We present an x-ray magnetic circular dichroism (XMCD) study of a [Ce(10 Å)/Fe(30 Å)] multilayer
performed at the Ce-M4,5 absorption edges. In this system the Ce-4f electrons are strongly hybridized
with the valence band. XMCD experiments show that they carry an ordered magnetic moment. The
differences of the shape of the XMCD signals of a typical g-like compound (CeCuSi) and of the Ce/
Fe multilayer highlight the role of hybridization in determining the ground state of cerium atoms in the
multilayer, which results in a mixing of J = 5/2 and J = 7/2 coupled states

Bertran, F.

Dias, A.-M.

Felsch, W.

Finazzi, M.

Groot, F.M.F. de

Kappler, J.-P.

Kierren, B.

Krill, G.

Sainctavit, Ph.

Schulte, O.

De Groot, F. M. F.

Dias, A. M.

Sainctavit, P.

Kappler, J. P.

University of Groningen

Direct Evidence of the Role of Hybridization in the X-Ray Magnetic Circular Dichroism of α-Ce

FINAZZI, MARCO

F. de Groot

A. Dias

B. Kierren

F. Bertran

P.h. Sainctavit

J. Kappler

O. Schulte

W. Felsch

G. Krill

Archivio istituzionale della ricerca - Politecnico di Milano

We present an x-ray magnetic circular dichroism (XMCD) study of a [Ce(10 Angstrom)/Fe(30 Angstrom)] multilayer performed at the Ce-M(4,5) absorption edges. In this system the Ce-4f electrons are strongly hybridized with the valence band. XMCD experiments show that they carry an ordered magnetic moment. The differences of the shape of the XMCD signals of a typical gamma-like compound (CeCuSi) and of the Ce/ Fe multilayer highlight the role of hybridization in determining the ground state of cerium atoms in the multilayer, which results in a mixing of J = 5/2 and J = 7/2 coupled states.</p

ARTS repository - University of Groningen

We present an x-ray magnetic circular dichroism (XMCD) study of a [Ce(10 Å)/Fe(30 Å)] multilayer

performed at the Ce-M4,5 absorption edges. In this system the Ce-4f electrons are strongly hybridized

with the valence band. XMCD experiments show that they carry an ordered magnetic moment. The

differences of the shape of the XMCD signals of a typical g-like compound (CeCuSi) and of the Ce/

Fe multilayer highlight the role of hybridization in determining the ground state of cerium atoms in the

multilayer, which results in a mixing of J = 5/2 and J = 7/2 coupled states

NARCIS 

Direct Evidence of the Role of Hybridization in the X-Ray Magnetic Circular Dichroism of a-Ce

