication of transformer cores [1-3]. The large magnetocrystalline anisotropy offered by the ferrites is employed for non-reciprocal circulators and phase shifters [4, 5]. The ability to tune the permeability of ferrites is used for the designing of frequency tunable microstrip antenna. Further they permit the generation of highly preferable circular polarization of the radiated electromagnetic fields [6, 7]. The above properties have made it an attractive candidate for the power sector and the communication industry. Other than their excellent dielectric properties, ferrites also show remarkable magnetic properties. For magnetic switching, nanoferrites displaying negligible coercivity and a good squareness ratio are extremely desirable since these properties ensure minimal magnetic energy loss during magnetizationdemagnetization process . Further, the sharpness of the gradient of the M-H curve serves as a measure of its switching response; the steeper the gradient the faster saturation is achieved. However, development of such ferrites possessing low coercivity combined with ferrimagnetic behavior at room temperature is still a subject of scientific interest. Among the various classes of ferrite materials, Ni ferrites, forming an inverse spinel structure have been a subject of analysis due to their excellent magnetic and dielectric properties. Ni2+ ions occupy B sites while Fe3+ ions are distributed over A and B sites [9-11]. The introduction of a dopant such as Zn and Cu alter the magnetic properties such as saturation magnetisation and electrical conductivity . However, it is the addition of trivalent transition elements such as Cr and Mn which is of interest. Cr3+ ions show a marked preference for B-site as has been reported by others [13-15]. It is the replacement of Ni2+ ions or the Fe3+ ions by the Cr3+ ions which brings about a marked change in the local geometry of the crystal. The large difference in ionic radii of the ions may induce a compressive or a tensile strain in the lattice. Additionally the presence of trivalent ions leads to the possibility of formation of oxygen/cationic vacancies [16, 17]. The presence of defects and the distortions in the crystal bring about a change in the
magnetic properties of the spinel such as change in the net magnetic moment and magnetocrystalline anisotropy. In the present study, we have synthesized a series of Ni ferrite doped with Cr ions with the generic formula Ni1-xCrxFe2O4 with ‘x’ having values of 0.05, 0.10 and 0.15 using soft-chemical route. X-ray diffraction indicated the formation of the pure spinel phase without any additional peaks. Raman spectroscopy was employed to study the vibrational modes of the samples. Mössbauer spectroscopy was carried out to determine the distribution of Fe3+ in the A and B sites. The overall magnetic properties of the samples were studied by SQUID measurements. Very low magnetic anisotropy coupled with a good squareness ratio was obtained in the M-H plots. The above mentioned properties indicate the potential of the material for magnetic switching. 2. Experimental procedure The samples of Cr doped Ni ferrite Ni1-xCrxFe2O4 were synthesized via the standard chemical coprecipitation technique. The reagents Ni(NO3)2.6H2O, FeCl3, Cr(NO3)3.9H2O were dissolved in a stoichiometric ratio in de-ionized water. The solution was stirred using a magnetic stirrer while NaOH was added drop by drop till the pH of the solution reached 7. The pH was increased to 10 so as to enable complete precipitation. The solution was digested at 80oC for two hours so that no component was left unreacted. It was washed dried and ground into fine powder. The ground sample was fired at 1100 oC in a furnace and slowly cooled to obtain fine black particles of the ferrite. All the characterizations were done with these powders. The X-ray diffraction of the samples Ni1-xCrxFe2O4 (x= 0.05, 0.10, 0.15) was carried out using (Bruker D8 Advance) X-Ray Diffractometer at room temperature (300 K). The source of radiation was Cu-Kα having a wavelength of 1.5406 Å with the data collected at a scanning speed of 0.020 per sec in the angular range of 15o ≤ 2θ ≤ 70o. The Raman spectroscopy of the polycrystalline samples was carried out using Raman spectrometer (Renishaw Instruments) at room temperature using 514.5 nm line of Argon ion laser (0.5
