Getting FM in semiconductors is not trivial. Recall why we have FM in metals: Band structure leads to enhanced exchange interactions between (relatively)

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Presentation transcript:

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 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% Mn P=3.5x10 20 /cm 3 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 Ga 0.95 Mn 0.05 As. 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 - description Tanaka., J. Cryst. Growth 278, 25 (2005)

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 (2000) DMS: magnetic properties

So far, have increased Tc up to higher values (~ 175 K) in GaMnAs 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 T C 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)