https://doi.org/10.52131/jmps.2023.0401.0033 20 Journal of Materials and Physical Sciences Volume 4, Number 1, 2023, Pages 20 - 31 Journal Homepage: https://journals.internationalrasd.org/index.php/jmps Exploring Study of Magnetic and Electrical Properties of Tl3+ Doped Co0.5Ni0.5Fe2O4 Spinel Ferrites Touseef Ahmad1, M.U. Islam1*, I.H Gul2, Mutawara Mahmood Baig3, M. Ajmal1 1 Institute of Physics, Bahauddin Zakariya University, Multan, Pakistan 2 School of Chemical & Materials Engineering (SCME), NUST, Islamabad, Pakistan 3 School of Chemistry and Chemical Engineering, Yangzhou University, Yangzhou, Jiangsu, 225009 P. R. China ARTICLE INFO ABSTRACT Article History: Received: February 26, 2023 Revised: April 29, 2023 Accepted: May 06, 2023 Available Online: June 28, 2023 Co0.5Ni0.5Fe2-xTlxO4 (x=0, 0.05, 0.1, 0.15, and 0.2) spinel ferrites were prepared via the sol-gel technique. The XRD analysis revealed a single-phase spinel structure. The lattice constant ‘a’ increased from 8.223-8.269Å with doping of Tl+3 due to larger ionic radii of Tl+3 than Fe+3 ions. The mass susceptibility at 300K decreased from 7.46 x 10-3-4.15 x 10-3 cm3/g due tothe weakening of AB interactions followed by a decrease in Curie temperature from 453K to 408K. The electrical permittivity follows the Maxwell-Wagner model and Koop’s theory. The dc resistivity of ferrites at 300K increased from 1.82x109-9.23x109 Ω-cm with increasing Tl+3 contents due to increased hopping lengths. The activation energy obtained from Arrhenius plots increased from 0.126 to 0.131eV, increasing Tl+3contents. Keywords: Sol-Gel Auto Combustion Spinel Ferrites X-Ray Diffraction Magnetic Susceptibility Dielectric Properties Resistivity © 2023 The Authors, Published by iRASD. This is an Open Access article under the Creative Common Attribution Non-Commercial 4.0 *Corresponding Author’s Email: dr.misbahulislam@bzu.edu.pk 1. Introduction Spinel ferrites have got prime importance due to their versatile properties. These magnetic materials have high resistivity (Goldman, 2006; Jalaiah, Mouli, Krishnaiah, Babu, & Rao, 2019) and low eddy current losses. The nanoparticles of spinel ferrites exhibit novel properties that are useful in biomedical applications. Spinel ferrites are low-cost and easy to fabricate for various engineering applications like recording media, microwave absorbers, sensors, etc (Kazimierczuk, 2009). Amongst the various methods available for the preparation of sol-gel technique is low cost and easy to handle. The homogeneous distribution of nanoparticles is obtained using the sol-gel method (Batoo, Kumar, & Lee, 2009; Cullity & Graham, 2011). Co-Ni spinel ferrites have attracted the attention of materials scientists (Abdel-Latif, 2012; Abdul-Aziz, Abrahem, & Khaleel, 2013; Asghar et al., 2018; Singha, Singhb, & Dosanjha, 2015; Velhal, Patil, Shelke, Deshpande, & Puri, 2015) due to high saturation magnetization and Curie temperature. Since Co is more anisotropic (Kambale, Shaikh, Kamble, & Kolekar, 2009; Ortiz-Quiñonez, Pal, & Villanueva, 2018) compared to Ni, Co-Ni spinel ferrites become superior to other spinels. The substitution of transition metal ions in the Co-Ni ferrites may augment the magnetic properties. Velhal et al.(Velhal et al., 2015) reported the Ni-doped Co spinel ferrites via low- temperature auto-combustion technique and reported that increasing the Ni content in Cobalt ferrite decreases the magnetization because of the lower magnetic moment of nickel than cobalt. Gaffoor et al.