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Observation of superparamagnetism in ultra-fine ZnxFe1−xFe2O4 nanocrystals synthesized by co-precipitation method
Materials Chemistry and Physics 134 (2012) 783e788
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Observation of superparamagnetism in ultra-fine ZnxFe1xFe2O4 nanocrystals synthesized by co-precipitation method Nitu Kumar a, c, *, Geetika Khurana a, b, Anurag Gaur c, R.K. Kotnala a a
National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Department of Physics, University of Puerto Rico, San Juan, Puerto Rico 00931-3343, USA c Department of Physics, National Institute of Technology, Kurukshetra 136119, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2011 Received in revised form 8 March 2012 Accepted 22 March 2012
Ultra-fine nanocrystals of ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) have been synthesized by chemical coprecipitation method at low temperature (w60 C). Rietveld fitted X-ray diffraction patterns confirm the formation of cubic spinel structure with Fd-3m space group. High resolution transmission electron microscopy (HRTEM) analysis reveals that the average particle size reduces with the substitution of higher Zn concentration from 12 to 3 nm for sample x ¼ 0.1 to 0.7. Vibrating sample magnetometer (VSM) results indicate that the blocking temperature (TB) for sample x ¼ 0.1 is 120 K and significantly decreases up to less than 5 K as we move towards the sample x ¼ 0.7. Moreover, the observed irreversible magnetization (ΔM ¼ MFC MZFC), coercivity and remanence are indistinguishable for sample x ¼ 0.7 down to 5 K, which infers the superparamagnetic behaviour in this sample in a wide range of temperature from room temperature down to 5 K. The reduction in peak to peak line width and enhanced magnetic field asymmetry observed in electron paramagnetic resonance (EPR) spectra also supports the presence of superparamagnetism in sample x ¼ 0.7. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructured Electron microscopy (HRTEM) Co-precipitation method
1. Introduction Nanocrystalline magnetic particles shows unique physical and magnetic properties as electronic states modified due to quantum confinement compared to that of the bulk materials [1]. Superparamagnetic ferrite nanoparticles having potential applications in magnetic sensors, optoelectronics, information storage, electronic devices and gas sensors [2e5]. Therefore, in the recent years it has drawn a lot of attention in scientific and technological community. The properties of ferrites are highly sensitive to the cation distribution, preparation conditions and substitution of different transition metals. The average diameter of the nanoparticles is strongly dependent on the type of ferrite and precipitation conditions (pH, temperature, etc). In bulk ferrites, having cubic spinel structure with chemical formula A2þB3þ2O4 (A ¼ Zn2þ and B ¼ Fe3þ), A and B occupy tetrahedral and octahedral sites respectively [6]. In above system, Zn2þ occupy A site and Fe3þ at B site which in turn shows antiferromagnetism at TN ¼ 10 K due to antiferromagnetic superexchange interaction between BeB ions. It is well known that the
* Corresponding author. National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India. Tel.: þ91 7838343057. E-mail address: [email protected] (N. Kumar). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.069
nanostructured zinc ferrite compound, which has metastable phase, presents mixed spinel structure ZnxFe1xFe2O4, where x is the so-called inversion degree. In this case, Zn2þ ions occupy the tetrahedral (A sites), Fe2þ ions occupy the octahedral (B-sites) while Fe3þ ions distributed over both sites to make system partially 2þ inverted. Hence, the substitution of Fe3þ A by Zn on the A site also 2þ reduces the amount of FeB on B site due to charge neutrality as shown below: 0 0 0 ½Fe Fe A ½FeFe FeFe B 4OO ¼ ½ZnFeðxÞ FeFeð1xÞ A ½FeFeð1þxÞ FeFeð1xÞ B 4OO
(1) FeFe
0
3þ
2þ
where and Fe Fe denotes the Fe and Fe ions at a normal Fe site respectively. When Fe3þ sites are replaced by Zn2þ, it is denoted 0 by Zn Fe using KrögereVink notation. Because of this, the density of itinerant electrons on the B sublattice is also reduced. Furthermore, the non-magnetic Zn2þ (3d10, S ¼ 0) replaces Fe3þ on the A-sites due to favourable electronic configuration for tetrahedral bonding to the oxygen ions, which affect the lattice parameters (a0) [7,8]. Therefore the magnetic exchange for this system is governed by a combination of antiferromagnetic superexchange (SE) and double exchange interactions (DE). There are three antiferromagnetic SE interactions JAA(AeOeA), JBB(BeOeB), and JAB(AeOeB) between the Fe3þ ions
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on the A and B sites, mediated by the oxygen (O) ions. The simple Neel model has been extended by Yafet and Kittel for better understanding of magnetic behaviour of these ferrites [9e11]. In addition, there is a ferromagnetic DE interaction mediated by the itinerant spin down t2g electrons hopping between the mixedvalent Fe ions on the B sites. It is earlier reported that zinc substitution plays a decisive role in tuning the various structural and magnetic properties of the ferrites when they are formed of single domain particle (10 nm) and the phenomenon of superparamagnetism arises [12,13]. As per this strategy, we synthesized the ultra-fine nanoparticles of ZnxFe1xFe2O4 with different zinc concentrations (x ¼ 0.1, 0.3, 0.5 and 0.7) to achieve the superparamagnetism in this compound. The synthesized samples were characterized for their structural and magnetic properties to understand the role of zinc concentration. We observed the systematic reduction in the crystallite size from 12 to 3 nm as zinc concentration varies from x ¼ 0.1 to 0.7, respectively. Interestingly, it was also found that ZnxFe1xFe2O4 sample with x ¼ 0.7 (having average particle size w3 nm) revealed the superparamagnetic behaviour in a wide temperature range from room temperature down to 5 K. 2. Experimental 2.1. Synthesis of ultra-fine ZnxFe1xFe2O4 nanoparticles The nanocrystalline samples of ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) were synthesized by a low temperature chemical coprecipitation method [14,15]. High purity salts (99%, Aldrich Company) of Fe(NO3)3$9H2O, ZnCl2 and FeCl2 in aqueous media were used as starting percursors. These salts were dissolved in double deionized water with constant stirring to get solution of 0.5 mol/L. The equimolar solutions of Fe(NO3)3$9H2O, ZnCl2 and FeCl2 were mixed in their stoichiometric ratio and homogenized at 50 C. Then, 25% ammonia solution was added drop by drop as a precipitating agent under constant stirring and pH was monitored carefully until it reached in the range 8.5e10. The mixture was then heated at 60 C for about 1 h. This duration is sufficient for transformation of hydroxides into spinel ferrites (digestion step). The precipitated particles were then washed several times with double distilled water to remove the salt residues and other impurities. It was further dried at 60 C for 1 h to obtain the nanoparticles, which were used for further characterizations.
