SELF-ASSEMBLY AND MAGNETISM OF NANOCLUSTER ARRAYS

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SELF
-
ASSEMBLY

AND
MAGNETISM

OF
NANOCLUSTER

ARRAYS



Axel Enders

Department of
Physics

and
Astronomy


and Nebraska Center
for

Materials and
Nanoscience

University
of
Nebraska, Lincoln, NE 68588

a.enders@me.com

R.
Skomski
, G.
Rojas
, X. Chen

J.
-
S. Kim, J. Kim

University of Nebraska


Lincoln

J
. Zhang, V.
Sessi
, J.
Honolka
,


I.
Brihuega
, C. Michaelis, and
K. Kern

Max
-
Planck
-
Institut für Festkörperforschung

Stuttgart
,
Germany


K.
Fauth
, G. Schuetz (
Stuttgart
)

S. Bornemann, H. Ebert (
Muenchen
)

A.
Buchsbaum, P. Varga (
Wien
)


ACKNOWLEDGEMENT

OUTLINE

Self
-
Assembled Model Structures of Fe on Pt



Adatoms
, Chains, Surface Alloys



Local Coordination, Hybridization and Magnetism


Deposited Co Clusters



Substrate
-
induced Anisotropy



Magnetic Moment of
Rh

Clusters in Contact with Surfaces



Ordering on
Nanotemplates

local:

Scanning Tunneling Microscopy

integral:

X
-
ray magnetic dichroism

magneto
-
optical Kerr effect

LOCAL AND INTEGRAL CHARACTERIZATION

30 nm

8 atomic rows

[111]

F
e

ON P
t
: STRUCTURAL PHASES

Pt(997):

Lee,
Kuhnke
, Kern, Surf. Sci. 2006

while superparamagnetic fit

yields MAE and spin block size

XMCD gives magnetic moments

N


㌮㔬 䵁䔠㴠〮㤠浥嘯慴潭

m
S

= 1.85µ
B
, m
L

= 0.28µ
B


0
°

70
°

H = 1T

FE
IMPURITIES

ON PT(111)

0.12 ML Fe

1

2

3

Ensemble of Fe
n
-
clusters on Pt(111)

(n=1,2,3) at T=6K


Correlation of anisotropy energy with
anisotropy of orbital moment

Simulation of magnetisation

curves m(B,T,
θ
) based

on ab
-
initio results

Calculation of anisotropy energy via magnetic torque T(n)

Fe1

Fe2

Fe3

m
Spin

3.49

3.33

3.2

m
Orbit

0.44

0.24

0.16

Eur. Phys. J. D 45, 529
-
534 (2007)

AB
-
INITIO MODELING

ATOMIC WIRES AT STEP EDGES

P. Gambardella, et. al, Nature 416 (2002) 301

-6
-4
-2
0
2
4
6
-90
-45
0
45
90
-4
-2
0
2
4

+43°
-57°
B
-field (T)

+70°

Fe

B
-field (T)
Co
+80°
angle (deg)
Co
Fe
+43°
Magnetization (
a.u
.)

Co
-
measurements:

P. Gambardella et al. Nature
416

(2002) 301


Magnetization at B = 1T

MONOWIRES

ON P
t
(997)

0.1 ML Fe/Pt(997),
dI/dV

PRB 74, 054408 (2006)

Komelj, Steiauf, Fähnle

PRB 73, 134428 (2006)

Co

Fe

SOC at Fe
and

Pt site

Fe
only

Pt
only

strong influence of Pt on

MAE in Fe wire:

-
large MCA

-
SRT into film plane

SUBSTRATE
-
CONTROLLED CHAIN MAGNETISM

easy

axis

ca. 80deg
with

respect

to
surface

normal!

F
e
-
P
t

SURFACE ALLOY

Deposition of 0.5 ML
Fe (a) and 0.25 ML Fe
(
b
) on Pt(997)

at 525 K

Honolka
, Enders, Kern
,
Fauth
,
Schuetz
,
Buchsbaum
,
Varga
,
Bornemann
, Ebert
,
Skomski
, PRL 2009.

