Dielectric constant of multiferroic pure and doped CuO nanoparticles

• We have studied theoretically the dielectric function ϵϵ of CuO nanoparticles.
• It has two kinks at TN1 and TN2 due to magnetoelectric coupling.
• ϵϵ decreases with magnetic field and the kinks disappear.
• ϵϵ is suppressed compared to the bulk case due to band gap expansion in the NP.
• Ni- or Mn-doping lead to increasing or decreasing of ϵϵ.

Within a microscopic model and a Green׳s function technique we have calculated the temperature and magnetic field dependence of the dielectric function ϵ(T,H)ϵ(T,H) in CuO nanoparticles (NPs) taking into account frustration, single-ion anisotropy, s–d coupling, spin–phonon interaction and linear magnetoelectric coupling. ϵ(T)ϵ(T) shows weak and broad anomalies at the two magnetic phase transition temperatures TN 2 and TN 1 due to the magnetoelectric coupling. ϵ(H)ϵ(H) decreases with increasing of H   and the kinks disappear. The changes of ϵϵ with ion doping are discussed. Comparisons to experimental data are made.