Crystal field excitations of YbMn₂Si₂

The crystal field excitations of the rare earth intermetallic compound YbMn₂Si₂ have been measured by inelastic neutron scattering over the temperature range 2.5-50 K. The YbMn₂Si ₂ spectra exhibit three low energy excitations (~3-7 meV) in the antiferromagnetic AFil region above the magnetic phase transition at T _N2 = 30(5) K. The crystal field parameters have been determined for YbMn₂Si₂ in the antiferromagnetic AFil region. A further two inelastic excitations (~9 meV, 17 meV) are observed below T _N2=30(5) K, the temperature at which the high temperature antiferromagnetic structure is reported to exhibit doubling of the magnetic cell. Energy level diagrams have been determined for Yb³⁺ ions in the different sites above (single site) and below the magnetic transition temperature (two sites). The excitation energies for both sites are shown to be temperature independent with the temperature dependences of the transition intensities for the two sites described well by a simple Boltzmann model. The spectra below T_N2 cannot be described fully in terms of molecular field models based on either a single Yb³⁺ site or two Yb ³⁺ sites. This indicates that the magnetic behaviour of YbMn ₂Si₂ is more complicated than previously considered. The inability to account fully for excitations below the magnetic phase transition may be due to an, as yet, unresolved structural transition associated with the magnetic transition.