C. Mondelli (ILL), R. Siebrecht (ILL and Ruhr-Univ. Bochum), G. Battaglin, E. Cattaruzza and F. Gonella (INFM Venice), F. D'Acapito (INFM Grenoble and ESRF), P. Mazzoldi (INFM Padua).
Ion implantation of metal elements in glass substrates can lead to the formation of nanometer-radius colloidal particles in a thin surface layer. The formation of nanoclusters depends on the chemical reactivity of the pair “implanted atom-dielectric host”. Metal nanocluster composite glasses (MNCGs) exhibit an enhanced intensity-dependent refractive index due to the optical Kerr effect: this feature could be exploited in all-optical switching device technology, with the aim to process light signals without conversion to electronic form. Thus allowing to operate such a device in fast time regimes (tenths of pico seconds). Moreover, MNCGs obtained by ion implantation of transition elements are important for their magnetic properties: when the size of magnetic particles is in the nanometer range inside a non-magnetic matrix, surface effects are dominant. These affect the magnetic properties significantly, in terms of both oxidation and anisotropy effects. For this range of magnetic particle size, the composite material offers new technological possibilities, for example in the field of magnetic recording substrates for high-density information storage. The knowledge of the cluster size distribution, as well as the spatial correlation function, is important in order to relate the morphological characteristics to the physical properties.
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| Figure 1: The experimental in-depth nickel distribution measured with Rutherford backscattering spectroscopy (RBS), left, and reflectivity, right. |
The investigated samples are amorphous fused silica-glass slides implanted sequentially
with Ni+ ions at two different energies and (nominal) fluxes, namely,
180 keV (1.7.1017 ions/cm2) and 70 keV (6.1016 ions/cm2).
Only glass slides without voids were chosen to serve as substrates for implantation.
The double implantation leads to a flatter depth distribution of the nickel atoms,
as suggested by simulations [1]. The average in-plane cluster distribution is constant over the whole
sample area, which is known from transmission
electron microscopy measurements during former experiments. The experimental in-depth
nickel distribution, as measured by Rutherford backscattering spectrometry is reported
in Fig. 1 on the left side. The retained implanted dose was about 2.5.1017
ions/cm2. Grazing incidence x-ray diffraction on these samples gives evidence
for fcc nickel nanoclusters. To learn about the sample's magnetic properties a non-destructive
method, sensitive to the depth-dependent magnetism is needed. In our case polarised
neutron reflectivity with spin analysis comprises all the demanded qualities, making
it to the method of choice. The analysis of spin dependent reflectivity data enables
one to reveal quantitatively the nuclear and magnetic depth profile at the same time.
Nevertheless, due to the relatively low total amount of magnetic material in the
sample the scattered magnetic signal may be difficult to sort out from the data.
Thus, in a first step one has to confirm that it is possible to reconstruct the nuclear
non-magnetic depth profile for such a sample with unpolarised neutron reflectivity.
We performed neutron reflectivity measurements on the diffractometer ADAM [2]. The data were taken at
the instrumental wavelength of 
= 4.41 Å and over a Q-range of 0.1 Å.
Figure 2 shows the reflectivity curve (open circles). The fit to the data is given
by the solid line. The basic hypothesis for the fit was to take into account the
lateral homogeneity of the potential. The depth variation of the potential in the
z-direction, roughly proportional to the local metallic element density, was approximated
by potential slabs of different scattering-length density and thickness. On the right
side of Fig. 1 the potential obtained from the fit is depicted: two maxima are clearly
visible, in agreement with the simulation results. The peaks are both shifted of
about 350 Å towards the surface with respect to the positions in the simulation,
because of the surface erosion effect of the incident ion beam during the sample
preparation. This result is in agreement with the Rutherford backscattering spectrometry
measurements shown on the left side of Fig. 1.
Furthermore, the roughness of the slabs in the fit can be related to the mean size
of the clusters which is sketched in Fig. 1. In fact, due to the high local nickel
concentration, Ni clusters are expected to be very close one to each other. Under
this assumption, we obtain reasonable values of the cluster size for all the implanted
regions, except for the deepest part, where a very large value was found for the
roughness. Considering the low nickel concentration in the tail of the in-depth distribution,
we interpret this result as due to a large average distance among the metallic clusters.
This unpolarised reflectivity experiment is the first step to set a more general
approach to MNCGs using neutron-based techniques, to study their magnetic properties.
We clearly demonstrated that with this technique it is possible to reveal the non-magnetic
nuclear depth profile. This result is in agreement with precedent Rutherford backscattering
spectrometry measurements. The first neutron reflectivity experiments with polarised
neutrons and spin analysis on similar samples with magnetic implantations were performed
recently. The data obtained looks promising and data treatment is currently in progress.
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Figure 2: Reflectivity measurement (open circles)
taken on ADAM (
= 4.41 Å), solid line fit to the data. |
[1] J.P. Biersack and L.G. Haggmark, Nucl. Intr. and Meth. 174 (1980) 257. [2] R. Siebrecht, A. Schreyer, U. Pietsch, H. Zabel, Physica B 241 (1998) 169.
