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Magnetism of 3d transition metal

nanoparticles on surfaces probed with synchrotronradiation from ensembles towards individual objects

Joachim Bansmann*,1

, Armin Kleibert2,4

, Mathias Getzlaff3

, Arantxa Fraile Rodrguez4

, Frithjof Nolting4

,Christine Boeglin

5

, and Karl-Heinz Meiwes-Broer2

1 Institut fur Oberflachenchemie und Katalyse, Universitat Ulm, Albert-Einstein-Allee 47, 89081 Ulm, Germany2

Institut fur Physik, Universitat Rostock, Universitatsplatz 3, 18051 Rostock, Germany3 Institut fur Angewandte Physik, Universitat Dusseldorf, Universitatsstr. 1, 40225 Dusseldorf, Germany4Swiss Light Source, Paul Scherrer Institut, 5232 Villigen, Switzerland5 Institut de Physique et de Chimie de Strasbourg, Universitede Strasbourg, CNRS UMR 7504, 23 rue de Loess, 67034 Strasbourg,

France

Received 5 November 2009, revised 7 December 2009, accepted 11 December 2009

Published online 18 January 2010

PACS 73.22.f, 75.30.Gw, 75.75.a, 75.25.z, 78.70.Dm, 81.07.b

* Corresponding author: e-mail [email protected], Phone: 49 731 50 25469, Fax: 49 731 50 25452

Mass-filtered Fe, Co and FeCo nanoparticles in the size regime

from 7 to 25 nm have been deposited under soft-landing

conditions onto ferromagnetic films, non-magnetic surfaces as

well as embedded into Al matrices. In situ X-ray magnetic

circular dichroism (XMCD) measurements reveal a ferromag-

netic behaviour of FeCo nanoparticles (size: 10 nm) on Si-

substrates at room temperature whereas the respective Co

nanoparticles are superparamagnetic. Besides measurements

on ensembles of nanoparticles, we have also carried out in situ

measurements on individual Fe nanoparticles using X-ray

photoemission electron microscopy at the Fe L3,2 edges. Fe

nanoparticles on Co films show a magnetic contrast depending

on the direction of the underlying poly-crystalline Co domains.

This technique also allows to record XMCD spectra on

individual nanoparticles. Spectroscopy and spectro-microscopy on magnetic particles.

2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

1 Introduction In the last decade, ferromagneticnanoparticles have attracted much interest due to theirpossible applications in data storage media, chemistry,biotechnology and medicine (see, e.g. Refs. [13]).Nanoparticles with a homogeneous size distribution havethe advantage of similar magnetic properties and are thusrequired for most technical applications. Besides chemistry-based preparations routes which are more appropriate forlarge-area technical applications (e.g. Refs. [46]), physics-based methods are often used for more fundamental research

and characterisation of their magnetic properties. Here, we

report on investigations on the magnetic properties (such asthe magnetic anisotropy energy) of well-defined mass-filtered 3d transition metal (TM) nanoparticles in a sizeregime from 7 to about 25 nm prepared by deposition ofpreformed particles on surfaces. X-ray magnetic circulardichroism (XMCD) is a well-established technique foranalysing the magnetism, particularly the magnetic spin andorbital moments using tuneable, circularly polarised syn-chrotron radiation at third generation synchrotron facilities.The method is capable of investigating magnetic nanopar-

ticles and clusters downto a size of onlya few atoms [7] with

Phys. Status Solidi B 247, No. 5, 11521160 (2010) / DOI10.1002/pssb.200945516 p s sbasic solid state physics

