Download - Neutron reflectometry
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Neutron reflectometry
Helmut FritzscheNRC-SIMS, Canadian Neutron Beam Centre, Chalk River, Canada
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Canadian Neutron Beam Centre
Outlook
Application/advantages of neutron reflectometry
Theoretical background
Instrumental setup
Experiments:• Photoactive azobenzene films• Hydrogen storage in MgAl films• Element-specific hysteresis curves in ErFe2 / DyFe2 multilayers
Supermirrors (non-polarizing and polarizing)
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Canadian Neutron Beam Centre
What can be measured with neutron reflectometry?
Film thickness (2 – 200 nm):swelling of polymer films due to water uptake film expansion during illumination of photoactive filmsfilm expansion during hydrogen absorptiongrowth of oxide layer
In-plane structures on nm and m scale
Scattering length density profile:profile of absorbed gas/liquidinterdiffusion magnetic structuresmagnetic field penetration into superconductors
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Canadian Neutron Beam Centre
Specific advantages of neutron reflectometry
Large penetration depth (for most materials):Buried layersIn-situ measurements (cryostats, cryomagnets, high-pressure cells, furnaces)
Spin and non-spin flip reflectivity:Magnetization reversal, magnetic structure
No diamagnetic background of substrate for ferromagnetic samples:Determination of absolute magnetic moment
High sensitivity to hydrogen:Determine hydrogen profile in hydrogen storage materials
Change of contrast by using isotopes:swelling of films during water (vapor or liquid) uptake (H2O / D2O)expansion of films during hydrogen absorption (H2 / D2)
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Canadian Neutron Beam Centre
Reflection and refraction
specularly reflected
refracted
incoming wave
Physical origin:
different index of refraction for two media
medium 1: n1
medium 2: n2
Refraction: Snell‘s law
n1 sin 1 = n2 sin 2
2
1
Reflection:
r = 1
r
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Canadian Neutron Beam Centre
Reflection and refraction:the critical angle
reflectedrefracted
medium 1: n1
medium 2: n2 90°
c
Critical angle: n1 sin 1 = n2 sin 90° sin c = n2 / n1
For 1 > c : no refracted beam exists, only a reflected beam
Total reflection (100% reflectivity) occurs in the medium with the larger n
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Canadian Neutron Beam Centre
Index of refraction for light
For light with = 656 nm:
Material n c (for n2=1)
Vacuum 1.00 -Water 1.33 48.8Quartz glass 1.46 43.2Benzene 1.50 41.8
What is the index of refraction for neutrons?
Note: the index of refraction depends on the wavelength
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Canadian Neutron Beam Centre
Index of refraction for neutrons
z
Ez
}Ekin,1
V SLD
Ekin,2
bkVm
kk
nVmk
mk
2
21
21
222
221
2121
22
bm
V 22 Fermi’s pseudopotential:
m: neutron mass: neutron wavelengthb: nuclear scattering length: density of atomsb: scattering length density (SLD)
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Canadian Neutron Beam Centre
Scattering lengths
X-rays
X-rays: b Z (electron density)
Neutrons
Neutrons: no systematics
Important: not absolute number but contrast of SLX-rays and neutrons are complementary probes
0 5 10 15 20 25 300
1020304050607080
b (f
m)
atomic number
Ne
Ca
H
FeCr
Ti
ArS
SiMg
OC
BeHe
Ni
0 5 10 15 20 25 30-5
