Magnetic properties of amorphous metals

Neutron scattering has provided unique information about the nature of magnetism in amorphous alloys. This paper reviews some of the results obtained principally on two ribbon-form metallic glass systems - (Fe_xNi_1?x)₇₅P₁₆B₅Al₃ and Fe_xB_1?x. The former exhibits three states depending on composition: pure ferromagnetism, re-entrant spin glass, and ordinary spin glass. The neutron results provide evidence for the coexistence of spin glass and ferromagnetic correlations in the intermediate regime. The alloys of Fe and B show Invar phenomena which result in anomalously small values of the effective spin-wave stiffness calculated from low-temperature magnetization and Mössbauer data. The long-wavelength spin waves as measured by the neutrons do not reflect the rapid demagnetization found in the bulk results, implying the presence of excitation processes other than long-wave-length spin waves.     

Magnetic energy losses in field-quenched amorphous ribbons

The influence of transversal anisotropy on the magnetic energy losses in CoFeSiB amorphous ribbons is investigated. This transversal anisotropy has been induced by applying a magnetic field in the transverse direction during the quenching process. The results show that the magnetic energy losses are reduced. This decrease is explained on the basis of the magnetization process.     

Short range order in powder of amorphous anodic yttrium oxide

The characteristics of the short-range order in a powder of amorphous yttrium oxide obtained by anodic oxidation are determined by the Finbak-Warren method. It is established that the first coordination number equals seven. Amorphous oxide is built by irregular coordination polyhedra characteristic of the hexagonal and monoclinic modifications of yttrium oxide.     

Electric field gradient in transition metal amorphous alloys

The mean atomic configurations in Hf- and Zr-based amorphous alloys prepared by different routes were studied using the perturbed angular correlations (PAC) technique. For all the as-prepared amorphous samples, the PAC spectra are well described by a wide distribution of frequencies revealing atomic configurations around the probes compatible with those in a dense random packing of ions (DRP). The measured electric field gradient strength V_zz seems to be independent of the preparation method and of the addition of second phase nanoparticles (CaO and ZrC) for the same early transition metal content. The V_zz values observed in Hf based alloys are systematically higher than that in Zr-based alloys of similar composition and show a greater variation with composition. These behaviors are discussed in terms of the different contributions to the EFG, its dependence with the density of states at the Fermi level and in comparison with similar trends observed in binary amorphous alloys. Also, the Ta probe role on the measured V_zz in Hf and Zr systems was analyzed.     

Magnetic properties and low frequency magnetoimpedance of novel amorphous CoFeTaBCr ribbons

In this report, we study the effect of Cr addition on the dc an ac magnetic properties of novel melt spun amorphous Co_43−x/2Fe_20−x/2Cr_xB_31.5Ta_5.5 (x = 0, 1, 2, 4) alloy series. The saturation magnetization decreases with increasing Cr content as well as the ac relative initial permeability, which showed a high relaxation frequency of 3 MHz. In addition, the magnetoimpedance effect was detected in this alloy series, with a maximum variation ratio of 3% at x = 0.     

A simple method for forming amorphous rare earth-transition metal alloy nanotube arrays

Arrays of rare earth-transition metal alloy Co₆₈Nd₃₂ and Co₉₆Sm₄ nanotubes, with amorphous structures and outer diameters of ca. 270–330 nm and with lengths of over 30 μm, have been fabricated for the first time by direct current electrodeposition with a mercury cathode via anodic aluminum oxide (AAO) template. The using of mercury cathode is a crucial condition for the growth of nanotubes and the co-reduction of Co²⁺, Nd³⁺ and Sm³⁺ in aqueous solution without any chemical modification of the pores of AAO, since the liquid mercury has characters of excellent conductivity and fluidity, high surface tension, high photoelectric work function, and high over-potential. A possible growth mechanism for the nanotubes is proposed. The as-prepared nanotube arrays display low coercivity and uniaxial magnetic anisotropy, and the easy magnetization direction is perpendicular to the nanotubes axes.     

