Carbon covered magnetic nickel nanoparticles embedded in PBT-PTMO polymer: Preparation and magnetic properties

Fine particles of a face-centered-cubic phase of Ni covered with a graphite layer were prepared and embedded in a PBT-block-PTMO polymer at a concentration of 0.1 wt%. The mean crystalline size of Ni varied from 8 to 30 nm. A magnetic resonance study of the obtained nanocomposites was carried out in the 4-300 K temperature range using an electron paramagnetic resonance spectrometer. An almost symmetrical and very intense magnetic resonance line was recorded for all the investigated samples. The resonance line was centered at g = 2.253(2) (the resonance field H-r = 3003(1) Gs) and had a peakto-peak linewidth Delta H-pp = 693(2) Gs at room temperature. The amplitude of the resonance line increased with a temperature increase in the low temperature range (T < 40 K) and in the high temperature range (T > 100 K) but was constant at intermediate temperatures. The resonance field Hr decreased and linewidth Delta H-pp increased as the temperature decreased from room temperature what was similar to the changes observed for other systems of nanoparticles. The thermal gradient of the resonance field, Delta H-r/Delta T, strongly depended on the temperature range. The temperature shift of the resonance field and the linewidth were analyzed in terms of the demagnetizing fields of nonspherical agglomerates. A strong change of linewidth and resonance field was registered below 40 K due to the freezing of the spin system's dynamical magnetic fluctuations. A comparison was made of the results obtained on the Ni/C with the previous measurements on gamma-Fe2O3 nanoparticles embedded in a copolymer. (C) 2009 Elsevier B.V. All rights reserved.