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Engineering 2D high-temperature ferromagnets with large in-plane anisotropy via alkali-metal decoration in a tetragonal CoSe monolayer

Yiran Peng, Yanfeng Ge, Yong Liu, Wenhui Wan

Abstract

Two-dimensional (2D) ferromagnetic materials with high Curie temperature ($T_{\rm c}$) and large magnetic anisotropy energy (MAE) are critical for nanoscale spintronics but remain rare. We propose, via first-principles calculations, that adsorbing alkali atoms ($A$ = Li, Na, K, Rb, Cs) onto a tetragonal CoSe monolayer transforms it into a series of stable 2D ferromagnetic metals, $A$CoSe, with an in-plane easy axis. Notably, LiCoSe is a half-metal. These functionalized monolayers exhibit dramatically enhanced ferromagnetism compared to the pristine layer, with $T_{\rm c}$ > 300 K and MAE > 800 $μ$eV/Co. The coupled alkali atoms amplify the local magnetic moment of Co ions, reinforce ferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) and superexchange couplings, and concurrently weaken the direct antiferromagnetic exchange between Co ions. Furthermore, tensile strain can further elevate the MAE (via band shifting) and increase $T_{c}$ (by strengthening the nearest-neighbor exchange $J_1$). Among them, NaCoSe exhibits the highest MAE and excellent strain-modulated $T_{c}$, rendering it the most promising candidate material. Our results establish alkali-metal decoration as an effective strategy for realizing 2D ferromagnets with high $T_{\rm c}$ and large MAE in tetragonal lattices.

Engineering 2D high-temperature ferromagnets with large in-plane anisotropy via alkali-metal decoration in a tetragonal CoSe monolayer

Abstract

Two-dimensional (2D) ferromagnetic materials with high Curie temperature () and large magnetic anisotropy energy (MAE) are critical for nanoscale spintronics but remain rare. We propose, via first-principles calculations, that adsorbing alkali atoms ( = Li, Na, K, Rb, Cs) onto a tetragonal CoSe monolayer transforms it into a series of stable 2D ferromagnetic metals, CoSe, with an in-plane easy axis. Notably, LiCoSe is a half-metal. These functionalized monolayers exhibit dramatically enhanced ferromagnetism compared to the pristine layer, with > 300 K and MAE > 800 eV/Co. The coupled alkali atoms amplify the local magnetic moment of Co ions, reinforce ferromagnetic Ruderman-Kittel-Kasuya-Yosida (RKKY) and superexchange couplings, and concurrently weaken the direct antiferromagnetic exchange between Co ions. Furthermore, tensile strain can further elevate the MAE (via band shifting) and increase (by strengthening the nearest-neighbor exchange ). Among them, NaCoSe exhibits the highest MAE and excellent strain-modulated , rendering it the most promising candidate material. Our results establish alkali-metal decoration as an effective strategy for realizing 2D ferromagnets with high and large MAE in tetragonal lattices.

Paper Structure

This paper contains 1 section, 3 equations, 4 figures, 1 table.

Table of Contents

  1. data avaliability

Figures (4)

  • Figure 1: (a) The top view and (b) side view of the $A$CoSe ($A$ = Li, Na, K, Rb, Cs) monolayers. (c) The spin configuration of the FM state and four different AFM states. Yellow and blue color represent the up and down spins, respectively. (d) The energies of AFM states relative to FM state.
  • Figure 2: (a) The -COHP values of Co-Se and Na-Se bonding in spin-up and spin-down channels for NaCoSe monolayer. For comparison, dashed lines (Co-Se(I)) is the Co-Se bonds in intrinsic CoSe monolayer. (b) The band structure and (c) the contribution of Co-$d$ orbitals to the density of states. (d) The contribution of Co-$d$ orbitals to the magnetic moment in NaCoSe and CoSe monolayers.
  • Figure 3: (a) The energy of NaCoSe monolayer with spin direction lying on the whole space and $xy$ plane. (b) The strain-dependent MAE of $A$CoSe ($A$ = Li, Na, K, Rb, Cs) monolayers. (c) The contribution from the different orbital hybridization to the MAE of NaCoSe under strains. (d) The DOS of the $d_{xy}$ and $d_{x^2-y^2}$ states of NaCoSe under strains ($\varepsilon$) of 0% and 4%.
  • Figure 4: (a) The exchange coupling constants $J_1$, $J_2$, $J_3$ between the nearest-neighboring, next-nearest-neighboring, and next-next-nearest-neighboring Co atoms. (b) The exchange coupling constants and $T\rm _c$ in strain-free $A$CoSe ($A$ = Li, Na, K, Rb, Cs) monolayer. (c) The exchange coupling constants of NaCoSe monolayer under different strains. (d) The $T\rm _c$ of $A$CoSe varying with strains.