Investigation on structural, electrical and magnetic properties of titanium substituted cobalt ferrite nanocrystallites

Accepted Manuscript Investigation on Structural, Electrical and Magnetic Properties of Titanium substituted Cobalt Ferrite nanocrystallites Persis Amaliya, S. Anand, S. Pauline PII: DOI: Reference:

S0304-8853(17)33806-4 https://doi.org/10.1016/j.jmmm.2018.07.058 MAGMA 64164

To appear in:

Journal of Magnetism and Magnetic Materials

Received Date: Revised Date: Accepted Date:

8 December 2017 1 July 2018 17 July 2018

Please cite this article as: P. Amaliya, S. Anand, S. Pauline, Investigation on Structural, Electrical and Magnetic Properties of Titanium substituted Cobalt Ferrite nanocrystallites, Journal of Magnetism and Magnetic Materials (2018), doi: https://doi.org/10.1016/j.jmmm.2018.07.058

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Investigation on Structural, Electrical and Magnetic Properties of Titanium substituted Cobalt Ferrite nanocrystallites A. Persis Amaliya, S. Anand, S. Pauline* Department of Physics, Loyola College (Autonomous), Chennai-600034, Tamilnadu, India *Corresponding author: [email protected]

Abstract The doping of parent compound with selective dopants usually enhances the properties of the parent compound. In this paper, Titanium doped cobalt ferrite nanoparticles (Co1-xTixFe2O4 with x=0-0.075) via the sol-gel route is reported. The effect of titanium ions on the properties of cobalt ferrite is detailed. Structural evolution is confirmed using XRD technique and the titanium ion induced changes are given in terms of lattice constant, grain size, X-ray density, tetrahedral and octahedral hopping length and tetrahedral and octahedral bond length. The X-ray diffraction pattern displayed the formation of cubic inverse spinal structure belonging to the Fd3m space group. FTIR analysis confirmed the presence of characteristic peaks of ferrites around 400 cm-1 and 600 cm-1. Raman spectroscopy revealed reduction in peak intensities of Raman active modes. SEM characterization had shown the formation of well resolved spherical particles. Elemental composition was evaluated by EDAX spectrum. TEM images also confirmed the spherical morphology of the nanocrystallites and crystallite size calculated is in agreement with that calculated from the XRD analysis. The SAED pattern shows uniform fringes of width 0.428 nm belonging (222) plane. The diffraction pattern of HRTEM confirmed cubic spinel structure of the samples. Magnetic properties are evaluated from room temperature hysteresis loops. Increase in the saturation magnetization and coercivity was observed with increasing titanium content. Gradual increase in impedance with the increase in titanium concentration was estimated by impedance spectroscopy. Keywords: Ti substituted Co ferrite, X-ray diffraction, HRTEM, VSM, Impedance analysis

1

1. INTRODUCTION Magnetic nanoparticles have received noteworthy attentions in various fields of engineering and biomedicine as their magnetic and electrical performance provides a wide range of applications. The ferrite spinel structure is based on a closed-packed oxygen lattice; the sites having tetrahedral and octahedral oxygen coordination are called A-site and B-site respectively. There are 8 A-sites where the metal cations are tetrahedrally coordinated with oxygen, and 16 Bsites which possess octahedral coordination. Spinels with divalent ions at tetrahedral sites and trivalent ions at octahedral sites are called normal spinel. In the inverse spinel structure, all the Co2+ ions occupy the octahedral site; half of the Fe3+ ions also occupy the same site while half of the Fe3+ ions stay in tetrahedral site [1-2]. Magnetically, spinel ferrites expose ferrimagnetic ordering. The magnetic moment of cation A and B are parallel within each site but they are antiparallel between A and B sites. CoFe2O4 is one of the important material belonging to the inverse spinel group (they are mixed ferrite in nm size) with high coercivity and moderate magnetization. Along with these properties, its physical and chemical stability make CoFe2O4 nanoparticles suitable for magnetic recording applications such as audio, videotape and high-density digital recording disks [3-4]. Modifying the properties of spinel ferrites was made possible by substituting or doping them with magnetic ions, non-magnetic ions, and rare earth ions and so on. This is an evergreen research field of nanoscience from where various novelty works are emerging day by day. Substitution of ions into spinel ferrites produces nanomaterials for various applications such as soft or hard ferrites, materials with low or high coercivity and low or high impedance. Cadmium doped cobalt ferrite with Superparamagnetic behaviour was synthesized by Ch. Venkata Reddy et al.[5] and Cd substitution increased saturation magnetization (Ms) and coercivity (Hci). N. M. Deraz [6] substituted aluminium in cobalt ferrite which displayed drop off in the coercivity with an enhancement in both magnetization and magnetic moment. Reduction in anisotropy caused gradual decrease in coercive field and saturation magnetostriction (λS) with Zn substitution in Co ferrite [7]. Mahesh Kumar et al[8] reported saturation magnetization, coercivity, and remanence or remanent magnetization (Mr) values changed with the substitution of nickel, zinc and manganese. Sanpo et al [9] revealed Zn and Ni substituted cobalt ferrite had better biocompatibility than that of pure cobalt ferrite; the substitution of zinc and copper in cobalt 2

ferrite nanoparticles notably improved antibacterial activity and copper-substituted cobalt ferrite nanoparticles exhibited the effective contact biocidal property means the potential for linking between a bacterial cell and a substratum surface. Erum Pervaiz et al [9] reported that rare Gadolinium ion substitution into cobalt ferrite nanoparticles changes magnetic properties and enhance electrical conductivity by dropping resistance. Dysprosium [10] substitution decreased saturation magnetization and coercivity.

