  Measurements of magnetic properties at the atomic scale require probes
capable of combining high spatial resolution with spin sensitivity.
Spin-polarized scanning tunneling microscopy (SP-STM) fulfills these conditions
by using atomically sharp magnetic tips. The imaging of spin structures results
from the tunneling magneto-conductance that depends on the imbalance in the
local density of spin-up and spin-down electrons. Spin-sensitive tips are
generally formed from bulk materials or by coating non-magnetic tips with a
thin magnetic layer. However, ferromagnetic materials generate stray magnetic
fields which can influence the magnetic structure of the probed system, while
the magnetization of antiferromagnetic materials is difficult to set tip by
externally applied magnetic fields. Here, we use functionalized
Yu-Shiba-Rusinov (YSR) tips prepared by attaching magnetic adatoms at the apex
of a superconducting cluster to image magnetic interactions at the atomic
scale. We demonstrate that YSR tips are capable of sensing different
magnetization directions, conferring them full magnetic sensitivity. We
additionally show that the finite size of the tip superconducting cluster makes
it robust against relatively strong magnetic fields, making YSR tips capable of
visualizing magnetic field driven transitions of the spin texture.
