nan zheng course: solid state ii instructor: elbio dagotto semester: spring 2008 department of...
TRANSCRIPT
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NAN ZHENG
COURSE: SOLID STATE IIINSTRUCTOR: ELBIO DAGOTTO
SEMESTER: SPRING 2008DEPARTMENT OF PHYSICS AND ASTRONOMYTHE UNIVERSITY OF TENNESSEE KNOXVILLE
Dilute Magnetic Semiconductors
(DMS)
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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
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Introduction: Spintronics and DMS
Spintronics: Spin-based electronics Idea: a combination of microelectronics and magnetic
storage technique.
Searching for Materials??
Mass Storage Integrated Circuit
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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
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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!
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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)]
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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)
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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
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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
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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
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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)]
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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
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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)
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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
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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