Semiconducting and half metallic Heusler compounds for multifunctional

applications

Claudia Felser

Materials for

Optical, Magnetic, and Energy

TechnologiesMOMENT

JST-DFG 2009

Heusler Compounds as Multifunctional Materials

•

Magnetic

material: Cu2

MnAl•

Halfmetallic

ferromagnet:

NiMnSb

•

Magneto-optical: PtMnSb•

Magneto-mechanic: Ni2

MnGa•

Superconductor: Pd2

YSn•

Semiconductors: CoTiSb

•

Heavy fermion: Fe2

VAl •

Li-conductor: LiMnSb

•

Magneto-electronic:

Co2

FeSi•

Thermo-electric: TiNiSn

•

Magneto-caloric: CoMnSb:Nb

19051983

2001

JST-DFG 2009

•

Concept

•

Semiconducting Half Heusler•Thermoelectric materials •Diluted Semiconductors

•

Half metallic Heusler compounds •High Curie temperatures•Ferrimagnets

•

High energy photoemission for Devices

•

Summary

JST-DFG 2009

•

Concept

•

Semiconducting Half Heusler•Diluted Semiconductors•Thermoelectric materials

•

Half metallic Heusler compounds •High Curie temperatures•Ferrimagnets

•

High energy photoemission for Devices

•

Summary

JST-DFG 2009

Rational Design

•

First TMR device (Inomata

et al.) 19% at RT•

TMR-device

with

MgO

(Marukame

et al. APL 90

(2007)

012508)

109% TMR at RT

⇒

88 % spin

polarisation

at 4K•

Point contact

80% MR (Coey

et al.)

Patent (Felser, Block, DE 101 08 760, H01 L43/08 )

Block, Felser, et al. J. Solid State Chem. 176, 646 (2003)

Half metallicityHigh density

of states

at EF

Large MR at room

temperatureIntermag

2002: Co2

Cr0.4

Fe0.6

Al CCFA

JST-DFG 2009

Synthesis

Semiconducting Heuslers – Stuffed ZnS

XYZ X2 YZ

Ti

Si2

AsGa

Co Sb

9 + 4 + 5 = 18

3 + 5 = 8

CuLi2 Sb

VFe2 Al

2*8 + 5 + 3 = 24

2*1 + 11 + 5 = 18

additional t2

-levels

JST-DFG 2009

Slater-Pauling Rule

Kübler 1983Galanakis

et al., PRB 66, 012406 (2002)

Magic valence electron number X2

YZ 24Valence electrons =^24 + sat. magnetization

Co2

FeAl2*9 + 8 + 3 = 29 Ms = 5μB

B. Balke

et al., Sci. Technol. Adv. Mater. 9 (2008) 014102.

EF

JST-DFG 2009

•

Concept

•

Semiconducting Half Heusler•Thermoelectric materials •Diluted Semiconductors

•

Halfmetallic

Heusler compounds •High Curie temperatures•Ferrimagnets

•

High energy photoemission for Devices

•

Summary

JST-DFG 2009

Half Heusler: 18 valence-electrons

Thermoelectrica

R. Asahi et al. J. Phys.: Cond. Mat. 20 (2008) 64227K. Miyamoto et al. Appl. Phys. Express 1 (2008) 081901 E. Toberer, Nature Mat. 7 (2008) 105 VK Zaitsev

et al. PRB 74 (2006) 045207

Typ M aterial Price in $/kg (metals)

V-VI Bi2Te3 140 IV-VI PbTe 99 Zn4Sb3 Zn4Sb3 4

p-M nSi1.73 24 n-M g2Si0.4Sn0.6 18 Si0.80Ge0.20 660

Silicides

Si0.94Ge0.06 270 Skutterutides CoSb3 11 Half-Heusler TiNiSn 55 n/p-Clathrate Ba8Ga16Ge30 1000

w ithout Ba Oxides p-NaCo2O4, 17

w ithout Na, OZintl Phasen p-Yb14M nSb11 92 Th3P4 La3-XTe4 160

Information H. Böttcher

0 200 400 600 800 10000,00,20,40,60,81,01,21,41,61,8 Bi

2(Te

0.8Se

0.2)3

CoSb3

(Zr0.5Hf0.5)0.5Ti0.5NiSn0.998Sb0.002

Si0.8Ge0.2

(Hf0.5Zr0.5)NiSn (OFZ) Mg2Si0.8Sn0.2

Figu

re o

f mer

it ZT

Temperature T K

JST-DFG 2009

Sur

vey

on H

iTT

–Mat

eria

ls

Half Heusler for Thermoelectrics

Balke

et al. PRB

77, 045209 (2008)Kandpal et al. J. Phys. D 39 (2006) 776

−6 −4 −2 0 2 4 6energy (eV)

0

10

20

30

40 TiCoSbVCoSnNbCoSn

0

10

20

30

40

DO

S (

stat

es e

V−

1 cel

l−1 )

VFeSbTiCoSbYNiSb

(a)

(b)

0 100 200 300 400 500 600 700 800

-600

-500

-400

-300

-200

-100

0 (Zr0.5Hf0.5)Ti0.5NiSn0.998Sb0.002

TiCo0.93+xSb TiCoSb0.95Bi0.05

See

beck

coe

ffici

ent S

(T) [

μVK

-1]

Temperature T [K]

TZTλσα 2

=α: Seebeck

coeffizient

σ: Electrical

conductivityλ: Thermo

conductivity

T: Temperatur (K)

JST-DFG 2009Thermoelectrica

Barth et al., in preparation (2009)

Improvement of the thermal conductivity Melt Spinning Ball milling –

Spark Plasma

MultilayerNanoparticlesRattlers such as Lithium-Ions 19 20 21 22 23 24

0

5000

10000

15000

20000

25000

30000

35000

m easured data Lorentz fit

FW H M : ~0.86Int.

