IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013 4711

Soft X-Ray Magneto-Optics: Probing Magnetism by Resonant ScatteringExperiments

Carlo Spezzani , Horia Popescu , Franck Fortuna , Renaud Delaunay , Romain Breitwieser , Nicolas Jaouen ,Marina Tortarolo , Mahmoud Eddrief , Franck Vidal , Victor H. Etgens , Massimiliano Marangolo , and

Maurizio Sacchi

Sincrotrone Trieste S.C.p.A., Trieste I-34012, ItalySynchrotron SOLEIL, Gif-sur-Yvette 91192, France

Centre de Spectrométrie Nucléaire et Spectrométrie de Masse, Université Paris-Sud, Orsay 91405, FranceLaboratoire de Chimie Physique—Matière et Rayonnement, UPMC Paris 06, CNRS UMR 7614, Paris 75005, France

Institut des NanoSciences de Paris, UPMC Paris 06, CNRS UMR 7588, Paris 75005, FranceLaboratoire International Franco-Argentin en Nanosciences, CNRS-CNEA, Paris 75005, FranceVeDeCom—Université de Versailles, Saint-Quentin en Yvelines, Versailles 78035, France

The most advanced X-ray sources (third generation synchrotrons, linear free-electron lasers and high-harmonic generation sources)widen the range of application of X-ray scattering techniques considerably. Beyond flux and brilliance, improvements in polarizationtuneability, degree of coherence and selectable time-structure promoted newmethods for investigating the electronic andmagnetic prop-erties of solids. The soft X-ray range (50–2000 eV) is well suited for studying magneto-optical effects in laterally confined submicron sizedobjects, either artificially built or self-assembled. First, by tuning the photon energy at a core resonance, one provides the X-ray scat-tering technique with element selectivity. Second, resonant excitations make the optical constants sensitive to the local magnetization byintroducing large off-diagonal elements in the dielectric tensor; since magnetic effects are stronger when the core excitation produces adipolar transition to final states involving the magnetic orbitals ( for the first row TM; 4 for RE), the most interesting resonances forX-ray magneto-optics [(2, 3) and (3, 4) ] are all located in the soft X-ray region. Finally, the wavelengths correspondingto soft X-rays are very well suited for scattering studies of nanometer- to micrometer-sized magnetic structures. We will present theresults of recent soft X-ray resonant scattering experiments, showing that the combination of element selectivity, magnetic sensitivityand structural analysis can help disentangling and understanding the magnetic properties of complex self-assembled periodic systems.Lastly, recent applications of coherent scattering to the X-ray holographic imaging of magnetic domains will be presented.

Index Terms—Coherent scattering, magnetic dichroism, X-ray scattering, X-ray spectroscopy.

I. INTRODUCTION

W HEN AN electromagnetic wave interacts with an ob-ject, a well-defined relation exists between the spatial

distribution of the optical constants within the object and the an-gular distribution of the scattered intensity. When using X-rayscattering to determine the sample atomic structure, one implic-itly relies on the hypothesis that the optical constants dependmainly on the local electron density, therefore the scattered in-tensity provides a map of the position and nature of the atomswithin the object. In fact, optical constants depend also on thedetails of the electronic and magnetic properties of the object,and this sensitivity can be enhanced by tuning the photon energyat a core resonance of one of the sample components, leadingto the so-called anomalous diffraction or resonant scattering.Therefore resonant scattering is also a spectroscopic technique,whose results provide, in addition to the average charge den-sity distribution, information about the ground and excited elec-tronic configurations of a given element in the sample.X-ray resonant magneto-optical effects are stronger when the

resonance involves directly the magnetic orbitals of the ele-ment via dipolar electron excitations, like for

Manuscript received February 17, 2013; revised March 20, 2013; acceptedMarch 26, 2013. Date of current version July 23, 2013. Corresponding author:M. Sacchi (e-mail: maurizio.sacchi@synchrotron-soleil.fr).Color versions of one or more of the figures in this paper are available online

at http://ieeexplore.ieee.org.Digital Object Identifier 10.1109/TMAG.2013.2256113

the transition metals (TM) of the first row, or inrare-earths. These resonances are all located in the so-called softX-ray region, whose extension can be identified roughly withthe energy interval from 40 to 2000 eV, or 0.6 to 30 nm in wave-length. These relatively long wavelengths prevent a general ap-plication of resonant diffraction to the study of single crystals,whose order parameters are usually too small to satisfy Bragg’slaw at these energies. Soft X-ray scattering finds applicationsin the analysis of specular [1] and diffuse reflectivity and in thestudy of Bragg diffraction from artificially structured materialsof appropriate periodicity, like multilayers [2], superlattices [3],[25] and arrays [4]. In the following, we will show examples ofrecent studies performed on 3d-TM at the resonance,but it should be mentioned that other examples exist in the lit-erature of experiments involving, for instance, thetransitions in Fe [5] and Ni [6].

