Nuclear Magnetic Resonance - application to spin polarized Heusler - - PowerPoint PPT Presentation
Nuclear Magnetic Resonance - application to spin polarized Heusler compounds Sabine Wurmehl, J. T. Kohlhepp, H. J. M. Swagten, B. Koopmans, M. Wjcik, B. Balke, C. G. F. Blum, G. H. Fecher, C. Felser, G. Jakob, H. Schneider, D. Ebke, G.
Nuclear Magnetic Resonance - application to spin polarized Heusler compounds
Sabine Wurmehl,
JST - DFG workshop, Kyoto, January 21st-23rd 2009
Materials: Heusler compounds
Ternary intermetallic compounds X2YZ
What is a Heusler compound?
Z (4a) Y(4b) X (8c)
Order: Atoms on proper position proper position X2YZ: X (8c) Y (4b) Z (4a) L21 ( )
m Fm3
X: Most electronegative transition metal Y: Transition metal Z: Main group element L21 Structure X2YZ (Prototype: Cu2MnAl) Spacegroup m Fm3
Z (4a) Y(4b) X (8c)
Why are Heusler compounds attractive?
High spin polarization Half-metallic ferromagnetism
Spintronic Spintronic applications applications
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 applications applications
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"
Nobel price in physics 2007
„…for the discovery of Giant Magnetoresistance" …for the discovery of Giant Magnetoresistance"
100% spin polarization at the Fermi-edge
Minority ↓ Majority ↑
Example: ↓ Bandgap at Fermi-edge ↑ DOS>0 at Fermi-edge Concept: Rob de Groot Material: NiMnSb Half Half-
metallic 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 applications applications
Different structure types Affects spin polarization Easy to tune properties The tailoring principle
Substitutional series: Tuning the properties of Heusler compounds by partial substitution of one constituent by another partial substitution of one constituent by another at one crystallographic position at 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
Example: Co2Mn(1-x)FexSi
Why are Heusler compounds attractive?
Z (4a) Y(4b) X (8c)
High spin polarization Half-metallic ferromagnetism
Spintronic Spintronic applications applications
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 observed Various 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) 326216 Miura et al. Phys. Rev. B 69 (2004) 144413
Structural Characterization
Structural characterization by conventional methods (e.g. XRD) not sufficient.
Exchange of atoms
Two types:
(Intentional exchange of atoms tuning of properties)
(Unintentional exchange of atoms)
How to distinguish between types? How to distinguish between types?
Exchange of atoms
Two types:
tuning of properties)
exchange of atoms)
How to distinguish between types? How to distinguish between types?
Method: Nuclear magnetic resonance (NMR)
Resonance frequency depends on Resonance frequency depends on local (magnetic and electronic) local (magnetic and electronic) environment of nucleus environment of nucleus
Nuclear Magnetic Resonance (NMR)
ωL= = γ γ B0
Topical review Wurmehl, Kohlhepp
Nuclear Zeeman splitting
A typical 59Co NMR spectrum
Thanks to
Different local environments Different local environments have different hyperfine fields have different hyperfine fields
Results: Co2Mn1-xFexAl Problem: Problem: Intentional exchange of atoms Distribution of atoms in Distribution of atoms in substitutional series? substitutional series?
Introduction
Co Co2Mn Mn(1
(1-
x)Fe
FexSi Si:
Robust half-
metallic ferromagnets with high thermal stability with high thermal stability
L21 ordered
(XRD, EXAFS, Mößbauer-spectroscopy) Wurmehl et al. Appl. Phys. Lett. 88 (2006) 032503 Balke et al. Phys. Rev. B 74 (2006) 104405 Si (4a) Mn/Fe(4b) Co (8c) L21 (Cu2MnAl) Fm3m
Motivation
Crystallography: L2 L21 structure requires random distribution random distribution of Mn Mn and and Fe Fe on the 4b Wyckoff position! Question: Distribution of Distribution of Mn Mn and and Fe Fe on 4b position?
