Mater. Res. Soc. Symp. Proc. Vol. 1398 © 2012 Materials Research SocietyDOI: 10.1557/opl.2012.699

Synthesis and Magnetic Properties of Cobalt Ferrite Nanoparticles Morad F. Etier1, Vladimir V. Shvartsman1, Frank Stromberg2, Joachim Landers2, Heiko Wende2, Doru C. Lupascu1

1Institute for Materials Science, University of Duisburg-Essen, Essen, Germany 2Experimental Physics, Faculty of Physics, University of Duisburg-Essen, Duisburg, Germany ABSTRACT Nanopowders of cobalt iron oxide (CoFe2O4) were successfully fabricated by the co-precipitation method followed by a technique to prevent particle agglomeration. Particle sizes were in the range of 24 to 44 nm. The size of cobalt iron oxide particles decreases with increasing the concentration of the precipitation agent. The crystal structure was confirmed by X-ray diffraction (XRD), the chemical composition by energy dispersive spectroscopy (EDS), and phase changes by thermogravimetric differential thermal analysis (TGA-TDA). The particle morphology was analyzed by scanning electron microscopy (SEM). Magnetic properties were investigated by SQUID magnetometry and Mössbauer spectroscopy. Being nearly monodisperse and non-agglomerated the prepared cobalt iron oxide powders are the base for synthesizing magnetoelectric composites embedded in a ferroelectric BaTiO3 matrix. INTRODUCTION Cobalt iron oxide, CoFe2O4, is one of the most widely used materials in magnetic recording due to its high coercivity (about 5400 Oe), moderate magnetization (84 emu/g), and good chemical stability. Cobalt iron oxide also exhibits high hardness, wear resistance, electrical insulation, high cubic magnetocrystalline anisotropy, high electromagnetic performance, and photomagnetism [1-3]. CoFe2O4 is a member of the inverse spinel structure family where the Co+2 ions occupy half of the octahedral coordination sites. Fe+3 cations occupy the other half of the octahedral sites as well as the tetrahedral sites. Below TC = 820 K [2] CoFe2O4 is in a ferrimagnetic state, where the magnetic moments from the Fe+3 ions partly cancel each another. The net contribution to magnetization is reduced [4]. The magnetic properties of cobalt iron oxide nanoparticles mainly depend on the annealing temperature. For increasing annealing temperature the particle size rises and the saturation magnetization and the coercive field increase [5].

Large magnetostriction makes CoFe2O4 promising for fabrication of composite multiferroics [6]. In these materials the magnetoelectric (ME) effect, i.e. change of the polarization under an external magnetic field, is engineered between the order parameters of piezoelectric and magnetostrictive constituents [7]. An external electric field applied to the composite will induce a mechanical strain in the piezoelectric constituent transferred to the magnetostrictive component at the respective interfaces, where it induces a change of magnetization. Analogously, an applied magnetic field results in a change of the polarization in the piezoelectric constituent. An enhanced ME coupling is expected in composites with relatively large, well-defined interface areas. This is the case, e.g., in composites with a core-shell structure, where a magnetostrictive CoFe2O4 nanoparticle is enclosed by a piezoelectric shell (e.g. BaTiO3 or Pb(Zr,Ti)O3) [8, 9]. To fabricate composites with high ME coupling synthesis of CoFe2O4 nanoparticles with controlled size is necessary.

There are a number of reports on synthesis of the cobalt iron oxide nanoparticles with different methods such as co-precipitation [5, 10-12], sol-gel auto combustion [13], forced hydrolysis [14] and the polyol technique [15]. Particle agglomeration is the most important difficulty and special techniques should be provided to prevent it. Here we report on non-agglomerated powders of cobalt ferrite nanoparticles synthesised similar to the co-precipitation method described by Zhang [11]. The method is relatively simple with low production cost and provides small particle size. SYNTHESIS The cobalt iron oxide nanoparticles were synthesized by dissolving 0.02 mol Fe(NO3)3·H2O and 0.01 mol Co(NO3)2·6H2O in 200 ml of distilled water. The solution was stirred for about 1 hour. Then the temperature was raised to 90oC. The color of the mixture is dark orange indicating dissolved ions of Fe+3 and Co+2. NaOH in the amount of 0.1 - 2.5 mol is added to the solution drop by drop. Black participates start to appear indicating formation of CoFe2O4 particles. The temperature was fixed at 90oC for 2 hrs to homogenize the solution. Then the solution was sonificated for 20 minutes and subsequently dried for one day at 100oC. Appropriate drying and washing of the powder prevented agglomeration. The powder was ground gently and washed several times with ethanol and acetone to remove remaining NaOH and other impurities. A core-shell multiferroic composite ceramics containing 60 % of BaTiO3 as a piezoelectric material and 40% of CoFe2O4 as a magnetostrictive material were synthesized using a sol-gel approach described elsewhere [9].