(Gaffour & Ravinder, 2014) synthesized the Co-doped NiFe2O4 spinel ferrites by the sol-gel route. They reported that increasing the Cobalt content in NiFe2O4 increases the lattice parameter and crystallite size. Amirabadizadeh et al.(Amirabadizadeh & Amirabadi, 2013) synthesized the Al-doped Ni0.6Co0.4Fe2O4 spinel ferrites via the sol-gel route. They reported that by increasing the content of Al+3 (0-0.7) in https://journals.internationalrasd.org/index.php/jmps mailto:dr.misbahulislam@bzu.edu.pk Touseef Ahmad, M.U. Islam, I.H Gul, Mutawara Mahmood Baig, M. Ajmal 21 Ni-Co spinel ferrites, the crystallite size decreases from 29nm to 10nm. Similarly, the saturation magnetization decreases from 61-10 emu/g with increasing Al+3 content in Ni-Co spinel ferrites. Farid et al.(M. Farid et al., 2015)reported the effect of Nd on the electrical properties of Ni-Co spinel ferrites prepared by the sol-gel route and observed that both the dc resistivity and activation energy increase with Nd doping in Ni-Co spinel ferrites. In the present work, Tl-substituted Co0.5Ni0.5Fe2O4ferrites are prepared by the sol-gel technique. To the best of our knowledge, the Tl-doped Co-Ni spinel ferrites have yet to be reported frequently in the literature. There was a need to investigate the Tl-doped Co-Ni ferrites. 2. Experimental Procedure Thallium (Tl) doped Co0.5Ni0.5Fe2O4 spinel ferrites were prepared via the sol-gel auto- combustion method. Analytical grade starting materials were used; Fe(NO3)3.9H2O, TlNO3, Ni(NO3)3.6H2O, Co(NO3)3.6H2O, and citric acid. Stock solutions of all the nitrate salts and citric acid were prepared in 100 mL of distilled water in a beaker. These stock solutions were added to a 1000 mL beaker and stirred at 70oC continuously to obtain a homogeneous solution. Ammonium hydroxide was added dropwise in the solution to maintain the pH =7. After complete evaporation, a thick gel is formed; stirring was stopped, and the gel was burnt at about 400oC by the auto combustion process. The complete combustion ofthe ash gives ferrite powder (be implemented from June; Kumar et al., 2015; Sajjadi, 2005). The ash was ground and sintered at 1050oC for 5 hours in a box furnace. The following Fig. (1) shows the steps for the sol-gel method. Stirring Stirring + heating Ammonia solution At 70oC drop wise Heated at 400oC Figure 1: Flow chart of sol-gel process 2.1. Characterizations Techniques 2.1.1. X-ray Diffraction X-ray diffraction is used to determine the phase purity of samples; d-values, lattice constant, and hopping length may be determined from XRD data. Bragg's law is used to determine the d-values of a specimen. Bragg’s law is given below (de Almeida et al.; Suryanarayana, Norton, Suryanarayana, & Norton, 1998): 2𝑑𝑠𝑖𝑛𝜃 = 𝑛𝜆 (1) Where 𝜃 is the Bragg angle, and d is the inter-planer spacing. The lattice constant is determined by using the following equation (Kumar et al., 2015): Stock solutions of nitrate salts + Stock solution of citric acid Homogeneous solution Gel format ion Auto Combustio n Evaporation Ground for 30 mints Ash formation Ferrite powder Journal of Materials and Physical Sciences 4(1), 2023 22 𝑎 = 𝜆 2𝑠𝑖𝑛𝜃 √ℎ2 + 𝑘2 + 𝑙2 (2) Where 𝑎= lattice constant, λ= 1.54Å and 𝜃= Bragg’s angle, and hkl are Miller indices. The volume of the spinel unit cell is determined by using the formula: 𝑉 = 𝑎3(Å3) (3) The distance between magnetic ions or hopping length is calculated by using the formulae (BABBAR, 1997): 𝐿𝐴 = 0.25 𝑎 √3 (4) 𝐿𝐵 = 0.25 𝑎 √2 (5) Where LA= Hopping length in A-site, LB= Hopping length in B-site, and ‘𝑎’ is lattice constant. 