L Molenkamp 04 16 05 - Condensed Matter...
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Spintronic Nanostructures Laurens W. Molenkamp Physikalisches Institut (EP3) Würzburg University
Transcript of L Molenkamp 04 16 05 - Condensed Matter...
Spintronic Nanostructures
Laurens W. MolenkampPhysikalisches Institut (EP3)
Würzburg University
Overview
- Introduction: Spin injection in Semiconductors
- Spin RTDs using II-VI DMSs
- Nanoconstrictions in GaMnAs
- Tunnel AMR in GaMnAs
- Conclusion and outlook
Materials at EP3: II-VI MBE
MBE of:(Al,Ga,In)(As,Sb)
(Zn,Be,Mn)Se on (Al,Ga)As substrates/heterostructures(Cd,Be,Mn)Se on InAs/GaSb substrates/heterostructures
(Hg,Mn,Cd)Te on ZnCdTe substratesmagnetic III-Vs: (Al,Ga,In)Mn(As,Sb)
Heuslers (NiMnSb on InGaAs/InP)(U)HV deposition/sputtering/CVD of
Fe, Co, Ni, Au, W, Al, Ti, Cr, SiO, SiO2, Si3N4
(mostly stacks without breaking vacuum)
Materials for Spintronics
Lithography
300 m2 CleanroomAll usual Optical Lithography, Aligners,
Evaporators, etc.E-beam: LEO 1525 FEG w/ Elphy+
pattern generatorDry etching: RIE (CHF3) and CAIBE(Cl2)
systemsResolution down to 5 nm
Spininjection: Load line model
Loadline: crossing point between I-V curves determines operating current of circuit
Spininjection: Load line model
-model transport as two independent spin channels
-Spin injection occurs because of different spin channel conductivity in the ferromagnet
Spininjection from a metal into a sc: impedance mismatch
for spin injection, one needs:
-ferromagnetic semicond.
-‚half-metal‘
-spin-preservingtunnelbarrier
II-VI Dilute magnetic semiconductors(DMS)
The 100% spin-aligner– II-VI Semiconductors with magnetic components
(e. g. ZnMnSe), which are non-magnetic orantiferromagnetically aligned at zero B-field.
– For B ≠ 0: Finite magnetization, ‘giant’ Zeeman splitting of up to 100 meV of which 10 - 20 meV in the conduction band
– Low Fermi energy at high doping due to impuritybands: half-metallic behavior at high B-field.
⇒ β = 100% for kΤ << ∆ΕC
+
∆−=
)(25)(*
02/5
max0 TTTk
BgBB
EggB
BMn
B
µµ
gMn: g factor for MnB5/2: Brillouin function for S=5/2(∆E)max: saturation spin splitting
energyT0: scaling temperature
(accounts for spin spin interaction)
First demonstration of electrical spin injection R.Fiederling et al., Nature 402, 787 (1999).
Spin-LED: the first spin injection device
The spin-LED exhibits very robust spin-injection,up to 90% efficiency.Modeling more complex than load-line, high field (drift) effects
Spin-LED Data
EC
ZnSe
BeZnSe ZnMnSe
Growth direction
BeZnSe
ZnSe
(Zn,Be,Mn)Se-based Spin-switch RTD
Voltage-adjustable spin selectiveinjector and detector?
Spin-switch RTD in the (Zn,Be,Mn)Se-System
GaAs Substrate
ZnSe (i)
Zn0.97Be0.03Se (1e19)
Zn0.97Be0.03Se (1e18)
Zn0.97Be0.03Se (1e18)
ZnSe (2e19)
ZnSe (2e19)
Zn0.7Be0.3Se (i)
Zn0.7Be0.3Se (i)ZnMnSe (i)
ZnSe (i)
30 nm15 nm10 nm5 nm9 nm5 nm
10 nm10 nm50 nm300 nm
EC
all doping n-type
Spin-switch RTD in the (Zn,Be,Mn)Se-System
The dots are fits assuming each spin-channel has the same resonance behavior as the B=0 T signal. The fits yield the exact resonance spliting.
0.00 0.05 0.10 0.15 0.200
25
50
75
100
125
6T
3T
0T
4% MnT=1.3K
Voltage (V)
Cur
rent
(µA
)
0.00 0.05 0.10 0.15 0.200
255075
100125150
6T
3T
0T
8% MnT=1.3K
Voltage (V)
Cur
rent
(µA
)
A. Slobodskyy et al., PRL 90, 246601 (2003)
Spin-switch RTD in the (Zn,Be,Mn)Se-System
Splitting follows Brillouin function that belongs to the Mn concentration:Spin-switch RTD functions in all respects as anticipated..
0 1 2 3 4 5 60
4
8
12
16
20 1.3K_8% 4.2K_8% 8K_8%
1.3K_4% 4K_4% 8K_4%
Zeem
an s
plitt
ing
(meV
)
Field (T)
III-V DMS: (Ga,Mn)As
– GaAs with typically 1 to 8% Mn– Grown at ~220° to prevent MnAs formation.– Exhibits carrier mediated ferromagnetism. – Mn substitues on Ga sites (acceptor) or goes in interstitially
(donor) – Tc around 70K as grown, 150 K with annealing is routine.
Current WR: Nottingham 173K; Tanaka 250K in 2D layers.– Always p-type .– Basically metallic transport T4k ~ Troom
– Complex anisotropy both in transport and magnetism.
