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 EuPd2 (Eu2+, high magnetic moment) and EuPd3 (Eu3+, vanishing magnetic moment) thin films on MgO(001) substrates using molecular beam epitaxy. Ambient X-ray diffraction confirms an epitaxial relationship of cubic Laves-type (C15) EuPd2 with an [111]-out-of-plane orientation, whereby eight distinct in-plane crystallographic domains develop. For simple cubic EuPd3 two different out-of-plane orientations can be obtained by changing the substrate annealing temperature under ultra-high vacuum conditions from 873K to 1273K for one hour. A small resistance minimum evolves for EuPd3 thin films grown with low temperature substrate annealing, which was previously found even in single crystals of EuPd3 and might be attributed to a Kondo or weak localization effect. Absence of influence of applied magnetic fields and magnetotransport measurements suggest a purely trivalent Eu valence for EuPd3 thin films, as found in EuPd3 single crystals. For EuPd2 magnetic ordering below ∼72K is observed, quite similar to single crystal behavior. Field dependent measurements of the magnetoresistance and the Hall effect show hysteresis effects below ∼0.4T and an anomalous Hall effect below ∼70K, which saturates around 1.4T, thus proving a ferromagnetic ground state of the divalent Eu.