Enhancement of critical current density and strong vortex pinning in high entropy alloy superconductor Ta[formula omitted]Nb[formula omitted]Hf[formula omitted]Zr[formula omitted]Ti[formula omitted] synthesized by spark plasma sintering

Abstract

We synthesized the high entropy alloy (HEA) Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound by planetary ball milling and the spark plasma sintering (SPS) method. The sintered HEA sample had a BCC crystal structure with small amount of secondary phases. The superconducting phase transition is observed in the electrical resistivity (Tc∼ 7.8 K) and magnetic susceptibility (Tc∼ 7.6 K) measurements, and the Tc was similar to that of an arc-melted sample. The zero-temperature limit of the upper critical magnetic fields Hc2(0)= 10.5 T and coherence length ξ = 5.66 nm were slightly decreased or comparable to the arc-melted HEA sample (Hc2(0)= 12.05 T and ξ= 5.23 nm). The field-dependent isothermal magnetization hysteresis M(H) exhibited the typical type- 2 superconducting behavior with magnetic flux avalanches in the low magnetic field region. The calculated critical current densities were significantly increased by 286 % (Jc= 32,606 A cm−2 at 2 K) ∼ 687 % (Jc= 73,200 A cm−2 at 4 K) compared to the arc-melted HEA sample (Jc= 11,401 A cm−2 at T= 2 K). The enhanced Jc of the sintered HEA sample is caused by enhancement of the pinning force due to point pinning as well as surface pinning effects. The vortex relaxation measurements showed stable remanent magnetization for over 10,000 s after magnetic fields were turned off, which is unlike conventional sueprconducting vortex relaxation behavior. The strong pinning force in the SPS sintered Ta1/6Nb2/6Hf1/6Zr1/6Ti1/6 compound is beneficial for practical applications because of the significantly enhanced critical current density Jc with stable remanent magnetization Mrem(t).