J. Phys. Chem. C 2008, 112, 5123-5128
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Linearly Polarized X-ray Absorption Investigation of Ultrathin NiOx/Pd(100) Films S. Agnoli,† F. Sedona,† P. Finetti,† G. A. Rizzi,† G. Granozzi,*,† F. Bondino,‡ M. Zacchigna,‡ and F. Parmigiani‡ Dipartimento di Scienze Chimiche and INFM Research Unit, UniVersita` di PadoVa, PADOVA, Italy, and Laboratorio Nazionale TASC INFM-CNR, BasoVizza-Trieste, Italy ReceiVed: December 11, 2007; In Final Form: January 22, 2008
Linearly polarized soft X-ray absorption spectroscopy experiments both on Ni L2,3 and O K edges have been carried out on NiO ultrathin films on Pd(100), from the c(4 × 2) defective monolayer up to fully relaxed films. In the defective monolayer a dichroic effect on the Ni L2,3 edge has been detected, and it has been associated to anisotropic structural effects (e.g., local crystal field symmetry) due to the spatial confinement. This produces a change in the energy sequence of the Ni 3d levels resulting in different transition probabilities for the various levels at different polarization. A support to this interpretation has been also obtained by theoretical computations. In the case of thicker films, a clear dichroic effect, reversed with respect to the one observed for a bulk like NiO(100) surface, has been observed for the L2 white line at all the explored thicknesses. This has been associated to a different spin structure of the ultrathin films, where the domains are preferentially aligned to the surface plane instead of being perpendicular to it. However, we have not observed the inversion of dichroism even for very thick fully relaxed films. The dependence of the dichroic effect on the preparation procedure of the films has been also explored. For the thicker films the dichroic effect is independent from the actual oxygen pressure used during the growth, while remarkable differences are found in the case of the thinner films.
1. Introduction The study of ultrathin oxide films, and in particular of their magnetic properties, has gained a great amount of interest in recent years due to their great technological potential in many applicative fields such as data recording media1 and spintronics.2 Within this context, NiO has been the object of several investigations designed to gain a better understanding of general topics such as antiferromagnetism (AF), giant magneto resistance (GMR), and electron correlation in narrow band systems. Moreover, NiO possesses some advantages for magnetoelectronic applications, e.g., its insulating properties, very high corrosion resistance, low sensitivity to composition, and low reset temperature. Bulk NiO is a type-II face-centered cubic (fcc) antiferromagnet (AF-2): the magnetocrystalline anisotropy favors ferromagnetic (FM) sheets in correspondence of the {111} planes, which are antiferromagnetically stacked along the 〈111〉 directions. Hence by consideration of NiO crystal symmetry, 24 possible domains come out.3 To characterize magnetic nanostructures, linear dichroism (LD) in soft X-ray absorption spectroscopy (XAS) has been recently widely used; because of its elemental, chemical, and structural sensitivities over a wide range of spatial dimensions (lateral and in-depth), LD is often capable of discriminating the size and orientation of the magnetic moments. This technique uses a synchrotron radiation (SR) linearly polarized (LP) beam and detects the difference in the absorption as a function of polarization. However, other phenomena can be a source of the dichroic behavior, such as anisotropic structural effects (e.g., local crystal field symmetry), and very often it is hard to * To whom correspondence should be addressed. † Universita ` di Padova. ‡ Laboratorio Nazionale TASC INFM-CNR.