mW) in the frequency range 100 cm-1 - 1000 cm-1 to study the vibrational and structural details of the crystal. The magnetic measurements of the samples were carried out using (Quantum Design) SQUID magnetometer at room temperature up to a field of 7 T. The Mössbauer spectroscopy measurements were carried out using a constant acceleration Mössbauer spectrometer calibrated using a 12 μm high purity Fe57 foil at room temperature. The spectra were measured in zero magnetic field and fitted using LGFIT2 program . 3. Results and discussion 3.1. XRD analysis The XRD patterns of Ni1-xCrxFe2O4 (x = 0.05, 0.10, 0.15) are designated as Cr5, Cr10 and Cr15 respectively with reference to the Cr concentration. The samples belonged to the space group Fd-3m with the pattern displaying all the characteristic peaks of ferrite, 311 being the most prominent peak. The experimental patterns were consistent with the standard diffraction pattern of NiFe2O4 (JCPDS card number 10-325) and displayed no extra peaks which confirmed the presence of single spinel phase in the samples. All the patterns were analyzed and fitted by Rietveld refinement technique with the GSAS program . The peak profiles were fitted using pseudo-Voigt function with Finger-Cox-Jephcoat asymmetry. The observed diffraction pattern were in good agreement with the calculated pattern verified by the value of chi square and the residue factors obtained from the refinement (Fig. 1). The values of the lattice parameter and the crystallite size obtained from Williamson-Hall plot of the refined data are shown in Table I. The lattice constant is found to decrease with increase in Cr concentration in the lattice. This can be ascribed to the replacement of larger Ni2+ (0.69Å) ions by the smaller Cr3+ (0.615 Å) ions. The Williamson-Hall plot is shown in Fig. 2. It is observed that the plots of all the samples display negative slope. The negative slope signifies the presence of compressive strain in the samples [20, 21]. The cationic vacancy shows an increase with increasing Cr3+ concentration. This again can be ascribed both to the charge imbalance and large difference in the ionic radii of Ni2+ and Cr3+ ions. The replacement
of larger Ni2+ ions by the smaller Cr3+ ions and the presence of defects such as cationic vacancies distort the lattice thereby inducing a compressive strain in the lattice. 3.2. Raman study Nickel ferrite has an inverse spinel cubic structure and belongs to the space group O7h (Fd-3m) with eight formula units per cell. The complete unit cell contains 56 atoms but the smallest Bravais cell contains only 14 atoms . The group theory predicts 42 vibrational modes out of which there are five Raman active modes: A1g+Eg+3T2g. The A1g mode indicates the symmetric stretch of oxygen atoms along Fe-O tetrahedral bonds; the Eg and T2g(3) modes are due to symmetric and asymmetric bending of oxygen with respect to Fe respectively, T2g(2) indicates the asymmetric stretch of Fe-O bond and T2g(1) indicates the translational movement of the tetrahedron (Fe3O4) . Fig. 3 shows the Raman spectra of all the samples. All the samples showed distinct peaks with a shoulder like feature on the lower wavenumber side. The peaks correspond to the Raman active modes of the space group 3 . The peaks along with the shoulder like appearances were deconvoluted into two peaks each and fitted using Lorentzian line shape. The appearance of shoulder like feature and peak broadening is attributed to the lowering of the space group symmetry. In case of NiFe 2O4 the cations Ni and Fe at the B site are arranged in a definite order. The short range cationic order leads to formation of domains with lower space group symmetry . The extra fitted peaks were found to match with the Raman modes of the P4122 space group. The site symmetry reduces from the cubic 3 to tetragonal P4122. The short range atomic ordering leads to the co-existence of the tetragonal phase along with the cubic phase. This short range order is below the detection limit of X-ray diffraction technique and as a result the macroscopic crystal appears to be cubic. The small amounts of doped Cr ions replace the Ni ions at the B site, which is reflected in the shifts in the Raman active modes corresponding to the B site symmetry. The shoulder left of A1(3) peak is identified as B2/E modes of P4122 symmetry which appear to shift to higher wavenumber with increase in Cr