FINAZZI, M

DEGROOT, FMF

DIAS, AM

KIERREN, B

BERTRAN, F

SAINCTAVIT, P

KAPPLER, JP

SCHULTE, O

FELSCH, W

KRILL, G

University of Groningen Research Database

DIRECT EVIDENCE OF THE ROLE OF HYBRIDIZATION IN THE X-RAY MAGNETIC CIRCULAR-DICHROISM OF ALPHA-CE

Utrecht University Repository

VOLUME 75, NUMBER 25 P HY S I CA L REV I EW LE T T ER S 18 DECEMBER 1995Direct Evidence of the Role of Hybridization in the X-Ray Magnetic Circular Dichroism of a-CeM. Finazzi,1 F.M. F. de Groot,2 A.-M. Dias,1 B. Kierren,3 F. Bertran,3 Ph. Sainctavit,1,4 J.-P. Kappler,1,5 O. Schulte,6W. Felsch,6 and G. Krill11Laboratoire pour l’Utilisation du Rayonnement Electromagnétique, Bâtiment 209D, Université Paris-Sud, 91405Orsay Cedex, France2Solid State Physics, University of Gröningen, Nijenborgh 4, 9747 AG, Gröningen, Netherlands3Laboratoire de Physique du Solide, CNRS URA DO 155, Université de Nancy 1, 54506 Vandoeuvre les Nancy Cedex, France4Laboratoire de Minéralogie et Cristallographie de Paris, CNRS URA9, Universités de Paris 6 et 7, 75252 Paris Cedex 05, France5Institut de Physique et Chimie des Matériaux de Strasbourg-Groupe d’Etude des Matériaux Métalliques, 23 rue du Loess, 67037Strasbourg, France6I. Physikalisches Institut, Universität Göttingen, Bunsenstrasse 9. 37073 Göttingen, Germany(Received 29 March 1995)We present an x-ray magnetic circular dichroism (XMCD) study of a [Ce(10 Å)/Fe(30 Å)] multilayerperformed at the Ce-M4,5 absorption edges. In this system the Ce-4f electrons are strongly hybridizedwith the valence band. XMCD experiments show that they carry an ordered magnetic moment. Thedifferences of the shape of the XMCD signals of a typical g-like compound (CeCuSi) and of the Ce/Fe multilayer highlight the role of hybridization in determining the ground state of cerium atoms in themultilayer, which results in a mixing of J ­ 52and J ­ 72coupled states.PACS numbers: 71.27.+a, 75.30.Mb, 75.50.Rr, 78.70.CkMagnetic circular dichroism in x-ray absorption(XMCD) is now widely used to investigate the localmagnetic properties of molecules or of solids both inthe bulk or at the surface. This technique combines theorbital and site selectivity of a conventional absorptionexperiment with the possibility of reversing the relativeorientation of the sample magnetization and the helicityof the photons. Due to the electric dipole selection rules,which govern the absorption process of polarized light,orbital- and element-projected magnetic moments can bededuced in both magnitude and direction [1].In this Letter, we want to focus on a fundamental prob-lem related to the magnetic properties of cerium atoms inhighly correlated systems. Our goal is to give clear evi-dence that XMCD experiments performed at the M4,5 ab-sorption edges of cerium are able to highlight the role ofhybridization in the 4f magnetic properties of such sys-tems. The archetype for a highly correlated system is thecase of cerium metal itself, in the so called a state [2]. Ina-Ce, the 4f electron states of cerium are strongly hy-bridized with the 5d conduction electrons, and this hy-dridization usually yields a nonmagnetic ground state evenat low temperature. Several experiments [3–5] and calcu-lations [6,7] lead to the conclusion that the ground stateof a-like Ce can be magnetically ordered in 3d inter-metallic compounds such as CeFe2 and CeCo5. Indeed,by XMCD experiments at the Ce M4,5 edges, we demon-strated unambiguously that a small 4f magnetic moment(about 20.15mB) develops on cerium in CeFe2 [4].X-ray absorption (XAS) experiments performed on Ce-Fe multilayer systems [8] at the Ce-L2,3 absorption edgeshave revealed that in these multilayers the cerium is inthe a phase in a region extending up to 25 Å from theinterface [9]. It has been suggested that Ce near the4654 0031-9007y95y75(25)y4654(4)$06.00interface adopts the a-phase electronic configuration andhence a smaller atomic volume to reduce the misfit andstrain at the interface. XMCD experiments performed atthe Ce-L2,3 edges clearly demonstrate the existence of amagnetic moment of 5d origin on the cerium atoms on adepth scale of at least 15 Å near the interface and coupledantiparallel to the moment of iron [9].We must emphasize that the spectral shape of any rare-earth isotropic M4,5 absorption edge is a fingerprint ofthe initial value of the total angular momentum quantumnumber J in the ground state (for the pure 4f1 state of Ce,J ­ 52 ). As a consequence of the Wigner-Eckart theorem,the