This review describes a new paradigm of electronics based on the spin degree of freedom of the electron. Either adding the spin degree of freedom to conventional charge-based electronic devices or using the spin alone has the potential advantages of nonvolatility, increased data processing speed, decreased electric power consumption, and increased integration densities compared with conventional semiconductor devices. To successfully incorporate spins into existing semiconductor technology, one has to resolve technical issues such as efficient injection, transport, control and manipulation, and detection of spin polarization as well as spin-polarized currents. Recent advances in new materials engineering hold the promise of realizing spintronic devices in the near future. We review the current state of the spin-based devices, efforts in new materials fabrication, issues in spin transport, and optical spin manipulation.

http://science.sciencemag.org/content/sci/294/5546/1488.full.pdf

Spintronics: a spin-based electronics vision for the future.

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

1. Introduction 20642. Synthesis of Magnetic Nanoparticles 20662.1. Classical Synthesis by Coprecipitation 20662.2. Reactions in Constrained Environments 20682.3. Hydrothermal and High-TemperatureReactions20692.4. Sol-Gel Reactions 20702.5. Polyol Methods 20712.6. Flow Injection Syntheses 20712.7. Electrochemical Methods 20712.8. Aerosol/Vapor Methods 20712.9. Sonolysis 20723. Stabilization of Magnetic Particles 20723.1. Monomeric Stabilizers 20723.1.1. Carboxylates 20733.1.2. Phosphates 20733.2. Inorganic Materials 20733.2.1. Silica 20733.2.2. Gold 20743.3. Polymer Stabilizers 20743.3.1. Dextran 20743.3.2. Polyethylene Glycol (PEG) 20753.3.3. Polyvinyl Alcohol (PVA) 20753.3.4. Alginate 20753.3.5. Chitosan 20753.3.6. Other Polymers 20753.4. Other Strategies for Stabilization 20764. Methods of Vectorization of the Particles 20765. Structural and Physicochemical Characterization 20785.1. Size, Polydispersity, Shape, and SurfaceCharacterization20795.2. Structure of Ferro- or FerrimagneticNanoparticles20805.2.1. Ferro- and Ferrimagnetic Nanoparticles 20805.3. Use of Nanoparticles as Contrast Agents forMRI20825.3.1. High Anisotropy Model 20845.3.2. Small Crystal and Low Anisotropy EnergyLimit20855.3.3. Practical Interests of Magnetic NuclearRelaxation for the Characterization ofSuperparamagnetic Colloid20855.3.4. Relaxation of Agglomerated Systems 20856. Applications 20866.1. MRI: Cellular Labeling, Molecular Imaging(Inﬂammation, Apoptose, etc.)20866.2.

Magnetic Iron Oxide Nanoparticles: Synthesis, Stabilization, Vectorization, Physicochemical Characterizations, and Biological Applications

In this chapter, we will restrict our attention to the ferrites and a few other closely related materials. The great interest in ferrites stems from their unique combination of a spontaneous magnetization and a high electrical resistivity. The observed magnetization results from the difference in the magnetizations of two non-equivalent sub-lattices of the magnetic ions in the crystal structure. Materials of this type should strictly be designated as “ferrimagnetic” and in some respects are more closely related to anti-ferromagnetic substances than they are to ferromagnetics in which the magnetization results from the parallel alignment of all the magnetic moments present. We shall not adhere to this special nomenclature except to emphasize effects, which are due to the existence of the sub-lattices.

Ferromagnetic materials

This Account summarizes techniques for fabrication and applications in biomedicine of microfluidic devices fabricated in poly(dimethylsiloxane) (PDMS). The methods and applications described focus on the exploitation of the physical and chemical properties of PDMS in the fabrication or actuation of the devices. Fabrication of channels in PDMS is simple, and it can be used to incorporate other materials and structures through encapsulation or sealing (both reversible and irreversible).

Poly(dimethylsiloxane) as a material for fabricating microfluidic devices.

We report on the epitaxial growth of a group-IV ferromagnetic semiconductor, Mn(x)Ge(1-x), in which the Curie temperature is found to increase linearly with manganese (Mn) concentration from 25 to 116 kelvin. The p-type semiconducting character and hole-mediated exchange permit control of ferromagnetic order through application of a +/-0.5-volt gate voltage, a value compatible with present microelectronic technology. Total-energy calculations within density-functional theory show that the magnetically ordered phase arises from a long-range ferromagnetic interaction that dominates a short-range antiferromagnetic interaction. Calculated spin interactions and percolation theory predict transition temperatures larger than measured, consistent with the observed suppression of magnetically active Mn atoms and hole concentration.

http://science.sciencemag.org/content/sci/295/5555/651.full.pdf

A Group-IV Ferromagnetic Semiconductor: MnxGe1−x

A superconducting point contact is used to determine the spin polarization at the Fermi energy of several metals. Because the process of supercurrent conversion at a superconductor-metal interface (Andreev reflection) is limited by the minority spin population near the Fermi surface, the differential conductance of the point contact can reveal the spin polarization of the metal. This technique has been applied to a variety of metals where the spin polarization ranges from 35 to 90 percent: Ni0.8Fe0.2, Ni, Co, Fe, NiMnSb, La0.7Sr0.3MnO3, and CrO2.

http://science.sciencemag.org/content/sci/282/5386/85.full.pdf

Measuring the Spin Polarization of a Metal with a Superconducting Point Contact

Finite-size effects in CoO, an antiferromagnetic insulator with localized moments, have been observed in CoO/Si${\mathrm{O}}_{2}$ multilayers. The N\'eel temperatures of the CoO layers, as determined by dc susceptibility measurements, show a finite-size scaling relation with a shift exponent $\ensuremath{\lambda}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}1.55\ifmmode\pm\else\textpm\fi{}0.05$. Increased magnetization with an oscillation in magnitude, due to uncompensated antiferromagnetic layers, has also been observed.

Finite-size effects and uncompensated magnetization in thin antiferromagnetic CoO layers.

We have fabricated a thin film magnetic system consisting of nanoscale Mn11Ge8 ferromagnetic clusters embedded in a MnxGe1−x dilute ferromagnetic semiconductor matrix. The clusters form for growth temperatures of ∼300 °C with an average diameter and spacing of 100 and 150 nm, respectively. While the clusters dominate the magnetic properties, the matrix plays a subtle but interesting role in determining the transport properties. Variable range hopping at low temperatures involves both nanoclusters and MnGe sites, and is accompanied by a negative magnetoresistance attributed in part to spin-dependent scattering analogous to metallic granular systems.

Magnetoresistance of Mn:Ge ferromagnetic nanoclusters in a diluted magnetic semiconductor matrix

Angular dependence of the exchange coupling in a NiFe/CoO bilayer film has been measured. Different angular dependence and symmetry have been observed for the exchange field ${H}_{E}$ and the coercivity ${H}_{C}.$ The values of ${H}_{E}$ and ${H}_{C}$ are due to unidirectional and uniaxial components of the magnetic anisotropy energy, in which the odd and the even terms lead to ${H}_{E}$ and ${H}_{C},$ respectively. The proposed free energy accounts for the experimental results.

T. Ambrose

Papers

A Group-IV Ferromagnetic Semiconductor: MnxGe1−x

Measuring the Spin Polarization of a Metal with a Superconducting Point Contact

Finite-size effects and uncompensated magnetization in thin antiferromagnetic CoO layers.

Magnetoresistance of Mn:Ge ferromagnetic nanoclusters in a diluted magnetic semiconductor matrix

Angular dependence of exchange coupling in ferromagnet/antiferromagnet bilayers