Relationship Between Stoichiometry and Properties of EuO Films

Abstract

The preparation of EuO films by co‐evaporation of Eu and Eu₂O₃ has been studied. The two most important parameters to control are the ratio r=Eu/Eu₂O₃ and the background pressure. The latter should be in the 10⁻⁶‐Torr range, and only by starting with high density (>95%) Eu₂O₃ is it possible to maintain this. As r increases from 0.8 to 1.4 we observe the following: (1) A large increase in T_c is observed from 51° to 117°K; (2) the absorption a increases from 0.85×10⁵/cm to 1.6×10⁵/cm and the peak shifts toward the red (from 5900 to 6000 Å); (3) the longitudinal Faraday rotation 2φ (with 25° incident angle at 6328 Å) increases from 0.9×10⁵ deg/cm to a maximum of 3.7×10⁵ deg/cm, followed by a slight decrease; and (4) H_c (10°K) decreases sharply from 140 Oe to a minimum of 43 Oe for r=0.94, followed by a slight increase. The best films which were single‐phase stoichiometric EuO, with bulk‐like properties, are obtained with r=0.94. These have well‐defined x‐ray diffraction patterns with lattice constants a₀=5.14 Å, T_c of 71°K, and the normal Faraday rotation (at 20°K with H=20 kOe) of 8.5×10⁵ deg/cm. In films with r>0.9 the increase of T_c is attributed to the presence of Eu‐metal which contributes conduction electrons to enhance the Eu²⁺–Eu²⁺ interaction. For ra₀=5.10 are obtained which have T_c=51°K. We propose a model where the Eu²⁺–Eu²⁺ exchange interaction is weakened by the formation of a metastable solution of Eu³⁺ in the EuO lattice. These new results will be compared with previously published data.