Ferromagnetic resonance with Scanning Tunneling Microscopy

• Applicant: Professor Dr. Wulf Wulfhekel
• Subject Area: Experimental Condensed Matter Physics
• Term: from 2017 to 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 366208634

• Final Report Year: 2020

Final Report Abstract

We modified our 4 K STM setup to apply rf-voltages of up to 12 GHz to the tip of the STM and automated the determination of the transmission of the rf-voltage from the source to the tunneling junction. By adjusting the rf-amplitude at the source, we can compensate for transmission losses and could achieve a constant rf-amplitude at the junction on the level of few percent over the entire frequency range of the instrument. We studied the gyration of magnetic vortices in Fe islands on W(110) and found a clear resonance, which agrees in amplitude with detailed micromagnetic calculation. The resonance frequency in the experiment was, however, significantly higher than that expected from the calculations possible due to the lack of interface and surface anisotropies as well as strain effects in the simulations. Further, we managed to switch locally small domains in the skyrmion lattice of Fe/Ir(111) using a strong rf-excitation. The switching rate shows a clear frequency dependence with a broad maximum around 0.5 GHz which coincides with our previous magnetic resonance measurements with low rf-excitation showing a resonance at 0.615 GHz. Finally, we found a unique materials system of Co/Ru(0001), which can host isolated skyrmions at vanishing fields, and which does not rely on 5d metals. Here, the DM interaction is weak but also is the anisotropy such that non-collinear spin structures are stabilized. Due to a relatively large TAMR effect, these can be imaged with conventional STM avoiding the magnetic dipole interaction of a magnetic tip with the sample. The large TAMR is due to a spin-polarized surface state experiencing the spin-orbit interaction at the surface. Ultimately, we studied skyrmion annihilation by magnetic fields.

Publications

• Large tunneling anisotropic magnetoresistance mediated by surface states, Phys. Rev. B. 97, 220406R (2018)
  M. Hervé, T. Balashov, A. Ernst, W. Wulfhekel
  
• Stabilizing spin spirals and isolated skyrmions at low magnetic field exploiting vanishing magnetic anisotropy, Nature Communications 9, 1015 (2018)
  M. Hervé, B. Dupé, R. Lopes, M. Böttcher, M. D. Martins, T. Balashov, L. Gerhard, J. Sinova, W. Wulfhekel
  
• Towards Laterally Resolved Ferromagnetic Resonance with Spin-Polarized Scanning Tunneling Microscopy, Nanomaterials 9, 827 (2019)
  M. Hervé, M. Peter, T. Balashov, W. Wulfhekel
  
• Epitaxial growth of Co on stepped Ru(0001): Stabilization of CoRu magnetic surface alloy, Surface Science 692, 121512 (2020)
  R. Lopes, M. Hervé, L. Gerhard, W. Wulfhekel, M. D. Martins
  
• Instability of skyrmions in magnetic fields, Applied Physics Letters 116, 262406 (2020)
  L. Mougel, P. M. Buhl, R. Nemoto, T. Balashov, M. Hervé, J. Skolaut, T. K. Yamada, B. Dupé, W. Wulfhekel