2.1.2. Magnetic Susceptibility The term magnetic susceptibility (χ) of magnetic materials is defined as the ratio of magnetization (M) to magnetic field intensity (H) (Nikam et al., 2015): 𝜒 = 𝑀 𝐻 (6) The magnetic susceptibility of ferrites also depends on the temperature, called the Curie-Weiss law. At Curie temperature, ferrites' magnetic susceptibility becomes constant, which is paramagnetic (Callister Jr, 2007). 2.1.3. Construction of Colpitts Oscillator as Susceptometer The LC oscillator can be used to find the magnetic susceptibility. The tank circuit of LC oscillators consists of two capacitors and an inductor. When the oscillator works, the solenoid's magnetic field is set up. When a magnetic material is put in the solenoid, it gets magnetized. The magnetic field strength is increased, and hence the inductance of the solenoid is increased (Kazimierczuk, 2009; Properties). The frequency of the LC oscillator is determined as (Boylestad & Nashelsky, 2009): 𝑓 = 1 2𝜋√𝐿𝐶 (7) Since the capacitance (C) is constant, the frequency of LC oscillator is inversely proportional to the square root of inductance: 𝑓 ∝ 1 √𝐿 (8) The inductance of the solenoid is increased when the ferrite sample is placed in the solenoid field, hence the frequency of LC oscillator is decreased. The circuit diagram for the Colpitts oscillator as susceptometer is shown in Fig.(2). The susceptometer was constructed using the following circuit diagram to measure the magnetic susceptibility. The tank circuit of the Colpitts oscillator consists of two capacitors and an inductor in the form of a solenoid. The frequency of the Colpitts oscillator is determined as follows: 𝑓 = 1 2𝜋√𝐿𝐶𝑒𝑞 (9) Where, 𝐶𝑒𝑞 = 𝐶1𝐶2 𝐶1+𝐶2 (10) Touseef Ahmad, M.U. Islam, I.H Gul, Mutawara Mahmood Baig, M. Ajmal 23 Figure 2: Colpitts Oscillator as Susceptometer circuit diagram The values of C1 and C2 were 0.33µF, and the solenoid had 550 turns twisted on ceramic paper with copper wire gauge 26 SWG. The length of the solenoid was 5cm, and the volume was 55 cm3. The op-amp as inverting amplifier was used for oscillations because the tank circuit provides the 180o phase inversion, and the 180o phase inversion is also provided by inverting amplifier. Thus the total phase inversion is 360o, which satisfies the Barkhausen criterion condition for oscillations. When the sample is inserted into the magnetic field of the solenoid, the frequency changes; this change in frequency was converted into magnetic susceptibility using the following equation (Figueroa et al., 2012; Vannette, 2009): ∆𝑓 𝑓 = − 1 2 𝑉𝑆 𝑉𝐶 4𝜋𝜒 (11) Where f = frequency of oscillator without sample, VS = volume of sample, VC= volume of a coil (solenoid), χ = magnetic susceptibility. The ∆f changes in the oscillator frequency: ∆𝑓 = 𝑓 / − 𝑓, Where f/ = oscillator frequency with a sample. The change in frequency (∆𝑓) with the ferrite sample is negative. This negative sign is canceled with the negative sign of the right-hand side of equation (7); hence the magnetic susceptibility is positive for ferrites. The