The crystalline phases and unit cell parameters of the powders were determined by X-ray diffraction (XRD) using a Siemens D-5000 diffractometer (Cu-Kα radiation with λ = 1.54056 Å) in the range of 2θ = 20o-80o with a step size of 0.01° and a time constant of 1s. The structure of the particles was analyzed by transmission electron microscopy (TEM, TECNAI F20). Powders were studied using thermogravimetric differential thermal analysis (TG-DTA, Mettler Toledo TGA/DSC 1 star) in the temperature range from room temperature to about 1050°C. The morphology of the powder was studied by scanning electron microscopy (SEM, Quanta 400 FEG) and the composition was determined by energy dispersive spectroscopy (EDS). Mössbauer spectroscopy measurements were carried out in a custom-built setup at temperatures between 80 K and 300 K.

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Figure 1. (a) XRD pattern of the as-prepared cobalt iron oxide nanopowder synthesized using 0.1 mol of NaOH. The square labels show the CoFe2O4 pattern according to the database (pdf file 22-1086) [16]. (b) HRTEM image of a single CoFe2O4 Particle and (c) the corresponding fast Fourier transform (FFT) pattern (courtesy by Anna Elsukova).

RESULTS AND DISCUSSION Figure 1a displays the XRD pattern of the as-prepared powder. It confirms pure CoFe2O4 phase with the inverse spinel structure. The cell parameter of the cubic unit cell determined using Jade software is a = 8.38 Å, which is the same as the bulk value (8.384 Å) [17]. The crystallite size, D, was estimated from the width of the strongest diffraction peak (311) using the Scherrer equation, D = Kλ/βcosθ, where K = 0.9 is the shape factor, λ, is the x-ray wave length, θ is the diffraction angle corresponding to the (311) plane, and β is the full width at half maximum. The fitted crystallite size is D = 20 nm. Figures 1b and 1c show a single CoFe2O4 particle of powders measured by transmission electron microscopy, the beam was in the direction of [011] and the interplanar spacing between the [110], [001] and [200] planes were measured and found to be 0.309, 0.501 and 0.443 nm respectively which are corresponding to the cubic structure.

EDS as shown in Figure 2a also confirms the formation of the powder with the required elements and composition. The experimental molar ratio between Co and Fe was found to be 1:2.15 which is comparable to the nominal value.

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Figure 2. (a) Energy dispersive spectroscopy for the as-prepared powder of cobalt iron oxide nanoparticles. (b) TGA-TDA analysis for the as-prepared powder of cobalt iron oxide nanoparticles.

Figure 2b shows TG-DTA results for a sample obtained using 0.625 mol NaOH. The total weight loss is about 7.66 %. It occurs in a broad temperature range 50-700 °C. The DTA data show an endothermic valley at 25-130 °C corresponding to a dehydration of the sample with a weight loss of about 2 % only. A broad exothermic peak at 200-500°C which is accompanied by a weight loss of 5.5 % evidences crystallization of cobalt iron oxide. Above 600°C there was no significant weight loss indicating completion of cobalt iron oxide formation.

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Figure 3. SEM images for the CoFe2O4 powders prepared with (a) 0.1 mol (b) 0.25 mol, (c) 1.25 mol, and (d) 2.5 mol NaOH. Insets show the particle size distributions fitted by a Gaussian function (dashed lines).

Figure 3 shows SEM images for CoFe2O4 powders precipitated using different concentrations of NaOH. The powders prepared with a moderate concentration of NaOH consist of nanometer size particles. The particle size distributions were determined using the AnalySIS software (Soft Imaging Systems) for optical image processing (insets in Figure 3). The distributions are relatively uniform with the average diameter of the particle D = (38 ± 6) nm, (29 ± 4) nm, (27 ± 3) nm, and (24 ± 4) nm for the samples synthesized using 0.1 mol, 0.25 mol, 1.25 mol, and 2.5 mol NaOH, respectively. While the particle size decreases in samples prepared with the larger amount of NaOH, the degree of agglomeration increases. The sample synthesized by using 2.5 mol NaOH shows large agglomerates of nanoparticles (Fig. 3d).

