Influence of Co content and thermal annealing on structural, magnetic and magneto elastic properties of nanocrystalline Fe–Co–Nb–B alloys

Structural, magnetic and magneto elastic properties of the nanocrystalline Fe84-xCoxNb7B9 (x=17, 25, 33) alloys have been studied using differential scanning calorimetry, X-ray diffraction, hysteresis and magnetostrictive measurements. Successive addition of Co reduces the stability of the alloys against the crystallization and also affects the structural, magnetic and magneto elastic properties. Observed magnetic behaviour suggests that in the coercive field exhibits for the increase of Co content in the alloy there is a changeover from third power dependence to sixth power dependence on grain size, ascribed to the presence of annealing induced anisotropies. Both experimentally obtained and estimated values of the saturation magnetostriction constant shows noticeable changes with increase in the annealing temperature and Co content in the parent alloy due to the increase of volume fraction of the nanograins, stresses and grain growth. Co content in nanocrystalline bcc Fe–Co phase ranges between 10–66%; whereas volume fraction of nanocrystalline phase and average grain diameter is in the range 15–70% and 8–20 nm, respectively.