Dilute Magnetic Semiconductors (DMS)

1. 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

2. 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

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

4. 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

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

6. 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 TC ~ 110K x ~ .05 [Ohno et al., APL 69, 363 (1996)]

7. DMS materials I: (Ga,Mn)As • Metal to Insulator Transition (MIT) • Resistance measurements on samples with different Mn concentrations: • Metal  R  as T  • Insulator  R  as T  • MIT happens at TCfor intermediate Mn concentrations (0.035~0.053) [Ohno, JMMM 200, 110 (1999)]

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

9. 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

10. 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)

11. Magnetic Mechanism and Physical Properties • Carrier-mediated mechanism: 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)] Doping magnetic atoms (eg. Mn: S=5/2) Itinerant carriers (holes or electrons), s=1/2

12. Scenario 2 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 1

13. 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)

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

15. 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