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
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
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
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….
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
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 [email protected]
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
•Multifunctional materials. •Mesoscopic and nanoscopic devices. •Magnetic oxides and related topics. •Dilute magnetic semiconductors and semiconducting heterostructures. •Intermetallic compounds. •General field theory applications, experimental and computational techniques in condensed matter.
AT THE FRONTIERS OF CONDENSED MATTER V
Current Trends and Novel Materials
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