Structure, composition and magnetic properties of size-selected FeCo alloy clusters on surfaces

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1 Appl. Phys. A (5) DOI: 1.17/s Applied Physics A Materials Science & Processing m. getzlaff 1, j. bansmann f. bulut 1 r.k. gebhardt 1 a. kleibert k.h. meiwes-broer Structure, composition and magnetic properties of size-selected alloy clusters on surfaces 1 Institut für Angewandte Physik, Universität Düsseldorf, Universitätsstr. 1, 45 Düsseldorf, Germany Institut für Physik, Universität Rostock, Universitätsplatz 3, 1851 Rostock, Germany Received: 5 bruary 5/Accepted: 6 July 5 Springer-Verlag 5 ABSTRACT The magnetic properties of 3d-metal clusters significantly differ from bulk behavior. This phenomenon is caused by a narrowing of electronic states and the high ratio of surface to volume atoms giving rise to enhanced magnetic orbital moments. alloys as soft magnetic materials are known to exhibit very high magnetic moments. The spin and orbital moments of iron and cobalt in size-selected alloy clusters on non-magnetic as well as magnetized nickel substrates have been investigated by X-ray magnetic circular dichroism with elemental specification. Structural properties were determined by scanning probe measurements. The preformed clusters maintain their original three-dimensional shape with a tendency to a slightly oblate occurrence, which can be explained by particle support interaction, and do not change to a more or less two-dimensional formation after deposition. PACS 73..-f; a; b 1 Introduction Magnetic properties of nanoparticles clearly differ in many ways from the respective bulk and thin-film materials and may depend strongly on their size, their coverage on surfaces and in matrices and their surrounding material. The shape which significantly determines electronic and magnetic behavior is often unknown even for free clusters. This behavior becomes more important for the system clusteron-a-surface, where the interaction of the magnetic metallic cluster with the (ferromagnetic) metallic surface has a significant influence on shape, electronic and magnetic properties. This concerns orbital and spin moments as well as magnetic anisotropies, hysteresis curves and even the magnetic order within nanoparticles. The pioneering experiments on magnetic phenomena of small clusters in the gas phase had been carried out about 1 years ago by Billas et al. [1, ] and Bucher et al. [3, 4], showing enhanced total magnetic moments that depend strongly on the number of atoms in the clusters. For free clusters, about 6 atoms per cluster are sufficient to gain the bulk value [, 5, 6]. In a different experiment which concentrated on the ratio of magnetic spin and orbital moments, we were able to demonstrate that deposited clusters Fax: , getzlaff@uni-duesseldorf.de even with more than 1 atoms exhibit differences to bulk material concerning magnetic properties (see e.g. [7]). The question arises of whether this enhancement is induced by hybridization between cluster and substrate and/or due to a change of the shape. For a (better) understanding, one has to examine the phenomenon of cluster surface collision [8] and influences due to particle support interaction. The final structure of a cluster after deposition depends on a lot of parameters. The important ones are the size and the kinetic energy of the incoming cluster. Furthermore, the cohesive energies of the cluster and surface as well as the bonds formed between cluster atoms and surface atoms have a significant influence. A detailed overview is given in [9]. The goal of our investigations is related to soft-magnetic 3d-binary alloy clusters with very high magnetization values. Today, possible applications of such bulk materials are solenoids where an extremely high saturation magnetization and a small coercive force is required. One of the materials that are commercially used for this purpose consists of nearly equal parts of iron and cobalt. This class of ferromagnets may exhibit drastic differences in electronic and magnetic properties for nanoscaled particles compared to bulk-like samples [1, 11]. In this article we will focus on exposed size-selected 3dmetal alloy clusters on surfaces deposited from a cluster beam under soft-landing conditions. The structural properties of nearly free clusters were determined by transmission electron microscopy (TEM) and of deposited nanoparticles by atomic force microscopy (AFM), whereas X-ray magnetic circular dichroism (XMCD) measurements are used for the determination of orbital and spin moments in magnetic alloy clusters. The size of the nanoparticles can be chosen prior to deposition and thus opens the possibility of studying the magnetic properties of clusters depending on their size and coverage. For carrying out such magnetically sensitive experiments it is of great importance to have a well-defined magnetization state in these particles. Unfortunately, small clusters are superparamagnetic at finite temperatures, whereas ferromagnetism is found in thin films of the same material. In order to define the magnetization state of such particles, an external magnetic field has to be applied that forces the magnetization along a given direction. Even clusters on surfaces which are large enough to show ferromagnetic behavior require external magnetic fields to become magnetically saturated along a desired