LARGE
ANISOTROPY

AND
INDUCED

PT
MOMENTS

for

comparison
:
FePt

cluster

layers
,
H
C

= 0.6 T

for grain sizes of

4
-
5nm

J
.A.Christodoulides

et al.
Phys.Rev.B

68

(2003) 054428;
S.Sun

et al. Science
287

(2000) 1989

increased XMCD

at Fe
L
3

in Fe
50
Pt
50

induced moments

in Pt

m
tot

= 2.4µ
B


MAE = 0.42
meV
/atom

alloy

stripe

T
XMCD

= 10K

alloy

magnetic field (T)

[
D
m
L
:
calculated

(
Ederer
,
Fähnle
, 2003) ]

Mertig

1995
,

Ravindran

2001

MAGNETIC ANISOTROPY IN 3
d
-
5
d

BINARY ALLOYS

Fully relativistic
ab
-
initio calculations on 2D alloy layers (H. Ebert):

2
x

1 alloy:
strong FM coupling along wires (
30meV/atom
)



weak FM coupling between wires (
0.5
meV
/atom
)


strong
Dzyaloshinski
-
Moriya interaction (
>1
meV
)

2
x

2 alloy:
0.15
meV

per Fe atom


Disordered surface alloy ML
:
0.09
meV

per Fe atom


Full Fe monolayer:
0.03
meV
/ per atom, out of plane


Key to large anisotropy:


Fe bridging Fe chains at 0.5
-

0.6 ML coverage


9 nm

W(110):

Carbon

induced 15
x

12 reconstruction

Co on C/W(110)

Fe on C/W(110)

CLUSTER SELF
-
ASSEMBLY

ON TEMPLATES

BUFFER LAYER ASSISTED GROWTH

Key references:

J.H. Weaver and G.D.
Waddill
, Science 251 (1991) 1444




30 K

30 K

100 K

3.9
±

2.8
nm

6.7
±

4.2
nm

9.9
±

7.6
nm

clusters of 2 ML Fe

30L

250L

CONTROL OVER CLUSTER SIZE AND MAGNETISM

100 x 100nm
2

100L

1.7 ML Fe

MBE grown

Eur. Phys. J. D 45, 515
-
520 (2007)

4.2K

... on Pt(111)

5K

5K

... on Ag(111)

Co clusters

(0.1 ML Co /

10
L
Xe
)

M

M

M

SUBSTRATE
-
CONTROLLED CLUSTER MAGNETISM

40nm
3

x 0.05 ML Co / 10 L Xe

3.2 nm

100


100 nm
2

after 3 subsequent cluster fabrication cycles:

TOWARDS

ORDERED

CLUSTER
LAYERS

Prepared at T = 300 K, STM at 77 K

TCPP
/ Ag(111)

TPP / Ag(111)

H2TPP
:

meso
-
tetraphenyl porphyrin

H2TCPP
:

meso
-
tetracarboxyphenyl porphyrin

N

Co

0.5 nm

TPP / Cu(111)

TPP / Ag(111)

METALLO
-
ORGANIC STRUCTURES

biomimetics

single
-
molecule
sensing and devices

bio
-
inspired
machines

photonics

single electron
(spin) technology

quantum
technology

nanoelectronics

smart
materials

adaptive

programmed
materials

molecular scale
handling

single

electron & spin

quantum
coherence

isolated
nanostructures

integrated
nanosystems

hybrid

assemblies

multi
-
level hierarchic

architectures

2010

2015

2020

nanomagnetism

advanced

materials

nanomedicine

bionanotechnology

quantum
computation

supramolecular
technology

in vivo

nano
-
tools

FUTURE TRENDS AND PERSPECTIVES

SUMMARY

Fe model structures on Pt:



Adatoms
: out
-
of
-
plane M



Wires: in
-
plane M



Surface alloy: steps, DM interaction

3d
-
5d hybridization determines anisotropy



Deposited compact clusters:



substrate
-
dependent anisotropy of Co on Pt(111), Ag(111)



Suppressed moments in
Rh

upon contact with Ag(111)



Cluster ordering with
nanotemplates

(BN
nanomesh
)




Synchrotron radiation


+ UHV + STM + 0.3 K + 20 T !