Robust bipolar magnetic semiconductors in one-dimensional nanowires of transition metal dihalide

One-dimensional (1D) materials with robust ferromagnetic ground state are difficult to achieve while provide a significant platform for potential spintronics device applications in future. Herein, a new family of 1D transition metal dihalide (TMCl$_2$; where TM = Cu, Ni, Co, Fe, Mn, Cr) nanowires are proposed by using first-principles calculations. Their dynamic stability is ensured by Born-Oppenheimer molecular dynamics simulations. The electronic structures demonstrate that both CoCl$_2$ and CuCl$_2$ nanowires are promising bipolar magnetic semiconductors (BMSs) and can be converted into 1D half-metal materials by a small amount of carrier doping. While the CrCl$_2$ nanowire is an antiferromagnetic semiconductor (AFS). The formation of BMS is attributed to the super-exchange coupling between the Co/Cu atoms through the 3\emph{p}-orbitals in the Cl atoms. By using Monte Carlo simulations, we found that the CoCl$_2$ nanowire has a Curie point of 6 K, while the CuCl$_2$ nanowire has a corresponding point of 14 K. Our results put forwards a strategy to realize 1D BMSs and to design the low-dimensional AF spintronics devices.