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. 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 21 st- 23 rd 2009

Contents • Introduction • Materials • Method • Results of NMR analysis • Bulk samples of Co 2 Mn 1-x Fe x Si • Thin films of Co 2 FeSi

Heusler compounds Materials:

What is a Heusler compound? Ternary intermetallic compounds X 2 YZ Y(4 b ) Z (4 a ) X: Most electronegative transition metal L2 1 Structure X 2 YZ (Prototype: Cu 2 MnAl) Y: Transition metal X (8 c ) Spacegroup Fm 3 m Z: Main group element Order: Atoms on proper position proper position X 2 YZ: X (8 c ) Y (4 b ) Z (4 a ) � L2 1 ( ) Fm 3 m

Why are Heusler compounds attractive? Different The tailoring structure types principle X (8 c ) Z (4 a ) Y(4 b ) Affects Easy High spin polarization spin polarization to tune properties Half-metallic ferromagnetism Spintronic Spintronic applications applications

Why are Heusler compounds attractive? Different The tailoring structure types principle X (8 c ) Z (4 a ) Y(4 b ) Affects Easy High spin polarization spin polarization to tune properties Half-metallic ferromagnetism Spintronic Spintronic applications applications

Nobel price in physics 2007 P. Grünberg und A. Fert „…for the discovery of Giant Magnetoresistance" …for the discovery of Giant Magnetoresistance"

Nobel price in physics 2007 Transport ph Transport phenomena enomena using using charge and spin of electrons charge and spin of electrons P. Grünberg und A. Fert P. Grünberg und A. Fert „…for the discovery of Giant Magnetoresistance" …for the discovery of Giant Magnetoresistance"

100% spin polarization at the Fermi-edge Example: ↓ Bandgap at Fermi-edge ↑ DOS>0 at Fermi-edge Concept: Rob de Groot Material: NiMnSb Majority Minority ↑ ↓ Half Half- -metallic ferromagnetism metallic ferromagnetism de Groot et al. Phys. Rev. Lett. 50 (1983) 2024

Why are Heusler compounds attractive? Different The tailoring structure types principle X (8 c ) Z (4 a ) Y(4 b ) Affects Easy High spin polarization spin polarization to tune properties Half-metallic ferromagnetism Spintronic Spintronic applications applications

Tailoring of properties 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

Example: Co 2 Mn (1-x) Fe x Si Balke et al . Phys. Rev. B 74 (2006) 104405

Example: Co 2 Mn (1-x) Fe x Si Robust half Robust half- - metallic ferromagnets metallic ferromagnets with high thermal stability with high thermal stability Balke et al. Phys. Rev. B 74 (2006) 104405

Why are Heusler compounds attractive? The tailoring Different principle structure types X (8 c ) Z (4 a ) Y(4 b ) Affects Easy High spin polarization spin polarization to tune properties Half-metallic ferromagnetism Spintronic Spintronic applications applications

Disadvantage: Different structure types (and their mixtures) Y(4 b ) Z (4 a ) X or Y (8 c + 4 b ) Z (4 a ) X, Y or Z (2 a ) Y or Z (1 a ) X X (1 b ) (8 c ) (a) (b) (c) (d) A2 (Tungsten) B2 (CsCl) DO 3 (Fe 3 Si) L2 1 (Cu 2 MnAl) Im3m Pm3m Fm3m Fm3m Various structure types were observed Various structure types were observed

Spin polarization depends on structure Spin polarization ↔ structure! Spin polarization 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. 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 atoms � tuning of properties) Requirement: Requirement: • Structure type ( Unintentional (Local) Method! (Local) Method! exchange of atoms) How to distinguish between types? How to distinguish between types?

Nuclear magnetic resonance (NMR) Method:

Nuclear Magnetic Resonance (NMR) Nuclear Zeeman splitting ω L = = γ γ B 0 Resonance frequency depends on Resonance frequency depends on local (magnetic and electronic) local (magnetic and electronic) environment of nucleus environment of nucleus Topical review Wurmehl, Kohlhepp J. Phys. D.: Appl. Phys. 41 (2008) 173002

A typical 59 Co NMR spectrum Different local environments Different local environments have different hyperfine fields have different hyperfine fields Thanks to H. Wieldraaijer

Results: Co 2 Mn 1-x Fe x Al Problem: Problem: Intentional exchange of atoms � Distribution of atoms in Distribution of atoms in substitutional series? substitutional series?

