Practical Applications In Spintronics Research Articles

Magnetic‐Structure‐Stabilized Polarization in an Above‐Room‐Temperature Ferrimagnet

AbstractAbove‐room‐temperature polar magnets are of interest due to their practical applications in spintronics. Here we present a strategy to design high‐temperature polar magnetic oxides in the corundum‐derived A2BB′O6family, exemplified by the non‐centrosymmetric (R3) Ni3TeO6‐type Mn2+2Fe3+Mo5+O6, which shows strong ferrimagnetic ordering withTC=337 K and demonstrates structural polarization without any ions with (n−1)d10ns0, d0, or stereoactive lone‐pair electrons. Density functional theory calculations confirm the experimental results and suggest that the energy of the magnetically ordered structure, based on the Ni3TeO6prototype, is significantly lower than that of any related structure, and accounts for the spontaneous polarization (68 μC cm−2) and non‐centrosymmetry confirmed directly by second harmonic generation. These results motivate new directions in the search for practical magnetoelectric/multiferroic materials.

Gate-Voltage-Controlled Spin and Valley Polarization Transport in a Normal/Ferromagnetic/Normal MoS2 Junction

Two-dimensional (2D) materials are extensively explored due to the remarkable physical property and the great potential for post-silicon electronics since the landmark achievement of graphene. The monolayer (ML) MoS2 with a direct energy gap is a typical 2D material and promising candidate for a wide range of device applications. The extensive efforts so far have focused on the optical valley control applications of ML MoS2 rather than the electrical control of spin and valley transport. However, the electrical manipulation of spin injection and transport is essential to realize practical spintronics applications. Here, we theoretically demonstrated that the valley and spin transport can be electrically manipulated by a gate voltage in a normal/ferromagnetic/normal monolayer MoS2 junction device. It was found that the fully valley- and spin-polarized conductance can be achieved due to the spin-valley coupling of valence-band edges together with the exchange field, and both the amplitude and direction of the fully spin-polarized conductance can be modulated by the gate voltage. These findings not only provided deep understanding to the basic physics in the spin and valley transport of ML MoS2 but also opened an avenue for the electrical control of valley and spin transport in monolayer dichalcogenide-based devices.

Semiconductor with intrinsic spin: a hybrid structure of zigzag edge graphene nanoribbon/single‐walled carbon nanotube

AbstractA hybrid structure of n ‐ZGRN/(m,m)SWCNT, namely n ‐ZGNR‐(m,m)SWCNT, is predicted. It is found that the n ‐ZGNR‐(m,m)SWCNT is a ferromagnetic semiconductor with intrinsic spin in which the manipulation of spin‐polarized currents can be achieved just simply by applying a gate voltage. Moreover, compared to n ‐ZGNR, the n‐ ZGNR‐(m,m)SWCNT possesses enhanced local magnetic moment. Semiconductors with intrinsic spin represent a new direction in the exploration of materials for spintronics and good prospects for practical spintronic applications. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

First-principle prediction of robust half-metallic Te-based half-Heusler alloys

Te-based half-Heusler systems are studied by first-principle calculations to search for alloys with stable half-metallic properties. We found that CoMnTe and FeMnTe are the most robust half-metallic (HM) ferromagnetic alloys among the 90 studied alloys, with HM gaps of 0.42 and 0.61eV, respectively, larger than that of any Heusler or half-Heusler alloys reported in the literature. The half-metallicity of CoMnTe and FeMnTe is found to be robust under large in-plane strains, which makes them suitable for practical spintronic device applications.

First principle study on the electronic and magnetic properties in Zn0.75Cr0.25M (M = S, Se, Te) semiconductors

Using the first-principle method, the structural, electronic and magnetic properties of Zn0.75Cr0.25M (M=S, Se, Te) have been investigated. The calculated formation energy and cohesive energy of Zn0.75Cr0.25M are negative and the absolute values of them decrease with the increase of atomic radius as well as the electronegativity of the anions. Zn0.75Cr0.25M display half-metallic characteristics with energy band gaps of 1.06, 0.65 and 0.83eV, respectively. The conventional gaps for minority spin are 2.91, 2.16 and 1.75eV for Zn0.75Cr0.25S, Zn0.75Cr0.25Se and Zn0.75Cr0.25Te respectively, which decrease with the increase of anion radius successively, similar to that observed in the binary compounds of ZnM. For all the three materials, exchange splitting energies Δx(pd) due to the effect of Cr 3d state are negative, implying that the effective potential is more attractive for minority spin than that for majority spin, and the absolute value of Δx(pd) decreases with the increase of atomic radius of anions. The calculated positive values of N0α suggest interactions between Zn-s and Cr-3d states are repulsive, while the negative values of N0β show that p–d interactions at the valence band are more attractive. All these conclusions demonstrate that Zn0.75Cr0.25M are promising candidates for practical applications in spintronics.

