Coexistence of altermagnetism and robust ferroelectricity in a bulk MnO wurtzite structure

Altermagnetism is a new class of material with zero net magnetization, but having a nonrelativistic spin-split band structure. Here, we investigate the multifunctional properties of the hexagonal wurtzite MnO (<i>w</i>-MnO). <i>w</i>-MnO has a direct band gap of 0.39 eV and the altermagnetic behavior is achieved with a spin splitting of up to 188 meV. The bulk <i>w</i>-MnO has an in-plane magnetic anisotropy energy of -0.17 meV per cell along the [100] direction and temperature-dependent magnetization reveals a Néel temperature of around 180 K. Moreover, a maximum spin Hall conductivity of -285(ℏ/<i>e</i>) S cm^-1 is obtained at a chemical potential of -0.24 eV. Along with the altermagnetism, we also find a large out-of-plane spontaneous polarization of 76.25 μC cm^-2. <i>w</i>-MnO exhibits a low energy barrier of 0.26 eV per formula unit (f.u.) for polarization switching and this is further decreased to 0.15 eV per f.u. under 5% epitaxial tensile strain. The molecular dynamics simulations suggest that <i>w</i>-MnO retains its ferroelectric order below 2100 K indicating excellent thermal stability. These findings suggest that the hexagonal <i>w</i>-MnO can be a promising candidate for multiferroic applications particularly in spintronic and ferroelectric devices with high thermal stability.