SPIN-DEPENDENT TRANSPORT IN NANOSTRUCTURES

Laura B. Steren

Condensed Matter Dept. Centro Atómico ConstituyentesSan Martin, Argentina

OUTLINE

- MOTIVATIONS

- INTRODUCTION TO SPIN-POLARIZED TRANSPORT PHENOMENA AND MAGNETORESISTIVE EFFECTS IN MAGNETIC NANOSTRUCTURES

- SPINTRONICS GIANT MAGNETORESISTANCE EFFECT MAGNETIC TUNNEL JUNCTIONS

- OXIDE-BASED SPINTRONICS

BASIC RESEARCH

MOTIVATIONS

-CONFINEMENT AND SIZE EFFECTS ON THE TRANSPORT PROPERTIES OF MAGNETIC NANOSTRUCTURES (ELECTRONIC BANDS, TRANSPORT MECHANISMS, …)

- STRONG CORRELATION BETWEEN MAGNETISM AND TRANSPORT (MAGNETIC ORDER, ANISOTROPIES,….)

-EFFECT OF INTERFACES AND SURFACES IN SPINTRONICS Ex. GMR TUNNEL JUNCTIONS/ SPIN FILTERS

APPLICATIONS

- DATA STORAGE

-BIOMEDICINE AND BIOTECHNOLOGIES

ELECTRONIC TRANSPORT & SIZE EFFECTS

N ~ 30nm noble metals (Au, Ag, Cu)

FM ~ 8nm transition metals (Fe,Co,Ni)

v. The mean free path depends on electronic bands, impurities, carriers scattering processes, etc

CHARACTERISTIC LENGTH IN ELECTRONIC TRANSPORT:MEAN FREE PATH OF THE CARRIERS BETWEEN COLLISIONS

BALLISTIC REGIME (QM)

DIFFUSIVE REGIME (SEMIC)SIZE

Compare with?

d

The surfaces and interfaces affect the solution of the Boltzmann equation when d/ 1

SEMICLASSICAL APPROACH applied to thin films

ROUGHNESS, COMPOSITION,….

FUCHS-SONDHEIMER (1952) Electric transport in thin films

REFLECTION

DIFUSSION

ELECTRICAL TRANSPORT IN MAGNETIC SYSTEMS

KNOWN FOR MANY YEARS….

Interaction between electric carriers and localized moments for example: magnetic impurities in metals. Other cases: Kondo, ferromagnets

S-L (spin-orbit)

In ferromagnetic metals

SPIN-DEPENDENT SCATTERING PROCESSES

SPIN-POLARIZED ELECTRONIC BANDS

+

MAGNETORESISTANCE in metals

Lorentz force acts on the electron trayectory (MR).

A.D. Kent et alJ. Phys. Cond. Mat. 13, R461 (2001)

Application of a H

ANISOTROPIC MAGNETORESISTANCE IN FM

Coupling between electronic current and the magnetization of the FM (SPIN–ORBIT)

DEPENDENCE ON THE ANGLE BETWEEN ELECTRIC CURRENT AND MAGNETIZATION

Application of H ….

HALL EFFECT IN FERROMAGNETIC MATERIALS

Anomalous: M, Rs prop ρorigin: SO

Ordinary:Bands/Scattering ()

Late in the 80’s, french and german research teams led by A. FERT and P. GRUNBERG, respectively, discovered the giant magnetoresistant effect in Fe/Cr multilayers following different motivations

WORKING WITH STRUCTURES OF NANOMETRIC SIZE NOT ONLY SERVE TO MINIATURIZE DEVICES BUT ALSO TO FINELY TUNE THE TRANSPORT PROPERTIES OF MATERIALS.

