1. 1 BROOKHAVEN SCIENCE ASSOCIATES The use of fast-modulated elliptically polarized soft x-rays in the detection of small polarization signals Cecilia Sánchez-Hanke S. L. Hulbert and C-C. Kao National synchrotron light source (NSLS) Brookhaven National Laboratory (BNL) SRI April 23-25 Baton Rouge supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DE-AC02-98CH10886

2. 2 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge Motivation to work with fast switching Introduction of EPW how it works (very fast) no big explanations but something about the stability shape of the polarization distribution Comparison with undulator Why is important high sensitivity Magnetism and small signals, well XMCD done already and not too much more to do.. now signals: induced magnetic moments, that have to be studied, Nanomagnets also small signals (unless studied in arrays) and together in scattering mode signals at interfaces and/or magnetic interfaces Phase sensitive detection XAS and MCD measured at the same time, saves time.. But at the same time is possible to correct for possible changes in the polarization ratio. X13A endstation (capabilities) measurements of the electron yield and photon yield. Scattering chamber on fast switching polarization, unique, because opens the possibilities for

3. 3 BROOKHAVEN SCIENCE ASSOCIATES Baltimore 7-11 JanuaryJanuary 7-11 Baltimore The physics behind the small magnetic signals current at X13A limit is observation of polarization on Nitrogen in Fe-Nitrides on a 20nm thin film. Nanomagnets (properties dependence on size) Induced magnetic moments, mechanisms (properties depend on distance/size/elements/hybridization) Devices with magnetic properties, with magnetic behavior on non magnetic element for example oxygen or nitrogen. Not only spectroscopy but also scattering. Magnetic interfaces (Sujoys paper) publication… Depth sensitivity depending on study with Photon yield (Photodiode + APD) Electron yield (Sample current + channeltron) SRI April 23-25 Baton Rouge Motivation: small magnetic signals Pic with HL of Nitrogen Pic with HL of Cu

4. 4 BROOKHAVEN SCIENCE ASSOCIATES Baltimore 7-11 JanuaryJanuary 7-11 Baltimore a) better signal to noise: b) What other effect is possible to see? Magnetic moment interactions with dependence, highly diluted or small effects to be able to differenciates what is exclusively happening at the interfaces Noise sources: e - instabilities time frame Electronic noise at the beamline better current amplifiers Beamline stability problem, depending on the size of the beam and the feature that is going to be investigated SRI April 23-25 Baton Rouge Motivation Cecilia: The main motivation to have fast switching, and switching as fast as possible is mainly because of the necessity of measuring small magnetic signals. Cecilia: The main motivation to have fast switching, and switching as fast as possible is mainly because of the necessity of measuring small magnetic signals.

5. 5 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge Signal from EPW directly into a photodiode, full intensity, And 20 x 20 entrance and exit slit… a) no chopper b) with chopper Signal that goes onto the sample Polarization ratio (between left and right elliptically polarized) Dependence with the switching rate. Dependence with beamposition EPW pic Circular- and Linearly polarized x-rays Fast switching polarization: fast inverting the current in electromagnets Beam-stability corrected respect to the other beamlines switching frequency 22 Hz testing 47Hz future faster up to 97 Hz

6. 6 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge In soft X-rays : insertion devices (two schemes) a) switching in time electromagnetic insertion device EPW (NSLS, X13), CPU (APS Sector 4) EPU’s, SPRING-8 or mechanical switch b) switching in space 2 canted EPU’s (BESSY-II, SLS) (also require a chopper) Fast switching the polarization EPW (X13 NSLS)

7. 7 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge Y X Y I =Z Polarization and stability Y t Angle (mrad) Flux density vertical horizontal Δ pol. = 1% Polarization ratio @10 –3 Δ pol. = 0.001% Polarization ratio @ 10 –3

8. 8 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge Beam position and stability Angle (mrad) Flux density Angle (mrad) Degree of polarization (%)

9. 9 BROOKHAVEN SCIENCE ASSOCIATES Top / bottom Permanent magnets Left / rightElectromagnets hard and soft x-rays Linear and circular 22 Hz 47 Hz (in test) future 97 Hz Pic. Delivery of X-rays into X13A.

10. 10 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge Comparison with undulator delivery and polarization profile and stability test Changes in time: t f switching frequency more problems are continuous change in polarization ratio t >> t f _ with long period change affects the comparison with different measurements t ≥ t f t =1% t f affects the difference signal introducing noise t f > t lower pass filters in lock-in

11. 11 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge Beam position and stability Angle (mrad) Flux density verticalhorizontal Cecilia: calculate the amount of beam we are having from EPW… in the horizontal and vertical and put that in shadow in the figure….

12. 12 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge Beam position and stability Angle (mrad) Flux density Angle (mrad) Degree of polarization (%)

13. 13 BROOKHAVEN SCIENCE ASSOCIATES Baltimore 7-11 JanuaryJanuary 7-11 Baltimore Experiment geometry SRI April 23-25 Baton Rouge

14. 14 BROOKHAVEN SCIENCE ASSOCIATES MGU EPW to X13A Mirror in place MGU EPW to X13B Mirror out Outlook X13A beamline Examples EPW and new mirror operation New mirror and beamline new alignment (August 2004) gain x 10 more intensity 50% of beamtime

15. 15 BROOKHAVEN SCIENCE ASSOCIATES SRI April 23-25 Baton Rouge