Magnetic properties of tin-doped magnetite nanoparticles

Physics Letters A 359 (2006) 66–69 www.elsevier.com/locate/pla

Magnetic properties of tin-doped magnetite nanoparticles Yan-Wu Lu a , Qin-Sheng Zhu b , Fang-Xin Liu c,∗ a Department of Physics, Beijing Jiaotong University, Beijing 100044, China b Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China c University of Science & Technology of China, Hefei 230026, China

Received 16 May 2006; accepted 29 May 2006 Available online 9 June 2006 Communicated by V.M. Agranovich

Abstract Structural and magnetic characteristics of Fe3−x Snx O4 (x < 0.3) nanoparticles synthesized using the precipitation exchange method have been investigated by X-ray diffraction, transmission electron microscope, Mössbauer spectra, X-ray photoelectron spectroscopy and magnetization measurement. The mean particle dimension decreases from 8 to 6 nm, the lattice parameters enlarge, the saturation magnetization decreases, as well as the magnetization and the coercive field increase, with increasing tin-content. The paramagnetic property of the specimens indicates that the replacement of Fe3+ by Sn4+ on the octahedral sites of Fe3 O4 causes a progressive lowering of the Curie temperature and the Curie temperatures of the materials are all lower than that of crystallite tin-doped magnetite. This striking debasing is due to the lessening of the grain size. This is the smallest size reported thus far for paramagnetic tin-doped magnetite particles. © 2006 Elsevier B.V. All rights reserved. PACS: 75.50.-y; 61.46.+w; 75.75.+a Keywords: Tin-doped magnetite; Composition and particle-size effect; Paramagnetic properties

1. Introduction The design, synthesis and characteristic of nanophase materials are the subject of intense current research [1]. This activity has been inspired in part by the realization that the physical and chemical properties of nanophase materials are sometimes dramatically different from those of the bulk counterparts and that nanophase materials often exhibit new or crossover phenomena [2]. In addition, small magnetic nanoparticles exhibit unique phenomena [3,4]. The doping of tetravalent tin into Fe3 O4 with the inverse spinel-related structure has attracted attention because of the possible use of the materials in transformer cores, magnetic memories, heterogeneous catalysts and electrochemistry, biomedicine and biotechnology [5]. Drokin et al. [6] presented results of the spectral temperature investigation of photoconductivity in synthesized polycrystalline specimens * Corresponding author.

E-mail addresses: [email protected] (Y.-W. Lu), [email protected] (F.-X. Liu). 0375-9601/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2006.05.080

of tin-doped magnetite Fe3−x Snx O4 . Berry et al. [7] presented the high temperature properties of tin-doped magnetite crystal. Previous methods of preparing tin-doped magnetite were mainly solid phase reactions, in which process reaction temperature was high to 1300 ◦ C; reaction time was long (ten hours); and the average grain size was large (∼ µm). Here we report preparation of Fe3−x Snx O4 nanoparticles by precipitation exchange (solid phase reaction in solution) method and investigate the quantum size and composition effects as well as magnetic properties of the samples. 2. Experiments The starting materials, the aqueous iron(III) chloride hexahydrate and tin(II) chloride were added in aqueous ammonia, respectively. Then the two solutions were mixed together at properly conditions so as to the coprecipitated doped-magnetite containing the required concentration of the metal ions is produced. The resulting product was filtered, washed with deionized water until Cl−1 was not being detected by AgNO3 , and

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Fig. 2. TEM micrograph (a) and SEAD patterns (b) of Fe2.8 Sn0.2 O4 nanoparticles. Fig. 1. X-ray diffraction patterns obtained from Fe3−x Snx O4 samples a (x = 0), b (x = 0.1) and c (x = 0.2).

