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R. S. Turtelli et al.: Superparamagnetic CoFe2O4 prepared via a modified oxalate method
Reiko Sato Turtellia, Muhammad Atifa, Jakob Krippelb, Roland Grössingera, Frank Kubelc, Wolfgang Linertb a Institut
für Festkörperphysik, Technische Universität Wien, Vienna, Austria für Angewandte Synthesechemie, Technische Universität Wien, Vienna, Austria c Institut für Chemische Technologien u. Analytik, Technische Universität Wien, Vienna, Austria b Institut
Superparamagnetic CoFe2O4 prepared via a modified oxalate method The modified oxalate precursor method was used for the preparation of nanocrystalline CoFe2O4. The structural investigations of as-produced powders performed by high-resolution transmission electron microscopy and X-ray diffraction revealed an average particle size of 8 nm. The nanocrystallites are pure CoFe2O4 and present a well crystallized spinel structure with lattice constant of 8.3583 Å. The particles are superparamagnetic exhibiting a blocking temperature around room temperature. The magnetization and the coercive field values obtained at 9 T are 56.8 emu · g – 1 and 17.0 kOe (at 4.2 K), respectively. The crystalline anisotropy determined from the coercive field as a function of temperature is 8.0 · 105 J · m – 3, which is much higher than that of bulk materials. Keywords: Magnetic nanoparticles; Cobalt ferrite; Superparamagnetism; High anisotropy; Oxalate method
1. Introduction CoFe2O4 is a ferrite material with outstanding magnetic properties which makes it interesting from the scientific point of view as well as for industrial applications. CoFe2O4 crystallizes with the space group Oh7(Fd3 m) into the partial inverse spinel structure, in which Co cation can be located in octahedral (B-site) or in tetrahedral sites (A-site). The site distribution of Co cation plays an important role in the magnetic and magnetostrictive behaviour [1]. This material exhibits significantly different magnetic properties as compared with other ferrites. It is ferrimagnetically ordered below 793 K where the magnetic moment orientation of cations between A- and B-sites is antiparallel causing a ferrimagnetic type of ordering. Since the magnetic moments of Co2 + and Fe3 + are 3 lB and 5 lB, respectively, the theoretical magnetic moment of CoFe2O4 can be calculated easily. This means that in an ideal case (inverse spinel structure) the cobalt moments determines the remaining magnetization. However, the real experimentally measured saturation magnetization of CoFe2O4 depends strongly on the preparation methods. We have obtained values of the saturation magnetization up to ~3.7 lB that corresponds 87.8 emu · g – 1 [2]. The difference in the theoretical and experimental values indicates that cobalt ferrite is neither fully normal spinel nor fully inverted spinel [1 – 3], because the cobalt atoms are distributed among both the A- and BInt. J. Mat. Res. (formerly Z. Metallkd.) 103 (2012) 9
sites. Beside the saturation magnetization, CoFe2O4 exhibits the highest magnetocrystalline anisotropy (K1 = 4.106 erg · cm – 3) as well as the highest magnetostriction among ferrites (for polycrystalline material values between 100 – 400 ppm depending on the production method and heat treatment condition, see e. g. [1 – 6]). This, for ferrites, anomalous behaviour is caused by the spin-orbit coupling of the Co2+, which depends on the crystallographic site where the Co ion is situated. Nanosized particles exhibit the above-mentioned bulk properties and additionally a reduced coupling which leads to superparamanetic behaviour. From a magnetic point of view the increased importance of the surface (as e. g. symmetry breaking, reduced exchange etc.) for nanoparticles causes reduced magnetization as well as reduced magnetostriction. However, above the so-called blocking temperature the grains decouple which makes them interesting for various applications. Consequently nanosized CoFe2O4 ferrite is a promising material to be used for magnetic recording as well as ferrofluids. Additionally decoupled nanoparticles can be used for medical applications such as contrast media or drug delivery particles. CoFe2O4 nanoparticles can be prepared via many different methods, such as, mechanical alloying and wet chemical routes. The advantage of chemical routes is generally that grains are produced without any stress effects. The wet chemical routes can be the standard sol-gel method, co-precipitation method or auto-combustion methods etc. The disadvantage of all these methods is that afterward they need a high temperature treatment at temperatures up to 1 000 8C in order to form the crystalline phase. Consequently grain growth occurs, which causes large grains, d > 10 nm. Besides well known methods such as co-precipitation which allows the production of fine grains d < 5 nm is \forced hydrolysis" [7, 8]. However physical and structural properties depend on preparation method and on the heat treatment [9]. Therefore, it is the purpose of the present work to produce nanosized grains (d < 10 nm) using a \modified" oxalate method and to investigate them structurally as well as magnetically.