II. MICROSTRUCTURAL ORDER IN MnAs/GaAs AND Fe/MnAsMAGNETIC COUPLING

The first example concerns Fe/MnAs layers. MnAs/GaAs(001) is a metal/semiconductor system where the in-terplay between epitaxial constraints and temperature-drivenphase transitions gives rise to a peculiar morphological andmagnetic behavior [7]. At C bulk MnAs undergoesa first order phase transition between the ferromagnetic (FM)-phase and the nonmagnetic (NM) -phase. When MnAs isgrown epitaxially on GaAs(001), the first order transition isreplaced by the coexistence of the two phases over an extended

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4712 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013

Fig. 1. Schematics of XRMSmeasurements onMnAs/GaAs(001), showing thestripe domains with period and the exchanged momentum of compo-

nents . Using circularly polarized X-rays, the scattered intensity is sen-sitive to the magnetization component. Both the external magnetic field andthe -MnAs easy magnetization axis are aligned along the direction, whichcorresponds to the intersection between the sample surface and the scatteringplane.

temperature range ( – C), a coexistence that takes theform of a regular set of stripes alternating and phases [8].The MnAs layer thickness determines the period of the

stripes and the height of the steps betweenthem ( – % of ), while and stripes widths withina period vary continuously with temperature. We have used softX-ray resonant scattering to study the evolution of the morpho-logical and magnetic properties of MnAs/GaAs, while varyingthe sample temperature [9], [10]. More recently, we usedMnAs/GaAs as a template for growing a Fe ferromagnetic overlayer.Details of the growth of Fe/MnAs samples, carried out at theInstitut des NanoSciences de Paris, were reported previously[11], [12].The magnetic and structural properties of our samples were

investigated by X-ray resonant magnetic scattering (XRMS) atthe Circular Polarization beamline of the ELETTRA storagering in Trieste, using the IRMA reflectometer [13]. Data werecollected in co-planar geometry, i.e., the three axes corre-sponding to the surface normal, to the incoming beam and tothe scattered beam were contained within the same verticalplane. The external magnetic field was applied along the inter-section between the sample surface and the scattering plane,corresponding to the easy magnetization direction of -MnAs,as sketched in Fig. 1.Measuring the intensity of the MnAs stripe-related diffrac-

tion peaks as a function of the scattering vector componentsand allows one to characterize the microstructure of the filmat a given temperature (Fig. 2). The period of the stripes andthe height of the steps between them can be determined by ana-lyzing the position in the -space of the diffraction peaks.The intensities of specular and diffracted peaks of several ordersas a function of temperature can be modeled easily and serve asa benchmark for a fine analysis of the average width of andstripes.Data shown in Fig. 2 are collected by tuning the photon en-

ergy to the Mn-2p resonance. Apart from a strong signal en-hancement, this has little influence on the analysis of the mi-crostructural parameters. The analysis of magnetic order makes

Fig. 2. Bottom: scattering map measured at 641 eV photon energy(Mn-2p resonance) for a 70 nm MnAs layer grown on GaAs(001). Bragg peaksare observed, corresponding to an in-plane order parameter of 360 nm. Center:temperature dependence of specular, first order and second order scattered in-tensity at resonance. Top: percentage of -phase versus temperature, as obtainedfrom the analysis of scattering data.

a better use of element selectivity, but, sinceMn is the only mag-netic element present in the sample, this bears little original in-formation with respect to, for instance, MOKE analysis.The element selectivity of resonant scattering becomes much

more relevant when considering the Fe/MnAs/GaAs(001)system. By using circularly polarized X-rays and tuningthe photon energy to the Mn-2p ( eV) or to the Fe-2p

SPEZZANI et al.: SOFT X-RAY MAGNETO-OPTICS 4713

Fig. 3. Element selective hysteresis curves measured by tuning the photonenergy at the Fe (squares) and Mn (circles) 2p resonances. Sample temperatureis (a) C and (b) C.