volume magnetic susceptibility (χ) of spinel ferrites was determined by: 𝜒 = − 2∆𝑓 𝑓 𝑉𝐶 4𝜋𝑉𝑆 (12) The volume magnetic susceptibility (dimensionless) is converted to mass susceptibility (χm) as (Cullity & Graham, 2011): 𝜒𝑚 = 𝜒 𝜌 ( 𝑚3 𝑘𝑔 ) (13) 𝜒𝑚 = 𝜒 𝜌 𝑋 1000 4𝜋 ( 𝑐𝑚3 𝑔 ) (14) Journal of Materials and Physical Sciences 4(1), 2023 24 Where ρ= mass/volume = bulk density of the specimen. The mass susceptibility (χm) was converted into molar susceptibility (χmol) as (Raghasudha, Ravinder, & Veerasomaiah, 2013): 𝜒𝑚𝑜𝑙 = 𝜒𝑚 𝑋 𝑀𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑤𝑒𝑖𝑔ℎ𝑡 ( 𝑐𝑚3 𝑚𝑜𝑙 ) (15) 2.1.4. Dielectric Study The capacitance and dissipation of spinel ferrite specimens were measured through LCR model 8108 with a frequency range 20 Hz to 1 MHz. The capacitance of the specimen was converted to dielectric constant (ε/) by using the following formula (Pervaiz & Gul, 2012): ′ = 𝐶𝑡 𝜀𝑜𝐴 (16) Where ε’ = dielectric constant, εo= permittivity of free space (8.85 x 10-12 F/m), C= capacitance of the sample, and t = pellet thickness. The dielectric loss was measured from the following equation: ′′ = ′ 𝑡𝑎𝑛𝛿 (17) Where ε’’ = dielectric loss and tan𝛿 = loss tangent. 2.1.5. Resistivity Measurement The well-known two-probe method was used to find the resistivity of spinel ferrites. For this purpose, the Keithley source meter model 2400 was used. The temperature varied from 300 to 520K during the measurements. The following relation was used to find the resistivity of spinel ferrites (Arshad et al., 2018): 𝜌 = 𝑅 𝐴 𝑡 (18) Where ρ= resistivity of pellet, R= resistance of pellet, A= area of the pellet, and t = thickness of the pellet. The resistance (R) was measured from the slope of the I-V graph. 2.1.6. Activation Energy The activation energy of spinel ferrites was determined by using the Arrhenius equation given below (Ramarao et al., 2018): 𝜌 = 𝜌𝑜 exp ( 𝐸𝑎 𝑘𝐵𝑇 ) (19) Where ρ is the resistivity of spinel ferrite at temperature T, ρo= resistivity of the sample at room temperature, Ea = activation energy, and kB = 1.38066 x 10-23 J/k (Boltzmann’s constant). By solving the equation (17), the activation energy was determined as(Devmunde et al., 2016): 𝐸𝑎 = 0.1987 𝑋𝑠𝑙𝑜𝑝𝑒𝑜𝑓𝑡ℎ𝑒 𝑔𝑟𝑎𝑝ℎ (𝑒𝑉) (20) 3. Results and Discussion 3.1. X-ray Diffraction Fig. 3 shows the XRD patterns of Co0.5Ni0.5TlxFe2-xO4 spinel ferrites with x= 0.0, 0.05, 0.1, 0.15, and 0.2. The XRD analysis of Tl-doped CoNi ferrites prepared by sol-gel auto combustion method and sintered at 1050oC for 5hours revealed the cubic spinel structure. The peaks were identified by comparing the d-values with JCPDS card No. 21-1152. Five peaks characteristic of the fcc structure were observed (220). (311), (400), (422), and (333) with no extra peak that revealed the single-phase cubic spinel structure of Tl-doped CoNi ferrites (Asghar et al., 2018). This is because the Tl is soluble in the lattice, which is substituted in small amounts with interval x=0.05. Since the minute amount of Tl is substituted, no shift in the XRD peaks is observed even though the ionic radius of Tl is larger than Fe. The lattice constant of Tl doped ferrites and unit cell volume are given in Table 1. Touseef Ahmad, M.U. Islam, I.H Gul, Mutawara Mahmood Baig, M. Ajmal 25 20 25 30 35 40 45 50 55 60 0 100 200 300 400 500 600 700 In te n s it y ( a .u .) 