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Effect of co-substitution of Co2+ and V5+ for Fe3+ on the magnetic properties of CoFe2O4
Accepted Manuscript Effect of co-substitution of Co CoFe2O4
2+
5+ 3+ and V for Fe on the magnetic properties of
P.N. Anantharamaiah, P.A. Joy PII:
S0921-4526(18)30727-0
DOI:
https://doi.org/10.1016/j.physb.2018.11.031
Reference:
PHYSB 311166
To appear in:
Physica B: Physics of Condensed Matter
Received Date: 21 October 2018 Accepted Date: 15 November 2018
2+ 5+ Please cite this article as: P.N. Anantharamaiah, P.A. Joy, Effect of co-substitution of Co and V 3+ for Fe on the magnetic properties of CoFe2O4, Physica B: Physics of Condensed Matter (2018), doi: https://doi.org/10.1016/j.physb.2018.11.031. 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 proof before it is published in its final 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.
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Effect of co-substitution of Co2+ and V5+ for Fe3+ on the magnetic properties of CoFe2O4 P. N. Anantharamaiah1* and P. A. Joy2 Department of Chemistry, Faculty of Mathematical and Physical Sciences, M. S. Ramaiah University of Applied Sciences, Bangalore 560058, India
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Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory (CSIRNCL), Pune 411008, India
Abstract
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Different chemical compositions in Co1+2xVxFe2-3xO4 (0≤x≤0.1) have been synthesized by the conventional solid-state reaction method and processed under identical conditions. The
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materials are investigated to assess the impact of change in the oxidation states of metal ions on structural and magnetic properties of cobalt ferrite. Rietveld refinement of the X-ray diffraction patterns revealed a single phase nature of all compositions with cubic spinel structure. The cubic lattice parameter was found to decrease non-linearly from 8.393 Å for x=0 to 8.377 Å for x=0.1, due to the effect of ionic size and valency of the substituted metal ions. Sintered co-substituted compositions exhibited smaller grains against unsubstituted
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counterpart, due to low melting point of raw material V2O5. Strong experimental evidence of V substitution for Fe at the tetrahedral sites of the spinel ferrite has been extracted from the Raman spectral analysis. Saturation magnetization (Ms) was observed to decrease from 452 kA/m for x=0 to 411 kA/m for x=0.1. Coercivity (Hc) and magnetocrystalline anisotropy (K1)
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were also found to follow the same trend. Variation of the structural and magnetic parameters is attributed to the changes in the oxidation state of Co from 2+ to 3+ as it is confirmed from
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the results of XPS analysis.
Key words: Cobalt ferrite, co-substitution, oxidation state, magnetic properties E-mail: [email protected]
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1. Introduction Cobalt ferrite (CoFe2O4) is one of the technologically important and extensively studied metal oxide based functional magnetic materials due to its versatile applications in the fields of sensors, actuators, transducers, magnetic drug delivery, high density information storage, catalysis, biological, etc [1-5]. CoFe2O4 has been reported as a mixed spinel structure,
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designated by (Co1-δFeδ)A[CoδFe2-δ]BO4, where the subscripts A and B represent metal ions located at the tetrahedral (A) and octahedral (B) sites in the general spinel formula AB2O4, respectively, and ‘δ’ is the degree of cation inversion [6]. Among the family of spinel ferrites, cobalt ferrite is well-known for its higher magnetostriction properties at room temperature,
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due to the contribution of spin-orbit coupling governed by the presence of Co2+ at the B-sites [7,8]. Also, cobalt ferrite shows relatively hard magnetic behaviour depending on the grain
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size [9] and interesting electrical properties [10-12]. Structural, magnetic, magnetostrictive and electrical properties of sintered polycrystalline CoFe2O4 can effectively be tuned by the substitution of suitable metal ions for Fe and/or Co [11,13-16]. However the degree of modification in the properties, after the metal ions substitution, pertains to multiple factors such as the nature (magnetic or nonmagnetic), size, valency (di-, tri-, tetra-, penta-, hexa-) and site preferences (tetrahedral or octahedral) of the substituents. In addition, the
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microstructural parameters of the sintered specimen would also have dominant influence on the above mentioned properties [15,17].
The substitution of metal ions, other than trivalent, for Fe3+ and/or Co2+ in CoFe2O4 is
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expected to change the oxidation states of the metal ions (Co2+ and Fe3+) of the parent compound to achieve the charge neutrality that may eventually yields an interesting property
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to the system. Dwivedi et al have showed weak ferroelectric properties in the cobalt ferrite after substitution of Mo6+ in the spinel lattice [18]. Das et al have systematically probed the origin of multiferroic behaviour in the Mo6+ substituted cobalt ferrite system through the neutron diffraction technique and concluded that the presence of d0-Mo6+ at the B-sites and conversion of small fraction of Fe3+ to Fe2+ in the crystal lattice are responsible for the multiferroic properties in the system [19]. Recently, Heiba et al have reported the structural and magnetic properties of nanosized CoFe2-xVxO4 and CoFe2-1.67xVxO4 systems, prepared by a sol-gel method [20]. The authors have suggested that changes in the oxidation states of Fe and V, cation vacancies, and distribution of cations among the A- and B-sites are responsible for variation in the structural and magnetic properties. V5+ is a non-magnetic metal ion and
ACCEPTED MANUSCRIPT has a strong preference for the tetrahedral site [21], similar to the non-magnetic ions such as Ga3+, Zr4+, and In3+ [22,23]. Therefore, the substitution V5+ into the lattice of CoFe2O4 is likely to impart similar changes in the magnetic properties that are reported for Ga, In and Ge-substituted cobalt ferrites [22,23]. Naresh et al reported the temperature dependence magnetostrictive properties of sintered
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Co1+xGexFe2-2xO4 compositions, prepared by the solid-state reaction method [24]. Substitution of tetra-valent Ge for tri-valent Fe in CoFe2O4 leads to an imbalance of charge between the cations and the anions. Therefore, to keep the system electrically neutral, equivalent amounts of Co2+ and Ge4+ have been co-substituted for Fe3+ and their impact on the magnetostrictive
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properties has been studies. Higher magnitudes of magnetostriction characteristics are reported for the composition x=0.1. Studies on the co-substitution of vanadium and cobalt for
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iron in cobalt ferrite have not been reported in the literature. It is expected that incorporation of V and Co for Fe in CoFe2O4 can bring a notable impact on the structural and magnetic properties. The objective of the present study is to understand the impact induced by the cosubstitution of one V5+ and two Co2+ for three Fe3+ in CoFe2O4 system, having nominal compositions Co1+2xVxFe2-3xO4 (0 ≤ x ≤ 0.1), prepared by the solid-state reaction method and processed under identical conditions, on the structural and magnetic properties. Moreover it is
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anticipated that the changes in the oxidation states of metal ions, during the sintering process, will have considerable impact on the structural and magnetic properties of sintered cobalt ferrite. The concept of co-substitution has been introduced to safeguard the electrical charge neutrally between the cations and the anions. The studies revealed that co-substitution of one
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V5+ and two Co2+ for three Fe3+ in CoFe2O4 led to notable changes in the structural and magnetic properties. XPS analysis revealed the presence of Co3+ in the co-substituted
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compositions. The changes in the structural and magnetic parameters are attributed to the changes in the oxidation state of Co from 2+ to 3+ as well as to the effect of cation distribution.
