  Interactions between the different degrees of freedom form the basis of many
manifestations of intriguing physics in condensed matter. In this respect,
quantifying the dynamics of normal modes that themselves arise from these
interactions and how they interact with other excitations is of central
importance. Of the different types of coupling that are often important,
spin-lattice coupling is relevant to several sub-fields of condensed matter
physics; examples include spintronics, high-TC superconductivity, and
topological materials. While theories of materials where spin-lattice coupling
is relevant can sometimes be used to infer the magnitude and character of this
interaction, experimental approaches that can directly measure it are rare and
incomplete. Here we use time-resolved X-ray diffraction to directly access the
spin-lattice coupling by measuring both ultrafast atomic motion and the
associated spin dynamics following the excitation of a coherent electromagnon
by an intense THz pulse in a multiferroic hexaferrite. Comparing the dynamics
of the two different components, one striking outcome is the different phase
shifts relative to the driving field. This phase shift provides insight into
the excitation process of such a coupled mode. This direct observation of
combined lattice and magnetization dynamics paves the way to access the
mode-selective spin-lattice coupling strength, which remains a missing
fundamental parameter for ultrafast control of magnetism and is relevant to a
wide variety of correlated electron physics.
