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Nuclear Magnetic Resonance -application to spin polarized Heusler compounds

Sabine Wurmehl,J. T. Kohlhepp, H. J. M. Swagten, B. Koopmans,M. Wòjcik, B. Balke, C. G. F. Blum, G. H. Fecher,

C. Felser, G. Jakob, H. Schneider,D. Ebke, G. Reiss

JST - DFG workshop, Kyoto, January 21st-23rd 2009

Contents

• Introduction• Materials• Method

• Results of NMR analysis• Bulk samples of Co2Mn1-xFexSi• Thin films of Co2FeSi

Materials:Heusler compounds

Ternary intermetallic compounds XX22YYZZ

What is a Heusler compound?

Z (4a) Y(4b)

X (8c)

Order: Atoms on proper positionproper position

X2YZ: X (8c) Y (4b) Z (4a) L21 ( )mFm3

X: Most electronegativetransition metal

Y: Transition metal

Z: Main group element

L21 Structure X2YZ (Prototype: Cu2MnAl)Spacegroup mFm3

Z (4a)Y(4b)

X (8c)

Why are Heusler compounds attractive?

High spin polarization

Half-metallic ferromagnetism

Spintronic Spintronic applicationsapplications

Different structure types

Affects spin polarization

Easy to tune properties

The tailoring principle

Z (4a)Y(4b)

X (8c)

Nobel price in physics 2007

„„…for the discovery of Giant Magnetoresistance"…for the discovery of Giant Magnetoresistance"

P. Grünberg und A. Fert

Nobel price in physics 2007

„„…for the discovery of Giant Magnetoresistance"…for the discovery of Giant Magnetoresistance"

Transport phTransport phenomenaenomenausing using

charge and spin of electronscharge and spin of electrons

100% spin polarization at the Fermi-edge

Minority ↓

Majority↑

Example:

↓ Bandgap at Fermi-edge

↑ DOS>0 at Fermi-edge

Concept: Rob de GrootMaterial: NiMnSb

HalfHalf--metallic ferromagnetismmetallic ferromagnetism

de Groot et al. Phys. Rev. Lett. 50 (1983) 2024

Z (4a)Y(4b)

X (8c)

Substitutional series:

Tuning the properties of Heusler compounds bypartial substitution of one constituent by another partial substitution of one constituent by another at one crystallographic positionat one crystallographic position

e.g. tuning the Fermi-edge in the middle of the gap

Tailoring of properties

Example: Co2Mn(1-x)FexSi

Balke et al. Phys. Rev. B 74 (2006) 104405

with high thermal stabilitywith high thermal stability

Example: Co2Mn(1-x)FexSi

Robust halfRobust half-- metallic ferromagnetsmetallic ferromagnets

Z (4a)Y(4b)

X (8c)

(c)DO3 (Fe3Si)

(d)L21 (Cu2MnAl)

Z (4a)Y(4b)

Y or Z (1a)

(a)A2 (Tungsten)

(b)B2 (CsCl)

X or Y (8c + 4b)Z (4a)

X, Y or Z (2a)

Various structure types were observedVarious structure types were observed

Disadvantage:Different structure types (and their mixtures)

Spin polarization depends on structure

Spin polarization Spin polarization ↔↔ structure!structure!Gercsi et al. J. Phys. Condens. Matter 19 (2007) 326216Miura et al. Phys. Rev. B 69 (2004) 144413

Structural Characterization

Structural characterization by conventional methods (e.g. XRD) not sufficient.

B. Balke, S. Wurmehl, et al.Appl. Phys. Lett. 90 (2007) 172501

Exchange of atoms

Two types:

• Partial substitution (Intentional exchange of atoms

tuning of properties)

• Structure type(Unintentional exchange of atoms)

How to distinguish between types?How to distinguish between types?

Exchange of atoms

Two types:

• Substitution (Intentional exchange of atomstuning of properties)

• Structure type (Unintentional exchange of atoms)

How to distinguish between types?How to distinguish between types?

Requirement: Requirement: (Local) Method!(Local) Method!