to avoid saturation effect. The a-diphenyl-b-picrylhydrazyl (DPPH) was used as a reference material for the determination of g-value. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. Structural analysis The Rietveld fitted X-ray diffraction patterns of as-synthesized ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) nanoparticles have been shown in Fig. 1, which confirm the formation of cubic spinel structure (Fd-3m space group) without any impurity phases and coinciding with the standard data card (ICDD PDF #82-1042) by using the Rietveld Program, FullProf.2k (Version 5.20 - Jul2011-ILL JRC) [16]. It was also observed that the diffraction peaks of sample x ¼ 0.7 become very broad which indicate the ultra-fine particle nature of the sample due to reduced particle size. By using the most intense (311) peak line widths observed from the Rietveld fits, the average crystallite sizes shown in Table 1, were estimated by Scherrer’s formula. The instrumental broadening, measured through standard Si sample, has been deducted from the FWHM value during crystallite size calculations by Scherrer’s formula. The average crystallite size estimated by Scherrer’s formula are found to be 15 0.85, 12 0.73, 8 0.52 and 5 0.26 nm for the sample x ¼ 0.1, 0.3, 0.5 and 0.7, respectively (as shown in Table 1). Furthermore, the lattice parameters obtained from the Rietveld fits are 8.3820 0.0006, 8.3884 0.0012, 8.3915 0.0018 and 8.4092 0.0032 Å for sample x ¼ 0.1, 0.3, 0.5 and 0.7 respectively, which shows the slight increase with increasing zinc concentration (as shown in Table 1). The lattice parameters determined from Rietveld refinement agree with their respective values reported in the literature [17]. The increase in lattice parameter from 8.3820 to 8.4092 Å for sample x ¼ 0.1e0.7 indicates the replacement of vacancies of the smaller Fe3þ ion (0.49 Å) by larger Zn2þ ion (0.60 Å) at A-site during the reaction [18].
2.2. Characterization Structural studies of as-synthesized nanoparticles were carried out using powder X-ray diffractrometer (Bruker D-8 advance) with A) at the slow scan rate of 0.02 s1 to CuKa radiation (l ¼ 1.540 confirm the phase formation. High resolution transmission electron microscopy (HRTEM) (Model: Tecnai G2 F30 STWIN field emission gun supported operated at the electron accelerating voltage of 300 kV) was used to determine the shape and particle size distribution of the nanocrystals. Magnetic characterization of all the samples were recorded by vibrating sample magnetometer (LakeShore: Model no. 7305) at room temperature and low temperature in a magnetic field from 1.2 to þ1.2 T and other parameters such as magnetization (M), coercive force (HC), and remanent magnetization (Mr) were evaluated for each sample. The zero-field cooled (ZFC) and field cooled (FC) measurements were carried out in the temperature range of 5 to 320 K under a constant magnetic field of 100 Oe. The measurements of magnetic resonance spectra were performed on a conventional X-band (n ¼ 9.36 GHz) Varian make E Line Century X-band EPR spectrometer (Model-E-112) with 100 kHz magnetic field modulation and 10 mW microwave power
Fig. 1. Rietveld refinement pattern of as-synthesized ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) nanoparticles with average diameter of 12, 10, 6 and 3 nm respectively. X-ray experimental data are shown as very broad line and solid line is the best fit to the data and tick marks show the positions for the allowed reflections. The lower curve represents the difference between the observed and calculated profiles.
