STRUCTURE AND MAGNETIC ORDER OF EUB6

We present a study of the structural and magnetic properties of single -crystalline EuB6. Temperature-dependent x-ray diffraction found no si gnificant anomalies at the onset of ferromagnetic order. The resistivi ty, de susceptibility, magnetization, and specific heat prove the crys tal to be of extraordinarily high quality. Two ferromagnetic transitio ns at T-cl=15.3 K and T-c2=12.5 K are observed in all investigated pro perties. The ordered state displays an unexpected anisotropy, and we d erive the magnetic phase diagrams for the three principal cubic direct ions. We argue that the two magnetic phases are connected by spin reor ientation, enabled by a reduction of the crystalline symmetry from cub ic. An additional anomaly in the specific heat is observed at low temp eratures, which we interpret as arising from the splitting of tile Eu ground-state multiplet in internal magnetic fields.