Introduction Co Co 2 Mn Mn (1 x) Fe Fe x Si Si: (1- -x) • Robust half Robust half- -metallic ferromagnets metallic ferromagnets with high thermal stability with high thermal stability • L2 L2 1 ordered (XRD, EXAFS, Mößbauer-spectroscopy) Mn/Fe(4 b ) L2 1 (Cu 2 MnAl) Fm3m Co (8 c ) Si (4 a ) Wurmehl et al. Appl. Phys. Lett. 88 (2006) 032503 Balke et al. Phys. Rev. B 74 (2006) 104405

Motivation Crystallography: L2 L2 1 structure requires random distribution random distribution of Mn Mn and and Fe Fe on the 4 b Wyckoff position! Question: Distribution of Distribution of Mn Mn and and Fe Fe on 4b position? on 4b position?

Synthesis of bulk materials • Arcmelting in Argon atmosphere • Temperature treatment in evacuated quartz tubes Polycrystalline bulk samples Polycrystalline bulk samples

390 Co 2 Mn 0.5 Fe 0.5 Si 385 6% 14% 380 Frequency (MHz) 22% 23% 375 Fit of NMR spectrum 18% 10% 370 4% 365 360 55 Mn Spin-Echo Intensity (arb. units)

Local environments of Mn First coordination shell Mn Co Second coordination shell Mn Si Third coordination shell: Third coordination shell:

Random atom model Example : Co 2 Mn 0.5 Fe 0.5 Si N: Number of nearest neighbour sites in third shell of 55 Mn: 12 n: Number of Fe atoms in third shell of 55 Mn (varied) x: Concentration of Fe (nominal: 0.5) Rel. area of resonance line (%) 25 25 Random atom model (%) 20 20 N ! 15 15 − = − N n n P ( n , x ) x ( 1 x ) − ( N n )! n ! 10 10 5 5 Difference (%) 0 0 2 2 0 0 Wurmehl et al. -2 -2 Appl. Phys. Lett. 91 (2007) 052506 0 2 4 6 8 10 12 Number of Fe next neighbours J. Appl. Phys. 103 (2008) 07D706

55 Mn NMR of Co 2 Mn 0.5 Fe 0.5 Si 55 Mn Spin-Echo Intensity (arb. units) Frequency (MHz) Each resonance line Each resonance line is attributed to certain numbers of Fe atoms is attributed to certain numbers of Fe atoms

Co 2 Mn 1-x Fe x Si (0.1 ≤ x ≤ 0.9) Open symbols: random atom model Filled symbols: experimental data

Measured Fe concentration 1.0 0.8 55 Mn NMR Fe concentration x 0.6 measured by 0.4 0.2 0.0 0.0 0.2 0.4 0.6 0.8 1.0 Nominal Fe concentration x Measured Fe concentration Measured Fe concentration x follows nominal values follows nominal values

Summary Co 2 Mn 1-x Fe x Si Structure of Co 2 Mn 1-x Fe x Si confirmed by 55 Mn NMR: L2 L2 1 type with random distribution of random distribution of Mn Mn and and Fe Fe on 4 b Wyckhoff position Only intentional Only intentional exchange of atoms Wurmehl et al. Appl. Phys. Lett. 91 (2007) 052506 J. Appl. Phys. 103 (2008) 07D706

Conclusion Co 2 Mn 1-x Fe x Si High crystallographic order proved by 55 Mn NMR � high impact on 1) half-metallic character 2) high degree of spin polarization Si Heusler compounds with x ≈ 0.5 Co Co 2 Mn Mn 1- x Fe Fe x Si Heusler compounds with x 0.5 -x are are ideal candidates for spintronics ideal candidates for spintronics

Results: Co Co 2 FeSi FeSi thin film samples Problem: Problem: Unintentional exchange of atoms � Off Off- -stoichiometry stoichiometry

Motivation Co Co 2 FeSi is predicted to be a half FeSi is predicted to be a half- -metallic ferromagnet metallic ferromagnet Bulk : • L2 1 structure and 6 μ B Wurmehl et al. Appl. Phys. Lett. 88 (2006) 032503 Balke et al. Phys. Rev. B 74 (2006) 104405 Films : Cu (8 c ) • Magnetic moments low (4.5 – 5.0 μ B ) Mn(4 b ) Al (4 a ) (d) Inomata et al. J. Appl. Phys. 99 (2006) 08T314 L2 1 (Cu 2 MnAl) Schneider et al. Phys. Rev. B 74 (2006) 174426 Fm3m • Tunneling Magnetoresistance is low (44% at RT, 68 % at 5K) Gercsi et al. Appl. Phys. Lett. 89 (2006) 082512 • Spin polarization Spin polarization only 49% only 49% (PCAR, Jullière) Gercsi et al. Appl. Phys. Lett. 89 (2006) 082512