Magnetic Interactions Associated with Spin-Defect States in Quasi-One-Dimensional MoBr₃

The interplay of charge, spin, and lattice degrees of freedom in solids leads to the emergence of elementary excitations. Particularly in one-dimensional (1D) systems, spontaneous symmetry breakings often accompany an abrupt phase shift in the order parameter, or soliton, which can be considered low-energy excitations. Thus, investigating spin-defect states localized at the soliton site in quasi-1D materials presents a novel approach to modulating magnetic properties at the nanoscale. In this study, we explore the magnetic interactions associated with the spin-defect states in dimerized quasi 1D MoBr₃ using density functional theory. Bulk MoBr₃ is characterized by weakly coupled one-dimensional chains, where each Mo³⁺ ion is surrounded by a Br octahedron, face-sharing with neighboring octahedra. The insulating antiferromagnetic ground state exhibits dimerization of Mo-Mo pairs with alternating short and long bonds, further stabilizing the insulating phase by the orbital-dependent dimer-induced bonding-antibonding splitting. Moreover, we find antiferro-coupled localized spin-defect states with a total spin of S=3/2 formed at the soliton site of the Mo-Mo dimer, leaving a single isolated spin site. We quantify magnetic interaction between Mo sites by obtaining the Heisenberg exchange parameters and find that magnetic couplings between intra-dimer sites, inter-dimer sites, and between the spin-defect and its neighboring spin sites favour AFM order. From the calculated exchange parameters, it is suggested that the magnetic moment of the spin defect site can be switched by external magnetic fields, which may offer potential applications in developing high-density data storage devices.<br/><br/><br/>This work is supported by the National Research Foundation of Korea by the Korea government (MSIT) (No. RS-2024-00358551) and by the Ministry of Education (No. 2021R1A6A1A03043957)