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Magnetic materials from co-precipitated ferrite nanoparticles
Materials Science and Engineering B 167 (2010) 85–90
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Magnetic materials from co-precipitated ferrite nanoparticles V. Musat a,∗ , O. Potecasu a , R. Belea b , P. Alexandru a a Center of Nanostructures and Functional Materials-CNMF, Faculty of Metallurgy and Materials Science, “Dunarea de Jos” University of Galati, 111 Domneasca Street, 800201 Galati, Romania b Faculty of Electrical and Electronics Engineering, “Dun˘area de Jos” University of Galati, 111 Domneasca Street, 800201 Galati, Romania
a r t i c l e
i n f o
Article history: Received 26 August 2009 Received in revised form 17 January 2010 Accepted 20 January 2010 Keywords: Co-precipitation Nanoparticle Manganese ferrite Compact magnetic material Magnetic anisotropy
a b s t r a c t Some of recent technological advances in electronics need very compact magnetic materials. The paper presents the morphology and the magnetic properties of very dense polycrystalline magnetic materials obtained from co-precipitated manganese ferrite nanoparticles. The ferrite nanoparticles, with average diameter in the range of 13–25 nm, were obtained through an original low-cost co-precipitation route from aqueous solution of Mn2+ and Fe3+ ions generated by redox reactions between stoichiometric amounts of MnO2 (piroluzite) and FeSO4 ·7H2 O raw materials. Very dense homogeneous polycrystalline magnetic materials with high square hysteresis loop (Br /Bs = 0.91) and low intrinsic coercivity were obtained using the co-precipitated un-doped manganese ferrite nanoparticles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Even ferrite materials have been under intense research for long time due to their useful electromagnetic properties in a large number of applications [1], recently, ferrite nanoparticles have attracted great attention due to the controllability of superparamagnetic and magnetic single domain behaviour [2]. For magnetic particle smaller than the critical size for multi-domain formation, the domain wall resonance is avoided and the material can work at higher frequencies [3]. The recent technological advances in electronics need more compact cores for work at higher frequencies [4]. The current interest has been to make nanosized ferrite particles to reduce energy losses associate to bulk powders and to get large shapes pressed materials with near theoretical density required by the most electronic application, which is difficult to obtain with wide size particles. Magnetic ferrite nanoparticles have also applications in magnetic high-density information storage/recording media and as ferrofluids or magnetic drug delivery. For the abovementioned applications, the particles must have not only suitable magnetic properties but also reduced size and uniform shape [5]. Despite the great number of papers, preparing ferrite nanoparticles suitable for new advanced applications is still a challenge [1–7]. Used as dispersed systems (ferrofluids) or as compact sintered materials, the ferrite particles in nanoscale can be produced by bottom-up nanotechnology approach, known as soft chemical methods, such as co-precipitation [3], sol–gel, hydrothermal syn-
thesis [8], reverse micelle synthesis [6,9], etc. Among these various methods, different co-precipitation routes are used [3,7,10–14]. MnFe2 O4 nanoparticles have been obtained by co-precipitation from Fe3+ and Mn2+ [12] or Fe2+ and Mn2+ [13,14] salts aqueous solutions, by co-precipitation of Mn2+ and Fe3+ metal ions in a toluene/water/sodium dodecylbenzenesulfonate microemulsion system [15], from aqueous solution using different types of buffer [16], or by a growth-assisted co-precipitation process [9], etc. The co-precipitation technique results in the better homogeneity of the ferrite nanoparticles. The non-conventional bottom-up nanotechnology methods, especially the wet chemical routes, allow the preparation of high reactive ferrite nanoparticles whose composition, microstructure, size and properties can be rigorously controlled in order to obtain the special requirements of various advanced applications [17,18]. In this paper we present some results concerning the morphology and magnetic properties of compact magnetic materials obtained from manganese ferrite nanoparticles prepared by an original co-precipitation route from MnO2 and FeSO4 ·7H2 O as raw materials. The solution undergoing co-precipitation contains Mn2+ and Fe3+ ions generated by redox reactions between the raw materials reactants. Very dense polycrystalline magnetic materials with square hysteresis loops and low intrinsic coercivity have been obtained using un-doped manganese ferrite nanoparticles. 2. Experimental 2.1. Ferrite nanoparticles
∗ Corresponding author. Tel.: +40 757070613; fax: +40 236460754. E-mail addresses: [email protected], [email protected] (V. Musat). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.038
The co-precipitation of the ferrite particles was performed by adding, under stirring, a 15% solution of NaOH (Merck) to the 2N
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V. Musat et al. / Materials Science and Engineering B 167 (2010) 85–90
solution of the cations (Mn2+ and Fe3+ ) obtained through simultaneous dissolution of stoichiometric amounts of MnO2 (piroluzite mineral) and FeSO4 ·7H2 O (Merck). The aging occurs in the absence of NH4 NO3 and without air bubbling. After ageing until 120 min at different temperatures in the range 70–95 ◦ C and cooling at room temperature (RT), the particles were separated from the supernatant solution by centrifugation, washed and dried in air at RT and then at 65 ◦ C. The resulted particles were analyzed by transmission electron microscopy (TEM), X-ray diffraction and Mössbauer spectroscopy. The transmission electron microscopy (TEM) images were obtained with a Jeol-100 CX II microscopy. The X-ray diffraction (XRD) patterns were recorded with a PHILIPS PW 1050 equipment using Cu K␣ radiation ( = 154, 178 nm). The mean crystallite size was calculated from the XRD data using the Debye–Scherer equation: D(3
