Effect of boron content on structure and magnetic properties in CoFe2O4 spinel nanocrystals

Journal of Alloys and Compounds 744 (2018) 528e534

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Effect of boron content on structure and magnetic properties in CoFe2O4 spinel nanocrystals _  b, Ahmet Ekicibil b Mustafa Akyol a, *, Idris Adanur b, Ali Osman Ayas¸ c, Faruk Karadag a

Department of Materials Engineering, Adana Science and Technology University, Adana, 01250, Turkey Department of Physics, Çukurova University, 01330, Adana, Turkey c Department of Mechatronics Engineering, Adıyaman University, Adıyaman, 02040, Turkey b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 8 November 2017 Received in revised form 8 February 2018 Accepted 10 February 2018 Available online 12 February 2018

We study the effect of boron content on the structural and magnetic properties of CoFe2O4 spinel nanocrystallines synthesized by sol-gel method. The crystal structure and phase identification of samples are studied by using X-ray diffraction experiment and Rietveld analysis. Rietveld refinement results reveal that all samples have cubic symmetry with space group Fd3m. The cationic distributions are obtained from Rietveld refinement that boron ions are settled into both tetrahedral and octahedral sites in spinel lattice. The crystallite sizes of samples are found in a range of 47e67 nm that is in the limit of single domain in such structure. All samples show ferromagnetic nature and magnetic transition was not seen in the temperature range of 5e400 K. The magnetic domains are pinned with adding boron ions into the CoFe2O4 spinel structure at low temperatures. Thus, an increment in the propagation field (Hp) and temperature (Tp) by boron content in CoFe2O4 structure is observed. In addition, the saturation magnetization (Ms) normalized by crystal size increases with increasing boron concentration. The temperature dependence of magnetic properties of the samples taken by experimental data are confirmed with the Neel-Arhenius model by adding thermal dependence of magnetocrystalline anisotropy term. The results indicate that boron-doping into the spinel structure enhances ferromagnetic coupling and suppresses super-exchange interaction between tetrahedral (X) and octahedral (Y) sites. © 2018 Elsevier B.V. All rights reserved.

Keywords: Spinel Magnetism Boron Nanocrystal

1. Introduction Magnetic spinel oxides chemically formulized as XY2O4 where X is a divalent cation the occupier of tetrahedral sites and Y is a trivalent cation the occupier of octahedral sites, have great attention because of their physical interest and important applications in high temperature ceramics, catalysis, semiconductors, electrochemical sensors, biomedical materials, active components of ferrofluids and several technological applications in nanotechnology [1e7]. An interesting aspect of spinel ferrite nanoparticles is their unique magnetic properties, due to controlling of magnetic response by the particle size and shape. Although there are various spinel nanoparticle materials used in biomedicine and magnetic recording, much number of researches have done to understand of nanomagnetic ferrites. Cobalt ferrite (CoFe2O4) has commonly used

* Corresponding author. Adana Science and Technology University, Department of Materials Engineering, 01250, Sarıçam, Adana, Turkey. E-mail address: [email protected] (M. Akyol). https://doi.org/10.1016/j.jallcom.2018.02.121 0925-8388/© 2018 Elsevier B.V. All rights reserved.

for magnetic recording applications and ferrofluids because of its unique magnetic properties that remarkable large first order magnetocrystalline anisotropy (K) constant compared to the other spinel materials [8e11]. CoFe2O4 materials have also used as contrast agents in Magnetic Resonance Imaging (MRI). It is desired for practical applications that the materials have larger magnetic anisotropy and larger magnetic moment even their particle size is reduced to nano-meter scale. Varying the cations on the X/Y sites could change the magnetic interaction in antiferromagnetic X-Y and exchange interaction Y-Y. Therefore, the research attention has been focused mainly on the enhancement of magnetic properties of CoFe2O4 by substituting on X and Y sites with several cations, like Cr, Zn, Ni, Mn [3,12e17]. To reduce antiferromagnetic interactions between d-d (Co2þ-Fe3þ) orbital electrons in spinel structure, nontransition metal (p-orbital) can be doped into one of the site of spinel structure. Thus, p-d exchange interaction can enhance the effective magnetization in the whole structure. Here, we systematically study the effect of boron content in CoFe2O4 spinel samples on structural and magnetic properties. The structural properties and morphology of the samples were studied

