Linearly Polarized X-ray Absorption Investigation of Ultrathin NiO x /Pd(100) Films S. Agnoli, ² F. Sedona, ² P. Finetti, ² G. A. Rizzi, ² G. Granozzi,* ,² 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 L 2,3 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 L 2,3 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 L 2 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 media 1 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 resis- tance (GMR), and electron correlation in narrow band systems. Moreover, NiO possesses some advantages for magnetoelec- tronic 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 sym- metry, 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 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 theoretically 5 and experimentally 6 that, by use of LP radiation, a LD signal can be observed for antiferromagnetically ordered films, which is proportional to <M 2 > (where M indicates the magnetic moment). As a matter of fact, exploiting LD at the Ni L 2 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 (T N ) 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 NiO x (100) strained monolayer (ML) (with x ≈ * To whom correspondence should be addressed. ² Universita ` di Padova. ‡ Laboratorio Nazionale TASC INFM-CNR. 5123 J. Phys. Chem. C 2008, 112, 5123-5128 10.1021/jp711641e CCC: $40.75 © 2008 American Chemical Society Published on Web 03/12/2008