Local tetragonal distortion in La_{0.7}Sr_{0.3}MnO_3 strained thin films probed by x-ray absorption spectroscopy

We report on an angular resolved X-ray Absorption Spectroscopy study of $La_{0.7}Sr_{0.3}MnO_{3}$ thin films epitaxially grown by pulsed laser deposition on slightly mismatched substrates which induce tensile or compressive strains. XANES spectra give evidence of tetragonal distortion within the $MnO_{6}$ octahedra, with opposite directions for tensile and compressive strains. Quantitative analysis has been done and a model of tetragonal distortion reflecting the strain has been established. EXAFS data collected in plane for tensile substrate confirm the change in the $Mn-O$ average bond distance and the increase of $Mn-Mn$ length matching with the enlargement of the cell parameter. From these results we conclude that there is no significant change in the $Mn-O-Mn$ angle. Our observations conflict with the scenarios which this angle is the main driving parameter in the sensitivity of manganite films properties to external strains and suggest that the distortion within the octahedra plays a key role in the modification of the transport and magnetic properties.Comment: 8 pages, 6 figure

Strain driven monoclinic distortion of ultrathin CoO films in the exchange-coupled CoO/FePt/Pt(001) system

International audienceThe structure and strain of ultrathin CoO films grown on a Pt(001) substrate and on a ferro-magnetic FePt pseudomorphic layer on Pt(001) have been determined with insitu and real time surface x-ray diffraction. The films grow epitaxially on both surfaces with an in-plane hexagonal pattern that yields a pseudo-cubic CoO(111) surface. A refined x-ray diffraction analysis reveals a slight monoclinic distortion at RT induced by the anisotropic stress at the interface. The tetrag-onal contribution to the distortion results in a ratio c a > 1, opposite to that found in the low temperature bulk CoO phase. This distortion leads to a stable Co 2+ spin configuration within the plane of the film

Depth dependent local structures in thin films unraveled by grazing incidence x-ray absorption spectroscopy

A method of using X-ray absorption spectroscopy (XAS) together with resolved grazing incidence geometry for depth profiling atomic, electronic, chemical or magnetic local structures in thin films is presented. The quantitative deconvolution of thickness-dependent spectral features is performed by fully considering both scattering and absorption formalisms. Surface oxidation and local structural depth profiles in nanometric FePt films are determined, exemplifying the application of the method.Comment: 5 pages, 3 figure

Emergence of ferromagnetism and Jahn-Teller distortion in low Cr-substituted LaMnO3

The emergence of a ferromagnetic component in $LaMnO_{3}$ with low Cr-for-Mn substitution has been studied by x-ray absorption spectroscopy and x-ray magnetic circular dichroism at the Mn and Cr K edges. The local magnetic moment strength for the Mn and Cr are proportional to each other and follows the macroscopic magnetization. The net ferromagnetic components of $Cr^{3+}$ and $Mn^{3+}$ are found antiferromagnetically coupled. Unlike hole doping by La site substitution, the inclusion of $Cr^{3+}$ ions up to x = 0.15 does not decrease the Jahn-Teller (JT) distortion and consequently does not significantly affect the orbital ordering. This demonstrates that the emergence of the ferromagnetism is not related to JT weakening and likely arises from a complex orbital mixing.Comment: 5 figure

Spin orientation in an ultrathin CoO/PtFe double-layer with perpendicular exchange coupling

International audienceWe studied by soft x-ray absorption spectroscopy the magnetization axis in an 4nm thin CoO (111) layer exchange-coupled to an ultra thin L1 0 PtFe layer with perpendicular magnetic anisotropy. The angular dependence of the linear magnetic dichroism at 10K and the relative variations of the spectral features provide a full description of the spin orientation in this antiferromagnetic layer. The spins are found in the film plane, pointing along the 110 direction. This results is discussed in relation to the film strain and preferential occupation of t 2g orbitals.The strong orthogonal coupling between Co and Fe spins should be at the origin of the robustness of the exchange bias effect foun in this bilayer system