Fine powder of Ti doped cobalt ferrite (Ti0.2Co1.2Fe1.6O4) was prepared by Kwang Pyo Chae et al [11] using sol gel method. Annealing temperature was varied from 473 K in steps of 100 K which inherently caused changes in the magnetic properties. Increase in saturation magnetization (Ms) and decrease in coercivity (Hci) was reported with an increase in annealing temperature. They reported a maximum saturation magnetization and coercivity of 62 emu/g and 1564 Oe respectively. In another paper Kwang Pyo Chae et al [12] also reported the particle size dependent magnetic property for Ti and Co substituted cobalt ferrites having the common formula TixCo1+xFe2-2xO4(0.0≤x≤0.7) by sol–gel method. Coercivity decreased drastically and saturation magnetization decreased slowly with the increment of x. Magnetic performance of the samples annealed at various temperatures was analyzed using Mossbauer spectroscopy taken at room temperature. Titanium substituted cobalt ferrite with general formula CoFe 2−xTixO4 [x=0.00, 0.05, 0.10, 0.15, 0.20, 0.25 and 0.30] was synthesized by Srinivasa Rao et al [13] using standard ceramic method. As the diamagnetic Ti ions replaced iron ions at the octahedral site, magnetic moment of octahedral site decreased; small and gradual reduction in Curie temperature was resulted with increase in Ti ion concentration. They explained it based on A-B exchange interaction which got weakened. The observed increase in dc resistivity was due to the presence of Ti4+–Fe2+ locking pairs. Hopping between Fe3+-Fe2+ reduced as the distance between them was extended in this case. Daiki Shigeoka et al [14] synthesized Co-Ti ferrite nanoparticles by wet chemical method which is widely used in hyperthermia treatment. Saturation magnetization and coercivity decreases with increase of composition parameter x of the sample Co1+xTixFe2–2xO4 (0.2≤x≤0.5). It was suggested that the sample with composition Co1.3Ti0.3Fe1.4O4 could be used for hyperthermia treatment due to its higher susceptibility at room temperature compared to samples with other composition.

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In the present study, the effects of titanium substitution on the magnetic and electrical properties of cobalt ferrite are reported. The samples are characterized using X-ray diffraction, Fourier transform infrared spectroscope (FTIR), Raman spectroscopy, Scanning electron microscopy (SEM) with EDX, Transmission electron microscopy (TEM), Vibrating sample magnetometer (VSM) and Impedance analyzer.

2. MATERIALS AND METHODS 2.1 MATERIALS Titanium doped cobalt ferrite nanoparticles were produced by sol-gel route using ferric nitrate (Fe(NO3)3.9H2O), cobalt nitrate (Co(NO3)2.6H2O), titanium IV isopropoxide (Ti[OCH(CH3)2]4) and ethylene glycol ((C2H4(OH)2). All the reagents used for synthesizing titanium doped cobalt ferrite nanoparticles were analytical grade and used as received from Merck without additional purification. 2.2 METHOD The titanium substituted CoFe2O4 nanoparticles were synthesized by one pot sol-gel route using the following procedure. In a typical experiment, cobalt nitrate, titanium nitrate and ferric nitrate were taken in different molar concentrations described by chemical formula CoFe2O4, Co0.975Ti0.025Fe2O4, Co0.95Ti0.05Fe2O4 and Co0.925Ti0.075Fe2O4 respectively. Required quantity of ethylene glycol (EG) was added and magnetically stirred at room temperature to get translucent solution. This solution was poured into a pettri dish and thermally treated around 60 – 80 ºC. The obtained gel was transferred to a silica crucible and demoisturised in the muffle furnace for 2 hours at 200 ºC. The consequential powder sample was calcined for 3 hours at 600 ºC and then stored in an air tight container for further study. 3. RESULTS AND DISCUSSION 3.1 XRD ANALYSIS The XRD patterns of nanocrystallite with specific formula Co 1-xTixFe2O4 (x=0, x=0.025, x=0.05 and x=0.075) using the data recorded over 2 range 20º - 70º are shown in fig.1. Typical reflections from the planes (220), (311), (400), (422), (511) and (440) 4

were observed for all compositions and the peaks were indexed to inverse cubic spinel lattice [JCPDS No: 22-1086]. There were no peaks of other phases present in the XRD pattern [G.A. El-Shobakya et al 2010].

Fig. 1 XRD pattern of Co1-xTixFe2O4 nanocrystallites

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Table 1 Parameters calculated from XRD profiles are given below

Parameters

CoFe2O4

Co0.975 Ti0.025Fe2O4

Co0.95 Ti0.05Fe2O4

Co0.925 Ti0.075Fe2O4

34

26

27

17

8.384

8.378

8.363

8.354

8.389

8.379

8.367

8.356

4.433

4.435

4.441

4.450

3.630

3.628

3.623

3.617

2.963

2.962

2.958

2.953

1.902

1.901

1.898

1.896

2.047

2.045

2.042

2.039

Average Crystallite size (D) nm Lattice parameter (a±0.001) Å Corrected Lattice parameter values using N-R plot (Å) X-ray density

(dx)×10-3 g/cm3 Tetrahedral hopping length LA ( Å) Octahedral hopping length LB ( Å) Tetrahedral bond length dAX ( Å) Octahedral bond length dBX ( Å)

6

Average crystallite size The average crystallite sizes of the as-prepared samples were calculated by Scherrer’s equation (1) mentioned below corresponding to the high intensity plane (311). nm------------------------ (1) Where, D is the crystallite size, β is the full width at half maximum, λ is the X-ray wavelength, θ is the Bragg angle and 0.9 is the value of shape factor k. In the present study, the size of crystallites reduced after the substitution of Ti 4+ into cobalt ferrite crystallites. Decrease in particle size of cobalt ferrite crystallites through the substitution of different ions such as Ni [15] and rare earth [16] were reported earlier. Bond dissociation energy is higher for Ti-O bond (662 kJ/mol) than Co-O bond (368 kJ/mol) and Fe-O bond (409 kJ/mol and as a result Ti-O bond is short and strong. Therefore substitution of Ti4+ reduces crystallite size of the as prepared sample. Lattice parameter` Crystal lattice is the 3D regular array of points about which the atoms, ions, or molecules composing a crystal are arranged. Being a cubic lattice, the lattice parameter a=b=c which is calculated using the equation (2) a d√(h2+k2+l2) Å -----------------------(2) From Bragg’s law λ=2d sin θ, d-spacing (dhkl) value is found using the below relation (3)

dhkl

Å

---------------------(3)

