PHYSICAL REVIEW B VOLUME 57, NUMBER 4 15 JANUARY 1998-II 4f and 5d magnetic moments in highly correlated Ce/La/Fe and La/Ce/Fe multilayers studied by x-ray magnetic circular dichroism M. Arend I. Physikalisches Institut, Universita¨t Go¨ttingen, Bunsenstrasse 9, 37073 Go¨ttingen, Germany M. Finazzi European Synchrotron Radiation Facility, Boi te Postale 220, 38043 Grenoble Cedex, France O. Schutte and M. Mu¨nzenberg I. Physikalisches Institut, Universita¨t Go¨ttingen, Bunsenstrasse 9, 37073 Go¨ttingen, Germany A.-M. Dias, F. Baudelet, Ch. Giorgetti, and E. Dartyge Laboratoire pour l'Utilisation du Rayonnement Electromagne´tique, Ba timent 209 D, Universite´ Paris-Sud, 91405 Orsay, France P. Schaaf II. Physikalisches Institut, Universita¨t Go¨ttingen, Bunsenstrasse 9, 37073 Go¨ttingen, Germany J.-P. Kappler Laboratoire pour l'Utilisation du Rayonnement Electromagne´tique, Ba timent 209 D, Universite´ Paris-Sud, 91405 Orsay, France and Institut de Physique et Chimie des Mate´riaux de Strasbourg, 23 Rue du Loess, 67037 Strasbourg, France G. Krill Laboratoire pour l'Utilisation du Rayonnement Electromagne´tique, Ba timent 209 D, Universite´ Paris-Sud, 91405 Orsay, France W. Felsch* I. Physikalisches Institut, Universita¨t Go¨ttingen, Bunsenstrasse 9, 37073 Go¨ttingen, Germany Received 23 May 1997 Experiments are reported of x-ray absorption and x-ray magnetic circular dichroism at the L2,3(2p 5d) and M4,5(3d 4f) absorption edges of Ce and La in the multilayers La/Ce 10Å /La/Fe 30Å n and Ce/La 10Å /Ce/Fe 30Å n. In these layers, Ce adopts an -phase-like electronic structure. Strong 3d-5d and 3d-4 f hybridization is effective at the interfaces. It induces an ordered magnetic moment on the 5d and 4 f states in the ground state of Ce and of the 5d states of La at room temperature, in antiparallel orientation to the Fe moment. By choosing the special laminar structure of the two lanthanides intercalated in between the Fe layers, the spatial extension of the Fe-induced magnetic order is directly probed. The 4 f polarization of Ce and the 5d polarization of La are limited to the direct interface with Fe. In contrast, the polarization of Ce-5d states extends deeply into the layers. There is evidence that the 5d states of -like Ce in these layered structures are very close to a ferromagnetic instability. Remarkably, magnetic order on the Ce-5d states can exist without a polarization of the 4 f states. Apparently the 4 f -(5d,6s) hybridization wins over the intraatomic 4 f -5d exchange interaction, and stabilizes a nonmagnetic 4 f ground state. Comparative studies on alloys CexFe1 x confirm this observation: near x 0.34 the polarization of the 4f states gets lost, but the 5d states are magnetically ordered. The special electronic and magnetic properties of Ce and La in the multilayer systems are reflected in the reversal of the macroscopic magnetization studied by the magneto-optical Kerr effect. S0163-1829 98 07504-3 I. INTRODUCTION ancy. Both aspects can be realized, depending on the degree of mixing of the 4 f and conduction-electron states, either in Magnetic ordering in ordinary rare-earth metals arises the or phases of pure Ce metal or in compounds with from exchange coupling between the local 4 f moments and transition metals. Dramatic differences in the physical prop- the conduction electrons through which the conduction band erties and very complex phase diagrams mirror the unlike acquires a net magnetization. While this mechanism is rela- electronic ground-state configurations.1 Theoretical work on tively well understood for elemental rare earths, intermetallic Ce metal has focused mostly on the transition from the fcc bonding and spin-orbit coupling complicate theory in com- phase to the isostructural but much denser ( 17% phase, pounds of these elements with transition metals. The situa- which may be driven thermally or by the application of pres- tion is even more complex for cerium because its 4 f -electron sure. It has become clear by now that a good account of this states are at the borderline between localization and itiner- transition is obtained by a model treating the 4 f -electron 0163-1829/98/57 4 /2174 14 /$15.00 57 2174 © 1998 The American Physical Society 57 4 f AND 5d MAGNETIC MOMENTS IN HIGHLY . . . 2175 states as being localized nonbonding in -Ce and itinerant carry an ordered magnetic moment on their 5d states, at bonding in -Ce.2,3 A strong hybridization of the 4 f states room temperature, in antiparallel orientation to the Fe-layer with the conduction band makes -Ce an archetype for a magnetization. A similar magnetic polarization was found highly correlated electronic system. The interest in this ele- for the 5d states of La in the reference multilayer system ment is undoubtedly linked to the fact that -phase Ce metal La/Fe. The branching ratio of the integrated L2 and L3 itself has a nonmagnetic ground state,4 but in compounds XMCD signals is close to 1. This shows that in both cases with d-band transition metals, like CeFe 2 or CeCo5, where the orbital contribution to the 5d magnetic moment is almost Ce adopts an -phase-like electronic structure, experi- zero, i.e., Lz 0.13 Hence the moment is essentially of pure ments5­8 and theoretical work9­11 reveal that its ground state spin origin. A recent XMCD study at the M4,5 edges has may be magnetically ordered, with a magnetic moment on its shown that Ce has also a small 4 f magnetic moment 4 f states. An important concept in the theoretical description ( 0.1 B) in this -like phase.14 of the electronic structure of these compounds is the hybrid- This paper presents results which are intended to obtain a ization between the 3d states of the transition metal and the closer insight into the magnetic polarization of the 5d and 4 f itinerant 4 f states of Ce. states of Ce in the strongly hybridized -like phase, as well In view of these properties, experimental studies of the as the 5d states of La, and the spatial extension of this po- magnetic polarization of the electronic 4 f states of Ce in larization near the interfaces in multilayers with Fe. This is highly correlated systems with magnetic d-band transition accomplished by XMCD measurements at the M4,5 and/or metals are of fundamental interest. This applies equally to L2,3 edges of thin ``probe layers'' of La and Ce which are the Ce-5d states which play a key role in the interaction inserted into the middle of the Ce and La counterpart sub- between the rare earth and the transition metal tuning the layers of varying thickness. These experiments are comple- magnetic properties.12 In this context, we have recently mented by similar x-ray-absorption and XMCD studies on grown multilayers combining cerium and iron, and lantha- thin films of metastable Ce xFe1 x alloys. In these systems, num and iron as a non-4 f reference, to probe the local elec- a variation of the Ce concentration x permits one to study tronic and magnetic properties of Ce and La near their inter- systematically the evolution of the degree