Cesar Lazo

Electrically tunable quantum anomalous Hall effect in graphene decorated by 5d transition-metal adatoms.

The combination of the unique properties of graphene with spin polarization and magnetism for the design of new spintronic concepts and devices has been hampered by the small Coulomb interaction and the tiny spin-orbit coupling of carbon in pristine graphene. Such device concepts would take advantage of the control of the spin degree of freedom utilizing the widely available electric fields in electronics or of topological transport mechanisms such as the conjectured quantum anomalous Hall effect. Here we show, using first-principles methods, that 5d transition-metal (TM) adatoms deposited on graphene display remarkable magnetic properties. All considered TM adatoms possess significant spin moments with colossal magnetocrystalline anisotropy energies as large as 50 meV per TM atom. We reveal that the magneto-electric response of deposited TM atoms is extremely strong and in some cases offers even the possibility to switch the spontaneous magnetization direction by a moderate external electric field. We predict that an electrically tunable quantum anomalous Hall effect can be observed in this type of hybrid materials.

Probing the Magnetic Exchange Forces of Iron on the Atomic Scale

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.

A theoretical study of the dynamical switching of a single spin by exchange forces

We demonstrate the possibility of dynamically switching the spin of a single atom or molecule with the magnetic tip of an atomic force microscope making use of the acting exchange forces. We choose single V-, Nb- and Ta-benzene molecules as model systems and calculate the exchange interaction with an Fe tip using density functional theory. The exchange energy displays a Bethe–Slater-type behavior with ferromagnetic coupling at large tip–sample distance and antiferromagnetic coupling at closer proximity. The exchange energies reach maximum values of a few tens of meV, which allows one to switch single spins by overcoming the energy barrier due to the magneto-crystalline anisotropy. The spin dynamics of the system was explored by solving the time-dependent Schrodinger equation with additional Landau–Lifshitz-like spin relaxation. We find that the distance dependence of the exchange interaction as well as the appearance of quantum tunneling results in different scenarios for the switching behavior, e.g. the tip can switch the adatom or lead to a stable superposition state with zero magnetization.

Spin valve effect in single-atom contacts

Magnetic single-atom contacts have been controllably fabricated with a scanning tunnelling microscope. A voltage-dependent spin valve effect with conductance variations of ≈40% is reproducibly observed from contacts comprising a Cr-covered tip and Co and Cr atoms on ferromagnetic nanoscale islands on W(110) with opposite magnetization. The spin-dependent conductances are interpreted from first-principles calculations in terms of the orbital character of the relevant electronic states of the junction.

Competing Forces during Contact Formation between a Tip and a Single Molecule.

Sn-phthalocyanine adsorbs on Ag(111) in a physisorbed or a chemisorbed configuration. Both structures are contacted with the tip of a combined scanning tunneling and atomic force microscope. The tunneling conductances of both configurations exhibit similar exponential variations with the tip-molecule distance. The short-range forces, however, display nontrivial distance dependencies. First-principles calculations reproduce the experimental results. Both attractive and repulsive interactions occur between the tip and different parts of the molecule due to a combination of bond formation and electrostatic interactions with the tip electric dipole. Consequently, deformations occur and the force varies in the resulting unexpected fashion.