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NERC SWAN. NRI e-Workshop Making semiconductors magnetic: A new approach to engineering quantum materials. Jairo Sinova ( TAMU ). Tomas Jungwirth ( TAMU, Institute of Physics, Czech Republic, U. of Nottingham ). OUTLINE. Motivation Ferromagnetic semiconductor materials:

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slide1

NERC

SWAN

NRI e-Workshop

Making semiconductors magnetic:

A new approach to engineering quantum materials

Jairo Sinova

(TAMU)

Tomas Jungwirth

(TAMU, Institute of Physics, Czech Republic, U. of Nottingham)

outline
OUTLINE
  • Motivation
  • Ferromagnetic semiconductor materials:
    • (Ga,Mn)As - general picture
    • Growth and physical limits on Tc
    • Related FS materials
  • Ferromagnetic semiconductors & spintronics
    • Tunneling anisotropic magnetoresistive device
    • Transistors
slide3

Ferromagnetic semiconductor research for spintronics:

  • Motivations and strategies
  • Find new effects in this new material and utilize in conventional metal-based spintronics
  • 2. Develop a three-terminal gatable spintronic device to progress from sensors & memories to transistors & logic
  • In the 2nd part of the talk we show examples of 1. & 2. and a combination of both principles
slide4

Ga

Mn

As

Mn

Ferromagnetic semiconductors

Need true FSs not FM inclusions in SCs

GaAs - standard III-V semiconductor

Group-II Mn - dilute magnetic moments

& holes

(Ga,Mn)As - ferromagnetic

semiconductor

slide5

Ga

Mn

As

Mn

What happens when a Mn is placed in Ga sites:

Mn–hole spin-spin interaction

As-p

5 d-electrons with L=0

S=5/2 local moment

intermediate

acceptor (110 meV)

 hole

Mn-d

hybridization

Hybridization  like-spin level repulsion  Jpd SMn shole interaction

In addition to the Kinetic-exchange coupling, for a single Mn ion, the coulomb interaction gives a trapped hole (polaron) which resides just above the valence band

slide6

Ga

Ga

Mn

As

As

Mn

Mn

Transition to a ferromagnet when Mn concentration increases

GaAs:Mn – extrinsic p-type semiconductor

EF

spin 

~1% Mn

<< 1% Mn

>2% Mn

DOS

Energy

spin 

onset of ferromagnetism near MIT

As-p-like holes localized on Mn acceptors

valence band As-p-like holes

Ga

Mn

As

Mn

FM due to p-d hybridization (Zener local-itinerant kinetic-exchange)

slide7

(Ga,Mn)As synthesis

high-T growth

  • Low-T MBE to avoid precipitation
  • High enough T to maintain 2D growth
  • need to optimize T & stoichiometry
  • for each Mn-doping
  • Inevitable formation of interstitial Mn-double-donors compensating holes and moments
  •  need to anneal out but without loosing MnGa

optimal-T growth

slide8

Interstitial Mn out-diffusion limited by surface-oxide

Polyscrystalline

20% shorter bonds

O

GaMnAs-oxide

x-ray photoemission

MnI++

GaMnAs

Olejnik et al, ‘08

10x shorther annealing with etch

Optimizing annealing-T another key factor

Rushforth et al, ‘08

outline1
OUTLINE
  • Motivation
  • Ferromagnetic semiconductor materials:
    • (Ga,Mn)As - general picture
    • Growth and physical limits on Tc
    • Related FS materials
  • Ferromagnetic semiconductors & spintronics
    • Tunneling anisotropic magnetoresistive device
    • Transistors
slide10

185K!!

Tc limit in (Ga,Mn)As remains open

“... Ohno’s ‘98 Tc=110 K is the fundamental upper limit ..” Yu et al. ‘03

2008

Olejnik et al

“…Tc =150-165 K independent of xMn>10% contradicting Zener kinetic exchange ...” Mack et al. ‘08

“Combinatorial” approach to growth

with fixed growth and annealing T’s

can we have high tc in diluted magnetic semicondcutors
Can we have high Tc in Diluted Magnetic Semicondcutors?

NO EXTRINSIC LIMIT

NO IDENTIFICATION OF AN INTRINSIC LIMIT

Tc linear in MnGa local (uncompensated) moment concentration; falls rapidly with decreasing hole density in heavily compensated samples.

(lines – theory, Masek et al 05)

Relative Mn concentrations obtained through hole density measurements and saturation moment densities measurements.

Qualitative consistent picture within LDA, TB, and k.p

Define Mneff = Mnsub-MnInt

slide12

Linear increase of Tc with Mneff = Mnsub-MnInt

High compensation

8% Mn

Tc as grown and annealed samples

Open symbols as grown. Closed symbols annealed

  • Concentration of uncompensated MnGa moments has to reach ~10%. Only 6.2% in the current record Tc=173K sample
  • Charge compensation not so important unless > 40%
  • No indication from theory or experiment that the problem is other than technological - better control of growth-T, stoichiometry
slide13

Other (III,Mn)V’s DMSs

Kudrnovsky et al. PRB 07

Delocalized holes

long-range coupl.

Weak hybrid.

Mean-field but

low TcMF

InSb

d5

Impurity-band holes

short-range coupl.

Strong hybrid.

