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. MotivationWhy 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 – T C 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] [010] [100] [001] [110] [111] _ _ a b a Mo = 3.147 Å b Mo = 4.451 Å a Fe = 2.866 Å b Fe = 4.053 Å  Mo = 2.95 J.m -2  Fe = 2.55 J.m -2 bcc (110) plane

5. average terrace width: ~ 200 Å step height: 2.1±0.1 Å High T annealing (1300 – 2400 K) in O 2 and ultra-high vacuum LEED and AES analysis used to confirm clean surface [111] Mo(110) surface

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

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

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

9. 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

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

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

12. [111] [001] 3 6 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

13. 3 4 2 3 4 Fe wedgesstrain 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

14. Relaxation of the film lattice parameter 111 eV94 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

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

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