  By systematically investigating the relaxation behavior of a model metallic
glass based on the extensive molecular dynamics (MD) simulations combined with
the dynamic mechanical spectroscopy method, a pronounced ultra-low temperature
peak on the loss modulus spectrum was discovered for the first time in MD
simulations. It was found that the relation peak occurs at a much lower
temperature than the typical temperature for the conventional beta relation
peak as reported in the literature. According to the atomic displacement
analysis, we unravel that the reversible atomic motions, rather than the
thermal vibrations or local structural rearrangements, mainly contribute to
this relaxation peak. We further identify the atomic level mechanism of this
fast relaxation process by characterizing the local geometrical anisotropy.
Furthermore, by tracing the dynamic behaviors of these "reversible" atoms, we
demonstrate the intrinsic hierarchy of the local relaxation modes, which are
triggered by atomic vibrations and gradually developed to the reversible and
irreversible atomic movements. Our findings shed light on a general picture of
the relaxation processes in metallic glasses.
