Saturation Magnetization and Size of Iron Particles Less than 100 A in Diameter

Spherical iron particles were prepared by electrodeposition into mercury under conditions which produced average diameters as small as 15 A. The complete magnetization curves were obtained as a function of temperature from 4° to 300°K in fields up to 150 000 oe. Fields of more than 50 000 oe at 76°K were required to determine unambiguously the saturation magnetization; even higher fields were needed at higher temperatures. The saturationmagnetization of larger particle size samples, obtained by aging,remained constant and corresponded to the total iron present. Particle size and size distributions were calculated from the magnetization curves by means of the Langevin function at sufficiently high temperatures so that the samples were completely superparamagnetic. These results were compared to the particle sizes calculated from the temperature dependence of the remanence to saturation ratio. The two methods showed substantial agreement and were consistent with the properties and sizes of larger particles previously determined by direct electron microscope examination. The mean particle diameters thus determined ranged from about 15 A for the freshly deposited samples to about 60 A for the aged samples. Beyond this range the onset of stable magnetic behavior in a large fraction of the particles made the particle size calculation by these methods unsuitable.