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Table 1 The variation of lattice parameters, average particle diameter, remanent magnetization, blocking temperature, EPR peak to peak line width and gyromagnetic constant of ZnxFe1xFe2O4 with x ¼ 0.1, 0.3, 0.5 and 0.7. Sample with Zn content
Lattice parameters (A0)
Average crystalline size by XRD (nm)
Average particle diameter by HRTEM (nm)
remanent magnetization Mr (emu gm1)
Blocking temp. (K)
EPR peak-to-peak line width (Gauss)
Gyromagnetic constant
x ¼ 0.1 x ¼ 0.3 x ¼ 0.5 x ¼ 0.7
8.3820 0.0006 8.3884 0.0012 8.3915 0.0018 8.4092 0.0032
15 0.85 12 0.73 8 0.52 5 0.26
12 1 10 1 61 31
1.10 0.37 0.17 0.02
120 98 60 5
1300 1150 850 155
2.2931 2.2660 2.1885 2.1673
The high resolution transmission electron microscopy (HRTEM) images of sample x ¼ 0.1 and 0.7 are shown in Fig. 2a and b. Selected area electron diffraction (SAED) patterns and particle size distribution histograms, obtained through TEM images, are shown in the insets of Fig. 2. Most of the particles in Fig. 2a and b have diameters of 12 1 and 3 1 nm for the sample x ¼ 0.1 and x ¼ 0.7, which depicts the good agreement with the average particle size observed through XRD data. The reduction in crystallite size with zinc content is mainly due to the strong chemical affinity of specific cations like Zn to the tetrahedral A site and the metastable cation distribution in nanoscale range of ferrite particles as discussed by other groups [19e22]. Based on these studies, we propose the following mechanism to explain the dependence of particle size on cation stoichiometry: In a complex system like the ferrites where many cations are involved, the nucleation and growth of particles are expected to be influenced by the probability of a cation available chemically in equivalent sites and by its affinity to these sites. In our study, during the formation of ZnFe2O4 nuclei, Zn would preferentially occupy the tetrahedral site and force the Fe to the octahedral site. Even in nanoscale range, the preferential occupancy of Zn dominates over the metastable cation distributions. Similarly, the restriction of Fe to occupy only the octahedral sites as in the case of ZnFe2O4, no longer applies and it can occupy the tetrahedral site also. Therefore, the growth of these ZnFe2O4 nuclei will be restricted, since the incorporation of either Zn or Fe by the nuclei will depend on the availability of their preferential sites. This suggests the decrease in particle size with the increase in Zn2þ concentration. 3.2. Magnetic properties The temperature dependent magnetization (MeT) measurements under ZFC and FC modes in an applied magnetic field of
100 Oe in the temperature range from 5 to 320 K are shown in Fig. 3. It is clear from figure that the onset irreversible magnetization (ΔM ¼ MFC MZFC) and blocking temperature (TB) shifts towards the lower temperature with concentration of Zn2þ. The value of blocking temperature for sample x ¼ 0.1 is 120 K while it decreases below to 5 K for the sample x ¼ 0.7. Moreover, for x ¼ 0.7 sample, both ZFC and FC curves almost overlap to each other down to temperature 5 K and ΔM is indistinguishable [23]. This behaviour of ΔM indicates that the beginning of freezing process shifts towards lower temperature and irreversible magnetization is negligible over the entire measurable temperature range from room temperature down to 5 K for sample x ¼ 0.7. Such interesting phenomenon is attributed to the ultra-fine particles of sample x ¼ 0.7 (having average diameter of 3(1) nm), smaller than the superparamagnetic critical dimension (10 nm), which causes the shifting of blocking temperature (TB) towards lower values and leading to superparamagnetic behaviour towards lower temperatures [24e26]. The magnetization of nanoparticles at room temperature varies strongly as a function of Zn2þ doping as shown in Fig. 4. In ultra-fine nanoparticles, surface spin-structure become prominent due to breaking of bonds at the surface and become highly disordered as compared to core spins [27]. The magnetic properties drastically decreases with the substitution of Feþ3 (3d5, S ¼ 5/2) by Zn2þ (3d10, S ¼ 0) on the A sublattice. However Zn substitution also weakens antiferromagnetic exchange JAB by diluting the A site moments. Furthermore, it may also converts Fe2þ(3d6, S ¼ 2) into Fe3þ on the B sublattice, resulting in a reduction in the itinerant charge carrier density. This weakens the ferromagnetic double exchange, competing with antiferromagnetic superexchange interaction on the B sublattice. This leads to an increase in the spin canting on the B sublattice with increasing Zn ions and results the observed reduction in the total magnetization.
Fig. 2. HRTEM images of ZnxFe1xFe2O4 samples: (a) x ¼ 0.1 and (b) x ¼ 0.7 with their selected area electron diffraction (SAED) patterns and particle size distribution histograms.
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Fig. 3. Temperature dependent magnetization (MeT) curves of ZnxFe1xFe2O4 samples: (a) x ¼ 0.1, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7 recorded in zero-field cooled (ZFC) and field cooled (FC) conditions with applied magnetic field of 100 Oe in the temperature range 5e320 K.
Fig. 4. Room temperature magnetization versus magnetic field (MeH) hysteresis loops of ZnxFe1xFe2O4 samples: (a) x ¼ 0.1, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7 in applied field up to 1. 2 T.
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The MeH curves of the x ¼ 0.7 sample recorded at room temperature also support the superparamagnetic behaviour in this sample at room temperature as shown in Fig. 4d [28,29]. The nonsaturation in magnetization, absence of hysteresis, very low remanent magnetization and coercivity (as shown in Table 1) observed in this sample supports the presence of superparamagnetism at room temperature. Furthermore, to certify the superparamagnetic behaviour in these samples at low temperature, we recorded the MeH measurements at 5 K for samples x ¼ 0.3, 0.5 and 0.7, which has been shown in Fig. 5aec. MeH plots for samples
Fig. 6. Room temperature EPR spectra of ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) recorded at 9.36 GHz frequency of applied signal with modulated frequency 100 kHz and 10 mW microwave power.
