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

Abstract

Spherical iron particles were prepared by electrodeposition intomercury under conditions which produced average diameters assmall as 15 A. The complete magnetization curves were obtainedas a function of temperature from 4° to 300°K in fields up to150 000 oe. Fields of more than 50 000 oe at 76°K were requiredto determine unambiguously the saturationmagnetization; evenhigher 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 themagnetization curves by means of the Langevin function atsufficiently high temperatures so that the samples were completelysuperparamagnetic. These results were compared to the particlesizes calculated from the temperature dependence of the remanenceto saturation ratio. The two methods showed substantial agreementand were consistent with the properties and sizes of largerparticles previously determined by direct electron microscopeexamination. The mean particle diameters thus determined rangedfrom about 15 A for the freshly deposited samples to about 60 Afor the aged samples. Beyond this range the onset of stablemagnetic behavior in a large fraction of the particles made theparticle size calculation by these methods unsuitable.