Spin Resolution and Evidence for Superexchange on NiO(001) observed by Force Microscopy

ROM 2013-9
Author: Florian Pielmeier // florian1.pielmeier@ur.de
Institute: Institute of Experimental and Applied Physics, University of Regensburg, Regensburg, Germany
Publication: Physical Review Letters 110, 266101 (2013) // http://prl.aps.org/abstract/PRL/v110/i26/e266101
Instrument: LT STM

The spin order of the nickel oxide (001) surface is resolved, employing non-contact atomic force microscopy at 4.4 K using bulk Fe- and SmCo-tips mounted on a qPlus sensor that oscillates at sub- 50 pm amplitudes. The spin-dependent signal is hardly detectable with Fe-tips. In contrast, SmCo-tips yield a height contrast of 1.35 pm for Ni ions with opposite spins. SmCo tips even show a small height contrast on the O atoms of 0.5 pm within the 2x1 spin unit cell, pointing to the observation of superexchange. We attribute the increased signal-to-noise ratio to the increased magnetocrystalline anisotropy energy of SmCo, which stabilizes the magnetic moment at the apex. Atomic force spectroscopy on the Ni ↑, Ni ↓ and O lattice site reveals a magnitude of the exchange energy of merely 1 meV at the closest accessible distance with an exponential decay length of λexc = 18 pm.

Authors: Florian Pielmeier, Franz J. Giessibl