Influence of heat treatment on the magnetic and piezomagnetic properties of amorphous and nanocrystalline Fe₆₄Ni₁₀Nb₃Cu₁Si₁₃B₉ alloy strips

FINEMET type alloy with substitution of 10 at.% of iron by nickel is studied from the point of view of the influence of the nanocrystallization process on the magnetic and magnetoelastic properties. Strip-shape samples of Fe₆₄Ni₁₀Nb₃Cu₁Si₁₃B₉ amorphous alloy were annealed in vacuum for 1 h in the temperature range of 350–550 °C. Hysteresis loops were measured in order to determine the amount of anisotropy energy and corresponding anisotropy field, and coercive field of each annealed sample. This coercitivity reaches a minimum (about 5 A/m) after annealing at 460 °C. Magnetic field dependencies of magnetoelastic properties were also investigated. We have found a maximum value of the ΔE effect and magnetomechanical coupling coefficient of 61% and 0.85, respectively, that are accompanied by a minimum value of the quality factor of about 2 for the sample annealed at 460 °C. Such good parameters are related with internal stress relief and structural relaxation of the samples. The subsequent drop of the magnetoelastic coupling coefficient for higher annealing temperatures is related directly with a drop on the magnetostriction value, from (22–26) × 10⁻⁶ to 13 × 10⁻⁶, due to the formation of a nanocrystalline structure in the samples. On the other hand, ultrasound velocities, measured at magnetic saturation of each sample, increase continuously between 4.9 and 5.6 km/s as the annealing temperature of the sample increases.