Electrical Control Of Spin Polarization Research Articles

Nonvolatile electrical control of spin polarization in the 2D bipolar magnetic semiconductor VSeF

Nonvolatile electrical control of spin polarization in two-dimensional (2D) magnetic semiconductors is greatly appealing toward future low-dissipation spintronic nanodevices. Here, we report a 2D material VSeF, which is an intrinsic bipolar magnetic semiconductor (BMS) featured with opposite spin-polarized valence and conduction band edges. We then propose a general nonvolatile strategy to manipulate both spin-polarized orientations in BMS materials by introducing a ferroelectric gate with proper band alignment. The spin-up/spin-down polarization of VSeF is successfully controlled by the electric dipole of ferroelectric bilayer Al2Se3, verifying the feasibility of the design strategy. The interfacial doping effect from ferroelectric gate also plays a role in enhancing the Curie temperature of the VSeF layer. Two types of spin field effect transistors, namely multiferroic memory and spin filter, are further achieved in VSeF/Al2Se3 and VSeF/Al2Se3/Al2Se3 multiferroic heterostructures, respectively. This work will stimulate the application of 2D BMS materials in future spintronic nanodevices.

Gate-tuneable and chirality-dependent charge-to-spin conversion in tellurium nanowires.

Chiral materials are an ideal playground for exploring the relation between symmetry, relativistic effects and electronic transport. For instance, chiral organic molecules have been intensively studied to electrically generate spin-polarized currents in the last decade, but their poor electronic conductivity limits their potential for applications. Conversely, chiral inorganic materials such as tellurium have excellent electrical conductivity, but their potential for enabling the electrical control of spin polarization in devices remains unclear. Here, we demonstrate the all-electrical generation, manipulation and detection of spin polarization in chiral single-crystalline tellurium nanowires. By recording a large (up to 7%) and chirality-dependent unidirectional magnetoresistance, we show that the orientation of the electrically generated spin polarization is determined by the nanowire handedness and uniquely follows the current direction, while its magnitude can be manipulated by an electrostatic gate. Our results pave the way for the development of magnet-free chirality-based spintronic devices.

Electrical control of spin polarization of transmission in pure-carbon systems of helical graphene nanoribbons

Compared with conventional magnetic ways, modulating a spin-polarized current through electrical means could greatly reduce nano-devices’ energy consumption and dimensions, which emerges as a new research area in spintronics. Inspired by the experimental progress on the synthesis of helically twisted graphene, we study the electronic transport of kekulene-like helical graphene nanoribbons through first-principles calculations. By applying a gate voltage, the system can be switched between spin-unpolarized and completely spin-polarized states, realizing an electrically controlled spin filter. Moreover, a fine modulation of the spin polarization can also be achieved, where transmission with any ratio of spin-up to spin-down electrons can be obtained, beyond the traditional spin filter. The analysis shows that it is the particular transmission spectra that play a key role, where two wide peaks with opposite spins reside partially overlapped around the Fermi level. They originate from the p orbitals of the zigzag edge part in the helical structure. Since the configuration only consists of carbon atoms, the electrical control of spin polarization is realized in a pure-carbon nano-system, showing great application potentials.

Magneto-ionic control of spin polarization in multiferroic tunnel junctions

Multiferroic tunnel junctions (MFTJs) with Hf0.5Zr0.5O2 barriers are reported to show both tunneling magnetoresistance effect (TMR) and tunneling electroresistance effect (TER), displaying four resistance states by magnetic and electric field switching. Here we show that, under electric field cycling of large enough magnitude, the TER can reach values as large as 106%. Moreover, concomitant with this TER enhancement, the devices develop electrical control of spin polarization, with sign reversal of the TMR effect. Currently, this intermediate state exists for a limited number of cycles and understanding the origin of these phenomena is key to improve its stability. The experiments presented here point to the magneto-ionic effect as the origin of the large TER and strong magneto-electric coupling, showing that ferroelectric polarization switching of the tunnel barrier is not the main contribution.

Electrical control of spin polarization of conductance in Mn-encapsulated Si nanotube

Compared with the use of magnetic methods, electrical manipulation of the spin polarization of a current could greatly reduce the devices’ dimensions and energy consumption. Using first-principles calculations, the spin-dependent electronic transport of a Mn-encapsulated Si nanotube contacted with Cu electrodes is investigated. As the gate voltage decreases, the conductance changes from spin unpolarized to spin polarized (the polarization could reach 90%). Electron transfer between Si and Mn atoms modulated by the gate voltage is found to be the physical mechanism. Because Si and Cu are fundamental materials in integrated circuits, these findings may be quite useful for developing Si-based spintronic devices.

Optical and electrical control of spin polarization of two-dimensional hole gases in p-type resonant tunnelling devices

In this work, we have investigated the spin polarization from two-dimensional hole gases (2DHG) formed in p–i–p GaAs/AlAs resonant tunnelling diodes (RTDs) under magnetic field parallel to the tunnel current. We have observed that the polarization degree from the quantum well (QW) and the 2DHG formed at the accumulation layer is highly voltage and light sensitive and exhibits a clear sign inversion. Our results indicate that the voltage dependence of the QW polarization degree is mainly due to an efficient hole-resonant tunnelling process through spin states of the QW. On the other hand, the voltage dependence of the 2DHG polarization degree seems to be dependent on the hole density which is controlled by the applied voltage across the RTDs.

Sub‐ns electrical control of spin polarization in a semiconductor by microscale current loops

AbstractWe demonstrate a method to electrically control the spin polarization in a diluted magnetic semiconductor (DMS) quantum well (QW) on a micrometer length scale. A variable magnetic field generated from a microscale current loop atop a single CdMnTe/CdMgTe QW magnetizes the Mn2+ ion spins in the DMS QW and, via the sp–d exchange interaction, polarizes the spins of photon‐generated carriers. In the absence of an external field, the spin polarization can be switched on a time scale clearly below 1 ns.

Multiple switching of spin polarization injected into a semiconductor by a multiferroic tunneling junction

A method for switching between multiple spin polarization of the electric current injected into a semiconductor is proposed, based on injecting spins from a diluted magnetic semiconductor through a multiferroic tunnel barrier. We show that it is important to combine a diluted magnetic semiconductor electrode with a multiferroic barrier to realize multiple switching of spin polarization on wide range. The reversal of either electric polarization or magnetization in a multiferroic barrier results in a sizable change in the spin polarization of the injected current both in magnitude and in sign, thereby providing a four-state electrical control of spin polarization. The electroresistance and electromagnetoresistance effects can be also realized in this structure. Our investigations may stimulate experimental studies of the multiferroic tunnel junctions.