discriminate between the magnetic and the structural factors. For bulk NiO, because of the presence of the above-described domains, no LD could be observed in principle; however, the presence of an interface, which reduces the cubic symmetry stabilizing certain domain relative to others, determines an observable LD effect. Actually, a LD effect has been observed in NiO cleaved crystals.4 In addition, it has been shown both theoretically5 and experimentally6 that, by use of LP radiation, a LD signal can be observed for antiferromagnetically ordered films, which is proportional to (where M indicates the magnetic moment). As a matter of fact, exploiting LD at the Ni L2 edge, the structure of magnetic domains on the NiO(100) surface in several NiO films have been spatially resolved,7,8 and the magnetic properties of the interface between NiO(100) and different magnetic overlayers have been investigated.9-11 Some papers have been focused on epitaxial NiO ultrathin films; Alders et al.12 have explored the magnetic properties of the NiO(100)/MgO(100), showing a strong dependence of the Ne´el temperature (TN) and of the spin structure from the film thickness. Ag(100) supported systems have been studied by different groups,13,14 focusing in particular on the effect of strain and finite thickness on the antiferromagnetic order. A paper where the role of the strain on the magnetic properties of CoO films grown on different substrates has been recently reported.15 Recently, we have studied the structural properties of NiO ultrathin films supported on Pd(100). These systems are characterized by a large lattice mismatch. As a consequence of this factor, we have found in the sub-monolayer coverage range a new interface stabilized structure characterized by a c(4 × 2) periodicity with respect to the substrate. Experimental and theoretical investigations have demonstrated that this phase consists of a NiOx(100) strained monolayer (ML) (with x ≈
10.1021/jp711641e CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008
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Figure 1. (a) XAS spectra of the c(4 × 2)-NiOx/Pd(100) ML in the Ni L2,3 region for the two polarizations: we used horizontally polarized radiation (>99%) changing the direction of the incident radiation from normal (θ ) 0°) to grazing (θ ) 60°) angle in order to acquire spectra with in-plane and out-of-plane polarization, as schematically outlined in the inset. The two spectra have been normalized to the maximum of the L3 white line. (b) Simulated Ni L2,3 XAS spectra assuming Ni2+ ions in a C4h environment (see text).
1.33) characterized by a rhombic array of metal vacancies.16,17 For higher coverage (100) oriented stoichiometric films can be obtained. The relevant strain between substrate and overlayer (7.3%) is almost completely released in the first stages of epitaxy (2-4 ML); however, photoelectron diffraction (PED)18 and spot profile analysis low-energy electron diffraction (SPA-LEED)19 measurements have demonstrated that fully relaxed layers are only present for thicknesses higher than 10-12 ML. In this paper we report a LP-XAS study of NiO/Pd(100) films, from the defective c(4 × 2) ML up to fully relaxed films. The main goals of the present paper are to provide further experimental data on films with a varying degree of strain and to explore the role of the preparative route on the actual value of the LD effect. 2. Experimental The experiments have been performed at the Beamline for Advanced diCHroism (BACH, BL 8.2, using a partial yield mode) at the Elettra Synchrotron Light Source at Trieste, Italy.20 The NiO/Pd(100) films have been deposited directly into the analytical ultrahigh vacuum (UHV) chamber. The Pd(100) crystal was cleaned by means of repeated cycles of Ar+ sputtering and annealing at 970 K with a final flash in 1 × 10-6 mbar O2. The c(4 × 2)-NiOx structure was obtained by reactively depositing 0.75 MLE NiO (PO2 ) 10-6 mbar) on Pd(100) at room temperature (RT) and successively annealing the sample at 250 °C in 5 × 10-7 mbar O2.21 The NiO layers were grown by depositing Ni in oxidative conditions using different O2 pressures ranging from 5 × 10-7 mbar O2 to 5 × 10-6 mbar, at 300 °C. The Ni deposition rates from the e-beam evaporator were estimated using a quartz crystal microbalance measurements and were comprised between 0.5 MLE/min and 2 MLE/min; 1 ML equivalent (MLE) is referred to the atom density of the Pd(100) surface and corresponds to 1.3 × 1015 metal atoms/cm2. Thin film samples used for this study were grown and then probed by XAS in the same UHV system. To observe dichroism, the XAS spectra have been recorded at RT with two different acquisition geometries; in both cases