First-principles study on electronic and magnetic properties of Cu-doped CdS

Based on the full-potential linearized augmented plane wave (FLAPW) method, the electronic structures and magnetic properties in Cu-doped CdS diluted magnetic semiconductors (DMSs) have been investigated. The results indicate that Cu-doped CdS systems show half-metallic character with a total magnetic moment of 1.0 μ B per supercell. In the case of two Cu atoms substituting for Cd atoms, the long-range ferromagnetism is observed, which results from Cu(3d)–S(3p)–Cd–S(3p)–Cu(3d) coupling chain. The estimated Curie temperature of Cu-doped CdS is predicted to be 400 K, higher than room temperature. These results suggest that Cu-doped CdS may be a promising half-metallic ferromagnetic material for practical applications in electronics and spintronics.

Cu and Co codoping effects on room-temperature ferromagnetism of (Co,Cu):ZnO dilute magnetic semiconductors

Homogeneous distribution of magnetic dopants in semiconductor hosts is highly desired for practical applications in spintronics. Herein, we show that codoping 2 at. % Cu with Zn0.98Co0.02O could change the magnetic behavior from paramagnetism to room temperature ferromagnetism. For Zn0.96Cu0.02Co0.02O nanocrystals prepared by a modified solid-state reaction method, the combination of x-ray absorption fine structure spectra at Co, Cu, and O K-edge reveals that the Cu and Co ions are substitutionally incorporated into the ZnO matrix and distribute randomly in the host. First-principles calculations further indicate strong hybridization between Co 3d states and Cu-induced donor impurity bands at the Fermi level, which effectively enhances the indirect ferromagnetic superexchange of Co ions and is responsible for the occurrence of ferromagnetism in (Co,Cu)-codoped ZnO.

Tunnel magnetoresistance in textured Co2FeAl/MgO/CoFe magnetic tunnel junctions on a Si/SiO2 amorphous substrate

Magnetic tunnel junctions with B2-ordered Co2FeAl full Heusler alloy as a ferromagnetic electrode were fabricated by sputtering on thermally oxidized Si/SiO2 amorphous substrates. A Co2FeAl/MgO/Co50Fe50 structure showed a highly (001)-textured structure and the tunneling magnetoresistance (TMR) ratio of 166% at room temperature and 252% at 48 K were achieved. The temperature dependence of TMR can be fitted with spin wave excitation model, and the bias voltage dependence of differential conductance demonstrated that the high TMR was mainly contributed by coherent tunneling. This work suggests the B2-Co2FeAl is one of the promising candidates for practical spintronic applications.

Spin-dependent transport in a Rashba ring connected to noncollinear ferromagnetic leads

We analyze spin-dependent transport through a quantum ring coupled to two ferromagnetic leads, whose magnetic moments lie in a common plane and form an arbitrary angle with respect to each other. The Rashba spin-orbit (RSO) interaction existed in the ring arms is taken into consideration. We calculate the linear conductance in terms of the Green’s functions method based on the equation of motion technique. It is found that due to the quantum interference effect arising from the RSO-induced spin precession phase factor, the conductance is greatly suppressed when the Fermi energy is aligned to the on-site energies of the ring, where the spin polarization and the tunnel magnetoresistance (TMR) have their maximums. The conductance, spin polarization, and the TMR are monotonously tuned by the relative angle of the leads’ magnetization directions, which shows the typical spin-valve effect. We pay special attention on the situation when one magnetic lead is polarized along z axis while the other one is pointing at x direction. The peak value of the TMR is suppressed now and can become either positive or negative when the on-site energies of the two ring arms are different from each other. This device is realizable with current technology and may practical applications in spintronics.

Epitaxial growth of Fe/MgO/Ge(0 0 1) heterostructures

We report on the growth of epitaxial Fe/MgO heterostructures on Ge(0 0 1) by Molecular Beam Epitaxy. The better crystal quality and interfacial chemical sharpness at the oxide–semiconductor interface have been obtained by growing MgO at room temperature, followed by a post-annealing at 773 K, on top of a p(2 × 1)-Ge(0 0 1) clean surface. The growth of Fe at room temperature followed by annealing at 473 K gives the best epitaxial structure with optimized crystallinity of each layer compatible with limited chemical interdiffusion. Tunneling devices based on the epitaxial Fe/MgO/Ge heterostructure have been micro-fabricated and tested in order to probe the electrical properties of the MgO barrier. The current–voltage characteristics clearly show that tunneling is the dominant phenomenon, thus indicating that this system is very promising for practical applications in electronics and spintronics.

Spin-injection device prospects for half-metallic Fe3O4:Al0.1Ga0.9As interfaces

Electrical spin-injection across the Fe3O4:Al0.1Ga0.9As interface has been measured. We quantify this effect in an In0.2Ga0.8As:GaAs spin-light emitting diode optical device. The optical polarization signal is maintained from 4.2 up to 200 K without influence of the metal–insulator Verwey transition in the bulk of the Fe3O4 film. An incomplete oxidation at the interface may be detrimental for this device, as it has a similar spin-injection efficiency to that of Fe:Al0.1Ga0.9As. Ambient temperature operation of this device may be possible although the present polarization levels remain too low for practical spintronic applications. We demonstrate the first step in the integration of molecular beam epitaxy-grown magnetic oxides into III–V semiconductor devices.

