Temperature-Dependent Biquadratic Exchange Coupling...
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Temperature-Dependent Biquadratic Exchange Coupling in Co/CuMn Multilayers Thomas Saerbeck, Nick Loh, Frank Klose Australian Nuclear Science and Technology Organisation - Bragg Institute PNCMI Workshop July 1-5, 2010
Transcript of Temperature-Dependent Biquadratic Exchange Coupling...
Temperature-Dependent Biquadratic Exchange Coupling
in Co/CuMn Multilayers
Thomas Saerbeck, Nick Loh, Frank KloseAustralian Nuclear Science and Technology Organisation - Bragg Institute
PNCMI Workshop July 1-5, 2010
Frank KlosePNCMI
Magnetic Coupling in Thin Films
• Exchange Bias Systems
Pinned FM
• Multiferroics
• Interlayer Exchange Coupling
• Magnetic Springs
La
Ba
Y
OMn
LCMO
YBCO
Cu
• Superconductors
• Patterned Films
• Magnetic Semiconductors
EuS
PbS
► J. Chakhalian et al., Nature Physics 2, 244 (2006)
► J. Hoppler et al., Nature Materials 8, 315 (2009)
▲ K. Theis-Broehl et al., New Journal of Physics 10, 093021 (2009)
▲ H. Kepa et al.,Phys. Rev. B 64, 121302 (2001)
► H. Bea et al., Phys. Rev. Lett. 100, 017204 (2009)
► K.V. O’Donovan et al. Phys. Rev. Lett. 88, 067201 (2002)
ΛΛΛΛmag= 2ΛΛΛΛchem
ΛΛΛΛchem
Frank KlosePNCMI
Magnetic Coupling in Thin Films
• Exchange Bias Systems
Pinned FM
• Multiferroics
• Interlayer Exchange Coupling
• Magnetic Springs
La
Ba
Y
OMn
LCMO
YBCO
Cu
• Superconductors
• Patterned Films
• Magnetic Semiconductors
EuS
PbS
► J. Chakhalian et al., Nature Physics 2, 244 (2006)
► J. Hoppler et al., Nature Materials 8, 315 (2009)
▲ K. Theis-Broehl et al., New Journal of Physics 10, 093021 (2009)
▲ H. Kepa et al.,Phys. Rev. B 64, 121302 (2001)
► H. Bea et al., Phys. Rev. Lett. 100, 017204 (2009)
► K.V. O’Donovan et al. Phys. Rev. Lett. 88, 067201 (2002)
ΛΛΛΛmag= 2ΛΛΛΛchem
ΛΛΛΛchem
Interlayer exchange coupling:
How do magnetic ions in the spacer layer impact the exchange coupling?
Multilayer system: Co/Cu94Mn6
Interesting feature: Bulk Cu94Mn6 is a spinglass @ <10 K
Frank KlosePNCMI
Temperature Dependent Giant Magnetoresistance in Co/Cu94Mn 6 Multilayers
Co
Co
Co
Co
CuMn
CuMn
CuMn
CuMn
From:
Y. Kobayashi et al., Phys. Rev. B, 59, 3734 (1999)
• Pure AFM ordering fails to explain observations: Biquadratic coupling?
• Influence of spin glass on RKKY coupling?
Change from FM (300 K ) to AFM (4 K)?
