Prague IoP group and theoretical studies of ferromagnetic materials and nanostructure with strong spin-orbit coupling

Institute of Physics ASCR Tomas Jungwirth, Alexander Shick, Karel Výborný, Jan Zemen,

Jan Masek, Jairo Sinova, Vít Novák, Kamil Olejník, et al.

64-node high-performance computer cluster

State of the art

molecular-bean epitaxy

& electron-beam lithography systems

Theoretical methods

Electronic structure

Analytical models (Rashba, Dresselhaus, spherical-Luttinger)

k.p semiphenomenological modelling (typical for semiconductors) extensive library of home-made routines

spd-tight-binding modelling (half way between phenomenological and ab initio) home-made relativistic codes

Full ab initio heavy numerics (transition metals based structures) standard full-potential libraries, home-made relativistic ab-initio codes

Observables micromagnetic parameters from total energy, thermodunamics, and linear response theories

Boltzmann and Kubo equations for extraordinary, anisotropic, and coherent transport

Device specific modeling Finite-element methods, Schrodinger-Poisson solvers, Monte-Carlo semiclassical methods, Landauer-Buttiker formalism

Semiconductor 2D electron and hole systems with spin-orbit coupled bands

Dilute-moment ferromagnetic semiconductors

AsAsGaGa

MnMn

Transition metal ferro and antiferromagnets

Materials

Research goal: Electric field controlled spintronics

HDD, MRAMcontrolled by Magnetic field

Spintronic TransistorsLow-V 3-terminal

devices

STT MRAMspin-polarized charge current

& Opto-spintronics

1. Exchange & spin-orbit coupling & direct link to spintronics (magnetotransport)

2. Semiconducting multiferroic systems

3. Spin dynamics in non-magnetic spin-orbit coupled channels

Paradigms

AMRAMR TMRTMR

TAMRTAMR

Exchange & spin-orbit coupling;complex link to transport

Exchange only; direct link to transport

)(MTDOS

Au

Exchange & spin-orbit coupling; direct link to transport

ab intio theoryTAMR is generic to SO-coupled FMs

experiment

Bias-dependent magnitude and sign of TAMR

Shick et al PRB ’06, Parkin et al PRL ‘07, Park et al PRL '08

Park et al PRL '08

spontaneous momentmag

netic su

sceptib

ility

Consider uncommon TM combinationsMn/W ~100% TAMR

Consider both Mn-TM FMs & AFMs

exchange-spring rotation of the AFMScholl et al. PRL ‘04

Proposal for AFM-TAMR: first microelectronic device with active AFM component

spin

-orb

it cou

plin

g

TAMR in TM structures

Shick, et al,unpublished

Shick, et al,unpublished

GM

MGG

C

C

e

MV

MVVCQC

QQU

)(&

)]([&2

)(0

20

electric && magneticmagnetic

control of CB oscillations

Source Drain

GateVG

VDQ

Devices utilizing M-dependent electro-chemical potentials: FM SET

SO-coupling (M)

[010] M[110]

[100]

[110][010]

~ mV in GaMnAs~ 10mV in FePt

Wunderlich et al, PRL '06

(Ga,Mn)As nano-constriction SET CB oscillations shifted by changing M(CBAMR)

Electric-gate controlled magnitude and sign of magnetoresistance spintronic transistor

&

Magnetization controlled transistor characteristic (p or n-type) programmable logic

Complexity of the relation between SO & exchange-split bands and

transport

SET

Resistor

Tunneling device

Chemical potential CBAMR

Tunneling DOS TAMR

Group velocity & lifetime AMR

Complexity of the device design

Magnitude and sensitivity to electric

fields of the MR

1. Exchange & spin-orbit coupling & direct link to spintronics (magneotransport)

2. Semiconducting multiferroic systems

3. Spin dynamics in non-magnetic spin-orbit coupled channels

Paradigms

Magnetic materials

Ferroelectrics/piezoelectrics Semiconductors

spintronic magneto-sensors, memories

electro-mechanical transducors, large & persistent el. fields

transistors, logic,sensitive to doping and electrical gating

Semiconducting multiferroic spintronics

Control via (non-volatile) charge depletion and/or strain effects

Ferromagnetic semiconductors

GaAs - GaAs - standard III-V semiconductorstandard III-V semiconductor

Group-II Group-II Mn - Mn - dilute dilute magneticmagnetic moments moments & holes& holes

(Ga,Mn)As - fe(Ga,Mn)As - ferrromagneticromagnetic semiconductorsemiconductor

Need true FSs not FM inclusions in SCs

Mn

Ga

AsMn

Mn-d-like localmoments

As-p-like holes

Mn

Ga

AsMn

EF

DO

S

Energy

spin

spin

GaAs:Mn – extrinsic p-type semiconductor

FM due to p-d hybridization

(Zener local-itinerant kinetic-exchange)

valence band As-p-like holes

As-p-like holes localized on Mn acceptors

<< 1% Mn ~1% Mn >2% Mn

onset of ferromagnetism near MIT

As-p-like holes

Ferromagnetism & strong spin-orbit coupling

LSdr

rdV

err

mc

p

mc

SeBH effSO

)(1

Strong SO due to the As p-shell (L=1) character of the top of the valence band

V

BBeffeff

pss

Beff Bex + Beff

Mn

Ga

AsMn

Rushforth et al., ‘08

Strain & SO

Electric field control of ferromagnetism

k.p kinetic exchange model predicst sensitivity to strains ~10-4

and hole-density variations of ~1019-1020 cm-3

slow and requires ~100V

Low-voltage gating (charge depletion) of ferromagnetic semiconductors

Owen, et al. arXiv:0807.0906

Switching by short low-voltage pulses

Mag

neti

zati

on

1. Exchange & spin-orbit coupling & direct link to spintronics (magnetotransport)

2. Semiconducting multiferroic systems

3. Spin dynamics in non-magnetic spin-orbit coupled channels

Paradigms

Datta-Das transistor

Spin dynamics in non-magnetic spin-orbit coupled channels

Datta and Das, APL ‘99

Spin-injection Hall effect transistor and spin-photovoltaic cell

Non-destructive detection of spin-dynamics along the channel

Compatible with optical and electrical spin-injection and tunable by electrical gates