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 YbMn2Si2 spectra exhibit three low energy excitations (~3-7 meV) in the antiferromagnetic AFil region above the magnetic phase transition at TN2 = 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 TN2=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 TN2 cannot be described fully in terms of molecular field models based on either a single Yb3+ site or two Yb3+ sites. This indicates that the magnetic behaviour of YbMn2Si2 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.