Method:Nuclear magnetic resonance (NMR)

Resonance frequency depends on Resonance frequency depends on local (magnetic and electronic) local (magnetic and electronic) environment of nucleusenvironment of nucleus

Nuclear Magnetic Resonance (NMR)

ωωLL= = γ γ BB00

Topical reviewWurmehl, KohlheppJ. Phys. D.: Appl. Phys. 41 (2008) 173002

Nuclear Zeeman splitting

A typical 59Co NMR spectrum

Thanks to H. Wieldraaijer

Different local environments Different local environments have different hyperfine fieldshave different hyperfine fields

Results:Co2Mn1-xFexAl

Problem:Problem:Intentional exchange of atoms

Distribution of atoms inDistribution of atoms insubstitutional series?substitutional series?

Introduction

CoCo22MnMn(1(1--x)x)FeFexxSiSi:

•• Robust halfRobust half--metallic ferromagnets metallic ferromagnets with high thermal stability with high thermal stability

• L2L211 ordered (XRD, EXAFS, Mößbauer-spectroscopy)

Wurmehl et al. Appl. Phys. Lett. 88 (2006) 032503Balke et al. Phys. Rev. B 74 (2006) 104405

Si (4a)

Mn/Fe(4b)

L21 (Cu2MnAl)Fm3m

Motivation

Crystallography:L2L211 structure requires random distributionrandom distribution ofMnMn andand FeFe on the 4b Wyckoff position!

Question:Distribution ofDistribution of MnMn andand FeFe on 4b position?on 4b position?

Synthesis of bulk materials

• Arcmelting in Argon atmosphere

• Temperature treatment in evacuated quartz tubes

Polycrystalline bulk samplesPolycrystalline bulk samples

Fit of NMR spectrum

360 365 370 375 380 385 390

18%22%

Frequency (MHz)

Co2Mn0.5Fe0.5Si

Local environments of Mn

First coordination shell

Second coordination shell

Third coordination shell:Third coordination shell:

Random atom model

Example: Co2Mn0.5Fe0.5SiN: Number of nearest neighbour sites in third shell of 55Mn: 12n: Number of Fe atoms in third shell of 55Mn (varied)x: Concentration of Fe (nominal: 0.5)

nnN xxnnN

NxnP )1(!)!(

!),( −−

Wurmehl et al.Appl. Phys. Lett. 91 (2007) 052506J. Appl. Phys. 103 (2008) 07D706

Number of Fe next neighbours0 2 4 6 8 10 12

55Mn NMR of Co2Mn0.5Fe0.5Si

Each resonance line Each resonance line is attributed to certain numbers of Fe atomsis attributed to certain numbers of Fe atoms

ity (a

Frequency (MHz)

Co2Mn1-xFexSi (0.1 ≤ x ≤ 0.9)

Open symbols: random atom modelFilled symbols: experimental data

Measured Fe concentration

0.0 0.2 0.4 0.6 0.8 1.00.0

Nominal Fe concentration x

Measured Fe concentration Measured Fe concentration xxfollows nominal valuesfollows nominal values

Summary Co2Mn1-xFexSi

Structure of Co2Mn1-xFexSiconfirmed by 55Mn NMR:

L2L211 type with random distribution ofrandom distribution of MnMn andand FeFeon 4b Wyckhoff position

Only intentionalOnly intentional exchange of atoms

Wurmehl et al.Appl. Phys. Lett. 91 (2007) 052506J. Appl. Phys. 103 (2008) 07D706

Conclusion Co2Mn1-xFexSi

High crystallographic order proved by 55Mn NMRhigh impact on

1) half-metallic character 2) high degree of spin polarization

CoCo22MnMn11--xxFeFexxSi Heusler compounds with xSi Heusler compounds with x≈≈ 0.50.5areare ideal candidates for spintronicsideal candidates for spintronics

Results:CoCo22FeSiFeSi

thin film samplesProblem:Problem:

Unintentional exchange of atomsOffOff--stoichiometrystoichiometry

Motivation

Bulk:• L21 structure and 6 μB

Wurmehl et al. Appl. Phys. Lett. 88 (2006) 032503 Balke et al. Phys. Rev. B 74 (2006) 104405