dried. After processing, the desired Sn and Fe wt% values of the powders were verified by inductively coupled plasma–atomic emission spectroscopy (ICP–AES); these values agreed within 1% with the values anticipated from the starting solutions. X-ray diffraction (XRD) was taken on a Rigaku DMAX γ A X-ray diffractometor. A Hitachi model H-800 transmission electron microscopy (TEM) was used to obtain the morphology and selected area electron diffraction (SAED). The magnetic behavior of iron in the specimens was studied using an Oxford MS-500 Mössbauer Spectrometer with a source of 57 Co. The magnetization measurement was performed using a vibrating sample magnetometer (BHV-55) at magnetic fields ranging from 0.5 to 896 kA/m. X-ray photoelectron spectra (XPS) were recorded with an ESCALAB MK II system using A1 Kα radiation (hν = 1486.6 eV) under an operating vacuum 1 × 10−9 Pa. All binding energies were corrected to the C 1s signal, which was set at 284.6 eV. The XPS peak decomposition was determined using Gaussian–Lorentzian curve fitting software provided by this XPS system. 3. Results and discussion XRD patterns collected at same temperature and under the same pH conditions show a set of broadened peaks (see Fig. 1) of a similar inverse spinel-related structure, having a lattice constant nearly equal to that of bulk Fe3 O4 [9]. No evidence for a discrete tin(IV) oxide phase was observed. The observed increase in the peak broadening is due to decreasing long-range order and decreasing grain size of the modified Fe3−x Snx O4 particles, may also result from a lattice distortion due to the introduction of tin elements. The mean crystallite sizes calculated according to the Scherrer formula from the broadening peaks, decrease from 8 to 6 nm with increasing tin-content, were confirmed by TEM. Representative TEM image [see Fig. 2(a)] shows that the particles are very small and uniform. The clear diffraction rings in the SAED pattern [see Fig. 2(b)] confirm that the sample has a spinel polycrystalline structure. These diffraction rings can be indexed as Fe3 O4 (111), (220), (311), (400), (511) and (440). No evidence for a discrete tin(IV) oxide phase was ob-

Table 1 The tin content x, lattice constant a, coercive field Hc , and saturation magnetization Ms for Fe3−x Snx O4 nanoparticles Sample

x

a (Å)

Hc (kA/m)

Ms (kA m2 /kg)

A B C

0.0 0.1 0.2

8.39 8.41 8.44

6.0 1.0 2.0

21.0 14.0 13.0

served. In addition, from the bright spots it also can be deduced that the sample is composed of fine grains with different crystal orientations. These results are also in agreement with the XRD measurement. The refinement of XRD spectra indicates an increase of the lattice parameters a of the Fe3 O4 structure (see Table 1). The unit cells of tin-doping magnetite are significantly larger than that of Fe3 O4 (a = 8.396 Å) [8] and the lattice parameter increases with incrementing tin content, these are consistent with the large octahedral ionic radii of Fe2+ (0.78 Å) and Sn4+ (0.69 Å) compared with Fe3+ ions (0.65 Å) [9]. Considering the dimensions of the tetrahedral A- or the octahedral B-sites and the sizes of the ionic radii: Sn4+ ion has a larger ionic radius than the Fe3+ ion, thus the B-site which is more voluminous than the A-site is occupied by the larger Sn4+ ion. As a consequence, Fe2+ ions enter tetrahedral sites. Higher-voluminous “extra Fe2+ ions” are entering the A-sites on increasing Sndoping. Two 57 Fe Mössbauer spectra (MS) recorded in situ from the nanoparticles of composition Fe2.9 Sn0.1 O4 following heating at 340 K in vacuo (pressure ca. 0.2 Pa) are collected in Fig. 3. The spectrum at 300 K (Fig. 3(a)) showed doublet structure of paramagnetic Fe3+ species, and the magnetic hyperfine fields of the sextets to be very smaller. The doublet indicates all the tin-doped nano-magnetites had being in the paramagnetic state and the materials being within a few degrees of the Curie temperature. MS recorded at 340 K (Fig. 3(b)) showed the material to be completely paramagnetic, and the material being within a few degrees of the Curie temperature. The results demonstrate that the magnetic transition takes place over an interval of 40 K. By fitting the magnetic fields at different temperature to the relationship B(T ) = Bo (1 − T /Tc )β [7], we can calculate, for the compound Fe2.9 Sn0.1 O4 , Tc is 300 ± 5 K, 370 K lower than the Curie temperature of crystallite tin-doped magnetite [7], and

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Fig. 3. Room-temperature MS of Fe2.9 Sn0.1 O4 nanoparticles.

440 K than pure magnetite [10]. The sharpness of the transition is consistent with the high degree of homogeneity of the sample as indicated by XRD. MS recorded from the compound Fe2.8 Sn0.2 O4 showed the Curie temperature to be far under the room temperature. The lowering of the Curie temperature with increasing tin content can be attributed to the substitution by tin on the octahedral sites of Fe3 O4 . The notable lowering of the Curie temperature of the Fe3−x Snx O4 nanoparticles than that of the Fe3−x Snx O4 crystalline originates the effect of particle size. The magnetic orders in the nanoparticles are well preserved above the transition temperature. However, the magnetic anisotropy is overcome by the thermal activation and an external magnetic field, and the magnetization direction follows the direction of applied magnetic field as a paramagnetic material does. As the temperature decreases, the thermal activation energy is reduced. When the temperature drops below the transition temperature, the magnetic anisotropy is no longer overcome and the magnetization direction within each nanoparticles aligns along the easy axis of the nanoparticles. When a stronger external magnetic field is applied, the magnetic anisotropy of the nanoparticles is overcome at a relatively lower temperature, and hence, the transition temperature shifts to a lower value. In the present system, particles of Fe3−x Snx O4 with an average size of 8–6 nm are surrounded with air. In such an assembly of single magnetic domain particle held at blocking temperature T and consisting of a distribution of volumes, we can define the critical particle volume Vc [11], Vc = 25kB T /K,

