Exciton-magnon interactions in Ni_cMg_1-cO single crystals

The effect of chemical composition and temperature on exciton-magnon interactions in Ni_cMg_1-cO single crystals was studied using optical absorption spectra in the region of the magnetic-dipole ³A_2g(G) → ³T_2g(F) and electric-dipole ³A_2g(F) → ¹E_g(D) transitions. The two zero-phonon lines at ∼7800 and ∼7840 cm^-1 on the low-energy side of the magnetic-dipole transition band were assigned to a pure exciton transition and an exciton-one-magnon transition at the center of the Brillouin zone. The intensity of the exciton-one-magnon peak decreases rapidly with increasing magnesium ion concentration and/or temperature, to vanish altogether at T = 6 K for c < 0.95 and at T = 130 K for c ≥ 0.99. Thus, the contribution of long-wavelength magnons in optical absorption spectra of Ni_cMg_1-cO becomes negligible at temperatures substantially lower than the Néel point T_N (the antiferromagnetic ordering temperature). This observation can be explained as being due to a substantial decrease in the characteristic spin-spin interaction length with increasing concentration of the diamagnetic magnesium impurity ions (static disorder) and/or with increasing amplitude of thermal atomic vibrations (dynamic disorder). At the same time, the peak at ∼14500 cm^-1, which lies in the electric-dipole transition region and corresponds to excitation of an exciton and two magnons at the Brillouin zone edge, remains visible up to the Néel temperature. This is accounted for by the short-wavelength magnons being sensitive to short-range magnetic order, which persists up to T_N.