Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.

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Spintronics: Fundamentals and applications

First, Monte Carlo modelling of electron-solid interactions is briefly reviewed regarding its historical development, followed by quantification in microbeam analysis with various types of signals generated by electron penetration in solids. Second, typical models stimulated by these improvements are explained by demonstrating some of the Monte Carlo calculations which are, the authors believe, still of practical interest and use. Then, a typical calculation procedure for Monte Carlo simulation is described in detail from the standpoint of a physicist who does his or her own programming. Third, up-to-date Monte Carlo calculations as applied to modern surface analysis, high-resolution scanning electron microscopy, Auger electron microscopy and X-ray photoelectron spectroscopy are also presented.

https://robots.iopscience.iop.org/article/10.1088/0034-4885/55/4/002/pdf

Monte Carlo modelling of electron-solid interactions

A new Monte Carlo simulation approach has been developed to describe electron scattering and secondary electron generation processes in solids. This approach is based on the uses of Mott's elastic scattering cross section and Penn's dielectric function. A very good agreement has been found on the energy distribution of backscattered electrons between theoretical calculations and accurate experimental measurement recently made by Goto et al. (1994). This fact confirms that the present Monte Carlo model is very useful for more comprehensive understanding of basic phenomena in electron spec-troscopy and microscopy, particularly in the sub-keV energy region where cascade secondary electrons play a dominant role. In this paper the details of the Monte Carlo procedure are described and further application to the mechanism of secondary electron generation is presented.

A Monte Carlo modeling of electron interaction with solids including cascade secondary electron production

The dependence of magnetization on the applied magnetic field and temperature was measured carefully near their Curie temperatures for two perovskite manganite samples: La0.67Ca0.33MnOδ and La0.60Y0.07Ca0.33MnOδ. It is suggested by the results that these materials can be utilized as both the thermal storage (passive regeneration) and as the working material (active regeneration) in an active magnetic regenerative refrigerator with very large temperature span, for their significant entropy change upon the application of a magnetic field and the easily tuned Curie temperatures.

Magnetocaloric effect in La0.67Ca0.33MnOδ and La0.60Y0.07Ca0.33MnOδ bulk materials

In a two-dimensional electron gas as realized by a semiconductor quantum well, the presence of spin-orbit coupling of both the Rashba and Dresselhaus type leads to anisotropic dispersion relations and Fermi contours. We study the effect of this anisotropy on the electrical conductivity in the presence of fixed impurity scatterers. The conductivity also shows in general an anisotropy which can be tuned by varying the Rashba coefficient. This effect provides a method of detecting and investigating spin-orbit coupling by measuring spin-unpolarized electrical currents in the diffusive regime. Our approach is based on an exact solution of the two-dimensional Boltzmann equation and provides also a natural framework for investigating other transport effects including the anomalous Hall effect.

Anisotropic transport in a two-dimensional electron gas in the presence of spin-orbit coupling

In order to explain patterns formed during crystallization of metal-semiconductor films and during bacterial colony growth, a random successive growth model where no long-range diffusion is necessary has been proposed. In this model the growth is controlled by two local conditions: growth probability of the neighboring sites around the cluster and occupation ratio of the sites of the cluster inside a small circle with the potential growth site as its center. Varying these conditions, fractal, dense-branching, and compact growth patterns have been obtained.

Random successive growth model for pattern formation

Etude des regions fractales apparaissant dans des couches doubles implantees et recuites. Les dimensions fractales et la densite fractale (nombre de fractales par unite d'aire) dependent de la temperature

Temperature dependence of fractal formation in ion-implanted a-Ge/Au bilayer thin films

Monte Carlo simulations of electron scattering in silver have been performed at 6 and 20 keV to investigate the relation between electron transport, backscattering and depth distribution of characteristic X-ray production of tracer elements. The simulation models used combine the use of elastic cross sections, either of Rutherford or of Mott type, with approaches to inelastic scattering given by either the Bethe stopping power or by the energy loss- and momentum transfer-dependent dielectric functions derived from optical constants. A comparison of the simulation results shows that the model with Mott cross section and dielectric function results in better agreement between calculated and experimental energy spectra of backscattered electrons. However, the dominant term in models of Phi ( rho z) curves is elastic scattering and the Mott cross section yields better agreement with experimental results.

http://iopscience.iop.org/article/10.1088/0022-3727/26/4/001/pdf

A comparison of Monte Carlo simulations of electron scattering and X-ray production in solids

(Amorphous Ge)/(polycrystalline Au) bilayer thin films with different Au grain size were prepared, and the pattern formation of Ge in annealed films has been investigated. It was found that as the average Au grain size increases from 40 to 410 nm, the Ge patterns change from ramified fractals to simple tip-splitting patterns and finally to round patterns. Furthermore, our experiment proves that the fractal formation in such films is truly dominated by the local-latent-heat, and that the pattern geometry is governed by the latent-temperature field range and the lattice network formed by the grain boundaries of the underlying Au film.

Wu Zi-Qin

Papers

Random successive growth model for pattern formation

Temperature dependence of fractal formation in ion-implanted a-Ge/Au bilayer thin films

A comparison of Monte Carlo simulations of electron scattering and X-ray production in solids

Experimental demonstration of the role of local latent heat in Ge pattern formation.

Multifractal behavior of solid-on-solid growth