Magnetic amorphous alloys obtained by rapid quenching of the melt are excellent soft magnetic materials with a wide range of technological applications. They also represent a significant challenge to the scientific understanding of magnetic materials, since most of the existing theories of solids assume lattice periodicity. For these reasons, the magnetic and other properties of amorphous alloys have been very actively studied over the last decade. In recent years increasing attention has been directed to the fundamental understanding of the structural, thermal and magnetic properties of the amorphous alloys. It is not only scientifically but also technologically important to achieve such an understanding, since the amorphous alloys are, in many respects, so different from conventional crystalline magnetic materials. The author attempts to summarise recent progress in research on magnetic amorphous alloys and critically assesses the present level of understanding of this new class of magnetic materials, focusing mostly on the transition-metal-metalloid glasses. He starts with a review of early developments and a discussion on the nature of glasses and glass formation and proceeds to an extensive discussion of their atomic structure, both from the experimental and theoretical points of view.

https://iopscience.iop.org/article/10.1088/0034-4885/47/12/002/pdf

Magnetic amorphous alloys: physics and technological applications

Aperiodic crystals: A contradictio in terminis?

Using molecular dynamics computer simulations we investigate how the glass transition and the properties of the resulting glass depend on the cooling rate with which the sample has been quenched. The system we study is a two component Lennard‐Jones model which is coupled to a heat bath whose temperature is decreased from a high temperature, where the system is a liquid, to zero temperature, where the system is a glass. The temperature Tb of this heat bath is decreased linearly in time, i.e. Tb=Ti−γt, where γ is the cooling rate, and we study the cooling rate dependence by varying γ over several orders of magnitude. In accordance with simple theoretical arguments and with experimental observations we find that the glass transition, as observed in the specific heat and the thermal expansion coefficient, becomes sharper when γ is decreased. A decrease of the cooling rate also leads to a decrease of the glass transition temperature Tg and we show that the dependence of Tg on γ can be rationalized by assuming that the temperature dependence of the relaxation times of the system is given by either a Vogel–Fulcher law or a power law. By investigating the structural properties of the glass, such as the radial distribution functions, the coordination numbers and the angles between three neighbor‐sharing particles, we show how the local order of the glass increases with decreasing cooling rate. The enthalpy H and the density ρ of the glass decrease and increase, respectively, with decreasing γ. By investigating the γ dependence of clusters of nearest neighbors, we show how the cooling rate dependence of H and ρ can be understood from a microscopic point of view. Furthermore we demonstrate that the frequency of icosahedral‐like structures is decreasing with decreasing cooling rate. We also show that the spectrum of the glass, as computed from the dynamical matrix, shows a shift towards higher frequencies when γ is decreased. All these effects show that there is a significant dependence of the properties of glasses on the cooling rate with which the glass is produced.

How do the properties of a glass depend on the cooling rate? A computer simulation study of a Lennard-Jones system

Interrelations between atomic and electronic structures. Liquid and amorphous metals as model systems

Layer compounds exhibit highly anisotropic structural and elastic properties. They are characterized by rather rigid layers, loosely stacked together perpendicular to each other. Accordingly, the phonon dispersion of layered compounds are characterized by low lying inter-layer modes and highfrequency intra-layer modes. Intercalation compounds composed of a regular sequence of host and guest layers provide a playground for the investigation of the layer lattice dynamics by systematically changing the guest species and the number of guest layers per unit cell. In this contribution a brief overview of some characteristic features of the lattice dynamics of layered compounds is provided.

https://iopscience.iop.org/article/10.1088/0953-8984/13/34/313/pdf

Phonons in layered compounds

Experimental Investigation of the Dispersion of Collective Density Fluctuations near Q p in a Metallic Glass

The dynamical structure factor of glassy Cu46Zr54 has been measured at room temperature, for momentum transfers between 1 and 8 AA-1 and energies (h(cross) omega =E0-E) between -45 and 45 meV, using neutron inelastic scattering. From the measured time-of-flight spectra a generalised frequency distribution has been determined. The experiment was repeated after crystallisation of the sample. The results show a strong increase of low-frequency modes in the amorphous sample compared with the polycrystal, and the frequency distribution of the metallic glass shows a slope approximately omega 4/3 between 4.7 and 7 meV, where the frequency distribution of the polycrystalline sample still shows a Debye type of spectrum.

Dynamical structure factor and frequency distribution of the metallic glass Cu46Zr54 at room temperature

The dynamical structure factors of the glassy alloy Mg70Zn30 and the same sample after crystallisation have been determined at room temperature using neutron inelastic scattering techniques. From the time-of-flight spectra, generalised frequency distributions were determined which were used to calculate the temperature dependence of some of the thermodynamic properties of both samples. For temperatures below 70K the vibrational ('lattice') part of the specific heat of the glass is larger than that of the polycrystalline alloy. The difference between the specific heats of the two samples increases with decreasing temperature. It is shown that this larger specific heat is related to the low-frequency vibrations found in the dynamical structure factor of the metallic glass.

http://iopscience.iop.org/article/10.1088/0022-3719/14/17/004/pdf

Dynamical structure factor and vibrational density of states of the metallic glass Mg70Zn30 measured at room temperature

The atomic dynamics of the metallic glass Mg70Zr30 and the same sample after crystallisation was investigated at room temperature using neutron inelastic scattering techniques. For the first time the dispersion of short-wavelength collective density excitations could be measured in the glassy metal for a few momentum transfers near the first maximum of the static structure factor. From the intensity distribution in the spectra the authors conclude that the modes observed are transverse collective density excitations in the glassy system (transverse phonons in the polycrystal). They compare their findings with molecular dynamics and analytical calculations on the same system.

https://iopscience.iop.org/article/10.1088/0022-3719/13/36/002/pdf

Short-wavelength transverse collective modes in a metallic glass

In contrast to the strong difference observed in the static-structure factors of icosahedral and glassy PdSiU samples of their dynamical properties as reflected in their generalized vibrational density of states are nearly identical and quite different from that of the fully crystallized samples. Low-energy modes are detected in the generalized vibrational density of states of the icosahedral phase, which lead to an enhanced intensity at energies below 10 meV as compared to that of the crystallized samples. Also the wavelength dependence of the low-energy modes in icosahedral PdSiU is very similar to that in the metallic glass.

H. Rudin

Papers

Experimental Investigation of the Dispersion of Collective Density Fluctuations near Q p in a Metallic Glass

Dynamical structure factor and frequency distribution of the metallic glass Cu46Zr54 at room temperature

Dynamical structure factor and vibrational density of states of the metallic glass Mg70Zn30 measured at room temperature

Short-wavelength transverse collective modes in a metallic glass

Low-energy modes in quasicrystalline and glassy Pd 58.8 Si 20.6 U 20.6 : A comparative study by neutron inelastic scattering