2p3s3p, 2p3p3p, and 2p3s3s resonant Auger spectroscopy from NiO

We have investigated the behavior of the 2p3s3p, 2p3p3p, and 2p3s3s Auger lines of NiO, a model compound in the class of strongly correlated 3d systems, while varying the photon energy across the Ni L3 and L2 absorption edges. The experimental data are discussed in comparison with a theoretical model based on a charge-transfer multiplet approach. When the excitation energy is below the L3 resonance, we observe the 2p3p3p and 2p3s3p peaks at a constant binding energy. This behavior is typical of nonradiative resonant Raman scattering. If the photon energy is increased further, the 2p3p3p and 2p3s3p lines rapidly transform into constant kinetic energy features, showing a normal Auger behavior. The transition from Raman- to Auger-like behavior takes place for photon energies lower than the ones corresponding to excitations of the photoelectron into ligand-hole states. This might indicate the participation of inelastic processes in the recombination of the core hole involving energies much smaller than the NiO gap, or the possible presence of nonlocal effects. On the high photon energy side of the L3 edge, the constant kinetic energy of the 2p3p3p and 2p3s3p peaks is systematically larger than the one observed for an excitation well above the L2,3 edges. We attribute this behavior to the intervention of an intermediate state of 2p^5 3d^10 character, which has very little weight but is strongly enhanced at resonance

Local Electronic and Magnetic Structure of Ni below and above TC: A Spin-Resolved Circularly Polarized Resonant Photoemission Study

We report the measurement of the local Ni 3d spin polarization, not only below but also above the Curie temperature (TC), using the newly developed spin-resolved circularly polarized 2p (L3) resonant photoemission technique. The experiment identifies the presence of 3d8 singlets at high energies and 3d8 triplets at low energies extending all the way to the Fermi energy, both below and above TC, showing that it is the orbital degeneracy of the 3d band and the Hund's rule splitting which is of utmost importance to understand Ni and other 3d ferromagnets

Spin-Resolved Photoemission on Anti-Ferromagnets: Direct Observation of Zhang-Rice Singlets in CuO

We demonstrate that it is possible to obtain spin-resolved valence band spectra with a very high degree of spin polarization from antiferromagnetic transition metal materials if the excitation light is circularly polarized and has an energy close to the cation 2p3/2 (L3) white line. We are able to unravel the different spin states in the single-particle excitation spectrum of CuO and show that the top of the valence band is of pure singlet character, which provides strong support for the existence and stability of Zhang-Rice singlets in high-Tc superconductors

Strain induced phase separation in La0.7Sr0.3MnO3 thin films.

La0.7Sr0.3MnO3 thin films having different thicknesses were grown by pulsed laser deposition with in situ reflection high energy electron diffraction diagnostics on LaAlO3 substrates. The mismatch between film and substrate gives rise to an in-plane compressive biaxial strain, which partially relaxes in films thicker than 30 unit cells. Accordingly, the ratio between the out-of-plane and the in-plane lattice parameter c/a varies between 1.06 fully strained and 1.03 partially relaxed. In-plane compressive strain favors the stabilization of the 3z2−r2 orbitals chain-type antiferromagnetic phase, thus giving rise to a sizeable x-ray absorption linear dichroism signal. The shape of the linear dichroism depends weakly on the c/a ratio, while its intensity strongly increases with c/a. At the same time, the metal-insulator transition temperature shifts from about 360 K towards lower temperatures with decreasing thickness, eventually reaching an insulating state for the 30 unit cells film. Low-temperature nuclear magnetic resonance spectra show a decrease of the MnDE doubleexchange metallic contribution with decreasing the thickness, which becomes negligible in the 30 unit cells thick film. The experimental results demonstrate a strain driven competition between two stable phases: the orbital ordered chain-type insulating antiferromagnetic and the orbital disordered metallic ferromagnetic. For intermediate values of the epitaxial strain the local minimum state of the system lies in a gap region between the two stable phases. Such a region has glassy characteristics with coexisting clusters of the two phases. The strain is used as a driving force to span the glassy region

Modifications In Magnetic Properties Of Bimn2 O5 Multiferroic Using Swift Heavy Ion Irradiation

