CoPt3 nanostructures prepared by
molecular beam epitaxy on
WSe2(0001) and NaCl(001)
Fabiola Liscio
SIMaP-INPGrenoble (Saint-Martin d’Hères - France)
Dipartimento di fisica dell’Università di “Roma Tre” (Rome – Italy)
Introduction
Epitaxial Co-Pt nanoparticles with regular size distribution
Advanced applications
Fundamental study
Media for ultra high
density magnetic
recording
Understand correlation among:
• chemical ordering
• morphology
• magnetic properties
Molecular beam epitaxy
Co-deposition using very
low rates of impinging
atoms:
controlled epitaxial growth
and chemical ordering
Physical properties
depend on:
• chemical composition
• substrate
(surface symmetry
and energy)
• deposition temperature
• deposition rate
Introduction
Well oriented nanoparticles alloys with regular size distribution
Advanced applications
Fundamental study
Media for ultra high
density magnetic
recording
Understand correlation among:
• chemical ordering
• morphology
• magnetic properties
Alloys: CoPt3 in L12–type ordered phase
Substrates: low energy’s surfaces
• WSe2(0001) [van der Waals surface]
• NaCl(001)
Effect of deposition temperature
Fixed fluxes
Physical properties
depend on:
• chemical composition
• substrate
(surface symmetry
and energy)
• deposition temperature
• deposition rate
Outline
„
WSe2(0001)/(CoPt3(111), 5÷30Å)
„
„
„
„
„
NaCl (001)/(CoPt3(001),5÷30Å)
„
„
„
„
Epitaxial growth
L12-type chemical ordering
Correlation between local structure and magnetic
properties
Morphology
Deposition temperature effects on epitaxial growth and
L12-type chemical ordering
Local structure
Morphology
Conclusion
WSe2(0001)/CoPt3(111)
Lamellar systems: WSe2
(0001) plane
Se-W-Se sandwich layers along c
ε=
aCoPt − as
3
as
WSe2(0001)
0Å
= 17 %
Large lattice
mismatch
2 nm
IT = 1.2 nA, UT = 0.4 V
STM image (It=1.2nA, U=0.4V)
WSe2(0001)/CoPt3(111): epitaxial growth
RHEED patterns after deposition of 30 Å
thick Co20Pt80 on WSe2(0001) @ 300÷700K
Co-deposition by MBE
ΦCo=0.005Å/s, ΦPt=0.02Å/s
P<1x10-8mbar
Epitaxial relationship
Tdep=300K
[112]
[110]
[1120] WSe2(0001) // [101] CoPt3(111)
[1010] WSe2(0001) // [112] CoPt3(111)
L12-type chemical order starts
Calculated RHEED pattern
already at 370K
370K
L12 phase growth along the [111] direction
and twin effect
570K
700K
Triangular spots correspond
to facet islands
WSe2(0001)/CoPt3(111): long range chemical order
3nm CoPt3 /WSe2 ( RHEED,
300nm CoPt3/Pt(111) ( RX)
RX)
Nanostructured film:
L12-type chemical ordering
starts at lower temperatures
Chemical order parameter S
1,2
1,0
Nanostructured films*
0,8
Continuous films
0,6
0,4
0,2
0,0
300
400
500
600
700
800
900 1000
Deposition temperature (K)
*A. Maier et al., J. Magn. Magn. Mat. 240, 377 (2002)
Partial long range chemical
order origins from:
• surface disorder effects
associated with Pt surface
segregation
• kinetic effects
WSe2(0001)/CoPt3(111): structural origin of PMA
3nm CoPt3 /WSe2 ( RHEED,
300nm CoPt3/Pt(111) ( RX)
RX)
Chemical order parameter S
1,2
1,0
Nanostructured films
0,8
0,6
Continuous films
PMA
Ru(0001)/CoPt3(111):
Ku= 0.6MJ/m3 at 670K
PMA stems from the existence
of anisotropic local ordering:
preference of Co-Co pairs in
the film plane*
Polarized x-ray absorption spectroscopy measurements
L12 chemical ordering destroys PMA
0,2 have been performed to understand the origin of PMA
0,4
PMA
0,0
300
400
500
600
700
800
900 1000
Deposition temperature (K)
*C. Meneghini et al., Eur. Phys. J. B 7, 347 (1999)
WSe2(0001)/CoPt3(111) :
Ku=0.2MJ/m3 at RT
Polarized X-ray Absorption Fine Spectroscopy
Co K-edge of CoPt3/WSe2(0001)
~
• Local probe
• Chemical selective
• Linear polarization allows to
distinguish the structural features in
the film plane and perpendicular to it.
