The effect of Co addition on magnetic and structural properties of nanocrystalline (Fe,Co)-Si-B-P-Cu alloys

Magnetic properties of amorphous and nanocrystalline Fe85.2−xCoxSi0.5B9.5P4Cu0.8 (x = 0, 4, 10, 15, 20, 25, 35, 40, 50 and 57 at%) alloys have been investigated and their detailed structural parameters have been quantitatively investigated by X-ray diffraction, transmission electron microscopy (TEM), small angle X-ray scattering (SAXS) and three dimensional atom probe (3DAP). The nanocrystalline structure that forms by annealing the melt-spun amorphous ribbon at above 400 °C comprises a bcc phase and a residual amorphous phase. Both coercivity and magnetostriction of the nanocrystalline state increase with increasing Co content, from Hc = 4 A m−1 to 29 A m−1 and from λs = 14 ppm-55 ppm, respectively. The saturation polarization Js reaches a maximum of about 1.88 T at a Co-content of about 25 at%. The dependence of magnetic properties on the Co content can be explained by the composite rule of the intrinsic magnetic properties of the constituent phases. We also found the grain size dependence of coercivity to change from the well-known D6 dependence to D3-dependence in the composition range where the magnetocrystalline anisotropy constant vanishes.