Tunable spin-diode with a quantum dot coupled to leads

Spin-dependent electronic transport through a quantum dot coupled to one ferromagnetic lead and one normal metal lead is investigated by using the master equation approach. Both the intradot spin-flip transition and Coulomb interaction are studied for the current polarization p = (I⇑ – I↓) / (I⇑ + I↓). It is found that p is suppressed to zero for a particular regime of one direction bias, while it is enhanced to a relative maximum value when the bias is reversed, which is called the spin-current diode effect. The bias regime of this effect is determined by the dot level position and the intradot Coulomb interaction strength. We give a physical explanation and several control methods for it. This device is realizable with current nanofabrication technology and should have practical applications in spintronics.

Local structures and concentration dependence of magnetic properties in Cr- and Mn-doped amorphous silicon ferromagnetic thin films

Local environments surrounding magnetic ions in Mn- or Cr-doped and hydrogenated amorphous Si films have been determined using extended x-ray absorption fine structure (EXAFS) method. These films are found to be ferromagnetic with high Curie temperatures above 250 K and therefore are of tremendous interest for practical spintronics applications. Our EXAFS results indicate that these material systems are nearly free of dopant clusters and oxides up to an unusually high dopant concentration of $20--22\text{ }\text{at}\text{.}\text{ }\mathrm{%}$. As the dopant concentration increases, the saturation magnetization in the Cr-doped samples is decreased while that in the Mn-doped sample remains practically unchanged. Antiferromagnetic coupling of magnetic ions and enhancement of carrier-mediated ferromagnetism are proposed to account for the different concentration dependence of magnetization in the Cr- and Mn-doped samples.

Zero-gap materials for future spintronics, electronics and optics

Electrons carry both charge and spin. The processing of information in conventional electronic devices is based only on the charge of the electrons. Spin electronics, or spintronics, uses the spin of electrons, as well as their charge, to process information. Metals, semiconductors and insulators are the basic materials that constitute the components of electronic devices, and these have been transforming all aspects of society for over a century. In contrast, magnetic metals, half-metals, magnetic semiconductors, dilute magnetic semiconductors and magnetic insulators are the materials that will form the basis for spintronic devices. Materials having a zero-energy band gap are a special class of these materials that exhibit some fascinating and superior electronic properties compared to materials with a non-zero energy gap. This article reviews a range of materials with zero-gap band structures, focusing on materials with quadratic and linear symmetrical dispersions, disorder-induced linear dispersions, asymmetrical linear dispersions and topological insulating states. These materials all have intriguing physical properties and numerous potential practical applications in spintronics, electronics, optics and sensors.

Controlling spin-dependent transport via inhomogeneous magnetic flux in double-dot Aharonov–Bohm interferometer

We study the spin-dependent electron transport through parallel coupled quantum dots (QDs) embedded in an Aharonov–Bohm (AB) interferometer connected asymmetrically to leads. Both the Rashba spin–orbit interaction (RSOI) inside one of the QDs, which acquires a spin-dependent phase factor in the tunnel-coupling strengths when the electrons flow through this arm of the AB ring, and an inhomogeneous magnetic flux penetrating the structure are taken into account. Due to the existence of the RSOI induced phase factor, magnetic flux and the interdot coupling, a spin-dependent Fano effect will arise. We pay special attention on the properties of the local density of states and the conductance when the electron phase factor is close to integer multiplies of a quantum of flux. It is shown that the roles and lifetimes of the bonding and antibonding states of the two spin components are very sensitive to the phase factor and can be well controlled accordingly. This manipulation of the spin degree of freedom relies on the existence of RSOI but can be fulfilled even when its strength is very weak. The proposed structure can be easily realized with present technology and might be of practical applications in spintronics devices and quantum computing.

Spins go their own way

A semiconductor device that integrates electron spin injection, transport, modulation and detection in a single structure provides an important step in versatility for both fundamental research and practical spintronic applications.

Room temperature ferromagnetism in III–V and II–IV–V 2 dilute magnetic semiconductors

A new class of ferromagnets—dilute magnetic semiconductors (DMS) based on III–V materials (e.g. In 1 - x Mn x As or Ga 1 - x Mn x As )—which are both semiconducting and ferromagnetic (FM) at low temperature, have attracted a great deal of attention (as working media for spintronics) following the discovery of ferromagnetism in Ga 1 - x Mn x As . However, practical spintronics applications of these DMS are severely limited by the fact that they are FM only at low temperature (below about 150 K). Here we present the results of our studies in III–V (InSb) and II–IV–V 2 (CdGeAs 2 and CdGeP 2) materials doped with Mn which exhibit robust ferromagnetism at room temperature. μ SR experiments show that this ferromagnetism is a bulk phenomenon.