1. SELF-ASSEMBLY AND MAGNETISM OF NANOCLUSTER ARRAYS Axel Enders Department of Physics and Astronomy and Nebraska Center for Materials and Nanoscience University of Nebraska, Lincoln, NE 68588 a.enders@me.com

2. R. Skomski, G. Rojas, X. Chen J.-S. Kim, J. Kim University of Nebraska – Lincoln J. Zhang, V. Sessi, J. Honolka, I. Brihuega, C. Michaelis, and K. Kern Max-Planck-Institut für Festkörperforschung Stuttgart, Germany K. Fauth, G. Schuetz (Stuttgart) S. Bornemann, H. Ebert (Muenchen) A.Buchsbaum, P. Varga (Wien) R. Skomski, G. Rojas, X. Chen J.-S. Kim, J. Kim University of Nebraska – Lincoln J. Zhang, V. Sessi, J. Honolka, I. Brihuega, C. Michaelis, and K. Kern Max-Planck-Institut für Festkörperforschung Stuttgart, Germany K. Fauth, G. Schuetz (Stuttgart) S. Bornemann, H. Ebert (Muenchen) A.Buchsbaum, P. Varga (Wien) ACKNOWLEDGEMENT

3. OUTLINE Self-Assembled Model Structures of Fe on Pt Adatoms, Chains, Surface Alloys Local Coordination, Hybridization and Magnetism Deposited Co Clusters Substrate-induced Anisotropy Magnetic Moment of Rh Clusters in Contact with Surfaces Ordering on Nanotemplates

4. local: Scanning Tunneling Microscopy integral: X-ray magnetic dichroism magneto-optical Kerr effect LOCAL AND INTEGRAL CHARACTERIZATION

5. 30 nm 8 atomic rows [111] Fe ON Pt: STRUCTURAL PHASES Pt(997): Lee, Kuhnke, Kern, Surf. Sci. 2006

6. while superparamagnetic fit yields MAE and spin block size XMCD gives magnetic moments N  3.5, MAE = 0.9 meV/atom m S = 1.85µ B, m L = 0.28µ B 0° 70° H = 1T FE IMPURITIES ON PT(111) 0.12 ML Fe 123

7. Ensemble of Fe n -clusters on Pt(111) (n=1,2,3) at T=6K Correlation of anisotropy energy with anisotropy of orbital moment Simulation of magnetisation curves m(B,T,θ) based on ab-initio results Minar Ebert Calculation of anisotropy energy via magnetic torque T(n) Fe1Fe2Fe3  Spin 3.493.333.2  Orbit 0.440.240.16 Eur. Phys. J. D 45, 529-534 (2007) AB-INITIO MODELING

8. ATOMIC WIRES AT STEP EDGES P. Gambardella, et. al, Nature 416 (2002) 301

9. Magnetization (a.u.) Co-measurements: P. Gambardella et al. Nature 416 (2002) 301 Magnetization at B = 1T MONOWIRES ON Pt(997) 0.1 ML Fe/Pt(997), dI/dV PRB 74, 054408 (2006)

10. Komelj, Steiauf, Fähnle PRB 73, 134428 (2006) Co Fe SOC at Fe and Pt site Fe only Pt only strong influence of Pt on MAE in Fe wire: -large MCA -SRT into film plane SUBSTRATE-CONTROLLED CHAIN MAGNETISM easy axis ca. 80deg with respect to surface normal!

11. Fe-Pt SURFACE ALLOY Deposition of 0.5 ML Fe (a) and 0.25 ML Fe (b) on Pt(997) at 525 K Honolka, Enders, Kern, Fauth, Schuetz, Buchsbaum, Varga, Bornemann, Ebert, Skomski, PRL 2009.

12. LARGE ANISOTROPY AND INDUCED PT MOMENTS for comparison: FePt cluster layers, H C = 0.6 T for grain sizes of 4-5nm J.A.Christodoulides et al. Phys.Rev.B 68 (2003) 054428; S.Sun et al. Science 287 (2000) 1989 increased XMCD at Fe L 3 in Fe 50 Pt 50 induced moments in Pt m tot = 2.4µ B MAE = 0.42 meV/atom alloy stripe T XMCD = 10K alloy magnetic field (T)

13.  m L : calculated (Ederer, Fähnle, 2003) ] Mertig 1995, Ravindran 2001 MAGNETIC ANISOTROPY IN 3d-5d BINARY ALLOYS Fully relativistic ab-initio calculations on 2D alloy layers (H. Ebert): 2 x 1 alloy: strong FM coupling along wires (30meV/atom) weak FM coupling between wires (0.5 meV/atom) strong Dzyaloshinski-Moriya interaction (>1 meV) 2 x 2 alloy: 0.15 meV per Fe atom Disordered surface alloy ML: 0.09 meV per Fe atom Full Fe monolayer: 0.03 meV/ per atom, out of plane Key to large anisotropy: Fe bridging Fe chains at 0.5 - 0.6 ML coverage

14. 9 nm W(110): Carbon –induced 15 x 12 reconstruction Co on C/W(110)Fe on C/W(110) CLUSTER SELF-ASSEMBLY ON TEMPLATES

15. BUFFER LAYER ASSISTED GROWTH Key references: J.H. Weaver and G.D. Waddill, Science 251 (1991) 1444 30 K 100 K

16. 3.9 ± 2.8nm6.7 ± 4.2nm9.9 ± 7.6nm clusters of 2 ML Fe 30L 250L CONTROL OVER CLUSTER SIZE AND MAGNETISM 100 x 100nm 2 100L 1.7 ML Fe MBE grown Eur. Phys. J. D 45, 515-520 (2007)

17. 4.2K... on Pt(111) 5K... on Ag(111) Co clusters (0.1 ML Co / 10 L Xe) M M M SUBSTRATE-CONTROLLED CLUSTER MAGNETISM

18. 3 x 0.05 ML Co / 10 L Xe 3.2 nm 100  100 nm 2 after 3 subsequent cluster fabrication cycles: TOWARDS ORDERED CLUSTER LAYERS

19. Prepared at T = 300 K, STM at 77 K TCPP / Ag(111) TPP / Ag(111) H2TPP: meso-tetraphenyl porphyrin H2TCPP: meso-tetracarboxyphenyl porphyrin NN Co 0.5 nm TPP / Cu(111) TPP / Ag(111) METALLO-ORGANIC STRUCTURES

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21. SUMMARY Fe model structures on Pt: Adatoms: out-of-plane M Wires: in-plane M Surface alloy: steps, DM interaction 3d-5d hybridization determines anisotropy Deposited compact clusters: substrate-dependent anisotropy of Co on Pt(111), Ag(111) Suppressed moments in Rh upon contact with Ag(111) Cluster ordering with nanotemplates (BN nanomesh)

22. Synchrotron radiation + UHV + STM + 0.3 K + 20 T !