The role of Nb addition to improve the hard magnetic properties in FePt magnet was investigated by characterizing microstructures of both Fe-39.5 at.%Pt binary and Fe-39.5 at.%Pt-0.75 at.%Nb ternary alloys using transmission electron microscopy and atom probe field ion microscopy. In the as-quenched ternary alloy, the FePt ordered domains with a tweed contrast were observed as well as in the binary alloy. However, the ordered domain size of the ternary alloy was slightly smaller than that of the binary alloy and comparable to the thickness of magnetic domain wall. Although the coercivity of the as-quenched ternary alloy was small, subsequent annealing produced high coercivity. Nb was rarely solved in the FePt ordered phase and was concentrated in spherical particles of sub-microns in size. These particles may work as a pinning site against magnetic domain wall motion. Another possibility is that the change in composition ratio between Fe and Pt may be responsible to the magnetic property change.
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