we used horizontally polarized radiation (better than 99%) changing the direction of the incident radiation from normal (θ ) 0°) to grazing (θ ) 60°) angle to acquire spectra with inplane and out-of-plane polarization, as schematically outlined in the inset of Figure 1a. 3. Results and Discussion Before discussing the LP-XAS data of the investigated films, we just briefly mention here the main features of the L2,3 XAS spectrum of bulk NiO.22 Such features are also presented by the XAS spectra of thick NiO/Pd(100) films (see the data reported in section 3.2), which are then assumed to reproduce a bulklike behavior. The electronic transition from the Ni2+ 3d8 ground state to the 2p3d9 multiplets (where 2p indicates a hole in the 2p level while the electron is excited to the partial empty d band) gives rise to the two white lines (L3, with the edge at ca. 853 eV, corresponding to a transition from the core Ni 2p3/2 level, and L2, with the edge at ca. 870 eV corresponding to a transition from the Ni 2p1/2 level). The fine structure of the two white lines is due to their multiplet structure, and the relative intensities are dictated by the optical selection rules. Further lower intensity and higher energy features are found as a consequence of the contribution from 2p3d10L multiplets (where L a hole in the valence band of the ligands, i.e., the oxygen localized band).22 3.1. The c(4 × 2)-NiOx ML. The Ni L2,3 XAS spectrum taken at RT in partial yield mode of the c(4 × 2)-NiOx ML is shown in Figure 1a for the two different polarizations. The XAS main features of the c(4 × 2) ML are rather different from the bulk NiO ones: the fine structure of the L2 and L3 peaks is highly changed with respect to the bulk one and is also strongly polarization dependent (see Figure 1a). Actually, we have detected not only a change in magnitude of the absorption coefficients but also a real change in the peak shape and position: the L3 line of the spectrum taken at grazing incidence (θ ) 60°) is clearly structured in two distinct features, which are separated by ∼0.7 eV (see labels a and b in Figure 1a), and has a broad tail in the higher-energy side; similarly, the L2 line
Investigation of Ultrathin NiOx/Pd(100) Films presents a high-energy shoulder (d), which was absent in the other polarization. In addition, we have not found experimental evidence of satellites due to the contribution from 2p3d10L multiplets. Certainly all these features are to be connected to the peculiar structure of such defective ML, and several factors can be in principle invoked to explain them: e.g., magnetic effects, anisotropic structural effects, spatial confinement, interaction with the metallic substrate, the metallic character of the ML, etc. A starting guideline to unravel such a complex interplay of factors could come from a previous analysis of the Ni 2p photoemission spectrum of the c(4 × 2) ML:23 the main results from this analysis have outlined the role of the spatial confinement and hybridization between the metallic substrate and the c(4 × 2) ML. On the other hand, the metallic character of the c(4 × 2) ML has been assessed by the valence band photoemission study.23 To exclude the magnetic origin of the observed LD effect it is sufficient to remember that the measurements were performed at RT, where the phase should not be magnetically ordered. As a matter of fact, Alders et al.12 have observed that TN goes from 520 K for the bulk to 470, 430, and 295 K for the 20, 10, and 5 ML NiO films on MgO(100), respectively. Thus, a simple extrapolation would suggest that the c(4 × 2)-NiOx ML phase should not be magnetically ordered at RT. This prediction has been supported by Haverkort et al.,24 who demonstrated that for a MgO-capped 1 ML thick NiO(100) film on Ag(100) the ordering temperature must be lower than 80 K. Incidentally, it is worth mentioning that the spin structure of the c(4 × 2)NiOx ML is not easily predictable because of the effect of Ni vacancies (that formally leave two holes in the four surrounding oxygen atoms) and the possible quenching of magnetic moments, due to hybridization with the substrate, are difficult to be determined a priori. Recently, the magnetic state of the c(4 × 2)-NiOx structure has been investigated by Ferrari et al.25 by means of DFT calculations: two different AF and one FM spin structures have been explored, the FM being the most stable; however, their energy separation is very modest (less than 0.050.1 eV) so that at RT all the different configurations would be allowed. Having excluded the effects of magnetic ordering, we are left to associate the dichroic behavior observed in the XAS spectrum of the c(4 × 2)-NiOx ML to anisotropic structural effects connected with the spatial confinement and/or to the peculiar electronic properties due to the ordered defect array. To ascertain the electronic effect of the ordered array of Ni vacancies on the XAS spectrum, we can take advantage of the results of the mentioned DFT calculations by Ferrari et al.25 According to them, the residual Ni ions maintain a 3d8 ground state, and holes are localized on the oxygen band. This fact is in qualitative agreement with the absence in our XAS data of the 2p3d10L multiplets observed in bulk NiO (see also the discussion of the thick film data), whose contribution can be expected to be disfavored by the presence of such holes localized on the oxygen localized band. Passing now to the spatial confinement effects, we have to remember that the system under investigation is formed by an almost coplanar bilayer (Ni and O).16 As a consequence, the crystal field experienced by the Ni ions has certainly a reduced symmetry and intensity with respect to the ions in the bulk. Crystal field effects on LP-XAS have been already discussed by Haverkort et al.24 in the case of a (1 × 1) stoichiometric NiO ML on Ag(100); the authors associated the dichroic effect to a change of the environment of Ni2+(3d8) from octahedral to