of hybridization faces with Fe.13,14 These multilayers showed sharp interfaces between the Ce-4 f and -5d states with the Fe-3d states and without noticeable interdiffusion.15 The desired information its bearing on the local magnetism on the Ce atoms. about the magnetic and electronic properties of the 4 f and 5d electrons was derived by absorption spectroscopy at the II. SAMPLE PREPARATION AND CHARACTERISTICS M4,5 and L2,3 edges of Ce and La with circularly polarized x rays, taking benefit from the orbital and site selectivity of the Multilayers composed of the structural periods xÅCe/ method and the appearance of magnetic dichroism near the 10ÅLa/xÅCe/30ÅFe and xÅLa/10ÅCe/xÅLa/30ÅFe in absorption edges involved. X-ray magnetic circular dichro- which the Fe-layer thickness is constant, and the intercalated ism XMCD reflects the phenomenon that the photoabsorp- 10-Å-thick La or Ce films serve as probe layers, and alloy tion cross section depends on the helicity of the incident light films CexFe1 x were grown by computer-controlled ion- left or right circularly polarized with respect to the direc- beam sputtering in an ultrahigh vacuum chamber base pres- tion of the magnetization. The underlying mechanism, gov- sure p 5 10 10 mbar . Highly pure sputtering gas Ar erned essentially by electric dipole transitions, implies that in (6N) and target metals La, Ce (3N), and Fe (4N8) were the case of rare-earth elements which is of interest here pho- used. For the preparation of the alloy films, an Fe plate of a toabsorption involves the 3d 4 f (M4,5) and 2p 5d(L2,3) size adapted to the desired concentration was mounted onto excitation thresholds in the soft- and hard-x-ray regions, re- the Ce target. After preparation, the composition x of these spectively. Hence the magnetic polarization of the 4 f and 5d films was determined by Rutherford backscattering spec- states of the rare earth can be directly and independently trometry; it ranged between 0.17 and 0.35, with an uncer- probed by this technique. Moreover, XMCD allows one, due tainty of 0.03. Partial pressures of reactive gases e.g., O2, to the existence of sum rules,16,17 to separate for each edge N2, H2O) were below 10 10 mbar during the deposition pro- the orbital and spin contributions to the magnetic moment in cess. Typical growth rates were near 1.0 Å /s for La and Ce, the ground state. However, it has been shown recently18 that and 0.5 Å /s for Fe. Kapton or parylene foil coated with a in the case of the 5d magnetic moment i.e., at the L2,3 40-Å-thick Cr buffer layer were used as substrates for the edges of a rare-earth element the application of these sum x-ray-absorption experiments, to permit measurements in rules is not possible if there are both localized 4 f and ex- transmission mode, both in the hard- and soft-x-ray regimes, tended 5d states in the ground state. Fortunately, there is respectively. X-ray diffraction, magneto- evidence6,19,20 that these sum rules work correctly, in the metry, and Mo¨ssbauer spectroscopy were performed on special case of cerium, as soon as the 4 f states are delocal- samples deposited on equally precoated Si 100 wafers. Be- ized i.e., hybridized with the conduction states . cause of the same buffer layer, the properties of the layered The measurements of x-ray absorption performed on samples do not depend on the substrate. For the preparation Ce/Fe multilayers at the Ce-L2,3 thresholds have revealed of the multilayers, the sample holder was cooled to about 90 that, on a considerable length scale near the interface with K by liquid nitrogen to minimize diffusion. The alloy films Fe, Ce adopts the electronic structure of the phase.13 This were deposited at room temperature. The total thickness of must be attributed to compressive strain induced on Ce by the multilayers and alloys was near 4000 Å in the case of the the large mismatch between the Ce and Fe layers at their studies at the L2,3 edges of Ce and La, and between 245 and interface. The corresponding XMCD spectra taken at the Ce- 400 Å with the total quantity of the rare earth amounting to L2,3 edges clearly demonstrate that these -like Ce atoms an equivalent of 150 Å in the case of the M4,5-edge x-ray- 2176 M. AREND et al. 57 absorption measurements. A capping layer of Cr typically 100 Å thick provided protection against oxidation. The structural quality of the interfaces between Fe and Ce or La in the multilayers was investigated by x-ray reflecto- metry in -2 geometry at small angles.15,21 To quantify interfacial roughness, the data were analyzed by employing results of the dynamical scattering theory.22 Good fits were obtained for a rms roughness full width at half maximum of typically 3.5­4 Å in the case of the Ce/Fe interfaces and 2­3 Å for the La/Fe interfaces. Because of the very similar atomic form factor of Ce and La, conventional x-ray diffrac- tion as employed here provides only limited information about the nature of the interfaces between La and Ce. But, as will be outlined below, the x-ray-absorption results in par- ticular permit one to preclude noticeable interdiffusion or roughness at these interfaces. It can be concluded that the samples with their respective Ce or La probe layers present a well-defined layer structure with sharp composition profiles. The structure of the individual layers and of the alloy films was determined by x-ray diffraction at large angles.15,21 The 30-Å-thick Fe sublayers of the multilayers grow in the bcc structure of the phase. Up to layer thicknesses of about 15 Å for La and 40 Å for Ce, the rare earths grow in an amorphous structure. This also applies, in particular, to the probe layers. Above these critical thicknesses, La and Ce grow in a fcc structure. The amorphous-to-crystalline transi- tion advances gradually: once critical thicknesses have been reached 15 or 40 Å , respectively , the signatures of -phase La or -phase Ce appear, and then grow in intensity as the layers become thicker. The CexFe1 x alloy films 0.17 x 0.35 are single phase and grow in an amorphous hence metastable structure, in agreement with the result reported previously23 crystalline solid solutions of Ce and Fe do not exist . We have previously shown for Ce/Fe Ref. 24 and La/Fe Ref. 21 multilayers that 57Fe Mo¨ssbauer spectroscopy pro- vides information about the interaction between the lan- thanides and iron at their interface. The experiments were extended to the samples with La and Ce probe layers. Due to the ferromagnetic nature of the layers, the Mo¨ssbauer spectra not shown here are magnetically split, with asymmetric ab- sorption lines indicating the presence of inequivalent Fe sites. Data analysis was performed as previously:21,24 the spectra were decomposed into a sextet representing the nor- FIG. 1. Magnetic hyperfine-field distributions p(Bhf) mal bcc part in the core of the Fe layers, and a probability corresponding to the contribution of the interfaces to the 57 distribution of magnetic hyperfine fields attributed to Fe af- Fe conversion-electron