CoxAg1−x core–shell nanoparticles: magnetic and magneto-optical studies
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The magneto-optical properties of 14-nm CoxAg1−x core–shell nanoparticles (x=0.7, 0.8, and 0.35) deposited on different substrates are investigated at room temperature in the photon-energy range from 0.8 to 4.8 eV. Particles with low Ag content show spectra very similar to pure Co nanoparticles while particles with high Ag content have totally different features, where the Ag plasma edge dominates the spectra. The spectral features of the polar Kerr rotation depend on particle composition. The ageing process and development of an oxide layer influence the particles’ core–shell structures and magnetization curves. Co-rich particles exhibit lower resistance to the oxidation process as compared to Ag-rich ones. The quality of the nanoparticles was checked by transmission electron microscopy in respect of time scale.
For several decades, scientists have been working hard to prepare well-defined small magnetic particles. Nanoparticles exhibit interesting magnetic properties, which define them as good candidates for numerous applications in a variety of areas of human life such as recording media, biology, chemistry, cosmetics, or optics. The particles can be prepared in various ways ranging from vacuum techniques to chemical methods. In addition, nanostructures can be embedded in a matrix (metal or insulator) or used as a powder or in a colloidal form . The main effort is dedicated to size-tunable particle preparation. A key issue for most applications is that the particles must be monodisperse and possess internal structure easily controllable by sample preparation . Especially high-density magnetic recording media and magneto-optical storage media demand well-defined nanostructures.
For a long time, scientists were trying to prepare particles with well-defined size which are stable and less reactive to oxygen than pure elements. So, bimetallic, hybrid, or composite materials became a very interesting topic of such investigations  as they show dependence of the physical properties on composition. Studies of thin films and granular materials prove the influence of non-magnetic noble materials on 3d materials and vice versa. For example, magnetism can be induced in noble elements due to the presence of 3d elements in the system [4, 5]. In addition, some theoretical investigations predicted the existence of metastable phases in the case of low-dimensional systems such as Ag3Co in thin films . Therefore, it is interesting to prepare particles with large magnetic moment and large anisotropy which are stable in air. This presents an essential requirement for nanoparticles used in applications [7, 8]. Recently, bimetallic core–shell particles were synthesized using colloidal chemistry methods .
In this paper, we would like to present room-temperature magneto-optical Kerr effect (MOKE) studies of 14±2 nm CoxAg1−x composite particles with composition x ranging from 0.35 to 0.8 as a complementary description of magnetic properties of the AgCo system.
CoxAg1−x composite particles were prepared by the standard Schlenck technique, for details see Ref. , based on thermal decomposition of dicobalt octacarbonyl in combination with a transmetalation reaction with water-free AgClO4using oleic acid and tridodecylamine as a surfactant. The samples can be prepared with different Co-to-Ag molar ratios. The samples used for MOKE measurements had a Co content of about x=0.35, 0.7, and 0.8, which is larger as compared to those mentioned in Ref. . For structural characterization, transmission electron microscopy (TEM) images were taken.
The starting colloidal solution of an approximate concentration of 0.01 mol was diluted four times and thereafter dispersed in toluene. A drop of about 3–5 μl of the final solution was deposited on Cu, Au, or Al substrates and left for spontaneous evaporation. In order to induce structured arrangements of particles, an external magnetic field of 0.5 T was applied parallel or perpendicular to the sample plane.
3 Results and discussion
3.1 Transmission electron microscopy (TEM)
Previous studies with HRTEM show polycrystalline face-centered-cubic (fcc) structure of Co and Ag with very well defined Co shell and Ag core . Neither evidence of alloying nor existence of a CoO shell were found.
3.2 Magneto-optical Kerr effect (MOKE)
In Fig. 3b, spectra of CoxAg1−x composite particles deposited on Al substrates from three different solutions containing a Co composition of x=0.35, 0.7, and 0.8 are collected at room temperature. For the lowest Co composition x=0.35 (■), the polar Kerr rotation is very small. Despite the particle size of 14 nm, the Co shell is only a thin layer of about 2 nm , which is not thick enough to develop ferromagnetism at room temperature. The only pronounced feature in the spectrum at 4.0 eV is produced by the Ag plasma edge. This feature is observed in any sample containing Ag. Note that the sharp feature in the polar Kerr rotation at 3 eV is an experimental artifact due to the change of optical filter and detector at that particular energy.
In order to corroborate the nature of the plasma peak at 4 eV, we plot in Fig. 3c the reflectivity spectra measured at near-normal incidence. They show indeed pronounced features at the Ag plasma edge at 4 eV.
