Influence of spin orbit coupling on the magnetic and spectroscopic properties of supported transition metal clusters

• Antragsteller: Professor Dr. Hubert Ebert
• Fachliche Zuordnung: Experimentelle Physik der kondensierten Materie
• Förderung: Förderung von 2003 bis 2009
• Projektkennung: Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 5406375

• Erstellungsjahr: 2011

Zusammenfassung der Projektergebnisse

Our fully relativistic ab initio calculations for small Co clusters on Pt(111) and on Au(111) show that the atomic spin and orbital magnetic moments decrease monotonously with increasing cluster size but they remain always enhanced when compared to the corresponding complete monolayers or bulk systems. In general the atomic magnetic moments depend strongly on coordination and they decrease with increasing number of neighboring atoms. This decay is much faster for the orbital than for the spin magnetic moments. The Pt surface atoms that are nearest neighbors to Co cluster atoms show an appreciable induced spin polarisation between 0.05-0.15 Bohr magnetons. The corresponding Au atoms are only weakly polarised coupled antiferromagnetically to the magnetic moments at cluster atoms. The exchange coupling among the cluster atoms is very strong exceeding the value of bulk Co. Additional Monte Carlo simulations revealed that already Co clusters consisting of about 10-20 atoms remain ferromagnetically ordered above 300 K. These findings also confirm the interpretations of experimental XMCD spectra that were based on the XMCD sum rules. It has been shown by our ab initio magnetic torque calculations that the impact of SOC on magnetic interactions within small clusters or extended nanostructures causes subtle anisotropic effects. The analysis of these results within an extended Heisenberg Hamiltonian has identified the role of various contributions as well as the limitations of such models. For Fe and Co dimers on Pt(111) the Dzyaloshinski-Moriya interaction was found to be pronounced owing primarily to the SOC of the substrate leading to non-collinear magnetic configurations for these dimers in spite a strong ferromagnetic coupling and out-of-plane magnetic anisotropy energy. These SOC induced effects can be quite profound in systems where magnetic atoms are separated by non-magnetic spacers with large SOC. This allows the isotropic exchange to become comparable in size with the DM couplings. Moreover, the magnetic structure around edges of magnetic clusters is likely to be significantly affected by these interactions. Comparing our results with experimental data shows that with our calculated magnetic properties of small Fe clusters on Pt(111) the results of experimental magnetisation curves can be reproduced in a very satisfying way. In collaboration with the experimental group we performed a detailed theoretical study of small Ru clusters deposited on a Fe/Cu(001) substrate. Here, our results for small Ru clusters deposited on Fe(001) and Fe/Cu(001) show an interesting behavior of the spin and orbital moment with respect to the local environment. The comparison between theoretical and experimental XAS and XMCD data showed that the non-compact Ru clusters are the magnetically most stable structures and that the reduced magnetic moment found for the Ru adatom is due to diffusion into the Fe film or adsorption at Fe surface steps.

Projektbezogene Publikationen (Auswahl)

• Electronic and magnetic properties of free and supported transition metal Clusters. Phase Transitions 78, 71 (2005) Phase Transitions: A Multinational Journal, Vol. 78. 2005, Issue 1-3, pp. 71-83.
  H. Ebert, S. Bornemann, J. Minar, M. Kosuth, O. Sipr, P.H. Dederichs, R. Zeller, I. Cabria
  
• Magnetic and spectroscopic properties of deposited transition metal clusters: A case study for Os on Fe(001). Computing Letters, Vol. 1. 2005, Issue 4, pp. 158 – 163.
  S. Bornemann, J. Minar, H. Ebert
  
• Magnetic properties of Co clusters deposited on Pt(111) and Au(111). Phase Transitions: A Multinational Journal, Vol. 78. 2005, Issue 9-11, Special Issue: International Symposium on the Structure and Dynamics of Heterogeneous Systems, SDHS-2004, pp. 701-709.
  S. Bornemann, J. Minar, S. Polesya, S. Mankovsky, H. Ebert, O. Sipr
  
• Magnetic properties of Co-clusters deposited on Pt(111). Applied Physics A, Vol. 82. 2006, Issue 1, pp. 139-144.
  J. Minar, S. Bornemann, O. Sipr, S. Polesya, H. Ebert
  
• Magnetic properties of free Fe clusters at finite temperatures from first principles. EPL - Europhysics Letters, Vol. 74. 2006, Number 6: 1074.
  S. Polesya, O. Sipr, S. Bornemann, J. Minar and H. Ebert
  
• Scanning tunneling microscopy and electronic structure of Mn clusters on Ag(111). Applied Physics A, Vol. 82. 2006, Issue 1, pp. 63-66.
  J. Kliewer, R. Berndt, J. Minar, H. Ebert
  
• Magnetic anisotropy of deposited transition metal Clusters. European Physical Journal D, Vol. 45. 2007, Issue 3 , pp. 529-534.
  S. Bornemann, J. Minar, J.B. Staunton, J. Honolka, A. Enders, K. Kern, H. Ebert
  
• Magnetic moments, exchange coupling, and critical temperatures of Co clusters on Pt(111) and Au(111). Journal of Physics: Condensed Matter, Vol. 19. 2007, Number 9, 096203.
  O. Sipr, S. Bornemann, J. Minar, S. Polesya, V. Popescu, A. Simunek, H. Ebert
  
• XMCD Spectra of Co Clusters on Au(111) by Ab-Initio Calculations. AIP Conference Proceedings, Vol. 882. 2007, 159.
  O. Sipr, J. Minar, S. Bornemann, H. Ebert
  
• Effects of spin-orbit coupling on the spin structure of deposited transition-metal Clusters. Physical Review B, Vol. 80. 2009, 014422.
  S. Mankovsky, S. Bornemann, J. Minar, S. Polesya, J. Staunton, A.I. Lichtenstein, H. Ebert
  
• Magnetism of FePt surface alloys. Physical Review Letters, Vol. 102. 2009, Issue 6: 067207.
  J. Honolka, T.Y. Lee, K. Kuhnke, A. Enders, R. Skomski, S. Bornemann, S. Mankovsky, J. Minar, J.B. Staunton, H. Ebert, et al.
  
• Magnetism of free and supported clusters: a comparative study. Central European Journal of Physics, Vol. 7. 2009, Issue 2, pp. 257-263.
  O. Sipr, J. Minar, H. Ebert
  
• On the importance of the magnetic dipole term Tz in analyzing X-ray magnetic circular dichroism spectra of Clusters. EPL - Europhysics Letters, Vol. 87. 2009, 67007.
  O. Sipr, J. Minar, H. Ebert
  
• Cobalt nano-clusters on metal supported Xe monolayers: influence of the substrate on Cluster formation kinetics and Magnetism. Phys. Rev. B 81. 2010, 195403.
  V. Sessi, K. Kuhnke, J. Zhang, J. Honolka, K. Kern, A. Enders, P. Bencok, S. Bornemann, J. Minar, H. Ebert
  
• Experimental and theoretical study of the magnetic properties and XMCD spectra of Ru Clusters deposited on Fe/Cu(001). physica status solidi (b), Special Issue: Clusters at Surfaces, Vol. 247. 2010, Issue 5, pp. 1180–1186.
  J. Minar, S. Bornemann, S. Mankovsky, H. Ebert, M. Martins, M. Reif, L. Glaser, W. Wurth
  
• Magnetic anisotropy of Fe and Co adatoms and monolayers: Need for a proper treatment of the substrate. Physical Review B, Vol. 82. 2010, 174414.
  O. Sipr, S. Bornemann, J. Minar, H. Ebert