Superconducting properties of amorphous MoX (X=Si, Ge) alloy films for Abrikosov vortex memory

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Abstract
Structural and superconducting properties of amorphous MoSi (∼55%Si) and MoGe (∼30% Ge) films (200 and 50 nm in thickness) prepared by rf magnetron sputtering were examined for the purpose of application to Abrikosov vortex memory devices. Amorphous single-phase films were obtained at concentrations of 20% Si and above, while a crystalline phase was detected even at a concentration of 30% Ge. The highest critical temperature Tc was 7.2–7.3 K in Mo-20% Si films. Magnetic penetration depth λ at 4.2 K and upper critical field dependence on temperature Bc2(T) were measured. The Bc2(T) behavior in the vicinity of Tc showed a tail-like shape, which most likely is the result of structural inhomogeneities in the prepared films. The λ dependence on Si concentration was satisfactorily explained through the Ginzburg–Landau–Abrikosov–Gorkov (GLAG) dirty limit theory using the corrected critical temperature T*c which is defined by the Bc2(T) results. Other superconducting parameters (GL parameter κGL and coherence length ξGL ) and bare electronic density of states N(0) were derived from the GLAG relationships using the temperature slope of Bc2(T). Flux pinning was examined through the critical current measurements at 4.2 K in the perpendicular applied field to the film surface. Preliminary results on edge pinning estimation were also presented. It was concluded that the most suitable film for application to a vortex storage region (VSR) in an Abrikosov vortex memory cell is a 30% Si film 50 nm in thickness, considering both Tc and flux pinning force. Write current behavior of a memory cell with an amorphous Mo-30% Si VSR was discussed on the basis of measured superconducting properties.