Intrinsic Magnetism of Grain Boundaries in Two-dimensional Metal Dichalcogenides

In two-dimensional (2D) atomic crystals, ubiquitous grain boundaries (GBs) have been shown to cause considerable degradation in material properties. Using first-principles calculations, we show that dislocations and GBs in 2D metal dichalcogenides MX$_{2}$ (M$=$Mo,W; X$=$S,Se) exceptionally exhibit substantial magnetism, in sharp contrast to other 2D materials. All dislocations are shown to have a high magnetic moment of 1.0 Bohr magneton, mainly contributed by the Mo 4d orbitals. GB composed of pentagon-heptagon pairs shows ferromagnetic spin ordering and undergoes transitions from semiconductor to half-metal and to metal as tilt angle increases; when the tilt angle is over 47$^{\circ}$, GB prefers square-octagon pairs and turns to antiferromagnetic semiconductor. A novel mechanism based on interplay between dislocation-induced localized states and local unbalanced stoichiometry of GB is revealed for elucidating the magnetic behavior. Our findings suggest that purposeful engineering of topological GBs can upgrade 2D MX$_{2}$ into promising magnetic semiconductors for spintronic applications.