2 Applied Physics A Materials Science & Processing direction, since the anisotropy axes are randomly distributed due to the deposition process. We demonstrate that large metal clusters can be magnetized remanently along a certain preferential direction simply by deposition onto well-ordered ultra-thin ferromagnetic films. This behavior is important for many spectroscopic methods that require well-defined saturated magnetization states in nanoparticles without the presence of external magnetic fields, for example any method based on electron spectroscopy. Experimental In recent years the Rostock group has developed a new, continuously working cluster source with a high material flux for the deposition of size-selected metal clusters under ultra-high-vacuum (UHV) conditions [1]. The apparatus is based on an arc discharge in a hollow cathode. Here and subsequently the clusters are formed by eroded material from the target (i.e. the hollow cathode) in an environment of rare gases (usually argon or an Ar-He mixture) at a pressure of several millibars. The source is surrounded by a cylindrical solenoid to guide the arc erosion by its magnetic field. After a supersonic expansion the free cluster beam passes two skimmers. Two separate pumping stages remove most of the rare gas used for nucleation. Due to the arc process, a high percentage of the clusters is charged, either positively or negatively. Assuming a defined charge state and a sufficiently narrow velocity distribution, it is possible to carry out a mass separation in a static electric field (in our experiment by means of a quadrupole deflector), thus generating a nearly monodispersed beam. The resolution reaches not more than m/m 1% in the range between 5nmand about 15 nm, which is sufficient for our purpose. The importance of this mass range is pointed out in [13]. The amount of cluster material is estimated by the electrical current induced by the metal cluster ions, which can be recorded using a small mesh behind the deflector [14]. A detailed description is given, for example, in [1, 15]. The complete apparatus represents an ultra-high-vacuumcompatible version. A base pressure of mbar (without cluster beam) can commonly be reached in the ultra-highvacuum chamber where the quadrupole deflector is located. This chamber is equipped with an ion pump in combination with a titanium sublimation pump. A small valve between the skimmers and the deflecting unit serves to maintain ultrahigh-vacuum conditions even if the target has to be replaced. This cluster source is, due to its ultra-high-vacuum compatibility, developed for depositing clusters directly from the beam onto surfaces. They are specifically tailored for subsequent investigation and characterization by the whole set of surface-sensitive techniques. The size-selected clusters were deposited on the one hand on a Si wafer and on the other hand on a magnetized Ni(111) thin film, which is prepared on a W(11) substrate. The kinetic energy of the nanoparticles prior to deposition is clearly below the threshold for fragmentation, usually less than.1evper atom. For the experiments a piece from a commercial product (VACOFLUX 5, distributed by Vacuumschmelze GmbH, Germany) was used as a target for producing nanoparticles. The stoichiometry of this type of cathode is 5 48 V. The non-destructive deposition has been investigated on weakly interacting substrates by transmission electron microscopy (TEM) and on more strongly interacting ones by atomic force microscopy (AFM). The size calibration of the clusters has mainly been carried out by TEM on size-selected clusters deposited onto commercial TEM grids covered with amorphous carbon. Evaluation of the data yields a narrow size distribution of the deposited clusters with a sharp limit to smaller cluster sizes without any hint of fragmentation. For studying magnetic properties of supported clusters on ferromagnetic substrates we choose X-ray magnetic circular dichroism (XMCD), a powerful technique for determining orbital and spin moments in soft X-ray photoabsorption [16 18], which requires tuneable, circularly