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Magnetic heterostructures as seen by x-rays and neutrons
Hartmut ZabelExperimentalphysik/FestkörperphysikRuhr-Universität Bochum, Germany
MMSPN, 7-9 December, 2001, Budapest, Hungary
Thin films for magnetism, electronics, spintronics, superconductivity, optics,
corrosion protection...
Magnetic filmsGMR heterostructures
S
S
Proximity effects and tunneling
Lateral structures
F - pinned
Exchange bias
• Dimensional scaling of Ho/Y
• Exchange bias of Co/CoO
• Enhancement of interface sensitivity
Topics:
ξx
ξz
ξy
Scaling behavior of the critical temperature with thickness:
ξx
ξz=0 ξy
Connection between correlation lengthand suszeptibility:
C
Cv
TTT −== − εεξξ ,0,
1)(
22±+
±=ξ
χχqq
0 10 20 30 40 50 60 70 80 90 100
1020304050
60708090
100
T/T c
Thickness t
( ) ( )( )
λ−
⋅=
∞−∞
0ttb
TtTT
C
CC
λ=1/νλ=1/νλ=1/νλ=1/ν
How does the Ho spin structure and the Néel temperature depend on interlayer coupling?
Ho
Single film
Ho
Y
Ho
Y
Ho
Coupledsuperlattice
Ho
YHx
Ho
YHx
Ho
Breaking the coupling with
hydrogen
V. Leiner, D. Labergerie, H. Zabel
Bulk Holmium Phase Diagram
W.C. Koehler et al. PR 151 (1966) 414G.P. Felcher et al. PRB 13 (1976) 3034M.J. Pechan and C. Stassis, JAP 55 (1984) 1900
30° lock + cone inc. helix para
0 20 131.5 T[K]
9.68 µB
-τ +τ
G [001]
I +τ
Magnetic peaks from a spin helix are expected at: �
±= GK
4 6 8 10 1214 1618 20
0,00
0,05
0,10
0,15
0,20
30Å
4 6 8 10 1214 1618 200,00
0,05
0,10
0,15
0,20
0,25
0,30
0,35
2θ [deg]
Intensity [a.
46Å
4 6 8 10 121416 1820
0,2
0,4
0,6
0,8
1,0
1,2
1,4
100 Å
6 8 10 12-0,020,000,020,040,060,080,100,120,14
1000 Å
RKKY-type Interactions in Ho/Y Superlattices
RKKY-type interlayer coupling mediated by Y conduction electrons
RKKY coupling between localised Ho 4f - moments
5s
5s
IEC
4f
4f
Ho
Ho
tYY
Magnetic τ-peak for a single thin Ho film:Y
Ho
Y
I
Q
Magnetic τ-peak for a coupled Ho/Y superlattice:
Ho τ-peak⊗ =
I I I
Q
Modulation by the chemical periodicity
Tunability of the exchange coupling in Fe/V superlattices with hydrogen
V. Leiner, K. Westerholt, B. Hjörvarsson, H. Zabel, to be published
Ker
r an
gle
Fe /V
-2000 -1000 0 1000 2000
-0,003
-0,002
-0,001
0,000
0,001
0,002
0,003
(3) (13)
p(H2) = 0 mbar
beforeH-loading
afterH-loading< 10 mbar
-0,003
-0,002
-0,001
-2000 -1000 0 1000 2000
0,000
0,001
0,002
0,003
8 mbar
Ker
r an
gle
Exchange Coupling in Fe/Cr Superlattices
Λ
Q||
Q⊥
Λ
Q||
Q⊥
R. Siebrecht, A. Schreyer, H. Zabel, 2000
( )+ ( )+10 Oe 215 Oe
0,0 0,5 1,0 1,5 2,0 2,5 3,00
20
40
60
80
100
120
ferroantiferro
(Fe2V13)100 m agnetic phase diagram
T C,N
(K)
∆q /q (%)
2201
22
L)Lk()Lkcos(J)L(J
F
FRKKY ∝=
L
J
Crtical temperature as a function of interlayer exchange coupling for Fe 2 ML/VHx 12 ML
Magnetic resonance x-ray scattering of Ho at theLIII - absorption edge (8074 eV), sp-channel
Troika beam line, ESRF
C. Sutter, E. Weschke, R. Meier, C. Schüßler-Langeheine, G. Grübel, D. Abernathy
31nm Ho16.4nm Ho
Resonant enhancement of the magnetic superstructure satellite at the Ho MV edge
Reflectivity of a 109-ML-thick Ho film (a) well below and (b) in the vicinity of the Ho MV edge
0.05 0.10 0.15 0.20 0.25 0.30
(000+τ) T = 40 K
(a)
(b)
τ
109 ML Ho/W(110)
Refle
cted
X-R
ay In
tens
ity
(arb
. uni
ts)
Scattering Vector (c*)
hν = 1338 eV
hν = 900 eV
at resonance
below resonance
1.5 2.0 2.5 3.0 3.5 4.0 4.5
x raysneutrons
46Å HoT=70 KIn
tens
ity (a
rb. u
nits)
q(nm-1)
2 3 4q(nm-1)
2 3 4q(nm-1)
E. Weschke, C. Schüßler-Langeheine, A. Yu. Grigoriev, H. Ott, G. Kaindl, V. Leiner and H. Zabel, to be published
Resonant enhancement of the magnetic superstructure satellite at the Ho MV edge, BESSY
(00.0 +τ) peak of a single 4.6 nm thick Ho(00.1) film
Critical scattering from a 4.6 nm thick Ho film
XRMD results, BESSY
Short range order diffuse scattering
E. Weschke, C. Schüßler-Langeheine, A. Yu. Grigoriev, H. Ott, G. Kaindl, V. Leiner and H. Zabel, to be published
Order parameter and inverse correlation length
106 108 110 112 114 1160
2
4
6
8
10
TN = 110 K
Integrated Intensity (arb. units)
Temperature (K)H
WH
M (1
0-3 Å
-1)
-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50
Ker
rRot
atio
n [a
. u.]
