Permanent Magnetic Properties of Iron-Cobalt-Phosphides

An investigation of the phase relations of the section Fe2P–Co2P in the system Fe–Co–P showed that at high temperature all (Fe, Co)2P alloys, including the final components Fe2P and Co2P, probably form a continuous range of solid solutions having the hexagonal Fe2P structure. At low temperatures Fe2P and Co2P form an almost complete range of solid solutions with the orthorhombic Co2P lattice. The Curie temperature of the hexagonal phase increases from about room temperature for Fe2P to a value of nearly 190°C for (Fe, Co)2P with an Fe:Co ratio of 70:30. The anisotropy field HA appeared to be a function of the Fe:Co ratio and possesses a maximum value of about 19 000 oe near an Fe:Co ratio of 90:10. Particles obtained by lixiviating an Fe–Co–Cu–P melt can possess a coercivity IHC up to 2000 oe. This value can be substantially increased by annealing. The temperature coefficient of IHC appears to be relatively large and seems to be linked to the change of K with temperature. The coercivity decreases appreciably after mechanical loading, e.g. pressing, but can be restored by annealing; the anisotropy field HA as well as the saturation magnetization σ∞ changes in the same sense. Anisotropic permanent magnets have been made with a BHmax value of about 2×106 gauss oe.