0
5
10
15
b (f
m)
atomic number
H
D
He
Li
Be
BC
N
O FNe
Na
Mg
AlSi
P
S
Cl
Ar
KCa
Sc
Ti
V
Cr
Mn
Fe
Co
58Ni
Cu
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Canadian Neutron Beam Centre
Index of refraction for neutrons:some examples
For neutrons with = 0.237 nm:
Material n b (10-4 1/nm2)
Vacuum 1.00 0Water (H2O) 1.000001 -0.561Si 0.999998 2.073Quartz glass 0.999997 4.185Heavy water (D2O) 0.999994 6.36658Ni 0.999988 13.16
Note: n 1-10-5
The deviation of nneutron from 1 is much smaller than for light,because the interaction of neutrons with matter is much weaker
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Canadian Neutron Beam Centre
Reflectometry setup on D3
S1
S2
S3S4
sample
PG filter
analyzer
detector
Focusing PG monochromator
Polarizing supermirror
Spin-down neutrons
spin flipper
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Canadian Neutron Beam Centre
Reflectometry setup on D3
S1
S2
S3S4
sample
PG filter
analyzer
detector
Focusing PG monochromator
Polarizing supermirror
Spin-down neutrons
spin flipper
Spin-up neutrons
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Canadian Neutron Beam Centre
The reflectometry experiment
detector
sample
slit system
q2
q: scattering vector: scattering angle
sin4
ir kkq
geometry:sample moves by detector moves by 2
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Canadian Neutron Beam Centre
The reflectometry experiment
detector
sample
slit system
q
Reflectometry:Measuring the reflected intensity as a function of q
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Canadian Neutron Beam Centre
Visualization of a reflectivity curve (Si wafer)
z
Ez
refle
ctiv
ity
q
nucbm
V 22
}
qc
bq ccc sin4
Si: c=0.11º (for =2.37 Å)58Ni: c=0.28º (for =2.37 Å)
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Canadian Neutron Beam Centre
Kiessig fringes
A u S izd
V
0.00 0.02 0.04 0.06 0.08 0.1010-5
10-4
10-3
10-2
10-1
100
Ref
lect
ivity
q (Å-1)
Oscillations due to total film thicknessq 1/d
q=2/d
qc
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Canadian Neutron Beam Centre
Multilayer Bragg peaks
bilayer Bragg peaks at q=2/t
q = n · 2/62.8 Å-1 = n · 0.1 Å-1
0.00 0.05 0.10 0.15 0.20 0.25 0.301E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
refle
ctiv
ity
q (Å-1)
Short period oscillations:Kiessig fringes
Fe
SLD
Cr
Fe
CrSi wafer
Fe
Cr}Bilayer thickness tt = 32.8 Å + 30 Å = 62.8 ÅIn total: 20 repetitions
•••
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Canadian Neutron Beam Centre
Magnetic interaction
magnuc VVV Bbm
22 magnuc bbm
22
Hext: external magnetic fieldB : magnetic inductionµ : magnetic moment of neutrons
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Canadian Neutron Beam Centre
PNR: bulk Fe
0.00 0.05 0.10 0.151E-6
1E-5
1E-4
1E-3
0.01
0.1
1 R+
R-
Runmag
refle
ctiv
ityq (1/Å)
Different reflectivity for spin-up and spin-down neutrons
Determination of the absolute magnetic moment possible
qc- qc
+
Vnuc
BVmag
BVmag
V
spin up (R+) spin down (R-)
Vnuc
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Canadian Neutron Beam Centre
PNR: Fe/Cr multilayers
1.3 nm C r
M gO (001)
2 .5 nm Fe
x 20
d struc
B
H ex t
2 .5 nm Fe1.3 nm C r
1 .3 nm C r
M gO (001)
2 .5 nm F e
x 20
d st ruc
B
H ex t
2.5 nm F e1 .3 nm C r
d A F
0.00 0.05 0.10 0.15 0.201E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
R+
R-
refle
ctiv
ity
q (1/Å)0.00 0.05 0.10 0.15 0.20
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
0.1
1
R+
R-
