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.

/pdf/spintronics-fundamentals-and-applications-2dx38n6phu.pdf

Spintronics: Fundamentals and applications

We report calculations of electron inelastic mean free paths (IMFPs) for 50–2000 eV electrons in a group of 27 elements (C, Mg, Al, Si, Ti, V, Cr, Fe, Ni, Cu, Y, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Ta, W, Re, Os, Ir, Pt, Au and Bi). This work extends our previous calculations (Surf. Interface Anal. 11, 57 (1988)) for the 200–2000 eV range. Substantial variations were found in the shapes of the IMFP versus energy curves from element to element over the 50–2000 eV range and we attribute these variations to the different inelastic scattering properties of each material. Our calculated IMFPs wee fitted to a modified form of the Bethe equation for inelastic electron scattering in matter; this equation has four parameters. These four parameters could be empirically related to several material parameters for our group of elements (atomic weight, bulk density and number of valence electron per atom). IMFPs and those initially calculated was 13%. The modified Bethe equation and our expressions for the four parameters can therefore be used to estimate IMFPs in other materials. The uncertainties in the algorithm used for our IMFP calculation are difficult to estimate but are believed to be largely systematic. Since the same algorithm has been used for calculating IMFPs, our predictive IMFP formula is considered to be particularly useful for predicting the IMFP dependence on energy in the 50–2000 eV range and the material dependence for a given energy.

Calculations of electorn inelastic mean free paths. II. Data for 27 elements over the 50–2000 eV range

The contribution that the technique of ferromagnetic resonance (FMR) has made to the understanding of the magnetic behaviour of ultrathin single films is reviewed. Experimental methods to measure FMR in situ in ultrahigh vacuum are presented. The temperature dependence of the magnetization, of the magnetic relaxation rate in the vicinity of the Curie temperature, and of the second- and fourth-order magnetic anisotropy energy (MAE) constants can be measured by FMR in situ for magnetic monolayers. Using the cases of Ni/Cu(001) and Gd/W(110) as examples, the role of the MAE for the quantitative description of temperature- and thickness-dependent reorientation transitions of the magnetization is discussed. Initial results for the anisotropy of the g-factor which is related to the anisotropy of the orbital moment (and the MAE) are presented.

https://iopscience.iop.org/article/10.1088/0034-4885/61/7/001/pdf

Ferromagnetic resonance of ultrathin metallic layers

Physical and chemical properties of bimetallic surfaces

The discovery of enhanced magnetoresistance and oscillatory interlayer exchange coupling in transition metal multilayers just over a decade ago has enabled the development of new classes of magnetically engineered magnetic thin-film materials suitable for advanced magnetic sensors and magnetic random access memories. Magnetic sensors based on spin-valve giant magnetoresistive (GMR) sandwiches with artificial antiferromagnetic reference layers have resulted in enormous increases in the storage capacity of magnetic hard disk drives. The unique properties of magnetic tunnel junction (MTJ) devices has led to the development of an advanced high performance nonvolatile magnet random access memory with density approaching that of dynamic random-access memory (RAM) and read-write speeds comparable to static RAM. Both GMR and MTJ devices are examples of spintronic materials in which the flow of spin-polarized electrons is manipulated by controlling, via magnetic fields, the orientation of magnetic moments in inhomogeneous magnetic thin film systems. More complex devices, including three-terminal hot electron magnetic tunnel transistors, suggest that there are many other applications of spintronic materials.

Magnetically engineered spintronic sensors and memory

The magnetization in ultrathin Fe layers (2.5-3.5 atomic layers) on Cu(100) reversibly switches between perpendicular (at low temperature) and in-plane magnetization (at higher temperature). The switching temperature decreases with increasing film thickness. The switching transition is attributed to the temperature dependence of the perpendicular anisotropy. The transition is accompanied by a loss of magnetization over a temperature range of 20-30 K and shows evidence for a canted-spin configuration

Reversible transition between perpendicular and in-plane magnetization in ultrathin films.

Spin-polarized photoemission spectra at low photon energies from ferromagnetic ultrathin Fe layers on Cu(100) show a substantial polarization of the Cu 3{ital d} peaks. This is attributed to spin-dependent attenuation in the Fe overlayer. Values of the spin-dependent mean free path at low electron energies are obtained.

Spin-dependent electron attenuation by transmission through thin ferromagnetic films.

The surface magnetism of Gd(0001) films is studied using spin resolved electron spectroscopies. Both a canted surface magnetization vector out of the plane of the film and an enhanced surface magnetic ordering temperature 60[plus minus]2 K higher than the bulk Curie temperature are observed. Our results show that the surface magnetic layer behaves as a separate entity from the bulk. The in-plane component of surface magnetization aligns ferromagnetically to the bulk. Spin resolved photoemission from the Gd 4[ital f] core levels shows [approx]65% spin polarization at 150 K, which extrapolates to [approx]91% at 0 K.

Magnetic reconstruction of the Gd(0001) surface.

Reduction of macroscopic moment in ultrathin Fe films as the magnetic orientation changes.

Magnetism in ultrathin (1–10 ML) Fe films grown on Cu(100) has been studied by spin‐polarized secondary electron emission spectroscopy. The variation of the magnetization with temperature and oxygen adsorption was investigated for various film thicknesses. The orientation of the magnetization for films between 5 and 6 ML thick switches reversibly between perpendicular (at low temperature) to in‐plane (at high temperature). The switching transition temperature decreases with increasing film thickness, and is accompanied by a loss of long‐range order over a range of 20–30 K. The transition is attributed to the temperature dependence of the perpendicular anisotropy. The effect of oxygen adsorption onto films with perpendicular remanence is to first suddenly turn the magnetization into the plane at a critical coverage, and then to kill the magnetization gradually with continued exposure. This indicates that the uniaxial surface anisotropy at the Fe‐vacuum interface plays a major role in the magnetization of t...

H. Hopster

Papers

Reversible transition between perpendicular and in-plane magnetization in ultrathin films.

Spin-dependent electron attenuation by transmission through thin ferromagnetic films.

Magnetic reconstruction of the Gd(0001) surface.

Reduction of macroscopic moment in ultrathin Fe films as the magnetic orientation changes.

Magnetism of ultrathin films of Fe on Cu(100)