X-ray characterization of buried layers and interfaces in EuO based spin-filter tunnel junctions

Abstract

Spin filtering tunnel junctions provide a convenient method for controlling and manipulating the spin of electrons in spintronic devices. In a tunnel junction, the tunneling current depends exponentially on the thickness of the tunneling barrier. Ultrathin (~20 angstroms) barriers are highly desirable because they lower the junction resistance. In addition to the barrier thickness, the spin filtering efficiency of the junction depends on the tunneling barrier's magnetic properties and the nature of the barrier-electrode interfaces. EuO's high TC (69 K) and large exchange splitting (2 ΔE = 0.54 eV) in the conduction band make it a material of choice for achieving a spin-polarized tunneling current. We examined the properties of ultrathin EuO films of different thicknesses with different electrodes. The chemical and magnetic characterization of buried layers and buried interfaces was done using interface sensitive x-ray techniques with element specificity and magnetic contrast. Of the many materials studied, we found that yttrium overlayers are the best electrodes to protect the EuO barrier and prevent formation of Eu2O3, a stable nonmagnetic oxide, at the top (EuO-electrode) interface. We examined the electrode-EuO interfaces of EuO films with Cu bottom electrode deposited on Cr/Cu buffer layer. We found that the roughness of the top and bottom interfaces (above and below the EuO layer) is random and nonconformal in nature. EuO films grown on Cr/Cu buffer layer have smoother top interfaces compared to those deposited directly on Si substrate or Al bottom electrode. Moreover, compared to the Cr/Cu or Cu/EuO interfaces, the top (EuO-electrode) interface is smoother. Here we present results from chemical and magnetic characterization of ultrathin EuO film and the impact of reduced barrier thickness on the magnetic properties of the ferromagnetic tunneling barrier.