The enhancement of ferromagnetism in 2D tetragonal CoSe by electric fields and Janus engineering

Abstract Two-dimensional (2D) cobalt-based anti-MXene materials have been synthesized, but their poor ferromagnetism limits their spintronic applications. In this study, we employed first-principles calculations to investigate the various methods to modulate the magnetic properties of monolayer CoSe. The competition between the Co–Co direct exchange interaction, Co–Se–Co super-exchange interaction, and Ruderman–Kittel–Kasuya–Yosida exchange interaction lead to the weak ferromagnetism of monolayer CoSe. The strain slightly enhances the Curie temperatures ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> <mml:mo stretchy="false">)</mml:mo> </mml:mrow> </mml:math> of monolayer CoSe, but the substrate is ineffective. In comparison, a vertical electric field E z of 0.5 V Å −1 can enhance <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> from 8 K to 161 K. This enhancement stems from reinforcing Co–Se–Co superexchange by electric fields. Among the Janus structures based on the CoSe, monolayer Co 2 SeS can exhibit a <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mrow> <mml:msub> <mml:mi>T</mml:mi> <mml:mrow> <mml:mi mathvariant="normal">c</mml:mi> </mml:mrow> </mml:msub> </mml:mrow> </mml:math> of 40 K, due to the interplay between strain and built-in electric field. Our results suggested that polar substrates or Janus engineering can promote the ferromagnetism of 2D CoSe for spintronic applications.