A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic...

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Jochen Stahn Laboratorium f¨ ur Neutronenstreuung ETH Z¨ urich & Paul Scherrer Institut A neutron polariser based on magnetically remanent Fe/Si supermirrors ILL, Grenoble 26. 04. 2006

Transcript of A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic...

Page 1: A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic properties (magnetostriction) reason: shape of sputter target and aperture (ca. 100×400mm

Jochen StahnLaboratorium fur NeutronenstreuungETH Zurich & Paul Scherrer Institut

A neutron polariser

based on

magnetically remanent Fe/Si supermirrors

ILL, Grenoble 26. 04. 2006

Page 2: A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic properties (magnetostriction) reason: shape of sputter target and aperture (ca. 100×400mm

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remanent polariser application principle

transmission, polarisation

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HM ≈ 150 Oe, Hg < 20 Oe

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polarisation field H/Oe

transmission, polarisation

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M

M

Hg

Hg

HM

HM

Si wafer with supermirrorelectro magnet

unpolarised / polarised neutrons

Page 3: A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic properties (magnetostriction) reason: shape of sputter target and aperture (ca. 100×400mm

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remanent polariser material

ferromagnetic material: Fe

– might show easy axis of magnetisation

– almost matches Si for |−〉

– low absorption (required for transmission, less radiation damage)

spacer material: Si

– low potential

– matches the substrate (for transmission)

– low absorption

– can be influenced (potential and stress) by reactive sputtering

but

– rather low contrast for |+〉

⇒ larger number of layers reqired

– total reflection for low q

Page 4: A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic properties (magnetostriction) reason: shape of sputter target and aperture (ca. 100×400mm

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remanent polariser ideal ml

(bf + pf)ρf ≫ bsρs, ps = 0, ferromagnet, spacer

(bf − pf)ρf = bsρs

R↑

R↓10−4

10−3

10−2

10−1

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refle

ctiv

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ω / °

f s

Page 5: A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic properties (magnetostriction) reason: shape of sputter target and aperture (ca. 100×400mm

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remanent polariser real ml: Fe/Si:N:O

(bf + pf)ρf ≫ bsρs, ps = 0, ferromagnet, spacer

(bf − pf)ρf ≈ bsρs

R↑

R↓10−4

10−3

10−2

10−1

1

0 0.5 1 1.5 2 2.5 3 3.5 4

refle

ctiv

ity

ω / °

f s

interdiffusion

⇒ mag. dead layers

⇒ 2nd order peak

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remanent polariser magnetic properties

anisotropic in-plane stress

causes anisotropic magnetic properties (magnetostriction)

reason: shape of sputter target and aperture (ca. 100× 400 mm2)

⇒ spread of angle of incidence of sputtered atoms is anisotropic

⇒ growth and thus strain formation is effected

⇒ easy axis of magnetisation requires strained filmes!

mag

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M

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polarization field H/Oe polarization field H/Oe

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remanent polariser performance

M ⇀ ⇁Hg M ⇀ ↽Hg

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P reflec

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tyR

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smission

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ω/◦

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Hg = 15 OeHM = 200 Oeλ = 4.74 A

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remanent polariser Hc vs. layer thickness

saturation in H = −700 Oe

transmission measured in guide fields

Hg = +5 . . . + 45 Oe

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net

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Hg/Oe

51015202530354045

spin up spin down

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←− ←

ω/◦ ω/◦

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remanent polariser off-specular scattering

polarised beam, no spin analysis

Fe/Si:N:O sm, m = 2.4

assymetric off-specular signal

⇒ weak spin-flip scattering

|+〉 |−〉

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Page 10: A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic properties (magnetostriction) reason: shape of sputter target and aperture (ca. 100×400mm

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remanent polariser applications: analyser

at Morpheus, Narziss (SINQ)

coating Fe / Si:N:O

m = 3, 599 layers

substrate Si-wafer, 0.6 mm

mirror size 200× 60 mm2

magnet size 200× 100× 100 mm3

BM 200 Oe

Bg 20 Oe

PT, ⇀ ⇁

> 97 %

PT, ⇀ ↽

> 95 %

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remanent polariser applications: switchable polariser

at Amor (SINQ)

– FeCoV/TiN on glas

– operated in reflection mode

– saturation fields: ±400 Oe

– guide field: +20 Oe

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remanent polariser applications: white beam polariser

at SANS I (SINQ)

coating Fe / Si:N:O

m = 2.4, 299 layers

substrate Si-wafer, 0.6 mm

mirror size 2 00× 6 cm2

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ion

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2 m collimation

3 m collimation

6 m collimation

15 m collimation

λ / nm

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remanent polariser conclusion:

we produced supermirrors with Fe and Si:N:O which

– polarise neutrons (P > 95 % to P = 99 %)

– can be operated in transmission and reflection mode

– show a magnetic remanence

– thus need guide fields of 20 Oe, only

– can be operated antiparallel to the guide field

the reactive gases N2 and O2 in Si are needed to

– match the potentials for |−〉

– tailor strain in Fe layers (anisotropic stress), but

– keep the overall stress small

limitations:

– stress limits the number of layers ⇒ m < 3

– FeSi layer causes 2nd Bragg peak

⇒ |−〉 contamination in reflection mode