d (CuMn) = 15 Å, 19 Å, 26 Å
Frank KlosePNCMI
Polarised Neutron Reflectometry
x
y
z
Hext
M ||
M ⊥⊥⊥⊥
M tot
φφφφ
ααααiααααfSpin up
Spin down
R++
R--R
-+
R+-
Specular Reflection:ααααi = ααααf
Qz = 4ππππ/λλλλ sin ααααi
Frank KlosePNCMI
PNR: Off-specular ScatteringLateral magnetic domains
• Different magnetization
• Different direction
Or structural domains
Frank KlosePNCMI
PNR: Off-specular ScatteringOff-Specular Reflection:
∆α∆α∆α∆αf ≠≠≠≠ 0000
QZ= 2ππππ/λλλλ (sin ααααi+ sin( ααααf+∆α∆α∆α∆αf))
QX = 2ππππ/λλλλ (cos( ααααf+∆α∆α∆α∆αf)- cos ααααi)
k i
k f
QX
QZZZZ Qtotaltotaltotaltotal
Frank KlosePNCMI
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01E-5
1E-4
1E-3
0.01
0.1
1
R++ R-- R-+ R+-
Nor
mal
ized
spe
cula
r re
flect
ivity
Incident angle ααααi (°)
Polarized neutron reflectivity at 300 K and 7 mT
Co
Co
Co
CoCu0.94Mn0.06
Cu0.94Mn0.06
Cu0.94Mn0.06
Cu0.94Mn0.06
Magnetization
1st order Bragg-peak
PeriodicityRoughness
T = 300 KB = 7 mT
1st order Bragg peak
⇒ FM coupling
Frank KlosePNCMI
Co
Co
Co
CoCu0.94Mn0.06
Cu0.94Mn0.06
Cu0.94Mn0.06
Cu0.94Mn0.06
1/2 order (AFM) peak 1st order
Bragg-peak
Antiferromagnetic coupling?
Half order peak
⇒ Double periodicity antiferromagnetism
Spin Flip Channel ⇒⇒⇒⇒ Canting
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01E-5
1E-4
1E-3
0.01
0.1
1
Nor
mal
ized
spe
cula
r re
flect
ivity
(ar
b. u
n.)
Incident angle ααααi (°)
R++ R-- R-+ R+-
T = 30 KB = 7 mT
Polarized neutron reflectivity at 30 K and 7 mT
Frank KlosePNCMI
Co/Cu0.94Mn0.06MultilayerOff-specular scattering (ααααi vs. ααααf) at low temperature
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Out
goin
g an
gle
α α α αf [°
]
Incident angle ααααi [°]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0O
utgo
ing
angl
e α α α α
f [°]
Incident angle ααααi [°]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Out
goin
g an
gle
α α α αf [°
]
Incident angle ααααi [°]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Out
goin
g an
gle
α α α αf [°
]
Incident angle ααααi [°]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Out
goin
g an
gle
α α α αf [°
]Incident angle αααα
i [°]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Out
goin
g an
gle
α α α αf [°
]
Incident angle ααααi [°]
T=30 K, B=7 mT
R++, simulation
T=30 K, B=7 mT
R--, simulation
T=30 K, B=7 mT
R-+, simulation
T=30 K, B=7 mT
R++
T=30 K, B=7 mT
R--
T=30 K, B=7 mT
R-+
Simulation: DWBA method (B. Toperverg)
Frank KlosePNCMI
PNR simulations at 30 K
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Out
goin
g an
gle
α α α αf [°
]
Incident angle ααααi [°]
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.0
Out
goin
g an
gle
α α α αf [°
]
Incident angle ααααi [°]
T=30 K, B=7 mT
R-+, simulation
T=30 K, B=7 mT
R-+
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.01E-5
1E-4
1E-3
0.01
0.1
1
Nor
mal
ized
spe
cula
r re
flect
ivity
(ar
b. u
n.)
Incident angle ααααi (°)
R++ R-- R-+ R+-
-1.0 -0.5 0.0 0.5 1.0
1E-4
1E-3
0.01
Nor
mal
ized
offs
pecu
lar
refle
ctiv
ity
Offspecular angle φφφφ (°)
R++ R-- R-+ R+-
Simulation: DWBA method (B. Toperverg)
Frank KlosePNCMI
Magnetization model at low temperature: Average domain size 0.43µµµµm
30°
30°
-30°
-30°
-30°
-30°30°
30°
60°60°
H
Projections
Spin-flip
Non Spin-flip
Co: B
Co: A
Co: B
Co: A
Layer:
“Biquadratic” 60 deg. coupling is present!