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a high accuracy (for an overview, see e.g. Refs. [811]). As amatter of fact, most of the XMCD measurements averageover a certain area on the sample, usually the area that isilluminated by the incoming radiation. In third generation

storage rings equipped with undulator beamlines these areasare about 100mm 100mm or larger. Thus, hundredthousands of individual nanoparticles are excited whichmight differ in their orientation with respect to the surface,their magnetic anisotropy axes and their shapes. However,even at microfocus beamlines with spots down to only a fewnanometer, one averages over a large number of particles. Asa consequence of these variations, the magnetic properties ofindividual nanoparticles might differ significantly from theaverage behaviour of all particles being detected in XMCDspectroscopy. Thus, imaging the magnetic properties ofindividual particles and, moreover, to combine this infor-mation with structural information on the particle being

investigated is a highly interesting goal for a detailedunderstanding of magnetic phenomena of nanoscaledobjects. It has already been demonstrated that certainmagnetic properties of individual nanoparticles can beinvestigated; one possibility has been shown by Jametet al. [12] who used a micro-SQUID set-up [13] to measurethe magnetic anisotropy energy of a single Co nano-particle with about 1000 atoms. Further attempts have beenmade by Lorentz-microscopy [14, 15], revealing magneticdomains in Fe nanoparticles as small as 25 nm or switchingevents in 10 nm sized FeCo nanoparticles. Furthermore,chiral TEM[16] may allow in the near future to analysethe magnetic spin and orbital moments of magnetic

nanoparticles by combining high-resolution transmissionelectron microscopy (HR-TEM) with a magnetic circulardichroism technique.

In our approach, we use photoemission electronmicroscopy (PEEM) in combination with circularlypolarised synchrotron radiation in order to analyse themagnetic properties of individual Fe nanoparticles on apolycrystalline Co film by resonantly exciting photo-electrons at the Fe L3,2 edges. The Co films allow tomagnetically saturate the Fe nanoparticles [17] without theneed of an external magnetic field which would interferewith PEEM.

2 Experimental The experiments have been carriedout at the electron storage rings BESSY in Berlin (Germany)and the Swiss Light Source SLS (Paul Scherrer Institut) inVilligen (Switzerland). A UHV compatible and transport-

able cluster source, which is described in the followingsection, serves forin situexperiments.

2.1 Cluster source The arc cluster ion source (ACIS)[18, 19] has been designed to produce mass-filtered metalnanoparticles in a size regime from 4 to 25 nm. The typicalsize distribution has a width of 15% of the mean particlediameter. The cluster source itself, as shown in Fig. 1,consists of three parts: the hollow cathode, where the clustersare created by an arc erosion in an inert seeding gas, apressure reducing part and a mass-filtering unit with anelectrostatic quadrupole deflector. In the first part, thematerial for the cluster production is eroded from the metal

cathode (in our case Fe, Co or FeCo of high purity), thecondensation of the particles starts in the seeding gas (argon)at pressures around 2040 mbar and is finally finished in theexpansion channel.

The particles, more than half of them being in a chargedstate, leave the aggregation part of the cluster source after asupersonic expansion. The rare gas required for the seedingprocess is pumped off in the next unit consisting of twoskimmers, and the remaining molecular beam enters themass-filtering unit. The electrostatic quadrupole selects onlycharged particles which are deflected and may pass the exitslit (at the flux monitor) depending on their kinetic energy.Since all particles have nearly identical velocities after the

supersonic expansion given by the velocity of theseeding gas(in our case of pure Ar around 500 m s1), the kinetic energyonly depends on the mass of the particle when they are singlecharged (which is usually the case). Thus, the quadrupoledeflector acts as a mass selector with a moderate resolution,but a quite high transmission efficiency. The continuousworking mode (no pulsed units such as lasers or pulse valves)moreover contributes to a high cluster deposition rate andenables the application of static electric fields for the massselection. The kinetic energies of the particles beforedeposition are in the order of 0.1 eV per atom and thusbelow the limit where fragmentation might occur [20].

Phys. Status Solidi B 247, No. 5 (2010) 1153

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Figure 1 (online colour at: www.pss-b.com) Schematic drawing of thearccluster ion source withits functionalunits.

