Clamping effect on temperature-induced valence transition in epitaxial EuPd2_2Si2_2 thin films grown on MgO(001)

Bulk EuPd$_2$Si$_2$ show a temperature-driven valence transition of europium from ∼+2 above 200 K to ∼+3 below 100 K, which is correlated with a shrinking by approximately 2% of the tetragonal crystal lattice along the two a-axes. Due to this interconnection between lattice and electronic degrees of freedom the influence of strain in epitaxial thin films is particularly interesting. Ambient x-ray diffraction (XRD) confirms an epitaxial relationship of tetragonal EuPd$_2$Si$_2$ on MgO(001) with an out-of plane c-axis orientation for the thin film, whereby the $a$-axes of both lattices align. XRD at low temperatures reveals a strong coupling of the thin film lattice to the substrate, showing no abrupt compression over the temperature range from 300 to 10 K. Hard x-ray photoelectron spectroscopy at 300 and 20 K reveals a temperature-independent valence of +2.0 for Eu. The evolving biaxial tensile strain upon cooling is suggested to suppress the valence transition. Instead temperature-dependent transport measurements of the resistivity and the Hall effect in a magnetic field up to 5 T point to a film thickness independent phase transition at 16 to 20 K, indicating magnetic ordering