Influence of cobalt content on the structure and hard magnetic properties of nanocomposite (Fe,Co)-Pt-B alloys

The influence of Co content on the structural and hard magnetic properties of two sets of nanocrystalline Fe52−xCoxPt28B20 (x = 0-26) and Fe60−yCoyPt25B15 (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 Mössbauer 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 Fe52−xCoxPt28B20 alloys while a disordered solid solution of fcc-(Fe,Co)Pt was a dominating phase in the Fe60−yCoyPt25B15 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 L10 tetragonal (Fe,Co)Pt nanocrystallites and magnetically softer phases of (Fe,Co)B (for Fe52−xCoxPt28B20) or (Fe,Co)2B (for Fe60−yCoyPt25B15). Detailed Mössbauer spectroscopy studies revealed that cobalt substituted for iron in both the L10 phase and in iron borides. The nanocomposite Fe60−yCoyPt25B15 alloys exhibited significantly larger magnetic remanence and maximum energy products but a smaller coercivity than those observed for the Fe52−xCoxPt28B20 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/m3 were obtained for the Co-free Fe52Pt28B20 alloy while the largest coercivity (HC > 950 kA/m) was observed for the Fe50Co10Pt25B15 and Fe30Co30Pt25B15 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.