Where, ‘d’ is inter planar distance; h, k, l are the Miller indices of the crystal plane

7

Accurate measurement of lattice parameter is necessary for the crystallographic characterization of nanocrystallites. There are several possible sources of error as described by many workers such as line divergence of the X-ray beams, refraction and absorption of X-rays by the specimen etc. in the measurement of

or d values. So the accuracy of determination of lattice constant is

dependent upon the accuracy in measurement of “d”, Hence most accurate value of lattice parameters are estimated from the Nelson-Riley plots . Lattice parameter was re-evaluated using extrapolating different indexed planes in a plot of lattice constant versus Nelson-Riley exploitation function (N-R) F (θ) → 0 in order to find accurate value up to ±0.001Ǻ. The intercept satisfying majority error points in Nelson-Riley plot (least square fit straight line determining the dislocation density) intersect the y-axis at F (θ) = 0; can provide us lattice constant free from all systematic errors [49]. N-R formula is empirical and expected to give accurate value of lattice constant when fraction error approaches zero as 2θ = 180° or θ = 90°. However it is impossible to measure (ɑ) at 2θ = 180°. So we have to plot within the measurable values and then extrapolate those values up to 2θ = 180° or 90° versus function of θ fitting for straight line graph (figure 2).

…………………………..(4)

8

Fig. 2: Lattice parameters versus Nelson-Riley function plot for (a) CoFe2O4, (b) Co0.975Ti0.025Fe2O4, (c) Co0.95Ti0.05Fe2O4 and (d) Co0.925Ti0.075Fe2O4 nanocrystallites

9

Fig. 3 Lattice parameters versus Ti doping concentration of Co1-xTixFe2O4 nanocrystallites Figure 3 shows the plot of lattice parameter versus doping concentration. The observed

linear decreases in lattice parameter with the increase in Ti concentration is attributed to the substitution of Co2+ ions of larger ionic radius (0.75 Å) by Ti4+ ions of smaller ionic radius (0.69 Å). The linear change in lattice parameter obeys Vegard’s law [17]; according to this law if incoming ion of smaller radii replaces ion of larger radii in the lattice, lattice contracts and opposite phenomenon takes place for incoming ion of larger radii. Srinivasa Rao et al [13] reported enhancement in lattice parameter with Ti substitution for Fe site of cobalt ferrite crystallites. It was explained on the basis that the radii of replaced Ti 4+-Fe2+ pair (0.69 Å) is greater than ionic radii of replacing 2Fe3+ ion (0.645 Å). In another report no change in lattice parameter was observed for same substitution and they have remarked that the ionic radii of Ti4+(0.68 Ǻ) is comparable to ionic radii of Fe3+(0.69 Ǻ) [18]. Chae et al [19] reported that due to smaller ionic radii of Co2+ and Ti4+ compared to Fe3+, lattice parameter decreased when x was increased from 0.00 to 0.7 in steps of 0.1 in Ti xCo1+xFe2-2xO4 system. The above discussion of decreasing or increasing trend of lattice parameter clearly shows 10

that in the present investigation Ti4+ ions will only replace Co2+ ions of larger ionic radius because if Ti4+ substitutes Fe3+, then it should result in an increment or no change in lattice parameter. The numerical value of ionic radius of Co2+, Fe3+ and Ti4+ ions vary in literature and hence we compared all of them and used most probable one. In line with Chae et al [19] lattice parameter was found to decrease with Ti substitution. X-ray density X-ray density was calculated using the following equation (4) [20] dx

g/cm3------------------------ (5)

Where, 8 represents the number of molecules present in the spinel lattice, M is the molecular weight of the composition and N is the Avogadro’s number (6.023×1023mol-1). X-ray density depends on lattice constant determined from XRD characterization and molecular weight of the sample. Lattice parameter and molecular weight decreases with Ti inclusion which has lower atomic weight and density (47.867 g/mol, 4.506 g/cm3) compared to that of cobalt (58.933 g/mol, 8.9 g/cm3) and iron (55.854 g/mol, 7.874 g/cm3). So increase in X-ray density for Ti doped samples is owing to decreasing value of lattice parameter which is inversely proportional to dx. Hopping length Tetrahedral hopping length (d A) and octahedral hopping length (dB) were deduced from below equations [21] dA=a√3/4 ------------------------- (6) dB=a√2/4 ------------------------- (7) dA and dB are distance between magnetic ions of the sample on A (tetrahedral) and B(octahedral) site respectively. Substitution of Ti did not bring considerable change in both tetrahedral and octahedral hopping length.

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Bond length Tetrahedral, octahedral bond length (d AX, dBX) were calculated from the subsequent equation dAX= a√3(u-1/4) ----------------------- (8) dBX= a√3[3u2-(11/4)u+43/64]1/2 ----- (9) The value of oxygen positional parameter u=0.38 was taken from literature. Oxygen positional parameter (u) is the distance between the oxygen ion and the face of the cube edge along the cube diagonal of the spinel lattice [22]. Since Ti ions of smaller radius replaces Co ions of larger radius, calculated bond lengths shown in table 3.1 shows no considerable change with increment in Ti concentration. Cationic distribution Different cationic distribution has consequence on the structural, magnetic and electrical properties of spinel ferrite. Site preference of cations is based on three factors. (i) The size of cation: small cations should occupy tetrahedral site which is smaller in size whereas large cations should occupy octahedral site which is larger in size. (ii) Electrostatic energy (Madelung energy): cations with high electrical charge (valence) should occupy octahedral site and cations with smaller valence should occupy tetrahedral site. (iii) The crystal field stabilization energy: this energy is attributed to the geometry of d orbital and the arrangements of these orbitals can established within the crystal structure. Ions with stable electronic configuration move to tetrahedral site whereas unstable ions move to octahedral site of the lattice [23]. Ti4+ is a small cation having ionic radii 0.68 Å, it is larger that of Co2+ (0.75 Å) and comparable to that of Fe3+ (0.69 Å). Due to its small size it could occupy tetrahedral site and because of high electrical charge (+4) and lack of d electrons (3d 2) it could occupy octahedral site. Therefore Ti ions might be accommodated in tetrahedral or octahedral or both sites. Prediction of cationic distribution in mixed ferrite is a challenging task because redistribution takes place when any ion is substituted for a cation for it.