Mo¨ssbauer spectra at 300 K of the fected by the interfaces interface component .25 The result- multilayers 30ÅCe/30ÅFe 67 a , 25ÅLa/30ÅFe 60 b , ing hyperfine-field distributions p(B 10ÅCe/10ÅLa/10ÅCe/30ÅFe 68 c and 10ÅLa/10ÅCe/ hf) are compared in Fig. 1 for four representative examples. The following is note- 10ÅLa/30ÅFe 68 d . The data in a and b are from References 24 and 21, respectively. worthy: i The contribution of the interface component to the total intensity of the spectrum corresponds to a nominal Fe-layer thickness of 8-9 Å per interface, both in Ce/ low fields (Bhf 150 kG with the distinct maximum repre- La/Ce/Fe and La/Ce/La/Fe as in the nonintercalated sents Fe atoms next to the interface. For the multilayers in multilayers.21,24 This points to a magnetic ``proximity ef- Fig. 1, it corresponds to 2.5 and 3 Å per interface in Ce/Fe fect'' on a length scale exceeding the local thickness fluctua- and Ce/La/Ce/Fe, respectively, and to 1 and 2 Å per interface tions, and mirrors a modification of the Fe electronic struc- in La/Fe and La/Ce/La/Fe. Obviously, the maximum is more ture near the interface due to the lanthanide-iron interaction. pronounced in the case where Ce sublayers are in contact ii The hyperfine-field distributions underlying the interface with Fe. This indicates that Fe interacts differently with Ce component of the spectrum, p(Bhf) present characteristic and La at the interfaces. Note that the insertion of the Ce structures. As we argued previously,21,24 these structures may probe layers into the La/Fe system increases the low-Bhf part be assigned to different parts of the interface. The region at in the distribution p(Bhf). Interestingly, such differences ap- 57 4 f AND 5d MAGNETIC MOMENTS IN HIGHLY . . . 2177 FIG. 2. Coercive fields HC a , fields for 90% of magnetic saturation HS,90% b , and remanent magnetizations MR normalized with the saturation magnetization MS c at 300 K determined from the longitudinal MOKE magnetization curves of the multilayers xÅCe/30Å Fe , xÅCe/10ÅLa/xÅCe/30ÅFe , xÅLa/30ÅFe and xÅLa/10ÅCe/xÅLa/30ÅFe vs total thickness of the rare-earth layers tRE tCe,La or 2xÅLa Ce 10ÅCe La , respectively.... 2178 M. AREND et al. 57 pear also in the values of the average magnetic hyperfine in the center13 at large tCe . The most startling feature is the field resulting on Fe from the total 57Fe Mo¨ssbauer spectra: very soft magnetic behavior observed at low Ce-layer thick- this quantity, which is typically 260 kG in Ce/Fe and nesses: the magnetization curves not shown are rectangular, 300 kG in La/Fe, is not affected by the insertion of a with coercive and saturation fields below 1 Oe and nearly 10-Å-thick La layer into the Ce, whereas it reduces to 285 100% remanence. On the basis of these curves it is reason- kG following the insertion of a 10-Å-thick Ce layer into the able to assume that magnetization reversal occurs essentially La. These values compare to 334 kG in bulk bcc Fe at 300 by the nucleation of 180° domains. Apparently the energy K. We shall come back to this feature in relation with the x-ray-absorption results discussed below. required is very low due to the presence of the phase like We have already shown previously13 that the macroscopic Ce interlayers. With the appearance of -phase-like Ce at spontaneous magnetization of the multilayers mirrors the tCe 20 Å,13 the coercive and saturation fields Hc and HS,90% trends observed for the average magnetic hyperfine fields. increase and the remanence MR decreases. The same mag- The Curie temperatures T netic hardening effect is induced by inserting a 10-Å-thick c of the multilayer samples are far above room temperature,21,26 but the Ce La layer into the center of the Ce layers at low t xFe1 x alloy films Ce , where Ce order magnetically below 210 K. Both the values of T is entirely like, i.e., in Ce/La/Ce/Fe. The magnetization c and of the saturation magnetization M behavior of the La/Fe multilayers is more complex. Up to t S agree with the results La found previously for these alloys.23 The magnetization rever- 10 Å , the material is very soft magnetically, but in con- sal in the multilayers was studied by means of the magne- trast to the Ce/Fe system the remanence MR is very small. toopical Kerr effect MOKE in the usual longitudinal setup, This points to a domain structure in the layers. Inserting the with the magnetic field applied in the plane of the film and in Ce probe layer at these tLa values hardens the multilayers the plane of incidence of the light beam from a He-Ne laser. magnetically, and increases MR drastically. Figure 2 shows the coercive field Hc , the remanence MR , The x-ray absorption spectra were measured in transmis- and the field for 90% of magnetic saturation, HS,90% , deter- sion mode both on the energy-dispersive spectrometer D 11 mined from the hysteresis curves of the various samples in- of the DCI storage ring (L2 and L3 edges of Ce and La and vestigated. From the shape of these curves magnetic cou- on the SU 22 beam line on the asymmetric wiggler of the pling of the Fe layers across the rare-earth layers can be Super-ACO storage ring (M excluded. Since the Fe-layer thickness is always the same, 4 and M 5 edges of Ce at LURE Orsay, France . For the soft-x-ray experiments (M the observed variations of the magnetic quantities in Fig. 2 4,5 edges it was important to choose a weakly absorbing substrate reflect the influence of the rare-earth sublayers. Indeed, they parylene foil, 1.5 m thick and to keep the multilayer and are closely related to the results from x-ray absorption Sec. III . But the behavior is complex, and here we restrict our- alloy films sufficiently thin in order to have about 50% of selves to draw the attention to a few observations. Note that transmission, as we have indicated above. The XMCD spec- the saturation fields remain below 120 Oe and the coercive tra were recorded in the temperature range between 4.2 K fields never exceed 18 Oe. The magnetization curves of the and room temperature, in magnetic fields up to 5 kOe applied Ce/Fe multilayers reflect the evolution in the Ce layers from along the x-ray propagation direction, at grazing incidence the single-phase structure ( like at low t D 11 or under an angle of 60° SU 22 with respect to the Ce to the two- phase composite structure like at the interface, and like layer planes. In the latter case, some samples were measured in a field of 20 kOe. The fields are high enough to saturate the magnetization. The rate of circular polarization of the light at 0.3 mrad below the positron orbit plane is estimated to be 70% for the measurements at the L2,3 edges, 25% at the M5 edge, and 17% at the M4 edge. The XMCD spectra are obtained by the difference between the absorption spectra recorded for two opposite directions of the magnetic field. Details of the data collection and the experimental setup are described elsewhere.27,28 III. RESULTS A. Isotropic x-ray-absorption spectra 1. Multilayers The ground-state electronic configuration of Ce in bulk elemental form or in systems with transition metals is re- FIG. 3. Normalized x-ray-absorption spectra at the Ce-L3 edge flected in the conventional x-ray-absorption spectra at its L (E 2,3 0 5723 eV of a Ce/La, a Ce/Fe, and a Fe/Ce/La/Ce multilayer and M thickness 4000 Å . Thin lines: phenomenological fit. The curves 4,5 edges which involve the dipole-allowed transitions were displaced vertically. At the bottom, the decomposition of a 2p 5d and 3d 4 f , respectively. Since the M4,5-edge spectrum is shown by the superposition of two Lorentzians line- spectroscopy is based on the transition of an electron into a widths 9 eV, thin lines and two arctan functions dotted lines . See 4 f level, it probes the 4 f configuration in the ground state text. obviously more directly than the L2,3-edge spectroscopy, in 57 4 f AND 5d MAGNETIC MOMENTS IN HIGHLY . . . 