Lecture schedule October 3 – 7, 2011
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#1
#2
#3
#4
#5
#6
#7
#8
#9
#10
Kondo effect
Spin glasses
Giant magnetoresistance
Magnetoelectrics and multiferroics
High temperature superconductivity
Applications of superconductivity
Heavy fermions
Hidden order in URu2Si2
Modern experimental methods in correlated electron systems
Quantum phase transitions
Present basic experimental phenomena of the above topics
#3] Giant Magnetoresistance: Experimentally Driven 1986-1989;
Theoretically Modeled 1989; IT Applications into 1990’s
First Commerical Hard-Disks with GMR Sensors (IBM) 1998
Effect parallel / antiparallel thin magnetic films separated by nonmagnetic spacer
Nobel Prize in Physics 2007: ‘‘Discovery of GMR’’
Basic physics involved
Divice applications – computers hard disks
Beyond the GMR
Film from Juelich
1988: … simultaneously, but independent …
“Does the electrical resistance
depend on the magnetization
alignment?”
Albert Fert
Peter Grünberg
Magnetic Multilayers (Fe) with Nonmagnetic Spacers (Cr)
Epitaxial Growth of Multilayers (Idealized)
Modern layer-by-layer fabrication techniques: Molecular Beam
Epitaxial (MBE) and/or Pulsed Laser Deposition (PLD)
Topical use in “interface superconductivity”: LaAlO3 / SrTiO3 -complex oxides. 2DEG  2D-SC (over few nm) at TC = 0.2K
Two possible geometries film fabrication
Small thicknesses
Small diameter
Original Magnetoresistance Measurements
Gruenberg et al.
Fert et al.
Density of States for Unpolarized and Polarized 3d Metal
M=0
M = (n↑ - n↓) ≠ 0
Paramagnet
Ferromagnet
Two Ferromagnets with a Nonmagnetic Spacer in between
SPACER
AF
aligned
SPACER
F
aligned
Send current through device. Which has smallest resistance, AF or F??
Parallel Resistor Model with Current of Up/Down Electrons
AF
ΔR ≈ 50% or less
F
Half-Metallic Ferromagnetic, e.g., Hauslers & Skutterudites
Spin polarized conduction electrons at Fermi surface (EF) – here 100% ↓conduction electrons
Multilayer with Two Half-Metallic Ferromagnets
Spacer
AF
Spacer
F
Spintronics: electronics based upon the spin degrees of freedom,
i.e., electron transport controlled / manipulated by spins.
Spin-Dependent Scattering Theory of GRM
Camley and Barnas PRL(1989) and Maekawa et al.JPSJ(1991)
C & B: Boltzmann eq. approach with spin dependent coefficient for specular
reflection, transmission and anisotropy diffuse scatterings (interface
roughness) at the Fe/Cr boundary. N = D/ D .
(ρ↑↓ - ρ↑↑) / ρ↑↑
M et al.: Spin dependent random exchange potential at interfaces (F/NM) and
performing a Born approximation:
1/τ = matrix elements of V(r)
Boltzmann eq. to calculate differences between F and AF coupling between
adjacent layers.
Now place an insulator between the two magnetic metals
Oxide tunnel junction:
New physics involve: Quantum mechanical tunneling of electrons.
Magnetic fields dependence of tunneling processes.
Theory of TMR
“Old” Jullier PL (1975), Mathon & Umerski PRB (1999)
Conductance ratios: RTRM = [(0)-1 - (Hs)-1] / (Hs)-1  40% at Rm.T
Electron tunneling from ferromagnet are spin polarized
Spin polarized tunneling: P = [D(EF) – D  (EF)] / [ D (E F) + D(EF)] via net
difference of up/down density of states at EF.
Julliere formula: RTMR = (2PLPR ) / [1 – PLPR] at Left and Right electrodes
Different if one has a nonmagnetic metallic interlayer between one of the
ferromagnetic electrodes and the insulator.
Due to quantum well states in the metallic interlayer that do not participate in
the transport. Only in spin down channel causing an spin asymmetry of
tunnel electrons.
IN RESERVE  to Juelich CARTOONS
Film from Jülich at time of Noble Prize ?
Magnetic interlayer exchange coupling (IEC)
Consider two ferromagnetic layers separated by a thin spacer layer:
Ferromagnet / Non-Ferromagnet / Ferromagnet
The ferromagnetic layers interact across the spacer and align …
… parallel …
“ferromagnetic
coupling”
… antiparallel …
“antiferromagnetic
coupling”
… at 90º…
“biquadratic or
90º-coupling”
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Oscillatory interlayer exchange coupling
only occurs for thin spacers with a thickness of a few nm
is observed for many metallic spacer layers
(see [1] for a “periodic table of interlayer coupling”)
oscillates as a function of the spacer thickness D
Scanning electron microscopy with spin analysis (SEMPA) [2]:
3) Domain picture of Fe
layer grown on Cr wedge
2) Wedge-shaped Cr spacer
1) Domain picture of Fe
single crystal (whisker)
with two domains
Cr spacer thickness D (ML)
[1] S.S.P. Parkin, Phys. Rev. Lett. 67, 3958 (1991)
[2] D.T. Pierce et al., Phys. Rev. B 49, 14564 (1994)
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Origin of oscillatory interlayer exchange coupling
Spin-dependent interface reflection gives rise to spin-dependent
quantum-well states (QWS). They only form for parallel alignment of the
FM layers, but not for antiparallel alignment!
Parallel alignment:
Antiparallel alignment:
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Quantum Well States
The energy related to k is quantized. Energy levels shift when the spacer
thickness D is varied. A new level crosses EF when D is changed by

