MORPHOLOGY AND STRAIN-INDUCED DEFECT STRUCTURE OF FE/MO(110) ULTRATHIN FILMS:
IMPLICATIONS OF STRAIN FOR MAGNETIC NANOSTRUCTURES
I. V. Shvets Physics Department
Trinity College Dublin
Motivation Why study this system?
• Magnetism of a low-dimensional system
– relationship with morphologye.g. magnetic percolation in a two-dimensional system
– magnetoelastic anisotropyeffects of lattice mismatch in heteroepitaxial systems
can drive spin reorientation transitions
• Wide variety of nanostructures can be grown
– nanowires, wedges, two-dimensional islands
Overview
• Similar to the Fe/W(110) epitaxial system
– both systems have ~10% lattice mismatch
– Fe wets both surfaces
• Magnetic properties of Fe/W(110) system well-known
– TC in first layer below 300 K
– strain driven spin reorientation transition in second layer
– dipolar coupling between nanowires grown at high temp.
– spin reorientation transitions in Fe wedges grown at high temperatures
Mo(110) surface
[001]
[010] [100]
[001]
[110]
[111]
_
_
a
b
aMo = 3.147 Å bMo = 4.451 Å
aFe = 2.866 Å bFe = 4.053 Å
Mo = 2.95 J.m-2 Fe = 2.55 J.m-2
%9.8
s
sf
a
aam
bcc (110) plane
average terrace width: ~ 200 Å step height: 2.1±0.1 Å
• High T annealing (1300 – 2400 K) in O2 and ultra-high vacuum • LEED and AES analysis used to confirm clean surface
[111]
Mo(110) surface
Growth at room-temperature
= 0.95 ML
= 2.4 ML
= 0.42 ML
= 1.8 ML
[110]
[001]
12 ± 1 Å
Dislocation formation in second Fe layer
first layer Fe atomSecond layer atom
extra row
13 Å
[110]
[001]
Two-dimensional dislocation network
2 ML
3 ML
dislocation network
2 ML3 ML 2 ML
Mo substrate
4
3
2
3
2
4
3
2
[111]
Two-dimensional dislocation network
• network is formed by overlap of dislocation lines that run along the [111] and [111] directions
• the tensile strain in the film is relieved by matching 12 Fe atoms to 11 Mo atoms along [001] direction and 14 Fe atoms to 13 Mo atoms along [110] direction
Fe nanowires grown at 495 T 525 K
= 1.2 ML = 1.5 ML
11 2
2
Fe stripe width: 30-60 ÅNo dislocation lines
Fe stripe width: 130-200 ÅDislocation lines
Fe nanowires
Mo substrate
Mo substrate
• Dipolar superferromagnetism between monolayer Fe nanowires
• Dipolar antiferromagnetism between double layer Fe nanowires
[111]
[001]
36
Fe wedges
= 2.4 ML film grown on Mo(110) at 515 ± 15 K
• islands propagate across several terraces
• flat (110) surface of each island - unbroken by steps
• islands elongated along the [001] direction
3
4
2
3
4
Fe wedges strain relief
• onset of dislocation network is a gradual process developing in the third Fe layer from an array of closely-spaced dislocations
• the tensile strain is relieved by matching 12 Fe atoms to 11 Mo atoms along the [001] direction and 14 Fe atoms to 13 Mo atoms along the [110] direction
Relaxation of the film lattice parameter
111 eV 94 eV[001]
[110]
[001]
[110]
• LEED patterns indicate the relaxation of the Fe film to the unstrained Fe(110) state
= 2.4 ML T = 515 ± 15 K = 3.5 ML T = 700 ± 15 K
Fe wedges
unstrained Fe(110)
TC ~ 300 KTC ~ 200 K
imaged topography
magnetic STM tip
Mo substrate
,,, UsII ,,, UsII
Effective polarisation (P):Effective polarisation (P): PI I
I I
PI I
I I
STM tunnel current with magnetic tip/sample:STM tunnel current with magnetic tip/sample:
Conclusions
• Film morphology may be manipulated by deposition temperature to produce a variety of nanostructures
• The magnetic order within these nanostructures is highly sensitive to the film strain
• The mechanism by which film strain is relieved is different for each of the various nanostructures i.e. nanowires, wedges, islands grown at 300 K
• Arrays of Fe nanowires or wedges can be grown on Mo(110) analogous to the Fe/W(110) system
• It is expected that these structures will display similar magnetic phenomena to those observed for the Fe/W(110) system
• Because of changes in the magnetic order of the Fe nanowires and wedges on the nanometer scale, these systems are good candidates for spin-polarised STM
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