A users viewpoint: absorption spectroscopy at a synchrotron Frithjof Nolting

Bottom layer “pinned” Hard layer or Exchange coupling Top layer “free” can be switched MRAM Hard disk head Magnetic data storage and recording Low resistance high resistance GMR Element

FM AFM Interface - Different models for AFM/FM coupling exist. - Different assumptions on AFM structure lead to complete different results. - Spin arrangement at the interface is important High Spatial Resolution Elemental Sensitivity Ferromagnetic Contrast Antiferromagnetic Contrast Surface/Interface Sensitivity How? X-ray absorption spectroscopy (XAS) with spatial resolution Photoemission Electron Microscope (PEEM)

Photoemission Electron Microscope I (nA) h Energy E F ~ ~ h (x-ray) core level valence band 2p 3/2 2p 1/2 Magnetic lenses e-e- 16° analyzer 20 kV X-rays Sensitive to: elemental composition chemistry structural parameters electronic structure magnetic properties ELMITEC GmbH

Magnetic microscopy S E XMCD (X-ray Magnetic Circular Dichroism) XMLD (X-ray Magnetic Linear Dichroism) LaFeO 3 Co 5 µm J. Stöhr et al. Science 1993A. Scholl et al. Science 2000 F. Nolting et al. Nature 2000

Exchange biased Co/NiO multilayer EPU beamline 4 at ALS nm Co 50 nm NiO 1.5 nm Ru 3 nm Co 30° Magnetic field Left and right circularly polarized light XMCD Hysteresis of Co and NiO XMCD spectra – switching polarization Pinned Moments ? H. Ohldag, A. Scholl, F. Nolting, E. Arenholz, S. Maat, A.T. Young, M. Carey, and J. Stöhr, Phys. Rev. Lett. 91(1), /1-4 (2003).

Ratio of A/B Antoine Barbier et al User experiment at SLS, July 2004

SIM Beamline Layout Undulator T. Schmidt 2 Elliptical undulators Pure permanent magnet 95eV < h < 2000eV >10 19 photons/s/mrad 2 /mm 2 /400mA 100 % circular polarization [ eV] reduced on higher harmonics Hor. & vert. linear polar. Optics U. Flechsig Plane grating monochromator E/  E > 8000 <5% 2 nd order light Switch helicity Focus 30x100  m 2 Endstation C. Quitmann & F. Nolting SLS endstation: –PEEM & LEEM  x~ nmspatial  E~150meV energy –Sample Prep chamber –User endstation Front end Prepchamber ID 1 ID 2 MonochromatorChopper Refocusing optics Microscope User endstation

TEY Reference signal Sample signal Magnet Sample = Norm Reference Moving gap and monochromator, stop, measure (1s – 1minute), moving … How do we measure

Change polarization and repeat Take difference of spectra Absorption spectrum requires frequent moving of gap and shift must not effect other beamlines transparent!

switching by moving phase switching by moving the gap tuneddetuned move up 56 mm move 2 mm move 0 mm detuned tuned circ plus circ minus chopper 120 s 1s - 8 s 100 Hz Modes for switching the polarization switching by using a chopper

Alignment of IDs - Horizontal 1. Horizontal overlap at Frontend ID1 ID2 Asymmetric bump 2D Frontend scan Closed bump using chicane magnets 2. Horizontal overlap at focus of experiment ID2 Intensity Horizontal position

Alignment of IDs - Vertical 3. Vertical (energy) overlap at focus of experiment ID1 ID2 not yet finished ! Photon energy Intensity ID2 Absorption spectrum schematic eV source mono Exit slit

Beam variation - Noise horizontal movementintensity variation XAS normalization reduces it by a factor of PEEM no normalization daysno problem hoursno problem sec - minutesbad mili secondsok vertical movementenergy variation no normalization! daysno problem hours bad secondsbad mili secondsok 10 μm about 2% 10 μm about 10 meV

Energy shift Energy shift of 10 meV Identical spectra 1 %

Orbit feedback Fast orbit feedback slow orbit feedback beam position in ID (Bergoz) X-ray position in beamline Shift and gap Orbit correctors?

Orbit feedback – effect on measurement Normalized signal, circular plus Difference circular plus and minus Increased noise!!!

Slow Orbit feedback Circular plus Circular minus beam position in ID (Bergoz) X-ray position in beamline Shift and gap Orbit correctors?

Slow Orbit feedback Not transparent ! 10 μm

Summary We can do great measurements at SLS  Transparent IDs are essential! Very difficult to make an EPU transparent. Have to rely on Orbit feedback  For the measurement “no” difference between slow and no Orbit feedback  Critical time scale second – hour (10 Hz – Hz) Intensity variation 0.1% 0.5 μm energy variation 1meV 1 μm  Slow Orbit feedback is not sufficient Fast Orbit feedback is great