Thickness dependence of critical current density in MgB₂ films fabricated by ex situ annealing of CVD-grown B films in Mg vapor

A study was performed to examine the J_c behavior as a function of thickness in MgB₂ films fabricated by ex situ annealing at 840 °C of boron films, grown by chemical vapor deposition, in Mg vapor. The film thicknesses range between 300nm and 10νm. The values of J_c range from 1.2 × 10⁷Acm^-2 for 300nm to 1.9 × 10 ⁵Acm^-2 for 10νm film thicknesses at 20K and self-field. The study shows that the critical current density (J_c) in MgB ₂ films decreases with increasing film thickness, similar to that observed in YBCO-coated conductors. Moreover, our study shows that critical current (I_c) reaches its maximum value of 728Acm^-1 width at ∼1νm thick MgB₂ films at 20K and self-field, which is, interestingly, the same thickness of pulsed-laser-deposited YBCO-coated conductors at which I_c reaches its maximum value. The high J _c values carried by our films show that the ex situ fabrication method can produce high quality MgB₂ films at low processing temperatures, which is promising for RF cavity applications and coated-conductor wires and tapes.