Acoustic characteristics of Ni–Nb–Zr–H glassy alloys by ultrasonic shear waves

The propagation characteristics of shear horizontally polarized (SH) waves passing through (Ni₄₂Nb₂₈Zr₃₀)_100–x H_x (x = 0–15.2) glassy alloys were investigated as a function of hydrogen content. With an increase in hydrogen content, the propagation time and main frequency of the receiving waves show increase and decrease, respectively, indicating expan‐ sion in average atomic distance which comes from solution of hydrogen. In sharp contrast to crystalline alloys, the decrease in damping ratio and the delay in phase with increasing hydrogen suggest a strong settlement of hydrogen into four‐coordination sites surrounded tetrahedrally by four Zr atoms and the resulting increase in dynamic elasticity, respectively. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Measurement of the elastic properties of evaporated C60 films by surface acoustic waves

Surface Acoustic Wave (SAW) pulses were excited in C₆₀ films deposited on quartz and silicon substrates using pulses from excimer lasers with wavelengths of 248 nm and 308 nm for excitation. An optical beam-deflection technique and polymer electret transducers were utilized to detect the propagation of the SAW pulse with high spatial and temporal resolution, allowing an accuracy of better than 0.1% for SAW velocity measurements. With this technique the frequency dependence of the SAW velocity was determined for a number of fullerite films and density, as well as elastic bulk properties of the films were derived by a theoretical analysis of the dispersion effect.     

Thickness-shear modes and magnetoelastic waves in a longitudinally magnetized ferromagnetic plate

Magnetoelastic (ME) waves and thickness-shear modes in the ferromagnetic plate are studied. Coupled vibrations of magnetization and shear elastic deformations excited simultaneously by a variable magnetic field propagate in two mutually perpendicular directions: parallel and normal to a surface. For parameters characteristic of isotropic ferromagnet with the sample magnetization and Zeeman field parallel to the surface, resonant frequencies of shear modes are computed and their dispersion law is examined. It is shown that the dependence of dimensional resonances frequencies on wave number k_z of ME wave propagating along saturating field direction occurs. The possibility of excitation of ME waves with different k_z explains multimode character of thickness ME resonances.     

Spin-Hall effect: Back to the beginning at a higher level

The phenomena of the spin-Hall effect, initially proposed over three decades ago in the context of asymmetric Mott skew scattering, was revived recently by the proposal of a possible intrinsic spin-Hall effect originating from a strongly spin-orbit coupled band structures. This new proposal has generated an extensive debate and controversy over the past 2 years. On August 2006 the first workshop on the spin-Hall effect was held at the Asian Pacific Center for Theoretical Physics. Its purpose was to bring together many of the leading groups in this field to resolve such issues and identify future challenges. We offer this short summary to clarify formerly controversial issues now settled and help refocus the research efforts in new and important avenues.     

Brillouin spectroscopy on monodomains of thin polycrystalline layers of molecular crystals

An unconventional Brillouin-spectroscopic technique is proposed to determine elastic stiffness tensor coefficients of molecular crystals. With 90A Brillouin measurements on thin monodomains of melt-crystallized films (about 30 μm thickness) the need to grow large single crystals is avoided. Measurements on sym-difluorotetrachloroethane and perfluoroeicosane are presented.     

Ab Initio Study on Hypothetical Silver Nitride

We perform the ab initio calculations based on norm-conserving pseudopotentials and density functional theory to investigate the structural, elastic, and thermodynamical properties for silver nitride （AgN） compound that is a member of the 4d transition metal group and has not been synthesized yet. The obtained results are compared with the other available theoretical data, and the agreement is, generally, quite good. We also present the pressure-dependent behaviour of some mechanical and thermodynamical properties for the same compounds.     

Reshock Response of 2A12 Aluminum Alloy at High Pressures

By means of mounting the specimen on a low-impedance buffer, reshock experiments were carried out on a 2A12 almninum alloy up to shock stresses＇ of 67.6 GPa. Reshock wave profiles from the initial shock stresses of 60.9-67.6 GPa were measured with a velocity interferometer, and it shows that the 2A12 aluminum alloy characterizes as quasi-elastic response during recompression process. The Lagrange longitudinal velocities along the reloading path from initial shock state were obtained from two shots of experiments, while the bulk velocities at corresponding shock stresses were determined via extrapolating from the public reported unloading plastic sound velocities. Combining the reshock and the release experimental results, the yield strength of 2A12 aluminum alloy at shock stress of 60.9 GPa was estimated to be about 1.7GPa.     

