Spintronics at Univ. Spintronics at Univ. L’AquilaL’Aquila

FirstFirstprinciplesprinciples

calculations:calculations:FLAPW, FLAPW, PWSCF,PWSCF,DMolDMol33

A. Continenza, S. Picozzi

NorthwesternNorthwesternUniv., USAUniv., USA(Y.J.Zhao,

A.J.Freeman,T.Shishido)

Univ. TriesteUniv. Trieste(M. Peressi,

A. De Bernardi)

Spintronics at Univ. Spintronics at Univ. L’AquilaL’Aquila

Univ. CataniaUniv. Catania(F. Priolo et al.)

ExperimentsExperiments(growth + cha-(growth + cha-racterization)racterization)

Univ.CamerinoUniv.Camerino(N.Pinto, F.Gunnella,

M. De Crescenzi)

F. D’Orazio,F. Lucari,

M. Passacantando,P. Picozzi

MaterialMaterials s

exploreexploredd

HeuslerHeuslerAlloys

(Co2MnSi,Co2MnGe,Co2MnSn)

Mn-doped II-IV-VII-IV-V22

(i.e. Mn: CdGePMn: CdGeP22)

Mn-doped I-III-VII-III-VI22

(i.e. Mn: CuGaSMn: CuGaS22)

Heusler/semiconductorHeusler/semiconductorInterfacesInterfaces

(I.e. Co2MnGe/GaAs,Co2MnGe/Ge)

Mn-doped Si, Ge, SiGeSi, Ge, SiGe

alloys

Mn, Cr and V-doped

BeTeBeTe

Spin injection at Spin injection at CoCo22MnGe/semiconductor MnGe/semiconductor

interfaces:interfaces:ab-initio studyab-initio study

Focus on Half-metallicity of heusler compound:

• Effect of defects • Effect of junction

S. Picozzi et al. JAP 94, 4723 (2003); PRB 66,094421 (2002)

? ?

Co-antisite

Co-Mn swap

Mn-antisite

38.000.33Mn antisite

36.001.17Co-Mn swap

38.370.84Co antisite

Mtot(B)Hf (eV)

Quite low formation EnergiesHalf-metallicity is kept (lost) with Mn (Co) antisites

GeMnCo

Defect effectsDefect effects

Interface effectsInterface effects

Strong perturbationsperturbations induced by the interfaceinterface Interface gap statesInterface gap states present at bothboth sidessides of the interface Interface gap statesInterface gap states in the Heusler Heusler side: half-metallicity is half-metallicity is locally lost !locally lost ! States decay away from States decay away from interface (3 to 5 layers):interface (3 to 5 layers): D(ED(EFF) vs z) vs zatat

Co2MnGeGaAs GaAs[001]

Schottky barrier height: Schottky barrier height: effect of the semiconductoreffect of the semiconductor

0.08 eV0.5 eV < B < 0.7 eVB = EF - VBMsemic

“Ohmic”“Tunnel”Spin-injection process

Almost ohmic

for holes

(EF close to VBM)

Schottky

(EF pinned in the gap)Type of contact

GeGaAs

Co2MnGe/GaAs Co2MnGe/Ge

Mn-doping in Ge and Si

Mn

Ge Ge

Ge

Ge Ge

Spin densitylocalized onMn sites

Induced negative mom.on nearestneighbor withevident p-character: AFM Mn- Gecoupling re-lated to Zener FM?

Oscillatory trend for inducedmoment as a function of distancefrom Mn impurity

Trends with Mn concentration in Si, Ge:

relevant propertiesFA: Difference between FM and AFM total energy: FM favoredSimilar behavior for Si and GeFA Increases with Mn conc x:Consistent with expts

Magnetic moments: MnGe keeps integer moment(equal to 3 B) for all xMnSi show variations with x:larger p-d hybridization for SiMagnetism essentially of 3d origin

“Real atoms”

Absolutemagnetic moments

Totalmagnetic moments

Formation energies Virtual

crystal

Mn-doping in SixGe(1-x) cells (3.13% Mn)

Mn/Digital alloys

•The Mn-doped layers produce a potential well•The depth of the potential well is affected by the Mn concentration: upon doubling of the concentration, the barrier doubles • A potential barrier is also present for carriers in the Ge-region (not dependent on spin).

Mn_ML

1 Ge spacer

3 Ge spacers

Mn-doped

planes

Carr

ier

pro

pert

ies

Carr

ier

pro

pert

ies

Charge on GeSpin. Dens.

on Ge

What can we do?

First principles calculations of:• Structural properties

heats of formation phase stability defects energetics

• Electronic and magnetic properties magnetizationmagnetic alignment

• Carrier propertiesCarrier confinment, spin-polarization

• Magneto-optics related quantities (MOKE, XMCD)• Conductivity tensor• STM and Spin-Pol. STM maps