Gabriel Vlasák

Direct measurement of magnetostriction of rapidly quenched thin ribbons

Abstract A special device for direct measurement of magnetostrictions λ ∥ and λ ⊥ has been constructed. The sensitivity of this apparatus for λ ∥ and λ ⊥ determination is 10 −8 ; the set-up ensures identical demagnetization factor for both measurements. The method and set-up is especially suitable for thin flat samples, such as alloys produced by rapid quenching. From the dependences λ ∥ ( H ) and λ ⊥ ( H ), the saturation magnetostriction λ S , volume magnetostriction ω and force magnetostriction ∂ω / ∂H are determined as function of external field H .

Influence of heat treatment on the magnetic and piezomagnetic properties of amorphous and nanocrystalline Fe64Ni10Nb3Cu1Si13B9 alloy strips

FINEMET type alloy with substitution of 10 at.% of iron by nickel is studied from the point of view of the influence of the nanocrystallization process on the magnetic and magnetoelastic properties. Strip-shape samples of Fe 64 Ni 10 Nb 3 Cu 1 Si 13 B 9 amorphous alloy were annealed in vacuum for 1 h in the temperature range of 350-550 °C. Hysteresis loops were measured in order to determine the amount of anisotropy energy and corresponding anisotropy field, and coercive field of each annealed sample. This coercitivity reaches a minimum (about 5 A/m) after annealing at 460°C. Magnetic field dependencies of magnetoelastic properties were also investigated. We have found a maximum value of the ΔE effect and magnetomechanical coupling coefficient of 61% and 0.85, respectively, that are accompanied by a minimum value of the quality factor of about 2 for the sample annealed at 460°C. Such good parameters are related with internal stress relief and structural relaxation of the samples. The subsequent drop of the magnetoelastic coupling coefficient for higher annealing temperatures is related directly with a drop on the magnetostriction value, from (22-26) x 10 -6 to 13 x 10 -6 , due to the formation of a nanocrystalline structure in the samples. On the other hand, ultrasound velocities, measured at magnetic saturation of each sample, increase continuously between 4.9 and 5.6 km/s as the annealing temperature of the sample increases.

Saturation magnetostriction determination of annealed Fe73.5Nb3Cu1Si13.5B9 alloy using two methods

Abstract The saturation magnetostriction ( λ s ) of Fe 73.5 Nb 3 Cu 1 Si 13.5 B 9 alloy samples was determined using the capacitance method and the transverse susceptibility method. The values of λ s decreased from (24.3–26.3) × 10 −6 to (0.5–2.5) × 10 −6 and then increased to 5.6 × 10 −6 for the sample annealed at 650°C. The differences of the two methods were from 0 to 20% in the amorphous state and somewhat higher for the nanocrystalline state.

Correlation between microstructure and magnetic properties of amorphous and nanocrystalline Fe73.5Cu1Nb3Si16.5B6

Abstract The correlation between microstructure, saturation magnetostriction constant λ s , coercive field H c and remanence of nanocrystallizing Fe 73.5 Cu 1 Nb 3 Si 16.5 B 6 amorphous alloy after 1 h annealing at 400–800 °C was studied. The amorphous ribbons (4 mm wide and 20 μm thick) were prepared in air by the single-roller chill-block melt-spinning method. The structure of partially crystallized alloys was investigated using differential scanning calorimetry (DSC), X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The saturation magnetostriction constant λ s was measured at room temperature using two methods: the three-terminal capacitance (TTC) method and the strain-modulated ferromagnetic resonance (SMFMR) method. Quasi-static hysteresis loop was measured in order to determine H c and λ s . Very soft magnetic behavior (H c = 0.7–0.9 A m −1 ) was observed for two-phase nanocrystalline material composed of amorphous matrix and α-Fe(Si) crystallites 12–15 nm in diameter obtained after annealing at 480–520 °C. The observed effect of the almost constant value of H c when a substantial decrease of λ s was measured is ascribed to the compensation of the decrease of magnetoelastic anisotropy by the increase of effective magnetocrys-talline anisotropy related to the observed increase of average grain diameter. The observed differences in λ s measured using TTC and SMFMR methods suggest a higher volume fraction of crystallites at the surface than inside the ribbon.

Evolution of Structure and Magnetic Properties of Rapidly Quenched Fe–B-Based Systems With Addition of Cu

Rapidly quenched Fe85B15 and Fe64Co21B15 with addition of 1 at.% Cu have been prepared by planar flow casting. Selected magnetic properties were measured in as-cast state and after annealing targeted to produce fine-grain structure of body-centered cubic (bcc)-Fe in amorphous matrix. The transformation process has been followed by electrical resistivity and magnetization measurements. The evolution of structure has been compared with alloys without the Cu addition. Suitable processing conditions are analyzed in order to obtain the control of size and content of crystalline phase. The results are discussed with respect to the possible enhancement of properties related to the microstructure and leading to potential application for power electronics or alternatively to convenient tailoring of magnetic characteristics using thermal treatment in external magnetic fields.

