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Temperature dependence of ac susceptibilities Ј and Љ are reported using frequencies f m = 0.1, 1, 99, 499, and 997 Hz for nanoparticles of Ni dispersed in silica ͑Ni/ SiO 2 :15/ 85͒ with the mean sizes D = 3.8, 11.7, 15, and 21 nm ͑ Ӎ 0.2 nm͒, as determined by transmission electron microscopy. The blocking temperatures T B , as determined by peaks in Љ versus T data, are fit to the Vogel-Fulcher law based on the following equation: T B = T o + T a / ln͑f o / f m ͒. Using the attempt frequency f o = 1.82 ϫ 10 10 Hz, T a ͑K͒ = 310 ͑21͒, 954͑17͒, 1334͑14͒, and 1405͑47͒ are determined for D = 3.8, 11.7, 15, and 21 nm, respectively, along with T o ͑representing the interparticle interaction͒ =0, 0, 6.6͑0.7͒, and 12.5͑2.5͒ K respectively. The magnitudes of T a = K a V / k yield the anisotropy constant K a increasing with decreasing D ͑or volume V͒ due to contributions from surface anisotropy. The validity of the theoretical result Љ = C ‫ץ‬ ͑ЈT͒ / ‫ץ‬T with C Ӎ / ͓2 ln͑f o / 2f m ͔͒ is checked and the calculated values of f o are consistent with experimental value of f o = 1.82ϫ 10 10 Hz.

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