•Spin-valve effects important for device applications
•Large magnetoresistance expected from ‘ballistic’ domain walls in (Ga,Mn)As(Flatté & Vignale APL, PRL 2001/2)
•Constrictions: pin and shrink domain wall (Bruno, PRL 1999)
Magnetic point contacts
Nanofab:
Negative e-beam lithography, dry etching
(Ga,Mn)As from Furdyna, Notre Dame
-80 -40 0 40 8014.9
15.0
15.1
R xx (k
Ω)
-80 -40 0 40 8048.6
48.8
49.0
49.2
49.4
49.6
49.8
Rxx
(kΩ
)
Magnetic Field (mT)
Experimental Results: Spin-valve effect
Switching dominated by shape anisotropyStrong domain wall pinning
Amplitude of effect strongly depends on resistance of constrictions
(same sample, constrictions narrowed by re-etching)
+1.5%
C. Rüster et al., PRL 91, 216602 (2003).
-100 -80 -60 -40 -20 0 20 40 60 80 100
79
80
81
82
83
84
85
Rxx
(kΩ
)
Magnetic Field (mT)
+8%
Modeling – Lower Resistance Regime
•Valet-Fert: spin accumulation at abrupt junction of regions with oppositemagnetization
•Parabolic band model:
=∝
ββ 2 MR
↓↑↓↑
↓↑
+
∆=
+−
=FF
spin
FF
FF
EEE
EEEE
β
• Experiment fits w/ reasonable numbers:
•∆Espin=30 meV (from MCD data)
•EF =150 meV (1%), 90 meV (8%) (parabolic hole band)
Model explains evolution of R and MR with etching
Michael Flatté, Zhi Gang Yu
spin polarization
-1200 -800 -400 0 400 800 12000.0
2.0x107
4.0x107
6.0x107
8.0x107
1.0x108
Rxx (Ω
)
Magnetic Field (Oe)
Etching into TMR regime: Giant spin-valve effect
Further etching: contacts become tunnel bariers –and magnetoresistance increases to 2000%!
Modeling – tunneling regimeAssumption: shallow parabolic barrier :
⋅
⋅⋅⋅−=
hLEmT H
2/3
2/1*2
22exp π
Transmission probability through a parabolic barrier :
barrier theof thicknessbarrier theofheight
==
LEHwith
Main point: different tunneling distances for minority and majority holes
•Use the spin splitting as in the low R samples, meV 30∆E =
Model yields as parameters: nm 11...17L meV; 31EB ≈≈
barriers for
holes!
TAMR in (Ga,Mn)As/AlOx/metal
-60 -40 -20 0 20 40 602860
2880
2900
2920
2940
2960
2980
3000
Res
ista
nce
(Ω)
B (mT)
AlOx
GaMnAs
Au Au
ContactDevice
A tunnel barrier between a non-magnetic metal (Au) and ferromagnetic(Ga,Mn)As can exhibit a huge magnetoresistance that can show the signature of a spin valve.
GaMnAs
Contact
Device
Dependence of the magnetoresistance effect on the in-plane field angle(angle with respect to [100]).
-100 -80 -60 -40 -20 0 20 40 60 80 100290030003100320033003400350036003700
170 deg.
0 deg.
R (Ω)
B(mT)100 80 60 40 20 0 -20 -40 -60 -80 -100
290030003100320033003400350036003700
350 deg.
180 deg.
R(Ω)
B(mT)
Spin-Valve like TMR in (Ga,Mn)As/AlOx/Non-magnet devices
Magnetization shows double switching.
B1
2 3
4
1
2 3
-30 -20 -10 0 10 20 30-1.0x10-5
-7.5x10-6
-5.0x10-6
-2.5x10-6
0.0
2.5x10-6
5.0x10-6
7.5x10-6
1.0x10-5
M(e
mu)
B(mT)
Cubic + small uniaxial anisotropy
Magnetic Behaviour
-Cubic + small uniaxial anisotropy
-“double step switching“
-Coherent rotation a la Stoner-Wohlfarth.
-Domain wall nucleation and propagation.
∆E
Barrier against coherent rotation
ε
Domain wall propagation energy
Total dos of valence band strongly anisotropic.
B // [100 ] B // [110 ]
Abolfath et al., Phys. Rev. B 63, 054418 (2000).
T. Jungwirth, J. Sinova
Theory: TAMR in (Ga,Mn)As/AlOx/metal devices
Increasing in-plane momentum conservation
[100]
0]1[1_
0]1[0_
0]11[__
00]1[_
10]1[_
[010]
[110]
°90
°0
: AngleΦ
~3000Ω
~2900Ω
Comparing Theory and Experiment
C. Gould et al., Phys. Rev. Lett. 93, 117203 (2004).
A READ-WRITE MEMORY DEVICE!
Resistance at B=0mTResistance at B=0mT
2.90
2.95
3.00
2.90
2.95
3.00
B=0mT R
(kO
hm)
T=4.2K, V=10mV270
90
0
[010]
[100]
TAMR in ‚GMR‘ Sample
top: B in plane
bottom: B perpendicular
C. Rüster et al, PRL 94, 027203 (2005).
TAMR in ‚GMR‘ Sample
Effect blows up for low T, low bias: Efros-Shklovski gap.
Conclusions
– Spin LED
– Spin RTD: voltage adjustable spin-switch
– Very large magnetoresistance in GaMnAs nano-constrictions
– TAMR results from double step switching + anisotropic dos
– gives bistable hysteresis for single FM layer; is huge for two FM layers
Collaborators:Charles Gould, Georg Schmidt, Karl Brunner,Romain Giraud, Peter Grabs, Katrin Pappert, Christian Rüster, Gisela Schott, Anatoly and Taras SlobodskyyTatjana Borzenko, Volkmar Hock,
Funding:BMBF, DFG (SFB 410), EU, DARPA SPINS Program, ONR
