We report a transverse conical spin spiral as the magnetic ground state of a double-layer Mn on a W(110) surface. Using spin-polarized scanning tunneling microscopy, we find a long-range modulation along the [001] direction with a periodicity of 2.4 nm coexisting with a local row-wise antiferromagnetic contrast. First-principles calculations reveal a transverse conical spin-spiral ground state of this system which explains the observed magnetic contrast. The canting of the spins is induced by higher-order exchange interactions, while the spiraling along the [001] direction is due to frustrated Heisenberg exchange and Dzyaloshinskii-Moriya interaction.

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The tunneling anisotropic magnetoresistance (TAMR) of single Co atoms adsorbed on a double-layer Fe film on W(110) is observed by scanning tunneling spectroscopy. Without applying an external magnetic field the TAMR is found by comparing spectra of atoms that are adsorbed on the domains and domain walls of the Fe film. The TAMR can be as large as 12% and repeatedly changes sign as a function of bias voltage. First-principles calculations show that the hybridization between Co d states of different orbital symmetries depends on the magnetization direction via spin-orbit coupling. This leads to an anisotropy of the density of states and thus induces a TAMR.

Using spin-polarized scanning tunneling microscopy ͑SP-STM͒, we observe spin frustration in one monolayer Cr on the triangular lattice of the Pd͑111͒ surface. Our STM measurements demonstrate pseudomorphic growth of the first Cr layer on Pd͑111͒ without intermixing. Using SP-STM in the constant current mode, we observe three different types of images depending on the tip magnetization indicative of a noncollinear ͑ ͱ 3 ϫ ͱ 3͒ magnetic superstructure with an angle of 120°between moments of nearest-neighbor Cr atoms. The 120°Néel ground state of Cr/Pd͑111͒ and the SP-STM images are explained based on our first-principles calculations.The rich magnetic phase space of chromium has fascinated scientists for many years 1,2 and continues to challenge our theoretical understanding of itinerant magnetism. In its bulk bcc phase, Cr exhibits an intriguing antiferromagnetic spin-density wave due to Fermi-surface nesting. At the surface, the magnetic moments are enhanced and topological antiferromagnetism has been proposed for Cr͑001͒, 3 i.e., antiferromagnetic coupling of adjacent layers with ferromagnetic ͑FM͒ alignment within each layer and experimentally confirmed. 4,5 Similarly, FM order within a monolayer ͑ML͒ of Cr on W͑001͒ has been predicted 6 while recent experiments have reported local-antiferromagnetic order in a Cr monolayer on W͑110͒ ͑Ref. 7͒ showing the crucial impact of surface orientation. On a triangular lattice or on a ferromagnetic surface, Cr nanostructures are a classical example of frustrated antiferromagnets. This triggered many theoretical studies reporting noncollinear magnetic ground states for clusters of a few atoms 8-10 or for ultrathin films. 11-13 While the absence of the Kondo effect in Cr trimers on Au͑111͒ ͑Ref. 14͒ is consistent with this picture, no direct observation of spin frustration in such systems has so far been reported.Here, we apply spin-polarized scanning tunneling microscopy ͑SP-STM͒ to prove the 120°Néel ground state of a Cr monolayer on the triangular lattice provided by the Pd͑111͒ surface. Our STM measurements show pseudomorphic growth of the first Cr layer without intermixing. By using SP-STM we find three different types of images depending on the tip magnetization providing a real space observation of the noncollinear ground state. We find a reversal of the magnetic contrast due to switching of the tip's magnetization further proving the magnetic origin of the contrast. Our firstprinciples calculations confirm the Néel state of the Cr monolayer on Pd͑111͒ due to a dominating antiferromagnetic nearest-neighbor exchange interaction. By simulating STM and SP-STM images we can explain the experimentally observed contrasts and obtain corrugation amplitudes in excellent agreement with the measured values.In the classical Heisenberg model with only nearestneighbor exchange interaction, an antiferromagnet on a triangular lattice exhibits a 120°Néel ground state, cf. Fig. 1͑a͒. However, for itinerant antiferromagnets such as Cr, exchange interactions beyond nearest neighbors nee...

The tunneling anisotropic magnetoresistance (TAMR) effect demonstrates the sensitivity of spin-polarized electron transport to the orientation of the magnetization with respect to the crystallographic axes. As the TAMR effect requires only a single magnetic electrode, in contrast to the tunneling magnetoresistance effect, it offers an attractive route to alternative spintronic applications. In this work we consider the TAMR effect at the single-atom limit by investigating the anisotropy of the local density of states (LDOS) in the vacuum above transition-metal adatoms adsorbed on a noncollinear magnetic surface, the monolayer of Mn on W(1 1 0). This surface presents a cycloidal spin spiral ground state with an angle of 173° between neighboring spins and thus allows a quasi-continuous exploration of the angular dependence of the TAMR of adsorbed adatoms using scanning tunneling microscopy. Using first-principle calculations, we investigate the TAMR of Co, Rh and Ir adatoms on Mn/W(1 1 0) and relate our results to the magnetization-direction-dependent changes in the LDOS. The anisotropic effect is found to be enhanced dramatically on the adsorption of heavy transition-metal atoms, with values of up to 50% predicted from our calculations. This effect will be measurable even with a non-magnetic STM tip.