x ¼ 0.1 and 0.3 having the saturation in magnetization, however the sample x ¼ 0.5 does not show the saturation with hysteresis. Furthermore, it is clearly observed for sample x ¼ 0.7 (Fig. 5c) that there is no hysteresis and saturation in magnetization for this sample, which supports the MeT results that x ¼ 0.7 sample show the superparamagnetic behaviour up to 5 K. 3.3. EPR analysis The EPR studies of ferrites give the clear picture of interaction of spin with electromagnetic waves [30]. The room temperature EPR spectra of synthesised samples (as shown in Fig. 6) were analysed using Lorentzian distribution function which indicates that with increasing concentration of Zn2þ, the peak to peak line width (ΔHPP) decreases, where as the resonance field shift towards the higher value. In this system Zn2þ concentrations plays the crucial role to reduce the particle diameter significantly. We have already discussed this in the structural part so we can say that it is a correlated system. The intensity of the resonance line gradually decreases with increasing Zn2þ concentration as the intensity of resonance peak in EPR spectra is related to iron content as reported by the other groups [31,32] and iron content is modified by varying the zinc content in this system. Therefore, we got maximum intensity for x ¼ 0.1 sample which has maximum iron content. Furthermore, asymmetry in the peaks has been observed for the sample above x ¼ 0.3 and it goes maximum for sample x ¼ 0.7, mainly due to reduction in particle size [33]. The diameter dependence of the EPR resonance frequency is well explained by a simple k$p band model [34]. The systematic decrease in peak-to-peak line width is observed up to x ¼ 0.5, but a sudden fall has been observed for sample x ¼ 0.7. The clear decrement in calculated gyromagnetic constant (g) values is also observed as given in Table 1. The high asymmetry and narrow width of resonance peak for x ¼ 0.7 sample indicate the superparamagnetism in this sample which is also supported by MeT and MeH magnetic measurements [35]. 4. Conclusions
Fig. 5. Magnetization verses magnetic field (MeH) hysteresis loops of ZnxFe1xFe2O4 samples at 5 K: (a) x ¼ 0.3, (b) x ¼ 0.5 and (c) x ¼ 0.7.
In summary, we synthesized the ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) nanoparticles via chemical co-precipitation method and studied their structural and magnetic properties. Rietveld fitted XRD patterns exhibit the single phase nature of all the compositions with spinel structure (Fd-3m space group). High resolution transmission electron microscopy (HRTEM) analysis reveals that the average particle size reduces with the substitution of higher Zn
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concentration. Magnetic measurements shows that the blocking temperature (TB) and irreversible magnetization (ΔM ¼ MFC MZFC) shifts towards the lower temperature as we move from the sample x ¼ 0.1 to 0.7. Furthermore, Magnetic and EPR results reveal the superparamagnetic behaviour in ultra-fine nanocrystals of ZnxFe1xFe2O4 with x ¼ 0.7 in a wide temperature range from room temperature down to 5 K. Acknowledgements The author Nitu Kumar gratefully acknowledges the Council of Scientific and Industrial Research (CSIR) India for the award of Senior Research Fellowship to carry out research work at National Physical Laboratory, New Delhi for pursuing his Ph.D thesis. The authors are thankful to Dr. Sukhvir Singh for extending technical support to record the HRTEM micrograph. The authors are also grateful to Director, National Physical Laboratory under CSIR for providing constant encouragement and granting permission to publish this work. References [1] [2] [3] [4] [5]
M. Zheng, X.C. Wu, B.S. Zou, Y.J. Wang, J. Magn. Magn. Mater. 183 (1998) 152. R. Lebourgeois, C. Coillot, J. Appl. Phys. 103 (2008) 07E510. Z.H. Yuan, L.D. Zhang, J. Mater. Chem. 11 (2001) 1265. G.A. Ozin, Adv. Mater. 4 (1992) 612. Z. Hongliang, Gu. Xiaoyun, Z. Diantai, W. Zhengkai, W. Naiyan, Y. Kuihong, Nanotechnology 19 (2008) 405503. [6] J.S. Mit, H.P. Wijn, J. Ferrites, Physical Properties of Ferrimagetic oxide in Relation to Their Tech. Applications, N.V. Philip’s Glocilampenfabricken, Eindhoven, Holland, 1959. [7] B. Zou, A. Rondinone, J. Zhang, Z.J. Liu, C. Zou, B. Rondinone, A.J. Zhang, Z.J. Liu, J. Am. Chem. Soc. 122 (2000) 6263. [8] G. Vaidyanathan, S. Sendhilnathan, R. Arulmurugan, J. Magn. Magn. Mater. 313 (2007) 293.