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2.2 Experimental set-up at electron storagerings and XMCD technique At BESSY, the clustersource was attached to several beamlines (e.g. UE46-PGM1,UE56/1-PGM and PM3) providing circularly polarised

radiation (6501000 eV). Investigations regarding highmagnetic fields (up to 5 T) with a superconducting 7 Tmagnet (IPCMS Strasbourg, France) were carried out at thebeamline UE46-PGM1. The PEEM measurements on themagnetic behaviour of individual nanoparticles wereperformed in ultra-high vacuum at the surface/interfacemicroscopy (SIM) beamline [21] of the Swiss Light Sourceusing an Elmitec LEEM III setup equipped with an energyanalyser. The PEEM provides a magnified image of theemitted secondary photoelectrons, thus generating a spatialmap of the absorption with a probing depth of a fewnanometers [22]. These measurements were performed atroom temperature (RT) without external magnetic fields.

The magnetic behaviour of the 3d (TM) nanoparticlesdeposited on surfaces has been analysed using XMCD inphotoabsorption (for an overview and applications, see Refs.[8, 23, 24]) which requires tuneable soft X-rays with circularpolarisation at theL3,2 absorption edges of the ferromagnetic3d materials Fe, Co and Ni. The XMCD sum rules [2426]allow to determine the magnetic orbital morb and spinmomentsmspin. The XMCD signal is recorded by measuringthe photoabsorption, in spectroscopy via the total electronyield, in microscopy via the partial electron yield. Technicaldetails regarding the data analysis are described in earlierpublications of our group [19, 27, 28].

3 Results and discussion We focus on the magneticproperties of 3d metal nanoparticles (Co, FeCo and Fe) onmagnetic and non-magnetic surfaces as well as particlesembedded in non-magnetic matrices. The correlation of theshape and structure of deposited, mass-filtered 3d TMnanoparticles with their magnetic properties investigated byXMCD has already been discussed in earlier publications(see, e.g. Refs. [19, 29, 30]).

3.1 Magnetic spin and orbital moments of Conanoparticles on surfaces For films [31] and nanopar-ticles [32], it has been pointed out, that both, the spin andespecially the orbital moments, are usually significantly

underestimated when the XMCD sum rules are appliedto spectra obtained by TEY detection. In a precedingpublication [30], we have in detail analysed the TEYdetection in XMCD on supported nanoparticles. First of all,due to the short mean free path of the excited electrons in Fe(1.7A) and Co (2.2A), the XMCD signal mainly stemsfrom the outer layers of the nanoparticles whereas the innerpart does not contribute to the magnetic signal. In order tocorrect for these underestimated magnetic moments (mTEY),we have developed an algorithm for spherically shaped Fenanoparticles that enables us to recalculate the magnetic spinand orbital moments. It has been shown that correctionfactors can be obtained by numerical simulations [30]. The

correction factors mTEY

/mm

(the superscript m denotes

moments calculated from reference data, for details seeRef. [30]) for the orbital and spin moments of Conanoparticles are shown in Fig. 2 as a function of the particlediameter. For the spin moment, the corrections factors areclose to 1 and thus, the deviations that have to be considered,are small. The correction factors for the orbital moments arehigher and reach a value of mTEYorb /m

m

orb 0.75 for Co

nanoparticles with a size close to 10 nm. Compared to Fe,the correction factors are generally smaller due to the lowerabsorption coefficient of Co.

As an overview, the corrected values for the ratio of themagnetic orbital and spin momentsmorb/mspinas well as thespin moments mspin of 7.6 nm Co nanoparticles depositedonto Ni(111) and Fe(110) films on W(110) and Au(111) aregiven in Table 1. First of all, the experimentally determinedvaluesdiffer significantly depending on the surfaces; the spinmoments by about10% (between 1.46 and 1.66 mB) andthe orbital moments even more (cf. Table 1). It should bementioned that the measurements are performed underslightly different conditions. On ferromagnetic Ni(111) and

1154 J. Bansmann et al.: Magnetism of 3d TM nanoparticles on surfaces

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Figure 2 (online colour at: www.pss-b.com) Correction factorsfor the experimentally determined magnetic spin mTEYspin (solid

black dots) and orbital momentsmTEYorb (open red dots) of sphericalCo nanoparticles with diameter D on surfaces. The dependence

originates from the limited escape depth of electrons in the TEYmode. Lines serve as a guide for the eyes.

