ERA NANOSCI: Three-terminal transport through single-molecule magnets

• Applicant: Professor Dr. Maarten Rolf Wegewijs
• Subject Area: Theoretical Condensed Matter Physics
• Term: from 2007 to 2011
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 40465500

• Final Report Year: 2012

Final Report Abstract

The properties of single molecule magnets (SMM) have been investigated in detail mostly for large collections of molecules. The ERA NANOSCI collaborative project “SMM-TRANS” – of which this report accounts the theoretical part – demonstrated that an individual single molecule magnet retain its peculiar magnetic properties when embedded in an electric circuit. The experimental partners realized electron transport through an individual single-molecule magnet (H. van der Zant, Kavli Institute of Nanoscience, Delft, the Netherlands) which was tailormade by chemical synthesis (A. Cornia, University of Modena, Italy), achieving three terminal electrical control. Inelastic tunneling features in the Coulomb blockade regime were identified by detailed magnetic field measurements. These were modeled using quantum kinetic equations (moderately coupled sample, next-to-leading order perturbation theory in the tunneling) and NRG calculations (strongly coupled sample, non-perturbative tunneling), using a charge-state dependent bi-axial magnetic anisotropy model for the SMM. Detailed comparison with the calculations allowed the unknown angle of the magnetic field with the molecular magnet’s easy axis to be estimated, in particular through the non-linear Zeeman effect that was reproduced in the calculations. In this way the strength of the magnetic anisotropy parameter could be extracted. The central, surprising result was that, the magnetic anisotropy could be increased by 180% by charging the molecule through the electrical gating, which is impressive considering the efforts spent by synthetic chemists to control this parameter. This possibility of electrically controlling the magnetic anisotropy barrier in a single molecular magnet presents an important step towards application of molecular magnetism in nanoscale devices. It allows a spin state to be “written” without magnetic anistropy (off-state), and then “storing” it for later use by turning the magnetic anisotropy back on. Besides modeling these experiments various possible influences on the transport through single molecule magnets were surveyed: the applied voltages and magnetic fields, the strength of the tunnel coupling of the single molecule magnet to the attached electrodes, the vibrational molecular excitations coupling to the spin, and the effects of the various types of magnetic anisotropy. It was found that two types of Kondo effect may arise, one suppressed by anisotropy and one generated by bi-axial anisotropy. Surprisingly, the effect of coupling to molecular vibrations was found to enhance the latter Kondo effect. These theoretical studies show that the fields of spintronics, molecular magnetism and nano-electromechanical systems (NEMS) come together in a new, promising way in single-molecule magnets.

Publications

• Vibrational detection and control of spin in mixed-valence molecular transistors. Phys. Rev. B 79, 075313 (2009)
  Reckermann, F., Leijnse, M. & Wegewijs, M. R.
  
• Electric field controlled magnetic anisotropy in a single molecule. Nano Lett. 10, 3307 (2010)
  Zyazin, A. S. et al.
  
• Interaction of spin and vibrations in transport through single-molecule magnets. Beilstein J. Nanotechnol. 2, 693 (2011)
  May, F., Wegewijs, M. R. & Hofstetter, W.