[9] L. Neel, Ann. Phys. (Paris) 3 (1948) 137. [10] Y. Yafet, C. Kittel, Phys. Rev. 87 (1952) 290. [11] D. Venkateshvaran, M. Althammer, A. Nielsen, S. Geprägs, M.S.R. Rao, S.T.B. Goennenwein, M. Opel, Gross, Phys. Rev. B 79 (2009) 34405. [12] C. Kittel, Phys. Rev. 70 (1946) 965. [13] I.S. Jacobs, C.P. Bean, in: G.T. Rado, H. Suhl (Eds.), Magnetism, vol. 3, Academics Press, New York, 1963. [14] I.H. Gul, W. Ahmed, A. Maqsood, J. Magn. Magn. Mater. 320 (2008) 270. [15] V. Kumar, Anu Rana, Nitu Kumar, R.P. Pant, Int. J. Appl. Ceram. Tech. 8 (2011) 120. [16] H.M. Rietveld, J. Appl. Crystallography 2 (1969) 65. [17] C.M. Srivastava, S.N. Shringi, R.G. Srivastava, N.G. Nanadikar, Phys. Rev. B 14 (1976) 2032. [18] Y. Tamaura, T. Kodama, T. Itoh, J. Am. Ceram. Soc. 73 (1990) 2539. [19] R. Iyer, R. Desai, V.R. Uapdhyay, Bull. Mater. Sci. 32 (2009) 141. [20] C. Rath, S. Anand, R.P. Das, K.K. Sahu, S.D. Kulkarni, et al., J. Appl. Phys. 91 (2002) 2211. [21] C. Rath, N.C. Mishra, S. Anand, R.P. Das, K.K. Sahu, C. Upadhyaya, H.C. Verma, Appl. Phys. Lett. 76 (2000) 475. [22] C. Rath, K.K. Sahu, S. Anand, S.K. Date, N.C. Mishra, R.P. Das, J. Magn. Magn. Mater. 202 (1999) 77. [23] B. Martinez, X. Obradors, L. Balcells, A. Rouanet, C. Monty, Phys. Rev. Lett. 80 (1998) 181. [24] S. Mørup, F. Bødker, P.V. Hendriksen, S. Linderoth, Phys. Rev. B 52 (1995) 287. [25] C. Xuebo, Gu Li, Nanotechnology 16 (2005) 180. [26] Z.L. Wang, Y. Liu, Z. Zhang (Eds.), Handbook of Nanophase and Nanostructured Material, Kluwer-Academic, New York, 2003. [27] M. Sparks, J. Appl. Phys. 36 (1965) 1570. [28] C. Upadhay, H. C.Verma, V. Sathe, A.V. Pimplale, J. Magn. Magn. Mater. 312 (2007) 17. [29] C.N. Chinnasamy, A. Narayanasamy, N. Ponpandian, K. Chattopadhyay, H.Guerault, J.M. Grenece, J. Phys. Condens. Matter 12 (2000) 7795. [30] F. Gazeau, J.C. Bacri, F. Gendron, R. Perzynski, Y.L. Raikher, V. Stepanov, E. Dubois, J. Magn.Magn. Mater. 186 (1998) 175. _ [31] D. Sibera, U. Narkiewicz, N. Guskos, G. Zo1nierkiewicz, J. Phys. Conf. Ser. 146 (2009) 012014. _ [32] N. Guskos, G. Zo1nierkiewicz, J. Typek, D. Sibera, U. Narkiewicz, Rev. Adv. Mater. Sci. 23 (2010) 224. [33] J.P. Singh, R.C. Srivastava, H.M. Agrawal, R.P.S. Kushwaha, P. Chand, R. Kumar, Int. J. Nanosci. 7 (2008) 21. [34] Kelly M. Whitaker, Stefan T. Ochsenbein, Victor Z. Polinger, Daniel R. Gamelin, J. Phys. Chem. C 112 (2008) 14,331. [35] Y. Sui, D.P. Xu, F.L. Zheng, W.H. Su, J. Appl. Phys. 80 (1996) 719.
Contents lists available at SciVerse ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Observation of superparamagnetism in ultra-fine ZnxFe1xFe2O4 nanocrystals synthesized by co-precipitation method Nitu Kumar a, c, *, Geetika Khurana a, b, Anurag Gaur c, R.K. Kotnala a a
National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India Department of Physics, University of Puerto Rico, San Juan, Puerto Rico 00931-3343, USA c Department of Physics, National Institute of Technology, Kurukshetra 136119, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 October 2011 Received in revised form 8 March 2012 Accepted 22 March 2012
Ultra-fine nanocrystals of ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) have been synthesized by chemical coprecipitation method at low temperature (w60 C). Rietveld fitted X-ray diffraction patterns confirm the formation of cubic spinel structure with Fd-3m space group. High resolution transmission electron microscopy (HRTEM) analysis reveals that the average particle size reduces with the substitution of higher Zn concentration from 12 to 3 nm for sample x ¼ 0.1 to 0.7. Vibrating sample magnetometer (VSM) results indicate that the blocking temperature (TB) for sample x ¼ 0.1 is 120 K and significantly decreases up to less than 5 K as we move towards the sample x ¼ 0.7. Moreover, the observed irreversible magnetization (ΔM ¼ MFC MZFC), coercivity and remanence are indistinguishable for sample x ¼ 0.7 down to 5 K, which infers the superparamagnetic behaviour in this sample in a wide range of temperature from room temperature down to 5 K. The reduction in peak to peak line width and enhanced magnetic field asymmetry observed in electron paramagnetic resonance (EPR) spectra also supports the presence of superparamagnetism in sample x ¼ 0.7. Ó 2012 Elsevier B.V. All rights reserved.