Figure 4. Mössbauer spectrum of a Co-ferrite powder (0.625 mol NaOH) at T = 80 K. Figure 4 shows the Mössbauer spectrum of the Co-ferrite nanoparticles at T = 80 K and in zero external field. The spectrum was fitted with a slightly broadened sextet [see linewidth Γ in Table 1] for the A-sites (green line) and a distribution of hyperfine fields for the B-sites (blue line) with fixed line widths of smaller size. It is well known that a fraction of Co2+ ions located on the tetragonal sublattice leads to a non unique nearest-neighbor A-site configuration. Therefore the distribution represents the various contributions with six, five and four Fe3+ ions on the A-sites which produce decreasing hyperfine fields due to reduced supertransferred hyperfine fields. The Mössbauer parameters [see Table 1] are in good agreement with the literature values for bulk CoFe2O4 [18]. The inversion parameter, s, can be calculated from the ratio of the relative spectral areas from A to B site, RAB, s = (1-RAB)/(1+RAB). For our values at T = 80 K the outcome is s = 0.1. It must be emphasized that this value is afflicted with a large error bar since the spectral contributions from the A and B sites are strongly superimposed. Table 1. Spectral Mössbauer parameters of the Co-ferrite powder at T = 80 K in zero external field, Bhf = magnetic hyperfine field, Δ = quadrupole splitting, δ = isomer shift, Area = relative spectral area, Γ = full line width at half maximum. Values for the distribution are average values. The isomer shifts δ are given relative to α-Fe at room temperature.

A site, sextet Bhf (T) Δ (mm/s) δ (mm/s) Area (%) Γ (mm/s) 50.68(4) -0.02(1) 0.38(1) 45 0.56(2) B site, Bhf distribution 52.49(7) 0.05(1) 0.48(1) 55 0.28

Figure 5 shows the field dependence of the magnetization measured for the samples with particle size 24, 27 and 29 nm at room temperature. For all samples, the coercive field is less than 250 Oe. This value is much smaller than for a bulk, but it agrees with other reports on CoFe2O4 nanoparticles [19]. The shape of the M(H) hysteresis loops can be interpreted as superparamagnetic behavior as previously reported for CoFe2O4 nanoparticles. A crossover to a similar behavior is observed in our samples for the smallest particle size. In this case the surface-volume ratio becomes larger and the magnetic characteristics are strongly affected due to the influence of thermal energy over the magnetic ordering. However, the finite magnetic remanence

indicates that the blocking temperature likely lies above room temperature for all samples. The remanent magnetization drops from 22 to 4.5 emu/g when the mean particle size decreases from 29 to 24 nm (see inset). At the same time, the powder with the smallest particles has the largest magnetization at μ0H = 1 T. The ratio of the remanent magnetization to the high-field magnetization for the three samples is less than 0.5 which is an indication of a randomly oriented uniaxial system [19] or the proximity to the transition temperature into the superparamagnetic state. The increase of the high-field magnetization for decreasing particle size might be related to a loss of total compensation between the magnetic moments of Fe3+ ions on the A and B sites of the spinel structure. Such imbalance becomes larger in smaller particles resulting in an extra magnetic moment. A similar effect has been observed e. g. in antiferromagnetic nanoparticles [21, 22]. The distribution of Fe+3 and Co+2 cations over the octahedral and tetrahedral sites may be affected by the amount of NaOH resulting in appearance of extra non-compensated magnetic moments in powders precipitated with more base. As we here report data taken at 300 K, the proximity to the superparamagnetic state is the more likely interpretation.

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Figure 5: Magnetization loop of cobalt iron oxide nanoparticles at T = 300 K (29 ± 4 nm ⇔ 0.25 mol; 27 ± 3 nm ⇔ 1.25 mol; and 24 ± 4 nm ⇔ 2.5 mol NaOH).

The powder prepared with 0.25 mol NaOH was used for the synthesis of magnetoelectric composites, because it showed low agglomeration. We previously published data on a BaTiO3/CoFe2O4 ceramic based on a commercial CoFe2O4 powder [9]. For the commercial powder we have never been able to de-agglomerate the starting powder due to the hardness of the agglomerates. The powder obtained here in a first attempt yielded a similar microstructure. A certain degree of re-agglomeration occurred during the slurry formation, gelation of BaTiO3 and sintering. Our first attempt yielded comparable microstructures to the previous data [9]. Ongoing work is establishing the necessary processing steps to maintain the non-agglomerated nanopowder structure of the CoFe2O4 starting powders. This is the content of forthcoming work.