refle
ctiv
ity
q (1/Å)
Structural peakStructural peak
AF peak
Ferromagnetic coupling:Magnetic period = chemical period
Antiferromagnetic coupling:Magnetic period = 2 x chemical period
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Canadian Neutron Beam Centre
In-situ setup for photoactive films
lenses
shutter mirror
Neutron reflectometry and Laser illumination at the same time
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Canadian Neutron Beam Centre
Results for azobenzene films
0.0 h
0.4 h2.5 h
8.0 h
Laser irradiation time
Smaller q larger film thickness
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Canadian Neutron Beam Centre
Co-sputtering of MgAl alloy films
Mg Al Pd
Vacuum Chamber
<100> Si Wafer
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Canadian Neutron Beam Centre
Hydrogen absorption
Hydrogen gas cylinderAbsorption cell for thin filmson wafers with up to 100 mm diameter
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Canadian Neutron Beam Centre
Hydrogen desorptionequipment
Reflectometry furnace:
Ar atmosphere or vacuum300 K < T < 670 K
sampleheater
thermocouple
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Canadian Neutron Beam Centre
Mg0.6 Al0.4 at 298 K
0.00 0.02 0.04 0.06 0.08 0.101E-6
1E-5
1E-4
1E-3
0.01
0.1
1
exp. data simulation
Si / 52 nm Mg0.6
Al0.40
/ 10 nm Pd
Ref
lect
ivity
q (Å-1)
T = 298 K
0 100 200 300 400 500 600 700 8000.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
Si / 52 nm Mg0.6
Al0.4
/ 10 nm Pd
SLD
(Å-2
)
z (Å)
Mg0.6Al0.4PdSiO2
Si
Fit:
Pd: t = 104 Å = 4.4 ÅMgAl: t = 520 Å = 15.7 Å
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0 100 200 300 400 500 600 700 8000.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
without hydrogen with hydrogen
Si / 52 nm Mg0.6Al0.4 / 10 nm Pd
SLD
(Å-2
)
z (Å)
Canadian Neutron Beam Centre
Absorption in Mg0.6 Al0.4
• increase of film thickness by about 20%• hydrogen content is 83 at.% = 3.2 weight %
SLDbH < 0
t
0.00 0.02 0.04 0.06 0.08 0.101E-6
1E-5
1E-4
1E-3
0.01
0.1
1 without hydrogen with hydrogen
Si / 52 nm Mg0.6Al0.4 / 10 nm Pd
Ref
lect
ivity
q (Å-1)
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Canadian Neutron Beam Centre
Annealing of a desorbed Mg0.7 Al0.3 film
Pd layer does not exist anymore after 9 h:Pd diffuses into the MgAl layer
0 100 200 300 400 500 600 700 8000.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
5.0x10-6
1 h @ 473 K 3 h @ 473 K 9 h @ 473 K
SLD
(Å-2
)
z (Å)
0.00 0.02 0.04 0.06 0.08 0.10
1E-5
1E-4
1E-3
0.01
0.1
1
1 h @ 473 K 3 h @ 473 K 9 h @ 473 K
Ref
lect
ivity
q (Å-1)
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Canadian Neutron Beam Centre
DyFe2 / ErFe2 multilayer:element-specific hysteresis
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1E-4
1E-3
0.01
0.1
1
0H = 6.0 T
q (Å-1)
Ref
lect
ivity
1E-4
1E-3
0.01
0.1
1
fit of R- -
fit of R+ +
H = 1.0 T
1E-4
1E-3
0.01
0.1
1
R- -
R+ +
H = 0.4 TMagnetization reversal at 100 KAfter saturation at µ0H = –6 T(6 nm DyFe2 / 6 nm ErFe2)40
0 1 2 3 4 5 6-1.5
-1.0
-0.5
0.0
0.5
1.0
1.5
DyFe2
ErFe2
averageM
(T)
0H (T)
ErFe2 and DyFe2 magnetizations are not parallelDyFe2: easy-axis loopErFe2: hard-axis loop
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Canadian Neutron Beam Centre
PNR is element-specific
ErFe2 DyFe2
R+ = R-
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0 TM
ErFe2 = 0 T
q (Å-1)
Ref
lect
ivity
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0.13 TM
ErFe2 = -0.13 T
1E-4