M. Akyol et al. / Journal of Alloys and Compounds 744 (2018) 528e534

by x-ray diffraction (XRD), Rietveld analysis and scanning electron microscopy (SEM) technique. The detail magnetic properties of samples have been studied by performing temperature (M-T) and field (M-H) dependence magnetization measurements. 2. Experimental process Bx:CoFe2O4 (x ¼ 0.0, 0.05, 0.10 and 0.15 labeled as CFBO-0, CFBO1, CFBO-2 and CFBO-3, respectively) magnetic nanoparticles were synthesized by using sol-gel technique with Co(NO3)2$6H2O, Fe(NO3)3$9H2O and boric acid as starting materials. In order to obtain the desired stoichiometry, appropriate amounts of starting materials were first dissolved in dilute HNO3 solution at 150  C, and then, citric acid and ethylene glycol were added to the mixture. A viscous residual was formed after slowly boiling the solution at 200  C. Afterwards the obtained residual was dried slowly at 300  C

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until a dry gel was formed. Finally, in order to remove organic materials produced during chemical reactions, the residual precursor was burned for one hour in air at 500  C. The resulting powders were calcinated for 5 h in air at 550  C, and thereafter, they were furnace cooled. After cooling, the resulting materials were grounded using an agate mortar to obtain fine powder. Afterwards, powder formed samples were pressed into disc shape and sintered at 1100  C for 24 h in air atmosphere and then furnace cooled to the room temperature (RT). The crystal structure of magnetic nanoparticles (MNPs) was carried out by X-Ray Diffraction (XRD) technique on PANalytical Empyrean diffractometer using Cu-Ka1 (l ¼ 1.54 Å) radiation, and then analyzed by Rietveld refinement. The surface morphology of MNPs was measured by FEI-Quanta 650 Scanning Electron Microscope (SEM) imaging technique. The element contents in the sample were determined by EnergyDispersive X-ray Spectrum (EDS). The temperature and magnetic

Fig. 1. X-ray diffraction patterns with Rietveld refinement of Bx:CoFe2O4 a) x ¼ 0.0, b) 0.05, c) 0.10 and d) 0.15 samples.

Table 1 Rietveld refinement agreement factor (c2), lattice constant (a), unit cell volume (V), crystallite size (D), boron amount in tetrahedral and octahedral sites in CoFe2O4 spinel structure and cation distributions obtained from Rietveld refinement of various concentration of B in CoFe2O4 samples. B (%)

0.0 5.0 10.0 15.0

c2

3.12 2.41 3.87 1.95

a (Å)

8.3492(2) 8.3351(1) 8.3356(2) 8.2783(3)

V (Å3)

582.02(4) 579.07(2) 579.18(4) 567.31(5)

D (nm)

65.7 ± 1.5 62.2 ± 1.8 67.1 ± 2.2 47.5 ± 1.9

Boron amount in

Cation Distribution

Tetrahedral site (%)±1.0

Octahedral site (%)±1.0

e 12 78 55

e 88 22 45

(Co0.98)[Fe2.08]O4.10 (Co0.97B0.006)[Fe1.93B0.044]O3.93 (Co0.89B0.079)[Fe1.97B0.021]O3.96 (Co0.91B0.083)[Fe1.95B0.067]O4.01

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field dependence of magnetization of the samples were determined by Physical Properties Measurement System (PPMS) with Vibrating Sample Magnetometer (VSM) module.

