Magnetism in EuAlSi and the Eu_1−xSr_xAlSi solid solution

The magnetic properties of EuAlSi, a compound comprising a honeycomb lattice of Al/Si atoms and a triangular lattice of Eu atoms, are presented. By means of single-crystal x-ray diffraction, we find that EuAlSi crystallizes in an AlB₂-type structure with space group P6/mmm and unit cell parameters a=4.2229(10)Å and c=4.5268(12)Å. Our magnetic measurements indicate that EuAlSi is a soft ferromagnetic material with T_Curie=25.8K. The susceptibility follows the Curie-Weiss law at high temperatures, which allowed us to determine the paramagnetic Curie temperature θ_P=36.2(1)K and an effective magnetic moment μ_eff=8.07(1)µ_B/Eu. This value is in agreement with the theoretical value of 7.9µ_B for Eu2+ free ion. Moreover, we have prepared the Eu_1−xSr_xAlSi solid solution, where the atoms in the triangular lattice were systematically exchanged, in order to study the evolution of the collective quantum properties from the ferromagnetic EuAlSi toward the superconducting SrAlSi. Across the Eu_1−xSr_xAlSi solid solution, the unit cell parameters change linearly, following Vegard’s law, and making the system reliable for studying composition dependence of the interplay between the crystal structure and physical properties. As the Sr content increases, i.e., x increases, we note a consistent reduction of μ_eff and T_Curie. Long-range magnetic order in Eu_1−xSr_xAlSi persists up to x=0.95, whereas superconductivity is only observed for samples with x>0.97.