Scaling behavior of the critical current density in MgCNi₃ microfibers

We present transport critical current measurements on microfibers consisting of a 80-nm thick layer of polycrystalline MgCNi₃ synthesized directly onto 7-μm diameter carbon fibers. Near the transition temperature T_c, the critical current density J_c is well described by the power law form [1−(T∕T_c)²]^α, where α=2 with no crossover to the Ginzburg-Landau exponent α=1.5. We extrapolate J_c(0)≈4×10⁷ A∕cm², which is an order of magnitude greater than estimates obtained from magnetization measurements of polycrystalline powders. The field dependence is purely exponential J_c(T,H)=J_c(T)exp(−H∕H₀) over the entire field range of 0 to 9 T. The unconventional scaling behavior of the critical current may be rooted in an anomalous temperature dependence of the London penetration depth.