We report the near edge x-ray absorption fine structure (NEXAFS) and x-ray magnetic circular dichroism (XMCD) studies at the Mn L3,2 edge of pulsed laser deposited pristine thin films of multiferroic BiMn2 O5. These investigations are furthermore testified for BiMn2 O5 thin films irradiated through 200 MeV Ag15+ ions with fluence value 5× 1011 ions/ cm2. Though the pristine film is primarily antiferromagnetic in nature, irradiation induces ferrimagnetism in it. Element specific characterizations, NEXAFS and XMCD demonstrate the evolution of Mn2+ state piloting to magnetic signal associated with it. © 2010 American Institute of Physics.1079Nan, C.-W., Bichurin, M.I., Dong, S., Viehland, D., Srinivasan, G., Multiferroic magnetoelectric composites: Historical perspective, status, and future directions (2008) Journal of Applied Physics, 103 (3), p. 031101. , DOI 10.1063/1.2836410Ramesh, R., Spaldin, N.A., Multiferroics: Progress and prospects in thin films (2007) Nature Materials, 6 (1), pp. 21-29. , DOI 10.1038/nmat1805, PII NMAT1805Eerenstein, W., Mathur, N.D., Scott, J.F., Multiferroic and magnetoelectric materials (2006) Nature, 442 (7104), pp. 759-765. , DOI 10.1038/nature05023, PII NATURE05023Gajek, M., Bibes, M., Fusil, S., Bouzehouane, K., Fontcuberta, J., Barthelemy, A., Fert, A., Tunnel junctions with multiferroic barriers (2007) Nature Materials, 6 (4), pp. 296-302. , DOI 10.1038/nmat1860, PII NMAT1860Cheong, S.-W., Mostovoy, M., Multiferroics: A magnetic twist for ferroelectricity (2007) Nature Materials, 6 (1), pp. 13-20. , DOI 10.1038/nmat1804, PII NMAT1804Hur, N., Park, S., Sharma, P.A., Ahn, J.S., Guha, S., Cheong, S.-W., Electric polarization reversal and memory in a multiferroic material induced by magnetic fields (2004) Nature, 429 (6990), pp. 392-395. , DOI 10.1038/nature02572Chapon, L.C., Radaelli, P.G., Blake, G.R., Park, S., Cheong, S.-W., Ferroelectricity induced by acentric spin-density waves in YMn2O5 (2006) Physical Review Letters, 96 (9), pp. 1-4. , http://oai.aps.org/oai?verb=GetRecord&Identifier=oai:aps.org: PhysRevLett.96.097601&metadataPrefix=oai_apsmeta_2, DOI 10.1103/PhysRevLett.96.097601, 097601Muoz, A., Alonso, J.A., Casais, M.T., Martínez-Lope, M.J., Martínez, J.L., Fernández-Díaz, M.T., (2002) Phys. Rev. B, 65, p. 144423. , PRBMDO 0163-1829,. 10.1103/PhysRevB.65.144423Blake, G.R., Chapon, L.C., Radaelli, P.G., Park, S., Hur, N., Cheong, S.-W., Rodriguez-Carvajal, J., Spin structure and magnetic frustration in multiferroic RMn2O5 (R=Tb,Ho,Dy) (2005) Physical Review B - Condensed Matter and Materials Physics, 71 (21), pp. 1-9. , http://oai.aps.org/oai/?verb=ListRecords&metadataPrefix= oai_apsmeta_2&set=journal:PRB:71, DOI 10.1103/PhysRevB.71.214402, 214402Shukla, D.K., Kumar, R., Sharma, S.K., Thakur, P., Choudhary, R.J., Mollah, S., Brookes, N.B., Choi, W.K., (2009) J. Phys. D, 42, p. 125304. , JPAPBE 0022-3727,. 10.1088/0022-3727/42/12/125304Vecchini, C., Chapon, L.C., Brown, P.J., Chatterji, T., Park, S., Cheong, S.W., Radaelli, P.G., (2008) Phys. Rev. B, 77, p. 134434. , PRBMDO 0163-1829,. 10.1103/PhysRevB.77.134434Shukla, D.K., Mollah, S., Kumar, R., Thakur, P., Chae, K.H., Banerjee, A., Choi, W.K., (2008) J. Appl. Phys., 104, p. 033707. , JAPIAU 0021-8979,. 10.1063/1.2964072Kumar, R., Arora, S.K., Kanjilal, D., Mehta, G.K., Bache, R., Date, S.K., Shinde, S.R., Patil, S.I., (1999) Radiat. Eff. Defects Solids, 147, p. 187. , REDSEI 1042-0150,. 10.1080/10420159908229008Houpert, C., Studer, F., Groult, D., Toulmonde, M., (1989) Nucl. Instrum. Methods Phys. Res. B, 39, p. 720. , NIMBEU 0168-583X,. 10.1016/0168-583X(89)90882-3Kumar, R., Samantra, S.B., Arora, S.K., Gupta, A., Kanjilal, D., Pinto, R., Narlikar, A.V., (1998) Solid State Commun., 106, p. 805. , SSCOA4 0038-1098,. 10.1016/S0038-1098(98)00122-7Biersack, J.P., Haggmark, L., (1980) Nucl. Instrum. Methods Phys. Res., 174, p. 257. , NIMRD9 0167-5087,. 10.1016/0029-554X(80)90440-1Shukla, D.K., unpublished dataWang, Z.G., Dufour, C., Paumier, E., Toulemonde, M., (1994) J. Phys.: Condens. Matter, 6, p. 6733. , JCOMEL 0953-8984,. 10.1088/0953-8984/6/34/006Izui, K., Furuno, S., (1986) Proceedings of the 11th International Congress on Electron Microscopy, p. 1299. , edited by T. Imura, S. Maruse, and T. Suzuki (Japanese Society of Electron Microscopy, Tokyo),Meftah, A., Brisard, F., Costantini, J.M., Hage-Ali, M., Stoquert, J.P., Studer, F., Toulemonde, M., (1993) Phys. Rev. B, 48, p. 920. , and, PRBMDO 0163-1829,. 10.1103/PhysRevB.48.920Thibaudau, F., Cousty, J., Balanzat, E., Bouffard, S., (1991) Phys. Rev. Lett., 67, p. 1582. , PRLTAO 0031-9007,. 10.1103/PhysRevLett.67.1582Toulemonde, M., Dufour, C., Paumier, E., (1992) Phys. Rev. B, 46, p. 14362. , PRBMDO 0163-1829,. 10.1103/PhysRevB.46.1436

