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 tunnel magnetoresistance (TMR) effect in magnetic tunnel junctions (MTJs)1,2 is the key to developing magnetoresistive random-access-memory (MRAM), magnetic sensors and novel programmable logic devices3,4,5. Conventional MTJs with an amorphous aluminium oxide tunnel barrier, which have been extensively studied for device applications, exhibit a magnetoresistance ratio up to 70% at room temperature6. This low magnetoresistance seriously limits the feasibility of spintronics devices. Here, we report a giant MR ratio up to 180% at room temperature in single-crystal Fe/MgO/Fe MTJs. The origin of this enormous TMR effect is coherent spin-polarized tunnelling, where the symmetry of electron wave functions plays an important role. Moreover, we observed that their tunnel magnetoresistance oscillates as a function of tunnel barrier thickness, indicating that coherency of wave functions is conserved across the tunnel barrier. The coherent TMR effect is a key to making spintronic devices with novel quantum-mechanical functions, and to developing gigabit-scale MRAM.

Giant room-temperature magnetoresistance in single-crystal Fe/MgO/Fe magnetic tunnel junctions

Magnetically engineered magnetic tunnel junctions (MTJs) show promise as non-volatile storage cells in high-performance solid-state magnetic random access memories (MRAM). The performance of these devices is currently limited by the modest (< approximately 70%) room-temperature tunnelling magnetoresistance (TMR) of technologically relevant MTJs. Much higher TMR values have been theoretically predicted for perfectly ordered (100) oriented single-crystalline Fe/MgO/Fe MTJs. Here we show that sputter-deposited polycrystalline MTJs grown on an amorphous underlayer, but with highly oriented (100) MgO tunnel barriers and CoFe electrodes, exhibit TMR values of up to approximately 220% at room temperature and approximately 300% at low temperatures. Consistent with these high TMR values, superconducting tunnelling spectroscopy experiments indicate that the tunnelling current has a very high spin polarization of approximately 85%, which rivals that previously observed only using half-metallic ferromagnets. Such high values of spin polarization and TMR in readily manufactureable and highly thermally stable devices (up to 400 degrees C) will accelerate the development of new families of spintronic devices.

Giant tunnelling magnetoresistance at room temperature with MgO (100) tunnel barriers

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

Electrons have a charge and a spin, but until recently these were considered separately. In classical electronics, charges are moved by electric fields to transmit information and are stored in a capacitor to save it. In magnetic recording, magnetic fields have been used to read or write the information stored on the magnetization, which 'measures' the local orientation of spins in ferromagnets. The picture started to change in 1988, when the discovery of giant magnetoresistance opened the way to efficient control of charge transport through magnetization. The recent expansion of hard-disk recording owes much to this development. We are starting to see a new paradigm where magnetization dynamics and charge currents act on each other in nanostructured artificial materials. Ultimately, 'spin currents' could even replace charge currents for the transfer and treatment of information, allowing faster, low-energy operations: spin electronics is on its way.

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The emergence of spin electronics in data storage

Giant magnetic tunneling effect in Fe/Al2O3/Fe junction

We have prepared epitaxial FePt ${L1}_{0}$ (001) nanoparticles covered with Ag and Pt overlayers and investigated their magnetic behaviors by means of anomalous Hall resistance measurements. The particle shapes are thin oblate spheroids with the aspect ratio (height/diameter) of 1/5. The size is ranging from 1 to 2.5 nm in height and from 5 to 30 nm in diameter. FePt ${L1}_{0}$ nanoparticles show extremely large coercivity ${H}_{c}$ of about 70 kOe at 10 K, which is close to the anisotropy field ${H}_{k}$ of highly ordered FePt ${L1}_{0}.$ This verifies that the very strong magnetic anisotropy ${K}_{u}$ of FePt ${L1}_{0}$ remains even in the size of several atomic layers along the c axis. For a particle diameter of ${D}_{m}l20\mathrm{nm},$ all the magnetic properties, such as the angular dependence of irreversible switching field, the magnitude of ${H}_{c},$ and their temperature dependence, are fully explained by the coherent rotation model, taking the thermal relaxation into account. Although both Ag- and Pt-coated particles follow the coherent rotation model, the latter always exhibits smaller ${H}_{c}$ than the former. Such a decrease in ${H}_{c}$ can be explained by assuming an enhancement of the effective magnetic moment caused by ferromagnetic polarization of Pt atoms at the Pt/FePt interface. As the particle size ${D}_{m}$ exceeds 20 nm, the magnetic behaviors deviate from the ideal coherent rotation model, suggesting that the magnetization reversal mode changes from coherent to incoherent rotation. The critical diameter ${D}_{m}\ensuremath{\sim}20\mathrm{nm}$ at which the reversal mode changes is in good agreement with the critical diameter predicted by the micromagnetic theory.

T. Miyazaki

Papers

Giant magnetic tunneling effect in Fe/Al2O3/Fe junction

Size dependences of magnetic properties and switching behavior in FePt L10 nanoparticles

Large magnetoresistance effect in 82Ni-Fe/Al-Al2O3/Co magnetic tunneling junction

Dependence of magnetoresistance on temperature and applied voltage in a 82Ni-Fe/Al-Al2O3/Co tunneling junction

Exchange-spring behavior in sputter-deposited α-Fe/NdFeB multilayer magnets