L.K. Varga

Structural and magnetic properties of metastable Fe1-xSix (0.15<x<0.34) alloys prepared by a rapid-quenching technique

Fe1-xSix (0.15<x<0.375) alloys have been prepared by a rapid-quenching technique. X-ray diffraction patterns show that these alloys consist of submicron grains between 100 and 300 nm. The DO3 cubic symmetry could be obtained for Si content up to 34 at.% Si. This feature is particularly interesting since alloys containing more than 26 at.% Si and cast with usual techniques consist of a mixture of Fe3Si and Fe5Si3. The magnetization, Curie temperature and resistivity data for these fine-grained alloys are consistent with those for bulk alloys for Si content below 25 at.% Si. In contrast, the structural and physical data for single-phase alloys with 28-34 silicon content are reported for the first time. Within this composition range, the magnetization and Curie point values roughly correspond to those suggested by the extrapolation of existing data. However, the resistivity shows an unexpected marked increase above 25 at.% Si. The values of resistivity, magnetization and effective anisotropy of soft-magnetic nanocrystalline Fe2Si are found to be 200 µΩ cm, 0.6 T and 15 kJ m-3 respectively, suggesting that this alloy has potential for high-frequency applications.

Characterization of luminescent silicon carbide nanocrystals prepared by reactive bonding and subsequent wet chemical etching

Fabrication of nanosized silicon carbide crystals is a crucial aspect for many biomedical applications. Here, we report an effective fabrication method of silicon carbide nanocrystals based on the reactive bonding method followed by electroless wet chemical etching. Our samples show strong violet-blue emission in the 410–450 nm region depending on the used solvents. Raman and infrared measurements unraveled the surface bonding structure of the fabricated nanoparticles being different from silicon carbide microcrystals. This might give an opportunity to use standard chemistry methods for biological functionalization of such nanoparticles.

Stress-induced magnetic anisotropy in nanocrystalline alloys

Abstract Stress-annealing experiments were extended to both nanocrystalline alloy families, Finemet and Nanoperm (Hitperm), and, for comparison, to amorphous Fe 62 Nb 8 B 30 alloy. For both Finemet and bulk amorphous, stress-annealing results in a strong induced transversal anisotropy (flattening of hysteresis loop) but yields longitudinal induced anisotropy (square hysteresis loop) in Nanoperm and Hitperm. These results are interpreted in terms of back-stress theory.

Influence of field annealing on the hysteretic behaviour of the giant magneto-impedance effect of Cu wires covered with Ni80Fe20 outer shells

Abstract The giant magneto-impedance (GMI) effect of Cu wires covered with a Ni–Fe/Ni–Co layered structure has been investigated in as-prepared and heat-treated states. A hysteretic behaviour of the GMI effect as a function of the applied field was observed. This hysteresis is related to the anisotropy field of the wires and can be reduced by field annealing.

Time and temperature dependence of nanocrystalline structure formation in a Finemet-type amorphous alloy

Abstract The magnetic softness and nanocrystalline structure formation of Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 Finemet alloy as a function of annealing temperature (500–600°C) and annealing time (5 s - 3 h) were investigated by means of magnetic measurements, DSC and X-ray diffractometry. Annealed at 550°C the grain size, the solute silicon content of α-Fe grains and the improvement in magnetic softness saturates as a function of time. The deterioration of the magnetic softness after annealing at 575°C is attributed to the Fe 3 Si compound which segregates from the α-Fe solid solution, giving a third intermediate peak on the well-known double-peaked DSC diagram. The crystallization products appearing after annealing at 600°C destroy the intergranular magnetic coupling and give rise to a magnetic hardening.

Magnetic properties of nanocomposites containing Fe-Ni or Fe dispersed in a Mn-Zn ferrite matrix

Summary form only given. Nanocomposites have been synthesised, by mixing in a planetary miller, of nanosized Fe/sub 25/Ni/sub 75/ or Fe and MnZn ferrite powders. Structural properties, grain size and lattice distortion were estimated by X-ray diffraction. Dispersion of the metallic particles was examined by TEM and SEM. Hysteresis loops were measured using a VSM. The powder preparation process increases the anisotropy constant of the ferrite component through the residual lattice distortion. The temperature dependence of magnetization was recorded by Faraday balance measurement, and well-defined Curie points are visible.