Films:• Magnetic moments low (4.5 – 5.0 μB)

Inomata et al. J. Appl. Phys. 99 (2006) 08T314Schneider et al. Phys. Rev. B 74 (2006) 174426

• Tunneling Magnetoresistance is low (44% at RT, 68 % at 5K) Gercsi et al. Appl. Phys. Lett. 89 (2006) 082512

•• Spin polarizationSpin polarization only 49%only 49% (PCAR, Jullière) Gercsi et al. Appl. Phys. Lett. 89 (2006) 082512

Al (4a)Mn(4b)

Cu (8c)

(d)L21 (Cu2MnAl)

CoCo22FeSi is predicted to be a halfFeSi is predicted to be a half--metallic ferromagnetmetallic ferromagnet

Expected NMR spectrum

4 Fe + 4 Si

L21 structure: One first shellenvironment

for the 59Co nuclei

Co2FeSi

One hyperfine field One singlesharp resonance line

50 100 150 200 250 300

6 8 10 12 14 16 18 20 22 24 26 28Hyperfine field (T)

Frequency (MHz)

Co2FeSi bulk sample

Synthesis of films

Load lock

Electronics

“Carouso”

Computer controlSputter guns with targets

Turbo pump (UHV)

rf/dc sputtering

Example: TU/e

NMR of Co2FeSi films (Exemplarily: Mainz)

100 120 140 160 180 200 220 240

10 12 14 16 18 20 22 2459

ity (a

Frequency (MHz)

Hyperfine field (T)

• RF magnetron sputtering • Deposited at 600°C• No capping and seed layer• Grown on MgO

Wurmehl et al. Submitted to J. Phys. D: Appl. Phys. (2009)

Films prepared by H. Schneider and G. Jakob, Johannes Gutenberg Universität, Mainz

Thin films in literature

Inomata et al. Phys. Rev. B 77 (2008) 214425

• Only high frequency linesOnly high frequency lines in Co2FeSi

• Previous NMR results:Fe-rich environments lead to high frequency satellites

Fe-rich environments in Co2FeSi

“Wrong” stoichiometry “Wrong” stoichiometry with Fe excess atoms??? with Fe excess atoms???

100 120 140 160 180 200 220 240

10 12 14 16 18 20 22 24

ity (a

Frequency (MHz)

Hyperfine field (T)

Fe - excess atoms

Two different types are possible:

• Fe rich at the cost of Co

• Fe rich at the cost of Si Co2Fe(Si1-xFex)

(Co2-xFex)FeSi

Check local structure ofCheck local structure ofcorresponding bulk “model” samplescorresponding bulk “model” samples

4 Co + 2 Fe6 Co + 0 Fe

6 Fe + 2 Si5 Fe + 3 Si4 Fe + 4 Si

5 Co + 1 Fe

Comparison with bulk spectra

Co2Fe(Si0.93Fe0.07)

(Co1.88Fe0.12)FeSi

6 Fe + 2 Si5 Fe + 3 Si

4 Fe + 4 Si

4 Co + 2 Fe

6 Co +0 Fe

5 Co + 1 Fe

Bulk samples prepared by C.G.F. Blum Group of Prof. C. Felser

•• OffOff--stoichiometricstoichiometric films of Co2FeSi out of stoichiometric targetsstoichiometric targets

• Similar results in films prepared by films prepared by different groupsdifferent groups

•• OffOff--stoichiometry only apparentstoichiometry only apparentusingusing local methods as NMRNMR

Principal problem while sputtering CoPrincipal problem while sputtering Co22FeSiFeSi

Summary Co2FeSi

Conclusion

Off-stoichiometry might explain

• too low spin polarization• too low TMR ratio • too low element specific magnetic moments

BUT: Results associated with“good” x-ray diffraction data

Troubleshooting

• Find optimized target to get stoichiometric films

• Optimize sputter conditions

• Prepare films by• Molecular beam epitaxy• Pulsed laser deposition

BUT: Sputtering is more common in industry

Film preparation monitored Film preparation monitored and optimized by NMRand optimized by NMR

Take home message

Towards Towards technical technical

application in application in spintronic spintronic

devicesdevices

you, you, for your attention!!!for your attention!!!