isotropic spectral shape will not change as long as theground state remains characterized by a single J value[10]. Thus any modification in the isotropic line shapeis direct evidence that levels with different values of Jcontribute to the ground state. As shown in our recentpaper [10], this remains true also for XMCD spectra. Theaim of this Letter is to demonstrate that the ground state ofa-Ce is described by a combination of the 4f0L, 4f1J­5y2,and 4f1J­7y2 configurations (where L represent an electronin the conduction band) as a result of the hybridization ofthe f electrons with the conduction band. This has alreadybeen suggested by previous theoretical works [11,12], buta direct experimental proof was still lacking.For g-like Ce compounds the crystal field parameter ismuch smaller than the energy separation between the twospin-orbit split J ­ 52 and J ­72 states (typically 10 meVagainst 250 meV, respectively [13]). In the case of a-likecerium, previous M4,5 XAS experiments [14] have proventhat the hybridization and the spin orbit can be of the sameorder of magnitude. The delocalization of the 4f elec-trons leads to the double result of reducing the effectiveseparation between the J ­ 52 and the J ­72 levels and© 1995 The American Physical SocietyVOLUME 75, NUMBER 25 P HY S I CA L REV I EW LE T T ER S 18 DECEMBER 1995increasing the influence of the crystalline environment. Arecent XMCD study shows the importance of the interplaybetween hybridization and crystal field in determining themagnetic properties of CeRh3B2 [15]. Thus a correct de-scription of the nature of the ground state of a-Ce needsto properly take into account both hybridization and crys-tal field effects with a basis set which should include the4f0L, 4f1J­5y2, and 4f1J­7y2 configurations. We shall re-fer to some general results through an advanced theoreti-cal model.The Ce-M4,5 XMCD data were obtained on the SU22beam line of the Super-ACO storage ring at LURE (Orsay,France). The Ce XAS spectra were recorded in thetransmission mode on a Ce-Fe multilayer consisting oftwelve [Ce(10 Å)/Fe(30 Å)] periods evaporated on a 7-mm-thick Be foil after deposition of a 50 Å chromiumbuffer, and was covered by a 200 Å thick chromiumlayer. The experiments are performed at 77 K underB ­ 650 mT magnetic field, which is high enough tosaturate the total magnetic moment in the plane of themultilayer [16]. The angle between the sample normaland the light propagation vector was 30–. The rate ofcircular polarization is estimated to be s25 6 1d% ands17 6 1d% at the M5 and M4 edges, respectively [17].The XMCD spectra are obtained by the difference betweenthe absorption data recorded for two opposite directionsof the magnetic field, which is reversed for each photonenergy.In Fig. 1 we present the Ce-M4,5 isotropic (upper panel)and XMCD (lower panel) spectra obtained for the mul-tilayer. As a reference, we also show the correspondingspectra of a typical g-like Ce system. Since metal g-Ceis not magnetic, we have chosen the spectra obtained onpolycrystalline CeCuSi in the total electron yield mode[4]. As evident in Fig. 1(a), the isotropic signal from themultilayer is characterized by a strong hybridization satel-lite emission (the so-called 4f0 channel) at hn ­ 890 and908 eV, which confirms that Ce in the multilayer is in thea phase. The analysis of the dichroic signal demonstratesthat the 4f electrons carry a magnetic moment. Whencompared to CeCuSi, the isotropic spectrum of the Ce/Femultilayer shows a larger energy separation between theM4 and M5 maxima. A second feature is the smearing outof the atomic multiplet fine structures at correspondenceboth with the M4 and the M5 edges. At the M4 the smallpeaks on the low photon energy side of the white line arenot resolved anymore, while at the M5 the doublet col-lapses into a single peak. The multilayer and the CeCuSicompound also present a different branching ratio betweenthe isotropic intensities of the M4 and M5 absorption edges.This result is well confirmed by transmission experimentson a 150 Å metal g-Ce film in experimental conditionsstrictly comparable to the ones for the Ce/Fe multilayer.Although the multiplet structures of the isotropic M4absorption-edge signal of the multilayer are smearedout, the corresponding XMCD signal presents a similarFIG. 