Synthesis of bulk materials
Argon atmosphere
treatment in evacuated quartz tubes Polycrystalline bulk samples Polycrystalline bulk samples
Fit of NMR spectrum
360 365 370 375 380 385 390
6% 14% 4% 10% 18% 22%
55Mn Spin-Echo Intensity (arb. units)
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: Co2Mn0.5Fe0.5Si N: Number of nearest neighbour sites in third shell of 55Mn: 12 n: Number of Fe atoms in third shell of 55Mn (varied) x: Concentration of Fe (nominal: 0.5)
n n N
x x n n N N x n P ) 1 ( ! )! ( ! ) , ( − − =
−
Wurmehl et al.
5 10 15 20 25 5 10 15 20 25
Random atom model (%) Number of Fe next neighbours
2 4 6 8 10 12
2
2
Difference (%)
55Mn NMR of Co2Mn0.5Fe0.5Si
Each resonance line Each resonance line is attributed to certain numbers of Fe atoms is attributed to certain numbers of Fe atoms
55Mn Spin-Echo Intensity (arb. units)
Frequency (MHz)
Co2Mn1-xFexSi (0.1 ≤ x ≤ 0.9)
Open symbols: random atom model Filled symbols: experimental data
Measured Fe concentration
0.0 0.2 0.4 0.6 0.8 1.0 0.0 0.2 0.4 0.6 0.8 1.0
Fe concentration x measured by
55Mn NMR
Nominal Fe concentration x
Measured Fe concentration Measured Fe concentration x follows nominal values follows nominal values
Summary Co2Mn1-xFexSi
Structure of Co2Mn1-xFexSi confirmed by 55Mn NMR: L2 L21 type with random distribution of random distribution of Mn Mn and and Fe Fe
Only intentional Only intentional exchange of atoms
Wurmehl et al.
Conclusion Co2Mn1-xFexSi
High crystallographic order proved by 55Mn NMR high impact on 1) half-metallic character 2) high degree of spin polarization Co Co2Mn Mn1-
xFe
FexSi Heusler compounds with x Si Heusler compounds with x≈ 0.5 0.5 are are ideal candidates for spintronics ideal candidates for spintronics
Results: Co Co2FeSi FeSi thin film samples Problem: Problem: Unintentional exchange of atoms Off Off-
stoichiometry
Motivation
Bulk:
Wurmehl et al. Appl. Phys. Lett. 88 (2006) 032503 Balke et al. Phys. Rev. B 74 (2006) 104405
Films:
Inomata et al. J. Appl. Phys. 99 (2006) 08T314 Schneider et al. Phys. Rev. B 74 (2006) 174426
Gercsi et al. Appl. Phys. Lett. 89 (2006) 082512
Spin polarization only 49%
Gercsi et al. Appl. Phys. Lett. 89 (2006) 082512
Al (4a) Mn(4b) Cu (8c)
(d) L21 (Cu2MnAl) Fm3m
Co Co2FeSi is predicted to be a half FeSi is predicted to be a half-
metallic ferromagnet
Expected NMR spectrum
4 Fe + 4 Si L21 structure: One first shell environment for the 59Co nuclei Co2FeSi One hyperfine field One single sharp resonance line
50 100 150 200 250 300 6 8 10 12 14 16 18 20 22 24 26 28
Hyperfine field (T)
59Co Spin-Echo Intensity (arb. units)Frequency (MHz)
Co2FeSi bulk sample
Synthesis of films
Load lock Electronics “Carouso” Computer control Sputter 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 24
59Co Spin-Echo Intensity (arb.units)
(a)
Frequency (MHz) Hyperfine field (T)
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 lines in Co2FeSi
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
59Co Spin-Echo Intensity (arb.units)(a)
Frequency (MHz) Hyperfine field (T)
Fe - excess atoms
Two different types are possible:
Co2Fe(Si1-xFex) (Co2-xFex)FeSi Check local structure of Check local structure of corresponding bulk “model” samples corresponding bulk “model” samples
4 Co + 2 Fe 6 Co + 0 Fe 6 Fe + 2 Si 5 Fe + 3 Si 4 Fe + 4 Si 5 Co + 1 Fe