Keywords: Nanostructured Electron microscopy (HRTEM) Co-precipitation method
1. Introduction Nanocrystalline magnetic particles shows unique physical and magnetic properties as electronic states modified due to quantum confinement compared to that of the bulk materials [1]. Superparamagnetic ferrite nanoparticles having potential applications in magnetic sensors, optoelectronics, information storage, electronic devices and gas sensors [2e5]. Therefore, in the recent years it has drawn a lot of attention in scientific and technological community. The properties of ferrites are highly sensitive to the cation distribution, preparation conditions and substitution of different transition metals. The average diameter of the nanoparticles is strongly dependent on the type of ferrite and precipitation conditions (pH, temperature, etc). In bulk ferrites, having cubic spinel structure with chemical formula A2þB3þ2O4 (A ¼ Zn2þ and B ¼ Fe3þ), A and B occupy tetrahedral and octahedral sites respectively [6]. In above system, Zn2þ occupy A site and Fe3þ at B site which in turn shows antiferromagnetism at TN ¼ 10 K due to antiferromagnetic superexchange interaction between BeB ions. It is well known that the
* Corresponding author. National Physical Laboratory, Dr. K.S. Krishnan Marg, New Delhi 110012, India. Tel.: þ91 7838343057. E-mail address: [email protected] (N. Kumar). 0254-0584/$ e see front matter Ó 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2012.03.069
nanostructured zinc ferrite compound, which has metastable phase, presents mixed spinel structure ZnxFe1xFe2O4, where x is the so-called inversion degree. In this case, Zn2þ ions occupy the tetrahedral (A sites), Fe2þ ions occupy the octahedral (B-sites) while Fe3þ ions distributed over both sites to make system partially 2þ inverted. Hence, the substitution of Fe3þ A by Zn on the A site also 2þ reduces the amount of FeB on B site due to charge neutrality as shown below: 0 0 0 ½Fe Fe A ½FeFe FeFe B 4OO ¼ ½ZnFeðxÞ FeFeð1xÞ A ½FeFeð1þxÞ FeFeð1xÞ B 4OO
(1) FeFe
0
3þ
2þ
where and Fe Fe denotes the Fe and Fe ions at a normal Fe site respectively. When Fe3þ sites are replaced by Zn2þ, it is denoted 0 by Zn Fe using KrögereVink notation. Because of this, the density of itinerant electrons on the B sublattice is also reduced. Furthermore, the non-magnetic Zn2þ (3d10, S ¼ 0) replaces Fe3þ on the A-sites due to favourable electronic configuration for tetrahedral bonding to the oxygen ions, which affect the lattice parameters (a0) [7,8]. Therefore the magnetic exchange for this system is governed by a combination of antiferromagnetic superexchange (SE) and double exchange interactions (DE). There are three antiferromagnetic SE interactions JAA(AeOeA), JBB(BeOeB), and JAB(AeOeB) between the Fe3þ ions
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on the A and B sites, mediated by the oxygen (O) ions. The simple Neel model has been extended by Yafet and Kittel for better understanding of magnetic behaviour of these ferrites [9e11]. In addition, there is a ferromagnetic DE interaction mediated by the itinerant spin down t2g electrons hopping between the mixedvalent Fe ions on the B sites. It is earlier reported that zinc substitution plays a decisive role in tuning the various structural and magnetic properties of the ferrites when they are formed of single domain particle (10 nm) and the phenomenon of superparamagnetism arises [12,13]. As per this strategy, we synthesized the ultra-fine nanoparticles of ZnxFe1xFe2O4 with different zinc concentrations (x ¼ 0.1, 0.3, 0.5 and 0.7) to achieve the superparamagnetism in this compound. The synthesized samples were characterized for their structural and magnetic properties to understand the role of zinc concentration. We observed the systematic reduction in the crystallite size from 12 to 3 nm as zinc concentration varies from x ¼ 0.1 to 0.7, respectively. Interestingly, it was also found that ZnxFe1xFe2O4 sample with x ¼ 0.7 (having average particle size w3 nm) revealed the superparamagnetic behaviour in a wide temperature range from room temperature down to 5 K. 2. Experimental 2.1. Synthesis of ultra-fine ZnxFe1xFe2O4 nanoparticles The nanocrystalline samples of ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) were synthesized by a low temperature chemical coprecipitation method [14,15]. High purity salts (99%, Aldrich Company) of Fe(NO3)3$9H2O, ZnCl2 and FeCl2 in aqueous media were used as starting percursors. These salts were dissolved in double deionized water with constant stirring to get solution of 0.5 mol/L. The equimolar solutions of Fe(NO3)3$9H2O, ZnCl2 and FeCl2 were mixed in their stoichiometric ratio and homogenized at 50 C. Then, 25% ammonia solution was added drop by drop as a precipitating agent under constant stirring and pH was monitored carefully until it reached in the range 8.5e10. The mixture was then heated at 60 C for about 1 h. This duration is sufficient for transformation of hydroxides into spinel ferrites (digestion step). The precipitated particles were then washed several times with double distilled water to remove the salt residues and other impurities. It was further dried at 60 C for 1 h to obtain the nanoparticles, which were used for further characterizations.
to avoid saturation effect. The a-diphenyl-b-picrylhydrazyl (DPPH) was used as a reference material for the determination of g-value. All the measurements were carried out at room temperature. 3. Results and discussion 3.1. Structural analysis The Rietveld fitted X-ray diffraction patterns of as-synthesized ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) nanoparticles have been shown in Fig. 1, which confirm the formation of cubic spinel structure (Fd-3m space group) without any impurity phases and coinciding with the standard data card (ICDD PDF #82-1042) by using the Rietveld Program, FullProf.2k (Version 5.20 - Jul2011-ILL JRC) [16]. It was also observed that the diffraction peaks of sample x ¼ 0.7 become very broad which indicate the ultra-fine particle nature of the sample due to reduced particle size. By using the most intense (311) peak line widths observed from the Rietveld fits, the average crystallite sizes shown in Table 1, were estimated by Scherrer’s formula. The instrumental broadening, measured through standard Si sample, has been deducted from the FWHM value during crystallite size calculations by Scherrer’s formula. The average crystallite size estimated by Scherrer’s formula are found to be 15 0.85, 12 0.73, 8 0.52 and 5 0.26 nm for the sample x ¼ 0.1, 0.3, 0.5 and 0.7, respectively (as shown in Table 1). Furthermore, the lattice parameters obtained from the Rietveld fits are 8.3820 0.0006, 8.3884 0.0012, 8.3915 0.0018 and 8.4092 0.0032 Å for sample x ¼ 0.1, 0.3, 0.5 and 0.7 respectively, which shows the slight increase with increasing zinc concentration (as shown in Table 1). The lattice parameters determined from Rietveld refinement agree with their respective values reported in the literature [17]. The increase in lattice parameter from 8.3820 to 8.4092 Å for sample x ¼ 0.1e0.7 indicates the replacement of vacancies of the smaller Fe3þ ion (0.49 Å) by larger Zn2þ ion (0.60 Å) at A-site during the reaction [18].
