Temperature dependence of the magnetization of ultrafine particles of nickel

Partially oxidized Ni ultrafine particles were prepared by the gas evaporation method, and the magnetization of the particles has been measured between 1.5 and 250 K in applied fields up to 15 kOe. The results are well described by spin-wave theory (SW) and Stoner excitement (SE) above a characteristic temperature T/sub s/ which depends on the sizes of particles. Discrepancy of the data from SW and SE below T/sub s/ was associated with the superparamagnetism of antiferromagnetic NiO particles on the surface of ferromagnetic Ni particles. For temperatures above T/sub s/, the spin-wave constants B were found to be 2.83*10/sup -6/ K/sup -3/2/ and 2.60*10/sup -6/ K/sup -3/2/ for particles with average diameters of 100 AA and 220 AA, respectively.<>