1 1)
=
0.9 cos , ˇ1/2
(1)
where D(3 1 1) is the average particle size of the crystallites calculated from (3 1 1) peak data, is the wavelength of the incident X-ray, is the corresponding Bragg angle and ˇ1/2 is the full width at the half maximum (FWHM) of the (3 1 1) peak. The uncertainties in the particle size determination were estimated from the errors in the fitting procedures while fitting the (3 1 1) peak, which lie in the range ±1 nm. The Mössbauer spectra were recorded at RT and at 4.2 K with a PROMEDA (Israel) conventional transmission spectrometer and a 25 m Ci Co57 (Rh) source. The spectrometer was calibrated by a standard ␣-iron absorber at room temperature. 2.2. Magnetic materials Manganese ferrite nanoparticles co-precipitated and 2 h aged at 95 ◦ C was pressed at 10.5 GPa as toroidal shape sample (9 mm × 6 mm × 4 mm) and sintered in air at 1250 ◦ C. The sintering of the samples was done by heating (1.5 ◦ C/min in the range 20–350 ◦ C and 5 ◦ C/min in the range 350–1250 ◦ C) and maintaining between 15 min and 4 h at 1250 ◦ C in air atmosphere followed by cooling in inert (N2 ) atmosphere. The density of the sintered samples was measured with the analytical balance PCE-AB 200 using a density kit PCE-AB-DB. The morphology on the fracture surface of the sintered samples was observed by scanning electron microscopy (SEM) using a JEOL-3 (Japan) microscope. The magnetic data were recorded at a maximum applied field of 250 A/m. 3. Results and discussion 3.1. Ferrite nanoparticles Fig. 1 shows the transmission electron microscopy (TEM) images of the ferrite nanoparticles. The size of nanoparticles, estimated from the TEM micrographs for over hundred particles, corresponds to mean diameters of about 17 ± 1, 25 ± 1 and 13 ± 0.5 nm for the samples obtained at 70, 80 and 95 ◦ C, respectively. One can notice that there is no continuous variation of the nanoparticle size with the temperature of preparation, smallest nanoparticles were obtained at 95 ◦ C. The X-ray diffraction patterns (Fig. 2) confirm the formation of single-phase cubic spinel structure in all the samples. From the full width half maximum of the (3 1 1) peak, using the Debye–Scherer equation, the mean crystallite size of 16, 27 and 14 nm have been calculated for the samples obtained at 70, 80 and 95 ◦ C, respectively. These average particle sizes estimated from XRD data are in good agreement with the values obtained from the TEM data. Comparing these values, one can assume that ferrite particles are single crystal nanoparticles.
Fig. 1. TEM micrographs of the manganese ferrite nanoparticles co-precipitated and aged at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
Figs. 3 and 4 show the experimental Mössbauer spectra of the investigated samples. The spectra obtained at 293 K (Fig. 3) consist in a broad non-lorentzian hyperfine magnetic component together with a central quadrupolar doublet. There is a distribution of hyperfine magnetic fields in the range of 500–360 kOe. The isomer shifts I.S. (0.3 mm/s, relative to Fe) and the quadrupolar splittings Q.S. (closed to zero) were practically the same for all magnetic sextets in the Mössbauer spectra of the investigated samples. The quadrupolar splitting of the central doublet is ∼0.7 mm/s and the characteristic isomer shift was found to be ∼0.4 mm/s in agreement with the results of Tang et al. [6]. The central doublet with Mössbauer parameters I.S. ∼0.4 mm/s and Q.S. ∼0.7 mm/s (specific for Fe3+ ions in octahedral co-ordination) is attributed to very small manganese ferrite particles. This behaviour confirms a simultaneous contribution of ferrimagnetic particles in the range of about
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Fig. 2. XRD patterns of the manganese ferrite nanoparticles obtained at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
Fig. 4. Mössbauer spectra at 4.2 K of the manganese ferrite nanoparticles obtained at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
20 nm or higher and of the superparamagnetic effects of much smaller ferrite nanoparticles. The Mössbauer spectra recorded at 4.2 K (Fig. 4) shows no more the central quadrupolar doublet observed in the spectra recorded at RT. That confirms the existence of the superparamagnetic effect of very small crystallites in the ferrite powders at ambient temperature. 3.2. Magnetic sintered materials
Fig. 3. Mössbauer spectra at 293 K of the manganese ferrite nanoparticles obtained at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
The sintered magnetic materials were obtained using the nanoparticles of ferrite prepared at 95 ◦ C. After pressing at 10.5 GPa and before sintering, the samples had an apparent density of 2.4 ± 0.2 g/cm3 . In order to determine the optimal sintering (compaction and crystallization) temperature, the initial (non-aged) co-precipitated powder was subjected to thermal annealing at different temperature values in the range 200–1250 ◦ C. Our result shows that, separated from the solution in which has been obtained, washed, dried and heated in air, the initial co-precipitate powder was transformed into single ferrite phase MnFe2 O4 at temperatures higher than 1000 ◦ C (curves h and i in Fig. 5), in agreement with the results obtained by Rashad [19]. In alkaline solution, the same precipitate rapidly generates the ferrite phase at temperatures lower than 100 ◦ C (Fig. 2). Depending on the duration of isothermal sintering at 1250 ◦ C, between 1 and 2 h, the ring samples reached outer diameter between 8 and 7.5 mm, inside diameter between 5 and 4.5 mm, height between 3.5 and 3 mm and a density of ∼5 g/cm3 (Table 1). The SEM images on the fracture surface of the sintered materials presented in Fig. 6 show the morphological changes of the compacted ferrite material during the isothermal heat treatment of sintering. Fig. 6a shows the channel network structure exist-