Large TcMF but

low stiffness

GaP

(Al,Ga,In)(As,P) good candidates, GaAs seems close to the optimal III-V host

slide14

III = I + II  Ga = Li + Zn

GaAs and LiZnAs are twin SC

n and p type doping through Li/Zn stoichiometry

LDA+U says that Mn-doped are also twin DMSs

No solubility limit for group-II Mn

substituting for group-II Zn !!!!

Masek, et al. PRB (2006)

outline2
OUTLINE
  • Motivation
  • Ferromagnetic semiconductor materials:
    • (Ga,Mn)As - general picture
    • Growth and physical limits on Tc
    • Related FS materials
  • Ferromagnetic semiconductors & spintronics
    • Tunneling anisotropic magnetoresistive device
    • Transistors
slide16

Au

AMR

TMR

~ 1% MR effect

~ 100% MR effect

Exchange split & SO-coupled bands:

TAMR

Exchange split bands:

discovered in (Ga,Mn)As Gold et al. PRL’04

slide17

Strong exchange splitting & SO coupling in (Ga,Mn)As

Ga

Mn

As

As-p-like holes

Mn

Standard MBE techniques for high-quality tunneling structures

slide18

TAMR in metal structures

experiment

Park, et al, PRL '08

ab intio theory

Shick, et al, PRB '06, Park, et al, PRL '08

Also studied by Parkin et al., Weiss et al., etc.

slide19

Gating of highly doped (Ga,Mn)As: p-n junction FET

(Ga,Mn)As/AlOx FET with large gate voltages, Chiba et al. ‘06

p-n junction depletion estimates

~25% depletion feasible at low voltages

Olejnik et al., ‘08

slide21

Persistent variations of magnetic properties with ferroelectric gates

Stolichnov et al.,

Nat. Mat.‘08

slide22

Electro-mechanical gating with piezo-stressors

Strain & SO 

Rushforth et al., ‘08

Electrically controlled magnetic anisotropies via strain

slide23

(Ga,Mn)As spintronic single-electron transistor

Wunderlich et al. PRL ‘06

Huge, gatable, and hysteretic MR

Single-electron transistor

Two "gates": electric and magnetic

slide24

AMR nature of the effect

Coulomb blockade AMR

normal AMR

slide25

Single-electron charging energy controlled by Vg and M

M

[010]

[110]

F

Q

VD

[100]

Source

Drain

[110]

[010]

Gate

VG

Q0

Q0

e2/2C

magnetic

electric &

SO-coupling 

(M)

control of Coulomb blockade oscillations

slide26

Theory confirms chemical potential anisotropies in (Ga,Mn)As

& predicts CBAMR in SO-coupled room-Tc metal FMs

  • CBAMR if change of |(M)| ~ e2/2C
  • In our (Ga,Mn)As ~ meV (~ 10 Kelvin)
  • In room-T ferromagnetchange of |(M)|~100K
  • Room-T conventional SET
  • (e2/2C>300K) possible
slide27

V

DD

V

V

A

B

V

B

V

A

Nonvolatile programmable logic

1

0

ON

OFF

Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device

1

0

ON

OFF

1

1

0

0

0

1

1

0

OFF

ON

OFF

ON

ON

OFF

ON

0

1

1

1

0

0

1

1

Vout

A B Vout

0 0 0

1 0 1

0 1 1

1 1 1

OFF

ON

OFF

1

1

0

0

“OR”

OFF

ON

OFF

ON

slide28

V

DD

V

V

A

B

V

B

V

A

1

0

Nonvolatile programmable logic

ON

OFF

Variant p- or n-type FET-like transistor in one single nano-sized CBAMR device

1

0

ON

OFF

Vout

A B Vout

0 0 0

1 0 1

0 1 1

1 1 1

“OR”

slide29

Device design

Physics of SO & exchange

Materials

FSs and metal FS with strong SO

Chemical potential

 CBAMR

SET

FSs

Tunneling DOS

 TAMR

Tunneling device

metal FMs

Group velocity & lifetime

 AMR

Resistor

slide30

Mario Borunda

Texas A&M U.

Alexey Kovalev

Texas A&M U.

Xin Liu

Texas A&M U.

Liviu Zarbo

Texas A&M U.

Matching TAMU funds

Bryan Gallagher

U. Of Nottingham

Laurens Molenkamp

Wuerzburg

Sankar Das Sarma

U. of Maryland

Tomas Jungwirth

Inst. of Phys. ASCR

U. of Nottingham

Joerg Wunderlich

Cambridge-Hitachi

Allan MacDonald

U of Texas

Other collaborators: Bernd Kästner, Satofumi Souma, Liviu Zarbo, Dimitri Culcer , Qian Niu, S-Q Shen, Brian Gallagher, Tom Fox, Richard Campton

conclusion checks of theory
Conclusion (checks of theory)

In the metallic optimally doped regime GaMnAs is well described by a disordered-valence band picture: both dc-data and ac-data are consistent with this scenario.

The effective Hamiltonian (MF) and weak scattering theory (no free parameters) describe (III,Mn)V metallic DMSs very well in the optimally annealed regime:

  • Ferromagnetic transition temperatures 
  •  Magneto-crystalline anisotropy and coercively 
  •  Domain structure 
  •  Anisotropic magneto-resistance 
  •  Anomalous Hall effect 
  •  MO in the visible range 
  •  Non-Drude peak in longitudinal ac-conductivity 
  • Ferromagnetic resonance 
  • Domain wall resistance 
  • TAMR 

TB+CPA and LDA+U/SIC-LSDA calculations describe well chemical trends, impurity formation energies, lattice constant variations upon doping