( eV) resonances, the scattered intensity is sensitive to theaverage magnetic moment carried by Mn atoms or by Fe atoms,respectively. As shown in Fig. 3, element selectivity is essentialfor an independent analysis of MnAs and Fe magnetizationbehavior. In addition, being a photon-in-photon-out technique,X-ray scattering has a large probing depth and is unaffected byapplied magnetic fields.Resonant reflectivity versus applied field curves of Fig. 3(a)

show that, below the phase coexistence temperature range, thereis a remanent parallel alignment between Fe and -MnAs mag-netizations. The Fe and Mn coercive fields are very different,indicating that, in spite of being in close contact, the two layersbehave independently from the magnetic point of view. Whenstripes form, the Fe hysteresis loop is profoundly modified: thesaturation field increases and remanent magnetization is antipar-allel (AP) with respect to MnAs. These element selective hys-teresis curves can be understood by observing that the effectivemagnetic field felt by the Fe on top of -MnAs is the sum of theexternal applied field and of a dipolar term , propor-tional to the MnAs magnetization, generated by the underlying-MnAs ferromagnetic stripe. Since can be rather large

Fig. 4. Top panel: schematic picture of the magnetic and structural evolutionof Fe/MnAs in a thermal cycle. Bottom panel: temperature dependent magneticsignal at the Fe 2p resonance for two thermal cycles with Cand C . is C.

(several hundreds of Oe), strong applied fieldsare required in order to impose a parallel alignment between Feand MnAs, while at lower fields the dipolar termdominates, switching the Fe magnetization antiparallel to theMnAs one.If we now consider a thermal cycle that takes place with no

external field applied, from data in Fig. 3 we expect themagneticalignment between Fe and -MnAs to switch from parallel toAP when the sample temperature rises, for instance, from 8 Cto 18 C. Since at C the -phase dominates, the signof the overall Fe magnetization is defined by the Fe one, whichis AP with respect to -MnAs. The important finding is that thisnew alignment remains stable when the temperature is loweredbelow the stripe formation range [14].Moreover, during the cooling process, the Fe fraction ex-

pands at the expense of Fe , until a complete AP alignment be-tween Fe and MnAs is attained. The weak interaction betweenFe and MnAs that we inferred already from Fig. 3(a) is there-fore confirmed and it corresponds to a low-temperature bi-stablemagnetic configuration of Fe/MnAs.Therefore, the Fe magnetization reversal takes place in a

thermal cycle, first heating then cooling, between two temper-atures and . Fig. 4 compares two cycles withjust below and just above the onset of the transition,namely C and C. The formerleaves the Fe magnetization unaffected, while the latter pro-duces a complete reversal at the end of the cycle, evidencingthe existence of a threshold value of which is necessaryto pass for the switching to take place.With these resonant scattering experiments we showed that

we can act on the Fe magnetization direction by varying the

4714 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013

morphology of the MnAs template via temperature driven stripeformation. This provides a method of potential applicative in-terest for a temperature-driven switching of the magnetization,without applying external fields [15]. What we actually do bychanging the temperature is to control the magnetic microstruc-ture of the template, hence its associated dipolar fields and, inthe end, the effective magnetic field acting on the Fe overlayer.

III. X-RAY IMAGING BY FOURIER TRANSFORM HOLOGRAPHY

X-ray images can be obtained by coherent scattering, a tech-nique that takes advantage of all the most peculiar character-istics of synchrotron sources: high degree of coherence (for alarge field of view), high flux (for collecting large angle scat-tering) and short pulse duration (for time resolution). The scat-tered wavefield contains all the necessary information for re-constructing an image of the scattering object, image intendedas a picture of the spatial distribution of the optical constantswithin the object. In a scattering experiment, though, only theintensity is recorded, i.e., the square modulus of the wavefield,while the phase information is lost. Reconstructing an imagein real space from a measurement of the intensity distributionin reciprocal space is often obtained by iterative procedures forphase retrieval; another approach is Fourier transform holog-raphy (FTH).FTH imaging relies on the interference between coherent

beams scattered by the object to be imaged and by a referenceaperture, whose size is the main limiting factor defining theoverall spatial resolution. When the sample is illuminated bya coherent wave-front, the spatial distribution of the opticalconstants within the object area generates a unique angulardistribution of the scattered wave and this information is codedin phase and amplitude at the detector level, via the interfer-ence with the reference wave. The Fourier transform of thetwo-dimensional scattering diagram (2D-FT) corresponds tothe autocorrelation function of the entire sample (object andreference). By an appropriate design, the cross-correlationbetween object and reference can be well separated from theself-correlation of each one of them, providing an image of theobject with a resolution limited primarily by the diameter of thereference aperture [16]. For imaging magnetic domains, onetunes the photon energy at a core absorption resonance and re-lies on X-ray magnetic circular dichroism (XMCD) [17]: sincethe absorption coefficient (hence the optical index) dependson the sign of the magnetization, each given magnetic domaindistribution within the object produces a specific diagram in co-herent scattering. The object-reference cross-correlation in the2D-FT of the scattering signal will image the magnetic domainstructure as dark/bright areas, the intensity being proportionalto the local projection of the magnetization along the circularpolarization vector of the incoming X-rays.The most common approach to FTH has the sample, the

