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

TL;DR

This study uses non-contact atomic force microscopy at cryogenic temperatures with specialized tips to resolve the spin order on NiO(001) surfaces, providing evidence for superexchange interactions and measuring exchange energies at the atomic scale.

Contribution

It demonstrates the use of SmCo-tips in AFM to detect spin-dependent contrasts and provides experimental evidence for superexchange on NiO(001), including quantification of exchange energy.

Findings

SmCo-tips produce a 1.35 pm height contrast for Ni spins.

Observation of a 0.5 pm contrast on oxygen atoms indicating superexchange.

Measured exchange energy of approximately 1 meV with an exponential decay length of 18 pm.

Abstract

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 this to the increased magnetocrystalline anisotropy energy of SmCo, which stabilizes the magnetic moment at the apex. Atomic force spectroscopy on the Ni up, Ni down 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 \lambda_exc = 18 pm.