Table 1 Ratio of the magnetic orbital and spin moments morb/mspinas well as the spin moments mspinof Co nanoparticles (size:7.6 nm) on different supports (room temperature) after correctingfor saturation effects in the TEY detection mode.

morb/mspin mspin/(mB/atom)

Co/Ni(111) [27, 19] 0.090 0.02 1.54 0.06Co/Fe(110) [27] 0.132 0.008 1.77 0.03Co/Au(111) (08) [28] 0.29 0.03 1.46 0.01Co/Au(111) (408) [28] 0.162 0.03 1.66 0.015bulk hcp Co [24] 0.099 1.55bulk fcc Co [33] 0.078 1.72

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Fe(110) films on W(110), the Co nanoparticles weremagnetised by direct exchange interaction with the under-lying film (without externally applied magnetic fields) alongthe easy magnetisation direction of these films (i.e. for

Ni(111)/W(110) along the W[001] direction, for Fe(110)/W(110) along the W[110] direction of the underlyingtungsten single crystal, for details regarding these films,see Ref. [27]. Moreover, the saturation magnetisation of Feand Ni films (thickness: 13ML) are quite different(corresponding bulk values Fe: 2.0 T, Ni:0.65 T) andthus, the Co nanoparticles might not be completelymagnetised due to the much lower exchange interaction ofNi. Additionally, the different symmetries of the films(quasi-hexagonal bcc(110) and fcc(111) symmetry) and thedifferent in-plane magnetisation directions of the films mightalso affect the magnetic properties of the deposited Conanoparticles. In the case of non-magnetic substrates, the Co

nanoparticles were magnetised with an external magneticfield (either by a superconducting magnet atB 1.5 T, as inthe case of Au(111), or via a small electromagnet withmagnetic fields below 100 mT at the position of the sample,cf. Fig. 3). As already discussed in detail in Ref. [28], themagnetic orbital moments of Co nanoparticles on Au(111)strongly change with the temperature and the angle ofincidence, however, the ratio of orbital to spin moment isalways strongly enhanced with respect to the correspondingbulk value. The smaller magnetic spin moment in normalincidence and magnetisation perpendicular to the surface(08) may be (partly) caused by the magnetic dipole termmT[3436] which automatically enters the corresponding

equation for mspin when using XMCD. In the case of Conanoparticles and Co films on Au(111), the magnetic dipoleterm does not vanish and has a significant influence whendetermining the spin moment [37], it especially lowers thevalue in normal incidence and a magnetisation perpendicularto the surface. For uncapped Co nanoparticles on Si, we werenot able to determine the spin moment due to the limitedexternal magnetic field (cf. Fig. 3).

In conclusion, we would like to point out, that themagnetic orbital and spin moments of deposited Conanoparticles strongly depend on the surface where depo-sition took place. The orbital moment in Co nanoparticles

with a size of 9.6 nm is generally larger than thecorresponding Co bulk value, not least due to the enhancedmagnetic orbital moments at ferromagnetic surfaces due tothe reduced symmetry.

3.2 Magnetic behaviour of Co and FeCo alloyparticles on silicon surfaces with a native oxide Inthe preceding section, we have reported on the orbital andspin moments of Co nanoparticles on (mainlyferromagnetic)surfaces and their deviation from the respective bulk values.In the first part of this subsection, we will compare themagnetic anisotropy energy of Co and FeCo alloy nano-particles, in the second part, the magnetic behaviour of Conanoparticles on surfaces and in matrices. Fe and Co

nanoparticles with a diameter of about 10 nm or less areexpected to show superparamagnetism at RT, the respectiveblocking temperatureTb is about 17 K (bcc Fe) and 160 K(fcc Co), when assuming bulk-like magnetic anisotropyvalues. The blocking temperature can be estimated fromNeelBrown model, cf. Eq. (1) (see, e.g. Wernsdorfer et al.[38]), which relates the uni-axial magnetic anisotropy energyEMAE KuV (Vvolume,Ku uni-axial magnetic anisotropyconstant) of a particle to the thermal energyEth kBT (Ttemperature,kBBoltzmann constant):

