  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.
