Influence of radiation defects on exciton-magnon interactions in nickel oxide

Influence of radiation defects on the optical absorption spectrum of nickel oxide (NiO) was studied at 6 K in the near-IR energy range of 7750-8300 cm ^-1 corresponding to the magnetic-dipole transition ³A _2g(F)→³T_2g(F) at nickel sites. NiO single crystals grown by the method of chemical transport reactions on the MgO(100) substrates were irradiated by the neutron fluences up to 5×10¹⁸ cm^-2. Two sharp lines were observed at the low-energy side of the band: the peak at 7805 cm^-1 is assigned to the pure exciton transition, whereas the peak at 7845 cm^-1, to the exciton-magnon excitation that occurs at the Brillouin zone-center (BZC). An increase of the defect concentration at higher fluences results in the lowering of the magnon-satellite-peak intensity. The long-wavelength BZC magnon absorption is sensitive to the long-range magnetic ordering, which becomes destroyed in the presence of the radiation defects. Therefore, the observed decrease of the peak intensity is attributed to the decrease of the spin-spin correlation length due to inhomogeneous broadening.