concentration. This can be explained on the basis of replacement of heavier Ni ions by the lighter Cr ions. A similar shift is observed for A1(1) and B2(2) peaks deconvoluted from the T2g(2) peaks which arises as a consequence of B site symmetry. The T2g(3) mode was deconvoluted into peaks which were indexed as A1(2) and B2(3) symmetry modes. Out of these modes, the B2 mode appears as a consequence of the presence Fe ions at the B site. These peaks which exhibit a negligible shift for Cr 5 and Cr10 samples show a prominent shift towards lower wave number for Cr15 sample. From the XRD data it is found that the magnitude of Fe vacancy in the Cr15 sample is very high compared to the Fe vacancies in Cr5 and Cr10 samples. This shift is attributed to the presence of Fe ion vacancies the octahedral site. All the Raman peaks have been indexed and the presence of additional and broadened peaks has been attributed to co-existence of P4122 and 3 space group. 3.3. Magnetic study Fig. 4 shows the M-H curve of the samples obtained at room temperature. The soft nature of the ferrite samples can be inferred from the narrow loop area of the M-H curve of both the samples. The saturation magnetisation, retentivity and coercivity for different samples are shown in Table III. It is interesting to note that all the three samples exhibit zero coercivity and a small squareness ratio at room temperature. The process of magnetisation consists of two phenomena namely domain wall movement and spin rotation. The domain wall movement is energetically favorable in multi-domain particles as opposed to spin rotation which is possible energetically in smaller single domain particle. With increase in grain size, the number of domains increases thereby increasing domain wall movement whose contribution to the magnetisation process is greater than due to spin rotation [26, 27]. This results in low coercivity in samples with large grain size. The coercivity is a measure of magnetic anisotropy exhibited by a magnetic structure and is dependent upon the effective anisotropy constant (
# !"" )
material. The total effective anisotropy energy barrier ∆%& is given as : ∆%& =
′ !"" '
where V is the volume. For a finite volume of particle, a negligible value of coercivity is an indication of very small anisotropic constant. The application of a small applied magnetic field causes the domain wall to realign, such that the spins are oriented along the easy axis of magnetisation because of the low magneto-crystalline anisotropy, which accounts for the low coercivity . Another interesting aspect is the steepness of the gradient of the magnetisation curve. The graph exhibits a switching nature; a very small amount of applied magnetic field drives the spins into saturation. Upon reversing the applied magnetic field the alignment of the spins is reversed with negligible loss of energy. The MS for Cr doped samples decreases slightly for increasing Cr content (x ≤ 0.10) and then a small increment is observed for x = 0.15. The magnetic moment in NiFe2O4 is a result of the of the strong A-B superexchange interaction between the Fe3+ ions located at the octahedral and tetrahedral sites . As Cr3+ (3 μB) is introduced in the sample, it substitutes Ni2+ (2 μB) ions at the B site which results in increase of magnetic moment of B sub-lattice. According to Neel's molecular field model the net magnetic moment in case of spinels is given as the algebraic sum of the magnetic moments of the sub-lattices A and B: MS = MB - MA, where MB and MA are the magnetic moments of sub-lattices A and B and MS is the net magnetization . Although upon increasing the Cr concentration (x = 0.10), the saturation magnetisation is found to decrease. The decrease in MS is attributed to the formation of Ni vacancies at the B site as found out through X-ray fitting [32, 33]. The Ni vacancies cause a decrease in magnetic moment of B site, thereby contributing to the decrease in net magnetic moment. Further addition of Cr 3+ results in increase of MS in case of Cr15. 3.4. Mössbauer spectra analysis Fig. 5 shows the Mössbauer spectra of the samples obtained at room temperature. The Mössbauer data was fitted with two well defined sextets for minimizing the value of chi-square. The two sextets correspond to the Fe ions present at the tetrahedral and the octahedral sites in the lattice. The absence of doublet in all the spectra rules out the presence of any superparamagnetic component in the samples
within the detection limit of Mössbauer spectroscopy. Table IV shows the hyperfine interaction parameters obtained the Mössbauer data. The Mössbauer spectroscopy measures the hyperfine interactions between the nucleus and the electronic charge distribution of the surrounding atom. Isomer shift is one such hyperfine interaction which arises as a result of interaction of the iron nuclei with the surrounding s orbital electronic charge density. The A site exhibited a lower isomer shift (0.12-0.18 mm/s) than the B site (0.44-0.49 mm/s) for all the samples. This confirmed the presence of tetrahedrally coordinated Fe3+ ions at the A sub-lattice and octahedrally coordinated Fe3+ ions at the B sub-lattice [34, 35]. The quadrupole splitting for