Highlights and future possibilities in ferromagnetic semiconductor research

Kevin EdmondsSchool of Physics and Astronomy,

University of Nottingham, UK

Diluted magnetic semiconductors

Non-magnetic semiconductor

crystal

Paramagnetic DMS

(random spins)

Ferromagnetic DMS

(ordered spins)

Add dopants Add holes

1978: Paramagnetic (II,Mn)VI semiconductors

1992: Ferromagnetic (In,Mn)As with TC ~10K (Ohno et al.)

1998: Ferromagnetic (Ga,Mn)As with TC ~100K (Ohno et al.)

-300 -200 -100 0 100 200 300

-20

-10

0

10

20

Mn043Ga1-x

MnxAs:

.OPJ 08/10/02 16:45:19

Graph1

5 K 20 K 50 K 75 K 150 K

Ga1-xMnxAs

Mn-substituted zinc-blende GaAs

fraction x ~0.01-0.08 randomly occupied

by Mn

● Mn is a p-type dopant in GaAs→ conductivity

● half-filled 3d electronic shell → S = 5/2 magnetic moment

Mn

Ga

As

.. ... ... .... .. .. .. ...

. ..

...

Ga

As

Mn

HeatedGaAssubstrate

.. .

..

.

..... . ..

Molecular beam epitaxy

Structure

GaAs 004

GaAs 444

substrate1 μm film

● Compressive strained (Ga,Mn)As on a GaAs(001) substrate● Strain increases with increasing %Mn

Ga1-xMnxAs – magnetism and transport

0 50 100 150 200

0.01

0.1

0.02

0.05

0.015

0.01

T (K)

(c

m)

0.08

0 20 40 60 800

10

20

30

40

x=0.01

x=0.015

x=0.02

x=0.08

M (

emu/c

m3 )

T (K)

x=0.05

for varying nominal Mn concentration x

Effect of annealing in air at 180oC

0 50 100 150 200 250 3001E-3

0.01

48h

13h

7h

3h

1h

Temperature (K)

(

cm)

0h

0 50 100 150 200 2500

10

20

30

40

50

60

70

Temperature (K)

Mag

netiz

atio

n (e

mu/

cm3)

(lower than the growth temperature!)

Interstitial Mn

● Compensating donor defect

● Antiferromagnetic coupling to substitutional neighbours

● Weakly bound → diffuses readily

Kinetic energy (eV)

N

O

Mn GaAs

Auger spectra:Enhanced surface Mn after annealing

as-grown

annealed 24h in air

See talk by T. Lima

K.M. Yu et al., PRB (2002); Edmonds et al. PRL (2004)

Post-annealed samples

0 2 4 6 80

50

100

150

200

TC (

K)

xeff

(%)

178 180 182 184 186 188 1900

5

10

15

20

0

2

4

6

Temperature (K)

Mag

neti

zati

on (

emu/

cm3 )

χ-1

TC

xeff = effective concentration of uncompensated substitutional Mn

Hole-mediated ferromagnetism

-0.02 -0.01 0.00 0.01 0.02

0.00

0.01

0.02

0.03

15 Oe 12 Oe 10 Oe 8 Oe 6 Oe 5 Oe 4 Oe 3 Oe 2 Oe 1 Oe 0.5 Oe 0.25 Oe

|MR

| (%

)

(T-TC)/T

C

0 50 100 150 200 2501.5

2.0

2.5

d x

x/d

T

.cm

/K)

T (K)

0

4

8

12

0

10

20

30

40

50

60

70

M (em

u/c

m3)

xx(m

.cm

)

-15 -10 -5 0 5 10 15-30

-20

-10

0

10

20

30

Rxy(

B (T)M. Wang et al., Appl. Phys. Lett. 104, 132406 (2013)

Hole-mediated ferromagnetismEF

Dilute limit:Mn impurity level

Valence band

itinerant states

localized states

Hole-mediated ferromagnetismEF

EF

Dilute limit:

Increasing %Mn

Mn impurity level

Valence band

?

itinerant states

localized states

Hole-mediated ferromagnetismEF

EF

EFEF

Dilute limit:

Increasing %Mn

Mn impurity level

Valence band

?

“Valence band model”

“Impurity band model”

Predicted dependence of TC on carrier density p:

T C per

Mn

Carriers per Mn0 1

itinerant states

localized states

Magnetic transition temperature TC versus carrier density p

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

10-3

TC / x eff (

K)

p/Neff

p measured using Hall effectJungwirth et al. PRB 72, 165204 (2005)

Wang et al. PRB 87, 121301 (2013)

p estimated from ion channeling measurements of defect concentrationsDobrowolska et al. Nature Materials 11, 444

(2012)

0.0 0.2 0.4 0.6 0.8 1.00

1

2

3

4

10-3

TC / x eff (

K)

p/Neff

Magnetic transition temperature TC versus carrier density p

Wang et al. Phys. Rev. B 87, 121301 (2013)

Ion channeling measurements

Hall effect measurements

Hole-mediated ferromagnetism

● “p” in the above is estimated from measurements of interstitial and substitutional Mn concentrations

● TC always increases when carrier concentration is increased by annealing

● Other defects are probably responsible for the reduced TC

Dobrowolska et al. Nature Materials 11, 444 (2012)

Insulator-metal transition in (Ga,Mn)As: K-edge polarized x-ray spectroscopy

Transition from localized Mn to delocalized As electronic states

6530 6540 6550 6560 65700

1

2

XM

CD

(%

)

X-ray energy (eV)

Increasing %Mn

0.3%

0.7%

1%

2%5%

Mn K edge

Manganese

11870 11880 11890

0.0

0.2

0.4

0.6

0.8

1.0

X-ray energy (eV)X

MC

D (

%)

As K edge

Increasing%Mn

2%

4%

5%

Arsenic

Wadley et al. Phys. Rev. B 81, 235208 (2010)

B = 6T

Future directionsTowards room temperature diluted magnetic semiconductors?

0 2 4 6 80

50

100

150

200

TC (

K)

xeff

(%)

(Ga,Mn)As TC increases with %Mn but currently limited to 190K

Room temperature ferromagnetism observed at the (Ga,Mn)As interface in bilayer structures

Fe(Ga,Mn)As

Maccherozzi et al. Phys. Rev. Lett. 101, 267201 (2008)

Olejnik et al.Phys. Rev. B 81, 104402 (2010)

Future directionsTowards room temperature diluted magnetic semiconductors?

Other material systems

Widely cited article: Dietl et al. Science 287, 1019 (2000)

● Predicts TC > 300K assuming delocalized hole-mediated ferromagnetism

● Some promising results for oxides (e.g., TiO2), but not well-understood

● Wide material space still to explore, e.g. “122” compoundsZhao et al. Nature Comms. 4, 1442 (2013)

Future directionsExploration of spintronic phenomena

e.g. electrical control of magnetism

Sawicki et al. Nature Phys. 6 22 (2010)Stolichnov et al. Nature Mater. 7 464 (2008)

Summary

● Mn-doped GaAs is a ferromagnetic semiconductor, with magnetic properties that are closely tied to the nature and concentration of charge carriers and magnetic ions

● Understanding the nature of defects in this material is essential for understanding, utilizing and optimizing its properties

AcknowledgementsRichard Campion, Mu Wang, Pete Wadley, Bryn Howells, Bryan Gallagher, Andy RushforthSchool of Physics and Astronomy, University of Nottingham

Tomas Jungwirth, Jan MasekInstitute of Physics ASCR, Prague

Joerg Wunderlich, Andrew FergusonHitachi Cambridge Laboratory