1. Isotropic (upper panel) and XMCD (lower panel)spectra obtained for a Ce(10 Å)/Fe(30 Å) multilayer and Ce-CuSi [4] at the Ce-M4,5 absorption edges. The intensity of theM5 edge isotropic spectra of the two samples has been arbitrar-ily set to unity. The XMCD spectra have been normalized to100% circular polarization rate.line shape as the g-like compound, and the atomic finestructure is still well resolved. A closer investigationreveals that the XMCD signal at the M4 edge of themultilayer is shifted towards high photon energies by0.15 eV. At the M5 edge we observe the presence of anegative peak absent in the dichroic spectrum of CeCuSi.This feature has not been observed before in other g- ora-Ce systems and is not reproduced by one-configurationatomic models at T ­ 0 [18]. Since the spectral shapeof the XMCD signal cannot be changed as long as theground state remains in a pure J ­ 52 value, and sincethe 4f0L configuration is not observed to contribute tothe dichroic signal [4], we have direct evidence of thepresence of the J ­ 72 term in the ground state of thecerium atoms in the multilayer.In Fig 2 we show the isotropic and XMCD spectraof the J ­ 52 and J ­72 configurations of Ce31, usingan atomic multiplet calculation of the 4f1J­5y2 ! 3d94f2and 4f1J­7y2 ! 3d94f2 transitions [18]. The sign of theexchange field acting on the spin of the 4f electrons ischosen in order to reproduce the antiparallel alignment of4655VOLUME 75, NUMBER 25 P HY S I CA L REV I EW LE T T ER S 18 DECEMBER 1995FIG. 2. Ce-M4,5 isotropic (top panel) and XMCD (lowerpanel) spectra obtained for the J ­ 52and J ­ 72statescalculated for the ion Ce31. The zero of the energy axis hasbeen chosen at correspondence with the maximum in the M5edge of the J ­ 52XMCD spectrum. The intensity of the M5edge of the J ­ 52isotropic spectrum has been set to unity.the Fe-3d and Ce-4f spin moments expected for Ce-Fesystems [5,6,9]. Unfortunately, the experimental XMCDand isotropic spectra cannot be described in terms of asimple combination of J ­ 72 or J ­52 because of thepresence of interference terms, but a qualitative analysisis still possible [11]. The dichroic signal of CeCuSi iswell described by the XMCD spectrum given by the J ­52 state [4]. On the contrary, the experimental isotropicM5 edge is not an exact replica of the calculated J ­52spectrum. This is due to a small hybridization of the felectrons with the valence band [11]. These effects arenot taken into account by the ionic model we used, andseem not to influence significantly the XMCD signal of g-like Ce. For the multilayer, the appearance of a negativepeak in the XMCD signal can be interpreted in termsof a contribution from the J ­ 72 configuration. Notealso that the possible presence of the J ­ 72 contributionwould lead to a decreasing of the M5yM4 branching ratioin the isotropic spectrum, as actually observed in ourexperimental data.4656In order to study the influence of hybridization on theXAS and XMCD of a-Ce, we used the charge transfermultiplet (CTM) model as developed for Ce compoundsby Jo and Kotani [19]. This model mixes 4f1 with 4f0Lfor the ground state and 3d94f2 with 3d94f1L for the finalstate. The ground and final state mixing is controlled bythe energy separations and the hybridization. For the cal-culation reported in Fig. 3 we used a 4f1 state at 1.7 eVbelow the 4f0L band, which was approximated by a sin-gle level. In the final state the center of gravity of the3d94f2 multiplet is 4.7 eV below the 3d94f1L multiplet.The spherical hybridization used was 0.38 eV. These num-bers are the same as the ones in the CTM calculation ofCeRh3 in Ref. [19]. To include nonspherical symmetry asmall octahedral crystal field was added, and an exchangefield of 5 meV was used to create the XMCD spectra. Theused crystal and exchange field values have no physicalsignificance and are taken just to show the effects of sym-metry reduction. With this choice of parameters, we obtaina ratio between the weights of the J ­ 72 and J ­52 termsFIG. 