2. Experimental
Various compositions in Co1+2xVxFe2-3xO4, with x ranging from 0 to 0.1, were synthesized by the solid-state reaction route [25]. Stoichiometric amounts of the metal oxides (Fe2O3, Co3O4,
and V2O5 with purity >99%, procured from Aldrich chemicals) were weighed accurately
into a mortar and pestle and milled for 2 hrs using acetone as a mixing medium. The resultant mixtures were transferred to alumina crucibles and heat treated at 850 oC for 10 hours
ACCEPTED MANUSCRIPT followed by milling and re-calcination at two different temperatures (1000 and 1100 oC) with same dwell time at each step with intermediate grindings. Subsequently, the powders were lubricated with a minimum amount of 2% PVA solution and pressed into disc form by applying a compaction pressure of 8 MPa and sintered at 1350 oC for 2 hours with heating and cooling rates of 5 oC per minute.
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The phase purity and structural information of all the sintered compositions were ascertained by X-ray diffraction studies made on a PANalytical X’pert PRO power X-ray diffractometer using Cu Kα source. The microstructures of the fractured inner regions of the sintered compositions were investigated using a scanning electron microscope (FEI, ESEM
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Quanta 200-3D). Raman spectra were recorded on the smooth surface of the sintered pellets using micro-Raman spectrometer (Horiba JY Labram HR 800) coupled with 632.8 nm He-Ne
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laser source, at a power of 7 mW on the specimen surface. X-ray photoelectron spectroscopy (XPS) studies have been carried out using a model VG Microtech multilab ESCA 3000 spectrometer assembled with Mg Kα source. Room temperature magnetization of the sintered samples was measured using a vibrating sample magnetometer (VSM model EG&G PAR 4500).
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3. Results and discussion 3.1 X-ray diffraction studies
X-ray diffraction patterns of all the sintered compositions are shown in Fig. S1
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(supplementary information). Rietveld refinement studies confirmed that all the sintered compositions are in the form of single phase with cubic crystal structure, without any
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traceable secondary phases, signifying the substituents (V and Co) are effectively incorporated into the spinel lattice. The result of the Rietveld refined XRD pattern of unsubstituted cobalt ferrite is shown in Fig. 1(a). In the case of the co-substituted compositions, with increasing the amounts of x, the peaks are shifted progressively towards higher diffraction angles, as shown in Fig. 1(b) for the fitted (311) peaks of all the studied compositions. The lattice parameter, deduced from the refinement studies, was found to be decreased non-linearly from 8.393 Å for x=0 to 8.377 Å for x=0.1 (Fig. 1(c)). An increase in the lattice parameter with increasing co-substitution content ‘x’ would be anticipated based on the general chemical composition wherein three Fe3+ ions (0.49 × 3 = 1.47 Å for 4-fold coordination and 0.645 × 3 =1.935 Å for 6-fold coordination) are being replaced by 2 Co2+
ACCEPTED MANUSCRIPT ions and one V5+ ion with the average ionic sizes of 1.515 Å for 4-fold coordination (0.355 Å for V5+ and 0.58 × 2 Å for Co2+) and 2.03 Å for 6-fold coordination (0.54 Å for V5+ and 0.754 × 2 Å for Co2+) [26]. However, the exponential decrease in the lattice parameter with ‘x’ is most likely to be due to the conversion of Co2+ to Co3+ [27]. In the present study, all the compositions are sintered in air where the concentration of oxygen would relatively be lower
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and therefore a small fraction of vanadium may be reduced to V4+/V3+ from V5+ by oxidizing equivalent amounts of Co2+ to Co3+ in the spinel lattice. This possible explanation has been presented based on the reported VFe2Ox system where the system was heat treated under low partial pressure of O2 that resulted a fraction of V5+ reduced to V4+ [28]. The generated Co3+ would be in low-spin electronic configuration with strong preference for the octahedral
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coordination environment [27]. Furthermore, the ionic radii of Co3+ and V4+ for 6-fold coordinations are 0.533 and 0.58 Å, respectively, and these sizes are relatively lower than the
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size of Fe3+ at the same coordination site.
Fig. 1
3.2 Raman spectral studies The Raman spectra of all the sintered compositions, recorded at ambient conditions, are presented in Fig. 2. Seven distinguishable characteristics modes are observed for the unsubstituted composition, higher than the number (A1g, Eg and 3T2g) predicted by factor
ACCEPTED MANUSCRIPT group analysis for the spinel based compounds, indicating that the prepared composition is in the form of inverse or mixed spinel structure [27]. The random distribution of di- and trivalent metals among the A- and B-sites is responsible for the extra modes in the inverse and mixed spinels [27]. In the present study, the assignment of the observed modes has been made based on the reports available in the literature [27]. For the unsubstituted CoFe2O4
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(x=0), the bands situated at ~690 cm-1 due to A1g(1) and ~613 cm-1 due to A1g(2) are attributed to symmetric breathing modes of FeO4 and CoO4 units, respectively. The T2g(3) mode at ~560 cm-1 corresponds to asymmetric bending motion of oxygen coordinated with Co2+ at the tetrahedral site whereas Eg modes (~300 and ~360 cm-1) are pertaining to the symmetric bending motion of the oxygen within the AO4 units (where A = di- and tri-valent
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metal ions). T2g(2) mode at ~470 cm-1 is corresponds to the motion of oxygen atoms coordinated to Fe3+ at the B-site (FeO6) [27] and low intense T2g(1) mode (~ 207 cm-1) arises
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due to translational motion of the BO6 units against the cations of A-sites.
In the case of co-substituted compositions, in addition to the 7 modes, a well pronounced signal at ~750 cm-1 is emerged. The band emanated at wavenumber ~750 cm-1 is assigned to symmetric breathing mode of VO4 tetrahedral unit and it is labeled as A1g(3), as shown in Fig.2. Since Raman spectral studies on the V-substituted sintered cobalt ferrites have not
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been documented in the literature, the assignment of ~750 cm-1 band to VO4 unit has been made based on the ionic sizes and bond lengths, wherein the ionic size of V5+ and V-O bond length are relatively smaller than the ionic sizes of Fe3+ and Co2+ and their bond lengths with oxygen. Shorter bond length will have higher stretching frequency, as suggested [22]. The
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intensity of the A1g(3) band at 750 cm-1 is increased substantially with increasing the amount of co-substitution suggesting that more of V ions are incorporated into the A-sites. Also, the
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intensity of the A1g(1) band due to FeO4 decreases whereas A1g(2) due to CoO4 increases with x, indicating re-distribution of Co2+ and Fe3+ within the A-sites. Unlike for the compositions x=0.025 and x=0.05, extra signals are observed for x=0.1 and are situated at ~198 cm-1 and ~330 cm-1. These signals are likely to be due to the cation redistribution and variable oxidation states of V and Co in the spinel lattice, but not the Raman spectral features of V2O5 [29]. The signal at ~330 cm-1 is most probably due to asymmetric bending motion of oxygen ions coordinated to V located at the tetrahedral site, similar to that of the Eg modes.
