  Magnetic impurities on superconductors induce discrete bound levels inside
the superconducting gap, known as Yu-Shiba-Rusinov (YSR) states. YSR levels are
fully spin-polarized such that the tunneling between YSR states depends on
their relative spin orientation. Here, we use scanning tunneling spectroscopy
to resolve the spin dynamics in the tunneling process between two YSR states by
experimentally extracting the angle between the spins. To this end, we exploit
the ratio of thermally activated and direct spectral features in the
measurement to directly extract the relative spin orientation between the two
YSR states. We find freely rotating spins down to 7mK, indicating a purely
paramagnetic nature of the impurities. Such a non-collinear spin alignment is
essential not only for producing Majorana bound states but also as an outlook
manipulating and moving the Majorana state onto the tip.