J. Phys. Chem. C, Vol. 112, No. 13, 2008 5125 square planar and to the consequent changes in the band envelope resulting in different transition probabilities for the various components at different polarization. The two sets of experimental data, i.e., our data and those of 1 ML of NiO on Ag(100), are somehow similar. However, in our case larger effects are observed, and a well-defined splitting of the L3 white line for the grazing polarization is evident. The observed LD behavior can be interpreted also in our system as a consequence of the different transition probabilities for the various components split by the lower symmetry. A support to this interpretation has been also obtained by a series of computations carried out by using the TT-MULTIPLETS program26 based on the Cowan code.27 In Figure 1b, we report the simulations in the two polarizations for a Ni2+ ions (3d8) in a C4h symmetry. The choice to model the absorber element as Ni2+ ion derives from the results of DFT calculations of the electronic structure of this phase25 that predict a 3d8 electronic configuration for the Ni atoms. Strictly speaking, the c(4 × 2)NiOx structure is slightly distorted from the adopted C4h symmetry and results in a C2V symmetry, but the effects of such distortion can be considered negligible. Moreover, from an experimental point of view, the presence of two domains rotated by 90°, due to the fourfold symmetry of the substrate, precludes the possibility to observe macroscopic differences by changing the polarization within the plane perpendicular to the surface normal (xy plane). To simulate the tetragonal crystal field, we have used three parameter 10Dq, Ds, and Dt, set to 3.5 eV, 0.19 eV, 0.4 eV, respectively. As it can be seen from Figure 1b, we can reproduce quite well the polarization differences of the L3 white line, which is the most distinctive feature of the c(4 × 2) structure. In more detail, it results that the formation of the two peaks in L3 is originated by the lifting of the degeneracy between the dz2 (which can be accessed by z polarized light) and dx2-y2 derived states (predominantly accessed by xy polarized radiation) operated by the tetragonal crystal field potential. As a final comment, we want to underline that the good agreement between theory and experiment has been obtained without the inclusion of an exchange splitting, which means that the observed LD is a pure nonmagnetic effect, exclusively due to the low-symmetry crystal field splitting. 3.2. NiO Ultrathin Films. In Figure 2a, we show the Ni L2,3 XAS spectra (θ ) 60°) of the ultrathin NiO/Pd(100) films as a function of the thickness obtained by depositing metal Ni in 5 × 10-6 mbar O2. The extensive work performed on these systems, as well as in situ measurements (LEED and XPS), indicate that these NiO films are stoichiometric (at least at the level detectable by photoemission) and well ordered with just a minor degree of epitaxial defects such as mosaics.18,19,21 On passing from low to high thicknesses, there are only minor changes, and the fingerprint typical of bulk NiO, present in nuce already for the thinner films, becomes progressively better resolved (see the labels of the highest coverage spectrum and compare them with the literature ones22). The less-defined structure of multiplet splitting observed at lower thicknesses can be imputed to the various different structural environment experienced by the Ni2+ ions in the early stages of epitaxy because of the presence of strained layers.18 A clear LD effect has been observed for the L2 white line at all the thicknesses starting from 4 MLE: in Figure 2b we report as an example the LD effect in the case of a 14 MLE film. The spectra have been normalized to one at the first peak, and, in order to quantify the dichroic effect, we have used the standard anisotropy parameter: LD (%) ) (1 - R260°/R20°) × 100, where R2 is the ratio between the first and second peaks at the L2
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Figure 3. LD (%) of the L2 edge of NiO/Pd(100) ultrathin films as function of thickness for three different deposition conditions.
Figure 2. (a) Ni L2,3 XAS spectra of NiO/Pd(100) ultrathin films (grown at 5 × 10-6 mbar of O2, at 300 °C) as a function of their thickness (θ ) 60°). The labels indicated for the thickest film reproduce those reported in ref 22. (b) Ni L2 XAS data according to the two different polarizations for a 14 MLE NiO/Pd(100) film.