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Fig. 2
Fig. 3
To gain further information on the cations distribution, all the spectra were normalized with respect to the most intense T2g(2) band and deconvoluted. The deconvoluted spectra of
ACCEPTED MANUSCRIPT all the compositions are presented in Fig. 3. The cations distribution of the unsubstituted cobalt ferrite can be estimated from the area under the peaks of A1g [27]. In the present study, for the unsubstituted cobalt ferrite, the ratio of the area under the peaks due to A1g(2) (CoO4) and A1g(1) (FeO4) is found as 0.29 from which the cation distribution can be assigned to (Co0.29Fe0.71)A[Co0.71Fe1.29]BO4. This distribution is nearly comparable to the cations
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distribution reported for the sintered cobalt ferrite systems [27]. However, it is difficult to estimate the cations distribution for the co-substituted compositions because the cosubstitution induces variable oxidation states in Co and V and the resultant ions would occupy different crystallographic sites leading to complex cations distribution. The variation
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of the relative area of the A1g peaks is shown in Fig. 4. As can be seen from the Fig. 4, the relative area due to FeO4 (A1g(1)) decreases, and the relative areas due to CoO4 (A1g(2)) and VO4 (A1g(3)) increase with increasing the values of x. However, the rate of decrease in the
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area of the A1g(1) (FeO4) is more pronounced compared to the rate of increase for both A1g(2) (CoO4) and A1g(3) (VO4) units. This apparently suggests that more and more Fe3+ ions are
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being replaced by Co2+ and V5+ at the tetrahedral sites.
Fig. 4
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3.3 Microstructure
Fig. 5
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It is obvious from the SEM photographs, presented in Fig. 5, that the microstructure of the unsubstituted cobalt ferrite would be tuned to a greater extent after incorporation of vanadium into the spinel crystal lattice. Despite the fact that all the compositions are prepared and processed under identical conditions, the co-substituted compositions showed relatively
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smaller and nearly uniform grains without intra-granular pores compared to that of the parent compound which comprises larger grains with inter- and intra-granular pores. The appearance
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of larger sized grains with intragranular pores in the case of unsubstituted cobalt ferrite is attributed to abnormal fusion of two or more smaller grains that eventually causes trapping of pores inside the grains, as suggested [30]. The presence of smaller grains in the case of cosubstituted compositions is likely to be due to the contribution of V2O5 raw material as it acts as a sintering aid at lower temperatures. However, at higher temperature V ions are incorporated comprehensively into the spinel lattice structure as there was no impurity phase in the XRD patterns of the sintered compositions. The grain size is increased with increasing the amount of co-substitution and the average grain sizes are obtained as 2, 4 and 6 µm, respectively, for the compositions x=0.025, 0.05 and 0.1 against 12 µm for x=0.
ACCEPTED MANUSCRIPT 3.4 Magnetic properties Fig. 6. shows the field dependence of magnetization (M vs H) curves of the sintered Co1+2xVxFe2-3xO4 compositions, measured at room temperature upto a maximum magnetic field strength 1200 kA/m (1.5 T). With increase in the content of V and Co, considerable changes in the nature of the M-H curves at lower as well as at higher fields have been
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noticed. As can be seen from the inset of Fig. 6, all the studied compositions display hysteresis loops, due to presence of a long-range ferrimagnetic order. Saturation magnetization (Ms), obtained by extrapolating M versus 1/H curve to 1/H=0, is found to be decreased with increasing the co-substitution content x (Fig. 7a), due to magnetic dilution
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induced in the system. The magnitude of the Ms in the metal ions substituted spinel ferrites depends on whether the substituent is located at A-site or at the B-site. Reported studies on
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the magnetic properties of the CoMxFe2-xO4 (M=Al, Ga, Zn, Mg) systems [13,14,22] suggest that the decreased in Ms for the Al- and Mg-substituted compositions is due to the magnetic dilution of the B-site by the presence of Mg and Al, whereas increased in Ms up to a particular composition in the case of Ga- and Zn-substituted compositions is primarily attributed to the A-site magnetic dilution by the presence of Ga and Zn.
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In the present study, Raman spectral analysis showed that with increasing the amount of co-substitution, more of V and Co are substituted for Fe at the A-sites and therefore an increase in Ms with increased x would be predicted due to the presence of non-magnetic vanadium at the A-site. The net magnetic moment for the unsubstituted cobalt ferrite is
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calculated as 4.16 µB based on the cations distribution estimated from the Raman spectral analysis. The net magnetic moments for the compositions with x=0.025, 0.05 and 0.1 can be estimated as 4.38, 4.61 and 5.06 µB, respectively, assuming that both Co and V are
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substituted for Fe at the A-sites. However, the observed decrease in the Ms with increasing x could be due to multiple factors including the changes in the oxidation states of the cations and their random distributions among the A- and B-sites. Some fraction of the Co2+ will be converted to Co3+ to attain the charge neutrality and the resultant Co3+ will be located at the B-sites. The Co3+ will be in low-spin electronic configuration with zero magnetic moment [13] and hence more magnetic dilution of B-site.
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Fig. 6
Fig. 7
First order magnetocrystalline anisotropy coefficient (K1) of all the compositions is calculated from the initial magnetization data using the method of law of approach to saturation [31]. The initial magnetization curves, particularly at higher field regions (H>500 kA/m), of the compositions including cobalt ferrite (x=0) are fitted to the relation, M = M 1 −
b
H
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; Ms is the saturation magnetization in kA/m; K1 is the first order
magnetocrystalline anisotropy coefficient; and µo is the permeability of free space (1.257 × 10-6 mkg/s2.A2). The constant 8/105 is for first order cubic anisotropy of randomly oriented polycrystalline materials. In the above equation, ‘Ms’ and ‘b’ are only the fitting parameters. Thus by knowing the values of Ms and b, extracted after fitting, K1 can be derived. A
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relatively good fit has been obtained for all the compositions and the results of the fitted curves are shown in Fig. S2 (supplementary information). For the unsubstituted cobalt ferrite (x=0), K1 is obtained as 3.46 × 105 J/m3 and is comparable to the values reported for the sintered polycrystalline cobalt ferrites [27,30]. With increasing the amounts of V and Co for
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Fe in the spinel lattice, K1 is decreased monotonically to 1.55 × 105 J/m3 for x=0.1. The variation of K1 as function of x is shown in Fig. 7(b). In cobalt ferrite, the magnitude of K1 is
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proportional to the concentration of Co2+ at the B-sites [23]. Therefore, any changes in the amount of Co2+ at the B-site as well as its oxidation state would directly influence the magnitude of K1. In addition, the reduction in the A-O-B super-exchange strength would also responsible for decline in the values of K1, as suggested for the various metal ions substituted polycrystalline cobalt ferrite systems [14,32,33]. In the present case, even though more and more Co, together with V, is substituted for Fe, the substantial drop in the values of K1 with
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‘x’ is most probably due to the reduction of the A-O-B super-exchange interactions. Furthermore, the presence of non-magnetic Co3+ at the B-site is likely to be accountable for the same [13,27].
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As illustrated in the Fig. 7(b), coercivity (Hc) of the composition with x=0.025 is nearly comparable to that of x=0 (~17.5 kA/m) and drops at a faster rate for x>0.025. The large drop
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in the Hc for x>0.025 is primarily attributed to a strong reduction in the A-O-B superexchange strength triggered by the presence of non-magnetic V at the A-sites and also due to the reduction in the magnetocrystalline anisotropy [13,14]. Interestingly, the variation of Hc with x follows approximately the same trend as that observed for K1. This is because Hc is proportional to the K1 through the Brown relation, Hc=2K1/µoMs [34]. Furthermore, in the MH curves, higher magnetization has been observed for the co-substituted compositions at low magnetic fields (< 400 kA/m) compared to that of the parent compound. This apparently supports the claim that reduction in the A-O-B superexchange strength is the dominant factor responsible for a large drop in the Hc for x>0.025. As documented in the literature, microstructure of the sintered body would also alter the magnitude of the Hc [17,30].
