Scaling behavior of the critical current density in $\mathrm{Mg}\mathrm{C}{\mathrm{Ni}}_3$ microfibers

We present transport critical current measurements on microfibers consisting of a $80\text{−}\mathrm{nm}$ thick layer of polycrystalline $\mathrm{Mg}\mathrm{C}{\mathrm{Ni}}_3$ synthesized directly onto $7\text{−}\mu \mathrm{m}$ diameter carbon fibers. Near the transition temperature $T_c$, the critical current density $J_c$ is well described by the power law form ${[1-{(T∕T_c)}^2]}^{\alpha }$, where $\alpha =2$ with no crossover to the Ginzburg-Landau exponent $\alpha =1.5$. We extrapolate $J_c\left(0\right)\approx 4\times {10}^7\mathrm{A}∕{\mathrm{cm}}^2$, which is an order of magnitude greater than estimates obtained from magnetization measurements of polycrystalline powders. The field dependence is purely exponential $J_c(T,H)=J_c\left(T\right)\exp (-H∕H_0)$ over the entire field range of $0\text{to}9\mathrm{T}$. The unconventional scaling behavior of the critical current may be rooted in an anomalous temperature dependence of the London penetration depth.

Figure 1

Resistivity normalized by its room-temperature value as a function of temperature for a carbon fiber coated with $80\mathrm{nm}$ of Ni and a similar Ni-coated carbon fiber processed in Mg vapor to form an $80\text{−}\mathrm{nm}$ $\mathrm{Mg}\mathrm{C}{\mathrm{Ni}}_3$ sheath. Inset: Scanning electron micrograph of $\mathrm{Mg}\mathrm{C}{\mathrm{Ni}}_3$ coated carbon fibers showing a cross-sectional view.

Figure 2

Log-log plot of the scaling behavior of the critical current density near $T_c$ in zero magnetic field for two $80\text{−}\mathrm{nm}$ fibers. The solid line is provided as a guide to the eye and extrapolates to a zero-temperature density $J_c\left(0\right)=4\times {10}^7\mathrm{A}∕{\mathrm{cm}}^2$. The dashed line has the Ginzburg-Landau slope of $3∕2$. Inset: Typical $I\text{−}V$ characteristics in zero field where the boxcar integrator output is plotted as a function of current pulse magnitude. The arrows depict the critical current thresholds at $T=7.45$, 7.3, 7.2, 7.1, 7.0, and $6.8\mathrm{K}$, going from left to right.

Figure 3

Reduced transition temperature as a function of applied current density for the samples in Fig. 2. The dashed line has slope 2 and represents the effect of Joule heating.

Figure 4

Semi-log plot of the magnetic field dependence of the critical current density at several temperatures: ▾ $6.8\mathrm{K}$, ▴ $6.0\mathrm{K}$, ◆ $5.5\mathrm{K}$, ● $5.0\mathrm{K}$. The $+$ symbols are $5.0\mathrm{K}$ data extracted from magnetization measurements of packed powder samples, Ref. 9. The field was applied longitudinally to the fiber. The solid lines represent exponential fits to the data from which a characteristic decay field $H_0$ is extracted. Inset: Inverse of characteristic field as a function of the inverse of the reduced temperature. The dashed line is provided as a guide to the eye.