Fractal behavior of amorphous metal structure under the action of tensile stress

We report a series of time-separated topograms of the stressed surface of a foil made of metallic glass Fe₇₀Cr₁₅B₁₅. The numerical analysis of the tortuosity of both normal profiles and horizontal sections of the surface relief demonstrates a fractal/self-affine geometry as beginning from the nanoscopic scale level. Unlike the foils made of convention metals whose surface profile do not exhibit any features of fractality, in the original metallic glass foil the scaling character of the profile depth distribution was revealed. This is a statistical representation of non-equilibrium nanoscopic structure resulting from the highly critical glass forming process in the amorphous alloy. The temporal variation of fractal dimension after applying the 500 MPa tensile stress includes a rise to 1.46 ± 0.06 at the initial stage of loading, dramatic drop to 1.12 + 0.03, and gradual increase to 1.22 ± 0.02 just before the formation of regular shear bands, that is in ∼1.5 h after the stress application. The mechanism of formation of the transient fractal structures is discussed in terms of mid-range order dynamics in disordered media.     

Photoinduced metal surface deposition in amorphous GeSAg films

Metallic silver photosurface deposition (PSD) and photoinduced stractural transformation in amorphous GeSAg films containing a large quantity of Ag have been studied. The photosensitivity of the films was greatly enhanced by the addition of gold, and also the morphology and distributing density of the deposits were markedly influenced. It has been found that the PSD effect can be reversed by a thermal annealing.     

Magnetic nanostructures in liquid and amorphous materials as probed by small angle neutron scattering

Small angle neutron scattering using polarized neutrons is introduced as a special type of contrast variation for magnetic systems. Examples of diluted magnetic systems are reviewed where low magnetic contrasts had to be analyzed beside strong nuclear contributions or vice versa. In Ferrofluids magnetic core–shell composite particles and magnetic aggregates could be precisely evaluated beside non-magnetic micelles and free surfactants of similar sizes. In more concentrated Co-Ferrofluids an external magnetic field induces a pseudo-crystalline hexagonal ordering which coexists with chain like arrangements of particles. In magnetic glasses magnetization density profiles have been monitored. Magnetically inactive deadlayers or weakly magnetic interfaces separate nanocrystalline ferromagnetic particles from amorphous (para- or ferromagnetic) matrices in soft magnetic materials based on Fe₃O₄-glass ceramics, Fe–Si–B and Fe–Nb–B alloys. Nanosized inclusions have been detected and identified to be at the origin of the hard magnetic properties of Nd based alloys.     

Consolidation of blended powders by severe plastic deformation to form amorphous metal matrix composites

Warm equal channel angular extrusion (ECAE) in 90° tooling is used to consolidate Vitreloy 106a plus tungsten powders. A fully dense amorphous matrix composite showing good infiltration of the amorphous phase in between crystalline particles and uniform consolidation is achieved after one ECAE extrusion in the supercooled liquid temperature region of the amorphous phase. The results demonstrate that ECAE appears to be a viable processing method for producing bulk metallic glass matrix-crystalline particle composites.     

Magnetic properties of liquid 3d transition metal–Si alloys

The magnetic susceptibility has been measured for liquid transition metal–Si alloys (TM_1−cSi_c, TM = Ni, Co, Fe, Mn) as a function of temperature. The magnetic susceptibilities of liquid Ni_1−cSi_c with c ⩾ 0.3, liquid Co_1−cSi_c with c ⩾ 0.4 and liquid Fe_1−cSi_c with c ⩾ 0.6 were found to be almost independent of temperature, which suggests that the Ni, Co and Fe ions on the Si side are in the non-magnetic state. Liquid Ni_1−cSi_c with c ⩽ 0.2, liquid Co_1−cSi_c with c ⩽ 0.3 and liquid Fe_1−cSi_c with c ⩽ 0.5 show the Curie–Weiss behavior with a reasonable value of effective Bohr magnetic number. Liquid Mn_1−cSi_c with c ⩽ 0.2 and c ⩾ 0.6 were found to be in their non-magnetic state. However, the Curie–Weiss behavior was observed for liquid Mn_1−cSi_c with c = 0.3, 0.4 and 0.5. The magnetic susceptibility of their liquid alloys in the non-magnetic state has been studied by using Anderson model. The density of 3d states at the Fermi level can be estimated from the data of magnetic susceptibility.     