ACCEPTED MANUSCRIPT 3.5 XPS analysis XPS is an effective tool to assess the valencies of the metal ions present in chemical compounds. In the present study, XPS spectral analyses have been carried out to investigate the change in the oxidation states of Fe, Co and V, and the corresponding XPS spectra (Fe2p, Co2p and V2p) of the selected sintered compositions are shown in Fig. 8. In the XPS spectra
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of Co, the peaks at ~780.6 eV and ~786.5 eV are attributed to 2p3/2 and 2p1/2, respectively, and both the peaks are accompanied by weak satellite signals (Fig. 8(a)). The binding energies of the Co2p3/2 and Co2p1/2 peaks of cobalt ferrite are in agreement with the values reported in the literature [27]. The intensity of the 2p3/2 satellite peak of the unsubstituted
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cobalt ferrite is relatively higher than that of the co-substituted compositions indicating the presence of higher concentration of Co2+ in the spinel ferrite. However, with increasing the
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amount of co-substitution, the peaks are shifted slightly towards lower binding energies and moreover the intensity of the 2p3/2 satellite peak decreases substantially, as shown in Fig. 8(b) that compares the binding energies and XPS peaks intensities of the compositions x=0 and x=0.1. The decrease in the intensity of the 2p3/2 satellite peak is primarily due to the conversion of Co2+ into Co3+, as reported [27, 35].
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No significant changes in the spectral features are observed in the case of Fe and the peaks at ~710.7 eV and ~724.5 eV correspond to 2p3/2 and 2p1/2, respectively, (Fig. 8(c)). The binding energies of the Fe2p3/2 and Fe2p1/2 peaks of the parent compound are in agreement with the values reported in the literature [27]. As shown in Fig. 8(d), a considerable variation
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in the spectral features has been observed in the XPS spectra of V2p. The peaks centered at 516.8 eV and 522.9 eV are attributed to V2p3/2 and Vp1/2, respectively, and the values are in concordance with the reported values for V5+/V4+ [28]. The peaks in the XPS spectrum of the
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composition x=0.1 are shifted slightly towards lower binding energies with respect to that of the peaks in the composition x=0.025, indicating decreasing the oxidation state of V in the higher co-substituted composition. Overall, the XPS spectral analysis revealed that with increasing the amount of co-substitution in the spinel ferrite lattice, oxidation state of Co is increased from +2 to +3 whereas that of V is reduced from +5 to lower oxidation states. These finding are strongly supporting the XRD and magnetic interpretations.
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Fig. 8
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4. Conclusions
The results presented in this paper indicated that co-substitution of Co and V for Fe in cobalt ferrite (Co1+2xVxFe2-3xO4 (0≤x≤0.1)) has considerable impact on the structural,
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microstructural and magnetic properties. The changes in the oxidation states of Co from +2 to +3 and V from +5 to lower states are confirmed from the XPS spectral analysis. Lattice parameter is found to decrease with increasing the amounts of co-substitution. The decrease
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is attributed to changes in the oxidation of states of Co and V in the crystal lattice and also the cation distributions. Raman spectral analysis indicated that V is substituted for Fe at the tetrahedral sites of the spinel ferrite. Better control over the microstructural parameters has been achieved for the sintered co-substituted compositions over unsubstituted counterpart. The magnetic parameters (Ms, Hc and K1) are also found to decrease non-linearly with increasing the amount of co-substitution, due to magnetic dilution and reduction in the A-O-B super-exchange interactions.
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Acknowledgement P. N. Anantharamaiah is grateful to the CSIR-NCL for the infrastructure and the facilities provided to complete this research work. Also, PNA is grateful to Sharath Devdas for his
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assistance during the samples preparation.
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structural, dielectric and impedance analysis of manganese substituted cobalt ferrite, Co1-xMnxFe2O4 (0.0≤x≤0.4), RSC Adv. 6 (2016) 20876-20885. 16. C. V. Ramana, Y. D. Kolekar, K. Kamala Bharathi, B. Sinha, K. Ghosh, Correlation between structural, magnetic, and dielectric properties of manganese substituted cobalt ferrite, J. Appl. Phys. 114 (2013) 183907.
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17. K. Khaja Mohaideen, P.A. Joy, Influence of initial particle size on the magnetostriction of sintered cobalt ferrite derived from nanocrystalline powders, J. Magn. Magn. Mater., 346 (2013) 96-102. 18. G.D. Dwivedi, K.F. Tseng, C.L. Chan, P. Shahi, J. Lourembam, B. Chatterjee, A.K.
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Ghosh, H.D. Yang, S. Chatterjee, Signature of ferroelectricity in magnetically ordered Mo-doped, Phys. Rev. B, CoFe2O4, 82 (2010) 134428.
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19. A. Das, G.D. Dwivedi, P. Kumari, P. Shahi, H.D. Yang, A.K. Ghosh, S. Chatterjee, Neutron diffraction study of multiferroic Mo-doped CoFe2O4, J. Magn. Magn. Mater. 379 (2015) 134428.
20. Z. K. Heiba, M. B. Mohamed, S.I. Ahmed, Cation distribution correlated with magnetic properties of cobalt ferrite nanoparticles defective by vanadium doping, J. Magn. Magn. Mater. 441 (2017) 409-416. 21. Z.K. Heiba, M.B. Mohamed, A.M. Wahba M.I. Almalowi, Effect of vanadium doping on structural and magnetic properties of defective nano-nickel ferrite, Appl. Phys. A, 124 (2018) 290-298. 22. P.N. Anantharamaiah, P.A. Joy, Effect of size and site preference of trivalent non-magnetic metal ions (Al3+, Ga3+, In3+) substituted for Fe3+ on the
ACCEPTED MANUSCRIPT magnetostrictive properties of sintered CoFe2O4, J. Phys. D: Appl. Phys. 50 (2017) 435005. 23. V.R. Monaji, D. Das, Influence of Zr doping on the structural, magnetic and magnetoelastic properties of cobalt-ferrites, J. Alloys Compd. 634 (2015) 99-103. 24. N. Ranvah, I.C. Nlebedim, Y. Melikhov, J.E. Snyder, D.C. Jiles, A.J. Moses, P.I.
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Williams, F. Anayi, S.H. Song, Temperature Dependence of magnetostriction of Co1+xGexFe2-2xO4 for magnetostrictive sensor and actuator applications, IEEE Trans Magn. 44 (2018) 3113-3016.
25. S.D. Bhame, P.A. Joy, Enhanced magnetostrictive properties of CoFe2O4
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synthesized by an autocombustion method, Sensors and Actuators A, 137 (2007) 256– 261.
26. R.D. Shannon, Revised effective ionic radii and systematic studies of interatomic
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distances in halides and chalcogenides, Acta Cryst. A 32 (1976) 751-767. 27. P.N. Anantharamaiah, P.A. Joy, Enhancing the strain sensitivity of CoFe2O4 at low magnetic fields without affecting the magnetostriction coefficient by substitution of small amounts of Mg for Fe, Phys. Chem. Chem. Phys., 18 (2016) 10516-10527.
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28. C.N. Chervin, J.S. Ko, B.W. Miller, L.D. ek, A.N. Mansour, M.D. Donakowski, T. Brintlinger, P. Gogotsi, S. Chattopadhyay, T. Shibata, J.F. Parker, B.P. Hahn, D.R. Rolison, J.W. Long, Defective by design: vanadium-substituted iron oxide nanoarchitectures as cation-insertion hosts for electrochemical charge storage, J.
