Tensile strain induced switching of magnetic states in NbSe2 and NbS2 single layers

Two dimensional crystals, befitting nanoscale electronics and spintronics, can benefit strain-tunable applications due to their ultrathin and flexible nature. We show by first-principles calculations that tensile strain can enhance the exchange splitting of spins in NbSe2 and NbS2 single layers. Particularly, a switch from antiferro-to ferro-magnetism is realized by strain engineering. Under strains lower than 4%, an anti-ferromagnetic state with opposite spins aligned on the next-nearest-neighbor rows of Nb atoms is favored in energy due to a superexchange interaction; with higher strains the ground state turns to be ferromagnetic with a double exchange origin. In contrast, the VSe2 and VS2 single layers, though with the same trigonal prismatic coordination, remain ferromagnetic even under compressive strains.