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.

• Received 17 April 2000

DOI:https://doi.org/10.1103/PhysRevB.63.012407