Opportunities for Soft X-Ray Spectroscopies at CELLS-ALBA

Transcription

1 Opportunities for Soft X-Ray Spectroscopies at CELLS-ALBA Eric Pellegrin On behalf of the CELLS Experiments Division Contents: Brief status of the ALBA Synchrotron Light Source A few recent examples: Strain effects on LSMO/STO thin films Breathing chemistry of FePc on Au(111) PEEM on STO, LSMO, and Ru(0001) surfaces XMCD on Co/CoO nanogranular thin films

2 E. Pellegrin The IFPAN ALBA WarsawBuilding 18/09/2014

3 ALBA Storage Ring Status Update: 100 ma top-up mode

4 Overview ALBA Phase I Beamlines Beamline Beamline use Field of activity Light source MSPD BL Material Science and Powder Diffraction Materials science NCD BL Non-Crystalline Diffraction Life sciences, Materials science Superconducting wiggler In-vacuum undulator XALOC BL Macromolecular Crystallography Life sciences In-vacuum undulator CLAESS BL X-Ray Absorption Spectroscopy Materials science Wiggler CIRCE BL Photoemission spectroscopy and microscopy Polarized electron spectroscopies Helical undulator (Apple II type) BOREAS BL Soft X-Ray Magnetic Circular Dichroism Polarized electron spectroscopies Helical undulator (Apple II type) MISTRAL BL Soft X-Ray Microscopy beamline Life sciences, Materials science Bending magnet In User Operation waiting for your beamtime proposals!

5 Hector End Station / Boreas Beamline: Surface symmetry-breaking & strain effects on orbital occupancy in LSMO/STO thin films D. Pesquera et al. Cooperation ICMAB / Elettra/ CELLS-ALBA

6 BL29 BOREAS Resonant Absorption & Scattering MARES end station HECTOR end station Dedicated to polarization-dependent spectroscopies of advanced materials. Two cutting edge end stations: HECTOR vector magnet (up to 2 / 6 Tesla) for absorption methods MARES UHV reflectometer for soft x-ray scattering & reflection Photon energy range: 80 to 4000 ev

7 Orbital Physics in Transition Metal Oxides Y. Tokura, Science, 288, (2000) Z.Fang, Phys. Rev. Lett. 84, (2000) Change of orbital (magnetic) configuration with hole doping Change of orbital (magnetic) configuration with strain

8 Interest in Catalysis Applications T. Wolfram, Phys.Rev.Lett. 30, (1973) J. Bockris, J. Electrochem. Soc.,131, (1984) J. Suntivich, Nat. chem. 3, (2011) J. Suntivich, Science, 334, (2011) Catalysts have the role to provide occupied surface states of the correct symmetry to allow the reaction to proceed with symmetry conservation. Active reaction surface This implies the existence of occupied surface states whose symmetry permits a net positive overlap to occur between surface states and antibonding orbitals of the gaseous reactant ORR activity as a function of e g filling Oxygen Reduction Reaction activity of transition metal oxide catalysts

9 Tuning Strain in LSMO Films La 0.7 Sr 0.3 Mn O 3 Lattice parameter in Å NGO LSMO LSAT Compressive strain LGO Tensile strain STO Intensity (arb.units) NGO1 (150 u.c.) LSAT1 (150 u.c.) LGO1 (150 u.c.) STO1 (70 u.c.) d subs (004)/ d

10 E.Stavitski, Micron 1993, (2010) A. Tebano, Phys.Rev.Lett, 100, (2008) Exploring Electronic Structure with X-Ray Linear Dichroism(XLD) 3z 2 -r 2 x 2 -y 2 2p 3/2 2p 1/2 Mn-L edge eV E (ev) I E(eV) XLD>0 I -I XLD< E (ev) E (ev) 650 E (ev)

11 Orbital Occupancy in LSMO Films Probing depth film substrate 3z 2 -r 2 x 2 -y 2 t>25 nm -0.3% 0% +0.5% +0.9% E E. (ev) Pellegrin IFPAN Warsaw 18/09/2014 ε XLD (a.u.) XLD (a.u.) XLD (a.u.) XLD (a.u.) NGO LSAT LGO STO Energy (ev) Strain induced orbital Additional contribution to orbital occupancy /

12 Orbital occupancy in LSMO films Integral area under XLD in L 2 energy region: L 3 L Area under XLD more 3z 2 -r 2 more x 2 -y 2 Substrates: NGO LSAT f (%) LGO STO LSMO thickness: 150 unit cells

13 Orbital occupancy in LSMO films Enhancing surface contribution with thinner films (larger surface/bulk ratio) t<4 nm Probing depth film substrate Films on LSAT STO LSAT1 150 uc LSAT2 4 uc XLD 3z 2 -r 2 film film substrate substrate film film film substrate substrate substrate E (ev) E (ev) x 2 -y 2 Larger 3z 2 -r 2 contribution in thinner films

14 Orbital occupancy in LSMO thin films Area under XLD NGO 150 u.c. 70 u.c. 8 u.c. 4 u.c. LSAT f (%) t (u.c.) Films on STO LGO STO 0.00 Area under XLD

15 NAPP End Station / Circe beamline: Breathing Chemistry in FePc C. Rogeroet al. Cooperation CSIC-UPV/EHU / CELLS-ALBA

16 BL24 CIRCE Photoemission Microscopy & Spectroscopy NAPP end station PEEM end station Half unit cell steps in LSMO/STO thin films Two terminations: La 1/3 Sr 1/3 O, MnO 2 NAPP XPS Ag sample/mg Kα From UHV (top) to 25 mbar N 2 (bottom) N1s Ag3d Variable polarization BL dedicated to advanced photoemission microscopy and spectroscopy. Two branches with dedicated state-of-the-art end stations: PEEM (photoemission electron microscopy) NAPP (near ambient pressure photoemission). Photon energy range: ev

