H.K. Lachowicz

Coexistence of various magnetic phases in nanocrystalline Fe-based metallic glasses

Abstract It is shown that, besides the promising prospects of applications, nanocrystalline Fe-based metallic glasses (nominal composition Fe73.5Cu1Nb3Si13.5B9) are interesting objects for basic research because of the variety of magnetic phases which they can show at elevated temperatures. These unusual particulate media show ferromagnetic behavior for temperatures below the Curie point of the residual amorphous matrix, however, in the temperature range above this point they may display superferromagnetic or superparamagnetic properties depending on the size and volumetric fraction of the nanoparticles created within the material by the appropriate annealing. The results of a number of experiments are presented, which show that all the magnetic phases mentioned above can be created in the nanocrystalline material considered.

Magnetostriction of amorphous magnetic materials

Abstract The current state of the art in the magnetostriction research on amorphous materials is presented, in particular in glassy ribbons and amorphous films. A brief description is given of the existing theoretical models as well as of the main experimental methods used to study magnetostriction in these materials. The review presents selected experimental results recently obtained which are important from the standpoint of understanding the mechanism governing magnetostriction as well as leading to the development of new materials for applications.

Magnetostriction and its temperature dependence in FeCuNbSiB nanocrystalline alloy

Abstract The temperature dependencies of the linear saturation magnetostriction of Fe73.5CuNb3Si15.5B7 nanocrystalline alloy have been measured for the samples exhibiting a wide range of the crystalline phase volumetric fraction obtained by a proper annealing procedure. It has been proved that the effective magnetostriction of this material can be interpreted as a volumetrically weighted balance among two contributions, a negative one (for the bcc-Fe(Si) nanocrystallites) and a positive one (for the residual amorphous matrix). The measured temperature dependencies of the effective magnetostriction have been analyzed in order to calculate both contributions to this quantity. This analysis was performed assuming linear as well as quadratic approximation of the dependence of the effective magnetostriction on the nanocrystalline fraction showing that the former reflects better the properties of the material. The above analysis also showed that, in order to obtain zero-magnetostrictive material at room temperature, the fraction of nanocrystallites should be relatively large (∼ 70–80%).

Comments on the indirect measurement of magnetostriction in low-magnetostrictive metallic glasses

Abstract Attention is payed to the anisotropy and magnetostriction, both induced by the tensile stress applied to the metallic glass-ribbons, of low-magnetostrictive alloys, in particular. An influence of both phenomena on the value of saturation magnetostriction, obtained by means of the indirect measuring methods based on the stress-dependence of magnetoelastic anisotropy is discussed. Conditions of measurement which are necessary to obtain correct results are presented.

The effect of particle size and surface-to-volume ratio distribution on giant magnetoresistance (GMR) in melt-spun Cu–Co alloys

Abstract Giant magnetoresistance (GMR), magnetization and structure of granular Cu 90 Co 10 alloy were investigated. The annealed sample is inhomogeneous with respect to the Co-particle sizes and interparticles distances and, therefore, not all particles behave superparamagnetically. The observed deviation from the expected theoretically quadratic dependence of GMR on magnetization can be attributed to differentiated surface-to-volume ratio of superparamagnetic particles as well as to the presence of a small amount of ferromagnetic phase which is created by relatively large Co-rich regions.

On the origin of stress-anneal-induced anisotropy in Finemet-type nanocrystalline magnets

Abstract It has recently been shown that the anisotropy in nanocrystalline Finemet -type magnets, induced by their crystallization under tensile stress, can originate from the magnetoelastic coupling within the crystallites (G. Herzer, IEEE Trans. Magn. 30 (1994) 4800) or can have its source in directional diatomic ordering which occurs also within the volume of nanocrystalline phase (Hofmann and Kronmuller, J. Magn. Magn. 152 (1996) 91). In the present work the temperature dependence of this anisotropy has been investigated for a series of initially amorphous Fe73.5Cu1Nb3Si15.5B7-ribbon samples (Vacuumschmelze, GmbH), nanocrystallized at various temperature-time-stress conditions. It has been assumed that the obtained results will allow to judge which of the mechanisms mentioned above governs in reality the considered anisotropy. An analysis of the results obtained shows that diatomic directional ordering within the volume of the nanocrystalline phase seems to be very likely the origin of the stress-anneal-induced anisotropy in FINEMET -type magnets. This analysis has been performed assuming that the anisotropy originating from the atomic ordering should scale with the square of saturation magnetization in the high-temperature range (in the vicinity of critical temperature-Curie point of nanocrystalline phase) and with the cube of this quantity at low temperatures. Quite satisfactory agreement has been obtained for the best fit of the experimental data to the polynomial consisting of the two above terms.

Temperature dependence of ferromagnetic resonance in granular Cu–Co alloy

Ferromagnetic resonance (FMR) spectra (derivatives of microwave power absorption) were measured in a granular, ribbon-shaped melt-spun Cu90Co10 alloy in a wide range of temperatures from 10 K up to room temperature. Prior to measurements the sample was annealed at 773 K for 1 h in order to create a well-developed granular structure. The results are interpreted using the Kittel FMR equations originally introduced for thin ferromagnetic films, as well as by using an Arrhenius-type exponential function for superparamagnetic relaxations. These results are also compared with the experimental data obtained from low-field susceptibility measurements in both, zero-field-cooling and field-cooling modes. It is shown that FMR spectroscopy is a useful tool for studying magnetic properties of granular magnets consisting of nanometer-scale single-domain particles, in particular, if the resonance spectra are measured in a wide range of temperatures, thus allowing for identification of the magnetic phases of the particle...

Nanocrystalline metallic glass-an unusual particulate medium

The expectation that nanocrystalline Fe-based metallic glass may show a variety of magnetic phases is confirmed experimentally. It is shown that, depending on the annealing conditions (resulting in controlled crystallite size as well as the volumetric fraction of the crystalline phase), the particles exhibit superparamagnetic or superferromagnetic behavior at temperatures above the Curie point of the amorphous matrix. At sufficiently low temperatures (around room temperature), the samples behave ferromagnetically as expected for typical nanocrystalline metallic glass. >

Evolution of transverse permeability with DC-field in Co-based metallic glass ribbons

A simple model is proposed that allows to calculate the penetration depth and the mean transverse permeability in magnetic elements of a rectangular cross-section they are used to study the giant magnetoimpedance effect. As an instance of the use of this model, both the mentioned quantities were calculated for the Co-based metallic glass ribbon. Their evolution with the applied axial DC-field at various frequencies of the AC-current, flowing along the ribbon, is presented. A comparison of the experimental data with those calculated by the model, shows that the latter gives only a qualitative approximation of the measured dependencies.

Magnetoresistance in nanocrystalline Fe-based metallic glass

The ferromagnetic anisotropy of resistivity as well as the effective resistivity of nanocrystalline Fe-based metallic glasses with different volumetric fraction of crystalline phase have been studied. It has been shown that the ferromagnetic anisotropy of resistivity can be considered as a sum of two independent contributions of the opposite sign-positive, originating from the residual amorphous matrix and negative from the crystallites. >