JOURNAL OF APPLIED PHYSICS 105, 1 共2009兲
Room temperature ferromagnetic ordering in indium substituted nano-nickel-zinc ferrite Sangeeta Thakur,1,a兲 S. C. Katyal,1 A. Gupta,2 V. R. Reddy,2 and M. Singh3 1
Department of Physics, Jaypee University of Information Technology, Waknaghat, Solan 173215, India UGC-DAE Consortium for Scientific Research, University Campus, Khandwa Road, Indore 452017, India 3 Department of Physics, Himachal Pradesh University, Shimla 171005, India 2
共Presented 11 November 2008; received 17 September 2008; accepted 29 October 2008; published online xx xx xxxx兲 Nano-nickel-zinc-indium ferrite 共NZIFO兲共Ni0.58Zn0.42InxFe2−xO4兲 with varied quantities of indium 共x = 0 , 0.1, 0.2兲 have been synthesized via reverse micelle technique. X-ray diffraction and transmission electron microscopy confirmed the size, structure, and morphology of the nanoferrites. The addition of indium in nickel-zinc ferrite 共NZFO兲 has been shown to play a crucial role in enhancing the magnetic properties. Room temperature Mössbauer spectra revealed that the nano-NZFO ferrite exhibit collective magnetic excitations, while indium doped NZFO samples have the ferromagnetic phase. The dependence of Mössbauer parameters, viz. isomer shift, quadrupole splitting, linewidth, and hyperfine magnetic field, on In3+ concentration has been studied. Mössbauer study on these nanosystems shows that the cation distribution not only depends on the particle size but also on the preparation route. Mössbauer results are also supported by magnetization data. Well defined sextets and appearance of hysteresis at room temperature indicate the existence of ferromagnetic couplings which makes nano-NZIFO ferrite suitable for magnetic storage data. © 2009 American Institute of Physics. 关DOI: 10.1063/1.3062956兴 I. INTRODUCTION
II. EXPERIMENT
Nanoparticles of spinel ferrites, which are a class of complex oxides, by virtue of their unique electronic or physical structure,1 display enhanced properties, which may be hard for technological applications.2,3 With increasing demand for planar devices, recent ferrite research has shifted to the fabrication and design of novel nanoparticles. Nanostructure processing provides a new opportunity to fabricate novel magnetic materials with new functionalities. Among various methods of fabricating ferrite nanoparticles, the reverse micelle technique appears as an attractive route which provides monodisperse particles showing surprisingly good crystallinity and enhanced magnetic characteristics. Reverse micelles, which are essentially nanosized aqueous droplets that exist in microemulsions with certain compositions, are known to present an excellent medium for the synthesis of nanoparticles with uniform morphology.4–6 So far, hardly any publication has reported the synthesis of indium doped nanonickel-zinc ferrite 共NZFO兲 particles via reverse micelle technique. Further, very little work has been reported on the Mössbauer investigations in the case of trivalent diamagnetic ions doped in stoichiometric compositions of NZFO ferrites. This has motivated us to carry out the structural, magnetic, and Mössbauer measurements on nanostructured Ni0.58Zn0.42InxFe2−xO4 共x = 0 , 0.1, 0.2兲 ferrites synthesized via reverse micelle technique.
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For reverse micelle synthesis, a solution comprising isooctane, water, and a mixture of the surfactant sodiumdioctyl sulffosccinate was used to form the micelles7— FeCl2 . 4H2O 共Merk 99.9%兲, NiCl2 . 6H2O 共Merk 95%兲, ZnCl2 共95%兲, and InCl3 共99.95%兲 were used in a molar ratio to provide the metal cations; ammonia hydroxide was used to precipitate a mixed metal hydroxide. The sample was then fired for 4 h at 500 ° C to vaporize the coordinated water and facilitate the conversion and oxidation of the metal hydroxides to the spinel ferrites. The phase structure of nanoferrites was analyzed by x-ray diffraction 共XRD兲 共Rigaku Geiger Flex 3 kW diffractometer兲 using Cu K␣ source. Transmission electron microscopy 共TEM兲 was used to analyze the morphology of nanonickel-zinc-indium ferrite particles. The Mössbauer spectra were recorded using a conventional constant acceleration Mössbauer spectrometer with 57Co in a Rh matrix as the Mössbauer source. Mössbauer spectra were analyzed by NORMOS fitting. Magnetic measurements were obtained from a super conducting quantum interference device at room temperature 共300 K兲. III. RESULT AND DISCUSSION
XRD patterns of the prepared ferrites show a typical spinel structure for samples x = 0 and 0.1 共Fig. 1兲. However, the sample x = 0.2 contains slight trace of hematite 共Fe2O3兲 in addition to spinel phase 共Fig. 1兲. Slight increase in pH value during the synthesis condition may lead to the formation of Fe2O3 phase.7 Crystallite diameter for the samples x = 0, 0.1, and 0.2 were determined as 8.4, 15, and 15 nm, respectively.
