The recent discovery that a spin-polarized electrical current can apply a large torque to a ferromagnet, through direct transfer of spin angular momentum, offers the possibility of manipulating magnetic-device elements without applying cumbersome magnetic fields. However, a central question remains unresolved: what type of magnetic motions can be generated by this torque? Theory predicts that spin transfer may be able to drive a nanomagnet into types of oscillatory magnetic modes not attainable with magnetic fields alone, but existing measurement techniques have provided only indirect evidence for dynamical states. The nature of the possible motions has not been determined. Here we demonstrate a technique that allows direct electrical measurements of microwave-frequency dynamics in individual nanomagnets, propelled by a d.c. spin-polarized current. We show that spin transfer can produce several different types of magnetic excitation. Although there is no mechanical motion, a simple magnetic-multilayer structure acts like a nanoscale motor; it converts energy from a d.c. electrical current into high-frequency magnetic rotations that might be applied in new devices including microwave sources and resonators.

Microwave oscillations of a nanomagnet driven by a spin-polarized current

In recent years, a number of different technologies have been proposed for use in reflective displays1,2,3. One of the most appealing applications of a reflective display is electronic paper, which combines the desirable viewing characteristics of conventional printed paper with the ability to manipulate the displayed information electronically. Electronic paper based on the electrophoretic motion of particles inside small capsules has been demonstrated1 and commercialized; but the response speed of such a system is rather slow, limited by the velocity of the particles. Recently, we have demonstrated that electrowetting is an attractive technology for the rapid manipulation of liquids on a micrometre scale4. Here we show that electrowetting can also be used to form the basis of a reflective display that is significantly faster than electrophoretic displays, so that video content can be displayed. Our display principle utilizes the voltage-controlled movement of a coloured oil film adjacent to a white substrate. The reflectivity and contrast of our system approach those of paper. In addition, we demonstrate a colour concept, which is intrinsically four times brighter than reflective liquid-crystal displays5 and twice as bright as other emerging technologies1,2,3. The principle of microfluidic motion at low voltages is applicable in a wide range of electro-optic devices.

Video-speed electronic paper based on electrowetting

Ferromagnetic materials exhibit intrinsic `easy' and `hard' directions of the magnetization. This magnetic anisotropy is, from both a technological and fundamental viewpoint one of the most important properties of magnetic materials. The magnetic anisotropy in metallic magnetic multilayers forms the subject of this review article. As individual layers in a multilayer stack become thinner, the role of interfaces and surfaces may dominate that of the bulk: this is the case in many magnetic multilayers, where a perpendicular interface contribution to the magnetic anisotropy is capable of rotating the easy magnetization direction from in the film plane to perpendicular to the film plane. In this review, we show that the (in-plane) volume and (perpendicular) interface contribution to the magnetic anisotropy have been separated into terms related to mechanical stresses, crystallographic structure and the planar shape of the films. In addition, the effect of roughness, often inherent to the deposition techniques used, has been addressed theoretically. Several techniques to prepare multilayers and to characterize their growth as well as methods to determine the magnetic anisotropy are discussed. A comprehensive survey of experimental studies on the perpendicular magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on. Two major subjects of this review are the extrinsic effects of strain, roughness and interdiffusion and the intrinsic effect of the crystallographic orientation on the magnetic anisotropy. Both effects are investigated with the help of some dedicated experimental studies. The results of the orientational dependence studies are compared with ab initio calculations. Finally, the perpendicular surface anisotropy and the in-plane step anisotropy are discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/59/11/002/pdf

