M. Poppi

Magnetic softening and nanocrystallization in amorphous Co-rich alloys

Abstract In this work we have analyzed the changes of the magnetic properties in the Co-based Metglas 2714A (made by Allied Chem. Corp., USA) caused by isothermal heat treatments in the range of temperature from room temperature to conventional crystallization temperature T cr = 550°C. The nominal composition of the amorphous alloy is Co 66 Fe 4 Ni 1 B 14 Si 15 . The analysis is made on toroidal samples prepared by winding lengths of amorphous ribbon of about 20 cm. The magnetic properties undergo variations depending on the treatment temperature, except for the saturation magnetization which remains unchanged. For heat treatments of about half an hour around 500°C, superior soft magnetic properties are obtained. Particularly, the initial permeability reaches values up to ten times the value of permeability in the as-received sample. Analysis by the transmission electron microscopy of the sample annealed around 500°C reveals the formation of a nanocrystalline phase, with average grain size of 2 nm, embedded in a residual amorphous matrix. The occurrence of permeability increases in concomitance with the formation of the nanocrystalline phase is ascribed to a drastic reduction in the local magnetocrystalline anisotropy randomly averaged out by the exchange interactions, similar to the case of the annealed Fe-based alloys containing Cu.

Nanocrystallization of ferromagnetic Co-rich amorphous alloys and magnetic softening

Ultra-fine grain structures were obtained in soft Co-based amorphous alloys subjected to suitable heat treatments within a restricted range of temperatures which begins well below the conventional crystallization temperature, , of the alloy. These structures lead to superior soft magnetic properties and consist of magnetic nanocrystallites with an average grain size in the range 10 - 12 nm embedded within a residual ferromagnetic amorphous matrix. The results refer to amorphous ribbons of Vitrovac 6025, which were obtained by the single-roller quenching technique and had the nominal composition . With regard to the coercive field and the initial reversible permeability, the best softening nanocrystallization effects occurred in toroidal samples heat treated for about half an hour at around . For the amorphous alloy, the nominal values of (the Curie temperature) and are 210 and , respectively. The results are interpreted on the basis of the random anisotropy model generalized to two-magnetic-phase systems.

Temperature dependence of magnetic properties of a Co-based alloy in amorphous and nanostructured phase

The magnetic properties of a Co-based amorphous alloy (Metglas 2714A) are compared with those of the corresponding nanostructured alloy in the temperature range 30 K to the Curie temperature. A minimum appears in the permeability versus temperature curve well below the Hopkinson peak for the amorphous alloy. This minimum is ascribed to a form of induced anisotropy due to structural instabilities. In contrast, the alloy containing the nanostructured phase, having excellent soft magnetic properties, shows a maximum below the Hopkinson peak which is interpreted as a combination of the variations of the anisotropy and the saturation magnetization with temperature. The anisotropy field distribution is also evaluated in the low-temperature range for both the amorphous and nanostructured phase of the alloy. The most probable anisotropy field for the nanostructured sample is ten times lower than that of the amorphous sample.

Influence of the structural instabilities on the anisotropy fields in Co-based amorphous alloys

Abstract The influence of structural instabilities, produced by thermal treatments on the magnetic anisotropy and initial permeability, is examined in a set of Co-based amorphous alloys. The analysis is made on square strips cut out from as-received ribbons in order to study the magnetic properties along two in-plane perpendicular directions having the same demagnetizing factor. Generally, in the as-received state the samples show an easy magnetization direction parallel to the original longitudinal direction of the ribbon. Annealing, performed at various temperatures, in the absence of applied magnetic fields and mechanical stresses, enhances the original anisotropy. Some exceptions are observed: in one of these alloys the influence of the annealing is very weak, whereas in another alloy the annealing treatment gives rise to a change of the easy axis from the longitudinal direction to the in-plane transverse direction. The evolution of the anisotropy field distribution with the annealing features is also evaluated. The results are discussed on the basis of irreversible changes of local atomic order.

Peculiar behavior of the magneto-impedance in Fe-based amorphous wires under torsion

Abstract The inductive ( ωL ) and resistive ( R ) components of the impedance of an amorphous wire (Fe 77.5 Si 7.5 B 15 ) subjected to torsional stresses are measured. For a critical value of the AC driving current L abruptly increases due to irreversible magnetization processes in the circumferential configuration. Suitable combinations of the AC driving current amplitude and frequency give rise to a peak in the resistive variations and a minimum in the inductance. The inductance shows a bistable asymmetric hysteresis loop ( L vs. H ). The torsional stress increases the coercivity of the longitudinal hysteresis loop of the wire but favors the reaching of the saturation magnetization at lower fields.

