M. Celasco

The effect of surface on the power spectrum of the Barkhausen noise in ferromagnetic materials

The Barkhausen noise power spectrum was investigated on laminated polycrystalline toroids of Fe and 4% SiFe as a function of sample thickness; that is, of surface-to-volume ratio. Upon decreasing the thickness strong variations of spectrum shape are observed-the cut-off frequency increases and the spectral density drops in the very-low-frequency range. On the other hand, only negligible variations of static magnetic properties are observed. According to a recent theory, the results are explained assuming that a change of thickness directly affects the number of elementary pulses clustering into a large Barkhausen discontinuity. The calculated variations of the spectrum intensity and cut-off frequency are in good agreement with the experimental results.

Experimental study of Bloch wall surface pinning in 3% Si-Fe monocrystalline lamination

Abstract Surface pinning of Bloch walls has important effects on the magnetization process of ferromagnetic laminations. For oriented materials it may be responsible for the largest part of the hysteresis loss, as shown by experiments made on laminations having different thickness. A new aspect of surface pinning concerns its influence on wall bowing and on the dynamic loss. The local excess loss generated by the surface Barkhausen jumps hinders the higher mobility of the wall near the surface predicted by the skin effect, and thus changes a few important conclusions about the behaviour of the dynamic loss when the magnetizing frequency is increased. In the present paper a study of the surface pinning is performed by means of an optical technique making use of the longitudinal Kerr effect and of two laser beams to apply cross-correlation analysis. Results about wall bowing and pinning field and the lamination surface are given.

Optical study of Bloch walls dynamics during the magnetization process of Fe-Si G.O. laminations under controlled induction waveform

By using an optical technique, the dynami cal behaviour of local Bloch wall structures characterized by differently spaced antiparallel domains on the same Fe-Si 3% grain-oriented lamination has been invest igated. A previously developed theory of the magnetic power loss in ferromagnetic laminations allows the correlation of the power spectrum of the time derivative of the optical signal with the local magnetic power loss. From this analysis it can be concluded that on a polycrystalline grain oriented material, magnetized under controlled waveform conditions, the local magnetic power loss is weakly dependent on the domain spacing, particularly when the magnetizing frequency is not very small. This effect, which is mainly due to a balancing effect of eddy currents on the Bloch walls speed is also responsible for the so called curvature anomaly of the loss per cycle vs.frequency plot in such materials.

Bloch walls motion homogenization effects due to a mechanical tension in Fe-Si G.O. laminations

Abstract An investigation of the magnetic power loss distribution over different grains in a Fe-Si 3% grain oriented lamination is made by using an optical technique recently developed. The study is done by evaluating the power loss as function of the magnetizing frequency under different physical conditions, characterized by the application of a mechanical tension to the lamination. It is seen that the local loss in well oriented grains decreases by effect of the tension much more than the average loss in the whole specimen.

Experimental study of local correlation effects between Bloch walls in FeSi 3% G.O. laminations

Abstract An optical technique for the study of correlations in the Bloch wall movements during the magnetization process of a ferromagnetic material is presented. The technique allows to visualize the domain patterns and to select two small regions on the same wall or on different walls for the study of the correlation. Some preliminary results concerning transverse correlation effects on contiguous walls separated by only 0.3 mm are presented.

Study of the dynamical power loss upon annealing in amorphous metglas ribbons

Abstract Magnetic power loss measurements on single-strip Metglas 2826 amorphous alloy before and after thermal treatment are reported. In both cases measurements were made with the strips under a slight tension to ensure a regular pattern of antiparallel domains. The results show that in these conditions the magnetic aftereffect does not appreciably influence the magnetic loss. However, they do not rule out the possibility that such an influence exists when a more complicated domain pattern and stress distribution exists within the specimen.

Correlation between power losses and viscosity field in amorphous magnetic alloys

Abstract Amorphous ferromagnetic alloys in the form of thin ribbons show exceptionally high dynamic losses, which cannot simply be explained by the high ratio between Bloch wall spacing and thickness of the ribbon. In the present paper preliminary experimental results are given about a possible contribution to losses caused by viscosity field effects on the magnetization dynamics of amorphous alloys.

Optical study of the coherence of Bloch walls motion during magnetization in Fe-Si 3% G.O. laminations

Through an optical technique making use of the Kerr effect, the motion of the Bloch walls on G.O. polycrystalline and single crystal Fe-Si strips is studied. The aim is to evidence the effects of internal fields on the wall motion, as produced by a driving field applied to the specimen. Results show that the motion of the walls in well oriented single crystal specimens is very little influenced by internal fields when displacements much larger then single Barkhausen jumps are taken into account. On the contrary, in polycrystalline G.O. specimens strong interaction effects arise, particularly in the low frequency components of the forced motion. Some implications of these results on the theory of the magnetic power loss are discussed.

Influence of the viscosity field on the power losses in amorphous magnetic alloys

Abstract Thermal annealing of an amorphous alloy showing large viscosity field effects, as Metglas 2826, produces a strong reduction of the dynamic loss, probably related to a corresponding reduction of the viscosity field. The effect is not related to changes in the magnetic domain structure due to the anisotropy induced when the material is annealed under magnetic field, but seems to be simply related to a stabilization of the amorphous atomic structure.