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Table 1 Characteristics of the magnetic materials sintered in air at 1250 ◦ C and cooling in inert (N2 ) atmosphere. Duration of isothermal sintering (min)
Density (±0.050 g cm−3 )
Intrinsic coercivity, Hc (A m−1 )
Remanent magnetic induction, Br (T)
Saturation magnetic induction, Bs (T)
Coefficient of rectangularity, R = Br /Bs
15 120 240
4.50 5.00 5.05
58.3 45.2 47.5
0.217 0.318 0.285
0.289 0.349 0.413
0.75 0.91 0.63
ing after 15 min of isothermal maintaining at 1250 ◦ C. During the heating of the samples until 1250 ◦ C (the isothermal stage of sintering), the aggregates of nanoparticles transform into polyhedron grains and the inter-particles spaces transform into a network of channels by superficial diffusion. This transformation is associated with 80–85% of the total densification. The channels network represents the route for the gases release. Fig. 6b shows the sample morphology after 70 min of isothermal maintaining at 1250 ◦ C. At the end of the first hour of maintaining at 1250 ◦ C, the network of channels is processed in isolated pores and the “secondary crystallization” starts, at a porosity of about 8%. In this stage, the grains increase by larger grains including the smaller ones (Fig. 6b), in a process controlled by the diffusion at the inter-granular limit. This
Fig. 5. XRD patterns of the powder co-precipitated at 95 ◦ C: non-aged (a), non-aged and air heated at 200 ◦ C (b), 350 ◦ C (c), 450 ◦ C (d), 500 ◦ C (e), 950 ◦ C (f), 1050 ◦ C (g), 1100 ◦ C (h) and 1250 ◦ C (i).
phenomenon is the result of the condition of balance between the three competing surface tension, , at the combination of the of three grains, expressed by the following relationship [20,21]: 2 3 1 = = and sin 1 sin 2 sin 3
i = 0,
(2)
where 1 , 2 and 3 are the dihedral angle from grains joining. According to the relationship (2), the faces of the large particles (more than six sides) receive a curvature inwards (concave) and the faces of the small particles (less than six sides) acquire a curvature outwards (convex). In this process, a gradient of defects concentration is achieved; the concave surface get a higher concentration of vacancies in relation to the convex surfaces. This gradient leads to the displacement of the inter-granular limits by the ions diffusion from the small grain with convex sides to large grain with concave sides. This phenomenon leads to the growth of the larger grain at the expense of smaller ones, by the “swallowing” small grains by the greatest ones (Fig. 6b). In Fig. 6c is observed that after 2 h of maintenance to 1250 ◦ C, the material transforms into polyhedral grains and roundish pores. This figure shows a very high compact and homogeneous polycrystalline microstructure with a porosity of about 3% and grains with size ranging between 3 and 5 m. The processes resulting on well-formed surface of the grains, sharp edges and spherical shape of the pores are controlled by volume diffusion. The magnetic material presented in Fig. 6c has a density of 5.05 g/cm3 , which represents 96% of the theoretical value. If the duration of the isothermal maintenance at 1250 ◦ C increases, the spherical pores are transformed into tetrahedral ones (Fig. 6d), as a result of the point defects (vacancies) accumulation in the planes {1 1 1}, {1 0 0} and {1 1 0} [22]. Fig. 6e shows some microstructural changes when the duration of the isothermal sintering increases at 4 h. The rounding edges of the grains and distortion of the crystalline grain shape take place. For the same chemical composition, the magnetic properties of the ferrites depend on the crystal structure, grain size and porosity. The structural changes described above have direct effect on the magnetic properties of the sintered materials, as can be seen from the hysteresis loops presented in Fig. 7 and the corresponding values of the magnetic parameters presented in Table 1. From Table 1 is observed that, when the duration of isothermal maintaining at 1250 ◦ C increases, the values of the saturation induction (Bs ) increases continuously while the remanent induction (Br ) increases until the sample corresponding to 2 h of maintenance and then decreases. The room temperature hysteresis loops of the samples shown in Fig. 7 have minimal areas and small coercivity indicating small hysteresis loss in these ferrite materials. The hysteresis loop of the material obtained after 2 h of maintenance at 1250 ◦ C (Fig. 7b) presents the highest degree of rectangularity (Br /Bs ≥ 0.9). Increasing the sintering duration to 4 h leads to lower the magnetic anisotropy by decreasing the internal stress and crystal anisotropy. As shown in Fig. 6e, increasing duration of maintaining at high temperature leads to rounding of grain edges and distortion of the crystalline grain shape, which leads to lower the magnetic anisotropy [23] and therefore to lowering the coefficient of rect-
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Fig. 6. SEM images on the fracture surface of the compacted samples sintered at 1250 ◦ C for 15 min (a), 70 min (b), 120 min (c), 180 min (d) and 240 min (e).
