a materials scientist’s view horst hahn - unlp...laterally structured thin films summary 5 0 n m5...

Mössbauer Spectroscopy and Nanomagnetism -

a Materials Scientist’s View

Institute for Nanotechnology

Forschungszentrum Karlsruhe

Joint Research Laboratory

Nanomaterials

Technische Universität Darmstadt

Horst Hahn

International Conference on Hyperfine Interactions

Iguassu Falls, 7.8.2007

Iguassu Falls, 7. August 2007 2

Constitutional and Thermal Defects in Intermetallics

How does a materials scientist get in contact with nuclear probe techniques?

Problems on diffusion and point defects

H. G. Müller, H. Hahn, Phil. Mag., 1984. A50, p. 71.

Some intermetallic B2 (CsCl)-compounds, such as PdIn, exhibit very

large concentrations of constitutional and thermal point defects, i.e.

vacancies and antistructure atoms

Probe atom: 111In 111Cd

Not an impurity

Constitutional defects

Pd50In50: no defects

Pd-rich compounds: PdIn

In-rich compounds: VPd

Thermal defects

•For 49at%<cPd<50.5at%: single

vacancies in each sublattice (VIn

remain invisible for PAC)

•Reaction with PdIn antistructure

defectsPd-rich sideIn-rich side


Contents

Introduction

DCEMS: a highly sensitive method with monolayer resolution

Metallic Nanostructures

Synthesis methods

Nanoparticles

Thin films and multilayers

Laterally structured thin films

Summary 5 0 n m5 0 n m

060821_Ellrich_Nano_2006.ppt


Interface and finite size effects

Some examples of properties

Magnetic properties

Chemical properties

Optical properties

Biofunctionality

Multifunctionality


Metallic nanostructures come in different morphologies

5 nm

Embedded nanocrystal

Rapid

quenching 20 nm

Self-assembling

Co-nanoparticles

Chemical

Synthesis

100 nm

Metallic Nano-rods

Pressure Infiltration

NanostructuredMaterials

Nanocrystalline Metals

Cold-rolling

Laterally structured metal dots

Anodic Oxidation 500 nm

Thin films

Molecular Beam Epitaxy

Fe

Fe

Al2O3

Ta


Morphology and applications

5 nm

Multilayers Nano-Dots Nano-Particles

XMR-sensors Data storagematerials

N S

Biotechnology, ferrofluids

5 0 n m5 0 n m

GMR-read heads


Depth Sensitivity

UHV Orange spectrometer

T = 40 % von 2

E/E ≈ 1%

typically 5 nm

SURFACE sensitivity

and

DEPTH sensitivity

due to electron energy loss


Depth sensitive Conversion Electron Mössbauer Spectrometer

1 m

Mössbauer-AbsorptionElectronEmission

Electron Detector

UHV Orange Spectrometer

Electron Source

MagnetCoils

UHV-Orange-Spektrometer

• high sensitivity

• high energy resolution

• short measuring time !

The UHV-Orange-Mössbauer-Spektrometer


Karslruhe Integrated Synthesis and Characterization System

Total view of open spectrometer showing

the current leads to the magnet coils

Mössbauer-source

Special sample holder

with cooling


Characteristics of DCEMS Orange Spectrometer

Transmission

Conversion

electrons

almost 2 collection

high sensitivity

temperature range from 10 to 450 K

extreme surface sensitivity

monolayer resolution with special samples

in-situ sample preparation

UHV (p 10-9 mbar)

no transmission geometry


How to obtain monolayer resolution

Typically, the depth resolution of DCEMS is approx. 5 nm (assuming

natural Fe is used)

The depth resolution can be improved by using the following sample

geometry:

Cover layer, i.e. oxide (for

TMR) or another metal (Pt)

One or two monolayers of

Fe57 (Mössbauer active)

Layer without any Fe or

pure Fe56 (Mössbauer

inactive)

Substrate

Mössbauer signal

originates from this

layer only, i.e.