2 (Degrees) (220) (311) (400) (422) (333) x=0.2 x=0.15 x=0.1 x=0.05 x=0 Figure 3: XRD patterns of Co0.5Ni0.5TlxFe2-xO4 (x=0.0-0.2) Table 1 XRD parameter of ferrite compostions with varied Thallium content Thallium content (x) Ferrite composition a(Å) Unit cell volume (Å3) LA (Å) LB(Å) 0.0 Co0.5Ni0.5Fe2O4 8.223 556.02 3.561 2.907 0.05 Co0.5Ni0.5Fe1.95Tl0.5O4 8.235 558.46 3.566 2.911 0.10 Co0.5Ni0.5Fe1.9Tl0.1O4 8.245 560.5 3.570 2.915 0.15 Co0.5Ni0.5Fe1.85Tl0.15O4 8.258 563.15 3.576 2.919 0.2 Co0.5Ni0.5Fe1.8Tl0.2O4 8.269 565.4 3.581 2.923 The lattice constant of Co-Ni ferrites increased from 8.223 to 8.269Å with the doping of Tl+3 ions in CoNi ferrites because of the larger ionic radius of the Tl+3 ions (1.025Å) than that of Fe+3 ions (0.67Å) as shown in Fig (4). 0.00 0.05 0.10 0.15 0.20 8.22 8.23 8.24 8.25 8.26 8.27 a (Å ) Tl content (x) Journal of Materials and Physical Sciences 4(1), 2023 26 Figure 4: Variation of lattice constant ‘a’ Vs Tl concentration for Co0.5Ni0.5TlxFe2-xO4 (x=0.0-0.2) ferrites Lattice constantly followed Vegard’s law (Suryanarayana et al., 1998). The hopping lengths LA and LB were calculated using equations (4) and (5) and are listed in Table 1. It is observed that the hopping lengths LA and LB increase due to lattice distortion produced when Tl is substituted. 3.2. Magnetic Susceptibility The temperature-dependent magnetic susceptibility of Tl-doped Co-Ni spinel ferrites was determined through the Colpitts oscillator susceptometer at 10 kHz frequency in the temperature range 290-520K.Fig.5shows the temperature-dependent magnetic susceptibility of Tl-doped Co-Ni ferrites.Fig.5 shows that the maximum magnetic susceptibility is 7.46x10-3 cm3/g for a sample with x=0, and the minimum magnetic susceptibility is for the specimen with x=0.2. The magnetic susceptibility continuously decreases with increasing Tl contents in ferrite. There are two sublattices in spinel ferrite called A and B-sublattice. Both the sublattices are antiferromagnetically aligned. The total magnetization (M) of spinel ferrites is given by equation (Shinde, 2016): 𝑀 = 𝑀𝐵 − 𝑀𝐴 (21) where MB= magnetization of B-sublattice and MA= magnetization of A-sublattice. 300 350 400 450 500 550 0.000 0.002 0.004 0.006 0.008  ( c m 3 /g ) Temperatute (K) x=0 x=0.05 x=0.1 x=0.15 x=0.2 Figure 5: Magnetic susceptibility Vs temperature for Co0.5Ni0.5TlxFe2-xO4 (x=0.0- 0.2) According to Neel's theory of ferrimagnetism, the A-B interactions are strongest in spinel ferrites than in A-A and B-B interactions. The Tl is non-magnetic and prefers B-sites due to its larger ionic radius (1.03Å) (Goldman, 2006). By increasing the content of Tl+3 ions in Co-Ni spinel ferrite the magnetization of B-sublattice (MB) decreased. Hence the total magnetization (M) is decreased. Thus due to the decrease of magnetization, the magnetic susceptibility is decreased with increasing content of Tl+3 ions. The magnetic susceptibility at room temperature, effective magnetic moments, and Curie temperature of ferrite specimens are given in Table 2. The Curie temperature of Co-Ni ferrite decreases with the increasing content of Tl+3 ions. The A-B interactions decrease with increasing Tl content in CoNi ferrites which causes the decrease in magnetization. Whereas the Curie temperature decreases with Tl contents. Fig.5 also shows that the magnetic susceptibility decreases with temperature. At lower temperatures, ferrites' thermal atomic vibrations are