The structure, thermal stability and magnetic properties of melt-spun Fe55Pt25B20-xPx (x = 0-5) alloys have been investigated. Addition of P increases the amorphous-forming ability of an Fe55Pt25B20 alloy, leading to the formation of the full amorphous phase. After annealing, the homogeneous nanosized structure consisting of hard L10-FePt and soft Fe 2B magnetic phases with average grain size of ∼40 nm are obtained for the alloys with x = 0 and 2, while a L10-FePt/Fe2B nanocomposite structure with an average grain size of ∼80 nm is obtained for the alloy with x = 5. As the P content increases, the coercivity ( iHc) of the L10-FePt/Fe2B nanocomposite alloys significantly increases, the remanence (Br) decreases. The alloy with x = 5 shows the largest iHc of 12.1 kOe, while the alloy with x = 2 exhibits higher Br of 53.5 emu/g and better demagnetization curve squareness, which is due to the enhancement in exchange coupling among the uniformly distributed nanosized Fe2B and L10-FePt phases.
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