angularity (Fig. 7c) from 0.91 to 0.63. This change is due to both a slight decrease of the remanent induction and the increase of the saturation induction (Table 1). The decrease of the internal stress reduces the hindrance to the movement of the walls domain resulting thereby in the increased value of the initial permeability. The high reactivity due to the very high area of the specific surface of the ferrite nanoparticles obtained at temperature below 100 ◦ C by wet bottom-up methods allow lower sintering temperature necessary to obtain compact materials close to the theoretical density, with homogeneous polycrystalline microstruc-
ture, well-formed grains, sharp edges and therefore a high magnetic anisotropy. In this study, the sintering of the co-precipitated particles was performed at a temperature of about 100 ◦ C lower than the temperature used by classical technology. Usually, the sintering temperature of the manganese ferrite powders obtained by classical ceramic technology is around 1360 ◦ C, to achieve high degree of compaction. The properties of the ferrite materials obtained by this wet chemical method show much better results when compared to the properties of the magnetic materials obtained by conventional method.
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nanoparticles, between ∼13 and ∼25 nm. The Mössbauer spectra suggest a simultaneous contribution of ferromagnetic nanoparticles and of very small superparamagnetic nanoparticles. Very dense, until 96% of theoretical density, polycrystalline magnetic materials can be obtained using co-precipitated ferrite nanoparticles. Magnetic materials with low coercivity (about 45 A/m) and high square hysteresis loop (Br /Bs = 0.91) was obtained using the co-precipitated manganese ferrite nanoparticles without adding doping elements to increase the magnetic anisotropy. The high surface area and the surface reactivity of the nanoparticles prepared by wet route cause a very good compactation by superficial diffusion, which is the mechanism with the smallest activation energy and provides premises to achieve near the theoretical density of the sintered material, at temperature and duration lower than in the case of powders obtained by classical ceramic technology. The non-conventional bottom-up nanotechnology methods allow the preparation, in more efficient economically conditions than the classical ceramic technology, of high reactive ferrite nanoparticles whose composition, microstructure, size and properties can be rigorously controlled in order to obtain the special requirements of various advanced applications. Acknowledgements Dr. Lucian Diamandescu from the Institute of Atomic Physics, Bucharest–M˘agurele, Romania and Dr. Thomas Grygar from the Academy of Sciences of the Czech Republic are warmly acknowledged for the Mössbauer spectra and TEM images, respectively. References
Fig. 7. Magnetic hysteresis loops of the manganese ferrite samples sintered at 1250 ◦ C for 15 min (a), 120 min (b) and 240 min (c).
4. Conclusion Manganese ferrite nanoparticles have been obtained through an original low-cost co-precipitation route from an aqueous solution of Mn2+ and Fe3+ ions generated by redox reactions between stoichiometric amounts of MnO2 (piroluzite) and FeSO4 ·7H2 O raw materials. All the particles prepared in the temperature range 70–95 ◦ C have spinel type crystalline structure of manganese ferrite and magnetic properties. The ageing in the above-mentioned temperature range modifies the average diameter of the obtained ferrite
[1] E.C. Snelling, C.M. Shrivastava, M.J. Patni (Eds.), Advances in Ferrites, vol. 1, Oxford & IBH, New Delhi, India, 1989, 579 p. [2] W. Jiang, H.C. Yang, S.Y. Yang, H.E. Horng, J.C. Hung, Y.C. Chen, C.-Y. Hong, J. Magn. Magn. Mater. 283 (2004) 210–214. [3] B.P. Rao, A.M. Kumar, K.H. Rao, Y.L.N. Murthy, O.F. Caltun, I. Dumitru, L. Spinu, J. Optoelectron. Adv. Mater. 8 (2006) 1703–1705. [4] Y. Liu, S. He, J. Magn. Magn. Mater. 320 (2008) 3318–3322. [5] S. Ammnar, et al., J. Mater. Chem. 11 (2001) 186–192. [6] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, J. Colloid Interface Sci. 146 (1991) 38–40. [7] I. Rhee, C. Chim, J. Korean Phys. Soc. 42 (2003) 175–177. [8] C. Rath, N.C. Mishra, S. Anard, R.P. Das, K.K. Sahu, C. Upadhyay, H.C. Verma, Appl. Phys. Lett. 75 (2000) 475–477. [9] S. Morrison, C.L. Cahill, S. Calvi, R. Swaminathan, M.E. McHery, V.G. Harris, J. Appl. Phys. 95 (2004) 6392–6395. [10] H. Yang, X. Zhang, A. Tang, G. Qiu, Chem. Lett. 33 (2004) 826–829. [11] B.P. Rao, C.O. Kim, C. Kim, I. Dumitru, L. Spinu, O.F. Caltun, Magn. IEEE Trans. 42 (2006) 2858–2860. [12] J. Pattanayak, J. Mater. Sci. Lett. 10 (1991) 1461–1464. [13] A. Mellissa, W. Denecke, W. Gußer, B. Buxbaum, P. Kuske, Mater. Res. Bull. 27 (1992) 507–509. [14] P. Mathur, A. Thakur, M. Singh, J. Magn. Magn. Mater. 