1

t0

f0exp KuV

kB

T : (1)

In this Arrhenius law, the frequency f0 is the attemptfrequency (for reversing the magnetisation) with a typicalvalue off0 10

9 s1 and t0the relaxation time. A system isoften considered as blocked when t0 is larger than about100 s; details are given in, e.g. Refs. [2, 3840]. Equation (1)can also be used to estimate the magnetic anisotropy requiredof a nanoparticle of a given size to be ferromagnetic at RT.Hcp Co nanoparticles with a diameter 9.6 nm should thushave a magnetic anisotropy constant ofK 13meV per atomand would be ferromagnetic at RT. In case of a cubicanisotropy with a constantK(as in fcc Co) one can replace Ku

in Eq. (1) by (K/4) [39]. This leads to remarkably differentblocking temperatures (or the required magnetic anisotropyconstants for observing ferromagnetic behaviour at RT)between fcc (cubic anisotropy) and hcp (uni-axial aniso-tropy) Co nanoparticles of the same size. In case of a cubicanisotropy, at least 51meV per atom is required to showferromagnetism at RT, which is clearly above the bulk valuefor fcc Co. For an overview, the respective magneticanisotropy values for bulk materials are given in Table 2.

In the following we will focus on the magnetic behaviourof Co and FeCo nanoparticles of identical size deposited onsilicon wafers with native oxide surfaces. The black dots inFig. 3 represent the relative magnetisation of Co nanopar-

ticles with a size of 9.6nm, the particles show a

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250 50 75 100

-1,0

-0,5

0,0

0,5

1,0

applied magnetic field mT/

-50 -25 250 50

-1,0

-0,5

0,0

0,5

1,0

M/MS

applied magnetic field mT/

normalisedXMCD

sig

nal(arb

.u

nits)

Figure 3 Normalised XMCD signals from a hysteresis curve of9.6 nm Co nanoparticles deposited onto a Si wafer. The solid linerepresents a Langevin fit for superparamagnetic Co nanoparticles

(mCo 1:5mB) of this size at 300 K. The inset shows a hysteresiscurve recorded from 9.6 nmFeConanoparticlealsodeposited onto a

Si wafer [46].

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superparamagnetic behaviour at RT. For comparison, aLangevin-type magnetisation curve is plotted in the sameviewgraph (solid line), the curve is calculated for Conanoparticles of this size (50,000 atoms per particles, spinmoment mCo 1:5mB per atom) at RT. The fact thatCo particles of this size are superparamagnetic at RT can

be ascribed to their fcc structure [19, 45]. As discussedabove, the cubic symmetry results in much lower blockingtemperatures when compared to hcp particles. Interestingly,FeCo (again 9.6 nm) nanoparticles deposited on a siliconwafer showed a ferromagnetic hysteresis curve (cf. inset inFig. 3) with a magnetic remanenceMr of aboutMr=Ms 0:2(Ms saturation magnetisation) and a coercive field Hc ofabout 8 mT.

The presence of a finite coercive field and a higherremanent magnetisations in FeCo alloy nanoparticlescompared to Co nanoparticles of the same size (anddeposited under the same conditions onto identical sub-strates) is unexpected. FeCo alloys are usually soft magnetic

materials with very high saturation magnetisation fields andvery low coercive fields [47]. Thus, the respective magneticanisotropy energies should be very small, at least muchsmaller than those of Co. Recent calculations [48], however,showed that tetragonally distorted FeCo alloys (films andparticles) might exhibit (with a Co concentration between 50and 70%) both, high magnetic saturation fields (around2.2 T) and very high magnetic anisotropies (up to a factor of20 larger than those of Co, cf. Table 2). Experiments byAndersson et al.[44] andWinkelmann et al. [49] have proventhe general prediction of strongly enhanced magneticanisotropies in FeCo alloys, e.g. Andersson et al. observedmagnetic anisotropies of 150meV per atom and saturation

fields of 2.2 T at RT. For comparison, the bulk magneticanisotropy constants of Fe and Co are also given in Table 2.