The Nobel Prize in Physics 2007 "for the discovery of Giant Magnetoresistance"

GIANT MAGNETORESISTANCE (1988)

MAGNETIZATION LOOP T=294K

MAGNETORESISTANCE T=294K

MAGNETORESISTANCE T=10K

CO

RR

ELA

TIO

N B

ETW

EEN

MA

GN

ETIS

M A

ND

MA

GN

ETO

- TR

AN

SP

OR

T!! GIANT MAGNETORESISTANCE

GMR = (Rap-Rp)/ Rp

PARALLEL/ANTIPARALLEL CONFIGURATION

TWO CURRENT MODEL ,

J. Barnas, A. Fuss, R.E. Camley, P. Grunberg, W. Zinn, Phys. Rev. B 42, 1990

SEMICLASSICAL APPROACH: BOLTZMANN EQUATION

SPIN -DEPENDENT BOUNDARY CONDITIONS

TO SOLVE THE BOLTZMANN EQUATION

AND CALCULATE THE ELECTRICAL CURRENT

SPIN-DEPENDENT DENSITY FUNCTION G

ICIP

I

CPP

COUPLED MULTILAYERS SPIN VALVES GRANULAR ALLOYS

A few years later, MAGNETIC TUNNEL JUNCTIONS

TMR = (Rap-Rp)/ Rp

AMR

TMR

CoFe/AlO/Co

Tunneling through the insulating barrierwithout spin-flip !

Experimental challenge: FABRICATION OF THE JUNCTIONS

1- SAMPLES GROWTH 2- NANOSTRUCTURATION

-SURFACE ROUGHNESS OF THE FM ELECTRODES -FM-I INTERFACE QUALITY-QUALITY OF THE TUNNEL BARRIER

OPTICAL LITHOGRAPHY+ ION ETCHING M. Sirena, CAB Bariloche

Oxides for Spintronics A. Barthelemy, M. Bibes, IEEE Trans. Electron Devices vol. X, August 2006

Au/NiFeO/LSMO

SPIN FILTER

The height of the barrier is spin-dependent due to the magnetic character of the spacer

M FM-MFM-I

Metallic multilayers CIP CPP

Magnetic tunnel junctions Insulating barriers Semiconducting barriers

different length scales

STRUCTURES

GMR IN METALLIC SYSTEMS ’88 TM/NM

MAGNETIC JUNCTIONS / SPIN FILTERS ‘92 TM/BINARY ALLOYS

DOMAIN-WALL IN WIRES ‘06TM OXIDES

IN THE BEGINNING THE TYPICAL MATERIALS USED IN SPINTRONICS DEVICES

The oxides composition could be changed and so as the substrate-induced strains in order to design nanomaterials with specific physical properties!!

FERROELECTRICITY FERROMAGNETISM/ AF METALICITY SUPERCONDUCTIVITY OPTICS

ADVANTAGES OF PEROVSKITE OXIDES: optimal properties for magnetic tunnel electrodes….

PEROVSKITES

ABO3

90s: Manganites

R. Ushibara, Phys. Rev. B 51, 14103 (1995)

A

B:Mn

O

Highly textured when grown on insulator single-crystalline substrates like SrTiO3 and MgO

GROWTH OF OXIDE FILMS BY SPUTTERING OR PLD

SUBSTRATE-INDUCED EFFECTS ON MAGNETIC AND TRANSPORT PROPERTIES

La0.6Sr0.4MnO3 films grown by sputtering

SrTiO3 full symbols, MgO: empty

Anisotropy fields

Coercive fields

t (nm)

HC

L.B. Steren, M. Sirena, and J. Guimpel, J. Appl. Phys., Vol. 87, No. 8, (2000)

KA1 = KV + 1/t * KS

LS-STO LS-MGO Kv[erg/cm3] -3.6x105 +2.6x105

Ks [erg/cm2] +0.89 +0.66

20

40

60

80

LS-MGOLS-STO

= m exp(/k

BT)

(m

eV)

0 200 4000.0

0.4

0.8

1.2

t (nm)

= * exp((Q/k

BT)

1/4)

Q1/

4 (a.

u.)

TRANSPORT PROPERTIES OF LSMO FILMS

Grown on MgO SrTiO3

L.B. Steren et al, J.Magn. Magn. Mater. 211, 28 (2000). J. Guimpel et al, Thin Solid Films 373, 102 (2000).