information from the

first and second

monolayer at the

interface

i.e. Ta/56Fe/2ML 57Fe/Al2O3/56Fe

or Ta/56Fe/2ML 57Fe/5ML Pt


Proof of monolayer resolution

Fe-Pt Interface

•magnetic properties of planar interfaces

Determine local magnetic properties

DCEMS – Mössbauer-spectroscopy as local probe method

Preparation of smooth interface

Ta – buffer layer


Fe-Pt interface

Two components clearly separable :

different temperature behaviour

different hyperfine fields

Assignment :

sub-interfacial layer [Fe(I-1)]

direct interface [Fe(I)]

Fe(I)Fe(I-1)

5 ML Pt

33 ML Fe56

1 nm Ta

Si/SiO2

= 0,39(7) mm/s; = 0,035(2) mm/s

= 0,13(3) mm/s; = 0,018(7) mm/s

well defined


Temperature dependence

Fe(I)Fe(I-1)

5 ML Pt

33 ML Fe56

1 nm Ta

Si/SiO2

Interfacial layer [Fe(I)]:

reduced ground state

hyperfine field

reduced Curie-Temperatur TC

Sub-interfacial layer [Fe(I-1)]:

almost bulk, BUT:

enhanced ground state

hyperfine field


Contents

Introduction

DCEMS: a highly sensitive method


Synthesis methods

Nanoparticles



Summary



Magnetic nanoparticles of different alloys

5 nm

FePt

5

nm

Fe45Co55 MS KA

< D >= 5.66 0.76 nm

100 nm

Fe25Co25Pt50


Synthesis of functionalized metallic nanoparticles

FePt nanoparticles

Pt(acac)2 Fe(CO)5

reductionthermal

decomposition

particle-ligand

TEM

4.1 nm

Challenges:

• Class of material

• Synthesis method

• Particle size

• Morphology

• Defects

• Functionalization

• Self organisation

• Structuring


Lokale Eigenschaften lokale Methoden

C O

O

C

OO

C

OO

CO

O

COO

?

Local structure ?

Local magnetic properties ?

Influence of surface ?

Mössbauer spectroscopy

EXAFS

Local methods needed

J. Ellrich, TU Darmstadt


Properties of FePt nanoparticles

-3 -2 -1 0 1 2 3

0,65

0,66

185 K

1,98

1,98

220 K

1,39

1,39

230 K

240 K

Co

un

ts /x10

6

3,70

3,70

0,69

0,69

250 K

-12 -8 -4 0 4 8 12

1,31

1,32

Velocity/mm s-1

10 K

33.3 T

46.5 T

4 nm

fast

slow

frozen

Brownian

motion

Particle moment

superpara-

magnetic

blocked

FePt particles

protected with

oleic acid

Brownian

motion of

entire particle

Magnetisation

of entire

particle

Ordering

within the

particles


Mössbauer-Spektroskopie

TB=55 K

Brownian motion of coupled particles

large Hyperfine fields :

oxides ?

surface ?

TB

2.4 nm Fe

F. Bodker, S. Morup, S. Linderoth, PRL 1994

-10 10


EXAFS Modellierung

Method :

generating atom positions

relaxation with EAM potentials

Simulation with FEFF 8.10

Fit of paths

Results :

Pt rich core (20%Fe)

Particle surface covered by

two monolayers Fe

Termination of surface by O

bond of Oleic acid via O to Fe


Contents

Introduction



Synthesis methods

Nanoparticles



Summary

5 0 n m5 0 n m


Synthesis methods – Thin films and multilayers

PVD: Physical Vapor Deposition

Molecular Beam Epitaxy

(layer-by-layer growth)

Sputtering

Thermal evaporation

CVD: Chemical Vapor Deposition

ALD: Atomic layer deposition

(self-limiting, sequential surface chemistry for

monolayer-monolayer growth)


Motivation: understanding interfaces in TMR systems

How is the barrier interface affected by

=> the preparation of the barrier?

=> annealing of the sample?

