  One dimensional systems strongly enhance the quantum character of electron
transport. Such systems can be realized in 5d transition metals Au, Pt and Ir,
in the form of suspended monatomic chains between bulk leads. Atomic chains
between ferromagnetic leads would open up many perspectives in the context of
spin-dependent transport and spintronics, but the evidence suggests that for
pure metals only the mentioned three 5d metals are susceptible to chain
formation. It has been argued that the stability of atomic chains made up from
ferromagnetic metals is compromised by the same exchange interaction that
produces the local moments. Here we demonstrate that magnetic atomic chains can
be induced to form in break junctions under the influence of light molecules.
Explicitly, we find deuterium assisted chain formation in the 3d ferromagnetic
transition metals Fe and Ni. Chain lengths up to eight atoms are formed upon
stretching the ferromagnetic atomic contact in deuterium atmosphere at
cryogenic temperatures. From differential conductance spectra vibronic states
of D$_2$ can be identified, confirming the presence of deuterium in the atomic
chains.
