A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic...
Transcript of A neutron polariser based on magnetically remanent Fe/Si ... · causes anisotropic magnetic...
Jochen StahnLaboratorium fur NeutronenstreuungETH Zurich & Paul Scherrer Institut
A neutron polariser
based on
magnetically remanent Fe/Si supermirrors
ILL, Grenoble 26. 04. 2006
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remanent polariser application principle
transmission, polarisation
0
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ω/◦
HM ≈ 150 Oe, Hg < 20 Oe
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-100 -50 0 50 100
mag
net
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M
polarisation field H/Oe
transmission, polarisation
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ω/◦
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•
•
M
M
Hg
Hg
HM
HM
Si wafer with supermirrorelectro magnet
unpolarised / polarised neutrons
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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
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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
1
0 0.5 1 1.5 2 2.5 3 3.5 4
refle
ctiv
ity
ω / °
bρ
f s
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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 / °
bρ
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
net
izat
ion
M
-1
-0.5
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-800 -400 0 400 800 -100 -50 0 50 100
polarization field H/Oe polarization field H/Oe
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remanent polariser performance
M ⇀ ⇁Hg M ⇀ ↽Hg
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pol
arisat
ion
P reflec
tivi
tyR
tran
smission
T
ω/◦
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P reflec
tivi
tyR
tran
smission
T
ω/◦
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
mag
net
isat
ion
M
-1
-0.5
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-100 -50 0 50 100polarisation field H/Oe
Hg/Oe
51015202530354045
spin up spin down
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0 0.2 0.4 0.6 0.8 1 1.2 1.4 0 0.2 0.4 0.6 0.8 1 1.2 1.4
tran
smission
T
←− ←
−
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
|+〉 |−〉
ω ω
2θ
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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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refle
ctiv
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risin
g ef
ficie
ncy
q / Å-1
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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
pol
arisat
ion
0.4 0.5 0.6 0.7 0.8 0.9 1.00.94
0.96
0.98
1.00
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