3. Results and discussions The structural properties of Bx:CoFe2O4 (x ¼ 0.0, 0.05, 0.10 and 0.15) magnetic nanocrystals are studied by performing Rietveld refinement of the XRD data. A good agreement between observed XRD patterns and calculated diffraction patterns found by Rietveld analysis is seen in Fig. 1aed. The quality of refined XRD patterns is indicated by Rietveld agreement factor (c2) in Table 1. The crystal structure of samples is found as cubic in Fd3m space group by Rietveld refinement. No change in the crystal structure of CoFe2O4 was noticed after adding B ions into the main lattice. Thus, it may be pointed out that the cubic structure of CoFe2O4 is not much disturbed by boron adding into the host lattice. In addition, no other diffraction peak is seen in the XRD patterns for all samples. The lattice constants shown in Table 1 monotonically decrease with increasing B concentration that consistent with the shift in the peaks which obviously indicates the B ions are settled into the lattice. Since ionic radius of B3þ (0.41 Å) is much smaller than Co2þ (0.7 Å) and Fe3þ (0.69e0.78 Å) ions, the unit cell is crinkled. Therefore, the lattice parameter decreases, as well. The cationic distributions are obtained from Rietveld refinement that boron ions are settled in both tetrahedral and octahedral sites of spinel lattice (see Table 1). Although B-ions are in both tetrahedral and octahedral sites in all samples, there is no correlation between occupation site and B-content. The structural formula of samples by taking into account of refinement results can be written as shown as in Table 1. The small brackets refer to the tetrahedral sites, and the square brackets to the octahedral sites. The XRD spectra is expanded in the range of 27 2q  45 to investigate in detail the effect of B-ions content in CoFe2O4 lattice (see Fig. 2a). The diffraction peaks of B-included samples shift to higher diffraction angle as compared to that of CoFe2O4 sample. The amount of shifts for the strongest peak of (311) planes of the B0.15:CoFe2O4 sample compared to that of CoFe2O4 sample is 0.35 as shown in Fig.2a. In addition, diffraction intensities of the peaks decrease almost  3 times when B amount is increased in the lattice (see Fig. 2a). This indicates the crystal quality of samples is getting worse with the B amount. We further work on the crystallite size (D) of samples by considering XRD data and DebyeeScherrer formula [18].

D¼

kl bcosq

(1)

where k is the crystallite shape factor (0.94), l is the x-ray wavelength (Cu-a ¼ 1.5406 Å), b is the peak full width at half maximum (in radians) at the observed peak angle q. In the calculation, we have considered five strongest peaks in XRD patterns. The average crystallite sizes of samples are found in a range of 47e67 nm and it is plotted as a function of B content in Fig. 2b where it decreases with increasing B concentration, except for x ¼ 0.10 in Bx:CoFe2O4. Although all samples are synthesized in the same conditions, the crystallite size of CFBO-2 sample is found the largest one within them (the origin of this increment is not clear). In overall of XRD analysis, the results clearly show that B-ions have been successfully settled into the both Co2þ and Fe3þ sites of CoFe2O4 lattice. Scanning electron microscope technique was performed to explore the morphology of the samples. Fig. 3aed show SEM images of Bx:CoFe2O4 (x ¼ 0.0, 0.05, 0.10 and 0.15) samples, respectively. It can be seen from figures that the grains are not uniformly distributed in the samples and there is no correlation between B amount and grain size. In addition to the morphology, the chemical composition is determined from energy-dispersive x-ray spectroscopy analysis combined with SEM system. The elemental analysis is consistent with the nominal composition that worked in this study. Chemical formula of samples by taking into account of EDS data are written as: Co1.02Fe2.12O3.87, B(3.2%):Co0.92Fe2.01O3.93, B(8.5%):Co0.84Fe1.96O3.54 and B(13.1%):Co0.94Fe2.11O4.15 for CFBO-0, CFBO-1, CFBO-2 and CFBO-3, respectively. Magnetic properties of the CoFe2O4 spinel samples with various B contents have been investigated by using a PPMS-VSM magnetometer. Temperature dependence of magnetization (M-T) was studied zero-field cooled (ZFC) and field-cooled (FC) conditions under 100 Oe magnetic field (see Fig. 4a). The magnetization data was collected by sweeping temperature between 5 and 400 K. One can be observed that although the magnetizations measured under ZFC conditions increase dramatically with temperature up to 400 K for all samples, the FC curves decrease unremarkable with increasing temperature. This indicates thermomagnetic irreversibility between ZFC and FC magnetization curves. It is expected that the blocking temperature are beyond the 400 K which is the maximum temperature of our experimental system. To see the effect of B on the magnetization, the ZFC curves are extended at low temperature range, and shown in Fig.4b. As seen from the figure that although the ZFC magnetization of CoFe2O4 sample linearly

