Potential For Spintronic Devices Research Articles

First principles study of electronic structures of Cd0.9375Co0.0625X (X = S, Se, Te) for magnetic, optical and thermoelectric device applications

The present article investigates the structural and electronic properties of Co doped (xCo = 6.25%) CdS/Se/Te diluted magnetic semiconductors to understand the optical and thermoelectric characteristics. The electronic properties, computed by applying generalized gradient approximation (GGA) and then modified Becke Johnson (mBJ) functional, are contrasted to identify the appropriate electronic parameters. The stable ferromagnetic states have been justified to arise due to the p-d hybridization that has been found responsible in inducing magnetic moments at the interstitial and at the non-magnetic sites. The computed direct band gap and the exchange constants (N0α and N0β) have suggested, respectively, the potential optical and spintronic device applications. The studied compounds operate within visible-ultraviolet energy range. The thermoelectric response improves with temperature, while deteriorated due to Co doping. The studied compounds exhibiting various significant physical properties evidence the potential consumption in various technologically important spintronic, optoelectronic and thermoelectric devices.

Spin depolarization dynamics of WSe2 bilayer**Project supported by the National Natural Science Foundation of China (Grant No. 11474276) and the Strategic Priority Research Program of the Chinese Academy of Sciences (Grant No. XDPB0603).

In this work, the spin dynamics of a centrosymmetric WSe2 bilayer has been investigated by the two-color time-resolved Kerr rotation together with helicity-resolved transient reflectance techniques. Two depolarization processes associated with the direct transition are discovered at a low temperature of 10 K, with the characteristic decaying time of ∼3.8 ps and ∼20 ps, respectively. The short decay time of ∼3.8 ps is suggested to be the exciton spin lifetime of the WSe2 bilayer, which is limited by the short exciton lifetime of the WSe2 bilayer and the rapid intervalley electron–hole exchange interaction between K+ and K− valley in the same layer as that of monolayer. The long decay time of ∼20 ps is suggested to be the spin lifetime of photo-excited electrons, whose spin relaxation is governed by the rapid intervalley scattering from the K valley to the global minimum Σ valley and the subsequent interlayer charge transfer in WSe2 bilayer. Our experimental results prove the existence of the spin-polarized excitons and carriers even in centrosymmetric transition metal dichalcogenides (TMDCs) bilayers, suggesting their potential valleytronic and spintronic device applications.

DFT calculations of strain and interface effects on electronic structures and magnetic properties of L1-FePt/Ag heterojunction of GMR applications

In this work, density functional theory (DFT) was employed to investigate the effect of strain and interface on electronic structures and magnetic properties of L10-FePt/Ag heterojunction. Two possible interface structures of L10-FePt(001)/Ag(001), that is, interface between Fe and Ag layers (Fe/Ag) and between Pt and Ag layers (Pt/Ag), were inspected. It was found that Pt/Ag interface is more stable than Fe/Ag interface due to its lower formation energy. Further, under the lattice mismatch induced tensile strain, the enhancement of magnetism for both Fe/Ag and Pt/Ag interface structures has been found to have progressed, though the magnetic moments of “interfacial” Fe and Pt atoms have been found to have decreased. To explain this further, the local density of states (LDOS) analysis suggests that interaction between Fe (Pt) and Ag near Fe/Ag (Pt/Ag) interface leads to spin symmetry breaking of the Ag atom and hence induces magnetism magnitude. In contrast, the magnetic moments of interfacial Fe and Pt atoms reduce because of the increase in the electronic states near the Fermi level of the minority-spin electrons. In addition, the significant enhancements of the LDOS near the Fermi levels of the minority-spin electrons signify the boosting of the transport properties of the minority-spin electrons and hence the spin-dependent electron transport at this ferromagnet/metal interface. From this work, it is expected that this clarification of the interfacial magnetism may inspire new innovation on how to improve spin-dependent electron transport for enhancing the giant magnetoresistance (GMR) ratio of potential GMR-based spintronic devices.

Room-Temperature Ferromagnetism and Extraordinary Hall Effect in Nanostructured Q-Carbon: Implications for Potential Spintronic Devices

We report extraordinary Hall effect and room-temperature ferromagnetism in undoped Q-carbon, which is formed by nanosecond pulsed laser melting and subsequent quenching process. Through detailed structure–property correlations in Q-carbon thin films, we show the excess amount of unpaired electrons near the Fermi energy level give rise to interesting magnetic and electrical properties. The analysis of the extraordinary Hall effect in Q-carbon follows nonclassical “side-jump” electronic scattering mechanism. The isothermal field-dependent magnetization plots confirm room-temperature ferromagnetism in Q-carbon with a finite coercivity at 300 K and a Curie temperature of 570 K, obtained by the extrapolation of the fits to experimental data using modified Bloch’s law. High-resolution scanning electron microscopy and transmission electron microscopy clearly illustrate the formation of Q-carbon and its subsequent conversion to single-crystalline diamond. Further, we found n-type conductivity in Q-carbon in the e...