absorption edge. In the case of all the spectra recorded, no matter the actual preparation is used, we have observed a LD effect which is reversed with respect to the one observed for bulklike NiO(100) films.12 One major issue is the nature of the LD effect, magnetic or not. Even if temperature-dependent measurements (not available at the moment) would be needed to prove that the LD is magnetic in nature, i.e., that TN is higher than RT for our films, we propose to interpret the present data as an indication that the spin structure of our ultrathin films is different from the bulk one, i.e., that the S domain is preferentially aligned to the surface plane instead of being perpendicular to it. This interpretation is suggested by reference to the temperaturedependent analysis performed by Alders et al.12 on NiO on MgO(100) films and by the experimental data reported by Finazzi et al.28 on NiO/Fe(100) films, where no significant temperature dependence of the spectra was noticed in the 180300 K range, indicating that the TN of the NiO/Fe films is significantly higher than RT. On the other hand, other RT
measurements in literature have assumed the magnetic origin of the LD effect in similar NiO films.14 The magnetic nature of the observed LD is also supported by the absence of an energy shift in the Ni L3 white line for the two different polarizations: actually, as suggested by Haverkort,24 such a shift would be an indication of crystal field effects, as observed also in our data of the c(4 × 2) ML. Tracking the origin of this change of the magnetic ordering is a very complex topic. A similar phenomenology was observed in the case of MgO-capped NiO films grown on Ag(100).13 In this case the authors have found that strained films (∼3 ML) exhibit reversed LD, while fully relaxed films (∼30 ML) show a dichroism of the same sign of bulk NiO. They explained the results by calculating the classical dipole interaction energy in a case of fcc collinear AF films, treating the strain and the finite thickness of the system as a perturbation.29 The authors have shown that in the case of AF NiO subjected to a uniform tetragonal strain ) (c/a) - 1 (c and a are the lattice parameters along [001] and [010], respectively) the degeneracy between different domains is lifted: compression ( e 0) stabilizes S⊥ domains, while expansion ( g 0) stabilizes S| domains. On the other hand, the effect of finite thickness τ (number of MLs) acts similarly to the well-known shape anisotropy of FM so that the stability boundary between S⊥ and S| is given by the line ) (1.34 - 1.67τ). This simplified model, effective in explaining the mentioned change in the sign of dichroism for MgO-capped NiO/Ag(100) as a function of thickness, fails in the case of our system. Actually, NiO films on Pd(100) totally release strain above 1012 MLE as demonstrated by scanning tunneling microscopy, PED, and SPA-LEED experiments,18,19,21 but we do not observe the reversal of LD even for a 44-MLE film, where the film must be fully relaxed. Moreover, we do not observe for thickness higher than 20 MLE any change in the LD (%) value (constantly pinned to -23%), contrary to the expectations based on the simple model considering only the finite thickness and the strain. Recently, also noncapped NiO/Ag(100) thin films have been investigated, and as in our experiments, even thicker films (72 ML) have shown a reversed dichroism without any recovery of the bulklike behavior.14 In summary, with the current state of
Investigation of Ultrathin NiOx/Pd(100) Films
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Figure 4. Ni L2 XAS spectra of a 6 MLE NiO/Pd(100) film (grown at 10-6 mbar of O2, at 300 °C, and left in UHV for a long period) prior (left) and after (right) desorption of contaminant hydroxyls; the inset shows the O 1s photoemission peak, which demonstrates the removal of hydroxyls after the annealing procedure (dotted line).
the knowledge, there is no simple explanation at hand to interpret the reversed LD behavior of the herein report ultrathin films. However, in the following we want to contribute to the ongoing discussion, by providing further experimental evidence that should be taken into account for developing a consistent interpretation of the LD effect, especially for very low thickness. To understand other possible sources leading to the observed LD, we have explored if different oxygen pressures can have a direct effect. The results are reported in Figure 3, where we show the LD (%) as a function of thickness for different films obtained by depositing Ni in a background oxygen pressure ranging from 5 × 10-7 mbar to 5 × 10-6 mbar. It is worth mentioning that, according to our precedent work, it results that, for pressure higher than 2 × 10-7 mbar O2, NiO ultrathin films are perfectly oxidized (at least as far as photoemission can tell us), and also in the present work, we have crosschecked by XPS the correct stoichiometry of the films. It turns out that the LD (%) is strongly dependent on the deposition conditions in the low thickness regime (films thinner than 15 MLE) and that it reaches a rather constant value, disregarding the actual oxygen pressure, above 20 MLE. In the case of the higher pressure (5 × 10-6 mbar) the amount of LD (%) rapidly decreases (in absolute value) with the thickness (as the strain does), reproducing the same trend observed in the case of the NiO/Ag(100) system, even if the we have found a LD (%) that is two times bigger; on the contrary, for the other two preparations, namely, 5 × 10-7 and 10-6 mbar, the LD (%) has a reversed behavior, increasing in absolute value, from low to high coverage. The reported data as a whole say that relevant differences are found in the low thickness range, where many important phenomena can take place, such as the progressive strain release, crystal field effect due the tetragonal distortion, relevant hybridization with the substrate, and epitaxial defects (mosaics, dislocations, etc.). Among the many factors, it seems reasonable to assess that the strain release and crystal field should have only a minor dependence from the oxygen pressure. On the contrary it is plausible that the presence of chemically different defects or modification of the electronic structure at the interface can be driven by a change in the oxygen chemical potential. Also a
