I. Škorvánek

STRUCTURE, HYPERFINE INTERACTIONS, AND MAGNETIC BEHAVIOR OF AMORPHOUS AND NANOCRYSTALLINE FE80M7B12CU1 (M=MO, NB, TI) ALLOYS

57Fe Mossbauer spectrometry is used to elucidate the structural arrangement and nature of hyperfine interactions in amorphous and nanocrystalline forms of Fe80M7B12Cu1 (M=Mo, Nb, and Ti) alloys. Paramagnetic at room temperature as-quenched M=Mo amorphous alloy shows a distribution of quadrupole splitting (TC=265 K). After partial crystallization, a wide range of hyperfine interactions is observed in the residual amorphous matrix of all samples. The hyperfine field distributions corresponding to the amorphous phase and interfacial zone provide a quantitative basis for a design of topography of hyperfine interactions. Closer inspection of magnetic interactions is made by the help of unconventional radio frequency (rf) Mossbauer technique. The rf Mossbauer results show that the nanocrystalline Fe phase has a larger anisotropy than that of the parent amorphous phase. Conventional magnetic measurements, differential scanning calorimetry, x-ray diffraction, transmission electron microscopy, and scanning tunneli...

Annealing effects on the magnetic properties of nanocrystalline FeNbB alloys

Abstract The influence of microstructure on magnetic behavior in nanocrystalline Fe 80.5 Nb 7 B 12.5 alloys is investigated in a series of specimens with different volume fractions of crystalline phase. The magnetic behavior is studied by VSM magnetometry and by hysteresis loop measurements. A marked increase of Curie temperature of the residual amorphous matrix is observed with increasing volume fraction of BCC Fe nanocrystals. An abrupt deterioration of the soft magnetic behavior takes place at the beginning of the crystallization process. The minimum value of coercivity is found after annealing of specimens in the temperature range 610–630°C, i.e. just before the onset of the crystallization of the residual amorphous matrix.

Laser-induced structural modifications of FeMoCuB metallic glasses before and after transformation into a nanocrystalline state

The effects of laser treatments on the structural and magnetic properties of metallic ribbons have been studied using the melt-spun Fe76Mo8Cu1B15 alloy in as-quenched and nanocrystalline states. 57Fe Mossbauer effect techniques, comprising transmission geometry measurements (TM) and detection of conversion electrons (CEMS), have been employed in addition to magnetization measurements, differential scanning calorimetry and x-ray diffraction. The Curie temperature of the as-quenched alloy was about 70 °C. The distributions of hyperfine magnetic fields as well as quadrupole splitting obtained from TM and CEM spectra have revealed the possibility of observing laser-induced structural modifications even at room temperature when the system is only weakly magnetic. Consequently, both types of hyperfine interactions have been detected and they are nearly in equilibrium (having the same strength or occurring to the same extent). After treatments with a pulsed XeCl excimer laser (with a homogeneous beam of 5×5 mm2, 308 nm, 55 ns, 1 Hz), the significance of magnetic dipole interactions rises as a function of the number of laser pulses (up to 64) and the laser beam fluence (up to 3 J cm-2). No traces of laser-induced crystallization have been found. In the nanocrystalline Fe76Mo8Cu1B15 alloy, surface crystallization was already completely removed after the first pulse of 1 J cm-2.

Mössbauer study of the magnetic properties of nanocrystalline Fe80.5Nb7B12.5 alloy

The nanocrystalline body-centered-cubic (bcc)-Fe phase was formed by controlled 1 h annealing of the amorphous Fe80.5Nb7B12.5 alloy at temperatures ranging from 490 to 650 °C. The microstructure and magnetic properties of the nanocrystalline alloy were investigated by Mossbauer spectroscopy, differential scanning calorimetry, and quasistatic hysteresis loop measurements. Conventional Mossbauer spectroscopy allowed identification of phases and the determination of their relative content. The specialized radio frequency (rf)-Mossbauer technique, which employs the effects induced by the rf magnetic field (rf collapse and rf sideband effects) allowed us to distinguish the magnetically soft amorphous and nanocrystalline phase from the magnetically harder microcrystalline Fe. The rf-Mossbauer experiments performed as a function of the rf field intensity allowed determination of the anisotropy fields in each phase of the nanocrystalline alloy (amorphous matrix, nanoscale bcc-Fe grains). The measurements of the h...

Magnetic study of neutron irradiated FeCrSiB metallic glass

Abstract The infleunce of neutron irradiation on the magnetic properties of Fe 80 Cr 2 Si 4 B 14 metallic glass is investigated by means of 57 Fe transmission Mossbauer spectroscopy and J-H hysteresis loop measurements. It is shown that the irradiation leads to a rearrangement of atoms giving rise to an increase in the average value of the hyperfine field distribution. Beside this, there are indications from both Mossbauer and hysteresis loop measurements that elastic stress centres are produced during the process of neutron irradiation as a result of atom mixing. A tendency of the mean magnetic moment to turn out of the ribbon plane as well as an increase in the coercivity force are observed. Before analysing the results of the magnetic study possible sources of radiation damage are discussed. They originate mostly from fast neutrons and high energy α and lithium particles.

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.

Magnetocaloric Effect of Amorphous Gd 65 Fe 10 Co 10 Al 10 X 5 (X = Al, Si, B) Alloys

Gd-based amorphous alloys with addition of metals and metalloids are interesting materials from the point of view of magnetic and magnetocaloric properties. Moreover, they usually possess high corrosion resistance and good mechanical properties. A series of Gd₆₅Fe₁₀Co₁₀Al₁₀X₅ (X = Al, Si, B) alloys was synthesized by melt-spinning. The amorphous structure of ribbons was confirmed by X-ray diffraction. Calorimetric curves allowed to determine crystallization temperatures, which are in the range of 260°C-303°C. The temperature dependences of the magnetic entropy changes were calculated from series of isothermal magnetization curves using Maxwell relation. Maximum value of the magnetic entropy change for the magnetic field change from 0 to 5 T is 7.1 J kg^-1 K^-1 for X = B, whereas the related refrigerant capacity (RC) is 748 J kg^-1. For Gd₆₅Fe₁₀Co₁₀Al₁₅ and Gd₆₅Fe₁₀Co₁₀Al₁₀Si₅ alloys, the maximum values of magnetic entropy change reached 6.0 and 5.9 J kg^-1 K^-1, while the obtained values for RC parameter were 700 and 698 J kg^-1, respectively.

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.

Melt-Spun Fe–Co–B–Cu Alloys With High Magnetic Flux Density for Relax-Type Magnetometers

Melt-spun Fe63 Co21 B15Cu ribbons were annealed at temperatures between 573 K and 623 K in longitudinal and transverse-magnetic field in order to prepare a representative set of relaxed amorphous and partially crystallized samples having uniaxial anisotropy. The optimal magnetic characteristics for the relaxation sensor were obtained after longitudinal field annealing for 1 h at 593 K, which corresponds to early crystallization stage in the heat treated amorphous material. The magnetic flux density after such heat treatment reaches 1.83 T and the value of coercive field is 4.2 A/m. The corresponding relaxation characteristics obtained by using ferroprobe designed as a flat, double-layer coil tightly surrounding the ribbon core with the dimensions of 60 × 3 mm show good prospects for the potential use of these alloys as core materials in the relax-type fluxgate magnetometers mainly due to the extending of their linear measuring range as compared to the currently used commercial materials while keeping the similar sensitivity.

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.