1
GaMnAs GaMnAs Nanostructures
Bryan Gallagher, Kevin Edmonds, Richard Campion,
Tom Foxon, Thomas Jungwirth, Laurence Eaves,
Kaiyou Wang, Lixia Zhao, Devin Giddings, Oleg
Makarovsky, Nicola Farley, Jeanho Yang, Amalia
Patane. (University of Nottingham) Jörg
Wunderlich, Mohammed Khalid, Kenchi Ito, Shazia
Yazin, David Williams (Hitachi Cambridge) Thomas
Jungwirth, Jairo Sinova, Allan Macdonald
(Praha,Texas) Mike Sawicki, Thomas Dietl
(Warsaw)
2
Semiconductors Spintronics
Ferromagnetic semiconductors (FSs) Compatible
with Existing Technologies. Integrate Magnetic,
Semiconducting Optical Properties. New
Functionalities.

• Already demonstrated in FS devices
• Efficient Spin injection.
• Polarised Light Emitting Diodes.
• GMR, TMR and resonant tunnelling.
• Gate Current controlled ferromagnetism.

Require (i) well understood ferromagnetic
semiconductors with Curie temperatures well above
room temperature and (ii) functioning nano-scale
ferromagnetic semiconductor devices
3
Origin of ferromagnetism in (GaMn)As
Substitutional Mn2 in GaAs Acceptor valence
band holes S 1/2 (itinerant) A d 5
magnetic moment S 5/2 (localised)
Long-range magnetic order is due to local
exchange interactions between Mn2 acceptors and
GaAs holes. The carrier density determines the
key magnetic properties of (Ga,Mn)As
4
Problems for GaMnAs (late 2002)

• Curie Temperature limited to 110K.
• Only metallic for 3 to 6 Mn
• High degree of compensation
• Unusual Magnetisation(Temperature)
• Significant Magnetisation deficit

110K could be a fundamental limit on TC 4 Yu
et al, PRB (2002)
But are these intrinsic properties of GaMnAs ??
5
Problems for Growth of GaMnAs
Incorporate high levels of Mn by low temperature
MBE.
Anti-site As defects Double donor Q2e
Substitutional Mn Q-2e and local 5/2 moment
Interstitial Mn Double donor Q2e
High densities interstitial Mn in as grown GaMnAs
(Experiment Yu et al PRB 2002, Theory Erwin
Petukhov PRL 2002)
6
Nottingham GaMnAs Growth

• Low Temperature (1500C to 2500C) 2D MBE Growth
• As2 rather than As4 to Minimise As antisite
  defect densities (Foxon Joyce, Surface Sci.,
  50, 434 (1975)).
• Two gallium cell procedure for near exact
  stoichiometry and surface temperature stability
• Use low temperature (180C ! ) annealing.

7
Effects of low temperature annealing
Hayashi et al APL 2001. Potashnik et al APL 2001.
Edmonds et al APL 2002

• Low temperature Annealing
• Increases Tc and saturation moment.
• Changes form of M(T).
• Increases hole density by up to a factor of 3.
• Mobility changes by less than 10.
• Conductivity proportional to hole density.

Annealing at 2500C
Potashnik et al
8
Origin of annealing effects
Edmonds et al PRL (2004) ( Nottingham, Warsaw,
North Carolina)
Monitor annealing by in-situ resistivity
measurements.
Annealing much quicker for thinner GaMnAs layers
Model of interstitial Mn diffusing to the free
surface, gives excellent agreement with the data.
9
Energy Barrier for Mn interstitial
Measurements at a series of anneal temperatures
give experimental energy barrier
Density functional theory calculations give
theoretical energy barriers
Activation energy Measured 1.4 0.2
eV. Calculated 0.8 eV for isolated Mn
interstitials. 1.3 eV for interstitial
-substitutional anti ferromagnetically coupled
pairs.
25nm thick 4.7 Mn Temperature 160K to 200K
10
Mn at the surface
3 kV Auger spectra 6 Mn, 50 nm film
Auger spectra samples annealed in air clearly
show that Mn is moving to the free surface
11
Obtaining Hole densities
12
Black single band free hole Hall Effect.Others
different scattering scenarios Hall Effect
Slope within 20 of single band free hole values

