The spin Hall effect (SHE) of light is very weak because of the extremely small photon momentum and spin-orbit interaction. Here, we report a strong photonic SHE resulting in a measured large splitting of polarized light at metasurfaces. The rapidly varying phase discontinuities along a metasurface, breaking the axial symmetry of the system, enable the direct observation of large transverse motion of circularly polarized light, even at normal incidence. The strong spin-orbit interaction deviates the polarized light from the trajectory prescribed by the ordinary Fermat principle. Such a strong and broadband photonic SHE may provide a route for exploiting the spin and orbit angular momentum of light for information processing and communication.

http://science.sciencemag.org/content/sci/339/6126/1405.full.pdf

Photonic spin hall effect at metasurfaces

Magnetism is a very fascinating and dynamic field Especially in the last 30 years it has experienced many major advances in the full range from novel fundamental phenomena to new products Applications such as hard disk drives and magnetic sensors are part of our daily life, and new applications, such as in non-volatile computer random access memory, are expected to surface shortly Thus it is timely for describing the current status, and current and future challenges in the form of a Roadmap article This 2014 Magnetism Roadmap provides a view on several selected, currently very active innovative developments It consists of 12 sections, each written by an expert in the field and addressing a specific subject, with strong emphasize on future potential This Roadmap cannot cover the entire field We have selected several highly relevant areas without attempting to provide a full review - a future update will have room for more topics The scope covers mostly nano-magnetic phenomena and applications, where surfaces and interfaces provide additional functionality New developments in fundamental topics such as interacting nano-elements, novel magnon-based spintronics concepts, spin-orbit torques and spin-caloric phenomena are addressed New materials, such as organic magnetic materials and permanent magnets are covered New applications are presented such as nano-magnetic logic, non-local and domain-wall based devices, heat-assisted magnetic recording, magnetic random access memory, and applications in biotechnology May the Roadmap serve as a guideline for future emerging research directions in modern magnetism

https://iopscience.iop.org/article/10.1088/0022-3727/47/33/333001/pdf

The 2014 Magnetism Roadmap

Many high-performance materials and novel devices consist of multiple components and are naturally or intentionally nano-structured for optimal properties and performance. To understand their structure-property relationships fully, quantitative compositional analysis at length scales below 100 nm is required, a need that is often uniquely addressed using soft X-ray microscopy. Similarly, the interaction of X-rays with magnetic materials provides unique element-specific contrast that allows the determination of magnetic properties in multi-element antiferromagnetic and ferromagnetic materials. Pump-probe-type experiments can even investigate magnetic domain dynamics. Here we review and exemplify the ability of soft X-ray microscopy to provide information that is otherwise inaccessible, and discuss a perspective on future developments.

Near-edge X-ray absorption fine-structure microscopy of organic and magnetic materials

Key design parameters of 64 Mb STT-MRAM at 90-nm technology node are discussed. A design point was developed with adequate TMR for fast read operation, enough energy barrier for data retention and against read disturbs, a write voltage satisfying the long term reliability against dielectric breakdown and a write bit error rate below 10-9. A direct experimental method was developed to determine the data retention lifetime that avoids the discrepancy in the energy barrier values obtained with spin current- and field-driven switching measurements. Other parameters detrimental to write margins such as backhopping and the existence of a low breakdown population are discussed. At low bit-error regime, new phenomenon emerges, suggestive of a bifurcation of switching modes. The dependence of the bifurcated switching threshold on write pulse width, operating temperature, junction dimensions and external field were studied. These show bifurcated switching to be strongly influenced by thermal fluctuation related to the spatially inhomogeneous free layer magnetization. An external field along easy axis direction assisting switching was shown to be effective for significantly reducing the percentage of MTJs showing bifurcated switching.

