LAUSR

Abstract The influence of Co content on the structural and hard magnetic properties of two sets of nanocrystalline Fe 52−x Co x Pt 28 B 20 (x = 0–26) and Fe 60−y Co y Pt 25 B 15 (y = 0–40) alloys was studied. The alloys were prepared as ribbons by the rapid quenching technique. The nanocomposite structure in the alloys was obtained by annealing at 840–880 K for 30 min. Structural characterization of the samples was performed using the Mossbauer spectroscopy and X-ray diffraction. Magnetic properties of the samples were studied by the measurements of the hysteresis loops and of the magnetization at increasing temperatures. An amorphous phase prevailed in the as-quenched Fe 52−x Co x Pt 28 B 20 alloys while a disordered solid solution of fcc-(Fe,Co)Pt was a dominating phase in the Fe 60−y Co y Pt 25 B 15 ribbons. Differential scanning calorimetry measurements revealed one or two exothermic peaks at temperatures up to 993 K, depending on the composition of the alloys. Thermal treatment of the samples led to the formation of the magnetically hard ordered L1 0 tetragonal (Fe,Co)Pt nanocrystallites and magnetically softer phases of (Fe,Co)B (for Fe 52−x Co x Pt 28 B 20 ) or (Fe,Co) 2 B (for Fe 60−y Co y Pt 25 B 15 ). Detailed Mossbauer spectroscopy studies revealed that cobalt substituted for iron in both the L1 0 phase and in iron borides. The nanocomposite Fe 60−y Co y Pt 25 B 15 alloys exhibited significantly larger magnetic remanence and maximum energy products but a smaller coercivity than those observed for the Fe 52−x Co x Pt 28 B 20 alloys. Co addition caused a reduction of the magnetization and the energy product in both series of the alloys. The largest magnetic remanence of 0.87 T and the highest energy product (BH) max  = 80 kJ/m 3 were obtained for the Co-free Fe 52 Pt 28 B 20 alloy while the largest coercivity (H C  > 950 kA/m) was observed for the Fe 50 Co 10 Pt 25 B 15 and Fe 30 Co 30 Pt 25 B 15 alloys. Differences in the hard magnetic properties of the nanocomposite alloys were related to different phase compositions influencing the strength of inter-phase exchange coupling interactions.