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Solid state reaction synthesis of NiFe2O4 nanoparticles
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Journal of Magnetism and Magnetic Materials 320 (2008) 857–863 www.elsevier.com/locate/jmmm
Solid state reaction synthesis of NiFe2O4 nanoparticles Abdullah Ceylana,b, Sadan Ozcanb, C. Nic, S. Ismat Shaha,c, a
Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA b Physics Engineering Department, Hacettepe University, Beytepe, Ankara 06800, Turkey c Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA Received 20 April 2007; received in revised form 29 July 2007 Available online 15 September 2007
Abstract Ni-ferrite (NiFe2O4) nanoparticles have been synthesized via a solid state reaction process. Ni and Fe bi-metallic nanoparticles in the form of Ni33Fe67 alloy nanopowder are first synthesized by simultaneous evaporation of the required amounts of pure Ni and Fe metals followed by rapid condensation of the evaporated metal flux into solid state by means of an inert gas, helium, using the process of inert gas condensation (IGC). In order to form the NiFe2O4 structure, as-synthesized samples (Ni33Fe67) are annealed for 12 h in ambient conditions at different annealing temperatures. Structural analyses show that NiFe2O4 starts to form at around 450 1C and gets progressively well defined with increasing annealing temperatures yielding particle with size ranging between 15 and 50 nm. Besides successfully forming NiFe2O4, NiO/Fe3O4 core/shell nanoparticles have also been synthesized by adjusting the annealing conditions. Three different structures, Ni33Fe67, NiO/Fe3O4, and NiFe2O4, obtained in this study are compared with respect to their structural and magnetic properties. r 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Tt; 75.30.Et; 75.75.+a Keywords: Ni-ferrite nanoparticles; Inert gas condensation
1. Introduction Nanocrystalline spinel ferrites have been the subject of many studies. These materials have attracted much attention recently due to the flexibility of controlling the physical behavior of spinel ferrites. By changing the cation type, it is possible to obtain significantly different physical properties in these ferrites. Ferrites have been used for permanent magnets, recording media, ferrofluids and in microwave applications [1–3]. In a spinel structure there are 56 ions, 32 oxygen and 24 metal ions in a unit cell. Eight metal ions occupy tetrahedral sites and 16 metal ions occupy octahedral sites. The general formula for the ferrite structure is given as ðM 1x Fex Þ½M x Fe2x O4 , where round and square Corresponding author. Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA. Tel.: +1 302 831 1618; fax: +1 302 831 4545. E-mail address: [email protected] (S. Ismat Shah).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.09.003
brackets correspond to tetrahedral and octahedral sites, respectively. M stands for divalent ions and x is the degree of inversion that is defined as the fraction of tetrahedral sites occupied by Fe3þ ions. The structure at which tetrahedral sites are occupied with only M2þ ions and octahedral sites are occupied with Fe3þ ions is called normal spinel. Magnetic ferrites are in fact inverse spinels where some of the divalent ions switch to occupy octahedral sites giving rise to various magnetic behaviors. If a material has both configurations then it is called mixed spinel. Nanosized NiFe2O4 is one type of ferrite that has been studied extensively. It shows peculiar structural and magnetic properties. Small particle size promotes a mixed spinel structure whereas the bulk form is an inverse spinel. As far as the magnetic properties of these materials are concerned, spin glass like behavior can be considered as the most interesting property that leads to high field irreversibility, shift of the hysteresis loops, and anomalous relaxation dynamics [4,5].
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High energy mechanical milling, co-precipitation, sonochemical precipitation and pulsed wire discharge are amongst the techniques that are used for the synthesis of NiFe2O4 nanoparticles [6,7]. There are various drawbacks of these techniques such as having complicated synthesis schemes, impurity formation (usually NiO), low material yield and strong agglomeration, which is mainly seen in wet processes [8,9]. Therefore, there is still a need for an alternative technique that can successfully produce ferrites with high yield, low agglomeration, and no impurity incorporation. In this work, we describe a new process based on the use of inert gas condensation (IGC) technique for the synthesis of pure NiFe2O4 nanoparticles. The process also yields two other magnetic structures (Ni33Fe67 and NiO/Fe3O4) under certain controlled conditions. Results on the investigation of structural and magnetic properties of NiFe2O4 as well as Ni33Fe67 and NiO/Fe3O4 nanoparticles are presented in a comparative manner.
samples is investigated by annealing the as-synthesized samples at different temperatures under atmospheric conditions. Structural properties of the samples are investigated by X-ray diffraction (XRD), and transmission electron microscopy (TEM). For elemental composition analysis of the samples, energy dispersive X-ray spectroscopy (EDS) is utilized. XRD studies are performed using a Rigaku D-Max B horizontal diffractometer using CuKa radiation. Transmission electron microscopy is carried out using a JEM 2010FX field emission transmission electron microscope operated at 200 kV. EDS analyses are performed using an EDAXs unit attached to the microscope. Magnetic properties of the samples are measured by using a Physical Properties Measurement System (PPMS) dc extraction magnetometer by Quantum Design Corporation.
