Synthesis, Characterization, and Magnetoresistive Properties of the Epitaxial PdFe/VN/PdFe Superconducting Spin-Valve Heterostructure.

A thin-film superconductor(S)/ferromagnet(F) F1/S/F2-type PdFe(20 nm)/VN(30 nm)/PdFe(12 nm) heteroepitaxial structure was synthesized on (001)-oriented single-crystal MgO substrate utilizing a combination of the reactive magnetron sputtering and the molecular-beam epitaxy techniques in ultrahigh vacuum conditions. The reference VN film, PdFe/VN, and VN/PdFe bilayers were grown in one run with the target sample. In-situ low-energy electron diffraction and ex-situ X-ray diffraction investigations approved that all the PdFe and VN layers in the series grew epitaxial in a cube-on-cube mode. Electric resistance measurements demonstrated sharp transitions to the superconducting state with the critical temperature reducing gradually from 7.7 to 5.4 K in the sequence of the VN film, PdFe/VN, VN/PdFe, and PdFe/VN/PdFe heterostructures due to the superconductor/ferromagnet proximity effect. Transition width increased in the same sequence from 21 to 40 mK. Magnetoresistance studies of the trilayer PdFe/VN/PdFe sample revealed a superconducting spin-valve effect upon switching between the parallel and antiparallel magnetic configurations, and anomalies associated with the magnetic moment reversals of the ferromagnetic PdFe and PdFe alloy layers. The moderate critical temperature suppression and manifestations of superconducting spin-valve properties make this kind of material promising for superconducting spintronics applications.