Low-Temperature Anomalies and Ferromagnetism ofEuB6

Measurements of the specific heat $C_p\left(T\right)$ of single-crystalline ${\mathrm{EuB}}_6$ reveal that the magnetically ordered ground state of this compound is reached via two consecutive phase transitions with onsets at 16 and 14 K, respectively. These transitions provoke a substantial change in the IR spectral range of the electronic excitation spectrum as evidenced by measurements of the optical reflectivity over a wide spectral range and at temperatures between 6 K and room temperature. The data analysis suggests that the magnetic ordering of ${\mathrm{EuB}}_6$ inflicts a highly unusual alteration of its electronic excitation spectrum.