Structural stability and magnetic-exchange coupling in Mn-doped monolayer/bilayer MoS2.

Ferromagnetic (FM) two-dimensional (2D) transition metal dichalcogenides (TMDs) have potential applications in modern electronics and spintronics and doping of TMDs with transition metals can enhance the magnetic characteristics. In this work, the structural stability, electronic states, and magnetic properties of Mn-doped monolayer/bilayer MoS2 are studied systematically by first-principles calculations. Substitutional Mn dopants at the Mo sites are energetically favorable in both monolayer and bilayer MoS2 under the S-rich condition which is common in the synthesis of MoS2 nanosheets. Two Mn dopants participate in the FM interaction in monolayer MoS2 and magnetic coupling of two Mn dopants via the double-exchange mechanism can be mediated by the nearest neighboring S. Magnetic coupling can be ascribed to the competition between the double-exchange, direct-exchange, and super-exchange interactions, which take place between two Mn dopants in bilayer MoS2 with the MniMnMo, MniMnS and MnMo-MnMo configurations. Our results reveal the geometrical dependence of magnetic-exchange coupling suggesting that Mn-doped monolayer/bilayer MoS2 has large potential in spintronic devices.

STRUCTURAL, SURFACE MORPHOLOGICAL AND MAGNETIC STUDIES OF Zn1−xFexS (x=0.00–0.10) DILUTED MAGNETIC SEMICONDUCTORS GROWN BY CO-PRECIPITATION METHOD

We have fabricated Zn[Formula: see text]FexS ([Formula: see text], 0.02, 0.04, 0.06, 0.08 and 0.10) diluted magnetic semiconductors using co-precipitation method. X-ray diffraction patterns depict that Zn[Formula: see text]FexS appears as a dominant phase with cubic zinc blende structure and nanoscale crystallite size. In addition, a secondary phase of rhombohedral ZnS also appears; however, no additional phase arises that primarily belongs to Fe dopant. Using Debye–Scherrer relation, the crystallite size is found to be in the range of 20–27[Formula: see text]nm, which is in good agreement with the crystallite size calculated using the Williamson–Hall (WH) plot method. The appearance of secondary phase provoked to study the residual strain using Stokes–Wilson equation, which is nearly consistent to that observed using WH plot method. The surface morphology, revealed using scanning electron microscopy, depicts non-uniform surface structure with a variety of grains and void dimensions. Hysteresis loops measured for Zn[Formula: see text]FexS at room temperature (RT) illustrate a paramagnetic behavior at higher fields; however, small ferromagnetic behavior is evident due to the small openings of the measured hysteresis loops around the origin. The measured RT ferromagnetism reveals the potential spintronic device applications of the studied diluted magnetic semiconductors.

Half-metallic compensated ferrimagnetism with a tunable compensation point over a wide temperature range in the Mn-Fe-V-Al Heusler system

The cubic Heusler compound Mn1.5FeV0.5Al with the L21 Heusler structure is the first fully compensated half-metallic ferrimagnet with 24 valence electrons. The ferrimagnetic state can be tuned by changing the composition such that the compensation point appears at finite temperatures ranging from 0 K up to 226 K, while retaining half-metallicity in the system. In this paper, the structural, magnetic and transport properties of the Mn-Fe-V-Al system are discussed. Magnetic reversal and a change of sign of the anomalous Hall effect were observed at the compensation point, which gives rise to a sublattice spin-crossing. These materials present new possibilities for potential spintronic devices because of their advantageous properties such as imperceptibility to external fields, lower power consumption and ultrafast switching in the THz region.

Lattice constant changes leading to significant changes of the spin-gapless features and physical nature in a inverse Heusler compound Zr2MnGa

The spin-gapless semiconductors with parabolic energy dispersions [1–3] have been recently proposed as a new class of materials for potential applications in spintronic devices. In this work, according to the Slater-Pauling rule, we report the fully-compensated ferrimagnetic (FCF) behavior and spin-gapless semiconducting (SGS) properties for a new inverse Heusler compound Zr2MnGa by means of the plane-wave pseudo-potential method based on density functional theory. With the help of GGA-PBE, the electronic structures and the magnetism of Zr2MnGa compound at its equilibrium and strained lattice constants are systematically studied. The calculated results show that the Zr2MnGa is a new SGS at its equilibrium lattice constant: there is an energy gap between the conduction and valence bands for both the majority and minority electrons, while there is no gap between the majority electrons in the valence band and the minority electrons in the conduction band. Remarkably, not only a diverse physical nature transition, but also different types of spin-gapless features can be observed with the change of the lattice constants. Our calculated results of Zr2MnGa compound indicate that this material has great application potential in spintronic devices.