(cps

)

ω (°)

30 40 50 60 70 80 900

5000

10000

15000

20000

25000

30000

35000

40000

S ubstra te

Int.

(cps

)

2θ (°)

T iN iS n on A l2O 3

S ubstra te(220)

0 100 200 300 400 500 600 700 800

10

100

1000

(Zr0.5;Hf0.5)Ti0.5NiSn0.998Sb0.002

TiCo0.93+xSb TiCo0.4Ni0.6Sb0.4Sn0.6

Res

istiv

ity R

(T) [

μΩm

]

Temperature T [K]0 100 200 300 400 500 600 700 800

0

2

4

6

8

10

(Zr0.5Hf0.5)0.5TiNiSn0.998Sb0.002

TiCoSb TiNi0.9Co0.1Sn0.9Sb0.1

TiCo0.93+xSb

Ther

mal

con

duct

ivity

κ(T

) [W

m-1K

-1]

Temperature T [K]

JST-DFG 2009

100 nm50 nm

10nm10 nm

First Heusler Nanoparticles: Co2 FeGa

Basnit

et al. J. Phys. D, (2009)

accepted

JST-DFG 2009For Spin Injection

XMCD-Investigation

on FeKroth

et al. APL 89 202509

(2006 )

Balke

et al. PRB

77, 045209 (2008)

JST-DFG 2009Design of Diluted Semiconductors

Ti

Si

AsGa

Co Sb

Mn

Fe/Mn

JST-DFG 2009

•

Concept

•

Semiconducting Half Heusler•Diluted Semiconductors•Thermoelectric materials

•

Half metallic Heusler compounds •High Curie temperatures•Ferrimagnets

•

High energy photoemission for Devices

•

Summary

JST-DFG 2009

Large temperature dependence of TMR ratio should be solved.

Tunneljunction

Sakuraba

et al. APL 89 (2006) 052508

Sakuraba

et al.

APL 88 (2006) 192508

TMR ratio = 67%@RT, 580%@2K Co2 MnSi

Co2 MnSiAl2 O3

0 50 100 150 200 250 3000

100

200

300

400

500

600

TMR

[%]

Temperature [K]

JST-DFG 2009

Half metallic ferromagnets - for Tunnelmagnetoresistance TMR - for CPP GMR

What do we need?

High Curie Temperature Ordered L21 structure Good interfaces Adjusted EF : middle of the gap no magnons

Designed electronic structure

JST-DFG 2009

High Curie Temperatures

Fecher, J. Appl. Phys. 99 (2006) 08J106Kübler

et al., Phys. Rev. B 76 (2007) 024414

Expected Curie temperature for Co2

FeSi

: > 1000K

JST-DFG 2009

Co2 FeSi

Magnetic

moment

in saturation: 5.97μB

±0.1μB at 5K

Extrapolation to 0K :Slater-Pauling rule: 6 μB

Curie Temperature

1120 K

-3 -2 -1 0 1 2 3

-6

-4

-2

0

2

4

6

-2.0k 0.0 2.0k-1.0%

-0.5%

0.0%

0.5%

1.0%

5K 300K 775K

Mag

netic

Mom

ent p

er u

nit c

ell

m [μ

B]

Magnetic Field H [106 A/m]

Wurmehl, et al ., APL 88 (2006) 032502

.Wurmehl, et al ., Phys. Rev. B

72 (2005) 184434

700 800 900 1000 1100 1200 13000

10

20

30

40

50

60

TC

1/χ(T)Θ = 1150 ± 50 K

σ(T)TC = 1100 ± 20 K

μ0H = 0.1Tm = 47 μg

Spec

ific

Mag

netiz

atio

n σ

[Am

2 kg-1]

Temperature T [K]

0

200

400

600

800

Inve

rse

Susc

eptib

ility

1/

χ

120 140 160 180

6 8 10 12 14 16 18 20 22 24 26 28

Hyperfeinfeld (T)

59C

o S

pin-

Ech

o In

tens

ity (a

rb. u

nits

)

Frequency (MHz)

JST-DFG 2009Heusler in Spintronic Devices: TMR

200

150

100

50

0

TMR

(%)

-1000 -500 0 500 1000Field (Oe)

500

400

300

200

Res

ista

nce

(Ω)

TMR: 223%, 300K, A470°C, Rs: 1.74e+02Ω, RA: 1.74e+04 Ω⋅µm2

10 x 10 µm2 MU28225A470L300-5m223

300 K223%

400

300

200

100

0

TMR

(%)