A Modified Free Volume Model for Characterizing of Rate Effect in Bulk Metallic Glasses

We investigate the plastic deformation and constitutive behaviour of bulk metallic glasses （BMGs）. A dimensionless Deborah number DeiD = tr/ti is proposed to characterize the rate effect in BMGs, where tr is the structural relaxing characteristic time of BMGs under shear load, ti is the macroscopic imposed characteristic time of applied stress or the characteristic time of macroscopic deformation. The results demonstrate that the modified free volume model can characterize the strain rate effect in BMGs effectively.     

Optical characterization of thermal-stress induced by spin waves in thin-film ferrimagnetic structures

It is shown that intense spin-dipole waves (SDWs) excited in thin yttrium iron garnet (YIG) films induce an in-plane thermal stress (σ) of 1-2 MPa in a YIG/GGG structure (where GGG is gadolinium gallium garnet). In YIG/GGG with normal magnetization, σ shifts its ferromagnetic resonance frequency by ≈1 MHz, which is comparable to the linewidth of the absorption curve of YIG/GGG resonators. The effect was characterized by an optical technique that detects σ in the GGG substrate. It was also demonstrated that this effect can be used for the optical-microwave spectroscopy of spin waves in thin ferromagnetic films, by using thermal mapping of SDWs in the substrate. We have shown that this opens up the possibility of determining the contribution of the two-particle magneto-elastic interaction to the microwave heating of the sample.     

The peculiarities of coupled electromagnetic and magnetoelastic waves in antiferromagnets

A spectrum of coupled electromagnetic, spin and elastic waves in a two-sublattice antiferromagnet with weak ferromagnetism is theoretically investigated. The influence of the g-factor anisotropy and the transverse and longitudinal relaxation in magnetic subsystem on the spectrum of coupled waves is considered. The most changes of dispersion laws occurred in long-wavelength approximation and near the spin reorientation point, then the vectors of ferromagnetism and antiferromagnetism reoriented onto another crystallographic axis. It is shown that the magnetoelastic, the Dzyaloshinsky and the dipole interactions, the anisotropy of g-factor, the external magnetic field and the longitudinal susceptibility determine the activation of quasiferromagnetic waves. The dispersion laws of quasielectromagnetic and quasielastic waves can change from linear dependence to square. At large damping in magnetic subsystem, one from these modes can become the pure relaxation one.     

Magnetostriction of TbxHo0.75−xPr0.25(Fe0.9B0.1)2 (x=0–0.3) compounds

The Tb_xHo_0.75−xPr_0.25(Fe_0.9B_0.1)₂ (x=0, 0.1, 0.15, 0.2, 0.25, and 0.3) compounds are found to stabilize in a cubic Laves phase structure. The lattice parameter, magnetostriction (at 10 kOe), and Curie temperature are found to increase with increasing Tb content. The compound with x=0.15 exhibits a possible anisotropy compensation between the Tb and (Ho/Pr) sublattices. The easy magnetization direction rotates towards the 〈1 1 1〉 from the 〈1 0 0〉 direction, with increasing Tb content. The splitting of the (4 4 0) peak accompanied by the spontaneous magnetostriction-induced rhombohedral distortion is observed for compounds with x?0.15 and the spontaneous magnetostriction (λ_1 1 1) is found to increase with Tb content.     

More on shear modulus collapse of lattices at high pressure

In his recent paper, Shear modulus collapse of lattices at high pressure, J. Phys. Cond. Matt. 16 (2004) L125, V.V. Kechin claims that the zero temperature shear modulus of a metallic solid vanishes at a high critical pressure, and the critical pressures for this shear modulus collapse lie in the range 0-250 Mbar for elemental metals. Here we demonstrate that Kechin's arguments contain an erroneous assumption, and therefore, do not prove that all metals become mechanically unstable at high pressures. Ab initio calculations and experimental results on a number of solids are analyzed to confirm our conclusion.     

