Post on 25-Jun-2020
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
Why are Heusler compounds attractive?
Z (4a)Y(4b)
X (8c)
High spin polarization
Half-metallic ferromagnetism
Spintronic Spintronic applicationsapplications
Different structure types
Affects spin polarization
Easy to tune properties
The tailoring principle
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"
P. Grünberg und A. Fert
Transport phTransport phenomenaenomenausing using
charge and spin of electronscharge and spin of electrons
P. Grünberg und A. Fert
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
Why are Heusler compounds attractive?
Z (4a)Y(4b)
X (8c)
High spin polarization
Half-metallic ferromagnetism
Spintronic Spintronic applicationsapplications
Different structure types
Affects spin polarization
Easy to tune properties
The tailoring principle
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
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
Why are Heusler compounds attractive?
Z (4a)Y(4b)
X (8c)
High spin polarization
Half-metallic ferromagnetism
Spintronic Spintronic applicationsapplications
Different structure types
Affects spin polarization
Easy to tune properties
The tailoring principle
(c)DO3 (Fe3Si)
Fm3m
(d)L21 (Cu2MnAl)
Fm3m
Z (4a)Y(4b)
X
(8c)
Y or Z (1a)
(a)A2 (Tungsten)
Im3m
(b)B2 (CsCl)
Pm3m
X
(1b)
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)
Co
(8c)
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
6%
14%
4%
10%
18%22%
55M
n S
pin-
Echo
Inte
nsity
(arb
. uni
ts)
Frequency (MHz)
23%
Co2Mn0.5Fe0.5Si
Local environments of Mn
Co
Mn
Si
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
0
5
10
15
20
25
0
5
10
15
20
25
Rel
. are
a of
reso
nanc
e lin
e (%
)
Ran
dom
ato
m m
odel
(%)
Number of Fe next neighbours0 2 4 6 8 10 12
-202
-202
Diff
eren
ce (%
)
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
55M
n S
pin-
Ech
oIn
tens
ity (a
rb. u
nits
)
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
0.2
0.4
0.6
0.8
1.0
Fe c
once
ntra
tion
xm
easu
red
by 55
Mn
NM
R
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)
Fm3m
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)
59C
o Sp
in-E
cho
Inte
nsity
(arb
. uni
ts)
Frequency (MHz)
Co2FeSi bulk sample
Synthesis of films
Load lock
Electronics
“Carouso”
Computer controlSputter guns with targets
Turbo pump (UHV)
User
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
Co
Spin
-Ech
o In
tens
ity (a
rb.u
nits
)
(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
Idea
• 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
59C
o S
pin-
Ech
o In
tens
ity (a
rb.u
nits
)
(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
-505
01020304050607080
(a)
0 1 2 3 4 5 6
-505 (c)
Diff
eren
ce (%
)
(d)
Rel
. are
a of
re
sona
nce
line
(%)
Pro
babi
lity
acco
rdin
g to
bin
omia
l dis
tribu
tion
(%)
Number of Fe excess atoms
Diff
eren
ce (%
)
(b)
Co2Fe(Si
1-xFe
x) (Co
2-xFe
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)
Co
(8c)
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)
Co
(8c)
(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
8 Al
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
59C
o sp
in e
cho
inte
nsity
(a. u
.)
Frequency (MHz)
4Fe+4Al
3Fe+5Al
5Fe+3Al
6Fe+2Al
7Fe+1Al
8Fe
2Fe+6Al
59Co NMR spectrum Co2FeAl
As cast sample
59Co NMR spectrum Co2FeAl
125 150 175 200 225 250 275 300
23%
59C
o sp
in e
cho
inte
nsity
(a. u
.)
Frequency (MHz)
4Fe+4Al
3Fe+5Al
5Fe+3Al
6Fe+2Al
7Fe+1Al
8Fe
2Fe+6Al
6%
20%
34%
11%
4% 2%
• 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
23%
Frequency (MHz)
4Fe+4Al
3Fe+5Al
5Fe+3Al
6Fe+2Al
7Fe+1Al
8Fe
2Fe+6Al
6%
20%
34%
11%
4% 2%
(A) Random atom model
59C
o sp
in e
cho
inte
nsity
(a. u
.)
0 1 2 3 4 5 6 7 80
5
10
15
20
25
0.4%0.4%
3% 3%
11% 11%
22% 22%
Pro
babi
lity
acco
rdin
g to
bin
omia
l dis
tribu
tion
(%)
Number Fe atoms
27%
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
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
2 3 4 5 6 7 8-10-505
10
(b)
Diff
eren
ce
Rel
. are
a of
re
sona
nce
line
(%)
Prob
abilit
y ac
cord
ing
to b
inom
ial d
istri
butio
n (%
)
Number of Fe atoms in the first shell of 59Co
Diff
eren
ce
(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
0
5
10
15
20
25
30
35
0
5
10
15
20
25
30
35
2 3 4 5 6 7 8-10-505
10
(b)
Diff
eren
ce
Rel
. are
a of
re
sona
nce
line
(%)
Pro
babi
lity
acco
rdin
g to
bin
omia
l dis
tribu
tion
(%)
Number of Fe atoms in the first shell of 59Co
Diff
eren
ce
nnN xxnnN
NxnP )1(!)!(
!),( −−
= −
0
5
10
15
20
25
30
35
2 3 4 5 6 7 8-10-50510
0
5
10
15
20
25
30
35
(a)
2 3 4 5 6 7 8-10-505
10
(b)
Diff
eren
ce
(d)
Rel
. are
a of
re
sona
nce
line
(%)
Pro
babi
lity
acco
rdin
g to
bin
omia
l dis
tribu
tion
(%)
Number of Fe atoms in the first shell of 59Co
Diff
eren
ce
(c)
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
(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
7% 7%
59C
o sp
in-e
cho
inte
nsity
(arb
. uni
ts)
Frequency (MHz)
3%
7%
11%
9%
4%
A2 contributions
Wurmehl et al. J. Phys. D: Appl. Phys. 41 (2008) 115007
125 150 175 200 225 250 275 300
(b)
59C
o sp
in e
cho
inte
nsity
(a. u
.)
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….