2.2. Characterization Structural studies of as-synthesized nanoparticles were carried out using powder X-ray diffractrometer (Bruker D-8 advance) with A) at the slow scan rate of 0.02 s1 to CuKa radiation (l ¼ 1.540 confirm the phase formation. High resolution transmission electron microscopy (HRTEM) (Model: Tecnai G2 F30 STWIN field emission gun supported operated at the electron accelerating voltage of 300 kV) was used to determine the shape and particle size distribution of the nanocrystals. Magnetic characterization of all the samples were recorded by vibrating sample magnetometer (LakeShore: Model no. 7305) at room temperature and low temperature in a magnetic field from 1.2 to þ1.2 T and other parameters such as magnetization (M), coercive force (HC), and remanent magnetization (Mr) were evaluated for each sample. The zero-field cooled (ZFC) and field cooled (FC) measurements were carried out in the temperature range of 5 to 320 K under a constant magnetic field of 100 Oe. The measurements of magnetic resonance spectra were performed on a conventional X-band (n ¼ 9.36 GHz) Varian make E Line Century X-band EPR spectrometer (Model-E-112) with 100 kHz magnetic field modulation and 10 mW microwave power
Fig. 1. Rietveld refinement pattern of as-synthesized ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) nanoparticles with average diameter of 12, 10, 6 and 3 nm respectively. X-ray experimental data are shown as very broad line and solid line is the best fit to the data and tick marks show the positions for the allowed reflections. The lower curve represents the difference between the observed and calculated profiles.
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Table 1 The variation of lattice parameters, average particle diameter, remanent magnetization, blocking temperature, EPR peak to peak line width and gyromagnetic constant of ZnxFe1xFe2O4 with x ¼ 0.1, 0.3, 0.5 and 0.7. Sample with Zn content
Lattice parameters (A0)
Average crystalline size by XRD (nm)
Average particle diameter by HRTEM (nm)
remanent magnetization Mr (emu gm1)
Blocking temp. (K)
EPR peak-to-peak line width (Gauss)
Gyromagnetic constant
x ¼ 0.1 x ¼ 0.3 x ¼ 0.5 x ¼ 0.7
8.3820 0.0006 8.3884 0.0012 8.3915 0.0018 8.4092 0.0032
15 0.85 12 0.73 8 0.52 5 0.26
12 1 10 1 61 31
1.10 0.37 0.17 0.02
120 98 60 5
1300 1150 850 155
2.2931 2.2660 2.1885 2.1673
The high resolution transmission electron microscopy (HRTEM) images of sample x ¼ 0.1 and 0.7 are shown in Fig. 2a and b. Selected area electron diffraction (SAED) patterns and particle size distribution histograms, obtained through TEM images, are shown in the insets of Fig. 2. Most of the particles in Fig. 2a and b have diameters of 12 1 and 3 1 nm for the sample x ¼ 0.1 and x ¼ 0.7, which depicts the good agreement with the average particle size observed through XRD data. The reduction in crystallite size with zinc content is mainly due to the strong chemical affinity of specific cations like Zn to the tetrahedral A site and the metastable cation distribution in nanoscale range of ferrite particles as discussed by other groups [19e22]. Based on these studies, we propose the following mechanism to explain the dependence of particle size on cation stoichiometry: In a complex system like the ferrites where many cations are involved, the nucleation and growth of particles are expected to be influenced by the probability of a cation available chemically in equivalent sites and by its affinity to these sites. In our study, during the formation of ZnFe2O4 nuclei, Zn would preferentially occupy the tetrahedral site and force the Fe to the octahedral site. Even in nanoscale range, the preferential occupancy of Zn dominates over the metastable cation distributions. Similarly, the restriction of Fe to occupy only the octahedral sites as in the case of ZnFe2O4, no longer applies and it can occupy the tetrahedral site also. Therefore, the growth of these ZnFe2O4 nuclei will be restricted, since the incorporation of either Zn or Fe by the nuclei will depend on the availability of their preferential sites. This suggests the decrease in particle size with the increase in Zn2þ concentration. 3.2. Magnetic properties The temperature dependent magnetization (MeT) measurements under ZFC and FC modes in an applied magnetic field of
100 Oe in the temperature range from 5 to 320 K are shown in Fig. 3. It is clear from figure that the onset irreversible magnetization (ΔM ¼ MFC MZFC) and blocking temperature (TB) shifts towards the lower temperature with concentration of Zn2þ. The value of blocking temperature for sample x ¼ 0.1 is 120 K while it decreases below to 5 K for the sample x ¼ 0.7. Moreover, for x ¼ 0.7 sample, both ZFC and FC curves almost overlap to each other down to temperature 5 K and ΔM is indistinguishable [23]. This behaviour of ΔM indicates that the beginning of freezing process shifts towards lower temperature and irreversible magnetization is negligible over the entire measurable temperature range from room temperature down to 5 K for sample x ¼ 0.7. Such interesting phenomenon is attributed to the ultra-fine particles of sample x ¼ 0.7 (having average diameter of 3(1) nm), smaller than the superparamagnetic critical dimension (10 nm), which causes the shifting of blocking temperature (TB) towards lower values and leading to superparamagnetic behaviour towards lower temperatures [24e26]. The magnetization of nanoparticles at room temperature varies strongly as a function of Zn2þ doping as shown in Fig. 4. In ultra-fine nanoparticles, surface spin-structure become prominent due to breaking of bonds at the surface and become highly disordered as compared to core spins [27]. The magnetic properties drastically decreases with the substitution of Feþ3 (3d5, S ¼ 5/2) by Zn2þ (3d10, S ¼ 0) on the A sublattice. However Zn substitution also weakens antiferromagnetic exchange JAB by diluting the A site moments. Furthermore, it may also converts Fe2þ(3d6, S ¼ 2) into Fe3þ on the B sublattice, resulting in a reduction in the itinerant charge carrier density. This weakens the ferromagnetic double exchange, competing with antiferromagnetic superexchange interaction on the B sublattice. This leads to an increase in the spin canting on the B sublattice with increasing Zn ions and results the observed reduction in the total magnetization.
