Single crystal growth and properties of the polar ferromagnet Mn1.05Bi with Kagome layers, huge magnetic anisotropy and slow spin dynamics

Abstract

The synthesis, structure, and properties of single crystalline ${\mathrm{Mn}}_{1.05}\mathrm{Bi}$ in the polar space group $Fdd2$ are reported. ${\mathrm{Mn}}_{1.05}\mathrm{Bi}$ is isostructural to the previously reported ${\mathrm{Mn}}_{1.05}{\mathrm{Rh}}_{0.02}\mathrm{Bi}$, with both ordered interstitials and ordered vacancies of Mn leading to Kagome-like layers. The ordering of the interstitials breaks inversion symmetry and forces the material into a polar space group. DC magnetization reveals ferromagnetic properties with a huge magnetic anisotropy, with the magnetization pinned along the $a$ axis (the stacking axis), and multiple magnetic transitions which retain this anisotropy. AC measurements confirm these transitions and show very sluggish spin dynamics along the $a$ axis, with a very large temperature-dependent out-of-phase response. Heat-capacity measurements reveal the presence of Schottky defects, and resistivity measurement confirms the transitions and reveal the material to be dominated by magnetic scattering. Overall, ${\mathrm{Mn}}_{1.05}\mathrm{Bi}$ shows magnetic properties markedly different from hexagonal, NiAs-type MnBi, driven by ordered interstitials and vacancies of Mn, stabilizing a likely complex magnetic structure with strongly temperature-dependent spin dynamics. This is supported by density-functional theory calculations, which suggest a strongly anisotropic noncollinear ground state driven by Kagome layers and asymmetric Mn environments.