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Mater. Chem A, 3 (2015) 12059-12068. 29. Y. Hu, Z. Li, Z. Zhang, D. Meng, effect of magnetic field on the visible light emission
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of V2O5 nanorods, Appl. Phys. Lett. 94 (2009) 103107. 30. M. Atif, R. Sato Turtelli, R. Grossinger, F. Kubel, Influence of manganese substitution on the microstructure and magnetostrictive properties of Co1-xMnxFe2O4 (x=0.0–0.4) ferrite, J. Appl. Phys. 113 (2013) 153902.
31. S. Chikazumi, Physics of ferromagnetism, 2nd edn. Oxford University Press, Oxford, 1997. 32. N. Ranvah, I.C. Nlebedim, Y. Melikhov, J.E. Snyder, P.I. Williams, A.J. Moses, D.C. Jiles, Temperature dependence of magnetic properties of CoAlxFe2-xO4. IEEE Trans. Magn. 45 (2009) 4261-4264. 33. P.N. Anantharamaiah, P.A. Joy, High magnetostriction parameters of sintered and magnetic field annealed Ga-substituted CoFe2O4, Mater. Lett. 192 (2017) 169-172.
ACCEPTED MANUSCRIPT 34. B.D. Cullity, Introduction to Magnetic Materials, Addison-Wisley, USA, 1972. 35. T. Mathew, N. R. Shiju, K. Sreekumar, B. S. Rao, C. S. Gopinath, C-Co synergism in
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Cu1-xCoxFe2O4 catalysis and XPS aspects, J. Catal. 210 (2002) 405-417.
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Captions for Figures:
Fig. 1. (a) Rietveld fit XRD pattern of the sintered cobalt ferrite. (b) enlarged patterns showing the
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(311) peak along with Kα1 and Kα2 components for all the compositions, and (c) variation of lattice parameter as a function of ‘x’ in Co1+2xVxFe2-3xO4.
Fig. 2. Raman spectra of different compositions in Co1+2xVxFe2-3xO4 compositions, recorded on sintered samples at ambient temperature.
Fig. 3. Deconvoluted Raman spectra of sintered Co1+2xVxFe2-3xO4 compositions.
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Fig. 4. Variation of the relative area under the peaks due to the A1g modes of CoO4, FeO4 and VO4 as a function of x in Co1+2xVxFe2-3xO4.
Fig. 5. SEM images of sintered Co1+2xVxFe2-3xO4 compositions. The scale is common for all images.
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Fig. 6. Field dependant magnetization hysteresis loops of the sintered Co1+2xVxFe2-3xO4 compositions.
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Inset shows an enlarged view of the hysteresis loops at low field regions. Fig. 7. Variation of (a) saturation magnetization, Ms, and (b) magnetocrystalline anisotropy coefficient, K1, and coercivity, Hc, as a function of ‘x’ in Co1+2xVxFe2-3xO4. Fig. 8. XPS spectra of Co1+2xVxFe2-3xO4 for (a) Co-2p, (b) normalized Co-2p spectra of x=0 and 0.1, (c) Fe-2p, and (d) V-2p.
2+
5+ 3+ and V for Fe on the magnetic properties of
P.N. Anantharamaiah, P.A. Joy PII:
S0921-4526(18)30727-0
DOI:
https://doi.org/10.1016/j.physb.2018.11.031
Reference:
PHYSB 311166
To appear in:
Physica B: Physics of Condensed Matter
Received Date: 21 October 2018 Accepted Date: 15 November 2018
2+ 5+ Please cite this article as: P.N. Anantharamaiah, P.A. Joy, Effect of co-substitution of Co and V 3+ for Fe on the magnetic properties of CoFe2O4, Physica B: Physics of Condensed Matter (2018), doi: https://doi.org/10.1016/j.physb.2018.11.031. 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 proof before it is published in its final 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.
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Effect of co-substitution of Co2+ and V5+ for Fe3+ on the magnetic properties of CoFe2O4 P. N. Anantharamaiah1* and P. A. Joy2 Department of Chemistry, Faculty of Mathematical and Physical Sciences, M. S. Ramaiah University of Applied Sciences, Bangalore 560058, India
2
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1
Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory (CSIRNCL), Pune 411008, India
Abstract
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Different chemical compositions in Co1+2xVxFe2-3xO4 (0≤x≤0.1) have been synthesized by the conventional solid-state reaction method and processed under identical conditions. The
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materials are investigated to assess the impact of change in the oxidation states of metal ions on structural and magnetic properties of cobalt ferrite. Rietveld refinement of the X-ray diffraction patterns revealed a single phase nature of all compositions with cubic spinel structure. The cubic lattice parameter was found to decrease non-linearly from 8.393 Å for x=0 to 8.377 Å for x=0.1, due to the effect of ionic size and valency of the substituted metal ions. Sintered co-substituted compositions exhibited smaller grains against unsubstituted
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counterpart, due to low melting point of raw material V2O5. Strong experimental evidence of V substitution for Fe at the tetrahedral sites of the spinel ferrite has been extracted from the Raman spectral analysis. Saturation magnetization (Ms) was observed to decrease from 452 kA/m for x=0 to 411 kA/m for x=0.1. Coercivity (Hc) and magnetocrystalline anisotropy (K1)
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were also found to follow the same trend. Variation of the structural and magnetic parameters is attributed to the changes in the oxidation state of Co from 2+ to 3+ as it is confirmed from
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the results of XPS analysis.
Key words: Cobalt ferrite, co-substitution, oxidation state, magnetic properties E-mail: [email protected]
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1. Introduction Cobalt ferrite (CoFe2O4) is one of the technologically important and extensively studied metal oxide based functional magnetic materials due to its versatile applications in the fields of sensors, actuators, transducers, magnetic drug delivery, high density information storage, catalysis, biological, etc [1-5]. CoFe2O4 has been reported as a mixed spinel structure,
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designated by (Co1-δFeδ)A[CoδFe2-δ]BO4, where the subscripts A and B represent metal ions located at the tetrahedral (A) and octahedral (B) sites in the general spinel formula AB2O4, respectively, and ‘δ’ is the degree of cation inversion [6]. Among the family of spinel ferrites, cobalt ferrite is well-known for its higher magnetostriction properties at room temperature,
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due to the contribution of spin-orbit coupling governed by the presence of Co2+ at the B-sites [7,8]. Also, cobalt ferrite shows relatively hard magnetic behaviour depending on the grain
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size [9] and interesting electrical properties [10-12]. Structural, magnetic, magnetostrictive and electrical properties of sintered polycrystalline CoFe2O4 can effectively be tuned by the substitution of suitable metal ions for Fe and/or Co [11,13-16]. However the degree of modification in the properties, after the metal ions substitution, pertains to multiple factors such as the nature (magnetic or nonmagnetic), size, valency (di-, tri-, tetra-, penta-, hexa-) and site preferences (tetrahedral or octahedral) of the substituents. In addition, the
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microstructural parameters of the sintered specimen would also have dominant influence on the above mentioned properties [15,17].
