Experimental Study of the Coercive Force of Fine Particles

The experiments described test the theory of fine-particle magnets. Particles of Fe, Co, and Ni, of size 100-2000A, were prepared by electrodeposition into a mercury cathode. Their sizes and shapes were determined with an electron microscope. Experiments on coercive force $\mathrm{vs}$ particle size show, in general agreement with theory, that the maximum coercive force of Fe particles occurs at about 150A and is about 1000 oersteds, for very small packing factors. The coercive force decreases rapidly for smaller particles, slowly for larger particles. The rapid decrease for particles smaller than 150A is attributed to thermal fluctuations; this is borne out by measurements at low temperatures.