small, and the coupling of magnetic forces is large. In ferrites, the magnetic moments are aligned due to coupling magnetic forces. The susceptibility is high at lower temperatures due to large Touseef Ahmad, M.U. Islam, I.H Gul, Mutawara Mahmood Baig, M. Ajmal 27 coupling magnetic forces. When the temperature is increased, the thermal atomic vibrations are increased and dominate the magnetic coupling forces that disturb the alignment of domains in ferrites. Thus the magnetic susceptibility decreases with temperature. The magnetic susceptibility is dropped to a minimum value above Curie temperature. The domains are oriented randomly above Curie temperature and the ferrite sample becomes paramagnetic (Callister Jr, 2007; Kazimierczuk, 2009). Fig.6 (a) & (b) show the plots of Tl content Vs magnetic susceptibilities at 300K and Curie temperatures, respectively. Table 2 Magnetic susceptibility vs temperature content of prepared samples Composition cm3/g) Curie Temp. Tc (K) µeff mol(cm 3/mol) Co0.5Ni0.5Fe2O4 7.46X10 -3 453 63.47 1.749 Co0.5Ni0.5Fe1.95Tl0.5O4 6.63 450 60.78 1.604 Co0.5Ni0.5Fe1.9Tl0.1O4 5.80 438 57.71 1.446 Co0.5Ni0.5Fe1.85Tl0.15O4 4.97 423 54.21 1.276 Co0.5Ni0.5Fe1.8Tl0.2O4 4.15 408 50.27 1.097 0.00 0.05 0.10 0.15 0.20 0.004 0.005 0.006 0.007 0.008 0.00 0.05 0.10 0.15 0.20 410 420 430 440 450 460  ( c m 3 /g ) Tl content (x) (a) T C (K ) Tl content (x) (b) Figure 6: (a) Plot of Tl content Vs Magnetic susceptibility at 300K for Co0.5Ni0.5TlxFe2-xO4 (x=0.0-0.2) ferrites (b) Plot of Tl content Vs Curie temperature of Co-Ni ferrites The effective magnetic moment (µeff) of Co0.5Ni0.5TlxFe2-xO4 (x=0.0-0.2) spinel ferrites was calculated using the following formula [28, 34](Zatsiupa et al., 2014): µ𝑒𝑓𝑓 = √ 3𝑘𝐵 𝑁𝐴µ𝐵 2 𝑋 √𝜒𝑚𝑜𝑙 𝑇(µ𝐵 ) (22) µ𝑒𝑓𝑓 = 2.828√𝜒𝑚𝑜𝑙 𝑇(µ𝐵 ) (23) Where µeff = Effective magnetic moment in Bohr magnetrons, kB= Boltzmann constant (1.38 x 10-16 erg/K), NA= Avogadro’s number (6.022 x 1023), and µB= Bohr magnetron (9.274 x 10-21 erg/G). The effective magnetic moment was observed to decrease with doping non-magnetic (Tl+3) ions in Co-Ni spinel ferrites, as listed in Table2. 3.3. Dielectric Constant The frequency-dependent dielectric properties were determined in the frequency range from 20Hz to 1MHz using equations (16) & (17). Fig.(7a) shows the plot of dielectric constant vs frequency. The dielectric constant (ε/) and dielectric loss (ε//) were observed to decrease with frequency and also observed to decrease with the concentration of Tl+3 ions. The interfacial polarization and conductivity of ferrites are due to the presence of Fe+3 and Fe+2 cations on the B-sites. With increasing the Tl+3 ions in Co-Ni ferrites, the content of Fe+3 ions decreases on the B-sites because the Tl+3 ions prefer B-sites due to their larger Journal of Materials and Physical Sciences 4(1), 2023 28 ionic radius. Thus the interfacial polarization is reduced, causing the dielectric constant's value to drop (M. T. Farid, Ahmad, Murtaza, Ali, & Ahmad, 2016). According to the Maxwell-Wagner model (1951) (Batoo et al., 2009), ferrites consist of two layers; the conducting grains and highly resistive grain boundaries. At lower frequencies, the resistive grain boundaries are more active and resist electrons hopping between Fe+2 and Fe+3 cations, causing interfacial polarization. At higher frequencies, the conducting grains are more active and allow the hopping of electrons from Fe+2 to Fe+3 cations; thereby, polarization decreases. Thus, the dielectric constant is decreased at higher frequencies (Batoo et al., 2009; Devmunde et al., 2016). According to Maxwell-Wagner theory, the dielectric loss is also reduced due to decreased space charge polarization, as shown in Fig. (7b). 