The substitution of metal ions, other than trivalent, for Fe3+ and/or Co2+ in CoFe2O4 is
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expected to change the oxidation states of the metal ions (Co2+ and Fe3+) of the parent compound to achieve the charge neutrality that may eventually yields an interesting property
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to the system. Dwivedi et al have showed weak ferroelectric properties in the cobalt ferrite after substitution of Mo6+ in the spinel lattice [18]. Das et al have systematically probed the origin of multiferroic behaviour in the Mo6+ substituted cobalt ferrite system through the neutron diffraction technique and concluded that the presence of d0-Mo6+ at the B-sites and conversion of small fraction of Fe3+ to Fe2+ in the crystal lattice are responsible for the multiferroic properties in the system [19]. Recently, Heiba et al have reported the structural and magnetic properties of nanosized CoFe2-xVxO4 and CoFe2-1.67xVxO4 systems, prepared by a sol-gel method [20]. The authors have suggested that changes in the oxidation states of Fe and V, cation vacancies, and distribution of cations among the A- and B-sites are responsible for variation in the structural and magnetic properties. V5+ is a non-magnetic metal ion and
ACCEPTED MANUSCRIPT has a strong preference for the tetrahedral site [21], similar to the non-magnetic ions such as Ga3+, Zr4+, and In3+ [22,23]. Therefore, the substitution V5+ into the lattice of CoFe2O4 is likely to impart similar changes in the magnetic properties that are reported for Ga, In and Ge-substituted cobalt ferrites [22,23]. Naresh et al reported the temperature dependence magnetostrictive properties of sintered
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Co1+xGexFe2-2xO4 compositions, prepared by the solid-state reaction method [24]. Substitution of tetra-valent Ge for tri-valent Fe in CoFe2O4 leads to an imbalance of charge between the cations and the anions. Therefore, to keep the system electrically neutral, equivalent amounts of Co2+ and Ge4+ have been co-substituted for Fe3+ and their impact on the magnetostrictive
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properties has been studies. Higher magnitudes of magnetostriction characteristics are reported for the composition x=0.1. Studies on the co-substitution of vanadium and cobalt for
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iron in cobalt ferrite have not been reported in the literature. It is expected that incorporation of V and Co for Fe in CoFe2O4 can bring a notable impact on the structural and magnetic properties. The objective of the present study is to understand the impact induced by the cosubstitution of one V5+ and two Co2+ for three Fe3+ in CoFe2O4 system, having nominal compositions Co1+2xVxFe2-3xO4 (0 ≤ x ≤ 0.1), prepared by the solid-state reaction method and processed under identical conditions, on the structural and magnetic properties. Moreover it is
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anticipated that the changes in the oxidation states of metal ions, during the sintering process, will have considerable impact on the structural and magnetic properties of sintered cobalt ferrite. The concept of co-substitution has been introduced to safeguard the electrical charge neutrally between the cations and the anions. The studies revealed that co-substitution of one
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V5+ and two Co2+ for three Fe3+ in CoFe2O4 led to notable changes in the structural and magnetic properties. XPS analysis revealed the presence of Co3+ in the co-substituted
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compositions. The changes in the structural and magnetic parameters are attributed to the changes in the oxidation state of Co from 2+ to 3+ as well as to the effect of cation distribution.
2. Experimental
Various compositions in Co1+2xVxFe2-3xO4, with x ranging from 0 to 0.1, were synthesized by the solid-state reaction route [25]. Stoichiometric amounts of the metal oxides (Fe2O3, Co3O4,
and V2O5 with purity >99%, procured from Aldrich chemicals) were weighed accurately
into a mortar and pestle and milled for 2 hrs using acetone as a mixing medium. The resultant mixtures were transferred to alumina crucibles and heat treated at 850 oC for 10 hours
ACCEPTED MANUSCRIPT followed by milling and re-calcination at two different temperatures (1000 and 1100 oC) with same dwell time at each step with intermediate grindings. Subsequently, the powders were lubricated with a minimum amount of 2% PVA solution and pressed into disc form by applying a compaction pressure of 8 MPa and sintered at 1350 oC for 2 hours with heating and cooling rates of 5 oC per minute.
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The phase purity and structural information of all the sintered compositions were ascertained by X-ray diffraction studies made on a PANalytical X’pert PRO power X-ray diffractometer using Cu Kα source. The microstructures of the fractured inner regions of the sintered compositions were investigated using a scanning electron microscope (FEI, ESEM
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Quanta 200-3D). Raman spectra were recorded on the smooth surface of the sintered pellets using micro-Raman spectrometer (Horiba JY Labram HR 800) coupled with 632.8 nm He-Ne
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laser source, at a power of 7 mW on the specimen surface. X-ray photoelectron spectroscopy (XPS) studies have been carried out using a model VG Microtech multilab ESCA 3000 spectrometer assembled with Mg Kα source. Room temperature magnetization of the sintered samples was measured using a vibrating sample magnetometer (VSM model EG&G PAR 4500).
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3. Results and discussion 3.1 X-ray diffraction studies
X-ray diffraction patterns of all the sintered compositions are shown in Fig. S1
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(supplementary information). Rietveld refinement studies confirmed that all the sintered compositions are in the form of single phase with cubic crystal structure, without any
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traceable secondary phases, signifying the substituents (V and Co) are effectively incorporated into the spinel lattice. The result of the Rietveld refined XRD pattern of unsubstituted cobalt ferrite is shown in Fig. 1(a). In the case of the co-substituted compositions, with increasing the amounts of x, the peaks are shifted progressively towards higher diffraction angles, as shown in Fig. 1(b) for the fitted (311) peaks of all the studied compositions. The lattice parameter, deduced from the refinement studies, was found to be decreased non-linearly from 8.393 Å for x=0 to 8.377 Å for x=0.1 (Fig. 1(c)). An increase in the lattice parameter with increasing co-substitution content ‘x’ would be anticipated based on the general chemical composition wherein three Fe3+ ions (0.49 × 3 = 1.47 Å for 4-fold coordination and 0.645 × 3 =1.935 Å for 6-fold coordination) are being replaced by 2 Co2+
ACCEPTED MANUSCRIPT ions and one V5+ ion with the average ionic sizes of 1.515 Å for 4-fold coordination (0.355 Å for V5+ and 0.58 × 2 Å for Co2+) and 2.03 Å for 6-fold coordination (0.54 Å for V5+ and 0.754 × 2 Å for Co2+) [26]. However, the exponential decrease in the lattice parameter with ‘x’ is most likely to be due to the conversion of Co2+ to Co3+ [27]. In the present study, all the compositions are sintered in air where the concentration of oxygen would relatively be lower
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and therefore a small fraction of vanadium may be reduced to V4+/V3+ from V5+ by oxidizing equivalent amounts of Co2+ to Co3+ in the spinel lattice. This possible explanation has been presented based on the reported VFe2Ox system where the system was heat treated under low partial pressure of O2 that resulted a fraction of V5+ reduced to V4+ [28]. The generated Co3+ would be in low-spin electronic configuration with strong preference for the octahedral
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coordination environment [27]. Furthermore, the ionic radii of Co3+ and V4+ for 6-fold coordinations are 0.533 and 0.58 Å, respectively, and these sizes are relatively lower than the
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size of Fe3+ at the same coordination site.
Fig. 1
3.2 Raman spectral studies The Raman spectra of all the sintered compositions, recorded at ambient conditions, are presented in Fig. 2. Seven distinguishable characteristics modes are observed for the unsubstituted composition, higher than the number (A1g, Eg and 3T2g) predicted by factor
ACCEPTED MANUSCRIPT group analysis for the spinel based compounds, indicating that the prepared composition is in the form of inverse or mixed spinel structure [27]. The random distribution of di- and trivalent metals among the A- and B-sites is responsible for the extra modes in the inverse and mixed spinels [27]. In the present study, the assignment of the observed modes has been made based on the reports available in the literature [27]. For the unsubstituted CoFe2O4
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(x=0), the bands situated at ~690 cm-1 due to A1g(1) and ~613 cm-1 due to A1g(2) are attributed to symmetric breathing modes of FeO4 and CoO4 units, respectively. The T2g(3) mode at ~560 cm-1 corresponds to asymmetric bending motion of oxygen coordinated with Co2+ at the tetrahedral site whereas Eg modes (~300 and ~360 cm-1) are pertaining to the symmetric bending motion of the oxygen within the AO4 units (where A = di- and tri-valent
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metal ions). T2g(2) mode at ~470 cm-1 is corresponds to the motion of oxygen atoms coordinated to Fe3+ at the B-site (FeO6) [27] and low intense T2g(1) mode (~ 207 cm-1) arises
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due to translational motion of the BO6 units against the cations of A-sites.
