Stress level in Finemet materials studied by impedanciometry

Abstract

In this work, a study of the stress relief in Finemet ribbons, Fe73.5Cu1Nb3Si16.5B6, as a function of the annealing temperature is presented. The as melt-spun samples are amorphous and become partially crystallized after annealing at appropriate temperatures. For temperatures TA⩾480 °C the samples are nanocrystalline, with a microstructure composed by α-Fe1−xSix (x∼0.2) crystallites (10 nm average diameter) embedded in an amorphous matrix. Nanocrystallization, associated with stress relief effects, improves the soft magnetic properties of this kind of material. The stress level was quantified using magnetorestriction (measured by SAMR), magnetoelastic anisotropy, and domain wall energy data obtained from impedance spectra measurements. A reduction of the internal stress from 15 to 0.2 MPa was verified when comparing the as-cast to the sample annealed at 580 °C. Improvement of the magnetic softness of the samples was also followed by the increase of the domain wall and magnetization rotation contributions to the overall effective permeability.

Domain structure in Joule-heated CoFeSiB glass-covered amorphous microwires probed by magnetoimpedance and ferromagnetic resonance

“…This is an indication that no domain-wall permeability is present, which would give rise to a single-peak response at low frequencies; more precisely, below the domain-wall relaxation frequency which is around 300 kHz. 8,9 The MI curves for the Joule-heated samples are very similar to the as-cast ones with the difference that the MI peak field and amplitudes change from sample to sample. The evolution of the maximum value of the MI (MIϭ100 ϫ͓Z(H)ϪZ(H max )͔/Z(H max )), with the measuring frequency is presented in the Fig.…”

Improving soft magnetic properties in FINEMET-like alloys. A study

“…The effect of the internal stresses, r i , has also to be computed in the value of Ku el since a total balance of the effective magnetostriction constant, k s , is not always possible, as we will see later on. Ribbons obtained in the as-quenched state have a high stress level reported in 30-15 MPa [12,13] that are reduced with subsequent heat treatment until the first stages of nanocrystallization (low m cr ), where there is an increase in r i due to the grain nucleation and a relatively low annealing temperature. At optimum annealing temperature with a relatively high m cr , stresses can be released up to 2.5-0.2 MPa.…”

Study of CoFeSiB glass-covered amorphous microwires under applied stress

“…[2][3][4] In particular, at high probe current frequencies, ferromagnetic resonance ͑FMR͒ can be observed in the impedance spectra, improving the capabilities of extracting micromagnetic parameters from magnetoimpedance measurements. 1 and the references therein͒.…”