Getting FM in semiconductors is not trivial. Recall why we have FM in metals: Band structure leads...
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Transcript of Getting FM in semiconductors is not trivial. Recall why we have FM in metals: Band structure leads...
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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)
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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
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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)
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Magnetic semiconductors - descriptionTanaka., J. Cryst. Growth 278, 25 (2005)
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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
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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
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TC > room T obtained in InAs:Mn QD sample.Enhancement caused by “good” disorder?
DMS QD samples Bhattacharya group, APL 85, 973 (2004)