The relations between microstructures and magnetic properties have been investigated in Pr17Fe76B5.5Cu1.5 magnets produced by a process of casting, hot-pressing and heat treatment. The crystallization of Pr6Fe13Cu during heat treatment at 753 K for 2 h increases the coercivity. This explains the enhancement of coercivity with the addition of small amounts of Cu. Pr6Fe13Cu has a tetragonal structure, space group 14/mcm, and lattice parameters a = b = 0.81 nm, c = 2.31 nm. Mössbauer spectroscopy and VSM measurements indicate that Pr6Fe13Cu is an antiferromagnetic compound with a Néel temperature of 391 K. Pr6Fe13Cu crystallizes in a peritectic reaction at 918 K, and is stable below that temperature. In Pr-Fe-B-Cu magnets, Pr6Fe13Cu is formed only within a low boron composition range (below 5.5 at%B). In the system Nd-Fe-Cu a compound Nd6Fe13Cu with the same crystal structure is observed.
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