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.

It is proposed that permanent magnets can be made of composite materials consisting of two suitably dispersed ferromagnetic and mutually exchange-coupled phases, one of which is hard magnetic in order to provide a high coercive field, while the other may be soft magnetic, just providing a high saturation J/sub s/, and should envelop the hard phase regions in order to prevent their corrosion. A general theoretical treatment of such systems shows that one may expect, besides a high energy product (BH)/sub max/, a reversible demagnetization curve (exchange-spring) and, in certain cases, an unusually high isotropic remanence ratio B/sub r//J/sub s/, while the required volume fraction of the hard phase may be very low, on the order of 10%. The technological realization of such materials is shown to be based on the principle that all phases involved must emerge from a common metastable matrix phase in order to be crystallographically coherent and consequently magnetically exchange coupled. >

The exchange-spring magnet: a new material principle for permanent magnets

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.

/pdf/the-emergence-of-spin-electronics-in-data-storage-1vrnm5549k.pdf

The emergence of spin electronics in data storage

On a generalised approach to first and second order magnetic transitions

The theory of critical phenomena in systems at equilibrium is reviewed at an introductory level with special emphasis on the values of the critical point exponents α, β, γ,..., and their interrelations. The experimental observations are surveyed and the analogies between different physical systems - fluids, magnets, superfluids, binary alloys, etc. - are developed phenomenologically. An exact theoretical basis for the analogies follows from the equivalence between classical and quantal `lattice gases' and the Ising and Heisenberg-Ising magnetic models. General rigorous inequalities for critical exponents at and below Tc are derived. The nature and validity of the `classical' (phenomenological and mean field) theories are discussed, their predictions being contrasted with the exact results for plane Ising models, which are summarized concisely. Pade approximant and ratio techniques applied to appropriate series expansions lead to precise critical-point estimates for the three-dimensional Heisenberg and Ising models (tables of data are presented). With this background a critique is presented of recent theoretical ideas: namely, the `droplet' picture of the critical point and the `homogeneity' and `scaling' hypotheses. These lead to a `law of corresponding states' near a critical point and to relations between the various exponents which suggest that perhaps only two or three exponents might be algebraically independent for any system.

https://iopscience.iop.org/article/10.1088/0034-4885/30/2/306/pdf

The theory of equilibrium critical phenomena

A new type of magnetic anisotropy has been discovered which is best described as an exchange anisotropy. This anisotropy is the result of an interaction between an antiferromagnetic material and a ferromagnetic material. The material that exhibits this exchange anisotropy is a compact of fine particles of cobalt with a cobaltous oxide shell. The effect occurs only below the N\'eel temperature of the antiferromagnetic material, which is essentially room temperature for the cobaltous oxide. An exchange torque is inferred to exist between the metal and oxide which has a maximum value at 77\ifmmode^\circ\else\textdegree\fi{}K of \ensuremath{\sim}2 dyne-cm/${\mathrm{cm}}^{2}$ of interface.

New Magnetic Anisotropy

The exchange interaction that gives rise to ordered magnetic states depends upon interatomic spacing. If the lattice is deformable, then a spontaneous distortion of the lattice will occur in the ordered state. We have calculated, in the molecular field approximation, the properties of a system in which the exchange energy dependence is given by ${T}_{c}={T}_{0}[1+\frac{\ensuremath{\beta}(v\ensuremath{-}{v}_{0})}{{v}_{0}}]$. ${T}_{c}$ is the Curie temperature appropriate to a lattice volume $v$ while ${v}_{0}$ is the equilibrium volume in the absence of magnetic interactions. The course of the magnetization with temperature of such a system depends upon the steepness $\ensuremath{\beta}$ of the exchange interaction dependence on interatomic distance, the compressibility $K$, and ${T}_{0}$. The behavior may be the usual second-order transition to paramagnetism, but it can in fact become a first-order transition with the properties usually associated thereto, e.g., latent heat and discontinuous density change. In the absence of an externally applied pressure, the transition will be of the first order if $\ensuremath{\eta}\ensuremath{\equiv}\frac{40NkK{T}_{0}{\ensuremath{\beta}}^{2}{[j(j+1)]}^{2}}{[{(2j+1)}^{4}\ensuremath{-}1]}g1$. In this inequality, $N$ is the number per unit volume of magnetic ions of angular momentum $j\ensuremath{\hbar}$ while $k$ is the Boltzmann constant.We have reviewed the experimental evidence on the nature of the first-order magnetic transition in MnAs. We find that this evidence indicates the transition to be one from ferromagnetism to paramagnetism rather than ferromagnetism to antiferromagnetism as has been generally assumed. Application of the theory noted above gives $\ensuremath{\eta}=2$ for this transition. In addition, we derive a value for the volume strain sensitivity, $\ensuremath{\beta}=19$ and infer the compressibility to be 2.2\ifmmode\times\else\texttimes\fi{}${10}^{\ensuremath{-}12}$ ${\mathrm{cm}}^{2}$/d.

Magnetic Disorder as a First-Order Phase Transformation

The coercive force predicted by theory for single-domain particles with shape anisotropy frequently far exceeds the observed value. From an examination of the results of Paine, Mendelsohn, and Luborsky on elongated iron particles, several approximate models are suggested which may exist in certain experimental situations. Detailed calculations are presented for a "chain-of-spheres" model remagnetizing by several mechanisms. Comparison with experiment on certain single-domain particles of known elongation favors the chain-of-spheres model as a suitable description of their magnetic behavior. A successful calculation is made of the coercive force of material prepared by several earlier workers. A comparison of the new models with the older ones indicates a direction for experimental advance.

An Approach to Elongated Fine-Particle Magnets

The abnormally long jump distance $\ensuremath{\lambda}$ which characterizes the ionic conduction of glasses as a function of electric field, $\ensuremath{\sigma}={\ensuremath{\sigma}}_{0}\mathrm{exp}(\ensuremath{-}\frac{q\ensuremath{\lambda}E}{\mathrm{kT}})$, and the fact that for tantalum oxide this distance changes by a factor of two at a critical field are explained in terms of a model in which ionic conduction is a two-stage process involving the creation and motion of interstitial metallic ions. Below a critical field both the number of interstitial ions and their mobility are rapidly varying functions of the field and the jump distance decreases discontinuously as the field increases. Above the critical field the number of interstitials depends much less strongly or not at all on the field, the field acts only to increase the ionic mobility, and the jump distance is of the order of the interatomic spacing. It is suggested that in tantalum oxide electrostatic repulsion between tantalum ions may provide the principle barrier to ionic motion.

C. P. Bean

Papers

New Magnetic Anisotropy

Magnetic Disorder as a First-Order Phase Transformation

An Approach to Elongated Fine-Particle Magnets

Ionic Conductivity of Tantalum Oxide at Very High Fields

LARGE MAGNETIC KERR ROTATION IN BiMn ALLOY