Temperature dependence of real part initial permeability (mu') of Fe75.5Cu0.6Nb2.4Si13B8.5 has been measured with different crystalline state of the material. Enhanced value of initial permeability by two orders of magnitude and very low value of relative loss factor of the order of 10(-5) has been observed with the variation of annealing temperature and time, which shows that Fe-based alloys of this composition with lower Cu and Nb content also bears ultra-soft magnetic properties like conventional FINEMET. The results have been correlated with the volume fraction of amorphous and crystalline phases obtained by differential scanning calorimetry. Temperature dependence of initial permeability curve for the annealing temperature, T-a < 475 degrees C, is characterized by the single amorphous phase. The curve is characterized by the two-phase structure for 475 degrees C < T-a < 625 degrees C. The curve is characterized by presence of iron boride phase for T-a > 625 degrees C. (c) 2006 Elsevier B.V. All rights reserved.