change in the film roughness according to different pressure could be relevant, although unlikely and of minor consequences since the NiO grows, in any case, in a 3D mode because of the large strain. We have also found that the LD (%) of the films is extremely sensitive to the surface composition in the case of very low thicknesses. As a matter of fact, the high reactivity of the NiO films toward hydrogen30 can lead to a partial hydroxylation due to the reaction with background gas, even in good UHV conditions. In Figure 4 we present the XAS of a sample contaminated by surface hydroxyls (it was left in UHV for a long period), as evidenced by the analysis of the spectrum of the O 1s photoemission peak which presents a very intense shoulder around 531 eV (see the inset in the Figure 4). The hydroxylated sample presents no significant LD effect; however, after a mild annealing at 300 °C in O2 (i.e., the thermodynamic conditions used in the initial deposition) the hydroxyls can be desorbed, and a significant LD (-8%) becomes evident (the residual shoulder at the high-energy side of the O 1s photoemission peak is due to the Pd 3p3/2 peak and not to hydroxyl groups). Another interesting experiment consists in measuring the LD (%) prior and after the annealing procedure, which is needed to improve the surface order after depositing the NiO at RT. It is known from previous studies that the most important effect of the annealing is a partial relaxation of the strain and a smoothing of the surface roughness.19 We have measured the LD (%) with our usual procedure, obtaining that in the case of a 5 MLE NiO film, the LD (%) has changed from -20% in the as deposited films to -15% after an annealing at 300 °C in O2. This result seems to confirm that the amount of tetragonal strain that affects the film directly reflects in the spin structure following the lines outlined in ref 29. 4. Conclusions In the present paper we have reported a detailed LP-XAS investigation on NiO/Pd(100) ultrathin films, from the defective ML up to bulk relaxed films. A clear dichroic effect has been obtained in all the cases, but the factors that generate it are different at the different coverages.
5128 J. Phys. Chem. C, Vol. 112, No. 13, 2008 For the defective ML (structurally originated by the high strain)16,17 the LD effect stem from anisotropic structural effects connected with the spatial confinement, as also confirmed by a series of computations. For the high coverage films, the LD effect is to be clearly associated to a magnetic nature, in line with the current literature interpretation. In the intermediate regime (4-10 MLE), the interpretation is not straightforward as the LD effect is strongly dependent also on the actual film preparation procedure. From the body of these experimental data, we can conclude that the strain is only one, and not always the determining one, among different sources capable to modify the LD of ultrathin films. Crystal field effects can also have an influence on dichroism, but it seems reasonable that they are active only at low thickness, where the deviation from octahedral symmetry is high. The removal of the strain and of the related tetragonal distortion at higher thickness (>10-12 MLE)19 results in reporting a LD (%) value independent of the actual preparation procedure. In relation to the unsolved interpretation of the upholding of the reversed dichroic behavior when the structural bulklike situation is reached (highest coverages, see also ref 14), frustration of the initial AF states by the presence of morphological defects, e.g., as steps or mosaics, remains the most plausible hypothesis for the origin of such a memory effect that pins the S domains to nonequilibrium orientation. Finally, we would like to mention another possible contribution to explain the reported anomalous behavior that could be traced back to the peculiar magnetic properties of Pd-NiO multilayers.31,32 Actually, the coupling between the AF NiO with Pd generates a FM system. However, whether this phenomenon could be of relevance also in our films is difficult to predict a priori. Acknowledgment. This work has been funded by European Community through the STRP project (Growth and Supraorganization of Transition and Noble Metal Nanoclusters) and by the Italian Ministry of Instruction, University and Research (MIUR) through the fund “Programs of national relevance” (PRIN-2003, PRIN-2005). References and Notes (1) (a) Service, R. F., Science 2006, 314, 1868. (b) J. F. Scott, J. F. Nat. Mater. 2007, 6, 256 - 257 (2007); (c) J. Nogues, J.; I. K. Schuller, I. K. J. Magn. Magn. Mat. 1999, 192, (1999) 203. (2) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; von Molna´r, S; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 2001.294, 1488. (3) According to a widely used nomenclature, they are divided in four principal T domains, corresponding to the possible 〈111〉 directions which are themselves divided into six S domains, corresponding to the six possible 〈11-2〉 directions. Domains with main spin component perpendicular (S⊥) to the (100) surface are those along [(1(1(2] directions, while those along
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