Valence instability and collapse of ferromagnetism in EuB 6 at high pressures

Despite the simplicity of their cubic crystal lattice, rare-earth hexaborides display complex physical properties including a (long debated) onset of metallization via magnetic polaron formation at T c 1 ≈ 15 K preceding ferromagnetic ordering at T c 2 ≈ 12 K. In this work, we used applied pressure to tune the interplay between electronic structure and magnetism in EuB 6 . We probed the magnetism, valence, and structure of EuB 6 under quasi-hydrostatic pressures up to 30 GPa using X-ray techniques. Our findings show evidence for collapse of ferromagnetism above 20 GPa following a monotonic increase of mean Eu valence. While X-ray diffraction measurements in the paramagnetic state at room temperature show that the lattice retains cubic symmetry, a measurable quadrupole interaction seen by time-domain synchrotron Mössbauer spectroscopy suggests a lowering of symmetry associated with magnetic ordering, becoming more prominent across the magnetic transition. The interplay between conduction band electron count and magnetism observed under applied pressure in EuB 6 opens possibilities for fine-tuning metallization and magnetic properties of similar Eu-based semi-metal systems.