Comparison with bulk spectra
Co2Fe(Si0.93Fe0.07) (Co1.88Fe0.12)FeSi
6 Fe + 2 Si 5 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
Off-
stoichiometric films of Co2FeSi
stoichiometric targets
films prepared by different groups different groups
Off-
stoichiometry only apparent using using local methods as NMR NMR Principal problem while sputtering Co Principal problem while sputtering Co2FeSi FeSi
Summary Co2FeSi
Conclusion
Off-stoichiometry might explain
BUT: Results associated with “good” x-ray diffraction data
Troubleshooting
BUT: Sputtering is more common in industry Film preparation monitored Film preparation monitored and optimized by NMR and optimized by NMR
Take home message
Towards Towards technical technical application in application in spintronic spintronic devices devices
Benjamin Balke Christian G. F. Blum Gerhard H. Fecher Claudia Felser Gerhard Jakob Horst Schneider Hajo Elmers Jürgen T. Kohlhepp Henk J. M. Swagten Bert Koopmans Gregory Malinowski Tim Ellis Patrick Jacobs FNA
Money: Money: DFG Forschungsstipendium WU 595/1 DFG Forschungsstipendium WU 595/1-
Marek Wòjcik
Daniel Ebke Günter Reiss
Conventional NMR vs NMR on ferromagnets
Methyl- Multiplett
with small lines
High internal magnetic fields NO external field required
with broad lines
Methylene
Methylen- Multiplett
Chemical shift δ (parts per million [ppm])
Methyl
160 180 200 220 240
Frequency (MHz)
Random atom model Fe excess atoms
10 20 30 40 50 60 70 80 1 2 3 4 5 6
5 10 20 30 40 50 60 70 80
(a)
1 2 3 4 5 6
5
(c) Difference (%) (d)
resonance line (%) Probability according to binomial distribution (%) Number of Fe excess atoms Difference (%) (b) Co2Fe(Si1-xFex) (Co2-xFex)FeSi
n n N
x x n n N N x n P ) 1 ( ! )! ( ! ) , ( − − =
−
x = 0.08 x = 0.06
Results: Co Co2FeAl FeAl bulk samples Problem: Problem: Unintentional exchange of atoms Structure types Structure types
Mixing of Fe and Al atoms
Co2FeAl:
B2 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) 08T314 Wurmehl et al. J. Phys. D: Appl. Phys. 39 (2006) 803
Motivation
Crystallography: B2 structure requires random distribution random distribution of Fe Fe and and Al Al on the 1a Wyckoff position! Questions: (A) (A) Distribution of Distribution of Fe Fe and and Al Al on the 1
position (B) (B) Structural contributions Structural 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)
Temperature treatment in evacuated quartz tubes. Polycrystalline bulk samples
Synthesis
(A) Distribution of atoms
Co2FeAl in literature: B2 type structure Requires random distribution random 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 environments B2 structure: 9 different first shell environments
…
4 Al + 4 Fe 1 Al + 7 Fe 6 Al + 2 Fe 8 Fe 2 Al + 6 Fe 8 Al
m(Al))
= m(Al) h + m(Fe) h
H(Co
1 1 1 0 ≈
Δ B2 structure: mixing of Fe Fe and Al Al
Mixing of magnetically extremely different elements Hyperfine field (transferred contributions):
(A) Spacing
Large spacing on the order of 20 Large spacing on the order of 20-
60 MHz
(A) Expected spectrum
Nine resonance lines Nine resonance lines Spacing roughly 20 Spacing roughly 20-
60 MHz
Literature
Inomata et al. J. Phys. D 39 (2006) 816
125 150 175 200 225 250 275 300
59Co spin echo intensity (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%
59Co spin echo intensity (a. u.)
