Superconductivity induced by spark erosion inZrZn2

Abstract

We show that the superconductivity observed recently in the weak itinerant ferromagnet $\mathrm{Zr}{\mathrm{Zn}}_{2}$ [C. Pfleiderer et al., 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 $\mathrm{Zr}{\mathrm{Zn}}_{2}$ crystals show that cutting by spark erosion leaves a superconducting surface layer. The resistive superconducting transition is destroyed by chemically etching a layer of $5\phantom{\rule{0.3em}{0ex}}\ensuremath{\mu}\mathrm{m}$ from the sample. No signature of superconductivity is observed in $\ensuremath{\rho}(T)$ of etched samples at the lowest current density measured, $J=675\phantom{\rule{0.3em}{0ex}}\mathrm{A}\phantom{\rule{0.2em}{0ex}}{\mathrm{m}}^{\ensuremath{-}2}$, and at $T\ensuremath{\geqslant}45\phantom{\rule{0.3em}{0ex}}\mathrm{mK}$. 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 $\mathrm{Zr}{\mathrm{Zn}}_{2}$.