2179 FIG. 4. Average occupation number of the Ce-4 f state, nf , and average linewidth W resulting from the phenomenological analysis of the L2,3 absorption edges obtained for Ce/Fe multilayers as a function of the Ce-layer thickness tCe . which it is involved only indirectly as a result of final state effects.29 Traditionally, this latter spectroscopy has been FIG. 5. Normalized x-ray-absorption spectra at the Ce-L more frequently applied, and here we first focus on the study 3 edge of the multilayers xÅLa/10ÅCe/xÅLa/30ÅFe n for several La- of the L2 and L3 edges. In Fig. 3 we display, as an example, layer thicknesses total thickness 4000 Å . Curves are vertically the L3 near-edge spectra of Ce in a Ce/La multilayer, in a displaced. Thin lines: phenomenological fit see text . Ce/Fe multilayer recently investigated,13 and in a Ce/Fe multilayer with a La probe layer. Obviously, the replacement the conduction-band states.31 He found that the L2,3 near- of La by Fe induces a profound modification of the absorp- edge spectra of Ce yield a reliable image of its ground-state tion spectrum: while that of the Ce/La multilayers shows the configuration. Hence the phenomenological approach to the ``white-line'' profile of the Ce- phase, those of the two L2,3 spectra, even if it is oversimplified, should provide a a multilayers with Fe exhibit a double-peak structure which satisfactory measure of the 4 f occupancy nf in the ground indicates the presence of Ce with an -phase-like electronic state, at least in the Ce- phase. However, recent experi- configuration. It should be noted that the intercalation of the ments strongly suggest that this L2,3 spectroscopy cannot La probe layer into the Ce sublayers does not visibly modify yield the correct value of nf if nf approaches 1.32 their absorption spectrum. Unfortunately, a rigorous interpre- In Fig. 4 we report, for the previously investigated Ce/Fe tation of the measured Ce-L2,3-edge spectra is a difficult task. multilayers, the variation of the average Ce 4 f -state occu- In the photoabsorption process the 4 f electron configuration pancy nf and of the average linewidth W, which is correlated interacts strongly with the 2p core hole. This leads to a re- with the 5d-bandwidth, with the thickness of the Ce sublay- ordering in energy of the 4 f levels. As a consequence, dif- ers, tCe . These data were not reported explicitly in the pre- ferent 4 f configurations in the ground state manifest them- vious paper.13 Above tCe 20 Å , there is a continuous in- selves as separate structures in the L2,3-edge profiles. Hence crease of nf with increasing tCe , as well as a leveling off in these profiles, even though they are determined by final-state W after an initial decrease. For tCe 20 Å, nf adopts the effects, are closely related to the initial 4 f configuration. In a minimum value reached by -Ce metal exposed to an exter- phenomenological approach,29 widely used by experimental- nal pressure;29,33 hence Ce is in a phase with an -like elec- ists, the configuration of the electronic ground state of Ce, tronic configuration. As we outlined previously,13 the ob- characterized by the ``effective occupation number of the served variation of the L2,3 absorption spectra with thickness Ce-4 f states'' nf 1, is deduced from a deconvolution of tCe reflects the evolution in the Ce layers from this -like the L2,3-edge spectra: The two-line structure related to phase to a two-phase structure with -like Ce at the inter- -phase-like Ce systems is interpreted as a superposition of faces with Fe up to 17 Å and -like Ce in the center of two white lines associated with the final states 2p54 f 15dn the Ce layers. As stated, a possible driving mechanism for and 2p54 f 05dn 1. The 4 f ground-state occupancy nf then is the formation of -phase-like Ce in the multilayers is pro- simply obtained from the relative intensities of the two white vided by stress created at the interfaces during the growth lines; their width is proportional to the width of the 5d band. process, due to the considerable mismatch between Ce and It is evident that this interpretation cannot be rigorously true. Fe. In fact, when the mismatch is negligible at the interfaces, More elaborate models are required to interpret the spectra. Ce is phase like as in the Ce/La multilayers Fig. 3 . In fact, Kotani and co-workers showed30 that a theoretical Figure 5 displays the absorption spectra near the L3 edge description of the Ce-L2,3 absorption process must involve of the Ce probe layer inserted into the La/Fe multilayers. The the interaction of the photoelectron and the electronic system double-peak structure which is clearly resolved even for the in the final state. Malterre extended their calculations by tak- highest La-layer thickness tLa 20 Å indicates that in each ing into account the hybridization of the photoelectron and sample Ce is in an -phase-like electronic configuration in 2180 M. AREND et al. 57 FIG. 6. Ce-4 f -state occupancy nf and average linewidth W resulting from the phenomenological analysis of the L3 absorption edges in the probe layers of the multilayers xÅLa/10ÅCe/ xÅLa/30ÅFe n as a function of the La-layer thickness tLa . these structures, presumably due to strain generated at the La/Fe interfaces. We can notice a discontinuous change in the 4 f occupancy nf for tLa between 10 and 12 Å . But, remarkably, the average linewidth W remains at the same high level near 10 eV Fig. 6 in contrast to Fig. 4, where it decreases considerably with the appearance of the two-phase structure in the Ce sublayers. This reveals clearly that the electronic structure of Ce remains like in the entire region covered by tLa . In contrast to the nonintercalated Ce/Fe mul- tilayers Fig. 4 , the variation of nf in Fig. 6 refers to a single Ce phase even though the overall variation nominally covers nearly the same range of values as in Fig. 4. This is not surprising since the thickness of the Ce probe layer is only 10 Å, which apparently is too low to permit the formation of a two-phase structure. These results are corroborated by the evolution of the iso- tropic Ce-M4,5 (3d 