(1)

where K is the anisotropy constant of the particle, kB , Boltzmann constant. Particles with volume larger than Vc have blocking temperatures higher than T and are, therefore, magnetically frozen. Only particles with volume smaller than Vc , can respond paramagnetically to an external field at temperature T . We can estimate the anisotropy constant K of the nanoparticles using Eq. (1) from the nanoparticle volume of 2.16 × 10−25 m3 corresponds to a diameter of 6 nm, and the temperature of 300 K. The estimated K of 4.79 × 105 J/m3 is qualitatively consistent with those (∼ 105 J/m3 ) given by Ref. [12].

Fig. 4. XPS recorded from Fe3−x Snx O4 nanoparticles.

Shown in Fig. 4 is the XPS of Fe (2p) for the as-prepared samples. The binding energies recorded from Fe3 O4 for the main peaks maximum 2p3/2 and 2p1/2 corresponding to the tetrahedral Fe3+ components are 710.6 and 724.3 eV, respectively, these are similar to data reported in the literature [13]. The peaks in the Fe (2p) XPS recorded from Fe2.9 Sn0.1 O4 and Fe2.8 Sn0.2 O4 were broader than those observed in the spectrum recorded from Fe3 O4 and the maxima of the Fe 2p3/2 envelopes were shifted to the lower energies of 710.3 eV and 710.0 eV respectively. A peak at 715.4 eV, which corresponds to the binding energy of the Sn 2p3/2 core level. The Fe (2p) spectrum recorded from Fe2.8 Sn0.2 O4 suggested an additional structure at 714.0 eV, which we associate with the presence of a Fe2+ satellite. The shifts to lower binding energy of the Fe 2p3/2 level, the increase in broadening of the Fe (2p) peaks and the development of satellite structure are all indicative [14] of increasing Fe2+ content as increasing amounts of tin are incorporated within the Fe3 O4 structure. This is more clearly demonstrated from the deconvolution of the Fe 2p3/2 peak. The single component in the Fe 2p3/2 peak of Fe3 O4 at 710.9 eV is characteristic of Fe3+ and, although Fe3 O4 contains both Fe2+ and Fe3+ , the surface is usually oxidized such that only the Fe3+ contribution is observed in the XPS [15]. In contrast the Fe 2p3/2 peak in the spectra recorded from Fe2.9 Sn0.1 O4 and Fe2.8 Sn0.2 O4 were deconvoluted to four peaks. The peaks at 710.8 eV and 715.5 eV correspond to Fe3+ and the Sn 2p3/2 level respectively. In addition, the peak at 709.1 eV, together with the satellite at 714.1 eV, are characteristic [13] of Fe2+ . Taken together the results show that the replacement of Fe3+ by Sn4+ in Fe3 O4 is achieved by substitution on the octahedral sites of the inverse spinel structure and is accompanied by the partial reduction of Fe3 O4 to Fe2+ .

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4. Conclusions

Fig. 5. Magnetization curves for the Fe3−x Snx O4 nanoparticles at room temperature: (a) (x = 0), (b) (x = 0.1), (c) (x = 0.2). The maximum applied magnetic field is 60 kA/m.

Fig. 5 shows the magnetization loops obtained at room temperature of Fe3−x Snx O4 nanoparticles under the maximum applied magnetic field of 896 kA/m, and the variations of coercivity Hc , and saturation magnetization Ms , as a function of the Sn4+ composition parameter x. From Figs. 3 and 5, we can see that for pure Fe3 O4 specimen, a larger hysteresis appears, and for tin-doped magnetite the hysteresis becomes smaller. This can be explained by the presence of anisotropy of Fe spins inside nanoparticles as well as in their local environments. The substitution of Sn4+ in stoichiometric magnetite, Fe3 O4 , introduces simultaneously an equivalent increase of the Fe2+ concentration, which modifies strongly the magnetic properties. For higher substitution levels, part of the Fe2+ -ions may also enter tetrahedral sites (A-sites). As shown in Table 1 and Fig. 5, the hysteresis increments with increasing tin-content. These facts can be interpreted in terms of the doping induced anisotropies of “extra Fe2+ ions” [16,17]. The introduction of one Sn4+ ion into the B-sublattice causes the disparition of two Fe3+ ions; (i) one by direct replacement and (ii) the other one by a valency change from three- into two-fold. Regarding the so-formed extra Fe2+ ions, their distribution on B- or A-sites has to be considered: i.e., in the case of Fe3−x Snx O4 , departing from x > 0.3, extra Fe2+ ions are placed on A-sites. It is obvious that the concentration of the tetrahedral Fe2+ ion affects the magnetic properties substantially. On increasing Sn doping, “extra Fe2+ ions” are expecting to enter the A-sites, thereby originating the observed modifications of the material. The saturation magnetization decreases increasing tin-content. As known, tin enters magnetite exclusively as Sn4+ into the octahedral sublattice [18]. Thus tetrahedral sites are no longer occupied exclusively by Fe3+ ions, whereas Fe2+ also appears randomly on the tetrahedral sites, these lead the saturation magnetization decreases as increasing tin-content.