Observation of Quantum Hall effect in an ultra-thin (Bi0.53Sb0.47)2Te3 film

We report the observation of the Quantum Hall effect from the topological surface states in both the Dirac electron and Dirac hole regions in a 4 quintuple layer (Bi0.53Sb0.47)2Te3 film grown on GaAs (111)B substrates. The Fermi level is sitting within the enlarged bulk band gap due to the quantum confinement of the ultra-thin film and can be tuned through the Dirac point by gate biases. Furthermore, the Hall resistance Rxy shows even denominator plateaus, which could be fractional Quantum Hall states. This may be due to the hybridization between the top and bottom surface states and suggests the possible way to manipulate the interaction of two surfaces for potential spintronic devices.

Ferromagnetic signature in vanadium doped ZnO thin films grown by pulsed laser deposition

Abstract

Investigation of ferromagnetic semiconducting and opto-electronic properties of Zn1−xMnxS (0 ≤ x ≤ 1) alloys: A DFT-mBJ approach

In this study, we report the mechanical, structural, electronic, magnetic and optical behaviors in Zn1−xMnxS (0 ≤ x ≤ 1), which are determined by employing Wein2K code. The ferromagnetic (FM) state stability of the Zn1−xMnxS alloys has been elucidated from the calculated values of enthalpy of formation. The elastic constant (C11, C12, C14) are calculated to find various useful mechanical parameters, which depend upon Mn concentrations. The calculated electronic band structure and density of states (DOS) have demonstrated that exchange splitting through p-d hybridization, arising due to Mn impurities, stabilize a ferromagnetic state. The exchange splitting of the bands is further elucidated from the sharing of magnetic moment, charge and spin, between the impurity cations and the host lattice anions. Various parameters like direct spin-exchange splitting Δx(d), exchange constants N0α and N0β have also confirmed a stable ferromagnetic state. Various calculated optical parameters have indicated that the studied compounds respond to visible and ultraviolet energies. Moreover, the calculated optical band gap and static dielectric constant ɛ1 (0) verify Penn's model. The studied compounds of Zn1−xMnxS have been shown theoretically that they find potential spintronic and optical device applications.

Inverse spin Hall effect of antiferromagnetic MnIr in exchange biased NiFe/MnIr films

Antiferromagnetic Mn3Ir, which is widely employed in exchange-biased applications, has attracted much attention recently due to its predicted and subsequently observed large spin Hall effect, therefore increasing its potential for spintronic devices in place of conventional paramagnetic 5d spin Hall metals. (Pt, Ta and W) Via the electrical detection of ferromagnetic resonance, we study a series of exchange biased NiFe/MnIr films for various MnIr thicknesses. In these systems, spin-pumped spin currents from NiFe are converted into dc voltages within MnIr via the inverse spin Hall effect (ISHE), which mixes with spin rectification voltages generated from NiFe. Through angular measurements, we separate these different voltage contributions to qualitatively detect non-zero ISHE in MnIr, which coexists with a non-zero unidirectional anisotropy. We find significant extrinsic damping contributions which prevent the accurate quantification of spin pumping-induced ISHE in MnIr films. The results show that spin currents may propagate and dissipate in MnIr films through ISHE in the presence of exchange bias.

Effects of the vacancy and doping on the half-metallicity in La0.5Sr0.5MnO3

The structural, magnetic and electronic properties of the perfect and defective La0.5Sr0.5MnO3 are investigated by performing the first-principle calculation. The negative value of formation energy implies the formation process of N atom substituting O atom in La0.5Sr0.5MnO3 are exothermic, indicating this structure could stably exist. Both the perfect and defective La0.5Sr0.5MnO3 are found to be half-metallic ferromagnets and potential in spintronic devices. For La0.5Sr0.5MnO3, the gap width of 2.902eV is extremely wide. While the gap width decrease with existing the defect and the gap width decreases according to the order of the perfect, VO and DN. There are different mechanisms determining the band gaps of the perfect and defective La0.5Sr0.5MnO3.