320 (2008) 1364–1369. [15] M. Bellusci, S. Canepari, G. Ennas, A. La Barbera, F. Padella, A. Santini, A. Scano, F. Varsano, J. Am. Ceram. Soc. 90 (2007) 3977–3983. [16] R. Aquino, F.A. Tourinho, R. Itri, M.C.F.L. Lara, J. Depeyrot, J. Magn. Magn. Mater. 252 (2002) 23–25. [17] M.C. Deng, T.S. Chin, Jpn. J. Appl. Phys. 30 (1991) L1276–L1279. [18] A. Goldman, Modern Ferrite Technology, second ed., Springer, US, 2006, 9780-387r-r28151-3. [19] M.M. Rashad, Mater. Sci. Eng. B 127 (2006) 123–129. [20] U. Linus, T. Ogbuji, Ceram. Int. 16 (1990) 195–200. [21] L.M. Letjuk, G.Y. Zuravlev, Himija i tehnologja ferritov, Himija, Leningrad, 1983. [22] A. Kwiatkowski, J. Rezka, W.A. Szwej, Sci. sintering 8 (1976), 53–63 and 157–162. [23] P. Perriad, B. Gillot, Solid State Ionics 67 (1993) 35–43.
Contents lists available at ScienceDirect
Materials Science and Engineering B journal homepage: www.elsevier.com/locate/mseb
Magnetic materials from co-precipitated ferrite nanoparticles V. Musat a,∗ , O. Potecasu a , R. Belea b , P. Alexandru a a Center of Nanostructures and Functional Materials-CNMF, Faculty of Metallurgy and Materials Science, “Dunarea de Jos” University of Galati, 111 Domneasca Street, 800201 Galati, Romania b Faculty of Electrical and Electronics Engineering, “Dun˘area de Jos” University of Galati, 111 Domneasca Street, 800201 Galati, Romania
a r t i c l e
i n f o
Article history: Received 26 August 2009 Received in revised form 17 January 2010 Accepted 20 January 2010 Keywords: Co-precipitation Nanoparticle Manganese ferrite Compact magnetic material Magnetic anisotropy
a b s t r a c t Some of recent technological advances in electronics need very compact magnetic materials. The paper presents the morphology and the magnetic properties of very dense polycrystalline magnetic materials obtained from co-precipitated manganese ferrite nanoparticles. The ferrite nanoparticles, with average diameter in the range of 13–25 nm, were obtained through an original low-cost co-precipitation route from aqueous solution of Mn2+ and Fe3+ ions generated by redox reactions between stoichiometric amounts of MnO2 (piroluzite) and FeSO4 ·7H2 O raw materials. Very dense homogeneous polycrystalline magnetic materials with high square hysteresis loop (Br /Bs = 0.91) and low intrinsic coercivity were obtained using the co-precipitated un-doped manganese ferrite nanoparticles. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Even ferrite materials have been under intense research for long time due to their useful electromagnetic properties in a large number of applications [1], recently, ferrite nanoparticles have attracted great attention due to the controllability of superparamagnetic and magnetic single domain behaviour [2]. For magnetic particle smaller than the critical size for multi-domain formation, the domain wall resonance is avoided and the material can work at higher frequencies [3]. The recent technological advances in electronics need more compact cores for work at higher frequencies [4]. The current interest has been to make nanosized ferrite particles to reduce energy losses associate to bulk powders and to get large shapes pressed materials with near theoretical density required by the most electronic application, which is difficult to obtain with wide size particles. Magnetic ferrite nanoparticles have also applications in magnetic high-density information storage/recording media and as ferrofluids or magnetic drug delivery. For the abovementioned applications, the particles must have not only suitable magnetic properties but also reduced size and uniform shape [5]. Despite the great number of papers, preparing ferrite nanoparticles suitable for new advanced applications is still a challenge [1–7]. Used as dispersed systems (ferrofluids) or as compact sintered materials, the ferrite particles in nanoscale can be produced by bottom-up nanotechnology approach, known as soft chemical methods, such as co-precipitation [3], sol–gel, hydrothermal syn-
