Searching atomic spin contrast on nickel oxide (001) by force microscopy

The (001) surface of NiO, an antiferromagnet at room temperature, was investigated under ultrahigh vacuum conditions with frequency modulation atomic force microscopy. The antiferromagnetic coupling between ions leads to a spin superstructure on (001) surfaces. Exchange interaction between the probe of a force microscope and the NiO (001) surface should allow us to image spin superstructures in real space. The surface was imaged with three different probing tips: nonmagnetic W tips, ferromagnetic Co tips, and antiferromagnetic NiO tips—and atomic resolution was achieved with all three of them in various distance regimes and in several channels. Atomic resolution is obtained with all tips, but evidence for spin contrast is lacking although oscillation amplitudes in the angstrom regime have been used, where optimal signal-to-noise ratio is expected.

Figure 1

(Color online) NiO structure (spins located at the nickel sites): the top view onto the (001) surface shows ferromagnetic rows in $\left[1\overline{1}0\right]$ direction which couple antiferromagnetically along the [110] direction.

Figure 2

(Color online) Large scale step structure on NiO(001) revealed with FM-AFM equipped with a NiO tip ($A\approx 2\mathrm{\AA }$, $\Delta f=+15\mathrm{Hz}$). Between wide flat terraces few screw dislocations are visible, such as the one indicated by a white arrow.

Figure 3

(Color online) qPlus sensors with tungsten (top), cobalt (bottom left), and nickel oxide (bottom right) tips as shown in (a) allow FM-AFM with atomic resolution on NiO(001) surfaces; imaging parameters: $A\approx 1\mathrm{\AA }$ and (b) $\Delta f=-20\mathrm{Hz}$ (W tip), (c) $\Delta f=-23\mathrm{Hz}$ (Co tip), and (d) $\Delta f=-25\mathrm{Hz}$ (NiO tip).

Figure 4

(Color online) Atomic resolution on NiO(001) obtained with frequency modulation (FM) AFM in the repulsive mode. The images were taken with a NiO tip oscillating with $A\approx 1\mathrm{\AA }$ at $\Delta f=+66\mathrm{Hz}$. (a) is a topographical picture whereas (b) presents the damping signals.

Figure 5

(Color online) (a) FM-AFM image of a NiO(001) surface taken with a NiO tip at $A\approx 1\mathrm{\AA }$ and $\Delta f=-98\mathrm{Hz}$ (unfiltered data). The presence of the two defects in the upper right and in the lower left corner shows that true atomic resolution is obtained, i.e., a single tip atom is responsible for imaging. The inset shows the central section of the Fourier transform of the topographical image. A peak at half the spatial frequency of one of the two base peaks would be visible if the contribution of the exchange interaction was larger than instrumental noise (see text). The inset also shows the Fourier transformed image (see text). However, we did not observe a distinguished peak at half the inverse lattice vectors as in Ref. 16. The data presented in (a) and (b) were taken within the same measurement session, a tip change indicated by a glitch and an overall contrast change occurred.

Figure 6

(Color online) (a) Sketch illustrating the effect of a misalignment between tip and sample spin; (b) comparison of density of spin-polarized states of the NiO surface and a metal tip revealing the improbability of an interaction; (c) sketch of a Ni front atom located above an O sample atom such that spin order is preserved from tip to sample; (d) a Ni front atom also sits on top of an O sample atom, but the spin order is broken.