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3.2 HRSEM ANALYSIS The morphology of as-prepared samples with differing composition was examined through high resolution scanning electron microscopy. The micrographs of the samples annealed at 600ºC were recorded. The samples were coated with a thin layer of gold (Au) before the analysis to protect sample from charging and for quality imaging and recording. Morphology was examined by secondary electron image taken at various magnifications as given below. At two different magnifications 60,000 X and 110,000 X images were taken. Homogeneous and well resolved crystallites having almost same size are uniformly distributed in above pictures and grains are well crystallized. Crystallites are spherical in shape and are in nanometer range with less agglomeration. Substitution of titanium did not affect shape of the crystallites; but reduction of grain size proved by XRD method could not be traced from HRSEM micrograph. See fig.4. 3.3 EDAX SPECTRUM ANALYSIS Determination of chemical composition of nanopowder is possible with Energy Dispersive X-ray Analysis (EDAX) characterization technique. EDAX spectra of pure sample shows the presence of constituents Co, Fe and O and EDAX spectra of Ti substituted samples shows the presence of Co, Fe, O and Ti. Gradual increase in Ti concentration was observed in the spectrum. No impurity content was detected in the spectrum. See fig.5.

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(a)

(b)

(c)

(d)

Fig. 4 HRSEM images of (a) CoFe2O4, (b) Co0.975Ti0.025Fe2O4, (c) Co0.95Ti0.05Fe2O4 and (d) Co0.925Ti0.075Fe2O4

14

(a)

(b)

(c)

(d)

Fig. 5 EDAX spectrum of (a) CoFe2O4, (b) Co0.975Ti0.025Fe2O4, (c) Co0.95Ti0.05Fe2O4 and (d) Co0.925Ti0.075Fe2O4

3.4 TEM ANALYSIS The following micrographs were recorded to analyze morphology and size of crystallites in detail by Transmission Electron Microscopy. The below presented micrographs in fig. 6 show the presence of small agglomeration of well spherically shaped nanocrystallites. Pictures also reveal that the sample contains monocrystallites with average diameter in nanometer range. Mostly crystallites with approximately uniform size are dispersed but in few places agglomeration of crystallites (large aggregates) was noticed. As the samples are mostly composed of magnetic substances,

15

the crystallites cling together by magnetic force of attraction. The average crystallite size of Co0.95Ti0.05Fe2O4 deduced from TEM analysis is 22 nm which is in analog with average crystallite size calculated from XRD profile. Electron diffraction image (Fig.7) confirms spinel structure of well crystallized nanocrystalline sample. No structural distortion was determined. Each ring in the image is an indicative of crystalline planes of the crystallite. Due to aging of the sample, bright spots are seen in the rings. Fringe width is nothing but interlayer distance of the as-produced crystalline material is visible in the fig. 8. The interplanar distances (d spacing) of each spot in the diffraction rings mentioned by serial number were obtained as back data from TEM characterization. ‘d values’ from XRD profile were compared with that of TEM and planes corresponding to all the rings of diffraction pattern were identified. Ti substituted cobalt ferrite (Co 0.95Ti0.05Fe2O4) was found to have inter-planar spacing of 0.428 nm for the plane (222). Though the valence of Ti4+ is different from Co2+ and Fe3+, substituting very less amount of Ti4+ into cobalt ferrite did not affect its structure.

16

(a)

(b)

(c)

(d)

Fig. 6 TEM images of Co0.95Ti0.05Fe2O4 nanocrystallites

17

Fig. 7 Diffraction image of Co0.95 Ti0.05Fe2O4 nanocrystallites

Fig. 8 Fringe width of Co0.95Ti0.05Fe2O4

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3.5 FTIR ANALYSIS Ti substituted cobalt ferrite powder was analyzed by Fourier Transform Infrared (FTIR) spectrophotometer in the region 4000–400cm-1. Fig. 9 shows typical pattern of FTIR transmittance spectrum of Co1-xTixFe2O4 nanocrystallites. The presence of the two absorption bands around 400 cm-1(ν2) and 600 cm-1(ν1) is a common feature of all the ferrites. Absorption band around frequency 600 cm-1 (ν1) is caused by stretching of the tetrahedral metal ion and oxygen bonding, whereas the absorption around frequency 400 cm -1 (ν2) is caused by metal-oxygen bond stretching at octahedral site. The frequency of transmittance changes for tetrahedral and octahedral site because of varying distance between Fe3+–O2−in the octahedral and tetrahedral sites. All the substituted samples had similar peaks compared to pure crystallites. The frequency mode ~600 cm-1 attributed to stretching vibrations of Fe3+-O2- at tetrahedral and characteristic absorption features of absorbed water on the surface were observed around frequency 3400 cm-1. Bands at 1400 cm-1and 1600 cm-1 are correspond to C-O stretching vibration. The band at 1100 cm-1 is attributed to pure ethylene glycol. Very weak bands at 2900 cm-1 assigned to stretching of CH2 groups are may be due to impurities. FTIR results are in good agreement with previous study [14,25].

Intensity of the peaks reduces exceedingly for Ti substituted cobalt ferrite sample as compared to that of pure sample. But the intensity of characteristic peak of ferrites near 600 cm-1 remains prominent for substituted samples. The variation in band position of ν2 was observed with Ti incorporation into spinel lattice. Incoming new ions redistribute cations already present in tetra and octahedral site which slightly shift the frequency mode assigned to tetrahedral site. Since ν2 mode falls below spectral range recorded site preference of Ti ions could not be determined from FTIR analysis[26].

