neutron reflectometry
DESCRIPTION
Neutron reflectometry. Helmut Fritzsche NRC-SIMS, Canadian Neutron Beam Centre, Chalk River, Canada. Application/advantages of neutron reflectometry Theoretical background Instrumental setup Experiments: Photoactive azobenzene films Hydrogen storage in MgAl films - PowerPoint PPT PresentationTRANSCRIPT
Neutron reflectometry
Helmut FritzscheNRC-SIMS, Canadian Neutron Beam Centre, Chalk River, Canada
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)
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
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)
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
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
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
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)
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
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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Reflectometry setup on D3
S1
S2
S3S4
sample
PG filter
analyzer
detector
Focusing PG monochromator
Polarizing supermirror
Spin-down neutrons
spin flipper
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
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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The reflectometry experiment
detector
sample
slit system
q
Reflectometry:Measuring the reflected intensity as a function of q
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)
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
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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Magnetic interaction
magnuc VVV Bbm
22 magnuc bbm
22
Hext: external magnetic fieldB : magnetic inductionµ : magnetic moment of neutrons
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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
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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In-situ setup for photoactive films
lenses
shutter mirror
Neutron reflectometry and Laser illumination at the same time
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
Canadian Neutron Beam Centre
Co-sputtering of MgAl alloy films
Mg Al Pd
Vacuum Chamber
<100> Si Wafer
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Hydrogen absorption
Hydrogen gas cylinderAbsorption cell for thin filmson wafers with up to 100 mm diameter
Canadian Neutron Beam Centre
Hydrogen desorptionequipment
Reflectometry furnace:
Ar atmosphere or vacuum300 K < T < 670 K
sampleheater
thermocouple
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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
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)
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)
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
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
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
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
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
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
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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supermirror
m-value: m = qc / qc, Ni
Ni
SLD
Ni
Ti Ti Ti
Ni
z
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
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
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