thesis [8], reverse micelle synthesis [6,9], etc. Among these various methods, different co-precipitation routes are used [3,7,10–14]. MnFe2 O4 nanoparticles have been obtained by co-precipitation from Fe3+ and Mn2+ [12] or Fe2+ and Mn2+ [13,14] salts aqueous solutions, by co-precipitation of Mn2+ and Fe3+ metal ions in a toluene/water/sodium dodecylbenzenesulfonate microemulsion system [15], from aqueous solution using different types of buffer [16], or by a growth-assisted co-precipitation process [9], etc. The co-precipitation technique results in the better homogeneity of the ferrite nanoparticles. The non-conventional bottom-up nanotechnology methods, especially the wet chemical routes, allow the preparation of high reactive ferrite nanoparticles whose composition, microstructure, size and properties can be rigorously controlled in order to obtain the special requirements of various advanced applications [17,18]. In this paper we present some results concerning the morphology and magnetic properties of compact magnetic materials obtained from manganese ferrite nanoparticles prepared by an original co-precipitation route from MnO2 and FeSO4 ·7H2 O as raw materials. The solution undergoing co-precipitation contains Mn2+ and Fe3+ ions generated by redox reactions between the raw materials reactants. Very dense polycrystalline magnetic materials with square hysteresis loops and low intrinsic coercivity have been obtained using un-doped manganese ferrite nanoparticles. 2. Experimental 2.1. Ferrite nanoparticles
∗ Corresponding author. Tel.: +40 757070613; fax: +40 236460754. E-mail addresses: [email protected], [email protected] (V. Musat). 0921-5107/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.mseb.2010.01.038
The co-precipitation of the ferrite particles was performed by adding, under stirring, a 15% solution of NaOH (Merck) to the 2N
86
V. Musat et al. / Materials Science and Engineering B 167 (2010) 85–90
solution of the cations (Mn2+ and Fe3+ ) obtained through simultaneous dissolution of stoichiometric amounts of MnO2 (piroluzite mineral) and FeSO4 ·7H2 O (Merck). The aging occurs in the absence of NH4 NO3 and without air bubbling. After ageing until 120 min at different temperatures in the range 70–95 ◦ C and cooling at room temperature (RT), the particles were separated from the supernatant solution by centrifugation, washed and dried in air at RT and then at 65 ◦ C. The resulted particles were analyzed by transmission electron microscopy (TEM), X-ray diffraction and Mössbauer spectroscopy. The transmission electron microscopy (TEM) images were obtained with a Jeol-100 CX II microscopy. The X-ray diffraction (XRD) patterns were recorded with a PHILIPS PW 1050 equipment using Cu K␣ radiation ( = 154, 178 nm). The mean crystallite size was calculated from the XRD data using the Debye–Scherer equation: D(3
1 1)
=
0.9 cos , ˇ1/2
(1)
where D(3 1 1) is the average particle size of the crystallites calculated from (3 1 1) peak data, is the wavelength of the incident X-ray, is the corresponding Bragg angle and ˇ1/2 is the full width at the half maximum (FWHM) of the (3 1 1) peak. The uncertainties in the particle size determination were estimated from the errors in the fitting procedures while fitting the (3 1 1) peak, which lie in the range ±1 nm. The Mössbauer spectra were recorded at RT and at 4.2 K with a PROMEDA (Israel) conventional transmission spectrometer and a 25 m Ci Co57 (Rh) source. The spectrometer was calibrated by a standard ␣-iron absorber at room temperature. 2.2. Magnetic materials Manganese ferrite nanoparticles co-precipitated and 2 h aged at 95 ◦ C was pressed at 10.5 GPa as toroidal shape sample (9 mm × 6 mm × 4 mm) and sintered in air at 1250 ◦ C. The sintering of the samples was done by heating (1.5 ◦ C/min in the range 20–350 ◦ C and 5 ◦ C/min in the range 350–1250 ◦ C) and maintaining between 15 min and 4 h at 1250 ◦ C in air atmosphere followed by cooling in inert (N2 ) atmosphere. The density of the sintered samples was measured with the analytical balance PCE-AB 200 using a density kit PCE-AB-DB. The morphology on the fracture surface of the sintered samples was observed by scanning electron microscopy (SEM) using a JEOL-3 (Japan) microscope. The magnetic data were recorded at a maximum applied field of 250 A/m. 3. Results and discussion 3.1. Ferrite nanoparticles Fig. 1 shows the transmission electron microscopy (TEM) images of the ferrite nanoparticles. The size of nanoparticles, estimated from the TEM micrographs for over hundred particles, corresponds to mean diameters of about 17 ± 1, 25 ± 1 and 13 ± 0.5 nm for the samples obtained at 70, 80 and 95 ◦ C, respectively. One can notice that there is no continuous variation of the nanoparticle size with the temperature of preparation, smallest nanoparticles were obtained at 95 ◦ C. The X-ray diffraction patterns (Fig. 2) confirm the formation of single-phase cubic spinel structure in all the samples. From the full width half maximum of the (3 1 1) peak, using the Debye–Scherer equation, the mean crystallite size of 16, 27 and 14 nm have been calculated for the samples obtained at 70, 80 and 95 ◦ C, respectively. These average particle sizes estimated from XRD data are in good agreement with the values obtained from the TEM data. Comparing these values, one can assume that ferrite particles are single crystal nanoparticles.