Relaxation and crystallization of mechanically alloyed amorphous Ni35Ti65 powder

The relaxation and crystallization of mechanically alloyed amorphous Ni₃₅Ti₆₅ powder was studied by differential scanning calorimetry. The results of isothermal measurements are discussed according to the Primak formalism in terms of an activation energy spectrum for relaxation, and using the Johnson-Mehl-Avrami formalism for crystallization kinetics. The results are compared with those obtained in an earlier study of melt spun specimens of the same composition.     

On the in-situ growth of aligned composites by means of disproportionation of amorphous metal systems

Results are reported of experiments aiming at the aligned disproportionation of amorphous metal systems in a negative temperature gradient. The crystallization behaviour is discussed in terms of nucleation and growth.     

Magnetic and magnetoelastic properties of cobalt–iron amorphous–nanocrystalline pulsed laser deposited thin films

We prepared pulsed laser deposited planar and cylindrical amorphous–nanocrystalline Co–Fe thin films using the corresponding target with composition Co_1−xFe_x, x = 0, 0.02, …, 1.0. Their room temperature spontaneous magnetization, M_s (film), was always a fraction of the M_s of the corresponding crystalline alloy, M_s (film) = γ M_s (crystal): γ ≈ 0.8 for pure Co, γ ≈ 0.88 for the Co₃₅Fe₆₅ film and γ ≈ 0.94 for pure Fe. Their isotropic magnetostriction coefficient, λ_s, was also determined. From pure Co to 30 at.% Fe λ_s values were similar to those corresponding to the crystalline alloys: from pure Co to 4 at.% Fe was negative and of the order of 10⁻⁶; λ_s increased to 10⁻⁵ up to 25 at.% Fe and achieved 10⁻⁴ from 30 at.% Fe to 90 at.% Fe; λ_s decreased to 10⁻⁵ for pure Fe. A chemical short-range order between the Co atoms surrounded by the Fe ones, increasing the magnetic moment of Fe atoms, was used to explain the observed behavior.     

Electroabsorption studies on amorphous silicon

The electroabsorption spectra of plasma deposited and chemical vapor deposited amorphous silicon films have been studied in comparison with their absorption. In all cases a peak with 350–400 meV halfwidth is observed similar to the results on chalcogenide glasses. The peak occurs at moderate values of the absorption constant near 10⁴ cm^?1 and follows the shift of the absorption edge with variation of the deposition temperature. No response has been obtained in the low absorption region below 1.5 eV. The field modulated reflectance which has been calculated from the electroabsorption data is about two orders of magnitude smaller than the electroabsorption signal. The electroabsorption spectrum is interpreted as resulting from the influence of the external field on the localized states only.     

Kinetics of crystallization of titanium based binary and ternary amorphous alloys

Metallic glasses are kinetically metastable materials. These amorphous materials can be transformed into a crystalline state by both isothermal and isochronal methods. The study of this transformation, and hence the thermal stability of metallic glasses, are important from an application view-point. In the present work, the non-isothermal crystallization kinetics of two titanium-based amorphous alloys namely, Cu₅₀Ti₅₀ and Ti₅₀Ni₃₀Cu₂₀, are reported. The activation energies for crystallization, E_c for both the systems have been evaluated using different non-isothermal methods viz. derived through Kissinger, Augis and Bennet and Ozawa. The values of E_c obtained using these methods are consistent for both the metallic glasses and it is found that E_c for the ternary metallic glass is considerably higher than the binary metallic glass. The increase in the activation energy on the substitution of Ni in the Cu–Ti metallic glass suggests the increase in the thermal stability.     

Magnetostatic bias in soft/hard bi-phase layered materials based on amorphous ribbons and microwires

Bi-phase soft/hard magnetic systems have been prepared by electroplating CoNi alloy with relative hard magnetic character (10² Oe coercivity) onto ultrasoft (coercivity less than 1 Oe) FeCo-based amorphous alloys prepared by rapid solidification techniques. Experiments have been performed in planar (ribbons) and cylindrical (microwires) configuration. Low-field hysteresis loops after premagnetizing to saturation are ascribed to the magnetization reversal process of the soft inner phase, while harder component remains close to its remanence. Such low-field loops exhibit a shift typical of magnetic bias phenomena which in this case is ascribed to the magnetostatic coupling originated through the stray fields created by uncompensated charges at the ends of the harder phase.