Abstract
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.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">></ETX>
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