Microwave Absorption in Nanocomposites (Revised Feb. 2014)

La_1.5Sr_0.5NiO₄ is well known as a dielectric material that has a colossal permittivity (up to 10⁷) and a weak paramagnet at room temperature. The permeability is about 1.005, which is just slightly larger than that of air. The weak magnetic moment together with the huge imbalance between permittivity and permeability seemed to negate La_1.5Sr_0.5NiO₄ as a promising candidate for electromagnetic absorption due to the lack of magnetic losses. However, we have found that La_1.5Sr_0.5NiO₄ nanopowder indeed has a reasonably strong microwave absorption capability in the range of 4-18 GHz. Apparently, impedance matching (|Z| = Z₀ = 377 Ω) is found to be responsible for the absorption resonance that shifts to lower frequencies with increasing the absorber´s thickness. To improve magnetic losses, as well as to balance out the dielectric and magnetic components, CoFe₂O₄ nanoparticles are gradually added to the La_1.5Sr_0.5NiO₄/CoFe₂O₄ composites. The influence of adding magnetic nanoparticles on reflection loss, resonance frequency, and matching effects will be discussed.