  Pt is known to show spontaneous formation of monatomic chains upon breaking a
metallic contact. From model calculations, these chains are expected to be spin
polarized. However, direct experimental evidence for or against magnetism is
lacking. Here, we investigate shot noise as a potential source of information
on the magnetic state of Pt atomic chains. We observe a remarkable structure in
the distribution of measured shot-noise levels, where the data appear to be
confined to the region of nonmagnetic states. While this suggests a nonmagnetic
ground state for the Pt atomic chains, from calculations we find that the
magnetism in Pt chains is due to 'actor' electron channels, which contribute
very little to ballistic conductance and noise. On the other hand, there are
weakly polarized 'spectator' channels, which carry most of the current and are
only slightly modified by the magnetic state.