Fig. 2. a) Expanded XRD spectra of CoFe2O4 and B(15%):CoFe2O4 samples and b) crystallite size as a function of boron concentration in CoFe2O4.

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Fig. 3. Scanning electron microscope images of Bx:CoFe2O4 for a) x ¼ 0.0, b) x ¼ 0.05, c) x ¼ 0.10 and d) x ¼ 0.15.

increases with temperature, the curves of B-included samples do not affected by temperature up to propagation temperature (TP). This might be caused of pinning magnetic domains by boron atoms. Moreover, the propagation temperature increases with increasing boron content (see inset of Fig. 4b). Above the propagation temperature, the ZFC curves increases rapidly up to the maximum temperature (400 K). Similar behavior was also found that the propagation temperature increases with B doping into the DyCo2 structure [19]. This supports our claim on the enhancement of pinned magnetic domains by B atoms in the structure. The magnetic field dependence of magnetization measurements in Bx:CoFe2O4 (x ¼ 0.0, 0.05, 0.10 and 0.15) spinel nanocrystals were performed at several temperatures without cooling field. Fig. 5aeb show the M-H curves measured at 5 and 300 K of all samples, respectively. The typical ferromagnetic hysteresis loops with

coercive field (Hc) at 5 K are observed for all samples. But, Hc values of samples are vanished at room temperature measurements. The values of Hc measured at 5 and 300 K temperatures (see Table 2) for CoFe2O4 sample are lower than previously reported results in similar structure [17,20]. This might be related to the forming of mixture shape of particles as shown in Fig. 3. Due to the lower symmetric coordination of atoms in spinel structure, the site of X and Y in spinel structure might create different local coupling that can be reason of the observed lower value of Hc. In addition, we observe a decrement in the Hc when B is added into the structure. To understand the effect of B on the domain pinning at low temperature, we determined the propagation field (HP) from the M-H curves at 5 K. The value of HP for samples is found as the point of magnetization starts rising from zero shown with an arrow in the inset of Fig.5a. A linear behavior between HP and B concentration is

Fig. 4. a-b. Temperature dependence magnetization of Bx:CoFe2O4 (x ¼ 0.0, 0.05, 0.10 and 0.15) spinel nanoparticles. Inset: Propagation temperature as a function of B concentration.

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Fig. 5. a-b) Magnetic hysteresis of Bx:CoFe2O4 (x ¼ 0.0, 0.05, 0.10) spinel nanoparticles at 5 and 300 K, respectively, c-d) Boron concentration dependence propagation field and normalized saturation magnetization, respectively.

