Francisites Cu3M(YO3)2O2X (M = Bi or rare earth, Y = Se, Te, X = Br, Cl, I) attract grate attention due to their interesting magnetic properties, such as metamagnetic transitions in relatively weak magnetic fields, magnetic phase transitions including spin reorientation, as well as model systems for studying two-dimensional and frustrated magnetism. In this work, a low-temperature optical spectroscopic study is presented of erbium francisite Cu3Er(SeO3)2O2Cl. The observed splittings of the spectral lines corresponding to the f f transitions in the Kramers Er3+ ion unambiguously indicates the magnetic ordering of the crystal at a temperature TN = 37.5 K. The temperature dependence of the splitting of the main doublet of the erbium ion is determined. The contribution of erbium to the heat capacity and magnetic susceptibility of Cu3Er(SeO3)2O2Cl is calculated.
Highly frustrated magnetism in relativistic
d4
Mott insulators: Bosonic analog of the Kitaev honeycomb model