Degradation of superconducting properties in MgB₂films by exposure to water

The effect of water exposure on MgB 2 is studied by submerging an 800 nm thick MgB 2 film into de-ionized water at room temperature for 1 h, 4 h, 10 h, and 15 h, and by analysing the resulting material using scanning electron microscopy and resistance versus temperature measurements. It is clearly observed that the T c onset of these films (obtained by an ex-situ reaction of an e-beam evaporated boron layer) remains unchanged throughout this process, indicating that at least a portion of the sample retains its original bulk-like properties. The data is consistent with an interpretation in which a portion of the exposed film - likely to be the region closest to the substrate - becomes superconducting only at ~ 25 K. It is possible that this low- T c region already exists in the as-prepared film, and we observe that its T c coincides with that of MgB 2 films obtained by annealing precursor films prepared by pulsed laser deposition. Therefore the data presented here not only illustrates the degradation of MgB 2 in water but also sheds light on the differences and similarities between films obtained via different routes.