2. Experimental
3.1. Structural analyses
IGC has been employed to synthesize the precursor Ni33Fe67 alloy nanoparticles which are subsequently annealed in ambient conditions to yield the NiFe2O4 nanoparticles. In our previous experiments, we have shown that it is possible to synthesize alloy nanoparticles of close melting point metals by simultaneously evaporating the metals to form vapors and then rapidly cooling the vapors into a metastable solid state phase [10]. The same idea has been used in this study to synthesize NiFe2O4 nanoparticles. Here, a controlled post-synthesis annealing in an oxidizing atmosphere is needed to oxidize the alloy particles to obtain the ferrite structure. As-synthesized samples are prepared in an inert gas (He) atmosphere. We tried to synthesize NiFe2O4 particles directly by using reactive gas condensation (RGC) in the presence of oxygen as the reactive gas. However, the experiments were unsuccessful. The reason was that Ni, with an electronegativity of 1.91 Pauling, oxidized in oxygen atmosphere faster than Fe which has an electronegativity of 1.83 Pauling. A microscopic mixture of NiO and Fe-oxide was obtained with RGC instead of the formation of a uniform nanosized Ni-ferrite structure. It is critical to first form an alloy of Ni and Fe prior to the oxidation so as to get the desired nanosized NiFe2O4 structure during annealing. Therefore, appropriate amounts of pure Ni and Fe were resistively evaporated using an Al2O3 coated tungsten boat kept at 1500 1C in the presence of 5 Torr He pressure. He was circulated by means of a roots blower with a speed of 1800 rpm. Details and the working principles of our system have been presented elsewhere [11]. Ni33Fe67 alloy nanoparticles are formed in situ and exposing the particles to atmospheric conditions leads the formation of a thin oxide layer at the surface of the particles. Finally, Ni33Fe67 nanoparticles are ex situ annealed in ambient conditions in a box furnace for 12 hs. The effect of annealing temperature (T an ) on the structural and magnetic properties of the
In order to confirm that the right composition Ni/Fe alloy was formed, EDS technique was utilized. A typical energy dispersive X-ray spectrum of the as-synthesized samples and cation ratio obtained based on the spectrum are given in Fig. 1. EDS analyses at several locations on the nanopowder samples were carried out to confirm the chemical homogeneity. The variation of the Ni/Fe values was in the range of the experimental error. Once the desired Ni/Fe atomic ratio was confirmed, assynthesized samples were annealed at different temperatures and the evolution of the NiFe2O4 structure was monitored using XRD. Patterns presented in Fig. 2 belong to the as-synthesized sample and the samples annealed at 350, 450, 550, 650 and 750 1C. The annealing temperatures were chosen based on the binary alloy phase diagram of Ni–Fe system [12]. According to the phase diagram, the
3. Results and discussions
Fig. 1. EDS spectrum of the as-synthesized sample with composition table.
ARTICLE IN PRESS A. Ceylan et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 857–863
two metals separate temperature range of analyses for 350 1C separation. In Fig. 3
into fcc and bcc phases in the 330–375 1C. In fact, the structural annealed sample reveal such a we summarize the results obtained
Fig. 2. XRD patterns of as-synthesized and samples annealed at 450, 550, 650, and 750 1C.
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from EDS and TEM analyses for the 350 1C sample. The bright field TEM and high angle annular dark field (HAADF) images clearly show the separation of two metals into two different structures by forming core-shell particles, as shown in Figs. 3c and d. The contrast in HAADF images is based on the atomic number of the metals and the density of the constituents [13]. Although the atomic numbers of Ni and Fe are very close to each other and it is very hard to distinguish metallic Ni and Fe via HAADF, upon annealing Ni forms NiO and Fe forms Fe3O4 (as seen in the XRD pattern, Fig. 3a) and that leads to the salient contrast difference between the two phases. Here, the problem is the distinction between the material of the core and the material of the shell. One can resolve this problem by comparing the lattice constants of the two phases. The lattice constant of Fe3O4 (8.375 A˚) is two times larger than that of NiO (4.176 A˚) which, in turn, means that in an HAADF image of NiO will appear darker than Fe3O4 due to the denser structure which creates larger electron density in the core region. The formation of NiO and Fe3O4 phases was also confirmed by selected area diffraction (SAD) analyses, Fig. 3b. Peaks in the XRD patterns of all samples annealed at temperatures 450 1C or above correspond to the NiFe2O4 structure. Particle size calculations based on NiFe2O4 (311)
Fig. 3. (a) XRD pattern, (b) SAD pattern, (c) HAADF image, and (d) Bright field TEM image of the sample annealed at 350 1C. Core/shell structure formation is evident.
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peak and using Scherrer’s formula show that the average particle size starts from 10 nm for 450 1C annealed sample and increases up to 50 nm for samples annealed at 750 1C. Bright field TEM image analyses also reveal that the particle sizes are 15–50 nm for the samples annealed at 450 and 750 1C, respectively. The small particle size difference between XRD and TEM results for 450 1C annealed samples is perhaps due to the fact that peak width in XRD patterns has contributions from various sources such as stress, compositional variations, etc. TEM analyses also show that the Ni33Fe67 alloy particles form 2 nm thick oxide layer (see inset image in Fig. 4, scale bar ¼ 5 nm) to stabilize their surface upon exposing to atmospheric conditions. IGC process usually yields clusters of small particles in powder form. When annealed, the original size of these clusters determines the ultimate size distribution and the shape of the particles since both of these quantities depend upon the number of available particles within an individual cluster. This effect has been observed in the annealed samples. TEM images shown in Fig. 4 exhibit the gradual effect of annealing on the particle size and shape. NiFe2O4 structure formation has also been confirmed by SAD patterns. The corresponding ring pattern of NiFe2O4 (see Fig. 4, SAD 450 1C) can be seen throughout the TEM sample which shows the uniformity of the NiFe2O4 structure in the samples.
