Epitaxial thin films of binary Eu-compounds close to a valence transition

Abstract

Intermetallic binary compounds of europium reveal a variety of interesting phenomena due to the interconnection between two different magnetic and 4f electronic (valence) states, which are particularly close in energy. The valence states or magnetic properties are thus particularly sensitive to strain-tuning in these materials. Consequently, we grew epitaxial EuPd$_2$ (magnetic Eu$^{2+}$) and EuPd$_3$ (nonmagnetic Eu$^{3+}$) thin films on MgO(001) substrates using molecular beam epitaxy. Ambient X-ray diffraction confirms an epitaxial relationship of cubic Laves-type (C15) EuPd$_2$ with an (111)-out-of-plane orientation, whereby eight distinct in-plane crystallographic domains develop. For simple cubic EuPd$_3$ two different out-of-plane orientations can be obtained by changing the substrate annealing temperature under ultra-high vacuum conditions from 600 °C to 1000 °C for one hour. A small resistance minimum evolves for EuPd$_3$ thin films grown with low temperature substrate annealing, which was previously found even in single crystals of EuPd$_3$ and might be attributed to a Kondo or weak localization effect. Absence of influence of an applied magnetic fields and magnetotransport measurements suggest a nonmagnetic ground state for EuPd$_3$ thin films, i. e., a purely trivalent Eu valence, as found in EuPd$_3$ single crystals. For EuPd$_2$ magnetic ordering below ~72 K is observed, quite similar to single crystal behavior. Field dependent measurements of the magnetoresistance and the Hall effect show hysteresis effects below ~0.4 T and an anomalous Hall effect below ~70 K, which saturates around 1.4 T, thus proving a ferromagnetic ground state of the divalent Eu.