1E-3
0.01
0.1
1 R+ +
R- -
MDyFe2
= -0.13 TMErFe2 = 0.13 T
nonmagnetic layers
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Canadian Neutron Beam Centre
PNR is element-specific
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0 TM
ErFe2 = 0 T
q (Å-1)
Ref
lect
ivity
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0.13 TM
ErFe2 = -0.13 T
1E-4
1E-3
0.01
0.1
1 R+ +
R- -
MDyFe2
= -0.13 TMErFe2 = 0.13 T
ErFe2 DyFe2
ErFe2 DyFe2
R+ = R-
~R+ ~R-H
D y F e 2
E rF e 2
nonmagnetic layers
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Canadian Neutron Beam Centre
PNR is element-specific
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0 TM
ErFe2 = 0 T
q (Å-1)
Ref
lect
ivity
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0.13 TM
ErFe2 = -0.13 T
1E-4
1E-3
0.01
0.1
1 R+ +
R- -
MDyFe2
= -0.13 TMErFe2 = 0.13 T
ErFe2 DyFe2
ErFe2 DyFe2
R+ = R-
~R+ ~R-H
D y F e 2
E rF e 2
nonmagnetic layers
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Canadian Neutron Beam Centre
PNR is element-specific
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0 TM
ErFe2 = 0 T
q (Å-1)
Ref
lect
ivity
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0.13 TM
ErFe2 = -0.13 T
1E-4
1E-3
0.01
0.1
1 R+ +
R- -
MDyFe2
= -0.13 TMErFe2 = 0.13 T
ErFe2 DyFe2
R+ = R-
ErFe2 DyFe2
~R- ~R+
ErFe2 DyFe2
~R+ ~R-H
D y F e 2
E rF e 2
H
D y F e 2
E rF e 2
nonmagnetic layers
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Canadian Neutron Beam Centre
PNR is element-specific
0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0 TM
ErFe2 = 0 T
q (Å-1)
Ref
lect
ivity
1E-4
1E-3
0.01
0.1
1
MDyFe2
= 0.13 TM
ErFe2 = -0.13 T
1E-4
1E-3
0.01
0.1
1 R+ +
R- -
MDyFe2
= -0.13 TMErFe2 = 0.13 T
ErFe2 DyFe2
ErFe2 DyFe2
R+ = R-
~R+ ~R-
ErFe2 DyFe2
~R- ~R+
H
D y F e 2
E rF e 2
H
D y F e 2
E rF e 2
nonmagnetic layers
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Canadian Neutron Beam Centre
supermirror
goal:Extend the range of neutron reflectionbeyond the regime of total reflection
concept:continuous Bragg reflection from a multilayercomposed of bilayerswith a variation of the thickness
realization:Ni/Ti multilayer, bNi = 10.3 fm, bTi = -3.4 fm100 bilayers,qc = 2 x qc, Ni
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Canadian Neutron Beam Centre
supermirror
m-value: m = qc / qc, Ni
Ni
SLD
Ni
Ti Ti Ti
Ni
z
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Canadian Neutron Beam Centre
Polarizing supermirror
concept:Using the supermirror concept with a magnetic/non-magnetic bilayerThe SLD of the bilayer is index-matched for spin-down neutronsno multilayer Bragg peaks for down-neutronsSpin-up neutrons show supermirror behavior with extended critical edge
Fe/Co
SLD
spin-up neutrons
Si
Fe/Co
Si
spin-down neutrons
Fe/Co
SLD
Si Fe/Co Si
Index matching
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Canadian Neutron Beam Centre
Polarizing supermirror:Fe-Co/Si
0.0 0.2 0.4 0.6 0.8 1.0 1.20
1000
2000
3000
4000
5000
R_down R_up
refle
cted
neu
trons
(cou
nts)
(deg)
supermirror 5127u with =0.472 nm
0.0 0.2 0.4 0.6 0.8 1.0 1.20
1000
2000
3000
4000
T_down T_up
trans
mitt
ed n
eutro
ns (c
ount
s)
(deg)
supermirror 5127u with =0.472 nm
Reflected intensity Transmitted intensity
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Canadian Neutron Beam Centre
Flipping ratio
0.0 0.2 0.4 0.6 0.8 1.0 1.20
20
40
60
80
100
transmission reflectivity
flipp
ing
ratio
(deg)
supermirror 5127u with =0.472 nm
RRRR
Flipping ratio =
usable range25