4. Magnetic properties As revealed by the structural analyses, within the NiFe2O4 nanoparticle preparation process three different magnetic structures are synthesized in the form of Ni33Fe67 alloy (as-synthesized, no annealing), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C or above) nanoparticles. The magnetic properties of these structures are discussed in the following sections. In Fig. 5, room temperature hysteresis loops of the NiFe2O4 samples are given to demonstrate the size effect. The general trend observed is that as the particle size increases with annealing temperature, the saturation magnetization approaches the bulk saturation magnetization, 52.9 emu/g, while the particles become magnetically soft, as is typical for a soft magnetic structure [14]. Sufficiently small magnetic particles are usually single domains with atomic spins completely aligned by exchange interactions. The rotational barriers due to magnetocrystalline, magnetoelastic, and shape anisotropy can trap particles in two or more metastable orientations, giving rise to hysteresis. Although 450 1C sample shows superparamagnetic behavior with no coercivity, for larger NiFe2O4 particles the coercivity starts from 50 Oe and increases up to 85 Oe as the particle size increases. As a consequence of small particle sizes, superparamagnetic behavior is observed in Ni33Fe67 alloy, NiO/Fe3O4
Fig. 4. Bright field TEM images of as-synthesized (with an inset showing Ni33Fe67 particle with a 2.2 nm oxide shell) and the NiFe2O4 nanoparticles obtained at different annealing temperatures. SAD pattern of 450 1C sample is also presented.
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thickness (t), the saturation magnetization of nanoparticles is given as 6t , (1) ss ¼ ss ð1Þ 1 d
Fig. 5. Room temperature hysteresis loops for NiFe2O4 nanoparticles obtained by annealing at 450, 550, 650, and 750 1C.
Fig. 6. Room temperature hysteresis loops for Ni33Fe67 alloy (assynthesized), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C) nanoparticles.
core/shell structures, and NiFe2O4 nanoparticles. In Fig. 6 room temperature M2H loops are shown for these structures. Ni33Fe67 and NiO/Fe3O4 saturate around 2T whereas NiFe2O4 (T an ¼ 450 C) does not reach saturation even at 3T. This is possibly due to growing shape and size anisotropies as a result of agglomeration of the particles during annealing. Saturation magnetization value for Ni33Fe67 alloy particles, 39.4 emu/g, is significantly smaller than expected value for its bulk counterpart which is calculated to be around 160 emu/g, based on Ni:Fe compositions and bulk saturation values of Ni (54.4 emu/ g) and Fe (221.7 emu/g) [14]. Reduction of the magnetization is due to the size confinement as well as magnetically dead thin oxide layer formation. For a constant dead layer
where ss ð1Þ corresponds to bulk saturation magnetization and d is the particle size [15]. Therefore, using the average particle size of 15 nm obtained from TEM analysis one gets 1.9 nm for the thickness of the dead layer being consistent with the size measured by TEM. On the other hand, the saturation magnetization for NiO/Fe3O4 core shell particles is the lowest amongst the three. The magnetization for NiO/Fe3O4 stems from the Fe3O4 layer rather than the antiferromagnetic NiO core. Although enhanced magnetization has been observed from NiO nanoparticles due to low coordination of surface spins, which is not the case in this work since Fe3O4 modifies the surface, we expect the observed magnetization to result from the ferrimagnetic Fe3O4 shell [16]. Therefore, it is reasonable to get lowest magnetization from the thin Fe3O4 layer of NiO/Fe3O4. As the structure changes to NiFe2O4, which is also ferrimagnetic, saturation magnetization increases. The increase in saturation magnetization is probably due to the increased volume of magnetically active material that is the whole volume of NiFe2O4 particle, as compared with 5 nm thick Fe3O4 layer of NiO/Fe3O4. All the three synthesized magnetic nanoparticles possess core/shell structures. In the case of Ni33Fe67 alloy and NiO/ Fe3O4 nanoparticles the core/shell structure is due to having two phases, namely ferromagnet/antiferromagnet or ferrimagnet/antiferromagnet, respectively. In NiFe2O4 the magnetically ordered/disordered regions form the core/ shell structure. The latter has been very well established by theoretical and experimental studies, including in our recent work [17,18]. It has been shown that the magnetic couplings, promoted upon field cooling between the ordered core and the disordered surface spins, give rise to unidirectional anisotropy that shifts the M2H loops. Therefore, in order to investigate shift of the M2H loops, in other words the exchange bias, we have carried out 2T field cooled (FC) M2H measurements at 5 K. As expected, all the three structures exhibited loop shifts, as shown in Fig. 7. NiO/Fe3O4 particles had 1361 Oe loop shift. It should be noted that the observation of exchange bias requires field cooling the sample from above Ne`el temperature T N of antiferromagnet NiO (523 K) whereas we have performed the measurement after field cooling the sample from room temperature. The reason why loop shift is observed for NiO/Fe3O4 is due to the decrease of the ordering temperatures of antiferromagnetic oxides for the sizes below 10 nm. In our samples the NiO core size is below 10 nm [19,20]. Ni33Fe67 and NiFe2O4 show very similar bias fields, 240 and 234 Oe, respectively. Since the nature of unidirectional anisotropy observed in these two structures is different and there are structural differences, such as core/shell sizes and shape, it is difficult to give a specific reason why the two structures experience same bias
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Fig. 7. Hysteresis loops for Ni33Fe67 alloy (as-synthesized), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C) nanoparticles taken at 5 K after field cooling the samples under 2T.