Magnetic anisotropy in metallic multilayers

Oscillatory interlayer coupling and giant magnetoresistance in Co/Cu multilayers

The impact of stress-driven structural transitions and of film strain on the magnetic properties of nm ferromagnetic films is discussed. The stress-induced bending of film-substrate composites is analysed to derive information on film stress due to lattice mismatch or due to surface-stress effects. The magneto-elastic coupling in epitaxial films is determined directly from the magnetostrictive bending of the substrate. The combination of stress measurements with magnetic investigations by the magneto-optical Kerr effect (MOKE) reveals the modification of the magnetic anisotropy by film stress. Stress-strain relations are derived for various epitaxial orientations to facilitate the analysis of the substrate curvature. Biaxial film stress and magneto-elastic coupling coefficients are measured in epitaxial Fe films in situ on W single-crystal substrates. Tremendous film stress of more than 10 GPa is measured in pseudomorphic Fe layers, and the important role of film stress as a driving force for the formation of misfit distortions and for inducing changes of the growth mode in monolayer thin films is presented. The direct measurement of the magneto-elastic coupling in epitaxial films proves that the magnitude and sign of the magneto-elastic coupling deviate from the respective bulk value. Even a small film strain of order 0.1% is found to induce a significant change of the effective magneto-elastic coupling coefficient. This peculiar behaviour is ascribed to a second-order strain dependence of the magneto-elastic energy density, in contrast to the linear strain dependence that is valid for bulk samples.

/pdf/the-correlation-between-mechanical-stress-and-magnetic-5fdi2tu8cb.pdf

The correlation between mechanical stress and magnetic anisotropy in ultrathin films

A series of epitaxial Co-Au and Co-Cu superlattices has been grown by molecular-beam-epitaxy techniques, with Co thicknesses varying from 5 to 40 \AA{} and constant Au and Cu thicknesses of 16 and 25 \AA{}, respectively. We compare the magnetic properties of both types of superlattice. Crossover to an easy axis normal to the film plane occurs at a thickness of 19 \AA{} of Co in the Co-Au series and 10 \AA{} of Co in the Co-Cu series. We show that the observed magnetic behavior of both of these systems can be accounted for by contributions from demagnetization, magnetocrystalline anisotropy, and magnetoelastic anisotropy. The magnetoelastic term depends on the strain present in Co layers, which in turn is a function of the Co-layer thickness. In our calculation we employ measured Co-layer strains, determined by x-ray scattering, in combination with the known elastic and magnetostriction constants of bulk Co.

Magnetic anisotropy in epitaxial Co superlattices.

We demonstrate the growth of epitaxially ordered Co-Au superlattices by ultrahigh-vacuum deposition techniques. The superlattice structure consists of hcp Co(0001) and fcc Au(111) layers with the in-plane Co [$11\overline{2}0$] crystallographic axis parallel to GaAs [001]. X-ray scattering, high-resolution transmission-electron microscopy, and measurements of the magnetic anisotropy show that the Co-Au interfaces are abrupt even in the presence of a large lattice mismatch.

Epitaxial Co-Au Superlattices

A series of epitaxial Co/Cu superlattices has been grown on GaAs(110) substrates by molecular-beam epitaxy. Detailed analysis of x-ray diffuse scattering scans along Co (101\ifmmode\bar\else\textasciimacron\fi{}l) reveals metastable fcc stacking of Co, with stacking coherence extending across Co layers at thicknesses below 20 A\r{}, while coherent interfaces are obtained at Co thickness up to 40 A\r{}. We discuss possible mechanisms for stabilizing the stacking sequence.

Coherent fcc stacking in epitaxial Co/Cu superlattices

Les resultats demontrent l'utilite de la resonance magnetique pour l'etude de la structure interfaciale dans des superreseaux a base de cobalt. Les satellites hyperfins n'identifient que des atomes Co interfaciaux avec trois plus proches voisins Cu. Ceci indique la presence d'interfaces atomiquement abruptes. Les couches de Co ont une symetrie d'empilement cubique a faces centrees

NMR study of interface structure in epitaxial Co-Cu superlattices.

Resistivity, magnetoresistance, and the Hall effect have been measured in two epitaxially ordered Co-Au superlattices at temperatures between 2 and 295 K. One sample is composed of 60 bilayers of Co 5 \AA{} Au 16 \AA{} and has a magnetic easy axis perpendicular to the film plane, while the other contains 30 bilayers of Co (30 \AA{})/Au(16 \AA{}) and has an easy axis parallel to the film plane. The magnetoresistance effect of the first sample (\ensuremath{\sim}15%) is an order of magnitude larger than in the second sample. In addition, the Co(5 \AA{})/Au(16 \AA{}) sample exhibits a large Hall voltage in zero applied magnetic field.

Hui He

Papers

Magnetic anisotropy in epitaxial Co superlattices.

Epitaxial Co-Au Superlattices

Coherent fcc stacking in epitaxial Co/Cu superlattices

NMR study of interface structure in epitaxial Co-Cu superlattices.

Magnetoresistance and Hall effect in epitaxial Co-Au superlattices