Monotonic decrease of T_cs with thinning of the superconducting MgB₂ layer in MgB₂/Ni and MgB₂/B alternately-layered thin films

We prepared MgB₂/Ni and MgB₂/B alternately-layered films using electron beam evaporation and coaxial vacuum arc evaporation techniques without any post-annealing. The thickness of each MgB₂ layer was designed to be 42, 24 or 15nm for both MgB₂/Ni and MgB ₂/B alternately-layered films. We confirmed that the layered structure was successfully obtained from the scanning transmission electron microscopic cross-sectional image of the MgB₂/Ni alternately-layered film. The critical temperature, T_c, of the alternately-layered film decreased as the MgB₂ layer became thinner, for both MgB ₂/Ni and MgB₂/B films. Moreover, the T_c was affected by only the MgB₂ layer thickness, and was independent of the inserted layer materials.