CONCLUSIONS CoFe2O4 powders with particle size around 24 - 44 nm were successfully synthesized by the co-precipitation method. This method is simple and produces monodisperse cobalt iron oxide with good crystallinity and magnetic properties at room temperature. Both the particle size and the agglomeration of the particles can be controlled by addition of NaOH. The sample of smallest particle size showed a decreased remanent magnetization, but the saturated magnetization increased. The latter effect is probably due to broken compensation between the magnetic moments of the two Fe sublattices, which becomes enhanced in smaller particles. The prepared particles show sufficiently low agglomeration to be used for fabrication of multiferroic core-shell composites. AKNOWLEDGMENTS The authors appreciate fruitful discussions with Yanling Gao and Dr. Firas Alawneh. We are grateful to Anna Elsukova for TEM measurements. Morad Etier acknowledges financial support by Deutscher Akademischer Austauschdienst. This work was partly supported by DAAD-GRISEC program (Grant 50750877). REFERENCES

1. H. Yue, L. Yong, F. Chunlong, Y. Zhi, Z. Lu, X. Rui, Y. Di and S. Jing, J. Appl. Phys. 108, 084312 (2010).

2. A. Franco and F. C. e Silva, Appl. Phys Lett. 96, 172505 (2010). 3. R. M. Mohamed, M. M. Rashad, F. A. Haraz, and W. Sigmund, J. Magn. Mag. Mater.

322(14) 2058 (2010). 4. W. D. Callister, Material Science and Engineering, an Introduction, 5th Ed. John Wiley &

Sons, p. 684 (2000). 5. M. M. el-Okr, M. A. Salem, M. S. Salim, R. M. El-Okr, M. Ashoush, and H. M. Talaat, J.

Magn. Magn. Mater. 323, 920 (2011). 6. R. M. Bozorth, E. F. Tilden, and A. J. Williams, Phys. Rev. 99, 1788 (1955). 7. C. A. F. Vaz, J. Hoffman, C. H. Ahn, and R. Ramesh, Adv. Mater. 22, 2900 (2010). 8. G. V. Duong, R. Groessinger, and R. S. Turtelli, J. Magn. Magn. Mater. 310, 1157

(2007). 9. V. V. Shvartsman, F. Alawneh, P. Borisov, D. Kozodaev, and D. C. Lupascu, Smart.

Mater. Struct. 20, 075006 (2011). 10. Y. Zhang, Z. Yang, D. Yin, Y. Liu, C. Fei, R. Xiong, J. Shi, and G. Yan, J. Magn. Magn.

Mater. 322, 3470 (2010). 11. Z. Zi, Y. Sun, X. Zhu, Z. Yang, J. Dai, and W. Song, J. Magn. Magn. Mater. 321, 1251

(2009). 12. K. Maaz, A. Mumtaz, S. K. Hasanain, and A. Ceylan, J. Magn. Magn. Mater. 308, 289

(2007). 13. B. G. Toksha, S. E. Shirsath, S. M. Patange, and K. M. Jadhav, Solid. State Commun.

147, 479 (2008).

14. N. Hanh, O. K. Quy, N. P. Thuy, L. D. Tung, and L. Spinu, physica B 327, 382 (2003). 15. Giovanni Baldi, Daniele Bonacchi, Mauro Comes Franchini, Denis Gentili, Giada

Lorenzi, Alfredo Ricci and Costanza Ravagli, Langmuir, 23, 4026-4028 (2007) 16. Natl. Bur. Stand. (U.S.) Monogr. 25, 9, 22, (1971). 17. E.P. Wohlfarth, Ferromagnetic Materials, Elsevier Science Publishers B.V. Vol. 3, p 196

(1982). 18. R. M. Persoons, E. De Grave, P. M. A. de Bakker, and R.E. Vandenberghe, Phys. Rev. B

47, 5894 (1993). 19. C. Vazquez-Vazquez, M. A. Lopez-Quintela, M. C. Bujan-Nunez, and J. Rivas, J.

Nanopart. Res. 13, 1663 (2011). 20. A. Virden, S. Wells and K. O’Grady, J. Magnt. Magnt. Mater. 316, 768 (2007). 21. L. Neel, J. Phys. Soc. Japan suppl. 17, 676 (1962). 22. S. Bedanta and W. Kleemann, J. Phys. D: Appl. Phys. 42, 013001 (2009).