J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 2 1, 2 0 0 2, 1135 – 1137
Effect of substitution of Mn ion on magnetic properties of Fe3 O4 nanocrystallites P. SARAVANAN, S. ALAM, L. D. KANDPAL, G. N. MATHUR Defence Materials and Stores Research and Development Establishment, DMSRDE P.O., G.T. Road, Kanpur 208 013, India E-mail:
[email protected];
[email protected]
Ferrites, the ferrimagnetic iron oxides, are a group of technologically important materials that are used in the fabrication of magnetic, electronic and microwave devices. They exhibit relatively high resistivity at carrier frequencies, sufficiently low losses for microwave application and a wide range of other electrical properties [1]. The nanoparticles of ferrites, such as spinel ferrites, possess great potential for applications since they are relatively inert and their magnetic properties can be fine-tuned by chemical manipulations [2]. In recent years, substituted ferrites with the spinel structure have been widely investigated due to their considerable importance to the electronic materials industry [3]. By introducing relatively small amount of foreign ions (Zn, Mg, Co, etc.) the structural and magnetic properties can be modified in ferrites [4–6]. Because the magnetic saturation is dependent on the site location and the d-electron structure of the transition metal cations, it is possible to systematically alter the net magnetic moment by chemical substitutions [7]. In the present communication, the above fact has been demonstrated with the substitution of Mn ion on the magnetic properties of Fe3 O4 colloidal nanocrystals. Fe3 O4 magnetic colloids were produced by the chemical co-precipitation method, i.e., mixing an acidic solution of FeCl2 · 4H2 O, FeCl3 · 6H2 O and MnCl2 · 4H2 O with a concentrated alkali solution of pH around 12 at ∼90 ◦ C. Reactions were carried out using Mn ion concentration of 0 wt%, 6 wt%, 12 wt% and 18 wt% and with a molar ratio of Fe(II)/Fe(III) = 0.5. The precipitates were isolated in the magnetic fields and supernatant liquid was removed from the precipitate by decantation. The cationic colloidal nanoparticles were obtained by neutralizing the anionic charges on the colloidal nanoparticles by adding dilute hydrochloric acid. The black Mn substituted Fe3 O4 precipitates obtained were strongly attracted to a magnet. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurements were performed with a 300 kV Jeol 3010 transmission electron microscope. Energy dispersive X-ray (EDX) analysis was performed using Links (ISIS) Si(Li) detector of Oxford instruments fitted to a Leica S-440i scanning electron microscope. X-ray diffraction (XRD) measurements were performed using a Seifart 3000 X-ray powder diffractometer equipped with CoKα radiation source employing a 0.02 ◦ step size. Magnetic measurements were carried out employing a VSM C 2002 Kluwer Academic Publishers 0261–8028
7300 Lakeshore vibrating sample magnetometer with maximum applied field of 8 × 105 A · m−1 . Fig. 1 shows the X-ray powder diffraction pattern identifying the phases formed at different Mn ion concentrations with Fe3 O4 colloids. The XRD pattern could be indexed to an inverse cubic spinel structure [8]. The broadening of diffraction lines is caused by a small crystalline size effect of the samples. The average grain sizes were determined from the XRD pattern parameters of the samples according to the Scherrer’s equation [9], D = kλ/(β cos θ ) where D is the average grain size, k is the shape fac˚ tor, λ is the X-ray wavelength equal to 1.5406 A, β is the full width at half maximum (FWHM) and θ is the diffraction angle. The average particle sizes determined from the peak broadening are shown in Table I. The chemical composition of the samples was determined by EDX and the obtained results are given in Table I. Line and spot profile analysis yielded Mn ion
Figure 1 XRD patterns for Fe3 O4 nanocrystals with Mn ion concentration of (a) 0 wt% (b) 6 wt% (c) 12 wt% and (d) 18 wt%.