A site is negligibly small for all the three samples. The Fe3+ at the tetrahedral sites maintain their cubic symmetry and the distortion is not enough for lifting the degeneracy of the 3d 6 energy levels which contribute to the quadrupole splitting [36, 37]. For the B-site the quadrupole splitting show relatively higher magnitude than those of A-site. This is attributed to the presence of Fe3+ vacancies present at the B sub-lattice which alter the bond lengths thereby creating an asymmetry in charge distribution surrounding the iron nuclei. In case of Cr10, highest quadrupole splitting is observed. The presence of Ni2+ vacancies in addition to Fe3+ vacancies at the B site cause an increased distortion of the octahedral sub-lattice which accounts for increase in the value obtained for quadrupole splitting. The increase in distortion will generate a compressive strain in the crystal. This is corroborated by the W-H plot for Cr10 sample exhibiting the highest compressive strain among the three samples. The Fe ions ratio was calculated as : () ,) = +. (* ,*
where nA/nB denotes the ratio of Fe ions present at the A site to the Fe ions present at the B site, IA/IB denotes the ratio of the area of the A site to the B site obtained from the Mössbauer spectra and ‘f’ is the recoilless fraction. The value of recoilless fraction was taken as 0.94 at the room temperature . The
ratio of the number of Fe ions nA/nB for Cr5 was close to 1 indicating an equal distribution of Fe3+ ions over the A and B sites. For Cr10, the Fe ions ratio was close to 0.76 indicating a migration of Fe3+ ions from the A site to the B-site. The increased concentration of Fe3+ ions displaced Ni2+ from the B sublattice to the A sub-lattice. However, the resultant saturation magnetisation was found to decrease which may be likely due to the presence of Ni2+ vacancies at the B sub-lattice which do not contribute to the magnetic moment of the B sub-lattice. Further, the distortion resulting from the vacancies may alter the local symmetry of the octahedral lattice such that the spins no longer remain collinear. In such a case, the canting of the spins due to increased distortion will tend to reduce the magnetization of the B sub-lattice and hence cause the reduction of net magnetization of the lattice. However, the Fe ions ratio (nA/nB) for Cr15 is 1.39 which exceeds the maximum possible value of 1 in case of Ni1-xCrxFe2O4. The ratio of area is sensitive to stoichiometry of the compound [39, 40] .The increase in ratio indicates the presence of cationic vacancy in the B site. The Fe vacancies at the B sites are surrounded by Fe3+ ions and these trapped Fe3+ ions contribute to the intensity of Fe3+ ions at A sublattice [41-43]. This results in an increase the ratio of Fe ions at the A site to the Fe ions at the B site. 4. Conclusions In conclusion, we have synthesized Ni1-xCrxFe2O4 (x = 0.05, 0.10 and 0.15) via chemical co-precipitation technique. The XRD patterns confirmed the presence of single spinel phase belonging to the space group Fd-3m. The diffraction pattern of the samples fitted with Rietveld refinement technique using GSAS program showed the presence of cationic vacancies. Analysis of the Williamson-Hall plot indicated the presence of compressive strain in all the samples which has been ascribed to the large difference in the ionic radii of Ni2+ and Cr3+ ions. Raman spectra and the Mössbauer study of the samples clearly indicated the presence of Fe3+/Ni2+ vacancies in the octahedrally coordinated site of the ferrite lattice. The magnetic measurements at room temperature showed the samples possessed low anisotropy. The addition of Cr3+ ions to the sample create charge imbalance which result in the formation of cationic vacancies. The ferrimagnetic nature coupled with very low coercivity and sharp magnetisation gradient at room
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Figure Captions Fig. 1. Rietveld refined X-ray diffraction patterns of all the samples. Fig. 2. Williamson-Hall plot of all the samples. Fig. 3. Raman spectra of all the samples. Fig. 4. Hysteresis curve of all the samples. Fig. 5. Mössbauer spectra of all the samples recorded at 300 K.
Table Captions Table 1 Lattice Parameters and refinement values obtained from Rietveld refinement of the XRD data Sample
Table 2 Raman mode assignment of Ni1-xCrxFe2O4 (x = 0.05, 0.10, 0.15) Raman modes(cm-1)
Table 3 Magnetic parameters calculated from SQUID study Sample Id
Table 4 Mössbauer parameters of the samples recorded at room temperature Sample
Site I. S.a
I.S.: Isomer shift; bQ.S.: Quadrupole splitting; cL.W.: Line width; dHint: Internal hyperfine field