Benjamin BalkeChristian G. F. BlumGerhard H. Fecher

Claudia FelserGerhard Jakob

Horst SchneiderHajo Elmers

Jürgen T. KohlheppHenk J. M. Swagten Bert KoopmansGregory MalinowskiTim EllisPatrick JacobsFNA

Money:Money:DFG Forschungsstipendium WU 595/1DFG Forschungsstipendium WU 595/1--11

Marek Wòjcik

Thanks toThanks to

Daniel EbkeGünter Reiss

Conventional NMR vs NMR on ferromagnets

Methyl-Multiplett

• High magnetic fields

• Small frequency range with small lines

• High internal magnetic fieldsHigh internal magnetic fieldsNO external field required

• Large variations in field/frequency with broad lines

• Enhancement-effect

Methylene

Methylen-Multiplett

Chemical shift δ (parts per million [ppm])

Methyl

160 180 200 220 240

Frequency (MHz)

Random atom model Fe excess atoms

01020304050607080

0 1 2 3 4 5 6

01020304050607080

0 1 2 3 4 5 6

-505 (c)

Number of Fe excess atoms

Co2Fe(Si

x) (Co

x)FeSi

nnN xxnnN

NxnP )1(!)!(

!),( −−

x = 0.08 x = 0.06

Results:CoCo22FeAlFeAl

bulk samplesProblem:Problem:

Unintentional exchange of atomsStructure typesStructure types

Mixing of

Fe and Al atoms

Co2FeAl:• Half-metallic ferromagnetic Heusler compound

• Tunneling Magnetoresistance about 50% at room temperature

• B2B2 type structure (XRD, EXAFS, Mößbauer-spectroscopy)

Al (4a)Fe(4b)

Fe or Al (1a)

Introduction

L21 Structure X2YZ (Protoyp: Cu2MnAl)

B2 structure Co2(FeAl)(Protoyp: CsCl)

Tezuka et al. J. Appl. Phys. 99 (2006) 08T314Wurmehl et al. J. Phys. D: Appl. Phys. 39 (2006) 803

Motivation

Crystallography:B2 structure requires random distributionrandom distribution ofFeFe and and AlAl on the 1a Wyckoff position!

Questions:(A) (A) Distribution of Distribution of FeFe and and AlAl on the 1on the 1aa positionposition(B) (B) Structural contributionsStructural contributions(C) Effect of annealing on local structure(C) Effect of annealing on local structure

Fe or Al (1a)

B2 structure Co2(FeAl)(Protoyp: CsCl)

• Arcmelting

• Optional:Temperature treatment in evacuated quartz tubes.

Polycrystalline bulk samples

Synthesis

(A) Distribution of atoms

Co2FeAl in literature:

B2 type structure

Requires random distributionrandom distribution of Fe and Al

Alters first shell environment of Co!Alters first shell environment of Co!B2 structure of Co2(FeAl)

Fe or Al (1a)

Al (4a)Fe(4b)

(A) First shell environment in the B2 structure

… ……

B2 structure: 9 different first shell environmentsB2 structure: 9 different first shell environments

4 Al + 4 Fe

1 Al + 7 Fe

6 Al + 2 Fe

8 Fe2 Al + 6 Fe

m(Al))-(m(Fe)-h=m(Al) h +m(Fe) h -)H(Co 1110 ≈Δ

B2 structure: mixing of FeFe and AlAl

• m (Fe)= 2.91 μB• m (Al) ≈ 0μB

Mixing of magnetically extremely different elements

Hyperfine field (transferred contributions):

(A) Spacing

Large spacing on the order of 20Large spacing on the order of 20--60 MHz60 MHz

(A) Expected spectrum

Nine resonance linesNine resonance linesSpacing roughly 20Spacing roughly 20--60 MHz60 MHz

Literature

Inomata et al. J. Phys. D 39 (2006) 816

125 150 175 200 225 250 275 300

Frequency (MHz)

