Electron paramagnetic resonance (EPR) in iron-oxide nanoparticles (∼2.5 nm) embedded in a polyethylene matrix reveals the sharp line broadening and the resonance field shift on sample cooling below $T_F\approx 40\mathrm{K}.$ At the same temperature a distinct anomaly in the field-cooled magnetization is detected. The temperature dependences of EPR parameters below $T_F$ are definitely different than those found for various nanoparticles in the superparamagnetic regime. In contrast to canonical bulk spin glasses, a linear fall-off of the EPR linewidth is observed. Such behavior can be explained in terms of the random-field model of exchange anisotropy.

This paper provides a convenient and simple method for fabricating of large amounts (kilogram-scale) of nanomaterials containing nanoparticles in a polymeric matrix. The encapsulation was done by thermal decomposition of metal containing compounds (MRn; M=Ba, Bi, Ca, Cd, Co, Cr, Cu, Fe, Mn, Mo, Nd, Ni, Pt, Re, Ti, Pb, Sr, Sm, W, Zn; R=CO, HCOO, CH

We have prepared nanocomposites consisting of narrowly sized metal-containing nanoparticles embedded in a polyethylene matrix and have established conditions for the fabrication of thick films and bulk materials from the synthesized polymer powders. Dielectric permittivity and resistivity measurements demonstrate that the electrical properties of the nanocomposites depend significantly on the nanoparticle size and content.The microwave absorption and permittivity of the materials are shown to vary little in a broad frequency range. The magnetization (including the remanent one) of the cobalt-containing nanomaterials is higher than that of the iron-containing samples.