2 4
D 
 Periodicity Q 

 2k
(Note: QRKKY  2kF )
2
D

after
M. Stiles
(Similar to an electron in a box, where E decreases with increasing D)
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What is the origin of spin-dependent reflectivity?
Spin-dependent reflectivity arises from the “potential landscape” seen by
the electrons due to the layered structure.
Example Co / Cu / Co: Similar band structure for majority electrons and
shifted band structure for minority electrons:
P. Lang et al., Phys. Rev. B 53, 9092 (1996)
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Giant magnetoresistance (GMR)
Ferromagnet
Metal
Ferromagnet
Electrical
resistance:
RP
<(>)
RAP
The electrical resistance depends on
the relative magnetic alignment of the ferromagnetic layers
GMR 
RAP  RP
RP
19% for trilayers @RT
80% for multilayers @ RT
GMR is much larger than the anisotropic magnetoresistance (AMR)
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First observations of GMR
Both experiments employ antiferromagnetic interlayer coupling to
achieve the antiparallel alignment
P. Grünberg, FZJ [1]
A. Fert, Paris-Sud [2]
GMR
AMR
[1] G. Binasch, P. Grünberg et al., Phys. Rev B 39, 4828 (1989)
[2] M.N. Baibich, A. Fert et al., Phys. Rev. Lett. 61, 2472 (1988)
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GMR of a spin-valve
Spin-valves make use of the
exchange bias effect at the
AFM/FM interface
6 nm Ni80Fe20
2.2 nm Cu
4 nm Ni80Fe20
7 nm FeMn
CIP-geometry
Ferromagnet
AntiFerromagnet
B. Dieny, J. Magn. Magn. Mater. 136, 335 (1994)
The steep slope at zero field makes
spin-valves sensitive field sensors.
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Microscopic picture of GMR: Spin-dependent scattering
1) Spin-dependent
scattering:
rmin  rmaj
2) Mott’s two current
model:
independent current
channels for spin-up
and spin-down
(no spin-flip
scattering)
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Microscopic picture of GMR: Scattering spin asymmetry
The origin of the spin-dependent scattering lies in the
spin-split band structure and density of states of 3d transition metals:
minority resistance rmin  majority resistance rmaj
For Co/Cu: rmin > rmaj
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Application of GMR: Magnetic field sensor
Example of a real layer structure [1]:
NAF = natural antiferromagnet, SAF = synthetic antiferromagnet
[1] K.M.H. Lenssen et al., J. Appl. Phys. 85, 5531 (1999)
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Application of GMR: Read heads in hard disk drives
Disk rotation
IBM-HGST Microdrive:
- 1 inch diameter
- 3600 rpm
- 119 Gbit/in2 (Bit size:180 x 30 nm2)
- 8 Gbyte in 2006 (340 Mbyte in 1999)
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Application of GMR in hard-disks
2) GMR is an interface effect
(AMR is a bulk effect):
 Thinner MR elements
 Less demagnetization
 Less wide MR elements
 Higher sensitivity
GMR
1) Stronger MR signal
 Better signal-to-noise
 Smaller bits can be read
AMR
Advantages of GMR-based read heads compared to AMR or
inductive read heads:
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What’s beyond GMR?
Apply Newton’s third law “Actio = Reactio”:
The electric current flow controls the magnetization state
Negative current
Positive current
 parallel alignment
 Antiparallel alignment
 Current-induced magnetization switching
by spin-transfer torque
J.C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev B 54, 9353 (1996)
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Advanced switching concept for spintronic devices
Spintronic devices employ the electron spin for data storage and
processing:
MRAM
“Spin-Transistor”
Bit
Bit
Gate
Advantages of current-induced switching over field-induced switching:
nano-scale addressability and favorable scalability
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Very New “Spin Caloritronics” by adding ΔT
E.G., Spin Seebeck effect - Sinova and Uchida et al. NM (2010)
ΔT
Voltage across Pt bar is due to Inverse Spin Hall Effect, transmitted along the F slab
by long-lived, long-range F spin waves.
What is the (Inverse) Spin Hall effect: JC =DISHE (JS x )
F pumps polarized spins into Pt bar. Spin current JS carrying a magnetic moment 
flows downward. As a result of the spin-orbit coulping (large in Pt) asymmetry
electron scattering occurs deflecting the  electrons in the same direction, i.e., to
the right. Thereby a charge current JC flows to the left generating a voltage + to -
FILM and THE END
STOP
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What’s beyond GMR?
Giant magnetoresistance (GMR):
The magnetization state controls to the electric current flow
Parallel alignment
Antiparallel alignment
 low resistive
 high resistive
RP = low
IP = high
RAP = high
IAP = low
- Resistance changes up to 80% at RT
- Widely employed in HDD read-heads and sensors
G. Binasch et al., Phys. Rev. B 39, 4828 (1989); N.M. Baibich et al., Phys. Lett. 61, 92472 (1988)
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Current-induced magnetization switching
The polarity of the electric current flow controls
the magnetization state.
Negative current
Positive current
 parallel alignment
 Antiparallel alignment
100 nm
contact diameter
Electron flux
 parallel
 antiparallel
High current densities: >107 A/cm2 or several mA per (100 nm)2
J.C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev B 54, 9353 (1996)
E.B. Myers et al., Science 285, 867 (1999); J.A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000)
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Nanopillar devices fabricated by e-beam lithography
top electrode
70 nm
2 nm free FM
6 nm spacer
20 nm fixed FM
bottom electrode
Wafer
H. Dassow, D.E. Bürgler et al., Appl. Phys. Lett. 89, 222511 (2006)
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Pioneering work by the Cornell group
Columnar structure (“nanopillar”) and measurement via GMR effect:
 = 130 nm
Au
low resistive,
parallel state for
negative current
Sputtered, polycrystalline system:
2.5 nm Co: thin, “free” FM layer
6.0 nm Cu: spacer
10.0 nm Co: thick, “fixed” FM layer
high resistive,
antiparallel state for
positive current
E.B. Myers et al., Science 285, 867 (1999); J.A. Katine et al., Phys. Rev. Lett. 84, 3149 (2000)
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Physical picture
A second FM layer with tilted magnetization polarizes the incident
current. One layer (Mfree) is easier to switch than the other (Mfixed):
electron flux