Nanometer Fe3−x Snx O4 (x < 0.3) with a narrow particle size distribution (6–8 nm) have been synthesized by the precipitation exchange method at relatively lower temperature, shorter reaction time for relatively large-scale preparation of nanoparticles. XRD, TEM, MS, XPS and magnetization data results clearly indicate that the low concentrations of tin in composition Fe3−x Snx O4 (x < 0.3) adopt the octahedral, as opposed to tetrahedral, sites of the inverse spinel structure. The saturation magnetization decreases, whereas the magnetization and the coercive field increase, with increasing tin-content, thus indicating a change of cation substitution. Substitution by tin on the octahedral sites of Fe3 O4 causes a progressive lowering of the Curie temperature, and the Curie temperature are all lower than room temperature, the lessening of the grain size brings another remarkable debasing of the Curie temperature. The magnetic behavior of the tin magnetite particles changed from ferromagnetic, as normal for magnetite, to paramagnetic when decreasing the particle size to nanometer scale at room temperature. This is the smallest size reported thus far for paramagnetic tin-doped Fe3 O4 nanoparticles. Acknowledgements This work is supported by National Natural Scientific Foundation of China under Grant No. 60376014. References [1] I. Nedkov, T. Merodiiska, L. Slavov, R.E. Vandenberghe, Y. Kusano, J. Takada, J. Magn. Magn. Mater. 300 (2006) 358. [2] A. Uheida, S.A. German, E. Björkman, Z. Yu, M. Muhammed, J. Colloid Interface Sci. 298 (2006) 501. [3] P. Poddar, J.L. Wilson, H. Srikanth, S.A. Morrison, E.E. Carpenter, Nanotechnology 15 (2004) S570. [4] K.S. Wilson, J.D. Goff, J.S. Riffle, L.A. Harris, T.G. St Pierre, Polym. Adv. Technol. 16 (2005) 200. [5] U. Häfeli, W. Schütt, J. Teller, M. Zborowski, Scientific and Clinical Applications of Magnetic Carriers, Plenum, New York, 1997. [6] N.A. Drokin, E.Y. Aksenova, Y.A. Mamalui, Sov. Phys. Solid State 26 (1984) 1111. [7] F.J. Berry, Ö. Helgason, K. Jónsson, S.J. Skinner, J. Solid State Chem. 122 (1996) 353. [8] B.A. Wechsler, D.H. Lindsley, C.T. Prewitt, Am. Mineral. 69 (1984) 754. [9] R.D. Shannon, Acta Crystallogr. Sect. A 32 (1976) 751. [10] L. Häggström, H. Annersten, T. Ericson, R. Wäppling, W. Karner, S. Bjarman, Hyperfine Interact. 5 (1978) 201. [11] L. Néel, Adv. Phys. 4 (1955) 91. [12] A. Hosseinpour, H. Sadeghi, Phys. Status Solidi B 227 (2001) 563. [13] T. Fujii, F.M.F. de Groot, G.A. Sawatzky, F.C. Voogt, T. Hibma, K. Okada, Phys. Rev. B 59 (1999) 3195. [14] C.S. Kuivila, J.D. Butt, P.C. Stair, Appl. Surf. Sci. 32 (1988) 99. [15] T. Choudhury, S.O. Saied, J.L. Sullivan, A.M. Abbot, J. Phys. D: Appl. Phys. 22 (1989) 1185. [16] V.A.M. Brabers, Physica B 205 (1985) 143. [17] Z. Kakol, J. Sabol, J.M. Honig, Phys. Rev. B 44 (1991) 2198. [18] F.J. Berry, Ö. Helgason, Hyperfine Interact. 126 (2000) 269.