Magnetic field aberration induced by cycle stress

Magneto-mechanical effect has been causing people's growing interest because of its relevance to several technology problems. One of them is the variation of surface magnetic field induced by stress concentration under the geomagnetic field. It can be used as an innovative, simple and convenient potential NDE method, called as magnetic memory method. However, whether and how this can be used as a quantitative measurement method, is still a virginal research field where nobody sets foot in. In this paper, circle tensile stress within the elastic region was applied to ferromagnetic sample under geomagnetic field. Experiment results on the relation between surface magnetic field and elastic stress were presented, and a simple model was derived. Simulation of the model was reconciled with the experimental results. This can be of great importance for it provides a brighter future for the promising Magnetic Memory NDE method—the potential possibility of quantitative measurement.     

Study of the magnetostrictive strains in Pr6Fe11Ga3 alloy

Magnetoelastic properties of the Pr₆Fe₁₁Ga₃ alloy are studied by magnetostriction and thermal expansion measurements. The effects of short- and long-range magnetic ordering processes about Curie temperature clearly appear in the temperature dependence of the spontaneous magnetostriction as two increasing steps with decreasing temperatures. Thermal variations of the total magnetocrystalline anisotropy introduce pronounce changes in the isofield curves of the forced magnetostriction as a negative minimum below 200 K, a compensation phenomena about 250 K, and a positive maximum between 250 K and T_c=320 K. The observed behavior of magnetostriction is discussed in terms of the competitive anisotropies of Pr and Fe sublattices and coupling magnetostrictive constants.     

Influence of H and N insertion on the magnetostriction and thermal expansion of YFe10V2Zx (Z=N,H) compositions

Experimental results on the thermal expansion and magnetostriction of YFe₁₀V₂ composites are reported and the influence of H and N interstitial atoms is studied. The anisotropic magnetostriction is about 30% larger in the composite than in the starting alloy. Also, the anisotropic magnetostriction remains positive after insertion of H (N) ion while the sign of volume magnetostriction changes by hydrogenation. The anisotropic magnetoelastic interactions are enhanced by insertion of H and especially N interstitial atoms. The results are discussed considering the effect of H and N, and of temperature on magnetic anisotropy and microstructure.     

Critical bond lengths and their role in spontaneous magnetostriction of R2Fe17CX (R=Y,Nd, Gd,Tb, Er)

High-energy high-flux synchrotron X-rays have been used to study the spontaneous magnetostriction of R₂Fe₁₇ (R=Y, Nd, Gd, Tb, Er) and their carbides in the temperature range 10–1100 K. Addition of interstitial carbon greatly increases both the Curie temperatures (T_C) and the spontaneous magnetostrain of the compounds, while reduces the anisotropy of the magnetostrain by expanding the distances between rare-earth and neighboring Fe sites. The increase of T_C with carbon is due to the increased spatial separation of the Fe hexagon layers. On the basal plane, the Fe hexagons are squeezed and the contribution of Fe sublattice to spontaneous magnetostriction is attenuated, while that of rare-earth sublattice is enhanced. The average bond magnetostrain around Fe sites are in linear relation with their hyperfine field intensities.     

Quantitative Model of Large Magnetostrain Effect in Ferromagnetic Shape Memory Alloys

A quantitative model describing large magnetostrain effect observed in several ferromagnetic shape memory alloys such as Ni₂MnGa is briefly reported. The paper contains an exact thermodynamic consideration of the mechanical and magnetic properties of similar type materials. As a result, the basic mechanical state equation including magnetic field effect is directly derived from a general Maxwell relation. It is shown that the magnetic field induced deformation effect is directly connected with the strain dependence of magnetisation. A simple model of magnetisation and its dependence on the strain is considered and applied to explain the results of experimental study of large magnetostrain effects in Ni₂MnGa.     

Martensitic transformation temperature hysteresis narrowing and magnetocaloric effect in ferromagnetic shape memory alloys Ni-Mn-Ga

The hysteresis narrowing and magnetocaloric effect (MCE) were studied in the Ni-Mn-Ga alloys. Ni₅₄Mn₂₄Ga₂₂ and Ni_51.9Mn₂₇Ga_21.1 were selected in order to obtain specific characteristic temperatures of alloys and to study an influence of several factors on the hysteresis width (δ) of basic austenite-martensite (AM) transformation. The effect of martensite crystal lattice softening due to intermartensitic transition (IT) taking place immediately after AM transformation on δ was examined in alloy 1. MCE was measured in alloy 2, in which Curie temperature (T_C) and austenitic start temperature (A_S) coincides. It was demonstrated that the hysteresis width δ reaches about 2 K, if IT is enough close to AM transition. MCE was found to have a maximum at T_C≈A_S.