10005000-500-1000Field (Oe)

1000

800

600

400

200

Res

ista

nce

(Ω)

TMR: 423.40%, 7K, A470°C, Rs: 1.91e+02Ω, RA: 1.91e+04 Ω⋅µm2

10 x 10 µm2 MU28-2-25A470L007-2m423

7 K423%

CFAS(30)

IrMn

MgO(2)CFAS (5)CoFe(1)

10505

10

10505

10

10505

10

10505

10

-10 -5 010505

10

(a)

Minority

Majority

(b)

(c)

Spin

reso

lved

den

sity

of st

ates

ρ(

E) [e

V-1]

(d)

(e)

Energy E − εF [eV]

Fecher, Felser J. Phys. D 40 1582 (2007)

Co2

FeSi1-x

Alx

N. Tezuka

et al.,Jpn. J. Appl. Phys. 46, L454 (2007)

JST-DFG 2009Heusler in Spintronic Devices: CPP-GMR

Inomata et al. to be published

CoFeB/MgO‐MTJ

Half‐metal +MgO

MTJ

CPP‐GMR

with 

half‐metal

CoFeB/MgO‐MT

J

Challenge: fitting

spacer

Courtesy

of Koki

Takanashi, Sendai

Interlayer exchange coupling!

JST-DFG 2009

Ferrimagnets

Application: Spintorque

Compensated ferrimagnet?

no net magnetization two magnetic sublattices with compensating moments

Warren E. Pickett Phys. Rev. B 57 (1998) 10613.

JST-DFG 2009

m2

m1

J ≈

1 –

100 MA/cm2

J ≈ ― αMs

HU

dħge

reduction

of α

& MS

Spin transfer switching

Courtesy

after

Shigemi

Mizukami

Sloczewski 1996

JST-DFG 2009

Halfmetallic Ferrimagnet

Kübler’s

RuleSlater Pauling Rule

Mn2

MnGa

Two magnetic sublattice•24 Valence electrons –

0 μB

•Mn3+

at octahedral site – 4

μB

•Mn

compensates

⇒Compensated ferrimagnet

Wurmehl, et al. J. Phys. Cond. Mat. 18 (2006) 6171Balke

et al. APL 90 (2007) 152504

JST-DFG 2009

Compensated Ferrimagnet: HeuslerMn2 MnGaLow Moment –

High Curie Temperature: low current for spinswitch

Tetragonal distorted Heusler: Mn3+

Jahn

Teller Ion

Balke

et al. APL 90 (2007) 152504Winterlik

et al. Phys. Rev. B 77 (2008)

054406

Compensated ferrimagnet: 1μBTheoretical Spinpolarisation: 88%Curie temperature: 730 K

JST-DFG 2009

Heusler and relates Structures

X2

MnZ

XMnMnZXCrCrZ

JST-DFG 2009

•

Concept

•

Semiconducting Half Heusler•Diluted Semiconductors•Thermoelectric materials

•

Halfmetallic

Heusler compounds •High Curie temperatures•Ferrimagnets

•

High energy photoemission for Devices

•

Summary

JST-DFG 2009

High Energy Photoemission

JST-DFG 2009

High Energy Photoemission: buried films

Fecher

et al. APL 92 195313 (2008)

MgOsubstrate

50 nm Co2 MnSi

1nm AlOx

MgO2nm, 20nm

Films Yamamoto SapporoMeasurements

SPring8

hν

= 7.94 keV

JST-DFG 2009

Film quality studied by High Energy PES

1nm AlOx

hν

= 7.94 keV

-14 -12 -10 -8 -6 -4 -2 00.0

0.2

0.4

0.6

0.8

1.0

1.2

as-grown annealed Bulk

Mn t2g ↑

Co t2g ↓Mn eg ↑

Si a1g ↑↓

Rel

ativ

e in

tens

ity

Energy E − εF (eV)

MgOsubstrate

30 nm Co2 MnSi

1nm AlOx

MgO2nm

Annealing and Irradiation improves the film qualityOuardi

et al. J. Phys. D (2009) accepted

JST-DFG 2009

Summary

Half Heusler compounds (Stuffed ZnS) are candidates for thermoelectric applications and for diluted semiconductors

Nanostructured Heuslers are need for low thermoconductivity

Heusler compounds are half metals with high Curie temperatures Co2YZCompensated ferrimagnetic Heuslers Mn2YZ with 24 Valenceelectrons

Halfmetallic ferrimagnets for spintorque application Mn2CoZHigh energy photoemission is an excellent tool to study devices

SPINHAXPES is needed

JST-DFG 2009Co-workers

JST-DFG Project: •

NIMS, Tohoku:

K. Inomata

•

Saporro: M. Yamamoto•

SPring8: K. Kobayashi

Dresden: S. WurmehlAugsburg: A. RellerFG 559: G. Jakob, B. Hillebrands, J. Kübler, Y. Ando

(Tohoku)

DFG-FG559, FE633, SP1166, BMBF: HEUSPIN, MULTIMAG