2. Sample preparation The oxalate precursor method was used for the preparation of cobalt ferrite. Chemical grade FeSO4 · 7H2O, CoSO4 · 7H2O, C2H2O4 · 2H2O and KNO3 were used as starting materials. Required quantities of cobalt sulphate and iron acetate solution were slowly added to oxalic acid solution to 1
IJMR_MK110791 – 4.5.12/stm media köthen
R. S. Turtelli et al.: Superparamagnetic CoFe2O4 prepared via a modified oxalate method
precipitate the required oxalate under continuous stirring. Stirring was performed for about 5 min and then the suspension was kept for 24 h. After that, the yellow precipitate was filtered and then washed with distilled water and ethanol to remove the extra surfactant from the solution. The dry precursors were annealed using potassium nitrate (KNO3) and taking two moles of KNO3 for one mole of dry precursor. The powders were mixed together properly and then put on the platinum plate which was heated by means of a Bunsen burner. After a few minutes, the yellow powder burns with a little sparking and turns into black cobalt ferrite fine powders.
3. Experimental methods X-ray diffraction (XRD) patterns were recorded by means of an Xpert Philips powder diffractometer (Goniometer Philips PW 3050/60) using Cu-Ka1,2 radiation in a BraggBrentano geometry, and an X’Celerator detector. The Xray generator Philips PW 3040/60 worked at a power of 40 kV and 40 mA and the goniometer was equipped with a graphite monochromator. Diffraction patterns were recorded in the angular range 58 to 1358 with a scan step size of 0.028. Collected data were refined using the Rietveld package TOPAS (Bruker AXS Topas V 2.1) based on the fundamental parameter approach, with diffractometer parameters and wavelength settings adjusted using an LaB6 standard. The microstructure of the samples was also investigated using high resolution transmission electron microscopy, (TEM). For the magnetic characterization, zero field cooling (ZFC) magnetization and hysteresis loops measurements were performed at temperatures 4.2 K £ T £ 400 K applying a maximum magnetic field of 5 – 9 T using a Physical Property Measurement System from Cryogenics.
Fig. 1. TEM high resolution micro structure of nanocrystalline CoFe2O4. The insert shows the XRD pattern as obtained on the same sample.
4. Results and discussion The insert of Fig. 1 shows the XRD pattern on single phase nanocrystalline CoFe2O4. The determined lattice constant of a = 0.8358 nm agrees well with literature data. The estimated mean grain size from the line broadening is about 8 nm. Additionally, the grain size was determined by means of high resolution TEM (see Fig. 1), which gives grain sizes between 5 and 10 nm. Figure 2 shows the magnetization as a function of temperature, applying a rather low external field of 25 to 100 Oe. The clear visible peaks indicate the change of the blocking temperature, TB, of the nanocrystalline material with the intensity of the external field [6]. Figure 3 shows the field dependence of the magnetization measured between 4.2 K and 400 K. At low temperatures the material develops a rather high coercive field (l0Hc = 1.7 T). Above TB the normalized magnetization M/M(at 9T) versus H/T plot gives identical curves for different temperatures. This can be expected for a nanocrystalline superparamagnetic material where the magnetic behaviour can be described using a Langevin function. Figure 4 gives an overview of the temperature dependence of the coercive field as well as of the magnetization at H = 9 T. The achieved magnetization is significantly below the expected value of microcrystalline CoFe2O4. The reduced magnetization which occurs at small grain sizes indicates that the magnetic alignment at the surface of the 2
Fig. 2. Temperature dependence of the magnetization (ZFC) of nanocrystalline CoFe2O4 measured in small external fields. The arrows indicate the blocking temperatures.
grains is not more simple ferromagnetic, the exchange becomes disturbed which reduce the achievable magnetization and cause a significant high field slope in M(H). At low temperatures the coercivity of non-interacting and randomly oriented superparamagnetic particles is expected to follow the relation 1=2 3K T 1 ð1Þ Hc ¼ 20 MS TB and TB = Ku / 25KB
(2)
where K is the first order magneto-crystalline anisotropy constant, is the average particle volume, kB is the Boltzmann constant and a = 0.48 for easy axis randomly oriented grains. Figure 5 shows the coercive field as a funcInt. J. Mat. Res. (formerly Z. Metallkd.) 103 (2012) 9
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R. S. Turtelli et al.: Superparamagnetic CoFe2O4 prepared via a modified oxalate method
5. Conclusions
Fig. 3. Field dependence of the magnetization of nanocrystalline CoFe2O4 measured between 4.2 K and 400 K. The insert shows M / M(at 9T) versus H / T which is typical for a by the Langevin function described superparamagnetic material above the blocking temperature.