polarized radiation. The incoming circularly polarized radiation impinges onto the surface at an angle of 3 with respect to the surface plane, in order to ensure that a large part of the magnetization vector points either parallel or antiparallel to the direction of the photon spin. This procedure guarantees a good signal to noise ratio. The data have been recorded using the total electron yield (TEY) from the sample. Experiments have been carried out at different beamlines at the BESSY II synchrotron facility, the dipole beamline PM3 as well as the helical undulator beamlines UE56/1-PGM and UE46-PGM. Usually, the magnetization of the sample is reversed at each recorded data point while keeping the helicity of the radiation constant. A degree of circular polarization of P circ =.9 has been assumed according to the values given by BESSY for helical undulator beamlines (third harmonics) and the PM3 dipole beamline. The cluster source has been attached to a two-chamber set-up consisting of a UHV-compatible preparation chamber equipped with the usual tools for surface science and a separate chamber with an external magnetic field of about 1kOe. Alternatively, the cluster source was mounted on a UHV apparatus at the UE46-PGM beamline (of the Hahn-Meitner-Institut Berlin) designed for spectroscopy on magnetic ultra-thin films and nanostructures. This chamber is equipped with a coil for reversing the magnetization state with a short current pulse of approximately 1 A. 3 Results and discussion The most frequently used method to grow particles on substrates is realized by atom atom aggregation on a surface, but this process dramatically depends on the properties of the substrate and its interaction with the deposited atoms. An alternative approach is the preparation of the particles as free clusters and the subsequent deposition. Using this procedure, the structure of clusters can be extensively characterized prior to deposition by different techniques. Moreover, their behavior can be adjusted by controlling size, density and morphology of the deposited particles, allowing unprecedented flexibility and enormous potential in the creation of new materials with tailored properties. For practical applications one must deposit the clusters on surfaces or embed them in matrices where the properties of the free clusters are modified by the particle support interaction. This raises interesting questions such as: what happens to the geometry of the clusters as they are deposited on substrates?

3 GETZLAFF et al. Structure, composition and magnetic properties of size-selected alloy clusters on surfaces For the experiments on magnetized surfaces, fcc(111) nickel films have been grown on a clean W(11) crystal under UHV conditions. The cleanliness and quality of both the substrate as well as the thin ferromagnetic films have been checked using low-energy electron diffraction technique (LEED). The films grow in the layer-by-layer mode on the W(11) surface at room temperature. The in-plane W[1] axis has to be chosen for a remanent magnetization. Growth and magnetic properties of these films have been studied in the past in detail (see e.g. [19 1]). 3.1 Structural properties of alloy clusters As ferromagnetic material the alloy 5 48 Ni was used. The morphology of the nearly free clusters was characterized using a high-resolution transmission electron microscope (HRTEM); for this purpose the material was deposited on a graphite-covered grid. Figure 1 displays the corresponding TEM result. It should be mentioned that the sample has been transferred to the TEM under ambient conditions, i.e. the samples have been exposed to air. The small clusters are spherically shaped (not shown here), whereas larger free ones have a cubic shape, which may be due to the surrounding oxidic shell. A detailed evaluation of these TEM results clearly shows that the size distribution exhibits a sharp lower limit. The diameter was determined to be (1 ± 1) nm. Due to the surface oxide layer the cluster size may be overestimated, i.e. clusters remaining under UHV conditions exhibit a diameter being smaller by up to about 1% than that gained by TEM investigations (see e.g. []). Such clusters were deposited on a native Si surface. An AFM image is shown in Fig.. The kinetic energy of the clusters of about 3 mev/atom is well within the low-energy FIGURE AFM image of clusters deposited on a native Si surface (1 µm 1 µm) regime [9]; therefore, no significant fragmentation or destruction occurs. Most of the clusters with a diameter of 1 nm as nearly free particles exhibit a height of about 1 nm after deposition. This observation directly points to oblate particles due to interaction with the surface. Assuming the same volume and the