An idiosyncratic surveyof

Spintronics

From 1963to the

Present

Peter M LevyNew York University

Three Nobel laureates I’ve worked with

Louis Néel - 1970

Albert Fert - 2007

John H. van Vleck - 1977

The early years

1963-85

The early period: 1960-85

• Grenoble 1963 Néel et al.Interlayer coupling

Proceedings of ICM’64

The early period: 1960-85

• Paris 1968-76 Fert & CampbellTwo current model of conduction in ferromagnetic metals

• 1971-82 Spin dependent tunneling

The early period: 1960-85

• Heterostructures 1970’s Esaki

The early period: 1960-85

• Metallic multilayers- 1980’s Schuller, Shinjo, Prinz,Grünberg

• Spin accumulation and injection -1987 Silsbee and Johnsonvon Son and Wyder

The early period: 1960-85

Metallic multilayers&

GMR

1985-95

• Interlayer coupling 1986, Grünberg, Salamon,Flynn, Kwo, Majkrzak, Yafet.

• Giant magnetoresistance [GMR] 1988, Fert, Grünberg

1985 - 1995

Spintronics- control of current through spin of electron

The two current model of conduction in ferromagnetic metals

Parallel configuration Antiparallel configuration

1988 Giant magnetoresistanceAlbert Fert & Peter Grünberg

Two current model in magnetic multilayers

Data on GMR

M.N. Baibich et al., Phys. Rev. Lett. 61, 2472 (1988).

• Spin currents in tunnel juctions 1989, Slonczewski

• CPP-MR 1991 Levy, Bass

Current in the plane (CIP)-MR

vs

Current perpendicular to the plane (CPP)-MR

• CPP-MR 1993 Valet-Fert

• Spin valves 1992, Speriosu, Dieny, Parkin

400 H (Oe)-40

400

110

H (kOe)-40

H // [ 011]

spin-valve

multi-layer GMR-metallic spacerbetweenmagnetic layers-current flows in-plane of layers

Co95Fe5/Cu[110]

ΔR/R~110% at RTField ~10,000 Oe

Py/Co/Cu/Co/Py

ΔR/R~8-17% at RTField ~1 Oe NiFe + Co nanolayer

NiFeCo nanolayerCuCo nanolayerNiFeFeMn

H(Oe)

H(kOe)[011]

S.S.P. Parkin

GMR in Multilayers and Spin-Valves

1995 GMR heads

From IBM website

• Larger GMR 1994-5, Parkin, Schad,Bruynseraerde

Oscillations in GMR:Polycrystalline vs.

Single Crystal Co/CuMultilayers

S.S.P. Parkin et al,Phys. Rev. Lett. 66, 2152(1991)

Polycrystalline

Single crystalline

S.S.P. Parkin

Sputter deposited on MgO(100), MgO(110)and Al2O3 (0001) substrates using Fe/Pt seedlayers deposited at 500C and Co/Cu at ~40C

From IBM website; http://www.research.ibm.com/research/gmr.html

Magnetic tunneljunctions

&MRAM

1995-2000

Tunneling-MR

Two magnetic metallic electrodes separated by an insulator; transport controlled by tunneling phenomena not by characteristics of conductionin metallic electrodes

2000 magnetic tunnel junctions used in magnetic random access memory

From IBM website;

http://www.research.ibm.com/research/gmr.html

• Reproducible MR with MTJ’s 1995, Moodera, Meservey,and Miyazake

1995-2000

• Magnetic Random Access Memory 1997 DARPA Initiative {IBM, Motorola, Honywell, NYU}

• Bias dependence of tunneling MR 1997 Zhang, Levy, Parkin. Inelastic scattering.

1995-20001995-2000

• Crystalline barriers, Oxides and semiconductors, 1997- Butler, MacLaren, and Mathon

1995-2000

• Predictions of very large TMR for MgO, 2001 Butler et al., Mathon and Umerski

1995-2000

2001

The calculated optimistic TMR ratio is in excess of 1000% for an MgO barrier

•Experimental confirmation of predictions of high TMR for MTJ’s with MgO barriers 2004, Yuasa

Ohno et al.

• CMOS technology, merging spintronics with semiconductors

• Spin injection into semiconductors- 2000 Schmidt et al., Fert & Jaffrès; the Spin transistor-1990 Datta & Das

1995-2000

Resistance mismatch

Spin transfer

2000-2005

2000-2005

• Spin currents produce torques-1996, Berger, Slonczewski

• How charge current produce spin currents which lead to torques acting on background magnetization; back in 1989 JC Slonczewski had the following idea for MTJ’s.

�

I p =2e

hT!"#µ# $T#"!µ![ ]

µ# % µ L µ! % µ R

ˆ T !"# & ˆ ' # ˆ t #!( )*

ˆ ' ! ˆ t !#( )

Particle current:

�

ˆ t !" =t

d+ t

mS

z

" /!t

mS#" /!

tmS

+

" /!t

d# t

mS

z

" /!

$

% &

'

( )

ˆ t !" = tdˆ 1 + t

m

! * +! S " /!

Density matrix:

�

ˆ ! =!" 0

0 !#

$

%

&

'

(

)

Rotated:

�

!"0

+ "zcos# $i"

zsin#

i"zsin# "

0$ "

zcos#

%

& '

(

) *

Transmission amplitude:

Charge current

�

Ic

=2e

2V

hTr

!

ˆ T

Spin current

�

Tr!! ! ˆ T "#$ %Tr!