Our FeCo alloy particles consist of about 50% Fe and48% Co, the remaining parts are mainly vanadium (1.8%)and nickel (0.3%), for details see also Refs. [17, 46]. Thesmall contributions fromthe 3dmetal (Ni and V) couldnotbedetected in XAS and are not expected to significantlyinfluence the magnetic anisotropy of the respective nano-particles. Based on the calculations from Burkert et al. [48],extremely high values for the magnetic anisotropy are notexpected, since the Co content is only slightly smaller thanthe Fe content. Nevertheless, strongly enhanced magneticanisotropy values are also predicted for materials consisting

of equal amount of Fe and Co, provided the lattice is

tetragonally distorted with ac/a axes ratio of more than 1.05.However, our data clearly indicate that FeCo nanoparticlesdeposited onto a Si-wafer show strongly enhanced magneticanisotropies compared to bulk FeCo materials. For ferro-

magnetic single domain particles without interaction, therespective coercive field Hc is related to the magneticanisotropy constantK, the saturation magnetisationMsandthe thermal energy as given in Eq. (2) [39]:

Hc 2K

Ms1

kBT

KV

: (2)

Based on this equation, we obtain a magnetic anisotropyenergy of about K 190 200 kJm3 for FeCo nanopar-ticle of 10 nm in diameter, considering T 300K, and acoercive field of Hc 8 mT. The fact, that these FeConanoparticles have a non-vanishing remanence at RT results,

most likely, from the presence of a distorted lattice (such asthe tetragonal distortion for FeCo predicted in Ref. [48] andobserved in Ref. [44]) and a reduced symmetry which leadsto uni-axial anisotropyKuand higher blocking temperaturesTb. Our experimentally observed value ofHc 8mTfitswellto corresponding data from Peng et al. [50] for Fe70Co30nanoparticles of the same size at RT. Based on theirtemperature-dependent data, Peng and co-workers deter-mined a magnetic anisotropy energy of about 40kJm3, avalue that coincides with the Fe bulk value and the respectivevalue of FeCo alloys with small contents of Co (cf.Ref. [51]).However, contrary to our approach with well-separatednanoparticles, the authors worked with cluster-assembled

films where magnetic dipole and exchange interactions playa dominant role. Hence, the results obtained from this studyare only partly comparable to our experimental data.

3.3 Co nanoparticles on Au(111) on surfacesand embedded into an Al matrix Magnetic particleson surfaces and embedded in matrices might show differentmagnetic properties due to their different symmetry andinterfaces which will be important in any application. As anexample, we present experimental data on Co nanoparticles(size: 7.6 nm) embedded in an Al matrix and discuss theresults with respect to data on Au(111) [28]. In Fig. 4, therelative magnetisation M/Ms (given by the normalised

XMCD signal at the L3 edge) is displayed for T 200K.Analogously to the results obtained earlier on uncapped Conanoparticles on Au(111) (cf. red triangles in Fig. 4), anexternal magnetic field of 1 T is required to saturate themagnetic spin moments. A Langevin-type magnetisationcurve [52] (black solid curve), plotted for 7.6 nm Co particleswith a magnetic moment of 1.5mB (T 200 K), agrees fairlywell with the experimental data points (black dots) for Conanoparticles in an Al matrix. Additional experimental datapoints for embedded Co nanoparticles (not displayed in thisgraph) recorded at 300 K for magnetic fields of 1 and 5 Tshow nearly identical results.

In the following, we will focus on the magnetic

anisotropies in the orbital moments of Co nanoparticles


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Table 2 Magnetic anisotropy constantsKfor bulk-like Fe and Co[4143] as well as for tetragonally distorted FeCo films [44] with a

c/a axes ratio of 1.18.