Influence of Structure Evolution on Magnetic Properties of Fe–Ni–Nb–B System

The effect of combined presence of iron and nickel has been studied in rapidly quenched amorphous (Fe-Ni) 81Nb7B12 system with the ratio of Fe/Ni=2/1 and 1/2 in as-quenched state and after annealing. Field dependencies of magnetostriction as well as the values of saturation magnetostriction were correlated with the evolution of nanocrystalline structure in amorphous matrix in the temperature range from ~ 700 to 800 K and after complete crystallization above 900 K. Intervals of stability and transformation regions were determined from temperature dependencies of electrical resistivity. The structure after annealing at selected temperatures was identified by X-ray diffraction (XRD), transmission electron microscopy (TEM), and electron diffraction. MO¿ssbauer spectroscopy was used only as a complementary method to demonstrate the behaviour and development of the paramagnetic (FeNi) 23B6 phase from the original as-quenched structure. MO¿ssbauer and magnetostriction measurements were performed at room temperature. The observed field dependencies of magnetostriction are a combination of magnetostrictions of ferromagnetic and paramagnetic phases formed during the transformation process, namely, the nanocrystalline cubic Fe-Ni phases and face-centered cubic (fcc)-type structure (FeNi)23B6, as identified from the structure analyses. The transition from purely ferromagnetic to partially paramagnetic state is well observed in the evolution of MO¿ssbauer spectra evolution. Different ratios of both ferromagnetic components Fe and Ni lead to a change of the structure of the nanocrystalline phases and thus also to a change in the magnetic behavior of the system.

Application of isochronal dilatation measurements for determination of viscosity of amorphous alloys

Abstract The viscosity of metallic glasses and its temperature dependence is well descriptive of the amorphous structure and reflects the structural changes or the influence of preparation conditions on the thermodynamic state of these materials. Determination of temperature dependence of viscosity from isothermal measurements is rather tedious. The present work describes a method of obtaining the values of viscosity from measurement of dilatation and isoconfigurational flow in isochronal regime. The viscosity η ( T ) is calculated from the sample length changes, which are given as a sum of contributions due to thermal dilatation, flow and elasticity. Thermal dilatation is determined from temperature dependence of dilatation of crystallized sample. Elastic length changes are expected to be small enough to be negligible. The viscosity η ( T ) is then η ( T )= Kl ( T ) σ /[d l ( T )/d t ], where K is a numerical constant, l ( T ) the sample length and σ is the applied specific load. This method of dilatation determination has been tested on Ni 25 Zr 55 Al 20 metallic glass. The measured viscosity curves were found to be in excellent agreement with isothermal viscosity values and furthermore exhibited smooth temperature dependence allowing to determine processes taking place in the material, e.g. glass transition.

Piezomagnetic properties of annealed Fe-Cu-Nb-Si-B strips

Abstract The magnetic field dependences of the piezomagnetic dynamics, elasticity moduli and magnetomechanical coupling in the Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 strips annealed in vacuum for 1 h at 300–600 °C were investigated. The maximum values of the magnetomechanical coupling coefficient (0.7) and maximum changes (ΔE effect), from 50–60 to 170–180 GPa, were observed after annealings at 440–460 °C.

Phase Transformations in Amorphous Bilayer Ribbons

Bilayer ribbons were prepared by rapid quenching from the melt using a double-nozzle technique. The composition of the layers was selected from the Fe/Co-Si-B and Fe-Cu-Nb-Si-B systems, respectively. Ribbons with typical thickness of 45-50 microns and width of 6 mm and 10 mm exhibited amorphous structure of both layers in as-quenched state. Temperature dependencies of electrical resistivity, dilatation and magnetization have been investigated in the amorphous state and during crystallization of both layers, which take place at different temperatures. The results combined with investigation of the structures formed in each layer and at the layer interface were compared to those of single-layer ribbons having the compositions of each layer, respectively.

Evolution of magnetostriction in Fe73.5−xNixCu1Nb3Si13.5B9 (x=0, 10, 20, 30, 40) alloy in the course of transformation

Abstract Magnetostrictions λ ∥ H , λ ⊥ H , ω ( H ), λ ∥ , λ ⊥ and ∂ ω /∂ H were studied on the Finemet alloy system Fe 73.5− x Ni x Cu 1 Nb 3 Si 13.5 B 9 . The dependencies λ ∥ H show that they are a superposition of at least two such dependencies originating from two phases with different degree of separation. From the values of λ ∥ , λ ⊥ and ω ( H ) the λ S and ∂ ω /∂ H values were determined, respectively. Temperature dependencies of λ S change with the concentration of Ni, decreasing from 25×10 −6 to 9×10 −6 ; ∂ ω /∂ H follows the internal processes taking place in the transformation. In amorphous state the spontaneous volume magnetostriction and λ S values decrease with increasing Ni content.