4f) absorption spectra reported in Fig. 7 a . For comparison, we also report the absorption spectrum at the M4 and M5 edges of La for one of the intercalated multilayers. In the case of Ce, both the M4 and M5 edges are characterized mainly by a two-peak structure which corre- sponds to the final states 3d94 f 2 main contribution and 3d94 f 1 satellite contribution at the high photon-energy side . They are directly related to the initial 3d104 f 1 and 3d104 f 0 configurations, since the absorption process in- volves the transition into the 4 f configuration. When com- pared to the spectrum of a typical -phase-like Ce system,34 FIG. 7. a X-ray-absorption spectra at the Ce-M4,5 edges of the the fine structure in the M4 and M5 components arising from multilayers 10ÅCe/29ÅFe 12 and xÅLa/10ÅCe/xÅLa/30ÅFe the atomic multiplets is smeared out. These features confirm n and at the La-M4,5 edges of the multilayer 5ÅLa/10ÅCe/5Å the -like character of Ce for all values of t La/30ÅFe 8 upper curve . The spectra were arbitrarily normal- La . The theoret- ical description of the M ized to unity at the M 4,5 absorption thresholds in highly 5 edge; they have been vertically displaced. correlated Ce systems appears to be much more clear than f 0: satellite related to the initial Ce configuration 3d104 f 0. b that of the L Occupation number of the Ce-4 f states, n 2,3 edges.35,36 Thus it is possible to adjust the f , deduced from the sat- experimental n ellite peak f 0 near the Ce-M f values, extracted from the M 4,5 spectra, in 5 edge of the spectra in a as described order to determine the correct value of n in the text. f in the ground state. For the case of strong hybridization (4 f bandwidth typically in the 2-eV range , we obtain the estimation of nf presented An important conclusion we can draw from the x-ray- in Fig. 7 b . The numbers are close to the ones resulting from absorption results is that they rule out noticeable diffusion at the L2,3-edge profiles Fig. 6 ; in particular, the discontinu- the interfaces between the Ce and La layers sandwiched in ous increase at a La-layer thickness between 10 and 12 Å is the multilayer structures. In fact, alloying of La and Ce hin reproduced. ders the -to- -phase transition of the latter element.37 A 57 4 f AND 5d MAGNETIC MOMENTS IN HIGHLY . . . 2181 simple argument for this observation derives from the differ- ence in the atomic volumes they are very similar for La and Ce in the phase, but quite dissimilar ( 17% in the phase and the minimization of strain energy. The isotro- pic x-ray-absorption spectra demonstrate that in the multilay- ers xÅLa/10ÅCe/xÅLa/30ÅFe] n and xÅCe/10ÅLa/x ÅCe/30ÅFe n Ce is in the -phase-like electronic con- figuration. Hence a perceptible mixing of this element with La at their interfaces can be excluded. In fact, Ce-La alloy formation and hence the appearance of -like Ce would particularly affect, for instance, the absorption spectra at low La-layer thicknesses tLa , but there the 4f occupation on Ce, nf , Figs. 6 and 7 b is close to the minimum value reached by -Ce metal29,33 and the 5d bandwidth is large and con- stant Fig. 6 . Appreciable diffusion at the Ce-La interfaces can also be ruled out on the basis of the hyperfine-field dis- tributions derived from the Mo¨ssbauer spectra Fig. 1 : the insertion of the probe layers essentially preserves the char- acteristic differences of the nonintercalated Ce/Fe and La/Fe multilayers. 2. Alloy films The isotropic L2,3- and M4,5-edge absorption spectra of Ce in the alloy films CexFe1 x reveal the double-peak structure and 4 f 0 satellites, respectively, which are important charac- teristics of an -like electronic structure. Figure 8 a shows that the M4,5-edge spectra are qualitatively very similar to the multilayer spectra displayed in Fig. 7 a . The amplitude of the 4 f 0 satellite decreases with increasing Ce concentra- tion x, which signals a progressive increase of the 4 f occu- pancy nf Fig. 8 b . As can be seen, a corresponding varia- tion nf(x) results from the analysis of the L2-edge absorption spectrum, even though the absolute values are somewhat higher. But the overall change is rather small; even for the alloy most concentrated in Ce, nf is not raised significantly. However, the XMCD spectra reveal a drastic variation of the 4 f magnetic moment in the ground state with FIG. 8. a X-ray-absorption spectra at the Ce-M the Ce concentration x see below . 4,5 edges of the alloy films CexFe1 x . The spectra were arbitrarily normalized to unity at the M5 edge; they have been vertically displaced. f0: sat- B. X-ray magnetic circular dichroism spectra ellite related to the initial Ce configuration 3d104 f 0. b Occupation number of the Ce-4 f states, nf , in the alloys CexFe1 x deduced 1. Multilayers from the satellite peak f 0 near the Ce-M5 edge of the spectra in a An important result of the previous investigation is that, circles and from the analysis of the isotropic L2-edge spectra in the Ce/Fe and La/Fe multilayers, the Ce and La atoms in squares . the interfacial region carry an ordered magnetic moment at room temperature, which is induced by Fe.13,14 Magnetic po- ``spacer layers'' of varying thickness which are in direct larization affects the 5d states of both lanthanides, and the contact with Fe. As representative examples of the spectra 4 f states of Ce. For both elements, the 5d moment is a spin measured at the L2,3 edges of the probe-layer elements at moment which is oriented antiparallel to the Fe moment. The room temperature, we display in Figs. 9 a and 9 b the L2 average values of the integrated dichroic L2 signals, normal- spectra of La, and the L3 spectra of Ce, as well as one ex- ized to the edge, are very close for Ce and La, but there is ample of a Ce-L2 spectrum. Obviously, the 5d states of both evidence that the moments are distributed differently over probe layers, La and Ce, are magnetically polarized by Fe. the thickness of the Ce or La layers in their growth direction For Ce in La/Ce/La/Fe, the XMCD signal consists of two see below . contributions, as the isotropic absorption signal Fig. 5 , with The magnetic polarization in these two lanthanides, roughly the same splitting of 10 eV. Magnetic polarization near the interfaces with Fe, is probed by XMCD experiments extends considerably into the spacer layers. Figure 10 shows performed on multilayers with the structural periods the variation of the integrated XMCD signal at the L2 edge xÅCe/10ÅLa/xÅCe/30ÅFe] and xÅLa/10ÅCe/xÅLa/ of the Ce and La probe- and spacer-layer atoms with the 30ÅFe . The XMCD signals were recorded both on the in- thickness of the spacers, tspacer . Two features are notewor- tercalated 10-Å-thick La or Ce probe layers and on the thy: First, the signal on the Ce probe layer in between the La 2182 M. AREND et al. 57 FIG. 10. Integrated normalized XMCD signal at the L2 edge of Ce and La probe and spacer layers in the multilayers xÅLa/ 10ÅCe/xÅLa/30ÅFe n and xÅCe/10ÅLa/xÅCe/30ÅFe n as a function of the La- or Ce-layer