title: Spin resolution and evidence for superexchange on NiO(001) observed by force microscopy.
abstract: The spin order of the nickel oxide (001) surface is resolved, employing noncontact 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 2×1 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.affiliation: Institute of Experimental and Applied Physics, University of Regensburg, D-93053 Regensburg, Germany. florian1.pielmeier@physik.uni-regensburg.de
authors.forename: Florian
authors.initials: F
authors.lastname: Pielmeier
authors.forename: Franz J
authors.initials: FJ
authors.lastname: Giessibl