nan zheng course: solid state ii instructor: elbio dagotto semester: spring 2008 department of...

NAN ZHENG

COURSE: SOLID STATE IIINSTRUCTOR: ELBIO DAGOTTO

SEMESTER: SPRING 2008DEPARTMENT OF PHYSICS AND ASTRONOMYTHE UNIVERSITY OF TENNESSEE KNOXVILLE

Dilute Magnetic Semiconductors

(DMS)

Outline

Introduction: spintronics and DMSDMS 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

Tc 110

Max TC ~ 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 TC 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 timeBelow 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

Tc 800

Tc 940

Highest in Dietl’s prediction

TC

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)

TC

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 2Scenario 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)AsVarious 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