thickness spacers measured at room temperature. Second, the amplitude of the XMCD signal on the -like Ce probe layer, after an initial decrease at low distances from the interface by a factor of 2, levels off to a nearly constant value where the discontinuous change of the 4 f -state occu- pancy nf occurs Fig. 6 . However, the magnetic polarization of the 5d states in -like Ce varies continuously, though the nf change and the shape of the XMCD signals is not visibly modified. From the signs of the XMCD signals at the two L edges positive for the L3 and negative for the L2 edge it can be concluded that the ordered 5d moment is antiparallel to the Fe moment for both probe layers, as in the spacer layers and in the lanthanide layers of the pure Ce/Fe and La/Fe systems.13 Furthermore, the branching ratio of the in- tegrated L2,3-edge XMCD intensities, I(L2)/I(L3), is very close to 1 both on the probe- and spacer-layer lanthanides see, for example, Fig. 9 b . This shows once more that the 4 f electrons of Ce remain delocalized for all La-layer thick- nesses, and that the sum rules16,17 can be applied. It implies that the 5d orbital moment is vanishingly small in these structures, i.e., Lz 0. Applying the second sum rule17 to Ce in xÅLa/10ÅCe/xÅLa/30ÅFe under the assumption that the magnetic dipole term Tz can be set to zero, we estimate the average 5d spin moment to be on the order of 0.13 B for x 0 and of 0.06 B for x 7 Å . The absolute values of the integrated L2,3 XMCD signals differ consider- ably in the spacer and probe layers in the two multilayer structures. This is emphasized in Figs. 11 a and 11 b where FIG. 9. Normalized XMCD spectra a at the La-L2 edge (E0 the integrated XMCD signals have been distributed, for the 5891 eV in the multilayers xÅCe/10ÅLa/xÅCe/30ÅFe n and sake of comparison, uniformly over the individual Ce and La b at the Ce-L3 edge (E0 5723 eV in the multilayers xÅLa/ layers. It is obvious that this simplistic illustration of the data 10ÅCe/xÅLa/30ÅFe n at room temperature. Curves vertically displaced. Lowest curve in b : normalized Ce-L does not represent the real 5d magnetic profile in the multi- 2-edge spectrum (E layers, but it permits an immediate overview and reveals 0 6164 eV for a 12-Å-thick La spacer. interesting features. It appears that the magnetic polarizabil- layers decays much more slowly than that on the La probe ity of the 5d electron states is higher in Ce than in La, Ce layer in between the Ce layers. This is also a strong indica- may be in direct contact with Fe or not. It is tempting to tion that the 5d magnetic moment i.e., the polarization on ``reconstruct'' the average integrated XMCD signals of the the -like Ce atoms remains important even at large dis- pure multilayers Ce/Fe and La/Fe by adding up the areas in tances from the Fe interfaces. Conversely, it appears that the Fig. 11 either on the Ce spacer layers in Ce/La/Ce/Fe and the 5d magnetic polarization vanishes rapidly on La with in- Ce probe in La/Ce/La/Fe or on the La layers correspondingly creasing distance from the interface: separating La from Fe in the other system see Fig. 11 a . The resulting values are by a 5-Å-thick Ce spacer or inserting the 10-Å-thick Ce displayed in Fig. 12 as a function of the inverse rare-earth- probe layer into La reduces the signal on La dramatically. layer thickness, together with the data for the nonintercalated 57 4 f AND 5d MAGNETIC MOMENTS IN HIGHLY . . . 2183 FIG. 11. Integrated normalized XMCD signals at the Ce- and La-L2 edges at room temperature distributed uniformly over the individual rare-earth layers in the multilayers Ce/La/Ce/Fe a and La/Ce/La/Fe b . Below in a : ``reconstruction'' of the Ce and La moments as described in the text. multilayers investigated previously.13 These average values direct measurements. The behavior of Ce is remarkable: a are very close for the pure systems. Since in pure Ce/Fe 10-Å-thick Ce layer inserted into La presents almost the -phase-like Ce develops in the center of the Ce layers when same Fe-induced 5d moment as if it was inserted into Ce their thickness passes 20 Å which is not polarized on the see Fig. 11 a , for example . On the other hand, La behaves 5d states,14 we only show the data up to that thickness for quite differently. this system. In each case, the signals obey a 1/tCe,La depen- One of the most remarkable results of the present inves- dence. While the reconstructed Ce/Fe data somewhat over- tigation is contained in Fig. 13, where the XMCD spectrum estimate the ones directly measured, the reconstructed La/Fe at the M4 and M5 edges of Ce at 77 K obtained for the values are distinctly smaller than the ones resulting from the multilayer 5ÅLa/10ÅCe/5ÅLa/30ÅFe 8 is compared to that of the multilayer 10ÅCe/30ÅFe 12 measured FIG. 12. Integrated normalized XMCD signal at the L2 edge of La and Ce in the La/Fe and Ce/Fe multilayers Fe-layer thickness FIG. 13. XMCD spectra at the Ce-M4,5 edges of the multilayers 29 Å at room temperature and reconstructed signals see text as a 10ÅCe/29ÅFe 12 and 5ÅLa/10ÅCe/5ÅLa/30ÅFe 8 mea- function of the inverse La- and Ce-layer thicknesses, 1/tLa and sured at 77 K. The spectra were normalized to 100% circular po- 1/tCe , respectively. larization rate, and have been vertically displaced. 2184 M. AREND et al. 57 concentration and on the temperature. This implies that the dichroic signal reflects ground-state properties of the Ce ions. The XMCD spectra at the M4,5 edges of Ce recorded for the alloys CexFe1 x Fig. 14 reveal the presence of an or- dered 4 f magnetic moment on Ce for the concentrations x 0.19 and 0.27, but there is no signal to within 0.2%, i.e., the amplitude of the noise for x 0.34, and hence no detect- able 4 f moment on Ce in this alloy. The difference between the shape of the dichroic spectra for the cases x 0.19 and 0.27, respectively, reflects simply the change in the hybrid- ization between the 4 f and the conduction electrons. For instance, we notice the presence of the negative peak at the M5 edge for x 0.19, as in the multilayer 10 Å Ce/30 Å Fe 12 Fig. 13 , and the shift of the M4 edge to lower photon energies as the Ce concentration increases from x 0.19­0.27. The most remarkable result in Fig. 14 is the disappearance of the ordered 4 f moment on Ce as x grows to 0.34 the amplitude of the XMCD signal, if any, must be at least a factor of 10 smaller than for x 0.27). Note that at this Ce concentration the alloy is still ferromagnetic, with a Curie temperature of 120 K, and the Ce-5d states carry an ordered magnetic moment Fig. 14, inset . This demonstrates again that the Fe-3d states are differently effective in gener- ating an ordered magnetic moment on the Ce-4 f and -5d states. FIG. 14. XMCD spectra at the Ce-M4,5 edges of three different alloy films CexFe1 x measured at 30 K. The spectra were normal- ized to 100% circular polarization rate and have been vertically IV. ON THE FE-INDUCED MAGNETIC POLARIZATION displaced. Inset: normalized XMCD spectra at the Ce-L2 edge mea- OF Ce AND La sured at 11 K. The experimental results presented in Sec. III provide in- previously.14 In both structures Ce is like. While, obvi- teresting information on the ground-state magnetic polariza- ously, the 4 f electrons of Ce in the nonintercalated Ce/Fe tion of Ce and La in multilayers with Fe, and of Ce in Ce-Fe multilayer top spectrum carry an ordered magnetic mo- alloy films. They supplement the previous investigations13,14 ment, a 5-Å-thick La spacer layer completely within 0.2% which have laid open the highly correlated nature of these suppresses the XMCD signal on the Ce layer bottom spec- systems. The most important aspects are the following. trum . Note that in contrast the XMCD amplitude at the L i In the Ce/Fe multilayers, a magnetic polarization of the 2,3 edges of Ce, which probes the magnetic polarization of the 5d states in -phase-like Ce can exist without a magnetic 5d states, is only reduced by a factor of about 2 by a 5-Å- polarization of the 4 f states. 