Stefano Sanvito Physics Department, Trinity College, Dublin 2, Ireland TFDOM-3 Dublin, 11th July 2002.

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