DMS

Dilute Magnetic Semiconductors (DMS):

NAN ZHENG COURSE: SOLID STATE II INSTRUCTOR: ELBIO DAGOTTO SEMESTER: SPRING 2008 DEPARTMENT OF PHYSICS AND ASTRONOMY THE UNIVERSITY OF TENNESSEE KNOXVILLE Dilute Magnetic Semiconductors (DMS)

Outline:

Outline Introduction: spintronics and DMS DMS materials (Ga,Mn)As (Ga,Mn)N Transitional metal doped oxide Magnetic mechanism studied by the Mean Field Approach Summary

Introduction: Spintronics and DMS:

Introduction: Spintronics and DMS Spintronics: Spin-based electronics Idea: a combination of microelectronics and magnetic storage technique. Searching for Materials?? Mass Storage Integrated Circuit

Introduction: Spintronics and DMS:

Introduction: Spintronics and DMS D iluted M agnetic S emiconductor (DMS): Traditional semiconductors doped with transition metals Why “Dilute” ? Small doping concentration (a few %) Why “Magnetic” ? Display ferromagnetisation Why “Semiconductor” ? While preserving the semiconducting properties

Introduction: Spintronics and DMS:

Criteria of ideal materials for spintronics: Room temperature ferromagnetisation Fit into current electronic technique Introduction: Spintronics and DMS Theoretical predictions by Dietl, Ohno et al. Various DMS displays room temperature ferromagnetism!

DMS materials I: (Ga,Mn)As:

DMS materials I: (Ga,Mn)As First DMS material, discovered in 1996 by Ohno et al using molecular beam epitaxy (MBE), a breakthrough in experiment. Curie temperature K at optimal doping Max T C ~ 110K x ~ .05 [Ohno et al., APL 69, 363 (1996)]

DMS materials I: (Ga,Mn)As:

DMS materials I: (Ga,Mn)As M etal to I nsulator T ransition ( MIT ) [Ohno, JMMM 200, 110 (1999)] Resistance measurements on samples with different Mn concentrations: Metal  R  as T  Insulator  R  as T  MIT happens at T C for intermediate Mn concentrations (0.035~0.053)

DMS materials I: (Ga,Mn)As:

DMS materials I: (Ga,Mn)As Annealing Effect (observed in other DMSs as well) Resistance  decreases with annealing time, up to 2 hrs, and then increases again Two regimes at annealing time Below 2h, T , FM , metallicity , lattice constant  WHY?? Origin related to defects, details unknown

DMS materials II: (Ga,Mn)N:

DMS materials II: (Ga,Mn)N First room temperature DMS discovered in 2001 using metal organic chemical vapor deposition (MOCVD) method. High curie temperature Experiment : up to K Theory : up to K Highest in Dietl’s prediction

DMS materials III: Transition metal doped oxide:

DMS materials III: Transition metal doped oxide Room temperature ferromagnetism discovered in Mn doped ZnO through reactive magnetron co-sputtering and fast annealing in 2001. Material: Mn doped ZnO Co doped TiO Reported up to 400K Hysteresis curve at Room temperature for Mn doped ZnO(Sn)

Magnetic Mechanism and Physical Properties:

Magnetic Mechanism and Physical Properties Carrier-mediated mechanism: Itinerant carriers (holes or electrons), s=1/2 Doping magnetic atoms (eg. Mn: S=5/2) Interaction between hole spin and Mn local moment is AFM, giving rise to an effective FM coupling between Mn spins [Dietl et al., PRB 55, R3347(1997)]

Magnetic Mechanism and Physical Properties:

Magnetic Mechanism and Physical Properties Two basic approaches to understand magnetism in DMS Mean Field Theory based on Zener model Clusters formed by magnetic atoms are responsible for ferromagnetism Scenario 2 Scenario 1

Magnetic Mechanism and Physical Properties:

Magnetic Mechanism and Physical Properties MF approach further explained: (A) High carrier density : Carrier (electrons or holes, depending on doping) mediated interaction leads to ferromagnetism. (B) Low carrier density : Percolation network is formed, carriers hop from site to site freely, aligning Mn moments within the cluster network. Pearton et al, Mat. Sci. Eng. R 40 (2003)

Magnetic Mechanism and Physical Properties:

Magnetic Mechanism and Physical Properties How good is Mean Field Theory? Its reliability is case dependent. Various MFT calculation for (Ga,Mn)As Various MFT calculation for (Ga,Mn)N

Summary and Outlook :

Summary and Outlook Room temperature DMS already realized, while explanation on the origin of ferromagnetism still under refinement. Further development on mean field approach in DMS: Monte Carlo simulations on local moment (eg. Mn) distribution Incorporation of defect structures (implied by annealing effect) Correlation effects in the hole sub-system