KkJ m3 K(meV/atom)

bcc Fe (bulk) 42 3.3hcp Co (bulk) 430 43fcc Co (bulk) 270 27

tetragonally distorted FeCo films 2900 150



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distance between them is significantly larger than the spatialresolution of the PEEM (about 50 nm). Additionally,the secondary photoelectron background signal from theunderlying silicon wafer at the FeL3,2absorption edges hasto be small compared to the signal from the Fe nanoparticles.Due to the element-specific excitation process, the method

allows to separate the magnetic signal of both, the Fenanoparticles and the Co film. The XMCD contrast dependson the relative orientation of the incoming circularlypolarised radiation with respect to the local magnetisationstate of the sample. In the case of the XMCD image shown inFig. 6b, the contrast is obtained from the asymmetry of twoimages with right- and left-handed circular polarisation, i.e.Is Is=Is Is. The XMCD image(Fig. 6b) recorded at the Fe L3 edge (708.1 eV) has beencollected at the same area as the image with the purechemical contrast (Fig. 6a), typical acquisition times of theFe XMCD images being 5 min. Magnetic images at the FeL2edge (721.2 eV) were also recorded (not shown here)

verifying that the observed intensity contrast is truly of

magnetic origin. Additionally to the PEEM images, we showan AFM image (Fig. 6c) of a single Fe particle (height:23nm)marked bythe box in the upper panels ofFig. 6.Moredetailed results of these investigations will be publishedelsewhere [57]. Using PEEM, it is also possible to measureXMCD spectra (in remanence) of individual Fe nanoparti-cles by collecting stacks of XMCDimages around the FeL3,2

edges for both photon helicities. The possibility of recordingsuch photoabsorption spectra has already been demonstratedin a recent publication ofex situ prepared Co nanoparticles inan Al matrix on Si-wafers with markers [58]. Here, we showresults on XMCD spectra recorded from Fe nanoparticlesdepositedin situonto a Co film on a Si wafer (cf. Fig. 7). Weobserve a well-pronounced contrast at the FeL3 edge, butonly a marginal contrast at the FeL2 edge. For a magneticeffect, the contrast at the FeL3and theL2edges should be ofopposite sign (e.g. cf. XMCD sum rules, [24]). In the case ofFe, the effect at theL2 edge should be smaller (a factor of 2 or3) thanatL3 edge (the detailed ratio determines the magneticorbital and spin moments). Although the contrast at the FeL2

edge is quite small and nearly vanishing, clear evidence isprovided that XMCD spectroscopy on individual three-dimensional nanoparticles down to 10 nm can be achieved.We like to mention that the Fe nanoparticles in this sequenceof data are partly oxidised (cf. double peak structure at theFe L3 edge) due to a very long illumination time withsynchrotron radiation. The additional shoulder observed at abinding energy of 710 eV is only visible in the case of partlyoxidised Fe, it is also responsible for the reduced magneticcontrast at the L2 edge. We usually observed such abehaviour after more than 12 h, but only on a small areaaround the spot on the incoming radition. When moving thesample laterally, we easily find a non-oxidised area for

further investigations.


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Figure 6 (online colour at: www.pss-b.com) (a and b) PEEMimages with elemental (a) and magnetic contrast (b) obtained from

Fe nanoparticles deposited onto a polycrystalline Co layer; (c) AFMimageofthesingleFenanoparticlemarkedintheupperimage,height

23 nm. Black and white contrast refers to different magnetisationdirectionsof the individualnanoparticles,the redarrow indicates thedirection of the incoming radiation.

Figure 7 (online colour at: www.pss-b.com) XMCD spectrarecorded with left (s) and right (s) circularly polarised soft X-rays from a single 12 nm Fe nanoparticle (indicated by a yellowsquareintheinset).Theredarrow(intheinset)indicatesthedirection

of the incoming photons.



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