Magnetic properties of FeCuNbSiB nanocrystallized by flash annealing under high tensile stress

We have investigated the creep induced anisotropy, magnetostriction, domain structure and coercivity of nanocrystalline Fe73.5Cu1Nb3Si15.5B7 crystallized by flash annealing under high tensile stress up to 800 MPa. The samples reveal a magnetic easy plane perpendicular to the stress axis with anisotropy constants up to Ku ≈ 12 kJ/m3 which even exceed the local magneto-crystalline anisotropy (K1 ≈ 8 kJ/m3) of the crystalline Fe–Si phase. Although coercivity increases with Ku, it remains reasonably small even for huge induced anisotropy constants. The coercivity mechanism can be understood from the interplay of the induced anisotropy and the random fluctuations of the local magneto-crystalline anisotropy of the crystallites. The tensile stress applied during annealing also affects the saturation magnetostriction constant λs. Thus, λs decreases with increasing magnitude of the annealing stress. This behaviour is compared to the elastic stress dependence of magnetostriction. The latter is well-known for amorphous Co-base alloys, but can also be observed in nanocrystalline alloys. The experimental results will be discussed theoretically in terms of the strain dependence of the magnetic anisotropy energy which ultimately provides the physical origin of magnetostrictive phenomena.