object hole and the reference holes integrated on one singleX-ray transparent Si N membrane (integrated mask/sampleapproach) [18]. This ensures imperviousness to vibrations withamplitudes well beyond spatial resolution expected values. Forinstance, a standard manipulator mounted in a turbo-pumpedUHV chamber is more than sufficient to reach sub-100 nm res-olution [18]. Moreover, no optical element is present, leaving

free space at will for implementing custom-designed sampleenvironments. This approach, though, has one major limitation,i.e., the area that can be imaged is fixed once for all during thesample preparation and other areas of the sample cannot beinvestigated. We implemented also a different approach [19],[20] where the sample and the holographic mask (object andreference apertures) belong to two separate Si N membranes.At the price of a more complex experimental setup (the maskand the sample must be aligned in situ with respect to eachother and with respect to the beam), this approach offers theadvantage of a much simpler sample and mask preparation.In particular, object and reference apertures of the holo-graphic mask are through holes, making the focused ion beam(FIB) machining much simpler with respect to the integratedmask/sample design. The major advantage, though, is that thewhole sample can be imaged sequentially by a controlled rela-tive displacement of sample and mask, something that wouldbe impossible in the integrated sample/mask approach.The examples shown here refer to (Co /Pd ) multilayers

(n ranging from 30 to 200) featuring perpendicular magneticanisotropy and, in as-prepared samples, meander domain struc-tures with a magnetic period ranging from 130 nm for to450 nm for . Prior to scattering experiments, in-planedemagnetization cycles were performed, favoring the parallelalignment of stripe domains along the field direction. Laterallyconfined objects were squares and rectangles (ranging from 300nm to 3 m in size) defined into the continuous Co/Pd multi-layers by FIB milling. Holography masks were obtained by FIBmilling the object and reference apertures in sputter-depositedX-ray opaque (Au /Cr ) films. Examples of a holog-raphy mask and a sample are shown in Fig. 5.Experiments were carried out at the SEXTANTS beamline

[21] of the SOLEIL synchrotron, using the IRMA-2 scatteringchamber [22]. The beamline covers the Co-2p energy range withfull control of the X-ray polarization state. Resonant scatteringmeasurements were performed at 778 eV (maximum Co-L ab-sorption), using circular polarization. In order to increase thetransverse coherence of the X-ray beam, the angular acceptanceof the beamline was limited to 40 40 rad using horizontaland vertical blades. Under these conditions, the transverse co-herence length exceeds 20 m in both directions [22]. In the sep-arate mask/sample case, the holography mask and the samplewere aligned with respect to the X-ray beam and brought inclose contact using encoded positioners. The scattered inten-sity was recorded using a CCD detector featuring a light-tightAl-filter; a movable beamstop blocked the intense transmittedbeam.Fig. 6 shows an example of integrated mask/sample experi-

ment, where the scattering diagram in (a) was collected usingright circularly polarized X-rays tuned at the Co-L absorptionmaximum (778 eV). Annular intense modulations correspondto the scattering from the round 1.5 m object hole defining thefield of view (FOV). Speckles that can be observed above andbelow the transmitted beam correspond to diffraction from or-dered magnetic domains: their position in reciprocal space isdirectly related to the average magnetic period. Very short pe-riod modulations are also observed, coming from the interfer-ence with the wavefield scattered by three reference apertures.

SPEZZANI et al.: SOFT X-RAY MAGNETO-OPTICS 4715

Fig. 5. SEM images of elements prepared by FIB milling. Top: holographymask with a 2 m circular object aperture and three reference apertures. Bottom:patterned (Co /Pd ) multilayer sample.

The self-correlation of the sample, at the center of the 2D-FT ofthe scattering diagram [Fig. 6(b)], contains information aboutthe object, but tangled in such a way that it is not trivial toretrieve it. On the other hand, each object-reference pair pro-duces two conjugate cross-correlation images, corresponding toa picture of the object depicted with a spatial resolution corre-sponding to the reference size. Reference-reference cross-corre-lations are also visible in Fig. 6(b). Since the chemical composi-tion of the sample is laterally homogeneous, the intensity mod-ulations within the FOVmust originate from modulations of themagnetization, whose orientation affects the optical constants atresonance. The details of one object-reference image are com-pared in Figs. 6(c)-(d) for opposite helicities of the X-rays, allother parameters being equal. Since the bright/dark modulationmeasures the projection of the Co magnetic moment onto thepolarization of the light, inverting the photon helicity producesan inversion of the contrast, as shown by the line profiles drawnacross the two images [Fig. 6(f)]. By taking their difference,one eliminates all contributions other than magnetic in origin,leading to the picture in Fig. 6(e). One can also express the mag-netic contrast in terms of asymmetry ratio, i.e., of difference di-vided by the sum of the images obtained for opposite helicities;Fig. 6(g) shows that a contrast in excess of 80% is obtained inour sample.The integrated mask/sample approach becomes a severe limi-

tation when one wants to image several objects prepared startingfrom the same film, as shown for instance in Fig. 5. To this end,we used separate mountings for mask and sample, as described

