It is known that magnetic and mechanical properties of amorphous alloys based on iron can be controlled by applying a suitable thermal annealing leading to the so-called structural relaxation and nanocrystallization. Recently, we have reported that for the Fe_86-xNb_xB₁₄ (2≤x≤8) amorphous alloys the structural relaxation - i.e. annealing out of free volume and formation of small iron clusters [1] - plays the main role in the process of optimization of soft magnetic properties. It makes possible to obtain a good soft magnetic material free of brittleness - the main disadvantage of nanomagnets. The aim of this paper is to study the influence of Nb content, structural changes and phase stability of amorphous structure on different physical properties. The following measurements were carried out: i) heat of transition amorphous-crystalline state (DSC, 300-800 K), ii) activation enthalpy of crystallization (DSC, magnetic balance), iii) Young modulus (vibrating reed apparatus, 300-800 K), iv) magnetic permeability (Maxwell-Wien bridge, H=0,5 A/m). Phase stability was studied by making use of magnetic after-effects-∆µ/µ (difference in permeability after demagnetization).
It was shown that the heat transition of amorphous-crystalline state, the optimized magnetic permeability and the change of Young modulus due to structural relaxation depend on Nb content. In all cases a remarkable maximum for x=6 at % of Nb, is observed. In contrary to this ∆µ/µ continuously decreases with increasing Nb content. These results show a correlation between free energy (frozen during sample fabrication) and the intensity of structural relaxation. It makes possibly to optimize soft magnetic properties in the relaxed amorphous phase free of iron nanograins.

1. G.Haneczok, J.E.Frąckowiak, A.Chrobak, P.Kwapuliński, J.Rasek, Physica Status Solidi (a) 202 (2005) 2574.

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