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Fig. 9 FTIR spectrum of Co1-xTixFe2O4 nanocrystallites

Table 2 FTIR bands and assigned vibrations

Entry

Band(cm-1)

Vibration

1

~400

Stretching vibration of the metal–oxygen band at octahedral site

2

~600

Stretching vibration of the metal–oxygen band at tetrahedral site

3

~1100

Ethylene glycol

4

~1400

C-O stretching vibration

5

~1600

C-O stretching vibration

6

~2300

C–H bond stretching

8

~3400

Hydroxyl group

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Table 3 Stretching vibration of M-O band at tetrahedral site

S.No

Sample

Tetrahedral mode frequency (ν1) cm-1

1

CoFe2O4

585

2

Co0.975Ti0.025Fe2O4

583

3

Co0.95Ti0.05Fe2O4

581

4

Co0.925Ti0.075Fe2O4

581

3.6 RAMAN STUDIES Raman spectra of the as prepared samples were taken by Renishaw Raman System 3000 using laser of excitation wavelength 532 nm (Fig.10). Particular vibration occurring at a specific wavelength corresponds to the mass of the atoms present in the molecule to be tested and the strength of the bond between atoms of the molecule. Shorter the bond and lower the mass correspond to substituting atom results in shifting of band towards higher wavelength and vice versa. Ferrites have cubic spinel structure which gives rise to 39 normal Raman modes in which 2A1g, 1Eg, 3F2g are Raman active modes [10]. Most intense peak and other peaks of Raman spectra changes with respect to exciting wavelength of laser. Peak at 680 cm -1 is the most intense peak in the majority spectra excited by laser having wavelength below 540 nm. Laser of excitation wavelength 532nm was used in the present study and thus obtained major peak at 680 cm-1 [27]. Many literatures define that the peaks above 600 cm-1 are characteristic of tetrahedron and peaks below 600 cm-1 are characteristic of octahedron. The Raman spectra of samples synthesized for present research reveals five Raman active modes A1g, Eg and 3T2g around 200, 300, 475, 575, 680 cm-1. The Eg (~300 cm-1) and T1g(3) (~200 cm-1) modes are due to the symmetric and asymmetric bending of oxygen -metal ion at the octahedral site, respectively. The T1g(2) mode (~475 cm-1) corresponds to the asymmetric stretching of

21

oxygen atom-metal ion at the octahedral site. The T1g(1) mode (~575 cm-1) represents the translation movement of the oxygen atom with respect to iron ion or in other words it represents Fe at the tetrahedral site. The A1g(1) mode (~680 cm-1) represents the symmetric stretching of oxygen atoms with respect to the metal ion at the tetrahedral site. There was a sub band to A1g(1) mode (~680 cm-1) at 620 cm-1 and named as A1g(2).

Fig. 10 Raman spectrum of Ti substituted cobalt ferrite nanocrystallites All the five Raman active modes belong to Fd3m phase are present in the spectrum which proves phase purity of the samples prepared.

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Table 4 Raman modes with corresponding intensity Raman shift in cm-1(Intensity in a.u)

Sample A1g(1)

A1g(2)

T1g(2)

T1g(1)

Eg

T1g(3)

CoFe2O4

682(3410)

612(2376) 475(3132) 580(1868) 302(1410) 192(1432)

Co0.975Ti0.025Fe2O4

692(1139)

618(597)

471(745)

572(569)

305(507)

208(378)

Co0.95Ti0.05Fe2O4

691(1044)

622(480)

469(659)

571(499)

309(504)

219(366)

Co0.925Ti0.075Fe2O4

692(840)

613(350)

471(620)

577(476)

299(461)

217(349)

From the above table, the following inference can be made. (1) Compared with the peak intensities of pure cobalt ferrite, all the peak intensities reduce for Ti substituted samples. (2) Intensities of all the peaks decrease for Ti substituted samples with increase in substituent (Ti) concentration. (3) A1g(1) and A1g(2) modes were shifted towards higher wavenumber infer short bond length of Ti-O bond and lower mass of Ti. Shifting of modes T1g(1) and T1g(2) towards lower wavenumber in substituted samples was noted but it was not considerable. Since cations in both tetrahedral and octahedral site rearranged with Ti inclusion, modes are shifted slightly.

Surface enhanced Raman spectroscopy study on Fe3O4 and CoFe2O4 by Guilherme V. M. Jacintho et al [27] reported that modes near 680 and 470 cm -1 have contributions from Fe3+-O motion while modes near 550 and 630 cm -1 have contributions from Co-O motion. In the Raman spectra of substituted samples displayed in fig.3.9, intensity of modes at 575 and 625 cm-1 decreases tremendously whereas modes at 475 and 680 cm-1 remains prominent.

23

This deduces that Ti ions replace Co ions, but since the percentage of replacement could not be known it cannot be predicted that whether few Fe ions are also replaced or not. 3.7 VSM ANALYSIS As the material taken for investigation is a ferrite, it is essential to analyze it using vibrating sample magnetometer. Hysteresis loops of as-prepared pure cobalt ferrite and Ti substituted cobalt ferrite Nano crystals synthesized at room temperature are given in figures 11 (a)-(b). Hysteresis loops of pure and cobalt ferrite Nano crystals modified by titanium recorded under a maximum magnetic field of 15,000 Gauss, exhibit ferrimagnetic property while the area of loop is proportional to the magnetic hysteresis loss. Saturation magnetization (Ms) and coercivity (Hci) was determined to increase whereas there was no much of a change in remanent magnetization (Mr) and squareness ratio (Mr/Ms). Remanent magnetization is also called retentivity when it is measured in units of magnetic flux density. The above measured and calculated values listed in table 5 depend on grain size, shape, preparation technique, type of cations and number of cation existing in tetrahedral and octahedral sites. Shadab Dabagh et al [28]remarked decreasing trend in magnetic measurement after doping CoFe2O4 with Cu-Al ions. Cu-Al replaces both Co and Fe ions and distributed at both sites. In the present study it may be taken that Ti ions replaced Co ions of A (tetrahedral) site or B (octahedral) site or both sites. It was reported that dilution of magnetic moment caused by octahedral accommodation of non-magnetic Ti ions with zero magnetic moment reduced saturation magnetization [11,13,14,18]. Generally in ferrites while Ti prefers octahedral site occupation, magnetic moment of that site dilutes (MB) and so net magnetic moment M= MBMA gets reduced. This was the explanation given by various scientists who synthesized Ti substituted Co ferrite with chemical composition Co1+xTixFe2-2xO4, Co1-xTixFe2−2xO4 and CoFe2−xTixO4. But here we report synthesis of Co1-xTixFe2O4 (x=0, x=0.025, x=0.05 and x=0.075).