Frank KlosePNCMI
Temperature dependence
1.0 1.2 1.4 1.6 1.8 2.01.0
1.2
1.4
1.6
1.8
2.0
T=4K
αα αα f [°
]
ααααi [°]
1.0 1.2 1.4 1.6 1.8 2.01.0
1.2
1.4
1.6
1.8
2.0
T=15K
αα αα f [°
]
ααααi [°]
1.0 1.2 1.4 1.6 1.8 2.01.0
1.2
1.4
1.6
1.8
2.0
T=45K
αα αα f [°
]
ααααi [°]
1.0 1.2 1.4 1.6 1.8 2.01.0
1.2
1.4
1.6
1.8
2.0
T=60Kαα αα f
[°]
ααααi [°]
1.0 1.2 1.4 1.6 1.8 2.01.0
1.2
1.4
1.6
1.8
2.0
T=75K
αα αα f [°
]
ααααi [°]
1.0 1.2 1.4 1.6 1.8 2.01.0
1.2
1.4
1.6
1.8
2.0
T=90K
αα αα f [°
]
ααααi [°]
2.2 2.0 1.8 1.6 1.4 1.2 1.0
Inte
grat
ed In
tens
ity [a
rb. u
n.]
Angle of Linescan along ααααf=-ααααi+3.17°
T = 4K T = 15K T = 45K T = 60K T = 75K T = 90K T = 100K
0
• Continuous decrease in intensity with increasing temperature
• Constant domain size
• Rotation of magnetization
T = 4 K T = 60 K
T = 45 K T = 90 K
T = 15 K T = 75 K
Frank KlosePNCMI
External field dependence at 8K, R-+ channel
1.0 1.5 2.0
1.0
1.5
2.0
30K, 7mT -+ channel
αα ααf [
°]
ααααi [°]
1.0 1.5 2.0
1.0
1.5
2.0
αα ααf [
°]
ααααi [°]
30K, 25mT -+ channel
1.0 1.5 2.0
1.0
1.5
2.0
30K, 70mT -+ channel
αα ααf [
°]
ααααi [°]
1.0 1.5 2.0
1.0
1.5
2.0
30K, 100mT -+ channel
αα ααf [
°]
ααααi [°]
1.0 1.5 2.0
1.0
1.5
2.0
30K, 150mT -+ channel
αα ααf [
°]
ααααi [°]
1.0 1.5 2.0
1.0
1.5
2.0
30K, 300mT -+ channel
αα ααf [
°]
ααααi [°]
7 mT
26 mT
70 mT
100 mT
160 mT
200 mT
Frank KlosePNCMI
Complementary methods: Magnetometry
5 10 15 200.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
χχ χχ" [(
emu/
mT
)*m
T]
Temperature (K)
Field cooled in 1T, ac field 575 Hz Field cooled in 1T, ac field 1000 Hz Not field cooled in 1T, ac field 575 Hz Not field cooled, ac field 1000 Hz
0 50 100 150 200
0.0
1.0x10-6
2.0x10-6
3.0x10-6
4.0x10-6
χχ χχ" [(
emu/
mT
)*m
T]
Temperature (K)
Field cooled in 1T (up), 1000Hz Zero field cooled (down), 1000Hz
-60 -40 -20 0 20 40 60-8.0x10-4
-6.0x10-4
-4.0x10-4
-2.0x10-4
0.0
2.0x10-4
4.0x10-4
6.0x10-4
8.0x10-4
Mom
ent (
emu)
Moment (emu) at 5K Moment (emu) at 30K Moment (emu) at 70K Moment (emu) at 110K Moment (emu) at 300K
Magnetic Field (mT)
Ac susceptibilityVibrating Sample Magnetometry
� Temperature dependent magnetization behaviour
� Spin glass transition??
low T
high T
Frank KlosePNCMI
X-Ray Magnetic Circular Dichroism
-50 -25 0 25 50
XM
CD
Inte
nsity
\ M
agne
tisat
ion
(arb
. un.