Fig. 6. Holographic imaging of a Co/Pd magnetic multilayer. (a) 2-D scat-tering diagram collected at 778 eV using circularly polarized X-rays. (b) FTof (a) showing the object self-correlation at the center and three object-refer-ence cross-correlations. Details of one cross-correlation image for left (c) andright (d) photon helicities and their difference. (e) Line-scan intensities for thetwo helicities are compared in (f), while (g) shows their magnetic asymmetryratio.

Fig. 7. Images of magnetic domains in the region of the continuous multilayer(a) and for two objects of size 2 2 m (b) and 1.2 1.3 m (c). Thickcircles indicate the FOV, while thinner lines in (b) and (c) sketch the objectborders.

above. Results for the continuous multilayer film and for twoisolated objects are shown in Fig. 7.For the continuous film, we observed a dominant parallel ori-

entation of the perpendicular magnetic domains, with the stripesrunning parallel to the in-plane demagnetization direction [23].A few regions can be found which display discontinuities inthe stripes organization, as shown in Fig. 7(a), but overall theorder is rather good and clear magnetic diffraction peaks can beobserved in resonant scattering. The measured magnetic periodof 130 nm is in excellent agreement with MFM observations.The measured spatial resolution is about 50–60 nm, which cor-responds to the limit set by the size of the reference apertures.For the 2 m square [Fig. 7(b)], up/down magnetization

domains do not follow an ordered pattern anymore. The demag-

4716 IEEE TRANSACTIONS ON MAGNETICS, VOL. 49, NO. 8, AUGUST 2013

netization direction is the same as in Fig. 7(a), approximatelyalong the square diagonal, but only a small fraction of thesample shows domains parallel to this direction. In the upperpart of the object, stripes form mostly parallel to the side of thesquare, while on the left side it is difficult to distinguish a clearpattern.The image of a smaller object [1.2 1.3 m rectangle,

Fig. 7(c)] shows even more clearly the tendency to lose theorder of the stripes: domains retaining the original orientationas in the continuous film can be observed along the rectanglediagonal only, with a width that does not exceed 500 nm. In therest of the sample, no clear magnetic pattern can be outlined.One should acknowledge that, for these samples, similar re-

sults could have been obtained by other techniques, notably byMFM; but one should not overlook that this kind of experi-ments aims at objectives of both immediate and mid-term rel-evance. Producing images of perpendicular magnetic domainsin nanostructured objects with 50 nm spatial resolution is, ifnot unique, relevant in itself. With respect to other techniques,X-ray holography provides the element selectivity of core res-onances, which, although of marginal interest in the case ofCo/Pd multilayers, can be relevant for studying complex mag-netic compounds. Moreover, being based on X-ray transmis-sion, FTH probes the entire thickness of the sample, anotheraspect that makes it complementary to MFM and to magneticmicroscopy techniques based on electron detection or on vis-ible light.On the longer term, this kind of work is interesting because

it sets the bases for holographic X-ray imaging of small mag-netic objects in the perspective of more complex applications,notably time-resolved imaging at modern X-ray sources, wherethe possibility of taking snapshots with a single X-ray pulse of100 fs duration has been demonstrated recently [24].

IV. CONCLUSION

We have shown examples of how soft X-ray magneto-op-tics can extend the capabilities of X-ray scattering beyondcrystallographic and, in general, structural analysis. Elementselectivity and magnetic sensitivity make resonant scattering anextremely useful tool for disentangling different contributionsto the complex magnetic behavior of multicomponent systems.Taking advantage of the excellent coherence properties ofmodern X-ray sources, scattering measurements offer alsoan original approach to element selective magnetic imaging,opening up new perspectives for time-resolved studies.

ACKNOWLEDGMENT

This work was supported in part by the RTRA Triangle de laphysique, by Grant 2009-082T FibNanoSynth, and by CNRS,within the PEPS SASLELX program. The authors thank Syn-chrotron SOLEIL and Sincrotrone Trieste for granting beam-time to their research projects.

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