observed that HP increases with increasing B atoms in the structure (see Fig.5c). This also supports our M-T results as discussed earlier. To see the saturation magnetization (Ms), we performed high field magnetic hysteresis measurement at 5 K. However, the magnetization could not completely saturate up to 7 T magnetic field. The reason might be the surface spin canting due to the increasing volume relatively to the particles [21]. The Ms values of samples measured at 5 and 300 K were found in a range of 75e103 emu/g (see Table 2). The saturation magnetization was normalized by crystal size (see Fig. 5d) to distinguish the B effect from the size effect, since the models and experimental studies show that the magnitude of magnetization is related to the size [17,20,22e24]. It can be easily seen that the Ms/D increases with boron concentration in CoFe2O4 spinel nanoparticles. This result indicates that boron which is a non-transition metal (p-orbital), enhances the p-d exchange

interaction in the spinel structure by reducing antiferromagnetic d-d (Co2þ) and (Fe3þ) interactions. Therefore, we observe an enhancement in the effective magnetization of the samples with boron amount in the lattice. Further, we calculate the effective magnetic moment from the following equation,

meff ¼

MMs NA b

(2)

where M is the molecular weight, NA is the Avogadro's number and b is the conversion factor (9.27  1021 Erg/Oe). The maximum moment value is found as 3.84 mB for CFBO-2 sample. Since the linear relation between particle size and moment, the reason of determined highest moment value of CFBO-2 is related to the relatively big particle size of it. Next, we have further studied the temperature dependence of

Table 2 Size, coercive field (Hc), saturation magnetization (Ms), effective magnetic anisotropy constant (K1) and effective magnetic moments (meff) of Bx:CoFe2O4 (x ¼ 0.0, 0.05, 0.10, 0.15) samples. Samples

CFBO CFBO-1 CFBO-2 CFBO-3

Size (nm)

65.7 ± 1.5 62.1 ± 1.8 67.1 ± 2.2 47.5 ± 1.9

Hc (Oe)

Ms (emu/g)

5K

300 K

5K

300 K

3129 ± 8 1025 ± 10 2025 ± 12 2047 ± 10

424 ± 16 473 ± 15 338 ± 10 326 ± 8

90.83 ± 0.3 85.78 ± 0.2 103.29 ± 0.4 83.18 ± 0.3

86.96 ± 0.2 77.35 ± 0.3 95.05 ± 0.3 75.64 ± 0.4

K1(0) (x106 J/m3) ±104

meff (mB)

2.28 2.13 2.58 2.75

3.65 ± 0.4 3.18 ± 0.3 3.84 ± 0.5 2.99 ± 0.4

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Acknowledgement This work was partially supported by Çukurova University (Adana/Turkey) under the Project No. of FBA-2016-6222. References

Fig. 6. Temperature dependence of the coercive field. The dashed lines are corresponding fit curves by using Eq. (3).

the coercive field, Hc, by plotting the experimental Hc(T) data as a temperature ranges from 5 to 400 K (see Fig.6). As expected that the decrement in Hc(T) with increasing temperature is observed in all samples. To fit our experimental data, we used couple of models. Since the Neel-Arrhenius model which is a simplest one [25], does not include particle size dispersion, this approximation was not able to fit our experimental data for any sample. If the materials have large particles and anisotropy, the thermal dependence of magnetocrystalline anisotropy could be added to the equation like in following relation [17,26];

Hc ðTÞ ¼

 1=2 i 2 2K1 ð0ÞeaT h 25kB T 1 2 m0 Ms VK1 ð0ÞeaT

(3)

where K1 is first magnetic anisotropy, V is the particle volume, m0 is magnetic permeability of free space, kB is Boltzman constant and a is a fitting parameter. As shown in Fig.6, the experimental data is admirable fitted with Eq. (3) for all samples. The effective magnetic anisotropy values given in Table 2 are good agreement with previous experimental and theoretical reports [27e29]. It is found that the magnetocrystalline anisotropy increases with B doping into the structure, except CFBO-1 sample. 4. Conclusions In conclusion, we have worked the effect of boron doping on structural and magnetic behavior of CoFe2O4 spinel nanoparticles synthesized by sol-gel technique. XRD patterns are fitted with cubic crystal structure of CoFe2O4. The crystallite size decreases with increasing boron concentration in Bx:CoFe2O4, except for x ¼ 0.10. Electron microscope images indicate non-monotonic grain size is observed with boron content in the structure. A comprehensive magnetic analysis has been performed to understand the magnetic properties of boron-doped CoFe2O4 spinel nanoparticles. The temperature dependence of coercive field is modeled via NeelArhenius relation by adding thermal dependence of magnetocrystalline anisotropy term. Our results show that B-ions in cobaltferrite spinel structure enhances ferromagnetic coupling and suppresses super-exchange interaction between tetrahedral (X) and octahedral (Y) sites.