24

(a)

(b)

(c)

(d)

Fig. 11 Hysteresis loops of Co1-xTixFe2O4 [a) x=0, b) x=0.025, c) x=0.05 and d) x=0.075]

25

Table 5 Magnetic measurement values of various composition of Ti substituted cobalt ferrite crystallites

S.

Sample

No

D

Ms(emu/g)

(nm)

Mr

Mr/Ms

(emu/g)

Hci

nB(μ B)

(Gauss)

1

CoFe2O4

34

51

25

0.49

1113

2.1425

2

Co0.975Ti0.025Fe2O4

26

71

31

0.44

1443

2.9721

3

Co0.95Ti0.05Fe2O4

27

74

38

0.51

1291

3.1013

4

Co0.925Ti0.075Fe2O4

17

76

33

0.43

1354

3.1889

Saturation magnetization The saturation magnetization (Ms) of pure cobalt ferrite sample is 51emu/g in accordance with earlier report by Xing-Hua Li et al [29]. But some reports like that of V. Vaithyanathan et al [18] recorded Ms for pure cobalt ferrite around 80 emu/g. There is a possibility of loss in saturation magnetization due to surface canting effect as nanocrystallites have smaller size. Substitution of Ti4+ ions for Co2+ ions of cobalt ferrite nanocrystallites have given rise to an increased saturation magnetization. Gradual and considerable increment in Ms was observed with increment in titanium concentration. Ferromagnetic material consists of small regions called magnetic domains in which magnetic moments of each domain align in a particular direction. The net magnetization of these materials is the sum of magnetic moments present in all the domains. When the particle size is below critical value, it behaves as single domain particle. All the spins are aligned in the same direction and so the particle is uniformly magnetized. Therefore total magnetic moment of the sample increased and hence M s increased [30]. In the case of cobalt ferrite nanoparticles the critical size for single domain was reported as 70 nm and it is material dependent [31]. Because in the present study the sizes of as-prepared samples lie in the range 17-34nm there is possibility of a single domain nature. Increasing trend of saturation 26

magnetization infer that spin canting on the surface as-prepared sample could be very less because surface spin disorder reduces the value of Ms. Coercivity Maximum of ~1450 Gauss of coercivity reported in the present experimental condition is greater than that of coercivity reported by Vaithyanathan et al[18] (398 Oe) and slightly less than the value reported by Chae et al (1564 Oe) for Ti substituted cobalt ferrite Nano crystals. Coercivity was found to decrease for Ti substituted for Fe [18] and for Co/Ti co substituted cobalt ferrite [14]. Coercivity of cobalt ferrite is connected with magneto crystalline anisotropy which is mainly attributed to Co2+ ions [32]. There is possibility of reducing magneto crystalline anisotropy in non-magnetic substituted cobalt ferrite when non-magnetic ion replaces Co2+ ions and thus reducing coercivity. But contrary to the above mentioned hypothesis, coercivity slightly increases for Ti doped cobalt ferrite system. Reducing of A-O-B super exchange interaction causes reduction in coercivity was remarked in Ti substituted for Fe site in cobalt ferrite Nano crystals [11,14]. Combination of lower coercivity and higher susceptibility estimated by Shigeoka et al [14] also insist the above hypothesis. In the present investigation, crystallites move towards single domain nature caused by the reduction in crystallite size with Ti substitution should be the reason for enhanced magnetic property. This character of as synthesized magnetic particles can be understood by remanence ratio or squareness ratio (Mr/Ms). The obtained values of remanence ratio of unsubstituted (0.49) and substituted samples (0.44, 0.51, and 0.43) are very close to remanence ratio (0.50) of a system with single domain. Single domain nature is not only proved by Mr/Ms value but also confirmed by critical size of crystallites to have single domain. Single domain nature of ferromagnetic material caused increment in coercivity. The reverse in magnetization will be through spin rotation rather than through the motion of domain walls due to the absence of domain walls.Though there is no perfect correlation between Hci and Mr/Ms could be drawn from VSM results of the present work but from the observation of values listed in table 3.5, it is well confirmed that when crystallite size

27

reduces coercivity increases. Reverse correlation between crystallite size and coercivity is also explained in literature [33]. Magnetic moment Magnetic moment per unit formula nB(μ B) was found by the below equation (9) nB

----- (10)

If cobalt ferrite has inverse spinel structure, cationic distribution will be, (Fe3+)A [Co2+Fe3+]B Magnetic moment of sublattice A (cations in tetrahedral position) M A=5μ B Magnetic moment of sublattice B (cations in octahedral position) MB=3μ B+5μ B=8μ B Net magnetic moment= MB - MA=3μ B If cobalt ferrite has mixed spinel structure, cationic distribution will be, (Co2+Fe3+)A [Co2+Fe3+]B Though both cations present in both sites, net magnetization will not be zero because the number of cations are different in both sites. In the present investigation, calculated magnetic moment per formula unit is nB=2.14 μ B. Enhancement in calculated magnetic moment for substituted crystallites because of increased saturation magnetization was observed in table 3.5.Total magnetic moment of ferrite is equal to the difference in magnetic moments of A and B sublattice i.e., M=M B-MA. Ti4+ ions were substituted for Co2+ ions and occupy both sites; increment in M could be the cause of more number of magnetic ions at octahedral site and almost single domain nature. Exchange interactions In general, magnetic characteristics of spinel ferrites are largely governed by three types of anti-ferromagnetic super exchange interaction, namely, J AA (A–O–A), JAB (A–O–B), and JBB (B–O–B), between cations on the A and B sites, each mediated by oxygen ions. Among them, J AB>> JBB, whereas J AA is small due to the large separation between two A-site 28

