I. V. Shvets Physics Department Trinity College Dublin

1. 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

2. Motivation Why study this system? • Magnetism of a low-dimensional system • – relationship with morphology • e.g.magnetic percolation in a two-dimensional • system • – magnetoelastic anisotropy • effects of lattice mismatch in heteroepitaxial systems • can drive spin reorientation transitions • Wide variety of nanostructures can be grown • – nanowires, wedges, two-dimensional islands

3. 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

4. Mo(110) surface [001] _ [001] [111] b a _ [110] [100] [010] bcc (110) plane 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

5. Mo(110) surface • High T annealing (1300 – 2400 K) in O2 and ultra-high vacuum • LEED and AES analysis used to confirm clean surface [111] average terrace width:~ 200 Å step height:2.1±0.1 Å

6. Growth at room-temperature  = 0.42 ML  = 0.95 ML  = 1.8 ML  = 2.4 ML

7. [001] [110] Dislocation formation in second Fe layer 13 Å [001] 12 ± 1 Å [110] extra row first layer Fe atom Second layer atom

8. Two-dimensional dislocation network 3 ML 2 ML dislocation network 2 ML 3 ML 2 ML Mo substrate

9. Two-dimensional dislocation network 2 4 [111] 3 3 3 2 4 2 • 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

10. Fe nanowires grown at 495  T  525 K 2 1 1 2  = 1.5 ML  = 1.2 ML Fe stripe width: 30-60 Å No dislocation lines Fe stripe width: 130-200 Å Dislocation lines

11. Fe nanowires • Dipolar superferromagnetism between monolayer Fe nanowires Mo substrate • Dipolar antiferromagnetism between double layer Fe nanowires Mo substrate

12. Fe wedges [001] 6 3 [111]  = 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

13. Fe wedges strain relief 2 3 3 4 4 • 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

14. Relaxation of the film lattice parameter [110] [110] [001] [001] 111 eV 94 eV  = 2.4 ML T = 515 ± 15 K  = 3.5 ML T = 700 ± 15 K • LEED patterns indicate the relaxation of the Fe film to the • unstrained Fe(110) state

15. Fe wedges STM tunnel current with magnetic tip/sample: Effective polarisation (P): magnetic STM tip imaged topography TC~ 200 K TC ~ 300 K unstrained Fe(110) Mo substrate

16. 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