Investigation of metallic/oxide interfaces in Pt/Co/AlOx trilayers by hard x-ray reflectivity

International audienceX-ray reflectivity (XRR) is used to determine the oxidation front at the nanometer scale in sputtered perpendicular semi tunnel junctions, as the form Pt/Co/AlOx, by varying the oxidation time tOx of the capping layer. From XRR simulations, we show that the nature of the stack is gradually defined according to the value of tOx. For low tOx values (<40 s), a simple Pt/Co/Al/AlOx multilayer is appearing whereas a Pt/Co/CoO/AlOx architecture takes place for higher tOx. The oxygen-induced magnetic properties obtained by extraordinary Hall effects measurements are explained by the structural results. The increase of Co-O bondings with tOx is at the origin of the appearing of the perpendicular magnetic anisotropy (PMA)

The contribution of x-ray specular reflectometry to the oxygen-induced magnetic properties in Pt/Co/AlOx

3 pagesInternational audienceTwo key parameters were analyzed in Si/SiO/Pt/Co/AlOx: the oxidation time of the Al layer resulting in AlOx, and the ex situ annealing temperatures varied in the 15 and 55 s and 20, 300, and 450 °C ranges, respectively. For intermediate annealing temperatures (∼300 °C), the quantitative analysis of specular reflectometry data shows that the progressive oxidation of layers by increasing the oxidation time goes along with an improvement of the homogeneity of the alumina layer. This outcome casts new light on the temperature dependence of magnetic properties of the samples. The remarkable temperature variation of the coercive field, extracted from extraordinary Hall effects in the 5-300 K range, is associated with structural change due to Co-oxygen bondings, which leads to strong pinning of Co spins in the low temperature regime

Short-range charge-order in RRNiO3_{3} perovskites (RR=Pr,Nd,Eu) probed by X-ray absorption spectroscopy

The short-range organization around Ni atoms in orthorhombic $R$NiO$_{3}$ ($R$=Pr,Nd,Eu) perovskites has been studied over a wide temperature range by Ni K-edge x-ray absorption spectroscopy. Our results demonstrate that two different Ni sites, with different average Ni-O bond lengths, coexist in those orthorhombic compounds and that important modifications in the Ni nearest neighbors environment take place across the metal-insulator transition. We report evidences for the existence of short-range charge-order in the insulating state, as found in the monoclinic compounds. Moreover, our results suggest that the two different Ni sites coexists even in the metallic state. The coexistence of two different Ni sites, independently on the $R$ ion, provides a common ground to describe these compounds and shed new light in the understanding of the phonon-assisted conduction mechanism and unusual antiferromagnetism present in all $R$NiO$_{3}$ compounds.Comment: 4 pages, 3 figures, accepted PRB - Brief Report Dec.200

Robust perpendicular exchange coupling in an ultrathin CoO/PtFe double layer: Strain and spin orientation

5 pagesInternational audienceWe report on the exchange coupling and magnetic properties of a strained ultrathin CoO/PtFe double layer with perpendicular magnetic anisotropy. The cobalt oxide growth by reactive molecular beam epitaxy on a Pt-terminated PtFe/Pt(001) surface gives rise to a hexagonal surface and a monoclinic distorted CoO 3 nm film at room temperature. This distorted ultrathin CoO layer couples with the PtFe(001) layer establishing a robust perpendicular exchange bias shift. Soft x-ray absorption spectroscopy provides a full description of the spin orientations in the CoO/PtFe double layer. The exchange bias shift is preserved up to the N'eel antiferromagnetic ordering temperature of TN = 293 K. This unique example of selfsame value for blocking and ordering temperatures, yet identical to the bulk ordering temperature, is likely related to the original strain-induced distortion and strengthened interaction between the two well-ordered spin layers