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Transcript
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)