fields. However, the reason why Ni33Fe67 and NiFe2O4 show significantly lower loop shifts compared with NiO/ Fe3O4 can be explained using the exchange bias expression given in the following equation: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 AAF K AF H EB ¼ , (2) M F tF where AAF and K AF denote the exchange stiffness and anisotropy constants of antiferromagnetic layer and M F and T F correspond to saturation magnetization and thickness of ferromagnetic layer [21]. Small saturation and thickness are expected to be the dominating factors for the increased loop shift. Additionally, the relatively larger thickness of the antiferromagnetic NiO layer positively affects the loop shift by increasing the anisotropy energy of the NiO layer. Finally, temperature dependencies of the magnetizations of the three structures have been investigated by zero field cooled (ZFC) and FC magnetization measurements using 5000 Oe applied field. Splitting of FC–ZFC curves has been observed from all three structures as a result of blocking/ unblocking of small particles, as shown in Fig. 8. Blocking temperature, T B , for the Ne33Fe67 alloy particles was about 30 K whereas the blocking/unblocking for the other two structures appeared at about the same temperature, 130 K. There are three main parameters that control blocking/ unblocking of the small particles: magnetocrystalline anisotropy, K, particle volume, V , and interparticle interactions [22]. T B for a constant applied field is a function of K and V in the form of T B ¼ KhV i=25kB . Given that the average particle sizes for these three samples are the same, based on TEM images, K controls the blocking temperature. Moreover, magnetocrystalline anisotropy proportionally reflects itself on M r =M s ratio. However, M r =M s values at 5 K, 0.59, 0.66, and 0.53 for Ni33Fe67, NiO/Fe3O4, and NiFe2O4, respectively, do not correlate with the observed
Fig. 8. Temperature dependence of magnetization for field cooled, FC, and zero field cooled, ZFC, Ni33Fe67 alloy (as-synthesized), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C) nanoparticles.
T B trend. Additionally, it is known that T B increases when the small particles are brought closer, as we observe in the annealed samples [23]. Therefore, the reason for different T B could only be the interparticle interactions that get stronger by the agglomeration of the particles upon annealing. 5. Conclusions Inert gas condensation along with post annealing has been investigated as an alternative technique for the synthesis of NiFe2O4 nanoparticles. It has been shown that it is possible to make NiFe2O4 nanoparticles starting from high purity Ni and Fe metals in a simple synthesis scheme. Structural analyses have revealed that proper annealing temperature is critical to get the desired nickel ferrite structure. Under non-optimal annealing temperature conditions the formation of NiO/Fe3O4 core/shell nanoparticles is observed. These core/shell structures show magnetic properties that are different from that of nickel ferrites. This study suggests that complete alloying of certain metals with the IGC process can lead to the formation of nanostructures that are useful for research and various applications. Acknowledgements A.C. would like to thank Dr. Mike Bonder for useful discussions. The work was partially funded by NSF OISE 0138151.
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References [1] R.S. Tebble, D.J. Craik, Magnetic Materials, Wiley, New York, 1969. [2] S. Krupicka, P. Novak, in: E.P. Wohlfarth (Ed.), Ferromagnetic Materials, 3, North-Holland, Amsterdam, 1982. [3] D. Wirtz, M. Fermigier, Phys. Rev. Lett. 72 (1994) 2294. [4] K. Raj, B. Moskowitz, R. Casciari, J. Magn. Magn. Mater. 149 (1995) 174. [5] R.H. Kodama, C.L. Seaman, A.E. Berkowitz, M.B. Maple, J. Appl. Phys. 75 (1994) 5639. [6] K. Shafi, et al., J. Phys. Chem. B. 101 (1997) 6409. [7] Y. Kinemuchi, K. Ishizaka, H. Suematsu, W. Jiang, K. Yatsui, Thin Solid Films 407 (2002) 109. [8] A.S. Albuquerque, et al., J. Magn. Magn. Mater. 226 (2001) 1379. [9] V. Sepelak, D. Baabe, D. Mienert, D. Schultze, F. Krumeich, F.J. Litterstb, K.D. Becker, J. Magn. Magn. Mater. 257 (2003) 377. [10] A. Ceylan, K. Jastrzembski, S. Ismat Shah, Metall. Mater. Trans. A 37 (2006) 1. [11] A. Ceylan, A.K. Rumaiz, S. Ismat Shah, J. Appl. Phys. 101 (2007) 094302.