! ! ˆ T $#"

! T " &Tr! [

! ! ˆ T $#" ]

! T $ &Tr! [

! ! ˆ T "#$ ]

! I s

=2e

2

h

! T "µ" '

! T $µ$[ ]

=2e

2

h

12

µ" + µ$( )! T " '

! T $[ ] + eV ( 1

2

! T " +

! T $[ ]{ }

Equilibrium spin current

�

! T ! "! T #[ ] $

! % ! &

! % #

None other than interlayer exchange coupling

Out of equilibrium spin current

�

! T ! +

! T "[ ]#$"

0 ! $ ! + $!0 ! $ "

�

(! ! "! a )(! ! "

!

b ) =! a "

!

b + i! ! " (! a #

!

b )

�

ˆ T !"# = td

2

ˆ $ # ˆ $ !

For tm=0

�

ˆ t !" = tdˆ 1 + t

m

! # $! S " /!

Spin current

�

�

! I

s

!

�

! I

s

!

�

! I

s

�

! I

s=

e

heV !

! T " +

! T #[ ]

Torque on an electrode

�

! y" #eV td

2

sin$%0

"%z

&

�

! y"

= ! y#(#$")

�

! ! " # $"(

! I

s$! I

s

")

! !

% # $"(! I

s

% $! I

s)

�

! ! "#

= $" (! I

s$! I

s

#)% ˆ # [ ]% ˆ # = $"

! I

s% ˆ # ( )% ˆ #

! ! "

&= $" (

! I

s

& $! I

s)% ˆ & [ ]% ˆ & = "

! I

s% ˆ & ( )% ˆ &

There’s something funny about the equilibrium spin current:

�

! I s

=2e

2

h1

2µ! + µ"( ) f (#)$ d#{ }

! T ! %

! T "[ ]

! T ! %

! T "[ ]&#F td

2 ! ' ! (! ' "

�

! I s!

2e2

h"Ftd

2 ! # $ %

! # &

Resolution:

�

H = !2

2m

" ! 2

+" #S$%(r & r$ )+" #S'%(r & r' )

2nd order perturbation of the free electron energy due to local moments, i.e., RKKY

�

!E = "J(r#$ )! S # %! S $ + i& %

! S # '! S ${ }

2nd order correctionto the energy

Produces precession of conductionelectrons spin

�

d! !

dt= i"

! H "! ! ,! ! [ ]

! H # $iJ(r%& )

! S % '! S &

d! !

dt( J(r%& )

! ! '

! S % '! S &( )

�

! I s!"

Ft

d

2 ! # $ %

! # & ! J(r$& )

! S $ %! S &( )

2000-2005

• Experimental confirmation of current driven magnetization reversal (switching)-CIMS-2000,

PHYSICAL REVIEW LETTERS VOLUME 84, 3149 (2000)Current-Driven Magnetization Reversal and Spin-Wave Excitations in CoCuCo PillarsJ. A. Katine, F. J. Albert, and R. A. BuhrmanSchool of Applied and Engineering Physics, Cornell University, Ithaca, New York 14853E. B. Myers and D. C. RalphLaboratory of Atomic and Solid State Physics, Cornell University, Ithaca, New York 14853

How can one rotate a magnetic layer with a spin polarized current?

By spin torques:Slonczewski-1996Berger -1996Waintal et al-2000Brataas et al-2000

Current induced switching of magnetic layers by spin polarized currents can be divided in two parts:

Creation of torque on background by the electric current, and

reaction of background to torque.

Latter epitomized by Landau-Lifschitz equation; micromagnetics

Former is focus of articles by PML between 2002-2005

Extension of Valet-Fert to noncollinear multilayers

Central idea

fout Tequil

fequilTout

Transmission of out-of-equilibrium distributions across interface

Conventional

Noncollinear multilayers one should also consider

The following does not enter in linear response:

fout

fout

fout foutTout

2000-2005

• Current driven motion of domain walls- 1986-89 Berger exp’t. confirmation 2003-, Fert, Ono, Ohno, Rüdiger.

• Additional theory 2004-2006,

• Gilbert vs LL form of damping, 2007 Wayne Saslow

• Additional theory 2004-2006,

2000-2005

• Spin transfer oscillators STO’s, orspin transfer driven FMR 2006 - Sankey, Buhrman & Ralph; Boulle, Barnas, Fert

2000-2005

• Relativistic treatment of torque, the transfer of orbital angular momentum 2006- Weinberger, Györffy

The present2006-

2006 -

• Spin Hall effect 1999- , Hirsch, SZ. Zhang, Jairo Sinova

• Semiconductors as barriers; ferromagnetic semi-conductors as electrodes in MTJ’s 1968~2000-Kasuya,Wachter, von Molnar, Methfessel, Mattis;2000- Ohno, Munekata, Dietl, Chiba, Das Sarma,Samarth,Awschalom, MacDonald, Sinova, Wunderlich, Halperin, Brataas, Inoue, Bauer.

• MTJ’s with spin filtering barriers, ~2004 - Moodera,Thales group (Barthélemy, Bibes, Gajek,…), Grünberg,

•

2006 -

• Multiferroics magnetoelectrics, 2005- Tsymbal, Thales.

• Carbon nanotubes ~2000- see review by Roche et al.RMP79,677 (2007). As applied to spintronics, see

• Graphene, massless Dirac Fermions ~2005- Kim,..see review by Geim et al. in RMP 2007, to appear(con-mat/0709.1163).

2006 -

• Molecular spintronics ~2000- Ratner, Reed, McEuen, Sanvito,

arXiv:cond-mat/070345v1; Provisionally accepted for publication in Journal of Physics D: Applied Physics

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