Getting FM in semiconductors is not trivial.
Recall why we have FM in metals:• Band structure leads to enhanced exchange interactionsbetween (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 atmuch 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
Dilute Magnetic semiconductors (DMS)
charge distribution of magnetic ions overlap
Direct exchange
Super-exchange
Magnetic ions interaction mediated by interaction with conduction elections. RKKY interaction
Indirect exchange
Magnetic ions interact by charge overlap with same non-magnetic ions
Exchange interactions
Dietl et al, Science 287, 1019 (2000)
5% MnP=3.5x1020/cm3
•Main family: III-V compoundsemiconductors.• Most common magnetic dopantin Mn (group II).• Result: III(Mn)-V compoundsare p-type.• Grown by low-temperatureMBE - not thermodynamicallystable.• Typical concentrationsomething like Ga0.95Mn0.05As.
•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).
Dilute Magnetic semiconductors (DMS)
Magnetic semiconductors - descriptionTanaka., J. Cryst. Growth 278, 25 (2005)
Sensitivity to carrierconcentration means it’spossible to have gateableferromagnetism!
Potentially very exciting forspintronics applications.
Major problems:• Temperature range is poor.• Materials compatibility is notvery good, either.
Ohno et al., Nature 408 944 (2000)DMS: magnetic properties
So far, have increased Tc up tohigher values (~ 175 K) inGaMnAs system….
- 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.)
To increase TC
Tanaka., J. Cryst. Growth 278, 25 (2005)
DMS: heterostructures
TC > room T obtained in InAs:Mn QD sample.Enhancement caused by “good” disorder?
DMS QD samples Bhattacharya group, APL 85, 973 (2004)
