Effect of thermomagnetic and thermomechanical treatments on the magnetic properties and structure of the nanocrystalline soft magnetic alloy Fe$_{81}$Si$_6$Nb$_3$B$_9$Cu$_1$

The structural and magnetic states of ribbon samples of the soft magnetic alloy Fe–Si–Nb–B–Cu (6 at% Si) have been investigated after the nanocrystallization at a temperature of 550$^\circ$C in a constant magnetic field (thermomagnetic treatment), in a field of mechanical tensile stresses (thermomechanical treatment), and without external effects. It has been shown that exposure to a constant magnetic field or a field of mechanical tensile stresses gives rise to a longitudinal anisotropy of magnetic properties. The magnetic hysteresis loop transforms and becomes close to rectangular. This is accompanied by a significant increase in the residual magnetic induction, which approaches the saturation magnetic induction. While the time required to complete the processes of nanocrystallization is as short as 20 min and, under thermomechanical treatment, the magnetic anisotropy is induced for 20 min, the time it takes to decrease significantly the coercive force of the alloys under thermomagnetic treatment is substantially longer (up to 60 min). After the thermomagnetic treatment, no lattice strains of $\alpha$-FeSi nanocrystals have been found. Either they do not exist at all, or their values are within the error of the X-ray diffraction experiment. In the samples subjected to annealing under tensile loading, anisotropic lattice strains of nanocrystals with the values increasing proportionally to the applied stress have been revealed. The highest strains reaching 1% have been observed after the annealing under a stress of 860 MPa.