Stabilizing isolated skyrmions at low magnetic fields exploiting vanishing magnetic anisotropy

Skyrmions are topologically protected non-collinear magnetic structures. Their stability and dynamics, arising from their topological character, have made them ideal information carriers e.g. in racetrack memories. The success of such a memory critically depends on the ability to stabilize and manipulate skyrmions at low magnetic fields. The driving force for skyrmion formation is the non-collinear Dzyaloshinskii-Moriya exchange interaction (DMI) originating from spin-orbit coupling (SOC). It competes with both the nearest neighbour Heisenberg exchange interaction and the magnetic anisotropy, which favour collinear states. While skyrmion lattices might evolve at vanishing magnetic fields, the formation of isolated skyrmions in ultra-thin films so far required the application of an external field which can be as high as several T. Here, we show that isolated skyrmions in a monolayer (ML) of Co epitaxially grown on a Ru(0001) substrate can be stabilized at magnetic fields as low as 100 mT. Even though SOC is weak in the 4d element Ru, a homochiral spin spiral ground state and isolated skyrmions could be detected and laterally resolved using a combination of tunneling and anisotropic tunneling magnetoresistance effect in spin-sensitive scanning tunneling microscopy (STM). Density functional theory (DFT) calculations confirm these chiral magnetic textures, even though the stabilizing DMI interaction is weak. We find that the key factor is the absence of magnetocristalline anisotropy in this system which enables non-collinear states to evolve in spite of weak SOC, opening up a wide choice of materials beyond 5d elements.