Frequency (MHz)
4Fe+4Al 3Fe+5Al 5Fe+3Al 6Fe+2Al 7Fe+1Al 8Fe 2Fe+ 6Al
6% 20% 34% 11% 4% 2%
(A) Origin of main lines
Main lines: Distribution of Distribution of Fe Fe and and Al Al in first shell of in first shell of Co Co Spacing Spacing ≈ 30 MHz 30 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
59Co spin echo intensity (a. u.)
1 2 3 4 5 6 7 8 5 10 15 20 25
0.4% 0.4% 3% 3% 11% 11% 22% 22%
Probability according to binomial distribution (%) Number Fe atoms
27%
n n N
x x n n N N x n P ) 1 ( ! )! ( ! ) , ( − − =
−
N=8 n=0-8 x≈0.5 Binomial distribution
B2 structure of Co2(FeAl)
Fe or Al (1a)
n n N
x x n n N N x n P ) 1 ( ! )! ( ! ) , ( − − =
−
N=8 n=0-8 x≈0.5
(A) Model vs. experiment
5 10 15 20 25 30 35
5 10 15 20 25 30 35
2 3 4 5 6 7 8
5 10
(b) Difference
resonance line (%) Probability according to binomial distribution (%) Number of Fe atoms in the first shell of
59Co
Difference
(A) Model vs. experiment
N=8 n=0-8 x≈0.5
X (8c) Fe(4b) Z(4a))
First shell in the L21 structure
n n N
x x n n N N x n P ) 1 ( ! )! ( ! ) , ( − − =
−
N=8 n=0-8 x≈0.5
(B) Model vs. experiment
5 10 15 20 25 30 35
5 10 15 20 25 30 35
2 3 4 5 6 7 8
5 10
(b) Difference
resonance line (%) Probability according to binomial distribution (%) Number of Fe atoms in the first shell of
59Co
Difference
n n N
x x n n N N x n P ) 1 ( ! )! ( ! ) , ( − − =
−
5 10 15 20 25 30 35 2 3 4 5 6 7 8
5 10 5 10 15 20 25 30 35
(a)
2 3 4 5 6 7 8
5 10
(b) Difference (d)
resonance line (%) Probability according to binomial distribution (%) Number of Fe atoms in the first shell of
59Co
Difference (c)
B2 and L21 contributions
N=8 n=0-8 x≈0.5
{ }
K K K = ⋅ + − − ⋅ =
− 4 , 4 , 2 2
1
) 1 ( ! )! ( ! ) , (
n n L n n N B
with l x x n n N N b x n P δ δ
1, n=4 0, n≠4
(B) Structural contributions
(B) Structural contributions
Distribution of Fe and Al not entirely random 90% 90% B2 B2 + + ≈ 10% 10% L2 L21 contributions contributions
Wurmehl et al. J. Phys. D: Appl. Phys. 41 (2008) 115007
Broad resonance line Broad resonance line corresponds to corresponds to A2 A2 structural contributions structural contributions
(C) Annealing
125 150 175 200 225 250 275 300
7% 7%
59Co spin-echo intensity (arb. units)
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)
59Co spin echo intensity (a. u.)
Frequency (MHz) (a)
as cast annealed
Co (8c) Fe(4b) Al(4a))
Mixing of Y and Z atoms
Co (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 environment for the 59Co nuclei
Between 200-250 MHz: Separation of CoAl after annealing process
(A) Sub-lines
90% B2 + 90% B2 + ≈ 10% L2 10% L21
1 contributions
contributions
distribution
in higher shells (cumulative higher shell effects) Sub Sub-
lines not apparent in thin films Only unintentional exchange of atoms Only unintentional exchange of atoms
Summary
Conclusions
High degree of order: Conservation of Conservation of half half-
metallic ferromagnetic properties (bulk) Higher long range order Higher long range order in bulk than in thin film samples in bulk than in thin film samples Improvement of structure: Improvement of structure: Better performance in spintronic devices…. Better performance in spintronic devices….