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[12] SGTE alloy databases, hhttp://www.crct.polymtl.ca/facti, 2004. [13] D.B. Williams, C.B. Carter, Transmission Electron Microscopy, vol. 1, Springer, Berlin, 1996, pp. 145. [14] S. Chikazumi, Physics of Ferromagnetism, Oxford University Press, New York, 1997. [15] Z.X. Tang, C.M. Sorensen, K.J. Klabunde, G.C. Hadjipanayis, Phys. Rev. Lett. 67 (1991) 3602. [16] R.H. Kodama, SalahA. Makhlouf, A.E. Berkowitz, Phys. Rev. Lett. 79 (1997) 1393. [17] R.H. Kodama, A.E. Berkowitz, E.J. McNiff Jr., S. Foner, Phys. Rev. Lett. 77 (1996) 96. [18] A. Ceylan, S.K. Hasanain, S. Ismat Shah, unpublished. [19] E.N. Abarra, K. Takano, F. Hellman, A.E. Berkowitz, Phys. Rev. Lett. 77 (1996) 3451. [20] T. Ambrose, C.L. Chien, Phys. Rev. Lett. 77 (1996) 1743. [21] A.P. Malozemoff, Phys. Rev. B. 35 (1987) 3679. [22] W. Luo, S.R. Nagel, T.F. Rosenbaum, R.E. Rosensweig, Phys. Rev. Lett. 67 (1991) 2721. [23] D. Kechrakos, K.N. Trohidou, J. Magn. Magn. Mater. 262 (2003) 107.
Journal of Magnetism and Magnetic Materials 320 (2008) 857–863 www.elsevier.com/locate/jmmm
Solid state reaction synthesis of NiFe2O4 nanoparticles Abdullah Ceylana,b, Sadan Ozcanb, C. Nic, S. Ismat Shaha,c, a
Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA b Physics Engineering Department, Hacettepe University, Beytepe, Ankara 06800, Turkey c Department of Materials Science and Engineering, University of Delaware, Newark, DE 19716, USA Received 20 April 2007; received in revised form 29 July 2007 Available online 15 September 2007
Abstract Ni-ferrite (NiFe2O4) nanoparticles have been synthesized via a solid state reaction process. Ni and Fe bi-metallic nanoparticles in the form of Ni33Fe67 alloy nanopowder are first synthesized by simultaneous evaporation of the required amounts of pure Ni and Fe metals followed by rapid condensation of the evaporated metal flux into solid state by means of an inert gas, helium, using the process of inert gas condensation (IGC). In order to form the NiFe2O4 structure, as-synthesized samples (Ni33Fe67) are annealed for 12 h in ambient conditions at different annealing temperatures. Structural analyses show that NiFe2O4 starts to form at around 450 1C and gets progressively well defined with increasing annealing temperatures yielding particle with size ranging between 15 and 50 nm. Besides successfully forming NiFe2O4, NiO/Fe3O4 core/shell nanoparticles have also been synthesized by adjusting the annealing conditions. Three different structures, Ni33Fe67, NiO/Fe3O4, and NiFe2O4, obtained in this study are compared with respect to their structural and magnetic properties. r 2007 Elsevier B.V. All rights reserved. PACS: 75.50.Tt; 75.30.Et; 75.75.+a Keywords: Ni-ferrite nanoparticles; Inert gas condensation
1. Introduction Nanocrystalline spinel ferrites have been the subject of many studies. These materials have attracted much attention recently due to the flexibility of controlling the physical behavior of spinel ferrites. By changing the cation type, it is possible to obtain significantly different physical properties in these ferrites. Ferrites have been used for permanent magnets, recording media, ferrofluids and in microwave applications [1–3]. In a spinel structure there are 56 ions, 32 oxygen and 24 metal ions in a unit cell. Eight metal ions occupy tetrahedral sites and 16 metal ions occupy octahedral sites. The general formula for the ferrite structure is given as ðM 1x Fex Þ½M x Fe2x O4 , where round and square Corresponding author. Department of Physics and Astronomy, University of Delaware, Newark, DE 19716, USA. Tel.: +1 302 831 1618; fax: +1 302 831 4545. E-mail address: [email protected] (S. Ismat Shah).
0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.09.003
brackets correspond to tetrahedral and octahedral sites, respectively. M stands for divalent ions and x is the degree of inversion that is defined as the fraction of tetrahedral sites occupied by Fe3þ ions. The structure at which tetrahedral sites are occupied with only M2þ ions and octahedral sites are occupied with Fe3þ ions is called normal spinel. Magnetic ferrites are in fact inverse spinels where some of the divalent ions switch to occupy octahedral sites giving rise to various magnetic behaviors. If a material has both configurations then it is called mixed spinel. Nanosized NiFe2O4 is one type of ferrite that has been studied extensively. It shows peculiar structural and magnetic properties. Small particle size promotes a mixed spinel structure whereas the bulk form is an inverse spinel. As far as the magnetic properties of these materials are concerned, spin glass like behavior can be considered as the most interesting property that leads to high field irreversibility, shift of the hysteresis loops, and anomalous relaxation dynamics [4,5].