In the case of co-substituted compositions, in addition to the 7 modes, a well pronounced signal at ~750 cm-1 is emerged. The band emanated at wavenumber ~750 cm-1 is assigned to symmetric breathing mode of VO4 tetrahedral unit and it is labeled as A1g(3), as shown in Fig.2. Since Raman spectral studies on the V-substituted sintered cobalt ferrites have not
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been documented in the literature, the assignment of ~750 cm-1 band to VO4 unit has been made based on the ionic sizes and bond lengths, wherein the ionic size of V5+ and V-O bond length are relatively smaller than the ionic sizes of Fe3+ and Co2+ and their bond lengths with oxygen. Shorter bond length will have higher stretching frequency, as suggested [22]. The
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intensity of the A1g(3) band at 750 cm-1 is increased substantially with increasing the amount of co-substitution suggesting that more of V ions are incorporated into the A-sites. Also, the
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intensity of the A1g(1) band due to FeO4 decreases whereas A1g(2) due to CoO4 increases with x, indicating re-distribution of Co2+ and Fe3+ within the A-sites. Unlike for the compositions x=0.025 and x=0.05, extra signals are observed for x=0.1 and are situated at ~198 cm-1 and ~330 cm-1. These signals are likely to be due to the cation redistribution and variable oxidation states of V and Co in the spinel lattice, but not the Raman spectral features of V2O5 [29]. The signal at ~330 cm-1 is most probably due to asymmetric bending motion of oxygen ions coordinated to V located at the tetrahedral site, similar to that of the Eg modes.
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Fig. 2
Fig. 3
To gain further information on the cations distribution, all the spectra were normalized with respect to the most intense T2g(2) band and deconvoluted. The deconvoluted spectra of
ACCEPTED MANUSCRIPT all the compositions are presented in Fig. 3. The cations distribution of the unsubstituted cobalt ferrite can be estimated from the area under the peaks of A1g [27]. In the present study, for the unsubstituted cobalt ferrite, the ratio of the area under the peaks due to A1g(2) (CoO4) and A1g(1) (FeO4) is found as 0.29 from which the cation distribution can be assigned to (Co0.29Fe0.71)A[Co0.71Fe1.29]BO4. This distribution is nearly comparable to the cations
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distribution reported for the sintered cobalt ferrite systems [27]. However, it is difficult to estimate the cations distribution for the co-substituted compositions because the cosubstitution induces variable oxidation states in Co and V and the resultant ions would occupy different crystallographic sites leading to complex cations distribution. The variation
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of the relative area of the A1g peaks is shown in Fig. 4. As can be seen from the Fig. 4, the relative area due to FeO4 (A1g(1)) decreases, and the relative areas due to CoO4 (A1g(2)) and VO4 (A1g(3)) increase with increasing the values of x. However, the rate of decrease in the
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area of the A1g(1) (FeO4) is more pronounced compared to the rate of increase for both A1g(2) (CoO4) and A1g(3) (VO4) units. This apparently suggests that more and more Fe3+ ions are
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being replaced by Co2+ and V5+ at the tetrahedral sites.
Fig. 4
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3.3 Microstructure
Fig. 5
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It is obvious from the SEM photographs, presented in Fig. 5, that the microstructure of the unsubstituted cobalt ferrite would be tuned to a greater extent after incorporation of vanadium into the spinel crystal lattice. Despite the fact that all the compositions are prepared and processed under identical conditions, the co-substituted compositions showed relatively
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smaller and nearly uniform grains without intra-granular pores compared to that of the parent compound which comprises larger grains with inter- and intra-granular pores. The appearance
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of larger sized grains with intragranular pores in the case of unsubstituted cobalt ferrite is attributed to abnormal fusion of two or more smaller grains that eventually causes trapping of pores inside the grains, as suggested [30]. The presence of smaller grains in the case of cosubstituted compositions is likely to be due to the contribution of V2O5 raw material as it acts as a sintering aid at lower temperatures. However, at higher temperature V ions are incorporated comprehensively into the spinel lattice structure as there was no impurity phase in the XRD patterns of the sintered compositions. The grain size is increased with increasing the amount of co-substitution and the average grain sizes are obtained as 2, 4 and 6 µm, respectively, for the compositions x=0.025, 0.05 and 0.1 against 12 µm for x=0.
ACCEPTED MANUSCRIPT 3.4 Magnetic properties Fig. 6. shows the field dependence of magnetization (M vs H) curves of the sintered Co1+2xVxFe2-3xO4 compositions, measured at room temperature upto a maximum magnetic field strength 1200 kA/m (1.5 T). With increase in the content of V and Co, considerable changes in the nature of the M-H curves at lower as well as at higher fields have been
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noticed. As can be seen from the inset of Fig. 6, all the studied compositions display hysteresis loops, due to presence of a long-range ferrimagnetic order. Saturation magnetization (Ms), obtained by extrapolating M versus 1/H curve to 1/H=0, is found to be decreased with increasing the co-substitution content x (Fig. 7a), due to magnetic dilution
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induced in the system. The magnitude of the Ms in the metal ions substituted spinel ferrites depends on whether the substituent is located at A-site or at the B-site. Reported studies on
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the magnetic properties of the CoMxFe2-xO4 (M=Al, Ga, Zn, Mg) systems [13,14,22] suggest that the decreased in Ms for the Al- and Mg-substituted compositions is due to the magnetic dilution of the B-site by the presence of Mg and Al, whereas increased in Ms up to a particular composition in the case of Ga- and Zn-substituted compositions is primarily attributed to the A-site magnetic dilution by the presence of Ga and Zn.
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In the present study, Raman spectral analysis showed that with increasing the amount of co-substitution, more of V and Co are substituted for Fe at the A-sites and therefore an increase in Ms with increased x would be predicted due to the presence of non-magnetic vanadium at the A-site. The net magnetic moment for the unsubstituted cobalt ferrite is
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calculated as 4.16 µB based on the cations distribution estimated from the Raman spectral analysis. The net magnetic moments for the compositions with x=0.025, 0.05 and 0.1 can be estimated as 4.38, 4.61 and 5.06 µB, respectively, assuming that both Co and V are
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substituted for Fe at the A-sites. However, the observed decrease in the Ms with increasing x could be due to multiple factors including the changes in the oxidation states of the cations and their random distributions among the A- and B-sites. Some fraction of the Co2+ will be converted to Co3+ to attain the charge neutrality and the resultant Co3+ will be located at the B-sites. The Co3+ will be in low-spin electronic configuration with zero magnetic moment [13] and hence more magnetic dilution of B-site.
