Magnetic Properties of EuS/Co Multilayers on KCl and BaF₂ Substrates

Abstract

EuS/Co multilayers are hybrid structures composed of a model ferromagnetic semiconductor EuS (Curie temperature Tc=16.5 K) and a ferromagnetic transition metal Co. Recent investigations of these multilayers revealed variety of important spintronic effects, such as an antiferromagnetic interlayer coupling in EuS/Co bilayers [1] and a very efficient spin filtering by EuS electron barrier in Al/EuS/Co tunneling structures [2]. These effects are expected to originate from the exchange interactions across EuSCo interface. In previous studies, polycrystalline or amorphous EuS/Co multilayers were grown on glass or on Si substrate with Pt/Co metallic buffer layer. In this work, we exploit the technological method developed for the epitaxial growth of EuS/PbS multilayers on KCl and BaF2 substrates, in order to study the magnetic properties of EuS/Co multilayers with monocrystalline EuS layer. EuS/Co/EuS trilayers and Co/EuS bilayers were grown on cleaved KCl (001) and BaF2 (111) substrates using high vacuum deposition technique employing electron guns for Co and EuS. Epitaxial 20-60 nm thick PbS buffer layer was deposited on the substrates from heated tungsten boat. The layer thickness covered the range 35-55 A for EuS and 40-250 A for Co. The structural properties of the layers were examined by xray diffraction method revealing monocrystalline PbS buffer and bottom EuS layer. Co and top EuS layers are expected to be polycrystalline. Magnetic properties of the layers were examined using magneto-optical Kerr (MOKE) and superconductor (SQUID) magnetometry techniques. In the case of MOKE experiments, the linearly polarized light from He-Ne laser was used in longitudinal MOKE geometry. Measurements of magnetic hysteresis loops were performed in temperature range T = 4-250 K and in external magnetic fields up to 2 kOe applied in the plane of the multilayer. All the investigated multilayers showed ferromagnetic properties at room temperature due to Co layer with the ferromagnetic transition in EuS layer clearly marked upon cooling below 16 K. In EuS/Co/EuS trilayers grown on KCl substrate the antiferromagnetic alignment of magnetization vectors of Co and EuS layers was experimentally observed as a characteristic low field plateau on magnetic hysteresis loops and a decrease of multilayer magnetization below 16 K. In contrast, no interlayer coupling was found in multilayers grown on BaF2 substrate. In Co/EuS bilayers, the characteristic temperature dependent shift of the magnetic hysteresis loops was found indicating the well known exchange bias effect. We assign this experimental finding to the exchange coupling between Co layer and an ultrathin (about 2 nm) top antiferromagnetic CoO layer known to spontaneously form on Co layers in the air.