Pavol Butvin

Magnetic response of FeNbCuBSi RQ ribbons to bi-axial strain

Abstract Nanocrystalline strip samples of the FeNbCuBSi class that are macroscopically heterogeneous due to surface /volume differences have been investigated. This heterogeneity is found to be a general property of the class. It represents a base for mutual force influence between the surface and the majority volume beneath. The bi-axial in-plane stress exerted by the ribbon surfaces on the volume is demonstrated first of all by a magnetoelastic anisotropy. The contribution of the creep-induced anisotropy, which can build up under the surface stress at post-treatment temperature, is also found possible.

Influence of heterogeneity on magnetic response of nanocrystalline ribbons

FeNbCuBSi nanocrystalline ribbons are known to show heterogeneities influencing their magnetic properties. To search for a heterogeneity-specific response, the incremental permeability μΔ method was used and domain structure observations served related arguments. Such a response can be found in the hysteresis shown by a plateau observed in the μΔ field variation. Domain structure observation confirmed the existence of an off-plane anisotropy due to the heterogeneity as well as the participation of wall movement when μΔ shows the plateau. Nanocrystalline ribbons, being two-phase materials, show different macroscopic heterogeneity (MH) if compared to a metallic glass.

A relax type wide range field sensor using Finemet

The task to extend the range and dynamic resolution of a proven digital magnetic field sensor is solved. The specific sensor-type requirements are followed when selecting the suitable material from among nanocrystalline ribbons of the FeNbCuBSi Finemet family. The materials are tested by measuring the hysteresis loops, the longitudinal incremental permeability and by the domain structure observation. The macroscopic heterogeneity and the magnetostriction of the ribbons show to be the key issues. Laboratory and field tests show that the best choice is the low-magnetostrictive nanocrystalline Finemet containing 16 at.% Si. The dynamic resolution attains 100 dB, the basic range of the sensor increases by 50% to 120 μT and higher service temperature is enabled if compared to the use of a Co-based metallic glass.

Indication of Intrinsic Macroscopic Forces Affecting Magnetic Properties of Fe-Nb/Mo-Cu-B-Si Ribbons

Si-rich (at.% Si >; 12) Fe-Nb/Mo-Cu-B-Si nanocrystalline ribbons are already successfully used in industry. Still the impact of intrinsic macroscopic stresses on magnetic anisotropy is not known in enough detail. Domain structure, hysteresis loops and surface chemistry have been studied in the as-annealed (nanocrystalline) state as well as the response to surface removal by etching. Creep-induced anisotropy established during annealing was found to be the essential source of the observed anisotropy. Apparently smooth gradient of transversal to longitudinal creep-induced anisotropy was identified from air-side center across the ribbon width as well as across the ribbon thickness. The relative rise of transversal anisotropy due to surface removal points to different surface etching efficiency. Raman spectroscopy confirmed the presence of ferrous oxides on both the ribbon surfaces. The existence of a defined oxide layer generating the macroscopic stress was not indicated.

As-cast anisotropy of amorphous CoFeCrBSi and FeNbCuSiB

Abstract The anelastic polarization (AP) is recognized as the principal source of features common to many rapidly quenched metallic ribbons. To elucidate the role of nonuniform as-cast AP in the behaviour of two rather different magnetic materials, a simple idea is proposed. It assumes and the experiments point to, the presence of AP compressive strain accompanying the obvious tensile one.

Magnetic and Surface Properties of High-Induction Nanocrystalline Fe-Nb-Cu-B/P-Si Ribbons

High-induction Si-poor Fe-Nb-Cu-B-Si Finemet ribbons annealed in inert gas ambience are known to build oxides and prefer surface crystallization. Phosphorus-3 at% of P instead of B was substituted to see how it can influence these surface effects. The off-axis magnetic anisotropy caused by surfaces squeezing the ribbon interior was reduced and it was found out that the major source of the squeeze is the surface crystallization and not the oxides. Phosphorus appears to hamper preferentially the surface crystallization. The substitution also improves the magnetic softness of vacuum-annealed ribbons by grain size reduction. Slight reduction of saturation induction is a minor sacrifice to the improvements.

Distinctive annealing behaviour of Si-poor FeNbCuBSi alloys

Abstract Unlike more Si-rich FeNb 3 Cu 1 B y − x Si x (where y = 18–22.5) alloys, for x ⩽ 10 at.% the building of hard-ribbon-axis magnetic anisotropy is observed after isothermal annealing of toroids at about 540 °C. The hysteresis loops increase their tilting with annealing time while the sense of the variation crosses over after a period depending on the Si contents. Primary increase of a.c. initial permeability is reverted after a short annealing period. Tiny ( d ⩽ 20 nm) α-Fe(Si) crystalline grains are formed in the initial stages of crystallization; their volume share is about 50% after 30 min annealing. Using transmission electron microscopy (TEM), it has been observed that after continued annealing a new crystalline phase is formed on the boundaries of the α-Fe(Si) grains. X-ray diffraction then shows extra reflections, indicating the presence of Fe borides. Several effects that could give rise to the observed behaviour are discussed.

Some evidence for ‘directional atomic pair ordering’ in a cobalt-based metallic glass

Abstract Time-of-flight neutron diffraction has been used to study structural anisotropy in a wide-ribbon sample of the multicomponent metallic glass Fe 4 Co 67 Cr 7 Si 8 B 14 prepared by continuous casting. A planar sample held at 45 ° to the incident beam was examined by two stationary counters at 90 ° left and 90 ° right, so that the directional structure factors, S l(Q), S w (Q), S t (Q) corresponding to the three orthogonal directions in the ribbon (length l , width w and thickness t ) were determined. It was found that the directional Structure Factor S t (Q) had stronger oscillations than S l (Q) and S w (Q) . Comparison of the directional S(Q) with the total structure factors for FeB and CoB glasses suggest that the result may be tentatively associated with ‘atomic directional pair ordering’ of the Fe-Fe first neighbour pairs in the t direction. The diffraction data have been analysed in the context of the atomic level stress model of metallic glass structures.

Structural and magnetic heterogeneity of ternary amorphous metallic alloys

Abstract The addition of 3d-metal (Co, Fe) to the weakly magnetic amorphous host (Pd 80 Si 20 ) causes the appreciable magnetic heterogeneity of the ternary alloy. The heterogeneity is reflected in various magnetic effects as seen by the measurements of magnetization, magnetic susceptibility, FMR, Hall effect and Mossbauer effect. It is found to be linked with structural heterogeneity.

Using metallic glasses for choke cores

The paper reviews major topics which concern the design of most common choke types from the point of view of materials research. Measurements of permeability (/spl mu/) frequency- and field-dependence as well as measurements on model chokes prove advantageous properties of metallic glasses. Preference of rotational processes in core magnetization is pointed out. Limitations in the use of high /spl mu/, of higher induction and in the feasibility of core-dimensions reduction are analysed too. >