ARTICLE IN PRESS 858
A. Ceylan et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 857–863
High energy mechanical milling, co-precipitation, sonochemical precipitation and pulsed wire discharge are amongst the techniques that are used for the synthesis of NiFe2O4 nanoparticles [6,7]. There are various drawbacks of these techniques such as having complicated synthesis schemes, impurity formation (usually NiO), low material yield and strong agglomeration, which is mainly seen in wet processes [8,9]. Therefore, there is still a need for an alternative technique that can successfully produce ferrites with high yield, low agglomeration, and no impurity incorporation. In this work, we describe a new process based on the use of inert gas condensation (IGC) technique for the synthesis of pure NiFe2O4 nanoparticles. The process also yields two other magnetic structures (Ni33Fe67 and NiO/Fe3O4) under certain controlled conditions. Results on the investigation of structural and magnetic properties of NiFe2O4 as well as Ni33Fe67 and NiO/Fe3O4 nanoparticles are presented in a comparative manner.
samples is investigated by annealing the as-synthesized samples at different temperatures under atmospheric conditions. Structural properties of the samples are investigated by X-ray diffraction (XRD), and transmission electron microscopy (TEM). For elemental composition analysis of the samples, energy dispersive X-ray spectroscopy (EDS) is utilized. XRD studies are performed using a Rigaku D-Max B horizontal diffractometer using CuKa radiation. Transmission electron microscopy is carried out using a JEM 2010FX field emission transmission electron microscope operated at 200 kV. EDS analyses are performed using an EDAXs unit attached to the microscope. Magnetic properties of the samples are measured by using a Physical Properties Measurement System (PPMS) dc extraction magnetometer by Quantum Design Corporation.
2. Experimental
3.1. Structural analyses
IGC has been employed to synthesize the precursor Ni33Fe67 alloy nanoparticles which are subsequently annealed in ambient conditions to yield the NiFe2O4 nanoparticles. In our previous experiments, we have shown that it is possible to synthesize alloy nanoparticles of close melting point metals by simultaneously evaporating the metals to form vapors and then rapidly cooling the vapors into a metastable solid state phase [10]. The same idea has been used in this study to synthesize NiFe2O4 nanoparticles. Here, a controlled post-synthesis annealing in an oxidizing atmosphere is needed to oxidize the alloy particles to obtain the ferrite structure. As-synthesized samples are prepared in an inert gas (He) atmosphere. We tried to synthesize NiFe2O4 particles directly by using reactive gas condensation (RGC) in the presence of oxygen as the reactive gas. However, the experiments were unsuccessful. The reason was that Ni, with an electronegativity of 1.91 Pauling, oxidized in oxygen atmosphere faster than Fe which has an electronegativity of 1.83 Pauling. A microscopic mixture of NiO and Fe-oxide was obtained with RGC instead of the formation of a uniform nanosized Ni-ferrite structure. It is critical to first form an alloy of Ni and Fe prior to the oxidation so as to get the desired nanosized NiFe2O4 structure during annealing. Therefore, appropriate amounts of pure Ni and Fe were resistively evaporated using an Al2O3 coated tungsten boat kept at 1500 1C in the presence of 5 Torr He pressure. He was circulated by means of a roots blower with a speed of 1800 rpm. Details and the working principles of our system have been presented elsewhere [11]. Ni33Fe67 alloy nanoparticles are formed in situ and exposing the particles to atmospheric conditions leads the formation of a thin oxide layer at the surface of the particles. Finally, Ni33Fe67 nanoparticles are ex situ annealed in ambient conditions in a box furnace for 12 hs. The effect of annealing temperature (T an ) on the structural and magnetic properties of the
In order to confirm that the right composition Ni/Fe alloy was formed, EDS technique was utilized. A typical energy dispersive X-ray spectrum of the as-synthesized samples and cation ratio obtained based on the spectrum are given in Fig. 1. EDS analyses at several locations on the nanopowder samples were carried out to confirm the chemical homogeneity. The variation of the Ni/Fe values was in the range of the experimental error. Once the desired Ni/Fe atomic ratio was confirmed, assynthesized samples were annealed at different temperatures and the evolution of the NiFe2O4 structure was monitored using XRD. Patterns presented in Fig. 2 belong to the as-synthesized sample and the samples annealed at 350, 450, 550, 650 and 750 1C. The annealing temperatures were chosen based on the binary alloy phase diagram of Ni–Fe system [12]. According to the phase diagram, the
3. Results and discussions
Fig. 1. EDS spectrum of the as-synthesized sample with composition table.
ARTICLE IN PRESS A. Ceylan et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 857–863
two metals separate temperature range of analyses for 350 1C separation. In Fig. 3
into fcc and bcc phases in the 330–375 1C. In fact, the structural annealed sample reveal such a we summarize the results obtained
Fig. 2. XRD patterns of as-synthesized and samples annealed at 450, 550, 650, and 750 1C.