For particles with higher Co content, x=0.7 (●) and 0.8 (▲), we notice in Fig. 3b an influence of particle composition on the polar Kerr rotation spectra expressed by an additional peak around 3 eV. The ratio of the magnitudes of the peaks at 3 and 4 eV is different. For a smaller amount of Ag, i.e. x=0.8, we do not observe a pronounced plasma peak of Ag but a strong influence of Co at 3 eV. For the sample with x=0.7, the dominant peak is around 4 eV. As mentioned before, the 4-eV peak is due to the optical properties of Ag and can also be seen in the reflectivity curves in Fig. 3c.
All CoxAg1−x MOKE spectra can be divided into two regions, one below and one above 2 eV. Below 2 eV, the MOKE signal has a negative sign as in thin films [12, 13] and bulk Co . This was discussed thoroughly in Ref. . Above 2 eV, the signal changes sign to positive. This behavior is unique to nanoparticle systems due to the competition between scattering and reflection phenomena .
Discussing the origin of the structures in the Kerr spectra, the feature around 3 eV is most likely connected with the particle size, since the position of this peak is independent of the substrate used. The origin of the feature is an optical enhancement effect as is evident from Fig. 4b and d where the reflectivity drops strongly for both Co concentrations up to approximately 3 eV. It is well known that a pronounced decrease in reflectivity will lead to defined structures in the polar Kerr spectrum [15, 16]. The feature at 4 eV, on the other hand, is related to the Ag plasma edge, as discussed before. From this results a dependence of peak intensity on composition. A similar phenomenon was observed as increase of optical density with increase of Ag particle content in a mixture of Ag and Co particles (not shown). A shift and quenching of plasmons with change of Co content in composite particles has been observed before . However, the measurements we performed here are not sensitive enough to see a difference in energy of the plasma edge with composition. At energies below 2 eV, the spectrum changes sign to negative and we can see a local minimum at about 1.4 eV reminiscent of the spectrum of pure Co particles .
Similarly, the position of the main Co peak going from Al to Cu and Au substrates shifts toward lower energy (it varies between 3.3 and 2.7 eV, respectively). Apparently, the modulation of Kerr rotation is sensitive to the underlying substrate material, as was shown for thin films . Such behavior was also observed for pure Co particles . The peak shift is more pronounced in samples with higher Co content where the Ag plasma-edge peak is weaker. Particles deposited on Cu have the same polar Kerr rotation and differ from Au and Al substrates. Less influence from the substrate composition can be due to fact that CoxAg1−x particles are larger as compared to pure Co particles and the observed features on the substrates are no longer critical for sample deposition and particle interaction. The signal is much weaker, so any change has also smaller values.
3.3 Magnetization measurements
The FC and ZFC magnetization loops measured at 10 K are very similar. However, comparing the magnetization loops measured at 10, 40, and 100 K, a different behavior is evident (Fig. 7b–d). At 10 K, the magnetization does not saturate up to 5.5 T, suggesting paramagnetic or rather superparamagnetic behavior of the particles. No clear indication of ferromagnetic interaction (hysteresis) was detected. There is a more pronounced ferromagnetic contribution visible at 40 and 100 K (steep, step-like increase of magnetization around zero field) besides an increasing diamagnetic contribution (negative slope) which originates from the core of the particle consisting of diamagnetic silver. At lower temperature, the observed negative slope is smaller, probably due to a paramagnetic contribution of the Co shell.
Summary of the experimental results of CoxAg1−x core–shell nanoparticles
Type of experiment
Spin disorder and enhancement of surface anisotropy
Electron energy-loss spectroscopy
Confirmation of AgcoreCoshell structure
13.1 nm, 15.2 nm
Lack of saturation magnetization up to 5.5 T
Co35Ag65, Co70Ag30, Co80Ag20
Below Co content 0.7 no EB was observed. For 0.8 Co EB is present, suggesting presence of oxide layer. MOKE results very similar to pure Co nanoparticles
We believe that the rotation changes sign as compared to a pure Co thin layer because we have a combination of scattering and reflection from the substrate and the particles leading to an optical enhancement effect.
The CoxAg1−x core–shell particles with Co content x=0.7 and 0.8 are much less stable than such with smaller Co content (x≤0.5). The films reproduce the surface and distribution dependence on polar Kerr rotation as previously seen for pure Co particles. A pronounced Ag plasma-edge peak is seen in the MOKE spectra. An exchange-bias effect observed for Co content x=0.8 can be explained by the development of a CoO shell at the particle surface which is not observable for Ag-rich samples.
The TEM images give evidence for an ageing process going on in the particles via creation of oxides or phase separation in the particles. Both of them can change the contrast in the particles as seen in the TEM images. Therefore, for higher Co concentration the stability of the particles must be improved in the near future.
We acknowledge the support of the European Union under contract no. HPRN-CT-1999-00150. B.K. would like to thank R. Feyerherm for SQUID measurements and N. Sobal for particle fabrication.
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