Ab initio study of AlxMoNbTiV high-entropy alloys

The Al(x)MoNbTiV (x = 0-1.5) high-entropy alloys (HEAs) adopt a single solid-solution phase, having the body centered cubic (bcc) crystal structure. Here we employ the ab initio exact muffin-tin orbitals method in combination with the coherent potential approximation to investigate the equilibrium volume, elastic constants, and polycrystalline elastic moduli of Al(x)MoNbTiV HEAs. A comparison between the ab initio and experimental equilibrium volumes demonstrates the validity and accuracy of the present approach. Our results indicate that Al addition decreases the thermodynamic stability of the bcc structure with respect to face-centered cubic and hexagonal close packed lattices. For the elastically isotropic Al(0.4)MoNbTiV HEAs, the valence electron concentration (VEC) is about 4.82, which is slightly different from VEC ∼ 4.72 obtained for the isotropic Gum metals and refractory--HEAs.

Systematic study of structural, transport, and magnetic properties of Ni52+xMn26―xAl22 (1 ≤ x ≤ 5) melt-spun ribbons

Structural, magnetic, and transport properties of Ni52+xMn26−xAl22 (1 ≤ x ≤ 5) melt-spun ribbons have been characterized by a variety of experimental techniques. As the composition changed from x = 1 to x = 5, the martensitic transition temperature T0 [ = (Ms + Af)/2] was found to increase from 277 K to 446 K which was attributed to an increase in the valence electron concentration e/a. In the martensitic state, all the samples demonstrate an anomalous semiconducting behavior of electrical resistivity ρ. This uncommon feature of the transport properties has been ascribed to the existence of a gap (Eg ∼ 0.1 eV) at the Fermi level. A crossover from semiconducting to metallic behavior of ρ observed in the martensitic state of Ni57Mn21Al22 is presumably related to a spin-density wave formation at the Neel temperature TN ≈ 300 K. Analysis of a low-temperature (T < 60 K) part of the resistivity curves and comprehensive magnetic measurements of a Ni57Mn21Al22 (x ≤ 5) sample provide grounds for the conclusion tha...

Thermal stability, crystallization kinetics, and grain growth in an amorphous Al85Ce5Ni8Co2 alloy

Thermal stability and crystallization kinetics of the melt-quenched amorphous Al 8 5 Ce 5 Ni 8 Co 2 alloy were investigated by x-ray diffraction and differential scanning calorimetry (DSC). The glass transition was followed by a supercooled liquid region (21 °C) and then by a two-step crystallization process. The final microstructure contained Al 3 Ce, α-Al, Al 3 Ni, and Al 9 Co 2 phases. Isothermal annealing of the as-quenched samples in the range of 275-285 °C showed that both crystallization reactions occurred through a nucleation and growth process. Continuous heating DSC measurements following pre-anneals for different times were also carried out to study the crystallization kinetics and the stability of the material. The Avrami analysis of the isothermal DSC-curves revealed that the 3-dimensional nucleation and growth process became more dominant with increasing annealing temperature. The average specific grain boundary energy corresponded to high-angle grain boundaries and indicated independent nucleation events.

Soft magnetic properties of nanocrystalline Fe100−xSix (15<x<34) alloys

Nanocrystalline Fe 100-x Si x (15 < x < 34) alloys have been prepared by melt spinning with high cooling rate. We have found by X-ray diffraction that the basic cubic symmetry could be extended up to 34at% Si. Around the composition of Fe 2 Si, a single-phase metastable B2 structure was found. If the quenching is not properly conducted, traces of hexagonal Fe 5 Si 3 appear, which is detrimental to the soft magnetic properties. Our M S , T C , K, D sp and p results complement the existing (up to 25at%) data and their soft magnetic properties are herein presented for the first time, The frequency dependence of the complex permeability in this high Si content region is comparable with that of NiZn ferrite.