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FIG. 1. XRD pattern on NZIFO particles.
TEM for the compositions 0.1 and 0.2 given in Figs. 2共a兲 and 2共b兲 show spherical particles, with a narrow particle size distribution in the range of 15–25 nm. TEM of NZFO also shows the spherical nature of the particles with the size distribution range of 8–12 nm.8 Figure 3共a兲 shows that the room temperature 57Fe Mössbauer spectra of nano-NZFO particles exhibit collective magnetic excitations. Figure 3共b兲 shows the p-B distribution derived from the Mössbauer spectra of NZFO particles. It can be seen that the spectrum has a wide range of hyperfine magnetic field 共HMF兲 distribution. Hyperfine field distribution shows a maximum near 4 T, extending up to ⬃58 T. The average hyperfine field for NZFO sample is 44.43 T. Mössbauer spectra of nano nickel-zinc-indium ferrite 共NZIFO兲 at x = 0.1 and 0.2 关Figs. 4共a兲 and 4共b兲, respectively兴 show well defined sextet with a small superparamagnetic doublet. The addition of indium creates an increasing distortion in the iron environment due to the next nearest neighbor effect, a distortion which lowers the electronic symmetry. The doublet thus split into a sextet with increasing indium content. This substitution causes a complete rearrangement in the cation distribution among the various valence states and coordination sites. The appearance of a well split sextet indicates the presence of a long range magnetically ordered phase with kV⬎ Eth. Thus the cation distribution results in a higher effective magnetic anisotropy barrier, bringing magnetic ordering at room temperature. The doublet can only be observed when the superparamagnetic relaxation of the nanoparticles occurs at a rate faster than the Mössbauer measurement time, giving a time average zero magnetization. Due to a distribution of energy barriers, some nano-
FIG. 2. TEM of 共a兲 x = 0.1 nanoparticles 共15–25 nm兲 and 共b兲 x = 0.2 nanoparticles 共15–25 nm兲.
J. Appl. Phys. 105, 1 共2009兲
FIG. 3. 共a兲 Mössbauer spectra of NZFO ferrite at room temperature and 共b兲 p-B distribution of the sample.
particles relax faster and the other slower at a given temperature. Consequently, the sextet peak and doublet peak can appear simultaneously. The experimental data for sample x = 0.1, were fitted with the superposition of three subspectra, two accounting for Fe3+ in the tetrahedral sites 共A兲 and octahedral sites 共B兲 of the spinel structure and one associated with the doublet 共M1兲. In addition to these, fourth spectra 共M2兲 is used for sample 0.2 for the traces of hematite. The corresponding Mössbauer parameters such as isomer shift 共IS兲, quadrupole splitting 共QS兲, hyperfine magnetic field, and linewidth, obtained by fitting the spectra, are listed in the Table I. The observed variations in hyperfine magnetic field can be qualitatively explained using Neel’s superexchange interactions.9 The replacement of Fe3+ by In3+ weakens FeA3+ – O – FeB3+ superexchange interactions and hence hyperfine field is expected to decrease with increasing concentration of indium. A similar decrease in hyperfine magnetic field with the successive incorporation of diamagnetic ions in place of Fe3+ ions in many kinds of ferrites has been reported.10–13 The values of IS for the A and B sites are consistent with high spin Fe3+ charge state.12 The values of QS for hyperfine spectra of all the samples are almost zero within the experi-
FIG. 4. Mössbauer spectra of NZIFO ferrite at room temperature: 共a兲 x = 0.1 and 共b兲 x = 0.2.
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TABLE I. Mössbauer parameters for nano-NZIFO particles.