ions. Owing to the difference in the size of cations, any alteration in their distribution changes the lattice constant and the oxygen parameter, thereby altering spin interactions that determine JAB [34]. According to Neel model, A-B interaction is the strongest interaction in ferrites A-A interaction is weaker than A-B and B-B interaction is the weakest of all. 3.8 IMPEDANCE ANALYSIS Since it is the electron and the electron spin that accounts for electrical or magnetic property of any material, doping ferrites with magnetic, non-magnetic or atoms with different valence will result in suitable combination of both magnetic and electrical properties. There are reports in literature on impedance study of ferrites [35] and substituted ferrites [36,37] for example frequency dependence ac conductance of CoFe2-xMxO4, x=0.2 with M =Zn, Zr & Cd was studied by Md. Ashiqur Rahman et al [38]. Some important electrical properties of the proposed materials were studied by the impedance measurement procedure using a computer-controlled PSM 1735: N4L impedance analyzer in a wide temperature (room temperature-500oC) and frequency (1-1000 kHz) ranges in air atmosphere. For characterization of these materials a proper relation between microstructure and electrical properties is essential. Complex Impedance Spectroscopy technique is used for simultaneous electrical and dielectric characterization of samples. In impedance spectroscopy the impedance data are generally plotted in complex plane. The variation of real with imaginary part of the impedance is known as Nyquist plots. To analyze the impedance spectra, data usually are modeled by an ideal equivalent circuit consisting of a resistor R and a capacitor C. The experimental impedance data points measured were fitted using software Zswimpwin, with an equivalent circuit. A circuit consists of a series collection of two sub-circuits (consisting of a resistor and capacitor connected in parallel), one representing grain effect and the other representing grain boundaries. In the present study, the impedance measurements have been the main focus for the samples Co1-xTixFe2O4 (x=0.025, 0.05, 0.075). Comparing to magnetic properties of cobalt ferrite and substituted cobalt ferrite, less attention has been given to the study of electrical/dielectric properties of the same. The present work will be useful for various suitability of the material for electrical and electronic applications in different frequency ranges. 29

Cole-Cole plot Cole-Cole plot is plotted between the real and the imaginary part of impedance. Through this plot, resistance and capacitance of microstructure i.e., grain and grain boundary and interfacial resistance of conducting electrodes can be separated. One, two or three semicircles are possible according to the electrical properties of the sample. Fig.12 represents complex impedance plots of pure and titanium modified cobalt ferrite Nanocrystallites. The Two well resolved consecutive semicircles or arcs are seen for pure Cobalt ferrite and also for a series of titanium substituted cobalt ferrite nanoparticles. These two semicircles are due to space charge and orientation polarization in ferrite materials. Left side arc at lower frequency is due to conductivity of grain boundary and right side arc at higher frequency is due to conductivity of grain interior. It is observed that, first a semicircle is formed completely, followed by an arc. It is seen that radius of the semicircle for the grain boundary increases steadily as the concentration of Ti increases and conduction takes place through both grains and grain boundaries [39]. Bode plot Bode plots of are drawn with frequency along X-axis and magnitude of impedance (|Z|) along the Y-axis. Fig. 13 (a), (b), (c) & (d) shows the bode plots for pure and modified Cobalt ferrite Nanocrystallites. Though the grain interior capacitances show very small variations, the grain boundary capacitances show a significant variation, but looking at the overall impedance there is an increase as compared to that of pure CoFe2O4. The nearly constant value for impedance in all the four samples in the frequency range of ≤ 100 kHz restricts its use in only that frequency range as a band pass filter.

30

(a)

(b)

(c)

(d)

Fig. 12 Complex impedance plots of Co1-xTixFe2O4 [(a) x=0, (b) x=0.025, (c) x=0.05 and (d) x=0.075]

31

(a)

(b)

(c)

(d)

Fig. 13 Bode plots of Co1-xTixFe2O4 ((a) x=0, (b) x=0.025, (c) x=0.05 and (d) x=0.075)

32

All the above graphs show plateau region till around 100 kHz and then show rapid variation beyond 100 kHz. The value of impedance gradually keeps increasing with increasing concentration of Ti ions into the lattice. Thus enhancement of impedance with Ti inclusion in cobalt ferrite was evident also from Bode plot. The real part (Z) and imaginary part (Z') of impedance as a function of frequency of Co1-xTixFe2O4 (0.0 ≤ x ≤ 0.075) nanocrystallites are shown in figures 14 and 15.

Fig. 14: Frequency dependent impedance, real part (Z) of Co1-xTixFe2O4 (0.0 ≤ x ≤ 0.075).

33

Fig. 15: Frequency dependent impedance, imaginary part (Z') of Co1-xTixFe2O4 (0.0 ≤ x ≤ 0.075).

Equivalent circuit Complex impedance plot is compared with a circuit consisting of discrete electrical components. This equivalent circuit reveals the physical phenomena taking place inside the system taken. It also represents conduction, charge accumulation and depletion. Circuit contains resistors and capacitors in some cases it also contains inductors. Equivalent circuit describing impedance data of both pure and Ti substituted cobalt ferrite crystallite is given in following fig.16. All the equivalent circuits reported here are based on Electrochemical Impedance Spectroscopy and its Applications by Andrzej Lasia et al [40].

