Reaction of europium and some transition metals deposited as ultrathin films

Abstract

Formation of rare earth (RE) and transition metal (TM) binary compounds, RExTMy, with different stoichiometry is well known [1]. Such compounds have been extensively studied during the last two or three decades, especially their magnetic properties. These compounds are interesting not only from a basic physics point of view but also from an applications standpoint. Certain important features occur in RExTMy thin films, such as large perpendicular magnetic anisotropy, a high Curie temperature, and a high coercive field makes these materials a promising candidate for potential applications in magneto-optical recording and/or permanent magnets used in on-wafer actuators and sensors. The aim of our work was to examine, over a wide range of Eu-TM (TM = Mn, Fe, Cr) concentrations, formation of ordered compounds or alloys in the form of thin films, and to determine their basic physical properties. A Molecular Beam Epitaxy systems equipped with XPS and RHEED, were used to prepare Eu-TM films. A series of 2-30nm thick Eu-TM films have been grown at room temperature by co-deposition or in multilayers form on MgO, GaAs or Si substrates with a 50 nm thick Mo buffer layer. We carried out electronic and crystallographic characterization of their properties using X-ray photoemission spectroscopy (XPS) and diffraction measurements. The magnetic and transport properties were investigated with the help of a SQUID magnetometer and a 4-point probe over a wide temperature range. Relative changes in the Eu 4rf/(Mn, Fe, Cr) 2p photoemission line area ratio, as well as chemical shifts of core levels monitored during the annealing process indicate mixing between europium and transition metals. The intermixing of the layers was also confirmed by the XRR measurements, where a formation of the uniform film from the point of view of electronic density was observed. The diffraction measurements indicate that the mixing of Eu-TM layers was the most efficient for the Eu/Mn system where a new Eu-Mn compound has been formed - EuMn2 exhibiting magnetic ordering at temperatures below 40K. Indication of formation of additional two intermetallic phases was found in the Eu-Mn system. The magnetic properties of Eu-Mn and Eu-Cr systems result mainly from the occurrence of divalent europium in samples, whereas in the case of the Eu-Fe system they are dominated by the presence of iron aggregated probably in nanoparticles.