Metastability of the Skyrmion Lattice in GdRu_{2}Si_{2} Revealed by Ultrasound.

We report on the low-temperature magnetoelastic properties of the skyrmion-lattice compound GdRu_{2}Si_{2} under magnetic fields applied along the [001] axis. Our sound-velocity measurements reveal a series of pronounced anomalies related to the skyrmion-lattice phase. The emergence of hysteresis and steplike features indicates intrinsic metastability of the skyrmion lattice in GdRu_{2}Si_{2}, deviating from other skyrmion-lattice compounds. Analysis of multiple elastic modes allows us to disentangle the compressional and shear responses, with the latter showing a stronger sensitivity to the metastability of the skyrmion lattice.