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Journal of Magnetism and Magnetic Materials 316 (2007) 103–105 www.elsevier.com/locate/jmmm
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Magnetic properties of Co nanoparticles in zirconia matrix M. Garcı´ a del Muroa,, Z. Konstantinovic´a, M. Varelab, X. Batllea, A. Labartaa Dpt. de Fı´sica Fonamental and Institut de Nanocie`ncia i Nanotecnologia (IN2UB), Universitat de Barcelona, Martı´ i Franque`s 1, Barcelona 08028, Spain b Dpt. de Fı´sica Aplicada i O`ptica and Institut de Nanocie`ncia i Nanotecnologia (IN2UB), Universitat de Barcelona, Martı´ i Franque`s 1, Barcelona 08028, Spain Available online 24 February 2007
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Abstract
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Granular films composed of nanometric Co particles embedded in an insulating ZrO2 matrix were prepared by pulsed laser deposition in a wide range of Co volume concentrations (0.06oxvo0.42). High-resolution electron microscopy (HREM) shows very sharp interfaces between the crystalline particles and the amorphous matrix, with no evidence of intermixing. The mean particles size and width of the distribution determined by fitting the low-field magnetic susceptibility and magnetization curves in the paramagnetic regime to a distribution of Langevin functions are in agreement with the parameters extracted from direct TEM observations. Ferromagnetic correlations between Co particles are evident in the field-cooled state when increasing Co concentration. The effective anisotropy constant estimated from magnetic measurements is about two orders of magnitude larger than the bulk value, and decreases as particle size increases. r 2007 Elsevier B.V. All rights reserved.
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PACS: 61.82Rx; 75.75+a; 68.37Lp; 75.70Rf
Keywords: Nanocrystalline material; Transmission electron microscopy; Magnetic property of nanostructure; Surface magnetism
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Granular films composed of magnetic nanoparticles embedded in an insulating matrix have been the subject of very active research due to their magnetic and transport properties, which have promising technological applications. The properties of nanoparticle systems are different from those of the bulk state, giving rise to a wide variety of new phenomena, such as finite-size and surface effects, interparticle interactions, enhanced properties and striking behaviours related to percolation processes [1,2]. In order to properly correlate the observed behaviour with the corresponding microstructure and compare with theoretical predictions, the experimental model system should have a narrow size distribution of immiscible nanoparticles that are very well defined with respect to the matrix. The samples used in this work were thin films (about 200–300 nm thick) composed of nanometric Co particles embedded in an insulating matrix of ZrO2 stabilized with 7 mol% Y2O3. The films were deposited at room temperaCorresponding author. Tel.: +34 934039201; fax: +34 934021149.
E-mail address:
[email protected] (M. Garcı´ a del Muro). 0304-8853/$ - see front matter r 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2007.02.018
ture onto different substrates by laser ablation of rotating targets comprised of sectors of zirconia and pure cobalt (see Ref. [3] for details). The substrates for TEM experiments were membrane windows of silicon nitride, which enabled direct observation of as-deposited samples. Fig. 1 shows TEM images obtained for different Co volume fractions (xv). The dark regions correspond to Co particles and the light regions to zirconia matrix. For low Co content, typical TEM images clearly show the presence of a regular distribution of Co spherical particles, with very well-defined interfaces with the matrix. Besides, highresolution TEM images gave evidence of crystalline structure inside the particles and amorphous structure in the matrix [4]. The size distribution of the particles has been fitted to a linear-logarithmic function, where significant parameters are the most probable value D0 and the width of the distribution s. The size of the particles varies very slowly until percolation, where large Co aggregates begin to appear. Magnetic characterization of the samples was done using a commercial SQUID magnetometer. The zero-field-cooled
ARTICLE IN PRESS M. Garcı´a del Muro et al. / Journal of Magnetism and Magnetic Materials 316 (2007) 103–105
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Fig. 1. Micrographs of Co–ZrO2 films. From left to right and top to bottom: xv ¼ 0.06, 0.12, 0.3 and 0.42. The third one is from SEM and the rest are from TEM. The darker contrast corresponds to Co particles. The particle size histograms obtained from image analysis are shown in insets. The solid lines are fits to a linear-logarithmic distribution function.