17 Near Ambient Pressure Photoemission 17 Maximum pressure at the sample 20 mbar Analyzer energy resolution ~ 5/10 mev (UPS/XPS) Variable beam incidence angle and variable polarization Beam size at the sample 100 μm x 30 μm (H x V) Infrared laser heating & Peltier cooling -30 C < T sample < 1200 C Residual gas analyzer to monitor reaction products Sample in horizontal position with fully horizontal transfer Ultrapure gases inlet manifold with three lines Liquids vaporizer (Bronkhorst CEM) Surface science preparation chamber

18 Breathing chemistry By courtesy of Dr. Celia Rogero(CSIC-UPV/EHU) Submitted for best experiment of ALBA UHV 0.01 mbar O 2 UHV

19 PEEM End Station / Circe Beamline: Surface Re-Structuring in STO(001), Electrical switching of magnetization in La 2/3 Sr 1/3 MnO 3, and LEED & Dark Field LEEM & µpes in Ru(0001) L. Aballeet al. Cooperation ICMAB / CELLS-ALBA

20 Self assembled distinct termination areas on SrTiO 3 (100) XPEEM & LEEM 20 MEM image: - SrO/TiO 2 areas - steps XPEEM spectromicroscopy at minimum electron escape depth confirms surface SrO vs TiO Intensity (a.u.) "Moons" "Background" AFM: topography (left) & lateral force (right) Electron energy (ev) Work function difference from MEM-LEEM transition shift < 70 mev C. Ocal, E. Barrena, S. Matencio, J. Fontcuberta (ICMAB-CSIC)

21 Electrical switching of magnetization 21 La 2/3 Sr 1/3 MnO 3 //PMN-PT (100) with OOP electrical poling at RT D. Pesquera, B. Casals, G. Herranz, J. Fontcuberta (ICMAB-CSIC)

22 LEED & Dark Field-LEEM & µpes Fermi surface of single Ru(0001) atomic terraces 22 X-ray incidence Ru (0001) LEED pattern, surface terminations, 20 um dark field LEEM images a b c d Fermi surface of a single atomic terrace (E phot = 140 ev) and comparison with HR-ARPES and theory L. Martín, J. de la Figuera, B. Martínez-Pabón, A. Mascaraque, L. Pérez, M. Abuín (IQFR-CSIC & UCM)

23 Hector End Station / Boreas beamline: XMCD on Co/CoO-MgO nanogranular core-shell systems C. Geet al. Cooperation Nanjing University / MPI CPfSDresden / CELLS-ALBA MgO CoO (AFM) Co (FM)

24 Exchange Bias in Co/CoO-MgO Core-Shell Systems Sample morphology from HRTEM Co 69 Mg 7 O 24 Co 80 Mg 6 O 14 AFM CoO shell MgO matrix FM Co metal core Nanostructured Co/CoO-MgOthin films on Si(111) made by magnetron sputtering in an oxygen partial pressure of 2 x 10-7 mbar.

25 Exchange Bias in Co/CoO-MgO Core-Shell Systems Resistivity & Exchange Bias Magnetic Hysteresis 25 Decrease by a factor of 10 7 M(T) at 2 koe H E = 2460 Oe M(H) after FC H C = 6202 Oe Vertical Shift with increasing cooling field T B = 185 K (CCMO1) FM coupling of pinned CoO UCS and Co spins

26 Co2p XMCD on Co/CoO-MgO Core-Shell Systems Co 69 Mg 7 O 24 Co 80 Mg 6 O 14 Co2p absorption spectra show typical line shape of Co 2+. Co2p dichroic spectrum shows multiplet structure typical for Co 2+ in octahedral symmetry. Rotatable uncompensated Co 2+ spins in nominally AFM CoO shell. Stabilized by MgO matrix. Again Co 2+ line shape in Co2p absorption spectra. Multiplet structure in Co2p dichroic spectrum significantly reduced. Remainder similar Co metal XMCD. Metallicity reduces CoO FM Contribution, pinned UCS, and H E

27 Co2p XMCD CFM Simulation and Experiment Overall good agreement between theory and experiment. 30% Co metal + 70% ferromagnetic Co 2+ in dichroic spectrum. Large part of ferromagnetic XMCD signal stems from CoO shell (due to limited TEY probing depth) Rotatable Co 2+ uncompensated spins on CoO

28 Acknowledgements Strain effects in LSMO/STO thin films: D. Pesquera, G. Herranz, F. Sanchez, J. Fontcuberta ICMAB Barcelona A. Barla ISM CNR Trieste F. Bondino, E. Magnano Elettra BACH beamline Trieste P. Gargiani, J. Herrero Martin, S. M. Valvidares, A. Barla ALBA Boreas beamline Surface re-structuring in STO and LSMO/STO surfaces: C. Ocal, E. Berena F. Sanchez, J. Fontcuberta ICMAB Barcelona L. Aballe, M. Foerster ALBA Circe beamline Electrical switching of magnetization in LSMO thin films: D. Pesquera, G. Herranz, F. Sanchez, J. Fontcuberta ICMAB Barcelona L. Aballe, M. Foerster ALBA Circe beamline XMCD in Co/CoO-MgO core-shell systems: C. Ge, X. Wang, W. Zu, Y. Du Nanjing University Z. Hu MPI CPfS Dresden W.-I. Liang, Y.-H. Chu National Chiao Tung University, Hsinchu (Taiwan)

29 Thank you for your time & attention and looking forward to seeing you in Barcelona.