Microwave Absorption in ${\rm La}_{1.5}{\rm Sr}_{0.5}{\rm NiO}_{4}/{\rm CoFe}_{2}{\rm O}_{4}$ Nanocomposites (Revised Feb. 2014)

La
_1.5
Sr
_0.5
NiO
₄
is well known as a dielectric material that has a colossal permittivity (up to 10
⁷
) 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
_1.5
Sr
_0.5
NiO
₄
as a promising candidate for electromagnetic absorption due to the lack of magnetic losses. However, we have found that La
_1.5
Sr
_0.5
NiO
₄
nanopowder indeed has a reasonably strong microwave absorption capability in the range of 4-18 GHz. Apparently, impedance matching (|Z| = Z
₀
= 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, CoFe
₂
O
₄
nanoparticles are gradually added to the La
_1.5
Sr
_0.5
NiO
₄
/CoFe
₂
O
₄
composites. The influence of adding magnetic nanoparticles on reflection loss, resonance frequency, and matching effects will be discussed.