Perpendicular magnetic anisotropy in granular multilayers of COPD alloyed nanoparticles

Co-Pd multilayers obtained by Pd capping of pre-deposited Co nanoparticles on amorphous alumina are systematically studied by means of high-resolution transmission electron microscopy, x-ray diffraction, extended x-ray absorption fine structure, SQUID-based magnetometry, and x-ray magnetic circular dichroism. The films are formed by COPD alloyed nanoparticles self-organized across the layers, with the interspace between the nanoparticles filled by the non-alloyed Pd metal. The nanoparticles show atomic arrangements compatible with short-range chemical order of L10 strucure type. The collective magnetic behavior is that of ferromagnetically coupled particles with perpendicular magnetic anisotropy, irrespective of the amount of deposited Pd. For increasing temperature three magnetic phases are identified: hard ferromagnetic with strong coercive field, soft-ferromagnetic as in an amorphous asperomagnet, and superparamagnetic. Increasing the amount of Pd in the system leads to both magnetic hardness increment and higher transition temperatures. Magnetic total moments of 1.77(4) µB and 0.45(4) µB are found at Co and Pd sites, respectively, where the orbital moment of Co, 0.40(2) µB, is high, while that of Pd is negligible. The effective magnetic anisotropy is the largest in the capping metal series (Pd, Pt, W, Cu, Ag, Au), which is attributed to the interparticle interaction between de nanoparticles, in addition to the intraparticle anisotropy arising from hybridization between the 3d-4d bands associated to the Co and Pd chemical arrangement in a L10 structure type

Orbital reconstruction and two-dimensional electron gas at the LaAlO3/SrTiO3 interface