Characterization: DCEMS, TEM, TMR,

XRR, and XPS

Fe

SiO2

2.8 (2) nm Al2O3

Fe = 20 nmFe

Ta

H. Schmitt, et al., APL 2006. 88: 122505, 1-3.TEM-image as prepared

10nm

Fe

Fe

Al2O3

Ta


Structural changes at the interfaces

Fe3+

Fe2+ (spinel)

Fe3+ (spinel)

Fe

SiO2

2.8 nm Al2O3

Fe = 20 nmFe

Taafter

oxidation

co

unts

/100

0

Velocity [mm/sec] H. Schmitt, et al., APL 2006. 88: 122505, 1-3.

Ta–Fe–Al2O3–Fe


Structural changes at the interfaces

Fraction of

57Fe atoms in

this

environment

in monolayers

(ML)

H. Schmitt, et al., APL 2006. 88: 122505, 1-3.



Effects of annealing on TMR effect

more homogeneous & thicker barrier

appearance of Fe3+ (over-oxidation)

increase of Al2O3 in the barrier

reduction of Fe3+

appearance of FeAl2O4

smooth interfaces

Observed

changes of

TMR effect

upon

annealing

No other technique

available to detect

changes in the (sub-)

monolayer level in

buried interfaces

H. Schmitt, et al., APL 2006. 88: 122505, 1-3.



TMR measurements

H. Schmitt, Ph.D. thesis, TU Darmstadt (2005)

Pd–Fe–Al2O3–Fe


Hyperfine field distribution

Pd

Fe

Fe Al O2 3

60nm

H. Schmitt, et al., JAP (2005) 97:113902-1-113902-5



Results: 3,5 ML tracer

Free surface

Fe57

bcc Fe

High field

Low field

Paramagnetic Fe

ionic Fe

Al

Oxygen

H. Schmitt, et al., JAP (2005) 97:113902-1-113902-5



Contents

Introduction



Synthesis methods

Nanoparticles



Summary


Fe-Pt system

Fe50Pt50:

K. Watanabe, H. Masumoto, Trans. Jap. Inst. Met. 24, 627 (1983)

Fe

Pt

Fe

Pt

Fe

a

c a = 0,385 nm

c/a = 0,96

L10

• ordered phase: L10-structure (fct)

• high magnetocrystalline anisotropy

Keff ≈ 5 x 107 J m-3

• order-disorder transition

• disordered phase fcc


FePt nanodots as magnetic recording media

Nanoporous, ultrathin Al2O3 mask

by Lei Yong / INT

MgO/{2ML57Fe/2MLPt}6 → ttotal ≈ 4 nm

Tgrow = 280°C

layer by layer growth → artificial L10 structure

highly ordered, magnetic nanostructures with sizes < 20 nm

perpendicular anisotropy @ RT

J. Ellrich, Lei Yong, H. Hahn,

patent pending

J. Ellrich et al., submitted to

Small (2007)


Laterally structured thin films or nanodots

Lei Yong, INT-FZK

Alumina

nanoporous

thin film

Y. Lei, et. Al, Chem. Mat. (2005) 17: 580


Influencing the dot size and shape

Y. Lei, et. Al, Chem. Mat. (2005) 17: 580


Local magnetic properties

DCEMS:

in-situ measurement

L10 AND fcc Fe-Pt environment

reflects sample structure

L10-Fe-Pt well defined

fcc Fe-Pt rather broad

distribution of sites

magnetization direction:

=12° for 22 % L10-phase

=30° for 78 % fcc-phase


Conclusions

Nuclear probe techniques provide very useful information on the local

structure and magnetic properties of nanostructures

DCEMS is a highly sensitive detection technique with monolayer resolution for

the characterization of the atomic and magnetic nature of the interfaces

XMR-properties depend strongly on the atomic and magnetic structure at the

interfacial regions in

nanoparticles

thin films and multilayers

5 0 n m5 0 n m



Acknowledgements

INT M. Ghafari, J. Ellrich, B. Stahl, H. Schmitt, A. Hütten,

R. Kruk, Lei Yong, H. Rösner, G. Wilde, R. Theissmann

TUD M. Winterer, K. Albe, M. Müller

Financial support by:

DFG, CFN, AvH, DAAD, BMBF, Bosch, SusTech Darmstadt

a materials scientist’s view horst hahn - unlp...laterally structured thin films summary 5 0 n m5...

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