Stefano Sanvito
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Transcript of Stefano Sanvito
Stefano SanvitoPhysics Department, Trinity College, Dublin 2, Ireland
TFDOM-3 Dublin, 11th July 2002
Spintronics
Diluted magnetic semiconductors
Conclusions
Funded by NSF/ONR/ACS/EI
Digital ferromagnetic heterostructures
Electronic Structure Ballistic Transport Effect of As antisites
Use the spin of an electron as well as its electric charge (GMR)
Semiconductors MetalsNon-magnetic Magnetic
Very long spin-diffusion length (0.1mm)
Need for magnetic semiconductors: Mn + III-V or II-VI
LOGIC DATA STORAGE
Ga
As
Mn
100
200
300
Subs
trat
e T
(C
)
x0.02 0.04 0.06
Formation of MnAs
Metallic
Insulating
Roughening
Polycrystalline
LT-MBE Growth
x
Hole-mediated ferromagnetism
Mn = local spin (5/2) + 1 hole
H.Ohno, JMMM 200, 110 (1999)
AsMnGa1 xx
SsNHH
0
In the mean-field approximation SSi
31218 pxpENSST Fc
NSsHH
0 effBsHH
0
No dependence of on Mn concentration
Small dependence of on doping the GaAs
For some samples the transport changes from hole to electron dominated
cT
cT
R.Kawakami et al. APL 77, 2379 (1999)
We perform density functional theory (DFT) calculations in the local spin density approximation (LSDA). Several implementations. For large systems: SIESTA
Localized multiple- Pseudo-atomic orbitals
Optimized Pseudopotential
Ceperley-Alder Exchange correlation
Large k-point sampling
Super-cells with up to 100 atoms
D. Sánchez-Portal, P. Ordejón, E. Artacho, and J.M. Soler, Int. J. Quant. Chem. 65, 453 (1997)
Ga
As
Mn
Super-cell method
What is the real dimensionality of the system ?
Why is ferromagnetism insensitive to what you do in the GaAs layers?
Are the carriers spin-polarized?
The DOS shows a gap for the minority band at the Fermi level
Large dispersion parallel to MnAs plane
Small dispersion perpendicular to MnAs plane
BZ
kkT
h
e)(
2Landauer-Büttiker formalism
Extract a TB Hamiltonian H and overlap matrix S from LSDA
Write H and S in tridiagonal form
Calculate the transport with Green’s function technique, generalized to non-orthogonal basis set and singular coupling matrices
S.S. et al. PRB 59, 11936 (1999)
CPP
CIP
Small
Large
Current in the planes
I
xx kk
open
Large
Small
The current is due to hopping between Mn planes
I
Macroscopic Average of Total Potential
What is the real dimensionality of the system ?
Why is ferromagnetism insensitive to what you do in the GaAs layers?
CIP current in plane
CPP current (small) = hopping between plane
Total potential suggests spin-selective confinement
Are the carriers spin-polarized? Digital magnetic heterostructures are half metallic
Is this picture valid when As antisites are present?
In real samples there is a large hole compensation. This is due to As antisites (As atoms at the Ga sites)
x
As antisites destroy the half metallic state
however ……
I
Is this picture valid when As antisites are present?
As antisites destroy the half metallic state of digital ferromagnetic heterostructures
If the As antisites are far enough from the MnAs planes, charge and spin separation:
Majority spin holes in the MnAs plane
Minority spin electrons in the GaAs spacer
Digital Ferromagnetic Heterostructures are 2D half-metal (if no As antisites are present)
As antisites in DFH destroy the half metallic state, but different spins are spatially separated
DFT is a powerful tool to study structural, electronic, magnetic and transport properties of diluted magnetic semiconductors
Stefano SanvitoPhysics Department, Trinity College, Dublin 2, Ireland
TFDOM-3 Dublin, 11th July 2002
Mapping onto a pairwise interaction model
ji
jinij SSJE
,
Remarkably the fit works fine
meV3.130 JmeV9.01 JmeV5.42 J
meV0.90 JmeV06.01 J
meV9.32 J
50% compensation