Cesar Lazo

Based on first-principles calculations, we predict that 5d transition metals on graphene present a unique class of hybrid systems exhibiting topological transport effects that can be manipulated effectively by external electric fields. The origin of this phenomenon lies in the exceptional magnetic properties and the large spin-orbit interaction of the 5d metals leading to significant magnetic moments accompanied with colossal magnetocrystalline anisotropy energies. A strong magnetoelectric response is predicted that offers the possibility to switch the spontaneous magnetization direction by moderate electric fields, enabling an electrically tunable quantum anomalous Hall effect.

We demonstrate that magnetic exchange force spectroscopy allows for a quantitative determination of the distance-dependent magnetic exchange interaction across a vacuum gap. Experiments were performed on the antiferromagnetic Fe monolayer on W(001) with magnetically sensitive tips and compared to first-principles calculations performed for different cluster tip models. For stable tips, which can be distinguished from unstable tips by analyzing the dissipation signal, very good agreement with theory is observed.

Applying magnetic exchange force microscopy with an Fe-coated tip, we experimentally resolve the atomic-scale antiferromagnetic structure of the Fe monolayer on W(001). On the basis of first-principles calculations, using an Fe nanocluster as a tip, we determine the distance dependence of the magnetic exchange forces. Significant relaxation of tip and sample atoms occurs, which depend sensitively on the local magnetic configuration. This shifts the onset of magnetic interactions toward larger separations and facilitates their observation. Implementing a multiatom tip in the calculations and accounting for relaxation effects are crucial to obtain the correct sign and distance dependence of the magnetic exchange interaction. By comparison with our calculations, we show that the experimentally observed contrast is due to a competition between chemical and magnetic forces.

Using first-principles calculations, we demonstrate that the vacuum spin polarization of commonly used Fe-coated scanning tunneling microscopy (STM) tips is positive at the Fermi energy---opposite to that of Fe surfaces---and is often lower than expected from magnetic thin films. We consider single Fe atoms and pyramids of five Fe atoms on Fe (001) and (110) surfaces as models of STM tips. While the spin polarization of the local density of states (LDOS) at the apex atom of all considered tips is negative close to the Fermi energy and dominated by minority $d$ electrons, the spin polarization of the vacuum LDOS, crucial for the tunneling current, is positive and controlled by majority states of $sp$ character. These states derive from the atomic $4s$ and $4p$ orbitals and provide a large spillout of charge density into the vacuum. If we replace the Fe apex atom by a Cr, Mn, or Co atom, the vacuum spin polarization remains positive at the Fermi energy, and it is much enhanced for Cr or Mn in the favorable antiferromagnetic spin alignment with respect to the Fe tip body. At energies above the Fermi level, the spin polarization can change sign due to the contribution from antibonding minority $d$ states. Single Mn and Fe atoms on a nonmagnetic tip provided, for example, by a Cu(001) surface display a similar vacuum LDOS with a small positive spin polarization in good agreement with recent experimental findings. For Cr-coated tips, we observe that the spin polarization can display a change in sign very close to the Fermi energy which can complicate the interpretation of the measured asymmetry in spin-polarized tunneling spectroscopy.