Wayne M. Saslow

Adiabatic domain wall motion and Landau-Lifshitz damping

Recent theory and measurements of the velocity of current-driven domain walls in magnetic nanowires have reopened the unresolved question of whether Landau-Lifshitz damping or Gilbert damping provides the more natural description of dissipative magnetization dynamics. In this paper, we argue that (as in the past) experiment cannot distinguish the two, but that Landau-Lifshitz damping, nevertheless, provides the most physically sensible interpretation of the equation of motion. From this perspective, (i) adiabatic spin-transfer torque dominates the dynamics with small corrections from nonadiabatic effects, (ii) the damping always decreases the magnetic free energy, and (iii) microscopic calculations of damping become consistent with general statistical and thermodynamic considerations.

An economic analogy to thermodynamics

We develop analogies between economic systems and thermodynamics, and show how economic quantities can characterize the state of an economic system in equilibrium. We argue that just as a physical system in thermodynamic equilibrium requires a nonmechanical variable (the temperature T) to specify its state, so does an economic system. In addition, both systems must have a corresponding conjugate quantity, the entropy S. We also develop economic analogies to the free energy, Maxwell relations, and the Gibbs–Duhem relationship. Assuming that economic utility can be measured, we develop an operational definition of an economic temperature scale. We also develop an analogy to statistical mechanics, which leads to Gaussian fluctuations.

Universal thermal radiation drag on neutral objects

We compute the force on a small neutral polarizable object moving at velocity v--> relative to a photon gas equilibrated at a temperature T. We find a drag force linear in v-->. Its physical basis is related to that in recent formulations of the dissipative component of the Casimir force, i.e., the change in photon momentum in emission and absorption between the moving body and the stationary thermal bath. We estimate the strength of this universal drag force for different dielectric response functions and comment on its relevance in various contexts, especially to radiation-matter coupling in the cosmos.

Exact surface solutions for semiconductors: The Dember effect and partial currents

Exact solution of the linearized equations for steady-state transport in semiconductors yields two modes that vary exponentially in space, one involving screening (without entropy production) and one involving diffusion and recombination (with entropy production). Neither mode is quasineutral. For constant surface photoexcitation with generation of electrons and holes, the steady-state response is a linear combination of these modes, subject to global electroneutrality. The resultant charge separation produces a voltage difference across the sample (the Dember effect).

Spin pumping of current in non-uniform conducting magnets

We show that magnetic disequilibrium within a magnetic domain (e.g., by a magnetic field driving a domain wall) implies spin pumping of current within that domain. This has experimental implications for samples both with conducting leads and that are electrically isolated. For a two-band magnet these results are obtained first by simple arguments, and then by using irreversible thermodynamics to derive the full dynamical equations, with up and down spins each providing conduction and magnetism. It is known that in regions where the equilibrium magnetization is non-uniform, voltage gradients can drive both adiabatic and nonadiabatic bulk spin torques. Onsager relations then ensure that magnetic torques likewise drive related amounts of adiabatic and nonadiabatic currents what we call bulk spin pumping. As for recent spin-Berry phase work, we find that within a domain wall, the ratio of the effective electromotive force to the magnetic field is approximately given by P(2μΒ/e), where Ρ is the spin polarization. The adiabatic spin torque and spin-pumping terms are shown to be dissipative. We also discuss the issue of Landau-Lifshitz damping vs Gilbert damping; both irreversible thermodynamics and Langevin theory with near-equilibrium thermodynamic fluctuations lead to Landau-Lifshitz damping.

Electrostatics of conducting nanocylinders

This article calculates the capacitance and transverse polarizability of nonideal conducting nanocylinders. It employs a semiclassical theory that characterizes the cylinders with a nonzero screening length l and a dielectric constant e≠1. Cylinders both with and without free surface states are considered. The results should be relevant to cylinders with a quasicontinuous distribution of energy levels such as nanowires and nanotubes. The theory yields very acceptable agreement with random phase approximation calculations for the radius dependence of the polarizability of single-wall carbon nanotubes. The same type of theory has already been applied to the capacitance of nanospheres, and in a companion article is applied to static screening by nanospheres.

Spin accumulation at ferromagnet/nonmagnetic material interfaces

Many proposed and realized spintronic devices involve spin injection and accumulation at an interface between a ferromagnet and a non-magnetic material. We examine the electric field, voltage profile, charge distribution, spin fluxes, and spin accumulation at such an interface. We include the effects of both screening and spin scattering. We also include both the spin-dependent chemical potentials {\mu}_{\uparrow,\downarrow} and the effective magnetic field H* that is zero in equilibrium. For a Co/Cu interface, we find that the spin accumulation in the copper is an order of magnitude larger when both chemical potential and effective magnetic field are included. We also show that screening contributes to the spin accumulation in the ferromagnet; this contribution can be significant.

Electromechanical implications of Faraday’s law: A problem collection

A collection of problems illustrating the electromechanical implications of Faraday’s law is presented. They are appropriate for well‐prepared freshmen and all undergraduate physics majors. A number of interesting examples are worked out analytically, including Thomson’s jumping ring demonstration. They are of interest in part because they include the effects of inductance and capacitance more fully than in the usual textbook treatments.

Two classes of Kramers-Kronig sum rules

Abstract Using the Kramers-Kronig relations, two classes of sum rules obeyed by the frequency-dependent dielectric function have been derived. They may provide helpful restrictions in determining the form of ϵ 1 ( ω ) at frequencies too high to be measured.

Dissipation due to pure spin current generated by spin pumping | NIST

Based on spin-dependent transport theory and thermodynamics, we develop a generalized theory of the Joule heating in the presence of a spin current. Along with the conventional Joule heating consisting of an electric current and electrochemical potential, it is found that the spin current and spin accumulation give an additional dissipation because the spin-dependent scatterings inside bulk and ferromagnetic/nonmagnetic interface lead to a change of entropy. The theory is applied to investigate the dissipation due to pure spin-current generated by spin pumping across a ferromagnetic/nonmagnetic/ferromagnetic multilayer. The dissipation arises from an interface because the spin pumping is a transfer of both the spin angular momentum and the energy from the ferromagnet to conduction electrons near the interface. It is found that the dissipation is proportional to the enhancement of the Gilbert damping constant by spin pumping.