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.

Addition of 25 percent by weight of 20-oersted Fe particles to 1250-oersted Fe particles decreases the coercive force to 500 oersteds: thus any experiment designed to check the predicted 10 000 oersteds for particles of 10-to-1 shape anisotropy requires an extremely high percentage of such particles. For both Fe and Co, the temperature variation of coercive force closely follows that of crystalline anisotropy and not that predicted for particles with shape anisotropy. Electron micrographs show that many of these particles are bean-shaped, with axis ratio 1.5. The extent of the present work is not sufficient for evaluation of the contribution of shape anisotropy to the coercive force of Fe.