4Fe+4Al

3Fe+5Al

5Fe+3Al

6Fe+2Al

7Fe+1Al

2Fe+6Al

59Co NMR spectrum Co2FeAl

As cast sample

59Co NMR spectrum Co2FeAl

125 150 175 200 225 250 275 300

Frequency (MHz)

4Fe+4Al

3Fe+5Al

5Fe+3Al

6Fe+2Al

7Fe+1Al

2Fe+6Al

• 7 main lines with mean spacing of 27 MHz• Sub-lines with spacing of 5 MHz

(A) Origin of main lines

Main lines:

Distribution of Distribution of FeFe and and AlAl in first shell of in first shell of CoCo

Spacing Spacing ≈≈ 30 MHz30 MHz

125 150 175 200 225 250 275 300

Frequency (MHz)

4Fe+4Al

3Fe+5Al

5Fe+3Al

6Fe+2Al

7Fe+1Al

2Fe+6Al

(A) Random atom model

0 1 2 3 4 5 6 7 80

0.4%0.4%

11% 11%

22% 22%

Number Fe atoms

nnN xxnnN

NxnP )1(!)!(

!),( −−

N=8 n=0-8 x≈0.5Binomial distribution

B2 structure of Co2(FeAl)Fe or Al (1a)

nnN xxnnN

NxnP )1(!)!(

!),( −−

N=8 n=0-8 x≈0.5

(A) Model vs. experiment

2 3 4 5 6 7 8-10-505

abilit

Number of Fe atoms in the first shell of 59Co

(A) Model vs. experiment

N=8 n=0-8 x≈0.5Conflict between Conflict between model and experimentmodel and experiment

X (8c)

Fe(4b)

Z(4a))

First shell in the L21 structure

nnN xxnnN

NxnP )1(!)!(

!),( −−

N=8 n=0-8 x≈0.5

(B) Model vs. experiment

2 3 4 5 6 7 8-10-505

nnN xxnnN

NxnP )1(!)!(

!),( −−

2 3 4 5 6 7 8-10-50510

2 3 4 5 6 7 8-10-505

only B2 contributions B2 and L21 contributions

N=8 n=0-8 x≈0.5

{ } KKK =⋅+−−

⋅= −4,4,22 1

)1(!)!(

!),( nnLnnN

B withlxxnnN

NbxnP δδ 1, n=40, n≠4

(B) Structural contributions

Distribution of Fe and Al not entirely random

90% 90% B2B2 + + ≈≈ 10% 10% L2L211 contributionscontributions

Wurmehl et al. J. Phys. D: Appl. Phys. 41 (2008) 115007

Broad resonance line Broad resonance line corresponds to corresponds to A2A2 structural contributionsstructural contributions

(C) Annealing

125 150 175 200 225 250 275 300

Frequency (MHz)

A2 contributions

Wurmehl et al. J. Phys. D: Appl. Phys. 41 (2008) 115007

125 150 175 200 225 250 275 300

Frequency (MHz)

(a) as cast

annealed

Co (8c)

Fe(4b)

Al(4a))Mixing of Y

and Z atomsCo (1b)

Fe or Al (1a)

Example: fourth shell in the L21 structure in the B2 structure

B2 structure: 25 different

fourth shell environments

L21 structure: One environmentfor the 59Co nuclei

Between 200-250 MHz:Separation of CoAl

after annealing process

(A) Sub-lines

• Distribution of Fe and Al not entirely random90% B2 + 90% B2 + ≈≈ 10% L210% L21 1 contributionscontributions

• Sub-lines related to distributiondistributionof Fe and Al in higher shellsin higher shells(cumulative higher shell effects)

SubSub--lines not apparent in thin filmslines not apparent in thin films

Only unintentional exchange of atomsOnly unintentional exchange of atoms

Summary

Conclusions

High degree of order:Conservation of Conservation of halfhalf--metallic ferromagnetic properties (bulk)metallic ferromagnetic properties (bulk)

Higher long range order Higher long range order in bulk than in thin film samplesin bulk than in thin film samples

Improvement of structure:Improvement of structure:Better performance in spintronic devices….Better performance in spintronic devices….

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