Combinatorial search of superconductivity in Fe-B composition spreads

We have fabricated Fe-B thin film composition spreads in search of possible superconducting phases following a theoretical prediction by Kolmogorov et al. Phys. Rev. Lett. 105, 217003 (2010). Co-sputtering was used to deposit spreads covering a large compositional region of the Fe-B binary phase diagram. A trace of superconducting phase was found in the nanocrystalline part of the spread, where the film undergoes a metal to insulator transition as a function of composition in a region with the average composition of FeB2. The resistance drop occurs at 4 K, and a diamagnetic signal has also been detected at the same temperature. From the field-dependent resistive transition behavior, we estimate the upper critical field to be approximately 2 T.