Fig. 2. HRTEM images of ZnxFe1xFe2O4 samples: (a) x ¼ 0.1 and (b) x ¼ 0.7 with their selected area electron diffraction (SAED) patterns and particle size distribution histograms.
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Fig. 3. Temperature dependent magnetization (MeT) curves of ZnxFe1xFe2O4 samples: (a) x ¼ 0.1, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7 recorded in zero-field cooled (ZFC) and field cooled (FC) conditions with applied magnetic field of 100 Oe in the temperature range 5e320 K.
Fig. 4. Room temperature magnetization versus magnetic field (MeH) hysteresis loops of ZnxFe1xFe2O4 samples: (a) x ¼ 0.1, (b) x ¼ 0.3, (c) x ¼ 0.5 and (d) x ¼ 0.7 in applied field up to 1. 2 T.
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The MeH curves of the x ¼ 0.7 sample recorded at room temperature also support the superparamagnetic behaviour in this sample at room temperature as shown in Fig. 4d [28,29]. The nonsaturation in magnetization, absence of hysteresis, very low remanent magnetization and coercivity (as shown in Table 1) observed in this sample supports the presence of superparamagnetism at room temperature. Furthermore, to certify the superparamagnetic behaviour in these samples at low temperature, we recorded the MeH measurements at 5 K for samples x ¼ 0.3, 0.5 and 0.7, which has been shown in Fig. 5aec. MeH plots for samples
Fig. 6. Room temperature EPR spectra of ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) recorded at 9.36 GHz frequency of applied signal with modulated frequency 100 kHz and 10 mW microwave power.
x ¼ 0.1 and 0.3 having the saturation in magnetization, however the sample x ¼ 0.5 does not show the saturation with hysteresis. Furthermore, it is clearly observed for sample x ¼ 0.7 (Fig. 5c) that there is no hysteresis and saturation in magnetization for this sample, which supports the MeT results that x ¼ 0.7 sample show the superparamagnetic behaviour up to 5 K. 3.3. EPR analysis The EPR studies of ferrites give the clear picture of interaction of spin with electromagnetic waves [30]. The room temperature EPR spectra of synthesised samples (as shown in Fig. 6) were analysed using Lorentzian distribution function which indicates that with increasing concentration of Zn2þ, the peak to peak line width (ΔHPP) decreases, where as the resonance field shift towards the higher value. In this system Zn2þ concentrations plays the crucial role to reduce the particle diameter significantly. We have already discussed this in the structural part so we can say that it is a correlated system. The intensity of the resonance line gradually decreases with increasing Zn2þ concentration as the intensity of resonance peak in EPR spectra is related to iron content as reported by the other groups [31,32] and iron content is modified by varying the zinc content in this system. Therefore, we got maximum intensity for x ¼ 0.1 sample which has maximum iron content. Furthermore, asymmetry in the peaks has been observed for the sample above x ¼ 0.3 and it goes maximum for sample x ¼ 0.7, mainly due to reduction in particle size [33]. The diameter dependence of the EPR resonance frequency is well explained by a simple k$p band model [34]. The systematic decrease in peak-to-peak line width is observed up to x ¼ 0.5, but a sudden fall has been observed for sample x ¼ 0.7. The clear decrement in calculated gyromagnetic constant (g) values is also observed as given in Table 1. The high asymmetry and narrow width of resonance peak for x ¼ 0.7 sample indicate the superparamagnetism in this sample which is also supported by MeT and MeH magnetic measurements [35]. 4. Conclusions
Fig. 5. Magnetization verses magnetic field (MeH) hysteresis loops of ZnxFe1xFe2O4 samples at 5 K: (a) x ¼ 0.3, (b) x ¼ 0.5 and (c) x ¼ 0.7.
In summary, we synthesized the ZnxFe1xFe2O4 (x ¼ 0.1, 0.3, 0.5 and 0.7) nanoparticles via chemical co-precipitation method and studied their structural and magnetic properties. Rietveld fitted XRD patterns exhibit the single phase nature of all the compositions with spinel structure (Fd-3m space group). High resolution transmission electron microscopy (HRTEM) analysis reveals that the average particle size reduces with the substitution of higher Zn
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concentration. Magnetic measurements shows that the blocking temperature (TB) and irreversible magnetization (ΔM ¼ MFC MZFC) shifts towards the lower temperature as we move from the sample x ¼ 0.1 to 0.7. Furthermore, Magnetic and EPR results reveal the superparamagnetic behaviour in ultra-fine nanocrystals of ZnxFe1xFe2O4 with x ¼ 0.7 in a wide temperature range from room temperature down to 5 K. Acknowledgements The author Nitu Kumar gratefully acknowledges the Council of Scientific and Industrial Research (CSIR) India for the award of Senior Research Fellowship to carry out research work at National Physical Laboratory, New Delhi for pursuing his Ph.D thesis. The authors are thankful to Dr. Sukhvir Singh for extending technical support to record the HRTEM micrograph. The authors are also grateful to Director, National Physical Laboratory under CSIR for providing constant encouragement and granting permission to publish this work. References [1] [2] [3] [4] [5]
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