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

The interaction of subpicosecond laser pulses with magnetically ordered materials has developed into a fascinating research topic in modern magnetism. From the discovery of subpicosecond demagnetization over a decade ago to the recent demonstration of magnetization reversal by a single 40 fs laser pulse, the manipulation of magnetic order by ultrashort laser pulses has become a fundamentally challenging topic with a potentially high impact for future spintronics, data storage and manipulation, and quantum computation. Understanding the underlying mechanisms implies understanding the interaction of photons with charges, spins, and lattice, and the angular momentum transfer between them. This paper will review the progress in this field of laser manipulation of magnetic order in a systematic way. Starting with a historical introduction, the interaction of light with magnetically ordered matter is discussed. By investigating metals, semiconductors, and dielectrics, the roles of nearly free electrons, charge redistributions, and spin-orbit and spin-lattice interactions can partly be separated, and effects due to heating can be distinguished from those that are not. It will be shown that there is a fundamental distinction between processes that involve the actual absorption of photons and those that do not. It turns out that for the latter, the polarization of light plays an essential role in the manipulation of the magnetic moments at the femtosecond time scale. Thus, circularly and linearly polarized pulses are shown to act as strong transient magnetic field pulses originating from the nonabsorptive inverse Faraday and inverse Cotton-Mouton effects, respectively. The recent progress in the understanding of magneto-optical effects on the femtosecond time scale together with the mentioned inverse, optomagnetic effects promises a bright future for this field of ultrafast optical manipulation of magnetic order or femtomagnetism.

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Ultrafast optical manipulation of magnetic order

We review the current state of knowledge of phase separation and phase equilibria in porous materials. Our emphasis is on fundamental studies of simple fluids (composed of small, neutral molecules) and well-characterized materials. While theoretical and molecular simulation studies are stressed, we also survey experimental investigations that are fundamental in nature. Following a brief survey of the most useful theoretical and simulation methods, we describe the nature of gas‐liquid (capillary condensation), layering, liquid‐liquid and freezing/melting transitions. In each case studies for simple pore geometries, and also more complex ones where available, are discussed. While a reasonably good understanding is available for phase equilibria of pure adsorbates in simple pore geometries, there is a need to extend the models to more complex pore geometries that include effects of chemical and geometrical heterogeneity and connectivity. In addition, with the exception of liquid‐liquid equilibria, little work has been done so far on phase separation for mixtures in porous media.

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Phase separation in confined systems

This review examines the stages of drying, with the emphasis on the constant rate period (CRP), when the pores are full of liquid. It is during the CRP that most of the shrinkage occurs and the drying stresses rise to a maximum. We examine the forces that produce shrinkage and the mechanisms responsible for transport of liquid. By analyzing the interplay of fluid flow and shrinkage of the solid network, it is possible to calculate the pressure distribution in the liquid in the pores. The tension in the liquid is found to be greatest near the drying surface, resulting in greater compressive stresses on the network in that region. This produces differential shrinkage of the solid, which is the cause of cracking during drying. The probability of fracture is related to the size of the body, the rate of evaporation, and the strength of the network. A variety of strategies for avoiding fracture during drying are discussed.

Theory of Drying

The demand for ever-increasing density of information storage and speed of manipulation has triggered an intense search for ways to control the magnetization of a medium by means other than magnetic fields. Recent experiments on laser-induced demagnetization and spin reorientation use ultrafast lasers as a means to manipulate magnetization, accessing timescales of a picosecond or less. However, in all these cases the observed magnetic excitation is the result of optical absorption followed by a rapid temperature increase. This thermal origin of spin excitation considerably limits potential applications because the repetition frequency is limited by the cooling time. Here we demonstrate that circularly polarized femtosecond laser pulses can be used to non-thermally excite and coherently control the spin dynamics in magnets by way of the inverse Faraday effect. Such a photomagnetic interaction is instantaneous and is limited in time by the pulse width (approximately 200 fs in our experiment). Our finding thus reveals an alternative mechanism of ultrafast coherent spin control, and offers prospects for applications of ultrafast lasers in magnetic devices.

Ultrafast non-thermal control of magnetization by instantaneous photomagnetic pulses.

The supercooling and freezing of a restricted liquid has been studied by the probing of its molecular dynamics with picosecond optical techniques. A newly developed transparent porous host material makes it possible to study the liquid viscosity as a function of the confining pore radius and temperature. The observed behavior is remarkably different from that of an ordinary liquid, and can be interpreted in terms of a simple model.

Geometrical supercooling of liquids in porous glass.

A newly developed integrated SQUID magnetic spectrometer yields direct high-resolution measurements of the optically induced magnetization in a 10 \ensuremath{\mu}m-diam sample of ${\mathrm{Cd}}_{0.8}$${\mathrm{Mn}}_{0.2}$Te. Both the magnitude and the picosecond dynamics of the magnetic response have been studied and are seen to be dramatically dependent on the energy and polarization of the optical excitation. The data show that the overall sample magnetization changes upon illumination, and that the perturbed spins equilibrate through spin-lattice relaxation.

Low-temperature magnetic spectroscopy of a dilute magnetic semiconductor

Liquid and solid supercooling of molecular oxygen has been observed and studied in well-characterized porous sol-gel glasses. The depression of the solid-solid phase transition temperature was measured as a function of pore size, showing the strong influence of confinement on the solid phases. The liquid freezing-point depression was also measured, and a picosecond optical technique was used to probe the dynamics of the supercooled liquid. The liquid's geometrical confinement is observed to have an important effect on its properties, leading to behavior unlike that of ordinary bulk fluids.

Supercooled liquids and solids in porous glass

Porous media with well‐defined pore diameters can be employed as hosts within which one may study a wealth of physical phenomena in well‐characterized confining geometries. In this paper we describe the procedures for forming variable pore size glasses via the sol‐gel process. Two general methods were used to form the gels from which the porous glasses were formed; the hydrolysis of the alkoxides and the gelling of colloidal silica, e.g., DuPont LUDOX. By controlling the pH of the gelling solution, the water/silica ratio and the firing temperature, glasses with pore diameters ranging from less than 2–12 nm were formed from the alkoxide hydrolysis. Larger pore diameters were obtained from colloidal silica gels, the largest being in the 45‐nm range in glasses made from leached alkali silicate gels. Extensive high resolution transmission electron microscopy analysis showed both ‘‘colloidal’’ and ‘‘polymeric’’ type glasses, in addition to an intraparticle structure in the 1–3 nm range. An example is given where subpicosecond optical birefringence techniques have been used to directly probe the rotational dynamics of different molecular liquids in several glasses having different pore sizes. Thus, a systematic evaluation of the liquid behavior as a function of its geometrical confinement is obtained and the results discussed.

The chemistry of and physics with porous sol‐gel glasses

Subpicosecond optical birefringence techniques have been employed to probe directly the rotational dynamics of topologically different molecular liquids confined within porous sol-gel glasses. The glasses have extremely high surface areas with uniform and controllable pore diameters, allowing a systematic evaluation of the liquid behavior as a function of its geometrical confinement. Results with wetting and nonwetting liquids reveal differences between the dynamics of the surface layer and that of the bulk, thereby providing a method of calculating equilibrium properties of the adsorbed film.

J. Warnock

Papers

Geometrical supercooling of liquids in porous glass.

Low-temperature magnetic spectroscopy of a dilute magnetic semiconductor

Supercooled liquids and solids in porous glass

The chemistry of and physics with porous sol‐gel glasses

Orientational behavior of molecular liquids in restricted geometries