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

Introduction: Spintronics and DMS Diluted Magnetic Semiconductor (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

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 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 Metal to Insulator Transition (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 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 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 Room temperature ferromagnetism discovered in Mn doped ZnO through reactive magnetron co- sputtering and fast annealing in 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 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 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 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 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 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