Nanocrystalline CoFe2O4 was successfully produced using a new oxalate method. The phase purity were determined by means of XRD and TEM. The magnetic investigations support the nanocrystalline state (low field magnetization versus T as well as temperature dependence of the hysteresis). The achieved saturation magnetization is significantly below that of microcrystalline bulk material indicating the increasing importance of the surface where the local order is strongly disturbed. The hysteresis below the blocking temperature shows a coercive field with a strong temperature dependence which is caused by high anisotropy. The temperature dependence of the coercive field could be analysed applying the well known formulas for superparamagnetic particles. This allows to calculate the mean anisotropy as well as the grain size. The mean grain size as was estimated from the magnetic data was 6.4 nm, which is in good agreement with the TEM and XRD results. References
Fig. 4. Temperature dependence of the coercive field as well as of the magnetization obtained at 9 T.
[1] R. Sato Turtelli, M. Atif, N. Mehmood, F. Kubel, K. Biernacka, W. Linert, R. Grössinger, Cz. Kapusta, M. Sikora: Mater. Chem. Phys. Vol 132 (2012) 832. DOI:10.1016/j.matchemphys.2011.12.020 [2] M. Atif, R. Sato Turtelli, F. Kubel, R. Grössinger, M. Kriegisch, T. Konneger: J. Appl. Phys. 111 (2012) 013918. DOI:10.1063/1.3675489 [3] G.A. Sawatzky, F. Van der Woude, A.H. Morrish: J. Appl. Phys. 39 (1968) 1204.DOI:10.1063/1.1656224 [4] S. Iida, H. Sekizawa, and Y. Aiyama, J. Phys. Soc. Japan 13 (1958) 59.DOI:10.1143/JPSJ.13.58 [5] I.C. Nlebedim, N. Ranvah, P.I. Williams, Y. Melikhov, F. Anayi, J.E. Snyder, A.J. Moses, D.C. Jiles: J. Magn. Magn. Mater. 321 (2009) 2528.DOI:10.1016/j.jmmm.2009.03.021 [6] I.C. Nlebedim, J.E. Snyder, A.J. Moses, D.C. Jiles: J. Magn. Magn. Mater. 322 (2010) 3938. DOI:10.1016/j.jmmm.2010.08.026 [7] N. Hanh, O.K. Quy, N.P. Thuy, L.D. Tung, L. Spinu: Physica B 327 (2003) 382.DOI:10.1016/S0921-4526(02)01750-7 [8] G.V. Duong, R. Sato Turtelli, N. Hanh, D.V. Linh, M. Reissner, H. Michor, J. Fidler, G. Wiesinger, R. Grössinger: J. Magn. Magn. Mater. 307 (2006) 313. [9] G.V. Duong, R. Sato Turtelli, W.C. Nunes, E. Schafler, N. Hanh, R. Grössinger, M. Knobel: J. Non-Cryst. Solids 353 (2007) 805.
(Received November 16, 2011; accepted April 16, 2012) Fig. 5. Coercive field as a function of T1 / 2.
Bibliography
tion of T1/2. In the range where the magnetization at 9 T could be considered to be constant a linear fitting was performed in order to determine the values of TB, K and consequently grain size d. The determined values of anisotropy K, d (particle size for spheres) and TB from equations above are: TB = 324 K, K = 8.0 · 105 J · m – 3 and d = 6.4 nm. The thus estimated blocking temperature agrees with that determined from the peak of ZFC measured with lowest field of 25 Oe shown in Fig. 2. The anisotropy constant is significantly higher than that published for bulk CoFe2O4. However the coercivity of this superparamagnetic material is also high at low temperatures, which can be taken as a hint for a high anisotropy due to local symmetry breaking. The grain size estimated from the temperature dependence of the coercive field agrees very well with that determined from XRD, as well as from TEM observation (see Fig. 1).
DOI 10.3139/146.110791 Int. J. Mat. Res. (formerly Z. Metallkd.) 103 (2012) 9; page 1 – 3 # Carl Hanser Verlag GmbH & Co. KG ISSN 1862-5282
Int. J. Mat. Res. (formerly Z. Metallkd.) 103 (2012) 9
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