The tunnelling anisotropic magnetoresistance (TAMR) effect describes 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 mag
netic electrode, in contrast to the tunnelling magnetoresistance effect, it offers an attractive route towards alternative spintronics applications. In this work we consider the TAMR effect at the single-atom limit by investigating the anisotropy of the local density of states in the vacuum above transition-metal adatoms adsorbed on a noncollinear magnetic surface, the monolayer of Mn on W(110). This surface presents a cycloidal spin spiral ground state with an angle of 173$^circ$ between neighbouring spins and thus allows a quasi-continuous exploration of the angular dependence of the TAMR of adsorbed adatoms using scanning tunnelling microscopy. Using first-principles calculations, we investigate the TAMR of Co, Rh and Ir adatoms on Mn/W(110) and relate our results to magnetization direction dependent changes in the local density of states. 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.