[1] Y. Koseoglu, A. Baykal, M.S. Toprak, F. Gozuak, A.C. Basaran, B. Aktas, Synthesis and characterization of ZnFe(2)O(4) magnetic nanoparticles via a PEG-assisted route, J. Alloy Comp. 462 (2008) 209e213. [2] M.J. Iqbal, M.R. Siddiquah, Electrical and magnetic properties of chromiumsubstituted cobalt ferrite nanomaterials, J. Alloy Comp. 453 (2008) 513e518. [3] H.-Y. He, Structural and magnetic property of Co1-xNixFe2O4Nanoparticles synthesized by hydrothermal method, Int. J. Appl. Ceram. Technol. 11 (2014) 626e636. [4] K. Zakrzewska, Mixed oxides as gas sensors, Thin Solid Films 391 (2001) 229e238. [5] B.N. Kim, K. Hiraga, K. Morita, Y. Sakka, A high-strain-rate superplastic ceramic, Nature 413 (2001) 288e291.   nas, A. Setkus, [6] A. Galdikas, Z. Martu SnInO-based chlorine gas sensor, Sensor. Actuator. B Chem. 7 (1992) 633e636. [7] C.V. Gopal Reddy, S.V. Manorama, V.J. Rao, Semiconducting gas sensor for chlorine based on inverse spinel nickel ferrite, Sensor. Actuator. B Chem. 55 (1999) 90e95. [8] A. Goldman, Handbook of Modern Ferromagnetic Materials, Kluwer Academic Publishers, Boston, 1999. [9] X.-H. Li, C.-L. Xu, X.-H. Han, L. Qiao, T. Wang, F.-S. Li, Synthesis and magnetic properties of nearly monodisperse CoFe2O4Nanoparticles through a simple hydrothermal condition, Nanoscale Res. Lett. 5 (2010) 1039. [10] V. Kuncser, W. Keune, M. Vopsaroiu, P.R. Bissell, B. Sahoo, G. Filoti, Easy axis distribution in modern nanoparticle storage media: a new methodological approach, J. Optoelectron. Adv. Mater. 5 (2003). [11] V. Kuncser, G. Schinteie, B. Sahoo, W. Keune, D. Bica, L. Vekas, G. Filoti, € ssbauer Magnetic interactions in water based ferrofluids studied by Mo spectroscopy, J. Phys. Condens. Matter 19 (2007), 016205. [12] S.E. Ziemniak, L.M. Anovitz, R.A. Castelli, W.D. Porter, Thermodynamics of Cr2O3, FeCr2O4, ZnCr2O4, and CoCr2O4, J. Chem. Thermodyn. 39 (2007) 1474e1492. [13] A.A. Bush, V.Y. Shkuratov, K.E. Kamentsev, V.M. Cherepanov, Preparation and €ssbauer characterization of Co1  x Ni x X-ray diffraction, dielectric, and Mo Cr2O4 solid solutions, Inorg. Mater. 49 (2013) 296e302. [14] J.N. Hu, W.Y. Zhao, R.S. Hu, G.Y. Chang, C. Li, L.J. Wang, Catalytic activity of spinel oxides MgCr2O4 and CoCr2O4 for methane combustion, Mater. Res. Bull. 57 (2014) 268e273. [15] K.P. Polyakova, V.V. Polyakov, G.Y. Yurkin, G.S. Patrin, Magnetic properties of polycrystalline films of CoCr2O4 and CoFe0.5Cr1.5O4 multiferroics, Phys Solid Stateþ 56 (2014) 692e694. [16] M. Ptak, M. Maczka, A. Pikul, P.E. Tomaszewski, J. Hanuza, Magnetic and low temperature phonon studies of CoCr2O4 powders