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from EDS and TEM analyses for the 350 1C sample. The bright field TEM and high angle annular dark field (HAADF) images clearly show the separation of two metals into two different structures by forming core-shell particles, as shown in Figs. 3c and d. The contrast in HAADF images is based on the atomic number of the metals and the density of the constituents [13]. Although the atomic numbers of Ni and Fe are very close to each other and it is very hard to distinguish metallic Ni and Fe via HAADF, upon annealing Ni forms NiO and Fe forms Fe3O4 (as seen in the XRD pattern, Fig. 3a) and that leads to the salient contrast difference between the two phases. Here, the problem is the distinction between the material of the core and the material of the shell. One can resolve this problem by comparing the lattice constants of the two phases. The lattice constant of Fe3O4 (8.375 A˚) is two times larger than that of NiO (4.176 A˚) which, in turn, means that in an HAADF image of NiO will appear darker than Fe3O4 due to the denser structure which creates larger electron density in the core region. The formation of NiO and Fe3O4 phases was also confirmed by selected area diffraction (SAD) analyses, Fig. 3b. Peaks in the XRD patterns of all samples annealed at temperatures 450 1C or above correspond to the NiFe2O4 structure. Particle size calculations based on NiFe2O4 (311)
Fig. 3. (a) XRD pattern, (b) SAD pattern, (c) HAADF image, and (d) Bright field TEM image of the sample annealed at 350 1C. Core/shell structure formation is evident.
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peak and using Scherrer’s formula show that the average particle size starts from 10 nm for 450 1C annealed sample and increases up to 50 nm for samples annealed at 750 1C. Bright field TEM image analyses also reveal that the particle sizes are 15–50 nm for the samples annealed at 450 and 750 1C, respectively. The small particle size difference between XRD and TEM results for 450 1C annealed samples is perhaps due to the fact that peak width in XRD patterns has contributions from various sources such as stress, compositional variations, etc. TEM analyses also show that the Ni33Fe67 alloy particles form 2 nm thick oxide layer (see inset image in Fig. 4, scale bar ¼ 5 nm) to stabilize their surface upon exposing to atmospheric conditions. IGC process usually yields clusters of small particles in powder form. When annealed, the original size of these clusters determines the ultimate size distribution and the shape of the particles since both of these quantities depend upon the number of available particles within an individual cluster. This effect has been observed in the annealed samples. TEM images shown in Fig. 4 exhibit the gradual effect of annealing on the particle size and shape. NiFe2O4 structure formation has also been confirmed by SAD patterns. The corresponding ring pattern of NiFe2O4 (see Fig. 4, SAD 450 1C) can be seen throughout the TEM sample which shows the uniformity of the NiFe2O4 structure in the samples.
4. Magnetic properties As revealed by the structural analyses, within the NiFe2O4 nanoparticle preparation process three different magnetic structures are synthesized in the form of Ni33Fe67 alloy (as-synthesized, no annealing), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C or above) nanoparticles. The magnetic properties of these structures are discussed in the following sections. In Fig. 5, room temperature hysteresis loops of the NiFe2O4 samples are given to demonstrate the size effect. The general trend observed is that as the particle size increases with annealing temperature, the saturation magnetization approaches the bulk saturation magnetization, 52.9 emu/g, while the particles become magnetically soft, as is typical for a soft magnetic structure [14]. Sufficiently small magnetic particles are usually single domains with atomic spins completely aligned by exchange interactions. The rotational barriers due to magnetocrystalline, magnetoelastic, and shape anisotropy can trap particles in two or more metastable orientations, giving rise to hysteresis. Although 450 1C sample shows superparamagnetic behavior with no coercivity, for larger NiFe2O4 particles the coercivity starts from 50 Oe and increases up to 85 Oe as the particle size increases. As a consequence of small particle sizes, superparamagnetic behavior is observed in Ni33Fe67 alloy, NiO/Fe3O4
Fig. 4. Bright field TEM images of as-synthesized (with an inset showing Ni33Fe67 particle with a 2.2 nm oxide shell) and the NiFe2O4 nanoparticles obtained at different annealing temperatures. SAD pattern of 450 1C sample is also presented.
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thickness (t), the saturation magnetization of nanoparticles is given as 6t , (1) ss ¼ ss ð1Þ 1 d
Fig. 5. Room temperature hysteresis loops for NiFe2O4 nanoparticles obtained by annealing at 450, 550, 650, and 750 1C.
Fig. 6. Room temperature hysteresis loops for Ni33Fe67 alloy (assynthesized), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C) nanoparticles.