Fig. 1. TEM micrographs of the manganese ferrite nanoparticles co-precipitated and aged at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
Figs. 3 and 4 show the experimental Mössbauer spectra of the investigated samples. The spectra obtained at 293 K (Fig. 3) consist in a broad non-lorentzian hyperfine magnetic component together with a central quadrupolar doublet. There is a distribution of hyperfine magnetic fields in the range of 500–360 kOe. The isomer shifts I.S. (0.3 mm/s, relative to Fe) and the quadrupolar splittings Q.S. (closed to zero) were practically the same for all magnetic sextets in the Mössbauer spectra of the investigated samples. The quadrupolar splitting of the central doublet is ∼0.7 mm/s and the characteristic isomer shift was found to be ∼0.4 mm/s in agreement with the results of Tang et al. [6]. The central doublet with Mössbauer parameters I.S. ∼0.4 mm/s and Q.S. ∼0.7 mm/s (specific for Fe3+ ions in octahedral co-ordination) is attributed to very small manganese ferrite particles. This behaviour confirms a simultaneous contribution of ferrimagnetic particles in the range of about
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Fig. 2. XRD patterns of the manganese ferrite nanoparticles obtained at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
Fig. 4. Mössbauer spectra at 4.2 K of the manganese ferrite nanoparticles obtained at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
20 nm or higher and of the superparamagnetic effects of much smaller ferrite nanoparticles. The Mössbauer spectra recorded at 4.2 K (Fig. 4) shows no more the central quadrupolar doublet observed in the spectra recorded at RT. That confirms the existence of the superparamagnetic effect of very small crystallites in the ferrite powders at ambient temperature. 3.2. Magnetic sintered materials
Fig. 3. Mössbauer spectra at 293 K of the manganese ferrite nanoparticles obtained at 70 ◦ C (a), 80 ◦ C (b) and 95 ◦ C (c).
The sintered magnetic materials were obtained using the nanoparticles of ferrite prepared at 95 ◦ C. After pressing at 10.5 GPa and before sintering, the samples had an apparent density of 2.4 ± 0.2 g/cm3 . In order to determine the optimal sintering (compaction and crystallization) temperature, the initial (non-aged) co-precipitated powder was subjected to thermal annealing at different temperature values in the range 200–1250 ◦ C. Our result shows that, separated from the solution in which has been obtained, washed, dried and heated in air, the initial co-precipitate powder was transformed into single ferrite phase MnFe2 O4 at temperatures higher than 1000 ◦ C (curves h and i in Fig. 5), in agreement with the results obtained by Rashad [19]. In alkaline solution, the same precipitate rapidly generates the ferrite phase at temperatures lower than 100 ◦ C (Fig. 2). Depending on the duration of isothermal sintering at 1250 ◦ C, between 1 and 2 h, the ring samples reached outer diameter between 8 and 7.5 mm, inside diameter between 5 and 4.5 mm, height between 3.5 and 3 mm and a density of ∼5 g/cm3 (Table 1). The SEM images on the fracture surface of the sintered materials presented in Fig. 6 show the morphological changes of the compacted ferrite material during the isothermal heat treatment of sintering. Fig. 6a shows the channel network structure exist-
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Table 1 Characteristics of the magnetic materials sintered in air at 1250 ◦ C and cooling in inert (N2 ) atmosphere. Duration of isothermal sintering (min)
Density (±0.050 g cm−3 )
Intrinsic coercivity, Hc (A m−1 )
Remanent magnetic induction, Br (T)
Saturation magnetic induction, Bs (T)
Coefficient of rectangularity, R = Br /Bs
15 120 240
4.50 5.00 5.05
58.3 45.2 47.5
0.217 0.318 0.285
0.289 0.349 0.413
0.75 0.91 0.63
ing after 15 min of isothermal maintaining at 1250 ◦ C. During the heating of the samples until 1250 ◦ C (the isothermal stage of sintering), the aggregates of nanoparticles transform into polyhedron grains and the inter-particles spaces transform into a network of channels by superficial diffusion. This transformation is associated with 80–85% of the total densification. The channels network represents the route for the gases release. Fig. 6b shows the sample morphology after 70 min of isothermal maintaining at 1250 ◦ C. At the end of the first hour of maintaining at 1250 ◦ C, the network of channels is processed in isolated pores and the “secondary crystallization” starts, at a porosity of about 8%. In this stage, the grains increase by larger grains including the smaller ones (Fig. 6b), in a process controlled by the diffusion at the inter-granular limit. This
Fig. 5. XRD patterns of the powder co-precipitated at 95 ◦ C: non-aged (a), non-aged and air heated at 200 ◦ C (b), 350 ◦ C (c), 450 ◦ C (d), 500 ◦ C (e), 950 ◦ C (f), 1050 ◦ C (g), 1100 ◦ C (h) and 1250 ◦ C (i).