Mfixed
electron flux

Mfree
Mfree rotates towards Mfixed
 parallel alignment
Mfixed
Mfree
Mfree rotates away from Mfixed
 antiparallel alignment
Note importance of reflected current and asymmetry of FM layers
X. Waintal et al., Phys. Rev. B 62, 12317 (2000)
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Current-driven magnetization dynamics
For Hextern > Hc there is only one stable alignment and no switching
 spin-transfer torque can excite oscillatory motions of Mfree with
frequencies of several GHz.
 GHz voltage signal due to GMR
 nano-scale, solid-state, on-chip microwave oscillator operating at RT
Mfree
QuickTime™ and a
Cinepak decompressor
are needed to see this picture.
dc current
Mfixed
constant Hextern
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Current-induced microwave spectra
2 nm Fe / 6 nm Ag / 10 nm Fe / 0.9 nm Cr / 14 nm Fe at 50 K
Ibias = 8 mA
B || easy axis
Quality factor f/f of up to 90
Microwave power per line is estimated to be of the order of 1 nW
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GMR and its impact on information technology
1986
1988
1995
1996
1998
2000
2001
2004
2006
2007
Discovery of magnetic interlayer exchange coupling (Grünberg)
Discovery of GMR (Grünberg, Fert)
Realization of TMR at room temperature (Miyazaki, Moodera)
Prediction of spin-transfer effects (Slonczewski, Berger)
First commercial harddisks with GMR sensors (IBM)
Experimental observation of spin-transfer effects (Cornell)
Commercial harddisks with AFC media (IBM, now HGST)
Giant TMR across epitaxial MgO barriers
MRAM based on TMR (Freescale)
Demo: MRAM based on giant TMR and spin-transfer (Hitachi)
… there is more to come: e.g. quantum information technology
… short transfer times from basic research to applications in mass
markets
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The goal for the future
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STOP
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Once upon a time, …
Once upon a time, in the early 1980’s …
“What happens if
I bring two ferromagnets close
–I mean really close–
together?”
N
S
S
N
?
Peter Grünberg
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Thanks to …
… Albert Fert
and
Peter Grünberg …
… for opening the door to spintronics and its applications!
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Typical hysteresis loops for different types of interlayer coupling
FM coupling
or
decoupled
AF coupling
90° coupling
Dominant
90° plus AF
coupling
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Experiment: Anisotropy (“The normal compass”)
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Experiment: Interlayer coupling (“The crazy compass”)
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Application of GMR: Read heads in hard disk drives
An animation explaining the application of GMR in readheads
of hard disk drives can be found at:
http://www.fz-juelich.de/
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Giant Magnetoresistance Effect
Physical Picture, Applications, and Future
Daniel E. Bürgler
Institut für Festkörperforschung, Elektronische Eigenschaften (IFF–9)
CNI – Center of Nanoelectronic Systems for Information Technology
Forschungszentrum Jülich GmbH, Germany
University of Leoben, Leoben, November 14, 2007
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Fundamental Physics of Nano. & Info. Technology – Dec. 2008
Difference between F and AF Configurations
ΔR ≈ 50% … Best to date 10%