  The quantum mechanical screening of a spin via conduction electrons depends
sensitively on the environment seen by the magnetic impurity. A high degree of
responsiveness can be obtained with metal complexes, as the embedding of a
metal ion into an organic molecule prevents intercalation or alloying and
allows for a good control by an appropriate choice of the ligands. There are
therefore hopes to reach an "on demand" control of the spin state of single
molecules adsorbed on substrates. Hitherto one route was to rely on
"switchable" molecules with intrinsic bistabilities triggered by external
stimuli, such as temperature or light, or on the controlled dosing of chemicals
to form reversible bonds. However, these methods constrain the functionality to
switchable molecules or depend on access to atoms or molecules. Here, we
present a way to induce bistability also in a planar molecule by making use of
the environment. We found that the particular "habitat" offered by an antiphase
boundary of the Rashba system BiAg$_2$ stabilizes a second structure for
manganese phthalocyanine molecules, in which the central Mn ion moves out of
the molecular plane. This corresponds to the formation of a large magnetic
moment and a concomitant change of the ground state with respect to the
conventional adsorption site. The reversible spin switch found here shows how
we can not only rearrange electronic levels or lift orbital degeneracies via
the substrate, but even sway the effects of many-body interactions in single
molecules by acting on their surrounding.
