  Controlled movement of nano-scale stable magnetic objects has been proposed
as the foundation for a new generation of magnetic storage devices. Magnetic
skyrmions, vortex-like spin textures stabilized by their topology are
particularly promising candidates for this technology. Their nanometric size
and ability to be displaced in response to an electrical current density
several orders of magnitude lower than required to induce motion of magnetic
domain walls suggest their potential for high-density memory devices that can
be operated at low power. However, to achieve this, skyrmion movement needs to
be controlled, where a key question concerns the coupling of skyrmions with the
underlying atomic lattice and disorder (pinning). Here, we use Resonant
Ultrasound Spectroscopy (RUS), a probe highly sensitive to changes in the
elastic properties, to shed new light on skyrmion elasticity and depinning in
the archetypal skyrmion material MnSi. In MnSi, skyrmions form a lattice that
leads to pronounced changes in the elastic properties of the atomic lattice as
a result of magneto-crystalline coupling. Without an applied current, the shear
and compressional moduli of the underlying crystal lattice exhibit an abrupt
change in the field-temperature range where skyrmions form. For current
densities exceeding $j_c^*$ the changes of elastic properties vanish, signaling
the decoupling of skyrmion and atomic lattices. Interestingly, $j_c^*$, which
we identify as the onset of skyrmion depinning, is about 20 times smaller than
$j_c$ previously measured via non-linear Hall effect. Our results suggest the
presence of a previously-undetected intermediate dynamic regime possibly
dominated by skyrmion-creep motion with important consequences for potential
applications.