appearance of an oblate ellipsoid, the limits of possible shapes are schematically shown in the right-hand part of Fig. 3. The scaling to the free particle (left-hand part of Fig. 3) is identical. Thus, the preformed clusters maintain their original threedimensional shape with a tendency to a slightly oblate occurrence and do not change to a more or less two-dimensional formation after deposition. The reproducibility of the source as well as the scaling between integrated charge flux at the sample and evaporated material was determined by two specific measurements with a simultaneous determination of both quantities. For the first sample the current was determined in situ during deposition to 1 7 A, for the second one to A. The ratio therefore amounts to.5. The counting of particles with the AFM resulted in 16 clusters per µm and 44 clusters per µm, respectively. The ratio of.4 is in good agreement with the complementary charge measurement. In the next step the height of each individual cluster was determined for both samples (see lower part of Fig. 4). FIGURE 1 High-resolution TEM image of a mass-filtered cluster with a mean size of 1 nm. The image size is 17 nm 17 nm. This measurement has been carried out by S. Stappert at the University of Duisburg-Essen FIGURE 3 Left: shape of a spherical particle with a diameter of 1 nm corresponding to the cluster as a free particle. Middle and right: limits of possible shapes of this cluster with the same volume after deposition assuming a height of 1 nm and the appearance of an oblate ellipsoid

4 Applied Physics A Materials Science & Processing number of clusters diameter of nearly free clusters [nm] height of deposited clusters [nm] FIGURE 4 Histograms (horizontal axis denotes the diameter and height, respectively) for mass-filtered clusters. The size distribution can be shifted by adjusting the voltage settings at the deflector. The details are explained in the text. Upper: histogram of nearly free particles representing a mean size of 1 nm. Lower: histogram of clusters after deposition exhibiting a diameter of 1 nm for free particles, i.e. before deposition (cf. upper part). Note the high amount of particles with a height of 1 nm, giving evidence of an oblate shape due to particle support interaction corresponding to singly charged clusters. Each height represents a height class, i.e. the particles with, for example, 15 nm possess an observed height between 14.5 nm and 15.5nm The red values correspond to the first sample, the blue ones to the second sample. The analysis resulted in a quantized dependence. For comparison, the histogram of identically prepared clusters before deposition is presented in the upper part of Fig. 4 exhibiting a mean size of 1 nm. The occurring quantization means that not every particle height occurred. The minimum height of 1 nm was found for a high percentage. This behavior can be understood by the working principle of the electrostatic quadrupole which does not act, in a strict sense, as a mass analyzer but in separating particles with identical m/e ratio for constant velocities. The dominating height of 1 nm corresponds to singly charged clusters. An improvement of the mass resolution will be carried out in the future. It was not possible to take AFM images for the deposition on the Ni thin film. Therefore, we used an alternative and independent type of experiment for a qualitative determination of the shape of the clusters after deposition. It was shown by Nakajima et al. [3] that in total electron yield (TEY) measurements saturation effects in the energy region of L,3 edges of 3d metals are present for bulk material and even for thin-film systems if the X-ray incidence angle is changed from the surface normal towards an in-plane direction. In contrast, from general symmetry arguments one would anticipate isotropic self-absorption effects in the case of (free) spherically shaped particles. Indeed, recent calculations by Fauth showed only a weak angular dependence in the self-absorption due to the presence of the substrate in the case of deposited nanoparticles [4]. Moreover, significant effects appear only in the case of particles with a diameter much larger than the mean free path of the created electrons (d 1 nm). For this experiment, clusters were deposited on a Ni thin film and the absorption was determined with an incidence angle of the X-ray photon beam of 55 to the surface plane (green symbols in Fig. 5) and with an angle of 1, thus exhibiting a significantly larger contribution within the surface plane (red symbols in Fig. 5). The spectra have been scaled in such a way that they coincide below as well above the absorption energy, which they are given with respect to. The background subtraction was carried out