#3] giant magnetoresistance: experimentally driven 1986-1989; theoretically modeled 1989; it...

Lecture schedule October 3 – 7, 2011

#1 Kondo effect #2 Spin glasses #3 Giant magnetoresistance #4 Magnetoelectrics and multiferroics #5 High temperature superconductivity #6 Applications of superconductivity #7 Heavy fermions #8 Hidden order in URu2Si2 #9 Modern experimental methods in correlated electron systems #10 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 non-magnetic 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 …

Albert Fert

“Does the electrical resistance depend on the magnetization

alignment?”

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

SP

AC

ER

SP

AC

ER

Send current through device. Which has smallest resistance, AF or F??

AF AF alignedaligned

F aligned

Parallel Resistor Model with Current of Up/Down Electrons

AF

F

ΔR ≈ 50% or less

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 S

pacer

AF

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 .

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.

(ρ↑↓ - ρ↑↑) / ρ↑↑

Now place an insulator between the two magnetic metals

New physics involve: Quantum mechanical tunneling of electrons. Magnetic fields dependence of tunneling processes.

Oxide tunnel junction:

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 ?

Forschungszentrum Jülichin der Helmholtz-Gemeinschaft

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 or90º-coupling”


Scanning electron microscopy with spin analysis (SEMPA) [2]:

Oscillatory interlayer exchange coupling

[1] S.S.P. Parkin, Phys. Rev. Lett. 67, 3958 (1991)[2] D.T. Pierce et al., Phys. Rev. B 49, 14564 (1994)

3) Domain picture of Fe layer grown on Cr wedge

1) Domain picture of Fe single crystal (whisker) with two domains

2) Wedge-shaped Cr spacer

Cr spacer thickness D (ML)

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


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:


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

D 2

Periodicity Q 2D

4

2k (Note: QRKKY 2kF )

(Similar to an electron in a box, where E decreases with increasing D)

after M. Stiles


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:

What is the origin of spin-dependent reflectivity?

P. Lang et al., Phys. Rev. B 53, 9092 (1996)


The electrical resistance depends onthe relative magnetic alignment of the ferromagnetic layers

19% for trilayers @RT

80% for multilayers @ RT

Giant magnetoresistance (GMR)

FerromagnetMetalFerromagnet

Electricalresistance:

RP <(>) RAP

GMR RAP RP

RP

GMR is much larger than the anisotropic magnetoresistance (AMR)


First observations of GMR

[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)

P. Grünberg, FZJ [1] A. Fert, Paris-Sud [2]

GMR

AMR

Both experiments employ antiferromagnetic interlayer coupling to achieve the antiparallel alignment


GMR of a spin-valve

B. Dieny, J. Magn. Magn. Mater. 136, 335 (1994)

6 nm Ni80Fe20

4 nm Ni80Fe20

7 nm FeMn

2.2 nm Cu

The steep slope at zero field makes spin-valves sensitive field sensors.

CIP-geometry

Spin-valves make use of the exchange bias effect at the

AFM/FM interface

Ferromagnet

Anti-Ferromagnet


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)


Microscopic picture of GMR: Scattering spin asymmetry

The origin of the spin-dependent scattering lies in thespin-split band structure and density of states of 3d transition metals:

minority resistance rmin majority resistance rmaj

For Co/Cu: rmin > rmaj


Application of GMR: Magnetic field sensor

[1] K.M.H. Lenssen et al., J. Appl. Phys. 85, 5531 (1999)

Example of a real layer structure [1]:

NAF = natural antiferromagnet, SAF = synthetic antiferromagnet


Application of GMR: Read heads in hard disk drives

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)

Disk rotation

fixed Ferromagnet

Current

Anti-Ferromagnetfree Ferromagnet


Advantages of GMR-based read heads compared to AMR or inductive read heads:

1) Stronger MR signal Better signal-to-noise Smaller bits can be read

2) GMR is an interface effect (AMR is a bulk effect): Thinner MR elements Less demagnetization Less wide MR elements Higher sensitivity

Application of GMR in hard-disks

GM

RA

MR


What’s beyond GMR?