4 f polarization requires a direct thick La spacer compare Fig. 5 in Ref. 12 and Fig. 9 b . contact of the Ce and Fe atoms at the interface Fig. 13 . There is no doubt that a comparative reduction in the M Similarly, in the Ce-Fe alloys, where Ce is like, the polar- 4,5 spectra would have to lead to an observable signal. This ization of the 4 f states gets lost if the number of Fe neigh- proves unambiguously that -like Ce has to be in direct bors becomes too small Fig. 14 . contact with Fe in order to acquire a magnetic polarization ii There is a profound difference between -like Ce and of its 4 f states. In contrast, the Ce-5d states are magnetically La in the multilayers with respect to the Fe-induced 5d mag- polarized across 20 Å of La Figs. 9 b and 10 . Thus the netism. The 5d moment on La decreases dramatically when driving mechanism for the origin of the 4 f moment is the the material is separated from Fe by a very thin Ce spacer direct 4 f -3d hybridization and not the 4 f -5d exchange in- layer. In contrast, the 5d moment on -like Ce remains im- teraction. This will be further addressed in Sec. IV below. portant even at large distances from the Fe interfaces Fig. 10 .It is clear that at the direct interfaces of the multilayers or 2. Alloy films locally in the alloys hybridization occurs of the 3d states of For all of the -like Ce alloys CexFe1 x , XMCD spectra Fe with the itinerant 5d and 4 f states of Ce and La. This measured at the L2,3 edges of Ce show the existence of an induces an ordered magnetic moment on the 5d and 4 f states ordered 5d magnetic moment below the Curie temperature. of the lanthanides, similarly to what was discussed for the These spectra Fig. 14, inset present the same double-peak case of their compounds with the ferromagnetic transition structure as the isotropic absorption spectra, and their signs metals by the Uppsala group.9,11,12,38 In the multilayers one reveal again the antiparallel orientation of the Ce-5d and Fe can expect, a priori, that the hybridization-induced magnetic moments. We have carefully checked that the integrated polarization is of short range. Indeed the experiments show XMCD signal at the L2 edge follows the functional depen- that magnetic order on the -like Ce-4 f states does not ex- dence of the macroscopic saturation magnetization on the Ce tend beyond the immediate interface and must be linked to a 57 4 f AND 5d MAGNETIC MOMENTS IN HIGHLY . . . 2185 direct overlap of the Ce-4 f and Fe-3d orbitals; it is sup- factor of 3 Figs. 10 and 11 b ... must be due to a different pressed if Ce is separated from Fe by a 5-Å-thick La layer mechanism. A plausible mechanism could be based on com- Fig. 13 . This is related to the behavior of the alloy films pressive stress on La which is generated at the interface with Ce xFe1 x . The loss of the 4f moment on Ce between -like Ce due to the considerable mismatch of the inter- x 0.27 and 0.34 Fig. 14 indicates that 4 f magnetism in atomic distances in the two lanthanides. The consequence is the ground state of an -like Ce atom in these alloys is an increase of the La-5d-bandwidth, hence a decrease of the directly controlled by the number of Fe atoms surrounding it. state density at the Fermi level. This translates into a reduc- Note that according to the L2,3 XMCD spectra, 5d magne- tion of the Stoner parameter and of the 5d polarization. It is tism is less sensitive to the effects of the local environment in these alloys. Similarly to what happens in the multilayers, clear then that the ``reconstruction'' of the average 5d mo- it survives the loss of the ordered 4 f moment. Since in the ment on La from these spacer layer signals, described in Sec. compound CeFe III B 1, leads to a value which is too small as compared to 2 the Ce atoms clearly carry an ordered 4 f moment,6­9 it may be conjectured that the local Ce-Fe envi- the moment directly measured in La/Fe Fig. 12 . Moreover, ronment and hence the electronic structure must be distinctly the same interfacial stress in La/Ce/La/Fe may be also re- different in the Laves-phase intermetallic alloy and in an sponsible for the Ce-induced enhancement of the hyperfine- amorphous alloy with the same composition. In fact, differ- field distribution on Fe, p(Bhf), at low values of Bhf Fig. ences in the local structure are also manifest in the different 1 d , and for the decrease of the average value of Bhf as mass densities23 and in the different Curie temperatures of compared to the nonintercalated system La/Fe. In fact here the two systems. For the alloy, Tc is about 100 K lower than the maximum in p(Bhf) at low Bhf cannot be due to hybrid- for the compound for CeFe2, Tc 230 K . The local struc- ization between the Fe-3d and the Ce-4 f states as in the ture of the amorphous Ce-Fe alloys was not determined in binary Ce/Fe system, since the absence of an ordered 4 f the course of our work. Unfortunately, the values of the par- moment on Ce in La/Ce/La/Fe mirrors the absence of a 3d- tial coordination numbers and interatomic distances available 4 f hybridization. in the literature for this material are contradictory.39 Figure 10 shows two regimes for the magnetic polariza- The magnetic polarization of the itinerant Ce-4 f states in tion of the 5d states in the -like Ce layer in La/Ce/La/Fe. direct interface with Fe in the multilayers must be related to These regimes are separated by the discontinuous transition the pronounced maximum in the distribution of the magnetic in the Ce-4 f -state occupancy nf occurring at La-layer thick- hyperfine fields at 50 kG in the Fe sublayers of the Ce/Fe nesses tLa between 10 and 12 Å Figs. 6 and 7 b : the inte- Ref. 24 and Ce/La/Ce/Fe systems Fig. 1 . This feature is grated XMCD signal on Ce decreases progressively with in- associated with the interface component of the Mo¨ssbauer creasing distance from the Fe interface up to this transition spectra. In fact, it must be ascribed to the hybridization be- regime I and then remains essentially constant regime II , tween the 3d states