by determining the absorption of the Ni film before deposition of the clusters over the whole energy range of interest. This procedure results in good statistics in spite of the small amount of nanoparticle material. For a direct comparison the intensity difference is plotted below the absorption spectra with element specificity (blue lines). It is obvious that the absorption for and coincides for both angles within less than a few percent in the entire energy range, while the Ni spectra clearly show the anisotropy of the self-absorption effects well known from film samples [3]. The nearly identical and spectra suggest a high degree of spherical symmetry of the nanoparticles. Note, however, that without absorption [a.u.] Ni E-E B (p 3/ )[ev] FIGURE 5 Angular dependence of the photoabsorption for clusters on a Ni thin film. Red (green) symbols correspond to an X-ray incidence angle of 1 (55 ) with respect to the surface plane. All spectra are shown with respect to each L 3 edge and scaled for a coincidence below and well above the resonances. The blue lines represent the difference in the absorption between both geometries (note the different type of spectra compared to those in the other figures). Whereas for and, the cluster material, the spectra coincide over the whole energy range, significant differences occur for the Ni thin film

5 GETZLAFF et al. Structure, composition and magnetic properties of size-selected alloy clusters on surfaces additional numerical simulations it is not possible to deduce a certain aspect ratio from the data in Fig. 5 as has been done from our AFM images. It should additionally be noted that the cluster shape is not a priori the same for the deposition on a native Si surface and on the Ni thin film, because not only the deposition process but also the surface energies may play a role for the cluster shape. Nevertheless, the determination results in a similar shape. 3. mposition of alloy clusters The chemical composition of the alloy clusters was determined by energy-dispersive X-ray measurements (EDX) using a TEM (cf. Fig. 1). The observed ratio of : amounts to 56 : 44 with a variation of 1%. Therefore, we can conclude that the stoichiometry after generation from the 5 48 V target is conserved for the free clusters with an accuracy of better than about 1%. For the determination of the stoichiometry for deposited clusters we used photoabsorption measurements. The corresponding spectrum of clusters on Si is shown in the upper part of Fig. 6. For the background subtraction we adapted the data for Si given in [5] (gray curve). The spectrum after carrying out this procedure is presented in the lower part of Fig. 6 (black curve). Using again the data table [5] the absorption curve can be adapted for and but neglecting the resonances at the L,3 edges (broken lines). The absorption above the edge is proportional to the number of atoms within the deposited clusters. The same holds for the contribution. Thus, the absorption ratio allows us to determine the stoichiometry within the absorption [arb. units] 4,6 4,4 4, 4, 3,8 3,6 3,4 cluster on Si Si background,7,6,5,4,3, : 45%,1 : 55%, photon energy [ev] FIGURE 6 Upper: absorption spectrum for alloy clusters on Si. The gray curve corresponds to the Si background using the data table [5]. Lower: after this background subtraction (black line) the stoichiometry of the nanoparticle after deposition can be determined to be 55 : 45 for : (see additionally Ref. [6]) clusters after deposition, resulting in a relationship of 55 : 45 for :, which is in good agreement with the EDX measurement. Due to the integral character of photoabsorption measurements the ratio of the deposited amount of : is estimated; thus, a segregation cannot be ruled out but is very unlikely. 3.3 Magnetic behavior of alloy clusters in contact with surfaces For analyzing the magnetic properties the technique of X-ray magnetic circular dichroism in photoabsorption has been applied, which offers information on the orbital and magnetic moments. For a detailed analysis, size-selected clusters with a mean size between 7.5nm and 1 nm have been deposited onto silicon wafers and onto ferromagnetic Ni(111) on W(11) films under UHV conditions. The data have been recorded in remanence after magnetization with an external magnet. Photoabsorption spectra from 1-nm clusters deposited onto a silicon substrate are displayed in Fig. 7 in the energy range of the respective p absorption levels, i.e. between 69 ev and 81 ev. The experimental data clearly show the absorption edges of both materials ( and ) with nearly identical intensities; M + and M denote data taken for opposite magnetization directions in remanence. The intensity differences reveal a ferromagnetic coupling