Hole densities from Hall Effect
13
Hole densities
Open symbols half closed as grown. Closed
symbols annealed
Low Compensation
Obtain Mnsub assuming change in hole density due
to Mn out diffusion
High compensation
Annealing can very significantly increases hole
densities.
14
Generation of Mnint during growth
Theoretical calculation of equilibrium Mnsubst
and Mnint for a growth temperatures of (Jan
Masek, Tomas Jungwirth)
15
Ferromagnetic Ground State?
Observed large magnetisation deficits taken as
evidence for frustration or non-collinear
ferromagnetic ground states. PRL 88,187202
137201 247202 89, 047201 (2002)
Saturation Magnetisation 4mB? (5mB per Mn,
-1mB per hole)
As grown Annealed
Total Mn concentration
16
Moment per Substitutional Mn

• Assume
• change in hole density due to Mn out diffusion
• anti-ferromagnetic
• MnInt- Mnsub pairs
• (iii) -1mB per hole

No indication of a magnetisation deficit Wang et
al. J Appl. Phys. 95 (11) 6512
Confirmed by MCD measurements APL. 84 (20) 4065
17 (2004) J Appl. Phys. 95 (11) 7166 (2004)
17
Tc as grown and annealed samples
Open symbols as grown. Closed symbols annealed
18
Increase of Tc with effective Mn
Effective Moment density, Mneff Mnsub-MnInt due
to AF Mnsub-MnInt pairs. Tc increases with Mneff
when compensation is less than ???. No
saturation of Tc at high Mn concentrations
Closed symbols are annealed samples
High compensation
19
Tc/xeff vs p/Mneff
TBA/CPA calculations including RKKY fluctuations,
superexchange disorder
20
Magneto-transport
Transport calculations within mean field theory
using Born approximation ( Jungwirth, Gallagher
et al. Appl. Phys. Lett. (2003))
Berry phase theory of AHE (Jungwirth et al PRL
2002)
Agreement between theory and experiment,
unparalleled in itinerant ferromagnetic
systems (see also J. Appl. Phys. 93, 10, 2003
cond-mat/0209123.)
21
Ferromagnetic Semiconductor Nanostructures
2000 MR GaMnAs double nanocontact structures
Ruster et al Wuerzburg PRL 2003

• Interpreted as TMR.
• High resistance state magnetisation of leads
  island anti-parallel.
• Low resistance state magnetisations parallel

22
GaMnAs Nanocontact TAMR
Phys. Rev. Lett. Giddings et al PRL March 28th
2005), cond-mat/ 0409209 (Hitachi Cambridge,
Nottingham, Prague, Texas AM)
5nm thick 2 Mn GaMnAs Hall bars
nanoconstrictions
30nm Constriction
30nm constriction
Tunnelling Conduction at low temperatures
voltages
GaMn
23
Apparent TMR response
Applied Field In Plane Perpendicular to Current
Convention Current 110 B 110 (B x) B?
110 (B y) B 001 (B z)
BLACK Sweep up RED Sweep down
24
Inverted TMR response??
Applied Field in Plane Parallel to Current
Low resistance state at saturation. Cannot be
normal TMR
25
Spin-Valves with one ferromagnetic contact?
Wuerzburg Gould et al PRL (2004) Ruster et al
PRL (2005)
Tunnelling anisotropic magnetoresistance (TAMR)
tunnelling density of states depends on relative
orientation of the magnetisation and the
crystallographic axes
26
Spin-orbit coupling and anisotropies
spin-split bands at M?0
(Abolfath et al., PRB '01)
(Dietl et al., Science '00)

• Magnetization orientation dependences
• Hole total energy over Fermi volume
• ? magnetic anisotropy
• Group velocities at the Fermi surface and
  density of states for scattering
• ? in plane magneto-resistance anisotropy
• Density of states at the Fermi energy
• ? anisotropic tunnel magneto-resistance

27
AMR TAMR
AMR in unstructured bar
TAMR in constriction
TAMR dominates in our nanocontact devices
magnetic response of constricted device bar are
very similar
28
TAMR in single nanocontacts
30nm constriction
Very large TAMR in single nanocontacts
1400
29
Calculated tunnelling probabilites
Wavevector dependent tunnelling probabilityT (ky,
kz) Red high T blue low T.
Magnetisation perpendicular to plane
Magnetisation in plane
Magnetisation in plane
our system
strong z-confinement (ultra-thin film) less
strong y confinement (constriction)
30
Key Points

• GaMnAs System well behaved and well understood.
  Tc up to 173K. Expect further increases. Material
  available for collaborators
• Large Tunnelling Anisotropic MR in nanocontacts.
  Effect may have applications (very simple
  device).
• Need to reappraisal previous GaMnAs single
  crystal metal TMR data.
• Acknowledge support of EPSRC EU FENIKS

Ferromagnetic Semiconductor Web project
http//unix12.fzu.cz/ms (Jan Kucera) Tabulations
of Nottingham data freely available