NZIFO x = 0.1
x = 0.2
QS 共mm/s兲
HMF 共T兲
Area ratio 共%兲
Site
WID
IS 共mm/s兲
B A M1
0.45 0.70 0.5
0.15 0.11 0.21
⫺0.030 ⫺0.009 0.55
48.385 45.33 ¯
54.38 40.44 5.18
B A M1 M2
0.46 1.18 0.4 0.25
0.20 0.16 0.19 0.23
0.022 0.12 0.61 ⫺0.130
47.88 42.40 ¯ 51.05
66.45 20.64 6.45 6.46
mental error and are attributed to the fact that overall cubic symmetry is maintained between Fe3+ ions and their surroundings with substitute In3+ ions in NZFO particles. A small paramagnetic doublet appeared in the nano-NZIFO particles on magnetic sextet, showing negligible QS, and IS values are consistent with Fe3+ valence state. The linewidth values corresponding to A and B sites have been found to increase with increasing concentration of indium. The broadening of lines could be attributed to slight changes in the varying environment surrounding Fe3+ ions in the same sublattice; such changes in environment of Fe3+ ions would lead to a change in the magnetic filed and, consequently, result in an appreciable broadening of the Zeeman lines. Figure 5共a兲 shows the variation of magnetization with the applied field for all the three samples. The magnetization of NZFO sample does not saturate even at the maximum field attainable 共H = 50 kOe兲, while NZIFO particles attain saturation magnetization in the applied field. For x = 0 sample the saturation magnetization, M sat 共22.29 emu/g兲 is determined by extrapolating the M versus 1 / H curve to 1 / H = 0. The M sat value for the x = 0.1 and 0.2 samples are 39.38 and 24.85 emu/g, respectively. Variation of saturation magnetization with increase in indium content is in accordance with earlier study on NZIFO particles.11 The particular lack of saturation in NZFO sample is attributed to surface spin-canting.7 The typical characteristic of superparamagnetic behavior, an absence of hysteresis, and almost nonmeasurable coercivity and remanence is observed for NZFO par-
ticles. The observation of hysteresis loops for NZIFO samples at the room temperature is consistent with the occurrence of a ferrimagnetic or a ferromagnetic ordering in these nanocrystals 关Fig. 5共b兲兴.The coercive field of samples x = 0.1 and 0.2 are 47.75 and 56.95 Oe, respectively. The observed values of the squareness ratio and remanent magnetization are 0.10 and 3.93 emu/g, respectively, for sample x = 0.1, and 0.15 and 3.77 emu/g, respectively, for the x = 0.2 sample. The low inferred values of squareness ratio and coercivity are in agreement with the well established soft magnetic character of NZIFO particles.12 Indium is diamagnetic and initially its ions migrate to the tetrahedral by 共A兲 site,11,13–15 replacing Fe3+共A兲 – O – Fe3+共B兲 3+ In 共A兲 – O – Fe3+共B兲, which decreases the magnetic moment at the A site and thus the overall magnetic moment increases. This consequently raises the saturation magnetization of the system. However for higher concentration of In3+ ions 共x ⬎ 0.1兲 some of them migrate to the B site also, which reduces the overall saturation magnetization of the particles.11 The decrease in the saturation magnetization for sample x = 0.2 may also be the formation of antiferromagnetic phase with spinel phase.7 It is clear that room temperature magnetic behaviors are consistent with Mössbauer measurements. Such a clearly defined hysteresis and well defined sextet at room temperature indicates that these nanoparticles are truly ferromagnetic. IV. CONCLUSIONS
Considering that the magnetic properties of all ferrimagnetic materials are dependent on the preparation route, the technique outlined here promises to be an important tool for synthesis of nanoferrites. Magnetic behavior of NZIFO particles is consistent with almost perfectly magnetically ordered nanoparticles without a significant dead magnetic layer. Nano-NZIFO particles attain saturation magnetization and exhibit ferromagnetically coupling at room temperature. This behavior, which is promising if such materials are to be used in technological applications, has never been observed for similar nanoparticles obtained by other routes. Work is in progress to get pure phase with different particle sizes of nano-NZIFO ferrite, which would make these nanomaterials serious candidates for magnetic storage data. S. Son et al., J. Appl. Phys. 91, 7589 共2002兲. M. Sugimoto, J. Am. Ceram. Soc. 82, 269 共1999兲. I. Safarik and M. Safarikova, in Nanostructured Materials, edited by H. Hofmann, Z. Rahman, and U. Schubert 共Springer, Wien, 2002兲, pp. 1–23. 4 D. O. Yener and H. Giesche, J. Am. Ceram. Soc. 84, 1987 共2001兲. 5 A. Nanni and L. Dei, Langmuir 19, 933 共2003兲. 6 L. M. Gan et al., J. Mater. Sci. 31, 1071 共1996兲. 7 S. Thakur et al., J. Magn. Magn. Mater. 321, 1 共2009兲. . 8 S. Thakur et al., Appl. Phys. Lett. 91, 262501 共2007兲. 9 L. Neel, Ann. Phys. 3, 137 共1948兲. 10 K. H. Rao and R. G. Mendiratta, J. Appl. Phys. 54, 1795 共1983兲. 11 B. P. Rao and K. H. Rao, J. Magn. Magn. Mater. 292, 44 共2005兲. 12 S. Chkoundali et al., J. Phys.: Condens. Matter 16, 4357 共2004兲. 13 R. K. Puri et al., J. Mater. Sci. 29, 2182 共1994兲. 14 S. Ghosh et al., Phys. Lett. A 325, 301 共2004兲. 15 A. Lakshman et al., J. Magn. Magn. Mater. 284, 352 共2004兲. 1 2 3
FIG. 5. 共a兲 Variation of magnetization with applied field at 300 K for NZIFO particles with x = 0, 0.1, and 0.2. 共b兲 Part of the curves near the origin showing remanence and coercivity.
