Temperature dependence of AC susceptibilities χ' and χ'' are reported using frequencies fm = 0.1 Hz, 1 Hz, 99 Hz, 499 Hz and 997 Hz for nanoparticles (NPs) of Ni dispersed in silica (Ni/SiO2:15/85) with the mean sizes D=3.8 nm, 11.7 nm, 15 nm and 21nm. (σ≃0.2nm), as determined by TEM. The blocking temperatures TB, as determined by peaks in χ'' vs. T data, are fit to the Vogel-Fulcher law based Eq.: TB=To+Ta/ln(fo/fm) with the attempt frequency fo=2.6×1010 Hz and Ta(To) = 320(0) K, 990(0) K, 1370(6) K and 1500(9) K for D = 3.8 nm, 11.7 nm, 15 nm and 21nm, respectively. These magnitudes of Ta=KaV/k yield the anisotropy constant Ka increasing with decreasing D (or volume V) due to contributions from surface anisotropy. The validity of the theoretical result χ''=C∂(χ'T)/∂T with C≃π/[2ln(fo/2πfm)] is checked. The non-zero but small magnitudes of To for the two larger particles suggest the presence of very weak interparticle interaction.
Citation: Journal of Applied Physics
Pub Type: Journals
nanoparticles, magnetism, susceptibility, TEM