Conventional two-dimensional electron gases are realized by engineering the interfaces between semiconducting compounds. In 2004, Ohtomo and Hwang discovered that an electron gas can be also realized at the interface between large gap insulators made of transition metal oxides [1]. This finding has generated considerable efforts to clarify the underlying microscopic mechanism. Of particular interest is the LaAlO3/SrTiO3 system, because it features especially striking properties. High carrier mobility [1], electric field tuneable superconductivity [2] and magnetic effects [3], have been found. Here we show that an orbital reconstruction is underlying the generation of the electron gas at the LaAlO3/SrTiO3 n-type interface. Our results are based on extensive investigations of the electronic properties and of the orbital structure of the interface using X-ray Absorption Spectroscopy. In particular we find that the degeneracy of the Ti 3d states is fully removed, and that the Ti 3dxy levels become the first available states for conducting electrons.Comment: 12 pages, 3 figure

Structural and Electronic Reconstructions at the LaAlO3/SrTiO3 Interface

A full understanding of the mechanism of the formation of a two-dimensional electron gas (2DEG) at the interface between insulating LaAlO3 (LAO) thin films and bulk SrTiO3 (STO) crystals is a prerequisite for the full exploitation of this class of materials. Here, by using a combination of advanced X-ray synchrotron-based spectroscopic and structural measurements, it is shown that a structural and electronic reconstruction of the interface occurs before the realization of the 2DEG

Energy and symmetry of dd excitations in undoped layered cuprates measured By Cu L3 resonant inelastic x-ray scattering

We measured the high-resolution Cu L3 edge resonant inelastic x-ray scattering (RIXS) of undoped cuprates La2CuO4, Sr2CuO2Cl2, CaCuO2 and NdBa 2Cu3O6. The dominant spectral features were assigned to dd excitations and we extensively studied their polarization and scattering geometry dependence. In a pure ionic picture, we calculated the theoretical cross sections for those excitations and used these to fit the experimental data with excellent agreement. By doing so, we were able to determine the energy and symmetry of Cu-3d states for the four systems with unprecedented accuracy and confidence. The values of the effective parameters could be obtained for the singleion crystal field model but not for a simple two-dimensional cluster model. The firm experimental assessment of dd excitation energies carries important consequences for the physics of high-Tc superconductors. On the one hand, we found that the minimum energy of orbital excitation is always ≥ 1.4 eV, i.e. well above the mid-infrared spectral range, which leaves to magnetic excitations (up to 300 meV) a major role in Cooper pairing in cuprates. On the other hand, it has become possible to study quantitatively the effective influence of dd excitations on the superconducting gap in cuprates

A determination of the pairing interaction in the high Tc cuprate superconductor Tl2Ba2CaCu2O8 (Tl2212)

We have measured the near-normal reflectance of Tl2Ba2CaCu2O8 (Tl2212) for energies from 0.1 to 4.0 eV at room temperature and used a Kramers-Kronig analysis to find the complex, frequency dependent dielectric function, from which the optical conductivity was determined. Using Thermal-Difference-Reflectance (TDR) Spectroscopy the reflectance of the sample in the normal state just above the superconducting transition, and in the superconducting state were then obtained. From these data we determined the ratio of the superconducting- to normal-state optical conductivities. Mattis and Bardeen had calculated this function within the BCS theory, where the gap is a fixed energy-independent quantity. Taking into account the retarded nature of the electron-phonon coupling results in a complex, energy dependent gap causing deviations from the Mattis-Bardeen plot at energies where the phonon coupling function is large. We find a typical deviation near the phonon energies in Tl2212, and in addition, at 1.2 and 1.7eV. The phonon, and these electronic terms can each be described by a coupling constant. None of which by itself gives rise to a high transition temperature, but the combination does. Using Resonant Inelastic X-Ray Scattering (RIXS) we find that the d-to-d excitations of the cuprate ion in Tl2212 fall at the same energies as the dips in the Mattis-Bardeen plot. We conclude that the high superconducting transition temperature of the cuprates is due to the sum of the phonon interaction, and interactions with the Cu-ion d-shell.Comment: Preprint submitted to Physica C. Paper presented at M2S-HTSC-VIII, Dresden, Germany, July 9-14,200