Structure and magnetic order of ${\mathrm{EuB}}_6$

We present a study of the structural and magnetic properties of single-crystalline ${\mathrm{EuB}}_6$. Temperature-dependent x-ray diffraction found no significant anomalies at the onset of ferromagnetic order. The resistivity, dc susceptibility, magnetization, and specific heat prove the crystal to be of extraordinarily high quality. Two ferromagnetic transitions at $T_{c1\mathrm{}}=15.3\mathrm{}$ K and $T_{c2\mathrm{}}=12.5\mathrm{}$ K are observed in all investigated properties. The ordered state displays an unexpected anisotropy, and we derive the magnetic phase diagrams for the three principal cubic directions. We argue that the two magnetic phases are connected by spin reorientation, enabled by a reduction of the crystalline symmetry from cubic. An additional anomaly in the specific heat is observed at low temperatures, which we interpret as arising from the splitting of the Eu ground-state multiplet in internal magnetic fields.