Evolution of physical properties of amorphous Fe–Ni–Nb–B alloys with different Ni/Fe ratio upon thermal treatment

Amorphous ribbons (Fe–Ni)81Nb7B12 with Ni/Fe = 0, 1/6, 1/3 and 1 were prepared by planar flow casting. Thermal treatment of samples was performed in vacuum at temperatures chosen to map the evolution of selected properties in the course of transformation from amorphous state. The coefficient of thermal dilatation exhibits changes at temperatures close to the glass transition, Curie and crystallization temperatures; these effects are enhanced or suppressed by cyclic thermal treatments up to the vicinity of these temperatures. The values of saturation magnetostriction λS allow to infer about processes taking place in the investigated materials, especially with respect to formation of new magnetic phases or magnetic anisotropy.Complex processes of structural transformations induced by thermal treatment are strongly affected by Ni percentage. A transitional, magnetically harder phase, which is formed at lower temperatures preferentially near surfaces of the Ni-richest alloy, produces characteristic hysteresis loop shape. This shape disappears after annealing at higher temperatures and enables the material to show the lowest coercivity of the whole alloy series. The saturation magnetic polarization reflects mainly the resulting Curie temperature, which falls with increasing Ni percentage. Magnetic hysteresis loops were also used in the study of dynamics of magnetic domains by MOKE. Domain shape evolution is shown in dependence on composition and thermal treatment as well as a function of applied magnetic field, ranging from remanent sample state to magnetic saturation.

Research highlights▶ Evolution of physical properties in rapidly quenched Fe–Ni–Nb–B system with varying Ni/Fe ratio were investigated. ▶ Dilatation, magnetostriction and magnetization measurements were used to identify the evolution of structure upon thermal treatment. ▶ MOKE observations identified domain structure of ribbon surfaces. ▶ Annealing at and slightly above 500 °C lead to the best soft-magnetic properties. ▶ Saturation is controlled by the shape and Curie temperature of crystalline nanograins. ▶ Magnetic domain shape and behavior is typical of a low-anisotropy soft magnet.