of and in the nanoparticles. However, the spectra do not reflect saturated magnetization states; element-specific hysteresis curves taken at the and p 3/ levels display a remanence M r of about % with respect to the saturation magnetization M s. The corresponding hysteresis curves for iron and cobalt are shown in the upper and middle parts of Fig. 8, respectively. For a better comparison, both curves are averaged and presented with identical scaling in the lower part. Now it becomes evident that they exhibit a nearly identical slope with a saturation field of about H s = 4 Oe. These results hint at a parallel alignment of the spins in both materials ( and ), i.e. a strong ferromagnetic coupling as one would expect from the enhanced total magnetic moment in alloys. absorption [arb. units] 8 7 M+ M photon energy [ev] FIGURE 7 Photoabsorption spectra for opposite magnetization directions (M + and M ) taken in remanence with circularly polarized radiation from the p core levels of particles with a diameter of 1 nm for free clusters deposited on a silicon substrate

6 Applied Physics A Materials Science & Processing relative magnetization [ M / M sat ] 1,,5, -,5-1, 1,,5, -,5-1, 1,,5, -,5-1, p 3/ p 3/ -4-4 external magnetic field [Oe] FIGURE 8 Element-specific hysteresis curves recorded at at hν = 77 ev and p 3/ core levels at hν = 78 ev of alloy nanoparticles with a diameter of 1 nm on a Si substrate. Upper: hysteresis curve sensitive to iron, middle: curve sensitive to cobalt, lower: averaged curves with identical scaling In order to investigate the orbital and spin moments, it is necessary to saturate the nanoparticles remanently, either in an externally applied magnetic field or by a strong ferromagnetic coupling to a magnetic substrate. For the results presented here, alloy clusters with a diameter of 7.5nm have been deposited onto a thin Ni(111) film evaporated on W(11). Details of the epitaxial growth and magnetic properties of nickel films on W(11) can be found elsewhere [19 1]; these films can be magnetized remanently in-plane along the W[1] axis. Figure 9 shows the respective photoabsorption spectra of the three metals involved,, and Ni. The experimental results obtained from Fig. 9 indicate high magnetic moments. Although the clusters with a diameter of 1 nm possess a volume (and thus the number of atoms) four times larger than those with a diameter of 7.5nm,they have a remanence significantly smaller, as can be seen directly in the corresponding XMCD data, i.e. the difference in the photoabsorption for opposite magnetization states, by comparing the results presented in Figs. 7 and 9. Thus, the influence of external magnetic fields applied from outside or induced by a magnetic underlayer realized in this case with the ferromagnetic thin Ni film becomes evident. By applying the sum rules, one can calculate elementspecifically the spin and orbital moments. The Slater Pauling curve predicts an averaged total magnetic moment of about.3 µ B for equal contributions of iron and cobalt in the binary alloy; the maximum value is reached for an alloy with a larger contribution of (about 7%). These calculations are in good agreement with data from the company that distributes the bulk alloy material, giving a value of.3tfor the magnetic polarization (the magnetic polarization in [T] is nearly identical to the total magnetic moment in [µ B ]). The analysis of our experimental results on clusters indeed confirms enhanced spin and orbital moments in both constituents and (see black bars in Fig. 1) when compared to their bulk values (gray bars). The left-hand bars correspond to, the right to and the middle to the mean values. The data show a spin moment in the contribution of about.5 µ B ; the corresponding bulk value is close to m S =. µ B. The same holds for the cobalt spin moment, which reaches nearly m S =. µ B in the alloy clusters on the Ni film. The bars in the middle of Fig. 1 show the averaged values of the weighted and contributions to the alloy clusters. The total spin moments are in good agreement with theoretical values given in [7]. According to our knowledge, no chemically resolved experimental data exist for the spin and orbital magnetic mo- photoabsorption [a.u.] XMCD signal 4-1 x3 x3 x3 x3 M+ M- Ni photon energy [ev] FIGURE 9 Upper: photoabsorption spectra of 7.5-nm particles deposited on a Ni(111) film on W(11) taken in remanence with circularly polarized radiation from the p core levels (for opposite magnetization directions M + and M ) (cf. also Ref. [7]). The iron and cobalt spectra have been enlarged by a factor of 3 and the Ni spectrum has been shifted. Lower: corresponding XMCD signal (intensity differences). Note that in spite of the small signals due to the small amount of