Dilute Magnetic semiconductors DMS Getting FM in semiconductors

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 Ga 0. 95 Mn 0. 05 As. 5% Mn P=3. 5 x 1020/cm 3 • 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 Ga. Mn. As system…. Tanaka. , J. Cryst. Growth 278, 25 (2005)

DMS QD samples TC > room T obtained in In. As: Mn QD sample. Enhancement caused by “good” disorder? Bhattacharya group, APL 85, 973 (2004)