core/shell structures, and NiFe2O4 nanoparticles. In Fig. 6 room temperature M2H loops are shown for these structures. Ni33Fe67 and NiO/Fe3O4 saturate around 2T whereas NiFe2O4 (T an ¼ 450 C) does not reach saturation even at 3T. This is possibly due to growing shape and size anisotropies as a result of agglomeration of the particles during annealing. Saturation magnetization value for Ni33Fe67 alloy particles, 39.4 emu/g, is significantly smaller than expected value for its bulk counterpart which is calculated to be around 160 emu/g, based on Ni:Fe compositions and bulk saturation values of Ni (54.4 emu/ g) and Fe (221.7 emu/g) [14]. Reduction of the magnetization is due to the size confinement as well as magnetically dead thin oxide layer formation. For a constant dead layer
where ss ð1Þ corresponds to bulk saturation magnetization and d is the particle size [15]. Therefore, using the average particle size of 15 nm obtained from TEM analysis one gets 1.9 nm for the thickness of the dead layer being consistent with the size measured by TEM. On the other hand, the saturation magnetization for NiO/Fe3O4 core shell particles is the lowest amongst the three. The magnetization for NiO/Fe3O4 stems from the Fe3O4 layer rather than the antiferromagnetic NiO core. Although enhanced magnetization has been observed from NiO nanoparticles due to low coordination of surface spins, which is not the case in this work since Fe3O4 modifies the surface, we expect the observed magnetization to result from the ferrimagnetic Fe3O4 shell [16]. Therefore, it is reasonable to get lowest magnetization from the thin Fe3O4 layer of NiO/Fe3O4. As the structure changes to NiFe2O4, which is also ferrimagnetic, saturation magnetization increases. The increase in saturation magnetization is probably due to the increased volume of magnetically active material that is the whole volume of NiFe2O4 particle, as compared with 5 nm thick Fe3O4 layer of NiO/Fe3O4. All the three synthesized magnetic nanoparticles possess core/shell structures. In the case of Ni33Fe67 alloy and NiO/ Fe3O4 nanoparticles the core/shell structure is due to having two phases, namely ferromagnet/antiferromagnet or ferrimagnet/antiferromagnet, respectively. In NiFe2O4 the magnetically ordered/disordered regions form the core/ shell structure. The latter has been very well established by theoretical and experimental studies, including in our recent work [17,18]. It has been shown that the magnetic couplings, promoted upon field cooling between the ordered core and the disordered surface spins, give rise to unidirectional anisotropy that shifts the M2H loops. Therefore, in order to investigate shift of the M2H loops, in other words the exchange bias, we have carried out 2T field cooled (FC) M2H measurements at 5 K. As expected, all the three structures exhibited loop shifts, as shown in Fig. 7. NiO/Fe3O4 particles had 1361 Oe loop shift. It should be noted that the observation of exchange bias requires field cooling the sample from above Ne`el temperature T N of antiferromagnet NiO (523 K) whereas we have performed the measurement after field cooling the sample from room temperature. The reason why loop shift is observed for NiO/Fe3O4 is due to the decrease of the ordering temperatures of antiferromagnetic oxides for the sizes below 10 nm. In our samples the NiO core size is below 10 nm [19,20]. Ni33Fe67 and NiFe2O4 show very similar bias fields, 240 and 234 Oe, respectively. Since the nature of unidirectional anisotropy observed in these two structures is different and there are structural differences, such as core/shell sizes and shape, it is difficult to give a specific reason why the two structures experience same bias
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Fig. 7. Hysteresis loops for Ni33Fe67 alloy (as-synthesized), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C) nanoparticles taken at 5 K after field cooling the samples under 2T.
fields. However, the reason why Ni33Fe67 and NiFe2O4 show significantly lower loop shifts compared with NiO/ Fe3O4 can be explained using the exchange bias expression given in the following equation: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 2 AAF K AF H EB ¼ , (2) M F tF where AAF and K AF denote the exchange stiffness and anisotropy constants of antiferromagnetic layer and M F and T F correspond to saturation magnetization and thickness of ferromagnetic layer [21]. Small saturation and thickness are expected to be the dominating factors for the increased loop shift. Additionally, the relatively larger thickness of the antiferromagnetic NiO layer positively affects the loop shift by increasing the anisotropy energy of the NiO layer. Finally, temperature dependencies of the magnetizations of the three structures have been investigated by zero field cooled (ZFC) and FC magnetization measurements using 5000 Oe applied field. Splitting of FC–ZFC curves has been observed from all three structures as a result of blocking/ unblocking of small particles, as shown in Fig. 8. Blocking temperature, T B , for the Ne33Fe67 alloy particles was about 30 K whereas the blocking/unblocking for the other two structures appeared at about the same temperature, 130 K. There are three main parameters that control blocking/ unblocking of the small particles: magnetocrystalline anisotropy, K, particle volume, V , and interparticle interactions [22]. T B for a constant applied field is a function of K and V in the form of T B ¼ KhV i=25kB . Given that the average particle sizes for these three samples are the same, based on TEM images, K controls the blocking temperature. Moreover, magnetocrystalline anisotropy proportionally reflects itself on M r =M s ratio. However, M r =M s values at 5 K, 0.59, 0.66, and 0.53 for Ni33Fe67, NiO/Fe3O4, and NiFe2O4, respectively, do not correlate with the observed
Fig. 8. Temperature dependence of magnetization for field cooled, FC, and zero field cooled, ZFC, Ni33Fe67 alloy (as-synthesized), NiO/Fe3O4 core/shell (T an ¼ 350 C) and NiFe2O4 (T an ¼ 450 C) nanoparticles.
T B trend. Additionally, it is known that T B increases when the small particles are brought closer, as we observe in the annealed samples [23]. Therefore, the reason for different T B could only be the interparticle interactions that get stronger by the agglomeration of the particles upon annealing. 5. Conclusions Inert gas condensation along with post annealing has been investigated as an alternative technique for the synthesis of NiFe2O4 nanoparticles. It has been shown that it is possible to make NiFe2O4 nanoparticles starting from high purity Ni and Fe metals in a simple synthesis scheme. Structural analyses have revealed that proper annealing temperature is critical to get the desired nickel ferrite structure. Under non-optimal annealing temperature conditions the formation of NiO/Fe3O4 core/shell nanoparticles is observed. These core/shell structures show magnetic properties that are different from that of nickel ferrites. This study suggests that complete alloying of certain metals with the IGC process can lead to the formation of nanostructures that are useful for research and various applications. Acknowledgements A.C. would like to thank Dr. Mike Bonder for useful discussions. The work was partially funded by NSF OISE 0138151.
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