doped with Fe(III) and Ni(II) ions, J. Solid State Chem. 212 (2014) 218e226. [17] T.E. Torres, E. Lima Jr., A. Mayoral, A. Ibarra, C. Marquina, M.R. Ibarra, G.F. Goya, el-Arrhenius model for highly anisotropic CoxFe3xO4 Validity of the Ne nanoparticles, J. Appl. Phys. 118 (2015), 183902. [18] R.S.S.B.D. Cullity, Elements of X-Ray Diffraction, third ed., Prentice Hall, 2001. [19] C.L. Wang, J. Liu, Y. Mudryk, K.A. Gschneidner Jr., Y. Long, V.K. Pecharsky, The effect of boron doping on crystal structure, magnetic properties and magnetocaloric effect of DyCo2, J. Magn. Magn Mater. 405 (2016) 122e128. [20] M. Rajendran, R.C. Pullar, A.K. Bhattacharya, D. Das, S.N. Chintalapudi, C.K. Majumdar, Magnetic properties of nanocrystalline CoFe2O4 powders prepared at room temperature: variation with crystallite size, J. Magn. Magn Mater. 232 (2001) 71e83. [21] V. Jagadeesha Angadi, A.V. Anupama, R. Kumar, S. Matteppanavar, B. Rudraswamy, B. Sahoo, Observation of enhanced magnetic pinning in Sm3þ substituted nanocrystalline MnZn ferrites prepared by propellant chemistry route, J. Alloy Comp. 682 (2016) 263e274. [22] K. Maaz, A. Mumtaz, S.K. Hasanain, A. Ceylan, Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route, J. Magn. Magn Mater. 308 (2007) 289e295. [23] B. Martínez, X. Obradors, L. Balcells, A. Rouanet, C. Monty, Low temperature surface spin-glass transition in Fe2O3 nanoparticles, Phys. Rev. Lett. 80 (1998) 181e184. [24] H. Kachkachi, M. Dimian, Hysteretic properties of a magnetic particle with strong surface anisotropy, Phys. Rev. B 66 (2002), 174419. [25] W.C. Nunes, F. Cebollada, M. Knobel, D. Zanchet, Effects of dipolar interactions on the magnetic properties of g-Fe2O3 nanoparticles in the blocked state, J. Appl. Phys. 99 (2006), 08N705. [26] H. Shenker, Magnetic anisotropy of cobalt ferrite Co1.01Fe2.00O3.62 and nickel cobalt ferrite Ni0.72Fe0.20Co0.08Fe2O4, Phys. Rev. 107 (1957) 1246e1249.

534

M. Akyol et al. / Journal of Alloys and Compounds 744 (2018) 528e534

[27] A. Franco Jr., F.C.e. Silva, High temperature magnetic properties of cobalt ferrite nanoparticles, Appl. Phys. Lett. 96 (2010), 172505. [28] N. Ranvah, Y. Melikhov, I.C. Nlebedim, D.C. Jiles, J.E. Snyder, A.J. Moses, P.I. Williams, Temperature dependence of magnetic anisotropy of germanium/cobalt cosubstituted cobalt ferrite, J. Appl. Phys. 105 (2009), 07A518.

€ ssinger, M. Siddique, M. Nadeem, Effect of Mn [29] M. Atif, R. Sato Turtelli, R. Gro substitution on the cation distribution and temperature dependence of magnetic anisotropy constant in Co1xMnxFe2O4 (0.0x0.4) ferrites, Ceram. Int. 40 (2014) 471e478.