2nm CoOCo
-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50
1. magnetization reversal after field cooling
-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50
2. reversal
-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50
Trained hysteresis
MOKE measurements of a CoO/Co bilayer
Magnetic field (Oe)
(00.1)
F. Radu, M. Etzkorn, T. Schmitte, R. Siebrecht, V. Leiner, K. Westerholt, H. Zabel
Polarized neutron reflectivity Y
XM
Q
θφ φ
nµ�
1. Non-spin flip(NSF) scattering: the Y-component
Y
XM
Q
θφ φ
nµ�
Ym,, M(θpRR 2)sin2 ∝=− −−++
2. Spin-Flip (SF) Scattering:
the X-component
Y
XM
Q
θφ φ
nµ�
xm,, M(θpRR ∝== −+−+ )cos
HE
M
HM=0
+,+ -,-+,-
Hex
Bc HH ±
H
Neutron Scattering:
+,+ -,-
+,- Hex
H
Magnetometry:
H
Rotation
H
Wall motion
CoOCo
2nm
PNR results from a CoO/Co bilayer
-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50
Ker
rRot
atio
n [a
. u.]
-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50
-1200 -900 -600 -300 0 300 600 900 12000,380,400,420,440,460,480,50
-1500 -1000 -500 0 500 1000 1500
10
20
30
40
50
60
70
80
90
Spin
Flip
Inte
nsiti
es [c
ts/se
c]
Magnetic Field [Oe]-1000 -900 -800 -700 -600 -500
10
20
30
40
50
60
70
80
90
Spin
Flip
Inte
nsiti
es [c
ts/se
c]
Magnetic Field [Oe]
(+,+)
(-,-)
(+,-)
EVA-Neutron Reflectometer at the ILL with a 3He polarisation analyser:
B. Nickel A. Rühm, W. Donner, J. Major, H. Dosch, A. Schreyer, H. Zabel, and H.Humblot, Rev. Sci. Instr. 72, 163 (2001)
d ≈1- 10 nm0.1nm
0,0 0,1 0,2 0,3 0,4 0,5 0,61E-81E-71E-61E-51E-41E-30,010,1
1
X-r
ay re
flect
ivity
Scattering vector
2π/d
Enhancement of interface sensitivity
PNR of 2 nm Fe/ 150 nm Nb on sapphireADAM neutron reflectometer, ILL
K. Theis-Bröhl, R. Siebrecht, H. Zabel, 2001
Substrate
Nb - filmFe
Substrate
Nb - filmFe
0,00 0,05 0,10 0,15 0,201E-5
1E-4
1E-3
0,01
0,1
1
FeNb MBE1214: low temperature T =1.5 K
uunddndun
Ino
rmal
ized
to m
onito
r
Q [Å1]
bcohMoV MoV MoVV V
Λ
bcohMoV MoV MoVV(D) V(D)
bcohMoV MoV MoVV(D) V(D)
Contrast matching for enhancing interface sensitivity
V. Leiner, B. Hjörvarsson, J. Birch, H. Zabel, submitted
Neutron wave guide (resonator) with polarized neutrons
Cd
Cd
Fe
FeTi
PSD
Reflector
V.K. Ignatovich, F. Radu, V.L. Aksenov, Yu.V.Nikitenko, Yu. M. Gledenov, P.V. Sedyshev
Frank Laboratory, Dubna
-1000 0 1000 2000 3000 4000-6,00E-008
-4,00E-008
-2,00E-008
0,00E+000
2,00E-008
4,00E-008
6,00E-008
8,00E-008
1,00E-007
1,20E-007
1,40E-007
1,60E-007
1,80E-007
2,00E-007
bcoh+pmag
bcoh bcoh-pmag
scat
terin
g le
ngth
th ickness
Magnetic film SubstrateSurface
Florin Radu
0,000 0,005 0,010 0,015 0,020 0,025 0,030
0,0
0,2
0,4
0,6
0,8
1,0
R R++ R+- R-- R-+
Ref
lect
ivity
Scattering vector Q
Resonances below the critical edge of total reflection
Magnetizationvector 45°
0,000 0,005 0,010 0,015 0,020 0,025 0,030
0,0
0,2
0,4
0,6
0,8
1,0
R Rpp Rpm Rmm Rmp
Ref
lect
ivity
Scattering vector
Magnetizationvector 0°
Resonances below the critical edge of total reflection
Summary� Neutrons reveal the spin structure and exchange
coupling in magnetic superlattices� Neutrons can distinguish between wall motion
and rotation during magnetization reversal� Resonant magnetic x-ray scattering provides
element selective information on magnetichysteresis and spin structures
� Interface sensitivity can be enhanced by resonant techniques and/or clever design of the sample