Current-induced magnetization switchingby spin-transfer torque

Apply Newton’s third law “Actio = Reactio”:The electric current flow controls the magnetization state

Negative current parallel alignment

Positive current Antiparallel alignment

J.C. Slonczewski, J. Magn. Magn. Mater. 159, L1 (1996); L. Berger, Phys. Rev B 54, 9353 (1996)


Advanced switching concept for spintronic devices

Spintronic devices employ the electron spin for data storage and processing:

Advantages of current-induced switching over field-induced switching:nano-scale addressability and favorable scalability

MRAMBit

“Spin-Transistor”

Gate

Bit

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


What’s beyond GMR?

- Resistance changes up to 80% at RT- Widely employed in HDD read-heads and sensors

Giant magnetoresistance (GMR):The magnetization state controls to the electric current flow

Parallel alignment low resistive

Antiparallel alignment high resistive

G. Binasch et al., Phys. Rev. B 39, 4828 (1989); N.M. Baibich et al., Phys. Lett. 61, 92472 (1988)

RP = lowIP = high

RAP = highIAP = low


Current-induced magnetization switchingThe polarity of the electric current flow controls

the magnetization state.

High current densities: >107 A/cm2 or several mA per (100 nm)2

Electron flux

100 nmcontact diameter

parallel antiparallel

Negative current parallel alignment

Positive current Antiparallel alignment

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)


Nanopillar devices fabricated by e-beam lithography

Wafer

70 nm

H. Dassow, D.E. Bürgler et al., Appl. Phys. Lett. 89, 222511 (2006)

20 nm fixed FM

2 nm free FM6 nm spacer

bottom electrode

top electrode


Pioneering work by the Cornell group

Columnar structure (“nanopillar”) and measurement via GMR effect:

Sputtered, polycrystalline system: 2.5 nm Co: thin, “free” FM layer 6.0 nm Cu: spacer10.0 nm Co: thick, “fixed” FM layer

= 130 nm

Au

low resistive, parallel state for negative current

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)


Physical picture

A second FM layer with tilted magnetization polarizes the incident current. One layer (Mfree) is easier to switch than the other (Mfixed):

Mfree rotates towards Mfixed parallel alignment

Mfree rotates away from Mfixed antiparallel alignment

Note importance of reflected current and asymmetry of FM layers

Mfree

electron flux

Mfixed

Mfree

electron flux

Mfixed

X. Waintal et al., Phys. Rev. B 62, 12317 (2000)


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

QuickTime™ and aCinepak decompressor

are needed to see this picture.

Current-driven magnetization dynamics

nano-scale, solid-state, on-chip microwave oscillator operating at RT

dc current

Mfree

constant Hextern

Mfixed


Current-induced microwave spectra

Quality factor f/f of up to 90Microwave power per line is estimated to be of the order of 1 nW

2 nm Fe / 6 nm Ag / 10 nm Fe / 0.9 nm Cr / 14 nm Fe at 50 K

Ibias = 8 mAB || easy axis


1986 Discovery of magnetic interlayer exchange coupling (Grünberg)

1988 Discovery of GMR (Grünberg, Fert)1995 Realization of TMR at room temperature (Miyazaki,

Moodera)1996 Prediction of spin-transfer effects (Slonczewski, Berger)1998 First commercial harddisks with GMR sensors (IBM)2000 Experimental observation of spin-transfer effects (Cornell)2001 Commercial harddisks with AFC media (IBM, now HGST)2004 Giant TMR across epitaxial MgO barriers2006 MRAM based on TMR (Freescale) 2007 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

GMR and its impact on information technology


The goal for the future


STOP


Once upon a time, …

Once upon a time, in the early 1980’s …

Peter Grünberg

N

S

S

N?

“What happens ifI bring two ferromagnets close

–I mean really close–together?”


Thanks to …

… Albert Fert and Peter Grünberg …

… for opening the door to spintronics and its applications!


Typical hysteresis loops for different types of interlayer coupling

FM couplingor

decoupled

AF coupling 90° coupling Dominant

90° plus AF

coupling


Experiment: Anisotropy (“The normal compass”)


Experiment: Interlayer coupling (“The crazy compass”)


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/


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

Fundamental Physics of Nano. & Info. Technology – Dec. 2008

Difference between F and AF Configurations

ΔR ≈ 50% … Best to date 10%

#3] giant magnetoresistance: experimentally driven 1986-1989; theoretically modeled 1989; it...

Documents

juelich slide

d e f d e f d e f d

polarized spin

paramagnet ferromagnet

nonmagnetic spacers

peter gr nberg slide

fermi surface e f

d metal