Effects of cobalt addition on magnetic properties and thermal stability of FeHfBCu-based alloys.

Nanocrystalline (Fe1-xCox)86Hf7B6Cu1 alloys, where x=0,0.2,0.4,0.5,0.6,0.8 and 1, were investigated as potential magnetically soft materials for high temperature applications. The crystalization process of as-quenched alloys, obtained by melt-spinning technology, was studied by differential thermal analysis (DTA). All alloys crystallize in two stages, occuring between 400 degrees C and 800 degrees C at the heating rate of 20 degrees C/min. Partial of complete replacement of Fe by Co results in a decrease of the 1st and 2nd crystallization stage temperature, T1 and T2 respectively. The enthalpy of secondary crystallization slightly increases with the increase of Co content. The as-quenched ribbons were subjected to isothermal annealing from 300 degrees C to 700 degrees C for 1 hour in vacuum. The optimal annealing temperature, defined as the temperature allowing for the lowest coercive field after annealing, for most of the alloys studied is 550 degrees C. The coercive field of as-quenched and annealed ribbons increases with increasing Co content. In order to study the thermal stability of the magnetic properties, the nanocrystalline alloys were subjected to isothermal annealing at 550 degrees C for a range of time up to 1000 h. The coercive field of alloys with Co content x = or <0.4 is below 50 A/m after annealing for 1000 h.