A strong coupling mechanism between ferromagnetism and piezoelectricity in 2D ferroelectric CrXSYBrZ with high carrier mobility

Abstract

Materials with both ferromagnetism and excellent piezoelectricity can be classified as multifunctional materials and have been a focus of recent studies. At present, reported research studies focus mainly on three-dimensional materials. Herein, we report a series of two-dimensional (2D) multifunctional materials. Using first-principles calculations, we designed and predicted that 2D CrXSYBrZ (X = Cr or Mo, Y = S or Se and Z = H, F, Cl or Br) exhibit ferromagnetism and excellent out-of-plane piezoelectricity, and also have high carrier mobility. Importantly, the out-of-plane piezoelectric stress coefficient e₃₃ of CrXBrYSZ is positively correlated with the difference in the sum electronegativity of the upper and lower parts in CrXBrYSZ. This physical mechanism is defined as the electronegativity difference effect, which reflects the intensity of the asymmetric charge transfer in CrXSYBrZ. Furthermore, the magnetic moment enhancement of Cr and Mo inhibits the asymmetric polarization charge transfer inside CrMoS₂Br₂ along the z-axis, and thereby significantly decreases the out-of-plane piezoelectricity of CrMoS₂Br₂. The coupling mechanism between ferromagnetism and out-of-plane piezoelectricity of CrMoS₂Br₂ is most obvious among CrXSYBrZ monolayers. Additionally, the out-of-plane piezoelectric stress coefficient e₃₃ and atomic magnetic moments of these monolayers can be manipulated through biaxial strain. Therefore, CrXSYBrZ exhibit potential applications in multifunctional electromechanical and magnetic nanodevices.