Magnetic ground state of semiconducting transition-metal trichalcogenide monolayers

Layered transition-metal trichalcogenides with the chemical formula A B X₃ have attracted recent interest as potential candidates for two-dimensional magnets. Using first-principles calculations within density functional theory, we investigate the magnetic ground states of monolayers of Mn- and Cr-based semiconducting trichalcogenides. We show that the second and third nearest-neighbor exchange interactions (J₂ and J₃) between magnetic ions, which have been largely overlooked in previous theoretical studies, are crucial in determining the magnetic ground state. Specifically, we find that monolayer CrSiTe₃ is an antiferromagnet with a zigzag spin texture due to significant contribution from J₃, whereas CrGeTe₃ is a ferromagnet with a Curie temperature of 106 K. Monolayers of Mn compounds (MnPS₃ and MnPSe₃) always show antiferromagnetic Néel order. We identify the physical origin of various exchange interactions, and demonstrate that strain can be an effective knob for tuning the magnetic properties. Possible magnetic ordering in the bulk is also discussed. Our study suggests that A B X₃ can be a promising platform to explore two-dimensional magnetic phenomena.