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(ZFC) and field-cooled (FC) curves of samples with different xv well below percolation are shown in Fig. 2. All the curves exhibit features that are characteristic of a narrow distribution of small particles. The ZFC peak position moves from 4.5 K for xv ¼ 0.12 to 36 K for xv ¼ 0.3, indicating how the mean particle volume slowly increases with Co content. For the sample with xv ¼ 0.42 (just after percolation), a very broad peak above 100 K was observed. The ZFC curves below percolation have been fitted (solid lines in Fig. 2) to a distribution of Langevin functions with a temperature-dependent cut-off, which models the progressive blocking of superparamagnetic Co particles as temperature is reduced: Z DB MðH; TÞ ¼ M S LðM S VH=kB TÞf ðDÞdD þ M res , (1) 0
where V is the particle volume and H is the applied magnetic field. It was assumed that all particles have a bulk value for MS (MS ¼ 0.15 mB/A˚3, where mB is the Bohr magneton). The particles are considered to have a linearlogarithmic size distribution f(D), where D is the particle diameter. The threshold diameter DB and the blocking temperature TB are directly related according to
D3B ¼ aBTB, where aB is a fitting parameter. Mres accounts for any temperature-independent magnetization that may arise from ferromagnetic impurities (droplets) and/or diamagnetic contribution from the substrate or sample holder. The contribution of the blocked particles (particles with diameters D4DB) has been neglected, since the ZFC curves were measured under a low magnetic field. The fitting parameters D0 and s are shown in Fig. 2. The obtained values demonstrate that, below percolation, the Co particles size is slightly increasing (from 1.2 nm for xv ¼ 0.12 to 2.7 nm for xv ¼ 0.3), while the width of the distribution is the same. Assuming the characteristic values of the mean time between consecutive attempts to overcome the energy barrier t0E1011 s and of the measuring time t of the experiment [4], the mean height of the anisotropy energy barrier can be estimated as /KVSEln(t/t0)kB /TBS, where kB is the Boltzmann constant and /TBS is the mean blocking temperature, which for a narrow size distribution can be approximated by Tp. From the experimental values of Tp and the fitted particles diameters, we obtain an effective anisotropy constant of 1.9 106 J/m3 for D0 ¼ 1.2 nm, 1.5 106 J/m3 for D0 ¼ 1.8 nm, and 1.3 106 J/m3 for both D0 ¼ 2.2 nm
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evidence of intermixing. The particles size distributions deduced from TEM images are relatively narrow linearlogarithmic functions with a little variation of D0 below percolation. Large particle aggregates begin to appear for xv ¼ 0.3, announcing percolation. Low-field magnetic susceptibility (ZFC curves) and magnetization curves in the paramagnetic regime were fitted to a distribution of Langevin functions. The fitted parameters are in agreement with TEM results for samples below percolation. As usual for small particles [5], high values of the effective anisotropy constant were obtained, and its value decreases with increasing particle size. Close to percolation, the SEM image shows particle aggregates while the observed magnetic behaviour was reproduced assuming a narrow distribution of small Co particles. Studies on the magnetooptical response of similar samples confirm this fact [6]. All the obtained results point out that PLD is a good technique to produce samples with a narrow distribution of small Co nanoparticles, and where particle size growth with increasing Co content is less pronounced than in several previous works (see Refs. [1,7,4] and references therein). This is probably due to the choice of the matrix, which would have a high efficiency to coat Co particles, avoiding coalescence just before percolation and allowing particle growth mostly by nucleation in a wide range of Co content.
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and D0 ¼ 2.7 nm. According to previous results (see e.g. Ref. [5]), the effective anisotropy constant for small particles is significantly higher than the bulk value (6.5 104 J/m3 for FCC Co), and its value decreases as particles grow, and hence surface effects become less significant. Magnetization as a function of applied field was recorded for several xv and various temperatures. In the superparamagnetic region, i.e. for temperatures well above the blocking temperature, the experimental curves were fitted to a distribution of Langevin functions. Z 1 MðHÞ ¼ M S LðM S VH=kB TÞf ðDÞ dD þ wres ðTÞH.
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Fig. 2. ZFC–FC magnetization curves measured under 50 Oe for different Co–ZrO2 films. From left to right and top to bottom: xv ¼ 0.12, 0.2, 0.27 and 0.3. The solid lines are fits to a distribution of Langevin functions, given by Eq. (1).
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The last term in Eq. (2) corresponds to a residual susceptibility, which is necessary to account for paramagnetic and/or diamagnetic residual contributions, coming essentially from the substrate and the matrix. The obtained values of the parameters of the particle distributions are the same as in the case of the ZFC curve. Eq. (2) points out that the magnetization only depends on the reduced parameter H/T, as long as the last term has been subtracted from the experimental data. This implies that in the superparamagnetic regime the magnetization curves should scale when M is plotted as a function of H/T. The obtained magnetization curves scale for xvo0.3, while for xv ¼ 0.3 the curves do not fully scale, evidencing interparticle interactions. In conclusion, pulsed laser deposition was used to prepare granular films of Co nanoparticles embedded in zirconia, in a wide volume concentration range of Co (0.06oxvo0.42). TEM observations reveal regular distribution of crystalline FCC Co nanoparticles with clearly defined interfaces with the amorphous matrix, with no
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