Junsaku Nitta

New perspectives for Rashba spin-orbit coupling

In 1984, Bychkov and Rashba introduced a simple form of spin-orbit coupling to explain the peculiarities of electron spin resonance in two-dimensional semiconductors. Over the past 30 years, Rashba spin-orbit coupling has inspired a vast number of predictions, discoveries and innovative concepts far beyond semiconductors. The past decade has been particularly creative, with the realizations of manipulating spin orientation by moving electrons in space, controlling electron trajectories using spin as a steering wheel, and the discovery of new topological classes of materials. This progress has reinvigorated the interest of physicists and materials scientists in the development of inversion asymmetric structures, ranging from layered graphene-like materials to cold atoms. This Review discusses relevant recent and ongoing realizations of Rashba physics in condensed matter.

Giant spin Hall effect in perpendicularly spin-polarized FePt/Au devices

Conversion of charge current into pure spin current and vice versa in non-magnetic semiconductors or metals, which are called the direct and inverse spin Hall effects (SHEs), provide a new functionality of materials for future spin-electronic architectures. Thus, the realization of a large SHE in a device with a simple and practical geometry is a crucial issue for its applications. Here, we present a multi-terminal device with a Au Hall cross and an FePt perpendicular spin injector to detect giant direct and inverse SHEs at room temperature. Perpendicularly magnetized FePt injects or detects perpendicularly polarized spin current without magnetic field, enabling the unambiguous identification of SHEs. The unprecedentedly large spin Hall resistance of up to 2.9 mOmega is attributed to the large spin Hall angle in Au through the skew scattering mechanism and the highly efficient spin injection due to the well-matched spin resistances of the chosen materials.

Spin-interference device

We propose a spin-interference device which works even without any ferromagnetic electrodes and any external magnetic field. The interference can be expected in the Aharonov–Bohm (AB) ring with a uniform spin-orbit interaction, which causes the phase difference between the spin wave functions traveling in the clockwise and anticlockwise direction. The gate electrode, which covers the whole area of the AB ring, can control the spin-orbit interaction, and therefore, the interference. A large conductance modulation effect can be expected due to the spin interference.

Spin-Transistor Electronics: An Overview and Outlook

Spin transistors are a new concept device that unites an ordinary transistor with the useful functions of a spin (magnetoresistive) device. They are expected to be a building block for novel integrated circuits employing spin degrees of freedom. The interesting features of spin transistors are nonvolatile information storage and reconfigurable output characteristics: these are very useful and suitable functionalities for various new integrated circuit architectures that are inaccessible to ordinary transistor circuits. This article reviews the current status and outlook of spin transistors from the viewpoint of integrated circuit applications. The device structure, operating principle, performance, and features of various spin transistors are discussed. The fundamental and key phenomena/technologies for spin injection, transport, and manipulation in semiconductors and the integrated circuit applications of spin transistors to nonvolatile logic and reconfigurable logic are also described.

Coercivity change in an FePt thin layer in a Hall device by voltage application

The coercivity (Hc) of a perpendicularly magnetized FePt layer was modulated by applying the voltage (Vapp) to a Hall device through MgO and Al–O insulating layers. A change in ∼40 Oe in Hc was observed by changing Vapp from −13 to 13 V. From the quantitative analysis of the voltage effect on Hc, the change in the anisotropy energy by voltage application was evaluated to be 18.6 fJ/V m, which was of the same order as the theoretical prediction. The role of the MgO layer for the voltage effect was also discussed.

A Josephson field effect transistor using an InAs‐inserted‐channel In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure

A Josephson field effect transistor (JOFET) was coupled with a two‐dimensional electron gas in a strained InAs quantum well inserted into an In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure. The characteristics of this JOFET are much improved over previous devices by using a high electron mobility transistor (HEMT)‐type gate instead of the usual metal‐insulator‐ semiconductor (MIS)‐type gate. The superconducting critical current as well as the junction normal resistance are completely controlled via a gate voltage of about −1 V; this provides voltage gain over 1 for a JOFET.

Mesoscopic Stern-Gerlach spin filter by nonuniform spin-orbit interaction

A spin filtering in a two-dimensional electron system with nonuniform spin-orbit interactions (SOI) is theoretically studied. The strength of SOI is modulated perpendicular to the charge current. A spatial gradient of effective magnetic field due to the nonuniform SOI causes the Stern-Gerlach-type spin separation. The direction of the polarization is perpendicular to the current and parallel to the spatial gradient. Almost 100% spin polarization can be realized even without applying any external magnetic fields and without attaching ferromagnetic contacts. The spin polarization persists even in the presence of randomness.

Control of magnetization states in microstructured permalloy rings

Magnetization processes of microstructured NiFe rings are studied by the fringe-field-induced local Hall effect and numerical model calculations. The changes in reversible and irreversible magnetization of single rings are detected with very high resolution. We observe that the type of magnetic transition depends on the ratio between the inner and outer ring diameter. For narrow rings, sharp transitions from so-called “onion” to the “vortex” state are observed. In rings with smaller inner diameter, the transitions are more complex. The creation of local vortices and their spatial movement by applying an external magnetic field are detected.

Experimental realization of a ballistic spin interferometer based on the Rashba effect using a nanolithographically defined square loop array

As quantum wells. Inthis experiment, we demonstrate electron spin precession in quasi-one-dimensional channels that iscaused by the Rashba effect. It turned out that the spin precession angle θ was gate-controllableby more than 0.75π for a sample with L = 1.5µm, where L is the side length of the SL. Largecontrollability of θ by the applied gate voltage as such is a necessary requirement for the realizationof the spin FET device proposed by Datta and Das [Datta et. al., Appl. Phys. Lett. 56, 665 (1990)]as well as for the manipulation of spin qubits using the Rashba effect.

Improving the mobility of an In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure by inserting a strained InAs quantum well

The mobility of two‐dimensional electrons in an In0.52Al0.48As/In0.53Ga0.47As inverted modulation‐doped structure improved by inserting an InAs quantum well into the InGaAs channel. This letter addresses the main cause of this mobility improvement. By optimizing the thickness of the InAs quantum well, its distance from the underlying InAlAs spacer layer, and the InAlAs spacer‐layer thickness, maximum mobilities of 16 500 cm2/V s at 300 K and 155 000 cm2/V s at 10 K are attained. The improvement in mobility is attributed to a decrease in scattering caused by ionized impurities, interface‐roughness, and trap impurities. This decrease is a result of the superior confinement of two‐dimensional electron gas in the InAs quantum well.