)
Field (mT)
Mn Hysteresis from L-edge XMCD signal
at 130 K at 80 k at 61 K at 30 K
1.2
1.4
1.6
Mn Reflectivity Mn XMCD signal
Ref
lect
ivity
(ar
b. u
n.)
630 640 650 660
-0.14
-0.07
0.00
XM
CD
Sig
nal (
arb.
un.
)
Photon Energy (eV)
Element specific hysteresis loops and magnetization behaviour
Frank KlosePNCMI
X-Ray Magnetic Circular Dichroism
0 50 100 150 200 2500.25
0.30
0.35
0.40
0.45
0.50
0.55
XMCD signal strengthWarming Cooling
Co Co Mn Mn
Co
Mag
netis
atio
n (a
rb. u
n.)
Temperature (K)
0.04
0.05
0.06
Mn
Mag
netis
atio
n (a
rb. u
n.)
• Indication of Mn polarization at all temperatures (parallel Co)
Frank KlosePNCMI
Loose Mn Spins
s
m1 m2z
( ))0()(21
1 FFJ ls −= π
( ) )2/()0()(21
2 ππ FFFJ ls −+=
( ) ( ) ( )θθθ 221 CosCos JJE +−=
1Slonczewski, J. C. (1993). "Origin of biquadratic exchange in magnetic multilayers." Journal of Applied Physics. 73 (10): 5957
Ferromagnetic due to direct exchange
-1.0
-0.5
0.0
0.5
1.0
Fraction
al M
n p
olarisation
20151050Depth in Spacer layer [Å]
-150
-100
-50
0
50
100
150
RK
KY
Int
era
ctio
n [K
] Co CuMn
Mn Polarisation (50K) Left RKKY Right RKKY
, U1, U2
Nick Loh & Bob Stamps
1
F(T,θ)
U(θ) = |U1+U2|
J2 > 0 => BQ coupling
Frank KlosePNCMI
Theoretical Considerations - Competing Coupling
CuMnCo
---
--
+++
++
Co (FM) Mn (LS) Co (FM)
m1 m2
S
Z
• FM orange peel coupling(temperature independent, correlated roughness => FM coupling at RT)
0.3
0.2
0.1
0.0
J 2 (
mJ
m-2
)
200150100500
Temperature (K)
0.10
0.05
0.0028242016
CuMn (Å)26 Å
19 Å
15 Å
RKKY(z) = - A F(T0) Sin(qf z + φ)/z2
A = 3400K, T0 = 700K, qf = 0.45A-1
, φ = 115º
J1
Temperature independent J1from orange peel coupling
• RKKY “loose spin”(temperature dependent => BQ coupling at low T)
Frank KlosePNCMI
Co/Cu0.94Mn0.06MultilayerSummary and conclusion
Polarized Neutron Reflectometry (PNR)
• Temperature dependent coupling in column like domains
• ~60° coupling angle at 30K ⇒ “biquadratic coupling”
• Stable domain size of ~0.43µm
Complementary Techniques
• Spin glass transition at ~10K does not affect BQ-coupling (~100K)
• XMCD shows highly polarised Mn (>50% - paramagnetic)
• Theory: competing coupling => orange peel (FM) and loose spins (BQ)
Frank KlosePNCMI
Acknowledgements
• D. Lott, A. SchreyerGKSS Research Centre, Max-Planck Str. 1, 21502 Geesthacht, Germany
• M. Ali, B.J. Hickey University of Leeds, School of Physics and Astronomy, Leeds LS29JT, UK
• B.P. Toperverg Ruhr-University Bochum, Department of Physics, 44780 Bochum, Germany
• R.L. Stamps, R. MaggaragiaUniversity of Western Australia, School of Physics, Crawley, WA 6009, Australia
• A. Mulders Curtin University, Perth
• N. Loh, T. Saerbeck, F. Klose,ANSTO, The Bragg Institute, PMB1, Menai, NSW 2234, Australia
