Formation and magnetic properties of nanocrystalline mechanically alloyed Fe-Co

Fe100−xCox powders were prepared by mechanical alloying of the elements in a planetary ball mill. They were investigated with respect to phase formation and magnetic properties using x-ray diffraction, differential scanning calorimetry, and measurements of the saturation magnetization and the coercivity. The measurement of the saturation magnetization proved the true formation of the bcc (x≤80) and fcc (x=90) solid solutions by mechanical alloying. A nonequilibrium microstructure originates from a grain-size reduction to minimum 20–30 nm and the introduction of internal strain up to 1% (root-mean-square strain). An improvement in the soft magnetic properties by the nanocrystalline state, as hoped for, does not occur, because the high amount of internal strain together with the high saturation magnetostriction of the Fe-Co alloys causes relatively high coercivities of 5–40 A/cm. Grain growth and strain relaxation induced by controlled heat treatment of the as-milled powders allowed the separation of the influence on coercivity of the nanocrystalline structure from that of the internal strain: Below about 30 nm grain size, grain growth results in a pronounced coercivity increase superimposed on the general coercivity decrease caused by strain relaxation. This effect can be explained by the random anisotropy model describing the magnetization process in nanocrystalline ferromagnets.