Magnetic ground state of semiconducting transition metal trichalcogenide monolayers

Abstract:

Layered transition metal trichalcogenides with the chemical formula ABX3 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 Mnand 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 N´eel 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 ABX3 can be a promising platform to explore 2D magnetic phenomena.

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Exchange interactions

 For each compound, we optimize the crystal structure for all four spin configurations [see Fig. 1 (c)-(f)] to find the lowest-energy state. Figure 3 shows the partial density of states (DOS) of three representative ground states: AF-zigzag (CrSiTe3), FM (CrGeTe3) and AFN´eel (MnPS3).

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To further extract the J’s, we chose to fix the lattice to that of the most energetically favorable spin configuration and computed the energies for different spin configurations. The exchange coupling constants were derived by mapping the DFT energies to the Heisenberg spin Hamiltonian (1),

 EFM/N´eel = E0 + (±3J1 + 6J2 ± 3J3)|S~| 2 

, EAF−zigzag/stripy = E0 + (±J1 − 2J2 ∓ 3J3)|S~| 2 ,

 (2) where E0 is the ground state energy independent of the spin configuration. Using these J’s, we also calculated the critical temperature by performing a Monte Carlo simulation of an Ising model on the 2D honeycomb lattice.4.