G
ε
α
Differences between
G in-plane and
G α
ε out-of-plane εstructure are
correlated to a magnetic anisotropy
α
WSe2(0001)/CoPt3(111): short range chemical order
Co4
Pt3
Co4
SS2 Co2
MS4
Pt3
Pt1
Co2
Pt3
SS3
SS1
Pt1
Co4
Pt1
Co0
a
In-plane and out-of-plane
contributions
Co2
WSe2(0001)/CoPt3(111): short range chemical order
Anisotropy
Isotropy
A structural anisotropy at short range at Td=300K:
• preferential Co-Co correlations in the (111)
planes extending up to third shell
• formation of Co rich thin disks
WSe2(0001)/CoPt3(111): morphology
Grazing incidence small angle x- ray scattering
Specular rod
I (q// , q⊥ ) ≈ F
2
× S (q// )
Facet
Correlation peaks
Form factor:
• shape
• size distribution
Interference function:
• separation distance
WSe2(0001)/CoPt3(111): morphology
Experimental data, αi=0.4°- nominal thickness 1nm
Td=570K
Td=700K
Nominal thickness 3 nm
Tg=700K
{001} facets
qz (nm-1)
{111} facets
70.5°
qy (nm-1)
qy (nm-1)
Td=570K
(H/R)
hexagonal truncated shape
(H/R)
when Td
when ε
54.7°
NaCl(001)/CoPt3(001)
NaCl(001)/CoPt3(001):
RHEED patterns after deposition of 30 Å
of Co20Pt80 on NaCl(001) @ 300÷520K
NaCl
<100>
<110>
Td effects on epitaxial growth and
L12-type chemical ordering
Co-deposition by MBE
ΦCo=0.005Å/s, ΦPt=0.02Å/s
P<1x10-8mbar
af − as
ε=
as
= 30 %
Free stress
Large mismatch
Td=300K
370K
570K
670K
Epitaxial growth for Td>370K
Epitaxial relationship
[100]NaCl(001) // [100]CoPt3(001)
[110]NaCl(001) // [110]CoPt3(001)
No L12-type chemical ordering,
but a new LRO at Td=670K
NaCl(001)/CoPt3(001): Local structure
Isotropy
Short range and isotropic chemical order
670K//
670K
Isotropy
370K//
370K
NaCl(001)/CoPt3(001): morphology
1 nm CoPt3 on NaCl(001) Tg=520K
Experimental data (αi=0.4°)
Simulation
CoPt3(0.5nm)/C(8.4nm) after NaCl dissolution
(200)
qz (nm-1)
e
d
e
6
Intensity (a.u.)
2R
d200
c
b
a
4x10
6
d
6
c
6
b
a
3x10
2x10
1x10
0
1
exp
fit cuboct
fit sphere
3 nm
H
20nm
2R
HRTEM images of single dot
reveals the (200) lattice fringes
2
-1
qy(nm )
(220)
H
1
2
-1
qz(nm )
D=4.4nm
H=3.5nm
Growth along [001]
NaCl(001)/CoPt3(001): morphology
1 nm CoPt3 on NaCl(001) Tg=520K
2R
c
b
a
e
6
d
6
c
3x10
2x10
6
exp
fit cuboct
fit sphere
qz (nm-1)
qz (nm-1)
d
6
Intensity (a.u.)
Incident beam // [110] NaCl
H
e
4x10
Experimental data (αi=0.4°)
Simulation
H α
h
R
{001}
{111}
H
Incident beam // [100] NaCl
2R
qz (nm-1)
Experimental data (αi=0.4°)
3 nm CoPt3 on NaCl(001) Tg=670K
54.7°
b
a
1x10
0
1
2
-1
qy(nm )
1
2
-1
qz(nm )
Td=520K
D=4.4nm
H=3.5nm
qy (nm-1)
Facet effects are seen only for
thick alloy deposits
Conclusion
„
Epitaxial growth of free stress nanostructures on WSe2(0001) (300÷700K)
and NaCl(001) (370÷670K) in spite of large mismatch parameter.
„
„
Morphology:
„
Hexagonal to triangular shape for nanostructures on WSe2
„
Truncated cubooctahedric islands with narrower size distribution on NaCl
Different behaviors of L12 order:
„
At Td< 370K, short range anisotropic chemical order only on WSe2, structural
origin of PMA
„
At Td> 370K partial L12 order on WSe2 and only short range order on NaCl
Why?
• Influence of growth direction
• Se surfactant effect (already observed during the Pt deposition with the
formation of Moiré*) would favor the atomic arrangements at Td<700K
*Surface Science 601, 2032 (2007)
Acknowledgments
• SIMAP, CNRS-UJF Saint Martin d’Hères ,France
M. Maret, J. P. Simon and B. Doisneau-Cottignies
• Department of Physics « E. Amaldi », Università di Roma Tre, Rome, Italy
C. Meneghini and S. Mobilio
• Department of Physics, University of Konstanz, Konstanz, Germany
D. Makarov
• Chemnitz University of Technology, Institute of Physics, Chemnitz, Germany
C. Brombacher and M. Albrecht
• Institut Néel, MCMF, CNRS, Grenoble, France
O.Proux and Y. Gauthier
Thank you for your attention!