Transcript 12/06/05

Dilute Magnetic semiconductors (DMS)
Getting FM in semiconductors is not trivial.
Recall why we have FM in metals:
• Band structure leads to enhanced exchange interactions
between (relatively) localized spins (d- or f-shell electrons).
• Conduction electrons can play a very important role.
In semiconductors,
• Carriers present are only there because of doping, and at
much lower concentrations.
• No natural localized spins.
Situation today:
• Add localized spins by doping (e.g. with Mn).
• Mechanism of FM still not universally clear.
• Curie temperatures still not great
Exchange interactions
Direct exchange
charge distribution of magnetic ions overlap
Super-exchange
Magnetic ions interact by charge overlap with
same non-magnetic ions
Indirect exchange
Magnetic ions interaction mediated by
interaction with conduction elections.
RKKY interaction
Dilute Magnetic semiconductors (DMS)
Dietl et al, Science 287, 1019 (2000)
•Main family: III-V compound
semiconductors.
• Most common magnetic dopant
in Mn (group II).
• Result: III(Mn)-V compounds
are p-type.
• Grown by low-temperature
MBE - not thermodynamically
stable.
• Typical concentration
something like Ga0.95Mn0.05As.
5% Mn
P=3.5x1020/cm3
•Note that these materials are quite heavily doped!
•II-VI materials have been much harder to work with
(unable to dope; exchange interaction difficult to
control).
Magnetic semiconductors - description
Tanaka., J. Cryst. Growth 278, 25 (2005)
DMS: magnetic properties
Sensitivity to carrier
concentration means it’s
possible to have gateable
ferromagnetism!
Potentially very exciting for
spintronics applications.
Major problems:
• Temperature range is poor.
• Materials compatibility is not
very good, either.
Ohno et al., Nature 408 944 (2000)
DMS: heterostructures
To increase TC
- Increase Mn concentration: Mn
provides magnetic moments.
(LTMBE to incorporate Mn.)
- Increase hole concentration:
holes mediate exchange coupling.
(Low temperature growth results
in defects and reduces hole
concentration – HS and
modulation doping.)
So far, have increased Tc up to
higher values (~ 175 K) in
GaMnAs system….
Tanaka., J. Cryst. Growth 278, 25 (2005)
DMS QD samples
TC > room T obtained in InAs:Mn QD
sample.
Enhancement caused by “good”
disorder?
Bhattacharya group, APL 85, 973 (2004)