METAL-INSULATOR TRANSITION INDUCED BY ANNEALING TREATMENTS UNDER OXYGEN ATMOSPHERE

50 100 150 200 2500

70

140

210

Temperature (K)

Increasing POX

(b) - MGO

Increasing POX

Mr

(em

u/c

m3 )

0

133

267

400(a) - STO

Mr

(em

u/c

m3 )

Bulk La0.96Sr0.04MnO3 is a canted antiferromagnetic insulator

La0.96Sr0.04MnO3 thin films show: FERROMAGNETISM

M. Sirena et al, J. Appl. Phys. 105, 33902 (2009).

All films display a ferromagnetic transition and their Tc and Mr increase with increasing oxygen content.The magnetic saturation Ms is almost the same for all the samples with a variation of less than 10%.

1E-3

0.01

0.1

1

10

100

Increasing POX

(a) - STO

(

.cm

)

100 200 300

0.1

1

10

100

TC

(b) - MGO

(

.cm

)

Temperature (K)

Increasing POX

= Our results indicate that the oxygenation dynamic of LaSr0.04MnO manganites depend on strain fields and defects induced by the substrates.

= The faster oxygenation dynamics observed in LSMO-STO films could be attributed to a high oxygen diffusion rate in strained films.

= On the other hand, oxygen vacancies in LSMO-MGO films are probably removed from dislocations

46.0 46.5 47.0 47.5 48.0 48.51k

10k

100k

1M

10M

100M

0.402 0.404 0.406 0.4080

2

4

6

LSMO (002)

STO (002)

m=5m=4

m=3m=2

m=1

2 (Degree)

I (a

.u)

t = 634 ÅR = 0.99976

Sin()

m

m Fit

0.0 0.4 0.8 1.2 1.6 2.0

0

4

8

12

16

20

0.0 0.1 0.2 0.3 0.4

1E-4

1E-3

0.01

0.1

1

experiments fit

= 71.3 Å

c = 0.34º

m2

sin2m (10-3)

m=4m=3

m=2

m=1

tLSMO

= 38.7 Å

tLNO

= 33.0 Å

R

4sin

exp sim M. Granada et al,

Appl. Phys. Lett. 91, 072110 (2007)

multilayers La0.7Sr0.3MnO3/LaNiO3

44 46 48 50 52 54

0.40 0.44-2

0

2

4

dLSMO/LNO

(002)

-2-1

+5+4+3

+2+1

STO (002)

I (a.

u.)

2 (Degree)

= 72.78 ÅR = 1

m

Sin()

m Fit

LaSrMnO

J.C. Rojas Sanchez et al, Appl. Surf. Science, (07)

Growth of metallic multilayers La0.7Sr0.3MnO3/LaNiO3 in order to look for giant magnetoresistance in oxide-based multilayers

RoughnessN LSMO=2.33ÅN LNO =1.53ÅC =0.05Å

-1000 -500 0 500 1000

-0.0002

0.0000

0.0002

M (

emu)

% (?X)

MH5kzfc MH5KFC@3

0 10 20 30 40 50 60 70 800

5

10

15

20

25

30

HE HcIzq HcDere HcFC

T (K)

HE (

G)

100

150

200

250

300

350

400

450

Hc (G

)

BROADENING OF THE HYSTERESIS AND SHIFT OF THE FC LOOPS

EXISTENCE OF EXCHANGE BIAS AT LSMO/LNO INTERFACES

EFFECTS OBSERVED AT FM/AFM INTERFACES:PINNING OF THE FM PHASE WHEN THE SAMPLE IS COOLED UNDER A MAGNETIC FIELD BELOW TN (AFM layer)

First report: Co/CoO

Ni- O

La

Mn - O

FUNDAMENTAL ROLE OF INTERFACES A/B o A/substrate

-Lattice mistmatch => deffects, strain- Interdifussion- Roughness

FM/AFM oxide-based multilayers Phys. Rev. 64, 94429 (2001); Phys. Rev. B 60, 485 (1999)

YBaCuO/LSMO N. Haberkorn et al, APL 84, 3927 (2004) dead layers, Phys. Rev. B 69. 134428 (2004) hole transfer from the high Tc to the manganite

S. Dong et al, Phys. Rev. Lett 2009 => weak FM/FM origin of EB

THEORY

IN OXIDES:

CHEMICAL COMPOSITION AND MAGNETIC STATE AT THE INTERFACES? X-ray absorption spectroscopy experiments!