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T A B L E I Characterization results for Fe3 O4 colloids
Sample 0 wt% Mn + Fe3 O4 6 wt% Mn + Fe3 O4 12 wt% Mn + Fe3 O4 18 wt% Mn + Fe3 O4
Diameter (nm) Composition (wt%) dTEM dXRD – Mn = 5.97 Fe = 94.03 Mn = 11.99 Fe = 98.01 Mn = 18.02 Fe = 91.98
Coercivity (kA · m−1 )
8.0 ± 1.5 8.2 13.8 ± 0.8 13.5
4.89 4.90
12.7 ± 1.7 13.3
5.03
14.2 ± 1.3 14.8
4.95
substitution in Fe3 O4 colloids within 0.3% of feed ratio of the metal salts. Fig. 2a to d depict the typical TEM micrographs of assynthesized Fe3 O4 magnetic colloids. As evident from the TEM micrographs, nearly spherical and uniformly dispersed particles were obtained in all cases and there was no distinctive change in morphology with the substitution of Mn. The particles of Fe3 O4 colloids have an average diameter about 8 nm and the particle sizes however increased slightly upon Mn doping; from 8 nm for 0 wt% of Mn to 14.2 nm for 18 wt% of Mn. The av-
erage sizes of the Fe3 O4 nanocrystals estimated from the TEM analysis are given in Table I and it can be seen that these results are in good agreement with the particle sizes obtained from the XRD studies. Selected area electron diffraction (SAED) performed on isolated nanocrystals yielded lattice parameters that were consistent with that of crystalline Fe3 O4 . In all cases, the SAED pattern exhibits five rings. The d-spacings of the rings are 0.286, 0.243, 0.203, 0.156 and 0.143 nm, which are attributed to (220), (311), (400), (511) and (440) reflections of crystalline Fe3 O4 (magnetite), respectively [10]. The d-spacings measured by SAED of Fe3 O4 colloids are consistent with the XRD studies. In order to estimate the changes in the magnetic moment by the Mn substitution with the Fe3 O4 colloids, room temperature magnetization measurements were carried out on dried powder samples. Fig. 3 shows the magnetization curves obtained for the powders of Fe3 O4 with various Mn concentrations. The measured saturation magnetization (Ms ) of Fe3 O4 particles was 46.48 A · m2 · kg−1 , which is apparently 51% less than that of the bulk value of magnetite (Ms(bulk) = 92 A · m2 · kg−1 ); indicating the superparamagnetic nature of the Fe3 O4 nanoparticles. In such particles, the
Figure 2 TEM micrographs of Fe3 O4 nanocrystals with Mn ion concentration of (a) 0 wt% (b) 6 wt% (c) 12 wt% and (d) 18 wt%. Insets show the corresponding SAED patterns.
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netic moment of ferrites [14, 15]. Any substitution of metal ions in ferrite prefer to occupy tetrahedral sites and reduces magnetic moment on the A-site sublattice and hence the resulting net magnetic moment will be increased; this increase would lead until the B-site sublattice have sufficient magnetic neighbors [14]. In the present study, the above fact can be corroborated to the changes accompanying the Mn doping. In conclusion, the magnetic properties of Fe3 O4 nanocrystallites vary markedly with substitution of Mn ion and the saturation magnetization of Fe3 O4 nanocrystals can be significantly altered by suitable doping of Mn.
References 1. A . G O L D M A N , “Modern Ferrite Technology” (Marcel Dekker,
Figure 3 Magnetization curves for the Fe3 O4 powders with various Mn ion concentrations. Inset shows variation of saturation magnetization (Ms ) against Mn ion content.
energy required for relaxation exceeds the ambient thermal energy (25 meV). Further, the decrease in Ms of Fe3 O4 nanoparticles could also be attributed due to a magnetically disordered surface layer [11, 12]. The small change in the coercivity values (ranged from 4.89 kA · m−1 to 5.03 kA · m−1 ) could be due to the changes in the average particle diameters [13]. As is evident from the Fig. 3, the Ms value of Fe3 O4 nanocrystals increases with the small Mn doping and tends to decrease with the higher Mn doping, ranged from 48 A · m2 · kg−1 for 0 wt% of Mn to a maximum of 62.45 A · m2 · kg−1 for 12 wt% of Mn and reduced to 59.28 A · m2 · kg−1 for 18 wt% of Mn. The initial increase in Ms with Mn ion content and the reduction in Ms for higher Mn ion content could be explained in terms of resultant of sublattice magnetic moments [14]. It is well known that the spontaneous magnetization in spinel ferrites is a result of the interactions of transition metal cations and oxygen anions. Consequently, the magnetic properties can be considered to be the sum of the magnetic interactions on the tetrahedral Asite sublattice and the octahedral B-site sublattice and these two sublattices are oppositely aligned and their resulting magnetic moment determines the net mag-
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Received 10 September 2001 and accepted 14 March 2002
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