  Based on first-principles calculations, we demonstrate that magnetism impedes
the formation of long chains in break junctions. We find a distinct softening
of the binding energy of atomic chains due to the creation of magnetic moments
that crucially reduces the probability of successful chain formation. Thereby,
we are able to explain the long standing puzzle why most of the
transition-metals do not assemble as long chains in break junctions and provide
thus an indirect evidence that in general suspended atomic chains in
transition-metal break junctions are magnetic.
