Effective magnetic anisotropy and internal demagnetization investigations in soft magnetic nanocrystalline alloys

Abstract In order to clarify the effect of the initial anisotropy K 1 of the nanograins on the effective 〈 K 〉 of two-phase nanocrystalline Fe 73.5 Si 13.5 B 9 Nb 3 Cu 1 (Finemet) and Fe 86 Zr 7 B 6 Cu 1 alloys, the coercivity, virgin and anhysteretic curves have been studied at room temperature after different annealing. The experimental determination of the effective 〈 K 〉 was found to be 280 and 500 J/m 3 , respectively, instead of the order of unity J/m 3 expected by random anisotropy model calculations. The internal demagnetization factor, as a measure of magnetic heterogeneities was found to have a negligible contribution to the effective 〈 K 〉 in both materials although it is higher in the FeZr-based one. The coercivity results are explained well by our adaptation of the Friedberg and Paul formula in which the ratio of the Curie temperatures are considered for the ratio of the two local exchange constants. Further more calculations are developed from this formula in order to connect qualitatively the coercivity and the internal demagnetizing factor