The use of fast-modulated elliptically polarized soft x-rays in the detection of small polarization signals Cecilia S á nchez-Hanke

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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

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Motivation to work with fast switching

Introduction of EPW how it works (very fast) no big explanationsbut 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 modesignals at interfaces and/or magnetic interfaces

Phase sensitive detectionXAS 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 yieldand photon yield. Scattering chamber on fast switching polarization, unique, because opens the possibilities for

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•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

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• 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.

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Signal from EPW directly into a photodiode, full intensity, And 20 x 20 entrance and exit slit…

a) no chopperb) with chopperSignal 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 beamlinesswitching frequency 22 Hztesting 47Hzfuture faster up to 97 Hz

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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)

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Y

X

Y

I =Z

Polarization and stability

Y

t

Angle (mrad)

Flux

den

sity

vertical horizontal

Δ pol. = 1% Polarization ratio @10 –3

Δ pol. = 0.001% Polarization ratio @ 10 –3

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Beam position and stability

Angle (mrad)

Flux

den

sity

Angle (mrad)

Deg

ree

of p

olar

izat

ion

(%)

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Top / bottom Permanent magnetsLeft / right Electromagnets

hard and soft x-raysLinear and circular

22 Hz 47 Hz (in test) future 97 Hz

Pic. Delivery of X-rays into X13A.

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Comparison with undulator delivery and polarization profile and stability test

•Changes in time: tf switching frequency more problems are continuous change in polarization ratio

•t >> tf _ with long period change affects the comparison with different measurements•t ≥ tf t =1% tf affects the difference signal introducing noise•tf > t lower pass filters in lock-in

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Beam position and stability

Angle (mrad)

Flux

den

sity

vertical horizontal

Cecilia: calculate the amount of beam we are having from EPW… in the horizontal and vertical and put that in shadow in the figure….

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Beam position and stability

Angle (mrad)

Flux

den

sity

Angle (mrad)

Deg

ree

of p

olar

izat

ion

(%)

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Experiment geometry

SRI April 23-25 Baton Rouge

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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 intensity50% of beamtime

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