Superconductivity induced by spark erosion in Zr Zn2

We show that the superconductivity observed recently in the weak itinerant ferromagnet Zr Zn2 [C. Pfleiderer, Nature (London) 412, 58 (2001)] is due to remnants of a superconducting layer induced by spark erosion. Results of resistivity, susceptibility, specific heat, and surface analysis measurements on high-quality Zr Zn2 crystals show that cutting by spark erosion leaves a superconducting surface layer. The resistive superconducting transition is destroyed by chemically etching a layer of 5 μm from the sample. No signature of superconductivity is observed in ρ (T) of etched samples at the lowest current density measured, J=675 A m-2, and at T≥45 mK. Energy-dispersive x-ray analysis shows that spark-eroded surfaces are strongly Zn depleted. The simplest explanation of our results is that the superconductivity results from an alloy with higher Zr content than Zr Zn2.