34

Fig. 16 Equivalent circuit for the complex impedance plots

Calculation The complex impedance Z(ω) of a system at an applied frequency (ω) is equal to the adding up of real and imaginary part as given below [41]. z(ω)=z‫׳‬+jz″

………(11)

Where Z‫ ׳‬and Z″ can be written as, z‫= ׳‬

+

…………(12)

z″=

+

…………(13)

Where Rg and Rgb are resistances from grain and grain boundary correspondingly which are nothing but the impedance at the intersection of semicircle or arc with the X-axis. ωg and ωgb are frequencies from grain and grain boundary respectively which are frequencies at the peak of semicircles. Capacitor is a component of a circuit through which a constant current flow is not possible, but electrical charge can accumulate in it, and accumulation is different at different applied voltages. The measure of electrical charge accumulation in a capacitor is called capacitance. Cg and Cgb are capacitances from grain and grain boundary respectively which are determined using following equation: Cg

……………...(14) 35

Cgb

………………(15)

Relaxation time or time constant for grain boundary (τgb) and grain (τg) corresponding to frequencies of grain and grain boundary were calculated using the following equations: τgb

.....…. (16)

τg

......…..(17)

The resistance from grain boundary (R gb) and grain (Rg), capacitance from grain boundary (C gb) and grain (Cg) and relaxation time of grain boundary (τgb) and grain (τg) are given below. Figure 17 shown the Plots of grain and grain boundary resistance vs. Ti concentration, grain and grain boundary capacitance vs. Ti concentration and grain and grain boundary relaxation time vs. Ti concentration

Table 6: Measured data from impedance analysis

Sample

Rgb

Cgb

τgb

Rg

Cg

τg

Z‫׳‬

(kΩ)

(nF)

(μs)

(kΩ)

(nF)

(μs)

(kΩ)

CoFe2O4

59.078

0.268 15.849

8.706

22.918

1.995

73.069

Co0.975Ti0.025Fe2O4

123.523

0.181 22.387

17.756

17.810

3.162

150.542

Co0.95Ti0.05Fe2O4

150.709

0.167 25.119

23.526

16.922

3.981

192.987

Co0.925Ti0.075Fe2O4

185.360

0.136 25.119

28.934

21.807

6.309

237.350

36

The relaxation times for the grain boundary and grains are significantly different. While the relaxation time for grain boundaries does not show much variation, a significant change is observed in relaxation time for the grain interior at high frequency. There are two important factors that contribute to the observed variations in impedance spectra analysis. The first is the creation of oxygen vacancies upon Ti substitution which may significantly contribute towards dielectric relaxation. The second is the size effect which is primarily due to the first factor. Electrical resistivity is very sensitive to lattice imperfections and vacancies. Oxygen vacancies may contribute in more than one way in changing the dielectric relaxation. The excess electron hopping in the lattice may result in dipole moment reorientation or creating conducting electrons through ionization that may cause electron hopping from site to site or different levels of oxygen deficiency in different spatial region leading to interfacial polarization (Maxwell-Wagner space charge polarization). Size effect like grain size reduction upon doping will result in decreased grain conductivity.

37

Fig.17: Plots of (a) grain and grain boundary resistance vs. Ti concentration, (b) grain and grain boundary capacitance vs. Ti concentration and (c) grain and grain boundary relaxation time vs. Ti concentration

38

Electrical conductivity in cobalt ferrite is because of hopping of electrons and holes generally occurring at octahedral site. Hopping of electrons between Fe2+↔Fe3+ has greater role than hopping of holes between Co3+↔Co2+ in the conduction process as Co ions have less mobility than Fe ions. Existence of cobalt in two valence states was proposed by Parker et al (1966). So conduction is mainly dependent on electron hopping. Conductivity increases with multiplication of Fe2+↔Fe3+ pair at octahedral site. No exchange interaction takes place between tetrahedral sites (A-A) because Fe2+ prefers to occupy octahedral site. So definitely reduction of Fe2+↔Fe3+ pairs at octahedral site can possibly result in the migration of Fe3+ ions from B to A site [42]. In addition to the above reason, titanium has single valency (+4) and it does not take part in exchange interaction. Electron or hole interaction occurs only between ions of same element which has multiple valence. So decreasing cobalt concentration and adding Ti4+ will definitely affect exchange interaction and as a result conductivity decreases. Non-magnetic Ti ion occupies octahedral site or both sites and so the interaction between tetrahedral and octahedral site reduces which results in weakening of interaction between ions. Hence conduction is reduced. Overall, the impedance and resistivity increased with cobalt ferrite nanocrystallites increased with increment in Ti substitution. 4. CONCLUSION Without distorting the spinel structure Ti was successfully incorporated into cobalt ferrite nanocrystallites by sol-gel method. Various concentration of titanium was substituted in cobalt ferrite and structural, magnetic and impedance properties were analyzed. XRD revealed cubic inverse spinel structure of as-synthesized crystallites, crystallite size reduced from 34 nm (pure CoFe2O4) to 17 nm (Co0.925Ti0.075Fe2O4). SEM and TEM characterization exposed spherical morphology and grain size is in agreement with those of calculated from XRD method. SAED pattern confirmed the presence of planes that of spinel structure observed in XRD technique. Minimizing of Raman modes (575 cm -1, 620 cm-1) correspond to Co-O motion inferred Ti ions replaced cobalt ions. Magnetic moment, saturation magnetization, retentivity and coercivity increased with Ti substitution due to Ti occupation 39

at tetrahedral site and also due to small crystallite size. The maximum coercivity of 1444 Gauss was observed for the sample with composition Co0.975Ti0.025Fe2O4 of 26 nm size and maximum saturation magnetization was observed for the crystallites of 17 nm (Co0.925 Ti0.075Fe2O4). Enhancement in saturation and magnetization made the sample to be used in magnetic recording. Impedance enhanced considerably since exchange interaction was weakened by Ti inclusion. The impedance steadily increased with increase in the concentration of Ti, but remained constant in the entire frequency range of 1Hz to 100 kHz, which restricts its use to less than 1MHz. Highest impedance was noticed when more Ti ions are substituted (Co0.925Ti0.075Fe2O4). Acknowledgement The authors are thankful to SAIF, IITM, Chennai for providing HRSEM analysis.

40

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Highlights  TixCo1-xFe2O4 with (x=0-0.075) via the sol-gel route is reported.  The effects of titanium substitution on the magnetic and electrical properties of cobalt ferrite are reported.  Crystallite size reduced from 34 nm (pure CoFe2O4) to 17 nm (Ti0.075Co0.925Fe2O4).  Minimizing of Raman modes (575 cm-1, 620 cm-1) correspond to Co-O motion inferred Ti ions replaced cobalt ions.  Magnetic moment, saturation magnetization, retentivity and coercivity increased with Ti substitution.  Impedance enhanced considerably since exchange interaction was weakened by Ti inclusion.

46