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Fig. 6
Fig. 7
First order magnetocrystalline anisotropy coefficient (K1) of all the compositions is calculated from the initial magnetization data using the method of law of approach to saturation [31]. The initial magnetization curves, particularly at higher field regions (H>500 kA/m), of the compositions including cobalt ferrite (x=0) are fitted to the relation, M = M 1 −
b
H
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; Ms is the saturation magnetization in kA/m; K1 is the first order
magnetocrystalline anisotropy coefficient; and µo is the permeability of free space (1.257 × 10-6 mkg/s2.A2). The constant 8/105 is for first order cubic anisotropy of randomly oriented polycrystalline materials. In the above equation, ‘Ms’ and ‘b’ are only the fitting parameters. Thus by knowing the values of Ms and b, extracted after fitting, K1 can be derived. A
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relatively good fit has been obtained for all the compositions and the results of the fitted curves are shown in Fig. S2 (supplementary information). For the unsubstituted cobalt ferrite (x=0), K1 is obtained as 3.46 × 105 J/m3 and is comparable to the values reported for the sintered polycrystalline cobalt ferrites [27,30]. With increasing the amounts of V and Co for
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Fe in the spinel lattice, K1 is decreased monotonically to 1.55 × 105 J/m3 for x=0.1. The variation of K1 as function of x is shown in Fig. 7(b). In cobalt ferrite, the magnitude of K1 is
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proportional to the concentration of Co2+ at the B-sites [23]. Therefore, any changes in the amount of Co2+ at the B-site as well as its oxidation state would directly influence the magnitude of K1. In addition, the reduction in the A-O-B super-exchange strength would also responsible for decline in the values of K1, as suggested for the various metal ions substituted polycrystalline cobalt ferrite systems [14,32,33]. In the present case, even though more and more Co, together with V, is substituted for Fe, the substantial drop in the values of K1 with
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‘x’ is most probably due to the reduction of the A-O-B super-exchange interactions. Furthermore, the presence of non-magnetic Co3+ at the B-site is likely to be accountable for the same [13,27].
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As illustrated in the Fig. 7(b), coercivity (Hc) of the composition with x=0.025 is nearly comparable to that of x=0 (~17.5 kA/m) and drops at a faster rate for x>0.025. The large drop
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in the Hc for x>0.025 is primarily attributed to a strong reduction in the A-O-B superexchange strength triggered by the presence of non-magnetic V at the A-sites and also due to the reduction in the magnetocrystalline anisotropy [13,14]. Interestingly, the variation of Hc with x follows approximately the same trend as that observed for K1. This is because Hc is proportional to the K1 through the Brown relation, Hc=2K1/µoMs [34]. Furthermore, in the MH curves, higher magnetization has been observed for the co-substituted compositions at low magnetic fields (< 400 kA/m) compared to that of the parent compound. This apparently supports the claim that reduction in the A-O-B superexchange strength is the dominant factor responsible for a large drop in the Hc for x>0.025. As documented in the literature, microstructure of the sintered body would also alter the magnitude of the Hc [17,30].
ACCEPTED MANUSCRIPT 3.5 XPS analysis XPS is an effective tool to assess the valencies of the metal ions present in chemical compounds. In the present study, XPS spectral analyses have been carried out to investigate the change in the oxidation states of Fe, Co and V, and the corresponding XPS spectra (Fe2p, Co2p and V2p) of the selected sintered compositions are shown in Fig. 8. In the XPS spectra
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of Co, the peaks at ~780.6 eV and ~786.5 eV are attributed to 2p3/2 and 2p1/2, respectively, and both the peaks are accompanied by weak satellite signals (Fig. 8(a)). The binding energies of the Co2p3/2 and Co2p1/2 peaks of cobalt ferrite are in agreement with the values reported in the literature [27]. The intensity of the 2p3/2 satellite peak of the unsubstituted
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cobalt ferrite is relatively higher than that of the co-substituted compositions indicating the presence of higher concentration of Co2+ in the spinel ferrite. However, with increasing the
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amount of co-substitution, the peaks are shifted slightly towards lower binding energies and moreover the intensity of the 2p3/2 satellite peak decreases substantially, as shown in Fig. 8(b) that compares the binding energies and XPS peaks intensities of the compositions x=0 and x=0.1. The decrease in the intensity of the 2p3/2 satellite peak is primarily due to the conversion of Co2+ into Co3+, as reported [27, 35].
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No significant changes in the spectral features are observed in the case of Fe and the peaks at ~710.7 eV and ~724.5 eV correspond to 2p3/2 and 2p1/2, respectively, (Fig. 8(c)). The binding energies of the Fe2p3/2 and Fe2p1/2 peaks of the parent compound are in agreement with the values reported in the literature [27]. As shown in Fig. 8(d), a considerable variation
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in the spectral features has been observed in the XPS spectra of V2p. The peaks centered at 516.8 eV and 522.9 eV are attributed to V2p3/2 and Vp1/2, respectively, and the values are in concordance with the reported values for V5+/V4+ [28]. The peaks in the XPS spectrum of the
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composition x=0.1 are shifted slightly towards lower binding energies with respect to that of the peaks in the composition x=0.025, indicating decreasing the oxidation state of V in the higher co-substituted composition. Overall, the XPS spectral analysis revealed that with increasing the amount of co-substitution in the spinel ferrite lattice, oxidation state of Co is increased from +2 to +3 whereas that of V is reduced from +5 to lower oxidation states. These finding are strongly supporting the XRD and magnetic interpretations.
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Fig. 8
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4. Conclusions
The results presented in this paper indicated that co-substitution of Co and V for Fe in cobalt ferrite (Co1+2xVxFe2-3xO4 (0≤x≤0.1)) has considerable impact on the structural,
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microstructural and magnetic properties. The changes in the oxidation states of Co from +2 to +3 and V from +5 to lower states are confirmed from the XPS spectral analysis. Lattice parameter is found to decrease with increasing the amounts of co-substitution. The decrease
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is attributed to changes in the oxidation of states of Co and V in the crystal lattice and also the cation distributions. Raman spectral analysis indicated that V is substituted for Fe at the tetrahedral sites of the spinel ferrite. Better control over the microstructural parameters has been achieved for the sintered co-substituted compositions over unsubstituted counterpart. The magnetic parameters (Ms, Hc and K1) are also found to decrease non-linearly with increasing the amount of co-substitution, due to magnetic dilution and reduction in the A-O-B super-exchange interactions.
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Acknowledgement P. N. Anantharamaiah is grateful to the CSIR-NCL for the infrastructure and the facilities provided to complete this research work. Also, PNA is grateful to Sharath Devdas for his
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assistance during the samples preparation.
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Captions for Figures:
Fig. 1. (a) Rietveld fit XRD pattern of the sintered cobalt ferrite. (b) enlarged patterns showing the
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(311) peak along with Kα1 and Kα2 components for all the compositions, and (c) variation of lattice parameter as a function of ‘x’ in Co1+2xVxFe2-3xO4.
Fig. 2. Raman spectra of different compositions in Co1+2xVxFe2-3xO4 compositions, recorded on sintered samples at ambient temperature.
Fig. 3. Deconvoluted Raman spectra of sintered Co1+2xVxFe2-3xO4 compositions.
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Fig. 4. Variation of the relative area under the peaks due to the A1g modes of CoO4, FeO4 and VO4 as a function of x in Co1+2xVxFe2-3xO4.
Fig. 5. SEM images of sintered Co1+2xVxFe2-3xO4 compositions. The scale is common for all images.
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Fig. 6. Field dependant magnetization hysteresis loops of the sintered Co1+2xVxFe2-3xO4 compositions.
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Inset shows an enlarged view of the hysteresis loops at low field regions. Fig. 7. Variation of (a) saturation magnetization, Ms, and (b) magnetocrystalline anisotropy coefficient, K1, and coercivity, Hc, as a function of ‘x’ in Co1+2xVxFe2-3xO4. Fig. 8. XPS spectra of Co1+2xVxFe2-3xO4 for (a) Co-2p, (b) normalized Co-2p spectra of x=0 and 0.1, (c) Fe-2p, and (d) V-2p.