SiO2 nanocomposites

SiO2 nanocomposites

Journal of Magnetism and Magnetic Materials 242–245 (2002) 1103–1105 Interactions and hysteresis behaviour of Fe/SiO2 nanocomposites J.J. Blackwellb,...

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Journal of Magnetism and Magnetic Materials 242–245 (2002) 1103–1105

Interactions and hysteresis behaviour of Fe/SiO2 nanocomposites J.J. Blackwellb, M.P. Moralesa, K. O’Gradyb, J.M. Gonza! leza, F. Cebolladac,*, c * M. Alonso-Sanudo a

Instituto de Ciencia de Materiales de Madrid, CSIC, 28049 Cantoblanco, Madrid, Spain b University of York, Heslington, York YO10 5DD, UK c ! Dep. de F!ısica Aplicada a las T.I., EUIT de Telecomunicacion-UPM, Cra. de Valencia, km 7, 28031 Madrid, Spain

Abstract We present a study of interactions in mechanically alloyed Fe/SiO2 nanocomposites. The DM plots and the switching field distributions reveal that increasing Fe concentrations lead to higher dipolar fields and larger dipolarly coupled regions. We propose a switching mechanism based on avalanche phenomena triggered by the reversal of individual particles, irrespective of the Fe concentration being above or below percolation. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Reversal mechanism; Magnetic interactions; Nanoparticles; Mechanical alloying; Percolation

1. Introduction Magnetic nanoparticles exhibit complex magnetic behaviour that is strongly dependent on size effects and on their specific structural features, giving rise to many striking properties [1]. In the case of Fe/SiO2 nanocomposites, many authors have reported unexpectedly high coercivities at room temperature, from a few hundred Oe to over 1 kOe [2,3]. Although in some cases the dependence of the coercivity on concentration can be correlated with the variation of the particle size, it is evident that magnetic interactions must be taken into account when considering the evolution of the magnetization process with Fe concentration. The analysis of interactions is a subject of ongoing study in the field of magnetic materials, their origin and strength being difficult to evaluate in most cases. The DM technique is a well-established tool that gives information about both matters: negative DM values are usually associated with dipolar interactions that tend

to demagnetize the samples, while positive values are due to magnetizing generally exchange interactions [4]. In this work we present an analysis of magnetic interactions in a series of Fe/SiO2 nanocomposites in an Fe concentration range x covering the low, medium and high (percolation, x > 0:5) regions. 2. Samples and experimental techniques A set of Fex/(SiO2)1x nanocomposites, with x between 0.2 and 0.6, was prepared by mechanically alloying the starting powders in a high-energy mill for 24, 40, and 60 h. The structural characterization of the samples was carried out by X-ray diffraction (XRD), using a Phillips PW1710 diffractometer and Cu-Ka radiation, and by transmission electron microscopy (TEM), by means of a 100 kV Phillips microscope. The magnetic measurements were performed in an EG&G VSM, with a maximum applied field of 1 T. 3. Results and discussion

*Corresponding author. Tel.: +34-91-336-7840; fax: +3491-336-7841. E-mail address: [email protected] (F. Cebollada).

The mean grain size of the samples, evaluated from the width of the XRD peaks, is about 17 nm, almost

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independent of composition and milling time. The TEM images revealed a broad particle size distribution, ranging from 5 to 40 nm, approximately. In most cases the particles were well dispersed in SiO2, although Fe clusters were also observable, particularly for x in the range from 0.4 to 0.6. The room temperature (RT) coercivity Hc of the samples (Fig. 1) has a maximum, for each milling time, and then decreases markedly when it approaches the percolation regime. Its highest value, Hc B400 Oe, was achieved for the sample with x ¼ 0:3 milled for 60 h and is well above that calculated for coherent rotation, about 180 Oe [5]. Hc cannot be correlated with the variation of the grain size as a function of either the milling time or the Fe concentration. In a similar way, the wide range of coercivity values that can be found in the literature [2, 3, 6] for samples with particle sizes of the same order indicates that other factors, including interaction effects, must be taken into account to understand the reversal mechanism of this system. The analysis of the magnetic interactions was carried out for the 60 h milled set of samples by measuring the isothermal and DC demagnetization remanence curves (IRM and DCD) at 77 K and RT, from which the DM plots and the switching field distributions (SFD) were derived. Figs. 2 and 3 show that the DM plots are all negative, which is due to demagnetizing (dipolar) interactions, and tend to merge into a single curve when the

percolation regime is approached. No qualitative differences are evident between the curves corresponding to samples either above or below the percolation threshold. Figs. 4 and 5 present the room-temperature SFDs determined from the IRM and DCD curves, respectively. Again, no qualitative changes are observable for samples either above or below the percolation threshold. This clearly indicates that, surprisingly, there is no change in the switching mechanism with Fe concentration. The SFDs decrease in width and are shifted to lower fields with increasing Fe concentration. Narrow distributions can be associated with high degree of coupling, similar to that taking place in exchange coupled systems [4], which shows that the degree of

Fig. 1. Coercivity as a function of Fe concentration.

Fig. 4. Magnetization SFDs corresponding to RT.

Fig. 2. DM plots corresponding to 77 K.

Fig. 5. Demagnetization SFDs corresponding to RT.

Fig. 3. DM plots corresponding to RT.

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to overcome to magnetize the samples. These structures no longer exist in the saturated state. The tendency to saturation of DH and the fact that the DM curves tend to merge into a single one when the percolation regime is approached suggest that the dipolar fields tend to a constant value.

4. Conclusions Fig. 6. DH as a function of concentration.

coupling increases with Fe concentration, as expected. However, the high degree of coupling should not be understood as leading to collective reversal processes in the sense of clusters formed by coupled particles reversing their magnetization in one step. This would lead to activation volumes larger than the particle size and to different switching mechanisms for samples either diluted or concentrated. In our opinion it is more likely that the reversal takes place through avalanches inside coupled regions, triggered by the reversal of individual particles. The more intense dipolar fields of the samples with higher Fe content would give rise to larger coupled regions and, consequently, to more narrow SFDs. Since this mechanism is based on the reversal of individual particles, it accounts for the fact that no qualitative differences in the switching mechanism are observed as a function of Fe concentration. This interpretation is also supported by the fact that the activation volume of these samples, calculated from magnetic viscosity measurements, is almost equal to the mean particle size, irrespective of the Fe concentration [7]. By comparing the magnetization and demagnetization SFDs corresponding to each sample, we observe that the magnetization spectrum is shifted to higher fields in the magnetization case. The shift DH can be measured from the field required to achieve 50% reversal in each SFD, and it increases with Fe concentration (Fig. 6). The shift DH can be understood as due to the formation of closed flux structures in the demagnetized state, with strong dipolar fields that the applied field has

Fe/SiO2 nanocomposites with coercivities of hundreds of Oe have been prepared. The analysis of their SFDs and of the magnetic interactions has revealed that the switching mechanisms do not depend on Fe concentration. We have proposed a switching mechanism based on avalanches triggered by the reversal of individual particles.

Acknowledgements F.C. acknowledges financial support by Spanish CICyT (Project Mat98-0965-C04-04).

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