3. Comparison between the experimental data of the Ce/Fe multilayer and the results of the model described in thetext. Top panel, Ce-M4,5 isotropic spectra; lower panel, XMCDspectra. The intensity of the two M5 edge isotropic spectrahas been set to unity. The origin of the energy axis of thetheoretical spectra has been chosen in order to align the positivepeaks at correspondence with the M5 edge of the theoretical andexperimental XMCD spectra.VOLUME 75, NUMBER 25 P HY S I CA L REV I EW LE T T ER S 18 DECEMBER 1995in the ground state equal to 0.10–0.15 [12]. In Fig. 3,the experimental data obtained on the Ce/Fe multilayer arecompared to the isotropic and XMCD spectra, calculatedaccording to this model. The experimental isotropic andXMCD line shapes are qualitatively well described at boththe M4 and M5 edges, even if the intensity of the M4 isoverestimated. For the isotropic spectra, the theoreticalmodel reproduces the variation of the M4-M5 energy sep-aration and the position of the f0 satellite. In the XMCDspectrum of a-Ce, the negative peak at the M5 edge is re-produced correctly, as well as the shift of the M4 XMCDmultiplet towards the higher photon energies. However,the intensity of the experimental dichroic signal is about afactor of 3 weaker than the experimental one. This maybe due to a decreasing of the 4f moment across the ceriumlayer. Nevertheless, we find a similar ratio between the ex-perimental and theoretical XMCD intensities of CeCuSi.For g-like Ce, this behavior reflects the reduction of the4f magnetic moment due to the crystal field-induced mix-ing of the Zeeman levels in the ground states. This phe-nomenon is probably underestimated by the model used inFig. 3. The presence of the peak labeled by the letter A inFig. 3(b) indicates that the XMCD effect is partly trans-ferred to the satellite structure. If the valence states of thesystem had been described with a continuous band insteadof a single discrete level, feature A would have been broad-ened into a shoulder.The discussion of the results of the application of thesum rules of Ref. [1] to the XMCD signal will be givenelsewhere. We would like to anticipate that kS4fz l is foundto be aligned antiparallel to kL4fz l and to kS3dz l.In conclusion, we have presented a XMCD studyof a [Ce(10 Å)/Fe(30 Å)] multilayer performed at theCe-M4,5 absorption edges. In this system, the Ce atomsare in the highly hybridized a-phase, characterized by astrong mixing between the 4f electrons with the valenceband, and carry an ordered magnetic moment. XMCDexperiments show the part of this moment is due to 4felectrons. The differences in the shape of the XMCDsignals of a typical g-like compound (CeCuSi) and ofthe Ce/Fe multilayer demonstrate that the XMCD spectrareflect the hybridization in the ground state of the ceriumatoms in the multilayer, which results in a mixing of J ­52 and J ­72 configurations. The experimental isotropicand XMCD spectra are well described in terms of a modelwhich combines the Anderson impurity Hamiltonian withthe interactions described by the Slater integrals, spinorbit coupling, and crystal field.[1] P. Carra, B. T. Thole, M. Altarelli, and X. Wang, Phys.Rev. Lett. 70, 694 (1993).[2] D. C. Kosenmaki and K.A. Gschneider Jr., in Handbookof the Physics and Chemistry of the Rare Earths, edited byK.A. Gschneider Jr., L. Eyring, and S. Hüfner (Elsevier,Amsterdam, 1987), Vol. 1, p. 337; A. Iandelli andA. Palenzona, ibid., Vol. 2. p. 1.[3] C. Giorgetti et al., Phys. Rev. B 48, 12 732 (1993).[4] J. P. Schillé et al., Phys. Rev. B 50, 2985 (1994).[5] S. J. Kennedy, P. J. Brown, and B.R. Coles, J. Phys.:Condens. Matter 5, 5169 (1993).[6] J. Trygg, J.M. Wills, B. Johansson, and O. Eriksson, Phys.Rev. B 50, 4200 (1994).[7] L. Nordström, O Eriksson, M. S. S. Brooks, andB. Johansson, Phys. Rev. B 41, 9111 (1990).[8] F. Klose, M. Steins, T. Kacsich, and W. Felsch, J. Appl.Phys. 74, 1040 (1993).[9] F. Klose et al., Phys. Rev. B 50, 6174 (1994).[10] J. P. Schillé et al., Phys. Rev. B 48, 9491 (1993).[11] G. Van der Laan et al., J. Phys. C 19, 817 (1986).[12] T. Jo and S. Imada, Phys. Scr. 41, 560 (1990).[13] J. J.M. Franse and R. J. Radwanski, in Handbook ofMagnetic Materials, edited by K.H. J. Buschow (North-Holland, Amsterdam, 1993), Vol. 7, p. 307.[14] J. C. Funggle et al., Phys. Rev. B 27, 4637 (1983).[15] Yamaguchi et al., Phys. Rev. B 51, 13 952 (1995).[16] J. Thiele et al., J. Magn. Magn. Mater. 119, 141 (1993).[17] D. Lefebvre, Ph. Sainctavit, and C. Malgrange, Rev. Sci.Instrum. 65, 2556 (1994).[18] J. B. Goedkoop et al., Phys. Rev. B 37, 2086 (1988).[19] T. Jo, and A. Kotani, J. Phys. Soc. Jpn. 57, 2288 (1988);Phys. Rev. B 38, 830 (1988).4657

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Direct Evidence of the Role of Hybridization in the X-Ray Magnetic Circular Dichroism of a-Ce

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NARCIS

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University of Groningen Research Database

Utrecht University Repository

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