cluster material (see black line in the upper part) the XMCD signal exhibits a very good signal-to-noise ratio allowing a precise determination of spin and orbital moments (cf. Fig. 1) FIGURE 1 Element-specific spin (left-hand bar in each pair corresponding to the left-hand scale) and orbital moments (right-hand bar in each pair corresponding to the right-hand scale) of mass-filtered alloy clusters (size: 7.5 nm) on Ni(111)/W(11) (black bars) in comparison to pure and bulk values (gray bars). The left-hand (right-hand) pair corresponds to bare () (gray) and the () contribution of the alloy particles (black), whereas the bars in the center show the averaged (i.e. mean) value from the iron and cobalt contributions of the clusters. It becomes evident that for both constituents the spin and orbital moments are enhanced for the alloy clusters compared to bare bulk material

7 GETZLAFF et al. Structure, composition and magnetic properties of size-selected alloy clusters on surfaces ments of bulk alloys. In order to distinguish between enhancements originating from the alloy material itself and from the effect related to clusters, we compare our results to calculations from Ebert and Battocletti [7] on a 5 5 bulk alloy and to experimental work [8]. We find a good agreement for the spin and orbital moments, while the experimental values for the moments exceed the calculated values by about 15% (spin moment) and 5% (orbital moment). The total magnetic moment with.43 µ B is therefore significantly enlarged compared to an averaging of the values of bare and and even slightly larger than that for bulk 5 5 with.39 µ B [8]; this enhancement is mainly caused by the iron amount in this alloy, whereas the magnetic moment due to the cobalt fraction remains nearly constant [9]. Taking into account our experimental uncertainties of ±. µ B for spin moments and ±.5 µ B for orbital moments and the typical underestimation of the orbital moment known from theory, we actually cannot claim to have observed significantly enhanced magnetic moments in our alloy particles. However, the experiments on such alloy clusters have shown that at least the corresponding bulk values are reached. Further studies of systems with different sizes and stoichiometries will show if clusters and particles are of technological relevance. 4 nclusions In conclusion, we have prepared size-selected softmagnetic alloy clusters under UHV conditions and deposited them onto non-magnetic as well as ferromagnetic surfaces. Scanning probe methods as well as element-specific experimental techniques in the soft X-ray regime have been used to explore the structural and magnetic properties of alloy nanoparticles after deposition on surfaces. We were able to produce alloy clusters with a stoichiometry that nearly equals the target material in our cluster source. The shape of the deposited clusters differs from free ones. Our investigations directly point to slightly oblate particles due to interaction with the surface. The magnetic moments of alloy particles have been analyzed, yielding high magnetic spin and orbital moments. The results were found to be generally in accordance with bulk calculations. The XMCD analysis will be extended to other cluster sizes and stoichiometries in order to compare these results with the Slater Pauling curve known from the bulk. Generally, an advanced theory related to the spin and orbital moments in large pure and alloy metal nanoparticles on surfaces would be highly desirable. ACKNOWLEDGEMENTS The work in the laboratories and at BESSY would not have been possible without the help of our co-workers and colleagues R.-P. Methling (now at INP Greifswald), V. Senz, J. Passig, K.L. Jonas and G. Holzhüter (University of Rostock) and Th. Maltezopoulos (University of Hamburg). Furthermore, we would like to thank E. Holub- Krappe, H. Maletta and D. Schmitz (Hahn-Meitner-Institut Berlin) for their help during the experiments at the HMI beamline UE-46PGM as well as S. Stappert and M. Farle (University of Essen-Duisburg) for carrying out the high-resolution TEM and EDX measurements on the alloy clusters. We gratefully acknowledge technical support by the staff of BESSY in Berlin and financial support by the Deutsche Forschungsgemeinschaft (DFG) via the priority call 1153 Clusters in ntact with Surfaces (Project Nos. GE 16/4-1 and BA 161/3-1). REFERENCES 1 I.M.L. Billas, J.A. Becker, A. Châtelain, W.A. de Heer, Phys. Rev. Lett. 71, 467 (1993) I.M.L. Billas, A. Châtelain, W.A. de Heer, Science 65, 168 (1994) 3 J.P. Bucher, D.C. Douglass, L.A. Bloomfield, Phys. Rev. Lett. 66, 35 (1991) 4 J.P. Bucher, L.A. Bloomfield, Int. J. Mod. Phys. B 71, 93 (1993) 5 A. Hirt, D. Gerion, I.M.L. 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