Rhombic skyrmion lattice coupled with orthorhombic structural distortion in EuAl4

The centrosymmetric tetragonal itinerant magnet EuAl4 exhibits an intricate magnetic phase diagram including rhombic and square skyrmion-lattice (SkL) phases in the external magnetic field. Here, we report a multi-axis dilatometric investigation of EuAl4 by means of a newly designed fiber-Bragg-grating technique complemented by a resonant x-ray scattering experiment, revealing anisotropic magnetostriction and magnetovolume effect associated with successive phase transitions. The rhombic and square SkL phases are found to possess similar to 0.10% and similar to 0.03% orthorhombic structural distortion within the ab plane, respectively. We propose that the coupling between the spin system and the lattice deformation should be essential for the structural instability in EuAl4, yielding a rich variety of topological spin textures with spontaneous rotational-symmetry breaking as well as a potential controllability of the SkL phases by uniaxial stress or pressure.