Dependence on the temperature of magnetic anisotropies in Fe-based alloys of Finemet

Abstract This paper deals with magnetic anisotropies in Fe-based nanostructured materials. Alloys of Finemet are studied which were obtained from amorphous ribbons by zero-field annealing and by annealing in longitudinal or transverse magnetic fields. A comparison is made between the anisotropies induced during the production processes and those induced by applying external stresses. The analysis is carried out, at temperatures in the range 20–300 K, through the behavior of the initial magnetic permeability and the saturation magnetization intensity. The anisotropy constant values show an increase with decreasing temperature that can be well described by an exponential dependence and are always one or two orders of magnitude lower than the local magnetocrystalline anisotropy. The results are considered on the basis of the random anisotropy model generalized to two magnetic phase systems.

Domain wall relaxation frequency and magnetocrystalline anisotropy in Co- and Fe-based nanostructured alloys

The magnetic behaviour and the frequency dependence of the initial complex permeability are analysed in very soft ferromagnetic Co- and Fe-based nanostructured alloys obtained by thermal treatments of amorphous ribbons. The spectra of the real and imaginary parts of the permeability show the typical features of domain wall relaxation and are characterized by a continuous distribution of relaxation times. In the framework of the random anisotropy model, the spread in the values of the relaxation frequency reflects a dispersion of the effective anisotropy constants and, in turn, a nonuniform distribution of the grain diameters. The results well agree with those deduced by the distribution function of the anisotropy fields and by the scanning and/or transmission electron microscopy analysis.

Temperature dependence of magnetic properties and phase transitions in a soft magnetic Co-based nanostructured alloy

We report the temperature dependence of magnetic properties and phase transitions in a soft-magnetic Co-based nanostructured alloy obtained from amorphous Vitrovac 6025. Homogeneous nanostructured samples are achieved by suitable thermal treatments below the conventional crystallization temperature and have ultrafine grain structures with average grain size of about 10 nm and volume fraction 0.6. The grains are embedded in a residual ferromagnetic amorphous matrix and this leads to superior soft-magnetic properties which, in our view, can be justified on the basis of the random anisotropy model. The temperature dependence of initial reversible permeability and coercive field emphasizes a different behaviour in the magnetic properties of nanostructured and amorphous samples. The sharpness in the Hopkinson peaks of the initial permeability is the best tool to show the first magnetic transition in both specimens: the results are compared with those detected by thermogravimetric curves and by magnetization intensity measurements. Differential thermal analysis is used to evaluate the transformation enthalpy of the amorphous-to-microcrystalline and the nano-to-microcrystalline processes, which both occur at about 820 K where another ferromagnetic phase appears.

Temperature dependence of structural and magnetic relaxation in amorphous and nanocrystalline Co-based alloys

Structural and magnetic relaxations in amorphous alloys containing a nanostructured phase are compared with those of the amorphous precursors Metglas 2714A and Vitrovac 6025, in the temperature range 20 K to the Curie temperature. The analysis is performed by considering the magnetic after-effect (MAE) of the reluctivity which is represented by its relative change Δr/r between two fixed times after demagnetization. The nanostructured samples, obtained by suitable thermal treatments of the amorphous alloys, have superior soft magnetic properties (much higher initial permeability and lower coercive field) but are characterized by a lower structural and magnetic stability in wide ranges of temperature. The characteristics of the MAE in nanostructured alloys seem to be connected with the mean radius of the grains. The activation energy spectra are evaluated on the basis of the logarithmic kinetics observed for MAE. It results that the formation of nanocrystallites makes more uniform the distribution of these energies without modifing significantly the range of their values.

Surface effects on the magnetic properties of nearly zero-magnetostriction amorphous ribbons subjected to nonuniform stresses

Abstract A remarkable initial permeability associated with a decrease of the disaccomodation has been obtained in nearly zero-magnetostrictive Metglas by applying weak elastic bending stresses. The stresses are produced by winding the ribbons to form toroids of different radii. The above effects depend on the way of winding the ribbon i.e whether the shiny surface of the ribbon is at the inside or the outside of the core. The discussion emphasizes a different role of the two surface layers of the ribbon on the behaviour of the samples. The results are explained on the basis of the hypothesis advanced by Hernando et al. who assume the λ s ≃ 0 condition in the above materials as due to the coexistence of different magnetostrictive phases on a macroscopic scale.