Magnetic properties of a directionally solidified eutectic alloy containing Co17Sm2 fibres

An eutectic alloy containing 44% by volume Co₁₇Sm₂ fibres has been located in the Co-Sm-Sn system and directionally solidified at speeds of between 1.7 and 550 m sec^–1. The spacing , between the fibres and their diameter, d, as a function of the growth rate, R, show the following relations: ²R = 42 m³ sec^–1d²R = 10 m³ sec^–1.Fibre diameters ranging from 1.6 to 0.14m have been obtained and coercive forces of up to 2.3 kOe, for the finest fibres, measured. The behaviour of the coercive force with changing diameter is compared with that in a Bi-MnBi eutectic and in powders made from the same compounds. The results can be interpreted by means of a simple theoretical analysis which shows that the attainment of high coercive forces in powder is restricted by the microscopic surface-defect density introduced during preparation, whilst in eutectics it is limited by the difficulties of growing fine regular structures at high speed.