phenomenon is the result of the condition of balance between the three competing surface tension, , at the combination of the of three grains, expressed by the following relationship [20,21]: 2 3 1 = = and sin 1 sin 2 sin 3
i = 0,
(2)
where 1 , 2 and 3 are the dihedral angle from grains joining. According to the relationship (2), the faces of the large particles (more than six sides) receive a curvature inwards (concave) and the faces of the small particles (less than six sides) acquire a curvature outwards (convex). In this process, a gradient of defects concentration is achieved; the concave surface get a higher concentration of vacancies in relation to the convex surfaces. This gradient leads to the displacement of the inter-granular limits by the ions diffusion from the small grain with convex sides to large grain with concave sides. This phenomenon leads to the growth of the larger grain at the expense of smaller ones, by the “swallowing” small grains by the greatest ones (Fig. 6b). In Fig. 6c is observed that after 2 h of maintenance to 1250 ◦ C, the material transforms into polyhedral grains and roundish pores. This figure shows a very high compact and homogeneous polycrystalline microstructure with a porosity of about 3% and grains with size ranging between 3 and 5 m. The processes resulting on well-formed surface of the grains, sharp edges and spherical shape of the pores are controlled by volume diffusion. The magnetic material presented in Fig. 6c has a density of 5.05 g/cm3 , which represents 96% of the theoretical value. If the duration of the isothermal maintenance at 1250 ◦ C increases, the spherical pores are transformed into tetrahedral ones (Fig. 6d), as a result of the point defects (vacancies) accumulation in the planes {1 1 1}, {1 0 0} and {1 1 0} [22]. Fig. 6e shows some microstructural changes when the duration of the isothermal sintering increases at 4 h. The rounding edges of the grains and distortion of the crystalline grain shape take place. For the same chemical composition, the magnetic properties of the ferrites depend on the crystal structure, grain size and porosity. The structural changes described above have direct effect on the magnetic properties of the sintered materials, as can be seen from the hysteresis loops presented in Fig. 7 and the corresponding values of the magnetic parameters presented in Table 1. From Table 1 is observed that, when the duration of isothermal maintaining at 1250 ◦ C increases, the values of the saturation induction (Bs ) increases continuously while the remanent induction (Br ) increases until the sample corresponding to 2 h of maintenance and then decreases. The room temperature hysteresis loops of the samples shown in Fig. 7 have minimal areas and small coercivity indicating small hysteresis loss in these ferrite materials. The hysteresis loop of the material obtained after 2 h of maintenance at 1250 ◦ C (Fig. 7b) presents the highest degree of rectangularity (Br /Bs ≥ 0.9). Increasing the sintering duration to 4 h leads to lower the magnetic anisotropy by decreasing the internal stress and crystal anisotropy. As shown in Fig. 6e, increasing duration of maintaining at high temperature leads to rounding of grain edges and distortion of the crystalline grain shape, which leads to lower the magnetic anisotropy [23] and therefore to lowering the coefficient of rect-
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Fig. 6. SEM images on the fracture surface of the compacted samples sintered at 1250 ◦ C for 15 min (a), 70 min (b), 120 min (c), 180 min (d) and 240 min (e).
angularity (Fig. 7c) from 0.91 to 0.63. This change is due to both a slight decrease of the remanent induction and the increase of the saturation induction (Table 1). The decrease of the internal stress reduces the hindrance to the movement of the walls domain resulting thereby in the increased value of the initial permeability. The high reactivity due to the very high area of the specific surface of the ferrite nanoparticles obtained at temperature below 100 ◦ C by wet bottom-up methods allow lower sintering temperature necessary to obtain compact materials close to the theoretical density, with homogeneous polycrystalline microstruc-
ture, well-formed grains, sharp edges and therefore a high magnetic anisotropy. In this study, the sintering of the co-precipitated particles was performed at a temperature of about 100 ◦ C lower than the temperature used by classical technology. Usually, the sintering temperature of the manganese ferrite powders obtained by classical ceramic technology is around 1360 ◦ C, to achieve high degree of compaction. The properties of the ferrite materials obtained by this wet chemical method show much better results when compared to the properties of the magnetic materials obtained by conventional method.
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nanoparticles, between ∼13 and ∼25 nm. The Mössbauer spectra suggest a simultaneous contribution of ferromagnetic nanoparticles and of very small superparamagnetic nanoparticles. Very dense, until 96% of theoretical density, polycrystalline magnetic materials can be obtained using co-precipitated ferrite nanoparticles. Magnetic materials with low coercivity (about 45 A/m) and high square hysteresis loop (Br /Bs = 0.91) was obtained using the co-precipitated manganese ferrite nanoparticles without adding doping elements to increase the magnetic anisotropy. The high surface area and the surface reactivity of the nanoparticles prepared by wet route cause a very good compactation by superficial diffusion, which is the mechanism with the smallest activation energy and provides premises to achieve near the theoretical density of the sintered material, at temperature and duration lower than in the case of powders obtained by classical ceramic technology. The non-conventional bottom-up nanotechnology methods allow the preparation, in more efficient economically conditions than the classical ceramic technology, of high reactive ferrite nanoparticles whose composition, microstructure, size and properties can be rigorously controlled in order to obtain the special requirements of various advanced applications. Acknowledgements Dr. Lucian Diamandescu from the Institute of Atomic Physics, Bucharest–M˘agurele, Romania and Dr. Thomas Grygar from the Academy of Sciences of the Czech Republic are warmly acknowledged for the Mössbauer spectra and TEM images, respectively. References
Fig. 7. Magnetic hysteresis loops of the manganese ferrite samples sintered at 1250 ◦ C for 15 min (a), 120 min (b) and 240 min (c).
4. Conclusion Manganese ferrite nanoparticles have been obtained through an original low-cost co-precipitation route from an aqueous solution of Mn2+ and Fe3+ ions generated by redox reactions between stoichiometric amounts of MnO2 (piroluzite) and FeSO4 ·7H2 O raw materials. All the particles prepared in the temperature range 70–95 ◦ C have spinel type crystalline structure of manganese ferrite and magnetic properties. The ageing in the above-mentioned temperature range modifies the average diameter of the obtained ferrite
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