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