A Study of Write Margin of Spin Torque Transfer Magnetic Random Access Memory Technology

Spin-transfer torques in magnetic heterostructures give rise to a number of dynamical processes that are not accessible with magnetic fields alone. A prominent example involves self-sustained magnetization oscillations, which are made possible through the compensation of magnetic damping by the transfer of spin angular momentum from a spin-polarized current. In this contribution, the theoretical aspects of magnetization oscillations driven by spin torques, such as spin waves and vortex gyration, are presented in detail and key experimental results are highlighted. It is shown how simple but useful models can be derived from fundamental theories of magnetization dynamics and used to describe a variety of stochastic and nonlinear phenomena.

Spin-Torque Oscillators

We present the first space- and time-resolved images of the spin-torque-induced steady-state oscillation of a magnetic vortex in a spin-valve nanostructure. We find that the vortex structure in a nanopillar is considerably more complicated than the 2D idealized structure often-assumed, which has important implications for the driving efficiency. The sense of the vortex gyration is uniquely determined by the vortex core polarity, confirming that the spin-torque acts as a source of negative damping even in such a strongly nonuniform magnetic system. The orbit radius is $\ensuremath{\sim}10\text{ }\text{ }\mathrm{nm}$, in agreement with micromagnetic simulations.

/pdf/images-of-a-spin-torque-driven-magnetic-nano-oscillator-3c98ylbowv.pdf

Images of a spin-torque-driven magnetic nano-oscillator

We present time-resolved x-ray images with 30 nm spatial and 70 ps temporal resolution, which reveal details of the spatially resolved magnetization evolution in nanoscale samples of various dimensions during reversible spin-torque switching processes. Our data in conjunction with micromagnetic simulations suggest a simple unified picture of magnetic switching based on the motion of a magnetic vortex. With decreasing size of the magnetic element the path of the vortex core moves from inside to outside of the nanoelement, and the switching process evolves from a curled nonuniform to an increasingly uniform mode.

/pdf/direct-observation-of-spin-torque-driven-magnetization-53yg0k5ais.pdf

Direct Observation of Spin-Torque Driven Magnetization Reversal through Nonuniform Modes

We show that the dissipationless spin current in the ground state of the Rashba model gives rise to a reactive coupling between the spin and charge propagation, which is formally identical to the coupling between the electric and the magnetic fields in the 2 + 1 dimensional Maxwell equation. This analogy leads to a remarkable prediction that a density packet can spontaneously split into two counter propagation packets, each carrying the opposite spins. In a certain parameter regime, the coupled spin and charge wave propagates like a transverse 'photon'. We propose both optical and purely electronic experiments to detect this effect.

/pdf/maxwell-equation-for-coupled-spin-charge-wave-propagation-ht2t98zh65.pdf

Maxwell equation for coupled spin-charge wave propagation.

We investigate spin-filtering effect in multilayered ferromagnetic (F)/semiconductor (S) heterostructures within the Landauer framework of ballistic transport. Spin-dependent transmission and polarization are calculated and analyzed for different magnetizations of three ferromagnetic layers in a F∕S∕F∕F structure proposed in this work. The results indicate that in such a multilayered configuration and when the magnetizations of the middle and the right ferromagnetic layers are antiparallel, the transmission for spin-up and spin-down electrons can be separated, which is quite different from the transport properties in the F∕S∕F structure, where electrons of different spin orientations have exactly the same contributions to transmission if the magnetic moments of the two ferromagnetic layers are antiparallel. It is also shown that the F∕S∕F∕F structure can have big values of the polarization than the F∕S∕F structure. The quantum size effect of the length of the middle ferromagnetic layer and that of the sem...

Spin filtering and spin-polarization reversal in multilayered ferromagnetic metal/semiconductor heterostructures

Typical spin-dependent devices proposed for information processing lack one of the most important features provided by charge based logic: they do not provide gain In this letter we show the basic concept of a spin amplifier and propose ways to amplify a spin current at room temperature

Xiaowei Yu

Papers

Images of a spin-torque-driven magnetic nano-oscillator

Direct Observation of Spin-Torque Driven Magnetization Reversal through Nonuniform Modes

Maxwell equation for coupled spin-charge wave propagation.

Spin filtering and spin-polarization reversal in multilayered ferromagnetic metal/semiconductor heterostructures

An amplifier concept for spintronics