Time-reversal-breaking Weyl nodal lines in two-dimensional A3C2 (A = Ti, Zr, and Hf) intrinsically ferromagnetic materials with high Curie temperature

Abstract

Most materials that feature nontrivial band topology are spin-degenerate and three dimensional, strongly restricting them from application in spintronic nanodevices. Hence, two-dimensional (2D) intrinsically spin-polarized systems with rich topological elements are still in extreme scarcity. Here, 2D A₃C₂ (A = Ti, Zr, and Hf) materials with the Pm2 type structure are reported as new ferromagnetic materials with intrinsic magnetism and good stability. Unlike the Weyl nodal lines existing in nonmagnetic 2D systems, A₃C₂ hosts time-reversal-breaking Weyl nodal rings (two Γ-centered, one K-centered, and one K′-centered) without spin–orbit coupling (SOC). These nodal rings still remained under SOC with magnetization along the z direction (easy magnetization axis). More interestingly, the Curie temperatures (T_C) of A₃C₂ were determined based on the Monte Carlo simulation. Ti₃C₂ features an extraordinary T_C (above 800 K), and those of Zr₃C₂ and Hf₃C₂ are above room temperature. Therefore, A₃C₂ materials are excellent platforms to study magnetic Weyl nodal lines in high T_C ferromagnetic 2D materials.