Depth probe ~ 5nm

These samples were d Esigned in order to test both LNO/LSMO and LSMO/LNO interfaces

Brittany B. Nelson-Cheeseman, University of California, BerkeleyDepart. Materials Science and Engineering, USA

0

1 LaM4

LNO(7.6nm)/LSMO LNO(3.7nm)/LSMO LNO(2.3nm)/LSMO

L2

Inte

ns

ity

(a

.u.)

L3

850 860 870 880-0.005

0.000

0.005

Ni L-edge 85K

Photon Energy (eV)

XM

CD

0

5

10 LaM4

LNO(7.6nm)/LSMO LNO(3.7nm)/LSMO LNO(2.3nm)/LSMO

L2

Inte

ns

ity

(a

.u.)

L3

850 860 870-0.005

0.000

0.005

Ni L-edge 85K

Photon Energy (eV)

XM

CD

-1.0 -0.5 0.0 0.5 1.0

-1.0

-0.5

0.0

0.5

1.0

LNO(3.7nm)/LSMO LNO(2.3nm)/LSMON

orm

aliz

ed A

sym

met

ry

Applied Field (T)

Ni

85K

PROBE OF THE MAGNETISM AND OXIDATION STATE OF Mn AND Ni separately

MAGNETIC Ni2+ AT INTERFACE!

BILAYERS

Normalized XAS (left) and absolute XAS (right) to directly compare lineshape, but also see relative concentrations of elements.

From the temperature dependence of the height of the barrier is deduced

MAGNETIC TUNNEL JUNCTIONS

LSMO/CaMnO/LSMOBariloche, Argentina

I-V curves

Temperature dependence of manganites magnetizationat interfaces!

BULK

INTERFACE

APPLICATION TO DATA STORAGE

Writing

Reading

Storage

2001, Hitachi AFC media

GMR reading heads

TMR W/R heads...

ROAD MAP FOR DATA STORAGE

Hitachi 2007 Min

iatu

riza

tion

an

d n

ew

tech

nolo

gie

s:

sp

intr

on

ics

CPP-GMR heads?

TMR heads (1992) recording

AFC media

GMR heads (1988)

AMR heads

Thin films heads

SPIN-DEPENDENT TRANSPORT PHENOMENA

Spin valves / Giant magnetoresistance GMR

Tunnel junctions/ Spin filters

MULTIFUNCTIONAL MATERIALS CHEMISTRY/PHYSICS

Oxides magnetism/electricity/multiferroics

Metal/semiconductor

DEVICES PHYSICS AND ENGINEERING (Top-down)

SUMMARY OF THE TALK

THERE IS A LOT OF THINGS TO DO!!

► DEVELOPMENT OF NEW MATERIALS

► STUDY OF QUANTUM AND LOW DIMENSIONAL PHENOMENA

► NEW APPLICATIONS

BIBLIOGRAPHY

MAGNETOELECTRONICS, Ed. by M. Johnson Elsevier 2004 NANOMAGNETISM AND SPINTRONICS, T. SHINJO, Elsevier

2009 SPINTRONICS: Fundamentals and applications; Reviews

of modern physics, vol. 76, april 2004. Oxide spintronics, IEEE TRANS. ELECTRON. DEVICES, VOL.

X, NO. XX, AUGUST 2006 ; M. Bibes and A. Barthelemy

Laura Steren steren@tandar.cnea.gov.ar

EXPERIMENTAL RESEARCH

Gabriela Alejandro (R) Julian Milano (R) Martin Sirena (R) Mara Granada (R)Marina Tortarolo (PD) Juan. C. Rojas Sanchez (GS)Federico Fernandez Baldis (GS)

TEAM CAB- CAC

In collaboration with:

V.H. Etgens, M. Marangolo, M. Eddrief (INSP,France)G. Leyva (CAC, Bs.As.) H. Pastoriza (CAB)G. Faini (LPN, France)

December 6-10, 2010   Buenos Aires, Argentina  

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