SpiDME meeting, Nijmegen, May 2007

Stefano Sanvito and Nadjib Baadji

Computational Spintronics Group

School of Physics and CRANN, Trinity College

People Dr. Nadjib Baadji (Uni. Strasbourg), April ‘07

Mr. Sankar Kesanakurthi (U. Hiderabad), April ‘07

Visits

Sanvito to Hamburg (Feb. 2007)

Outlook

A simple model for transport

Salen on Cu

SP-STM for molecules

Ab initio transport theory

L RL R

V 0

L

R VL

R

V ≠ 0

In equilibriumOut of equilibrium

2

L

R VL

R

V ≠ 02|EF-|

- 00 fNU

L

R VL

R

V ≠ 0

LL R

R

E +eV/2F

E -eV/2F

T(E)

E

I

H1

H0 H0

H1

HM (n)

H0

HRM

H0

H1

H0

H1

H0

HLM

RL

H= HM+H0 +H0 +H0 +….

HM+L (E)+R(E)

Lead’s Self-energy

Density Matrix

Current

Molecule Green function

A. R. Rocha and S. Sanvito, PRB 70, 094406 (2004)

D. Sánchez-Portal, P. Ordejón, E. Artacho, and J.M. Soler, Int. J. Quant. Chem. 65, 453 (1997)

KS-DFT Hamiltonian

We implemented NEGF in Siesta

Localized multiple-z Pseudo-atomic orbitals

(non-orthogonal)

Optimized Pseudopotential

Super-cells with up to 2,000 atoms

http://www.smeagol.tcd.ie/

A. R. Rocha et al., Phys. Rev. B 73, 085414 (2006); Nature Materials 4, 335 (2005)

Mailing list http://lists.tchpc.tcd.ie/listinfo/smeagol-discuss

Molecular Spin valves

Nature Mat. 4, 335 (2005)

Fe/MgO TMR junction

I. Rungger et al

DNA transport

A.R.Rocha et al., in preparation

Ni point contacts

A.R.Rocha et al., cond-mat/0701512

Problems with molecular transport

C. Toher et al., PRL 95, 146402 (2005)

Spin Torque

M. Stamenova et al., in preparation

0nA

40nA

80nA

0nA

50nA

100nA

Au on Au

V=250mV

d=0.4nm

Ni on Ni

V=250mV

d=0.4nm

0%

10%

20%

500 mV

250 mV

-30%

-10%

10%

-70%

-55%

-40%

-250 mV-60%

-45%

-30%

-500 mV

P=I I

I +I

I to tip

I from tip

Does the GMR mirror the polarization ?

-30%

-10%

10%

P=I I

I +I

-20%

-15%

-10%

R=IP IAP

IAP

250 mV

500 mV

250 mV

-250 mV

-500 mV

20%

10%

-45%

-50%

V=0

V=0

TIP

M+S

S

S

tip

tip V

V=0

TIP

M+S

V=400mV

TIP

M+S

Current to the tip

V=0

TIP

M+S

V=-400mV

Current to the S+M

TIP

M+S

Direct calculations of the tunneling currents are possible and include:

Some prospects of investigating the bonding of molecules on magnetic surfaces

Electronic Structure of the tip

Tip to sample interaction

Charging of the moleculae

Accurate determination of the spin-polarization

Non-collinear spin

Spin-orbit

Molecule

N,N'-BIS(SALICYLIDENE)ETHYLENEDIAMINO-TMWhere TM could be : Cu, Zn, Ni or Co

C1

C2C3

C1

C2

C3

Comparison between the DOS of the Salen

molecule and the hypothetical small molecule

E (eV)

DO

S (

arb

. u

nit

s)Big

Small

E (eV)

Big

Small

Molecule on Cu surfaces (un-relaxed)

Cu-salen on Cu(001)

Cu-salen on Cu(111)

DO

S (

arb

. u

nit

s)

E (eV)

Relaxation on Cu(001) surface

Unrelaxed structure Relaxed structure

DOS for different TM-salen

Cu

E (eV)

DO

S (

arb

. u

nit

s)4s13d10

E (eV)

DO

S (

arb

. u

nit

s)Zn 4s23d10

E (eV)

DO

S (

arb

. u

nit

s)Co 4s23d7

E (eV)

DO

S (

arb

. u

nit

s)Ni 4s23d8

Simulation STM imagesFree Cu-Salen

EF-0.2eV < E < EF

EF < E < EF

+0.2eVI molecule to tip I tip to molecule

Constant current STM imagesCu-Salen un-relaxed

EF-0.2eV < E < EF

EF < E < EF

+0.2eVI molecule to tip I tip to molecule

(a) (b)

(c) (d)

EF-0.2eV < E < EF

EF < E < EF

+0.2eVI molecule to tip I tip to molecule

Cu

Zn

This is very much work in progress

First find the right atomic configuration

Then simulate the current

Compare the images for different TM

Hopefully they will compare with experiments

integral of the DOS near Ef (pos. & neg. bias L-resolved DOS for Cu atom in the small molecule

L-resolved DOS for Zn atom in the small moleculeCu DOS in free mole. and in mole. on Cu (001)