Observation of superconductivity and ferromagnetism in high-entropy carbide ceramics

Generally, superconductivity (SC) is expected to be absent in magnetic systems. However, this assumption was disturbed by unconventional superconductors, such as cuprates, iron-based superconductors, and the recently-discovered nickelate, since their superconductivity is proposed to be related to the electron-electron interaction mediated by the spin fluctuation. The study of the correlation between SC and magnetism (mostly antiferromagnetism) has been an important topic in understanding the SC mechanism. However, the coexistence of superconductivity and ferromagnetism (FM) is still rare in both conventional and unconventional superconductors. In this work, we report the observation of these two quantum orderings in high-entropy carbide ceramics (Mo _0.2Nb _0.2Ta _0.2V _0.2W _0.2)C _0.9, (Ta _0.25Ti _0.25Nb _0.25Zr _0.25)C. A clear magnetic hysteresis loop was observed in these high-entropy carbides, indicating a ferromagnetic (FM) ground state, with such an FM ordering even existing above room temperature. A sharp superconducting transition is observed in (Mo _0.2Nb _0.2Ta _0.2V _0.2W _0.2)C _0.9 with a T_c of 3.4 K and upper critical field of ∼3.37 T. Meanwhile, superconductivity is suppressed to some extent, and the zero-resistance state disappears in (Ta _0.25Ti _0.25Nb _0.25Zr _0.25)C, in which stronger ferromagnetism is presented. The upper critical field of (Ta _0.25Ti _0.25Nb _0.25Zr _0.25)C is only ∼1.5 T, though they show a higher transition temperature near 5.7 K. The ferromagnetism stems from the carbon vacancies. The concurrence of SC and FM ordering within the same system, whether through coexistence or phase separation at the microscopic level, provides an exotic platform to study the correlation between superconductivity and magnetism, along with other emerging quantum phenomena, significantly benefitting the design of multifunctional electronic devices.