0.00 2.50x10 5 5.00x10 5 7.50x10 5 1.00x10 6 0 200 400 600 800 1000 1200 1400 0.00 2.50x10 5 5.00x10 5 7.50x10 5 1.00x10 6 0 200 400 600 800  / Frequency (Hz) x=0 x=0.05 x=0.1 x=0.15 x=0.2 (a) 0 20000 40000 60000 80000 100000 0 200 400 600 800 1000 1200 1400  / Frequency (Hz)  // Frequency (Hz) x=0 x=0.05 x=0.1 x=0.15 x=0.2 (b) 0.0 2.0x10 4 4.0x10 4 6.0x10 4 8.0x10 4 1.0x10 5 0 200 400 600 800  // Frequency (Hz) Figure 7: (a) Dielectric constant Vs frequency for Tl doped Co0.5Ni0.5Fe2O4 ferrites (b) Dielectric loss Vs frequency for Tl doped Co0.5Ni0.5Fe2O4 ferrites 3.4. DC Resistivity Fig. (8) shows the Arrhenius resistivity plots for Tl doped Co0.5Ni0.5Fe2O4 spinel ferrites measured by two probe methods. The resistivity of Co-Ni spinel ferrites was observed to increase with the doping of Tl+3 cations (Fig. (9a)). 2.0 2.5 3.0 3.5 12 14 16 18 20 22 24 In  (  -c m )→ T/1000 (K) → x=0 x=0.05 x=0.1 x=0.15 x=0.2 Figure 8: Arrhenius plots for Co0.5Ni0.5TlxFe2-xO4 ferrites Touseef Ahmad, M.U. Islam, I.H Gul, Mutawara Mahmood Baig, M. Ajmal 29 According to Verway’s mechanism(Arshad et al., 2018), the conductivity of ferrites is due to the presence of Fe+2 and Fe+3 cations on the B-site. As the Tl+3 contentsare increased, the concentration of Fe+3 cations decrease on the B-sites. The hopping length in B-site (LB) was found to increase with the doping of Tl+3 ions, as mentioned in Table 1. Thus the hopping of electrons between Fe+2 and Fe+3 cations decreases. So the resistivity of Co- Ni spinel ferrite is increased with doping Tl+3. The resistivity of Tl-doped Co0.5Ni0.5Fe2O4 was also observed to decrease with increasing temperature, confirming these ferrites' semiconducting behavior (Bhandare, Jamadar, Pathan, Chougule, & Shaikh, 2011; Pervaiz & Gul, 2012). The activation energy of spinel ferrites was determined from Arrhenius plots as shown in Fig.(9b). The activation energy of Co-Ni spinel ferrite increases with the doping of Tl+3 ions according to Verway’s hopping mechanism (Devmunde et al., 2016)—the plot of activation energy Vs. Tl contents follow the room temperature resistivity as shown in Fig.(9b). 0.00 0.05 0.10 0.15 0.20 0.25 2.0x10 9 4.0x10 9 6.0x10 9 8.0x10 9 1.0x10 10 0.00 0.05 0.10 0.15 0.20 0.25 0.126 0.128 0.130 0.132  (  - c m ) Tl Content (x) (a) E a ( e V ) Composition of Tl (x) (b) Figure 9: (a) Variation in resistivity Vs Tl content for Co0.5Ni0.5TlxFe2-xO4 ferrites (b) Plot of activation energy vs Tl content for Co0.5Ni0.5TlxFe2-xO4 ferrites Conclusions The lattice constant was observed to increase from 8.223 to 8.269 Å.The hopping length in the A-sites was found to increase from 3.561 to 3.581 Å, andin the B-sites increased from 2.907 to 2.923Å with doping Tl+3 ions.The magnetic susceptibility at room temperature decreased from 7.46 x 10-3 to 4.15 x 10-3 cm3/g, increasing the Tl content from 0 to 0.2. The effective magnetic moment was found to decrease from 63.47 to 50.27 B with doping Tl+3 ions in Co-Ni spinel ferrites. 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