Permanent-Magnet Properties of Elongated Single-Domain Iron Particles

Abstract

Previously described fine‐particle magnets have been based upon crystal anisotropy; this paper reports permanent‐magnet properties derived from the shape anisotropy of substantially elongated single‐domain iron particles. The shape anisotropy of these particles overcomes the limitation imposed on the energy of previous fine‐particle iron magnets by the low crystal anisotropy of iron. The predicted and observed properties of crystal anisotropy fine‐particle magnets are reviewed and compared with an ideal fine‐particle iron magnet based upon the shape‐anisotropy model of Stoner and Wohlfarth. Experimental results are reported for magnets made by aligning and compacting single‐domain iron particles 150 angstrom units in diameter, with a median length‐to‐diameter ratio of three to one, and an intrinsic coercive force before packing of 1600 oersteds. The effect of particle alignment and packing fraction on magnetic properties is described, and energy products above three million gauss‐oersteds are reported. These results are compared with properties predicted from theoretical considerations, and with existing permanent‐magnet materials.