of Fe with the 4 f states of Ce, since it is whatever the value of tLa is. The XMCD signal varies con- absent at the La/Fe interface. We have previously argued24 tinuously through the transition in nf , and its shape is not that the effect involves Fe atoms within a 1 atomic layer at sensitive to the actual value. It is noteworthy that this two- the direct interface with Ce. Magnetically, the direct inter- regime behavior in the Ce-5d polarization is reflected in the face appears as an ``alloy'' with a well-defined mean local magnetization reversal in La/Ce/La/Fe as a function of tLa environment. Due to the 3d-4 f hybridization, it must be sta- Fig. 2 : after an initial variation Hc , HS,90% , and MR level bilized by strong Ce-Fe bonding, similarly as in CeFe 2.9 off to an approximately constant value. The persistence of Note that this ``interface alloy'' is magnetically ordered at the considerable almost constant 5d-state polarization in room temperature with an ordered 4 f moment on the Ce-4 f -like Ce at large distances from the interface with Fe in states and a rather low magnetic hyperfine field on the Fe regime II is quite surprising. Note that in this regime the 5d sites centered around 50 kG . In contrast, the magnetic polarization of the La spacer layers is very small. It can be ordering temperatures of all known Ce-Fe compounds excluded that this magnetic polarization on Ce is induced CeFe2, Ce2Fe17) and alloys are distinctly below room tem- indirectly by the spin-split 3d states of Fe via a polarization perature. of the (s,p) conduction electrons in the La spacer layers by 5d magnetism in the multilayers appears to be much more a Ruderman-Kittel-Kasuya-Yosida-type mechanism. Such a complex than 4 f magnetism, and in some features quite sur- mechanism would lead to a decay of the XMCD signal, to- prising. We first address the La/Fe system. The 5d states of gether with an oscillatory variation, which is not observed. La in this structure are magnetically polarized essentially at Furthermore, it would give rise to a coupling between the the interface only, similarly as the Ce-4 f states in the Ce/Fe magnetizations of neighboring Fe layers which can be ex- multilayers, by direct hybridization with the spin-split 3d cluded by the MOKE studies. The constancy of the signal on states of Fe. This can be concluded from the dramatic reduc- Ce even up to a distance of 20 Å from the interface with Fe tion of the XMCD signal on La by only a very thin Ce spacer suggests strongly that magnetic order on the Ce-5d states in layer Fig. 10 . Indeed, the 5d moment of La decays rapidly the layered structure is an intrinsic property of the ground with its separation from Fe. It is reduced by a factor of 10 at state of -like Ce in regime II. This intriguing hypothesis a distance of 5 Å only. A similar conclusion was drawn by receives support from two additional observations: First, the Se ve40 from the results of x-ray resonant magnetic scattering same two-regime behavior of the XMCD signal as in Fig. 10 at the L2 edge of La in these multilayers. On the other hand, is observed in the La-free multilayers Ce/Fe if this signal is the strong reduction of the XMCD signal on La in La/Fe by referred to the -like-Ce part only.13 Second, the integrated the insertion of the Ce probe layer in the structure 5ÅLa/ signal on the Ce probe layer in La/Ce/La/Fe fits almost ex- 10ÅCe/5ÅLa/30ÅFe , for example, we have a reduction by a actly into the ``gap'' in the Ce spacer layers in the system 2186 M. AREND et al. 57 Ce/La/Ce/Fe if the La-layer contribution is omitted Fig. pears on the Ce-4 f states due to exchange with the spin-split 11 a . This indicates that the magnetic polarization of the 5d states if the 4 f hybridization is reduced and the 4 f states Ce probe layer is not mediated by any specific property of become more localized.32 the La spacer but is a characteristic of Ce itself, in a specific state with a 4 f -state occupation nf 0.9 Fig. 6 . This state is V. CONCLUSION reached after a discontinuous raise of nf near tLa 10 Å, which presumably is connected with a certain strain relax- We have demonstrated that XMCD offers the unique pos- ation. The electronic configuration of Ce is still phase like sibility to study independently ordered 4 f and 5d magnetism Fig. 5 but close to the transition into a -like phase, i.e., the in the ground state of -phase-like Ce in highly correlated 4 f states are on the verge of localization. This can be con- multilayers and alloys with Fe. In the multilayers, La spacer cluded from a comparison of the n layers intercalated between Ce and Fe reveal the profound f value in this regime and the pressure-valence phase diagram of Ce metal.33 Min differences between the 4 f and 5d states with respect to their et al.41 showed in a calculation that strong spin correlations magnetic polarization. They enlighten also the differences exist in -Ce. At the borderline of the transition to the for 5d magnetism in Ce and La. The most important result is phase, the Stoner enhancement factor is large comparable to the observation that -phase-like Ce is very close to an in- that of Pd , and -Ce is very close a magnetic instability; the stability toward intrinsic ferromagnetic order within the 5d energy difference between the paramagnetic and ferromag- states, if the 4 f -state occupation is at the borderline of the netic phases is very small. Hence it is conceivable that subtle transition to the -like phase. Furthermore, the reversal of electronic or strain effects in the multilayer environment the macroscopic magnetization of the composite structures is might induce long-range magnetic order in the -phase-like closely related to the special electronic and magnetic prop- Ce sublayers. Let us recall that this applies only to the 5d erties of the Ce and La sublayers resulting from the isotropic states since magnetic order on the itinerant 4 f states could and dichroic x-ray-absorption spectra. This is remarkable not be detected. The observation of an -phase-like isotropic since the absolute values of the Fe-induced 4 f and/or 5d L magnetic moments are only on the order of 0.1 B . 2,3 and M 4,5 absorption spectrum is an indication of an im- portant hybridization between the 4 f and (5d,6s) conduc- tion states of Ce. Apparently, this hybridization wins over ACKNOWLEDGMENTS the intraatomic exchange interaction between the 4 f and po- The work was supported by the Deutsche Forschungsge- larized 5d states and stabilizes a nonmagnetic 4 f ground meinschaft within SFB 345. M.A. benefited from a grant state. Indeed, preliminary experiments on these layered Ce- provided by the DAAD Program HSP II . The authors from based systems have shown that the relative weight of these Go¨ttingen are grateful for the kind hospitality of the Orsay competing mechanisms can be reversed: Magnetic order ap- group. *Corresponding author. Electronic adress: wfelsch@gwdg.de Materials, edited by K. H. J. 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