Static critical phenomena in ferromagnets with quenched disorder

Understanding the microscopic processes affecting the bulk thermal conductivity is crucial to develop more efficient thermoelectric materials. PbTe is currently one of the leading thermoelectric materials, largely thanks to its low thermal conductivity. However, the origin of this low thermal conductivity in a simple rocksalt structure has so far been elusive. Using a combination of inelastic neutron scattering measurements and first-principles computations of the phonons, we identify a strong anharmonic coupling between the ferroelectric transverse optic mode and the longitudinal acoustic modes in PbTe. This interaction extends over a large portion of reciprocal space, and directly affects the heat-carrying longitudinal acoustic phonons. The longitudinal acoustic-transverse optic anharmonic coupling is likely to play a central role in explaining the low thermal conductivity of PbTe. The present results provide a microscopic picture of why many good thermoelectric materials are found near a lattice instability of the ferroelectric type.

Giant anharmonic phonon scattering in PbTe

A review is given of the collinear and random magnetic structures which may be found in magnetically‐concentrated amorphous solids General consequences of a non‐crystalline lattice on the atomic moments, exchange interactions and single‐ion anisotropy are presented Magnetic structures with one and two magnetic subnetworks are then described, taking examples from the literature of each type Some discussion is also given of the low energy excitations and the behavior at the spin freezing or ordering temperature What happens to the familiar forms of magnetic order—ferromagnetism, ferrimagnetism, antiferromagnetism—in the absence of a crystalline atomic lattice? This article sets out to give an overview of magnetic order in non‐crystalline solids in an attempt to answer the question The magnetism of two classes of disordered metals have received particular attention during the past five years; amorphous ferromagnets, notably the Metglas‐type alloys, and spin glasses, typically dilute solutions of transition‐metal impurities in a non‐magnetic crystalline matrix The subject‐matter here will be magnetically‐concentrated amorphous materials, with the accent on non‐collinear spin structures A coherent picture of amorphous magnetism will be presented, but it must be admitted that not all the magnetic structures discussed have been equally well established Some of them are directly analogous to those found in crystals Others are peculiar to amorphous (or disordered) solids, whereas one crystalline magnetic structure seems impossible in a noncrystalline lattice

Amorphous magnetic order

Publisher Summary Metallic glasses are solids that have electronic properties normally associated with metals, but with atomic arrangements that are not spatially periodic. Noncrystalline and amorphous are equivalent terms used to describe the atomic scale structure of such materials. The term glass has often been reserved for amorphous solids formed by continuous solidification of a liquid, but is used in this review to refer to amorphous solids produced in a variety of ways. These include evaporation, sputtering, and electro- and chemical deposition, as well as quenching from the liquid state. The current status of research on structure of metallic glasses is reviewed in this chapter. Discussion is largely limited to metallic glasses which can be retained at room temperature. All of these contain at least two atomic components. Nominally pure, elemental metallic glasses have been prepared, but their metastability depends critically on impurity content; most of these crystallize well below room temperature.

Structure of Metallic Alloy Glasses

Publisher Summary This chapter provides an overview of the magnetoelastic properties of the highly magnetostrictive rare earth-Fe2 alloys. The chapter describes a general treatment of magnetostriction for the cases of hexagonal and cubic symmetry, which is applicable to the rare earth elements and the rare earth-iron compounds. The chapter presents the magnetostriction of binary rare earth-iron alloys and the magnetostriction of single crystal and polycrystal RFe2 compounds are compared to other magnetostrictive materials at room temperature. The chapter discusses a possible source of startling magnetostriction anisotropy, measurements of magnetization, sublattice magnetization, and magnetic anisotropy, and the role of intrinsic as well as extrinsic effects. It reports the effects of the strong magnetoelastic coupling on sound velocities and elastic moduli and observes extraordinarily large ∆E effects and changes in sound velocity in single crystals, polycrystals, and amorphous rare earth-Fe2 alloys. The chapter concludes with a discussion of the recent measurements of linear and volume magnetostriction on the amorphous form of the RFe2 alloys.

Chapter 7 Magnetostrictive rare earth-Fe2 compounds

Evidence is presented from neutron diffraction measurements for the existence of an amorphous spin-polarization distribution in a sputtered rare-earth-iron alloy of composition 33 at% Tb, 67 at% Fe

Direct Observation of an Amorphous Spin-Polarization Distribution

Softening of the transverse-optic mode in PbTe

Neutron diffraction and magnetization measurements on a sputtered specimen of 33% terbium, 66% iron have confirmed the existence of long range magnetic order and an amorphous spatial distribution of atoms and spin sites. Elastic diffraction data taken above and below the 409K Curie temperature have enabled the separation of nuclear and magnetic scattering contributions. The magnetic spin density function contains a very broad major peak, dominated by the Tb‐Tb pair distribution function, with a maximum near 3.6 A. Longer range correlations are very much weaker. The principal features of the atomic structure can be accounted for by considering only near neighbor correlations between terbium and iron atoms in a random close‐packed arrangement. Inelastic scattering studies have revealed a significant shift in the magnetic density of states to lower energy in the amorphous material relative to the crystal‐ line TbFe2 counterpart. No discrete spin‐wave excitations or critical scattering near Tc could be observed.

Magnetism in Amorphous Terbium‐Iron

Neutron diffraction and magnetization measurements on a sample of 33.3 at.% Tb, 66.7 at.% Fe prepared by sputtering show clear evidence for an amorphous atomic arrangement, and in addition exhibit an amorphous spin polarization distribution. Data presented both above and below the 388 K Curie temperature and a difference pattern show explicitly the diffuse scattering of both nuclear and spin origin. A low angle “tail” of purely magnetic origin is observed which may be related to the occurrence of longer range inhomogeneities or domain-like regions in these materials.

Amorphous Spin Polarization in a Tb-Fe Compound

Small-angle neutron scattering (SANS) has been used to investigate the onset of magnetic order and some aspects of the magnetic microstructure in a series of bulk amorphous rare-earth iron alloys prepared by DC rapid sputtering. SANS was also used to investigate crystalline (but atomically disordered) TbFe1.5Al0.5 and crystalline TbFe2. The measurements were made for the most part on a two-axis spectrometer at a wavelength λ =2.4 A with collimations of 20′–10′–10′. Some measurements were made on a three-axis spectrometer to obtain information on the inelasticity of the scattering.

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J. J. Rhyne

Papers

Direct Observation of an Amorphous Spin-Polarization Distribution

Softening of the transverse-optic mode in PbTe

Magnetism in Amorphous Terbium‐Iron

Amorphous Spin Polarization in a Tb-Fe Compound

Small-angle neutron scattering from rare-earth iron alloys