Magnetic and Crystalline Microstructures of Fe--Pt--B Nanocomposite Ribbons

We investigate magnetic and crystalline microstructures of melt-spun (Fe_0.675Pt_0.325)_100-xB_x (x=12, 14, 16, 18, 20) nanocomposite ribbons after optimal thermal treatment using a magnetic force microscope. The magnetic microstructures are characterized by darker spots adjacent to brighter ones in a sub-micro scale and in random distribution. It is found that the strength of the exchange coupling interaction between the crystals in the 10-100nm scale, implied by the maximum value (δM)_max of the Henkel plot, could be roughly described by the ratio of the average width of the magnetic spots w to the average crystal size for the ribbons. Moreover, we find that the intrinsic coercivity _jH_c of the ribbons is sensitive to their crystal sizes, and the smaller D, the higher _jH_c. Finally, by using roughness analysis, the curve of the root mean square values (δФ)_rms of the phase shift of the magnetic force images versus the boron content x is obtained, which is qualitatively consistent with that of the magnetization σ_12kOe of the ribbons versus x.