Interactions between mixed-valence Ni ions in stripe ordered La5/3Sr1/3NiO4

The aim of the work is to investigate how the spin dynamics in La2–xSrxNiO4 depend on the interaction between Ni3+ and Ni2+ ions in the stripe ordered phase of the compound. The experiments will be conducted on crystals with x = 0.33, where the correlation length for the ordering is longest. Preliminary studies show well-defined spin excitations associated with the stripe periodicity, but a gap at middle energies in the magnon spectrum remains unexplained by a linear spin-wave theory model. We will follow the spin excitations up to the maximum energy ~50 meV in order to measure the gap region in detail.