H. A. Alperin

Bio: H. A. Alperin is an academic researcher from Silver Spring Networks. The author has contributed to research in topics: Neutron scattering & Quasielastic neutron scattering. The author has an hindex of 2, co-authored 2 publications receiving 138 citations.

Direct Observation of an Amorphous Spin-Polarization Distribution

Abstract: 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

Giant anharmonic phonon scattering in PbTe

Abstract: 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.

Amorphous magnetic order

Abstract: 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

Structure of Metallic Alloy Glasses

Abstract: 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.

Chapter 7 Magnetostrictive rare earth-Fe2 compounds

Abstract: 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.