Magnetic, elastic and optical properties of zinc peroxide (ZnO2): First principles study

Using first principles method we elaborately discuss the magnetic, elastic and optical properties of pure, Zn and O vacant ZnO2. It is found that the electronic structure and band gap of ZnO2 is not sensitive to the active on-site Coulomb interaction term Ud, but found to be depending on the term Up. The role of orbitals subject to the correlation is thus completely opposite for the case of ZnO2 in respect of ZnO. Interestingly, the Zn vacancy converts ZnO2 as “d0 magnet”. Indeed, our analysis show that, Zn vacancy transmuted O22− state into O2δ+2− state, indicating the partially filled π∗ states are the governing reason for the d0 magnetism. Both HSE06 and PBE0 functional confirm the same. The similar phenomena has been observed for other peroxide materials XO2 (X=Mg, Ca, Sr, Ba) studied here. Our results suggest that this class of materials can be studied further to exploit its potential in spintronic devices. Further the elastic properties have been estimated for pure ZnO2 at different pressures and for Zn and O vacant ZnO2 to know the stability of the system. Zn vacancy in ZnO2 also tunes optical properties, indicating its potential application in other areas.

Oxygen vacancy induced room temperature ferromagnetism in Pr-doped CeO2 thin films on silicon.

Integration of functional oxides on Si substrates could open a pathway to integrate diverse devices on Si-based technology. Oxygen vacancies (Vo(··)) can strongly affect solid state properties of oxides, including the room temperature ferromagnetism (RTFM) in diluted magnetic oxides. Here, we report a systematical study on the RTFM of oxygen vacancy engineered (by Pr(3+) doping) CeO2 epitaxial thin films on Si substrates. High quality, mixed single crystalline Ce1-xPrxO2-δ (x = 0-1) solid solution films were obtained. The Ce ions in CeO2 with a fluorite structure show a Ce(4+)-dominant valence state in all films. The local crystal structures of the films were analyzed in detail. Pr doping creates both Vo(··) and PrO8-complex defects in CeO2 and their relative concentrations vary with the Pr-doping level. The RTFM properties of the films reveal a strong dependence on the relative Vo(··) concentration. The RTFM in the films initially increases with higher Pr-doping levels due to the increase of the F(+) center (Vo(··) with one occupied electron) concentration and completely disappears when x > 0.2, where the magnetic polaron concentration is considered to decline below the percolation threshold, thus long-range FM order can no longer be established. We thus demonstrate the possibility to directly grow RTFM Pr-doped CeO2 films on Si substrates, which can be an interesting candidate for potential magneto-optic or spintronic device applications.

Skyrmions in Multiferroic Insulator

Magnetic skyrmion is a topologically stable particle-like object, which appears as nanometer-scale vortex-like spin texture in a chiral-lattice magnet [1]. In metallic materials (MnSi, FeGe, Fe1-xCoxSi etc), electrons moving through skyrmion spin texture gain a nontrivial quantum Berry phase, which provides topological force to the underlying spin texture and enables the current-induced manipulation of magnetic skyrmion [2]. Such electric controllability, in addition to the particle-like nature, is a promising advantage for potential spintronic device applications. Recently, we newly discovered that skyrmions appear also in an insulating chiral-lattice magnet Cu2OSeO3 [3]. We find that the skyrmions in insulator can magnetically induce electric polarization through the relativistic spin-orbit interaction, which implies possible manipulation of the skyrmion by external electric field without loss of joule heating. The present finding of multiferroic skyrmion may pave a new route toward the engineering of novel magnetoelectric devices with high energy efficiency. In this talk, our recent attempts to drive skyrmions by external field are also introduced.

Morphology and Curie temperature engineering in crystalline La0.7Sr0.3MnO3 films on Si by pulsed laser deposition

Of all the colossal magnetoresistant manganites, La0.7Sr0.3MnO3 (LSMO) exhibits magnetic and electronic state transitions above room temperature, and therefore holds immense technological potential in spintronic devices and hybrid heterojunctions. As the first step towards this goal, it needs to be integrated with silicon via a well-defined process that provides morphology and phase control, along with reproducibility. This work demonstrates the development of pulsed laser deposition (PLD) process parameter regimes for dense and columnar morphology LSMO films directly on Si. These regimes are postulated on the foundations of a pressure-distance scaling law and their limits are defined post experimental validation. The laser spot size is seen to play an important role in tandem with the pressure-distance scaling law to provide morphology control during LSMO deposition on lattice-mismatched Si substrate. Additionally, phase stability of the deposited films in these regimes is evaluated through magnetometry measurements and the Curie temperatures obtained are 349 K (for dense morphology) and 355 K (for columnar morphology)—the highest reported for LSMO films on Si so far. X-ray diffraction studies on phase evolution with variation in laser energy density and substrate temperature reveals the emergence of texture. Quantitative limits for all the key PLD process parameters are demonstrated in order enable morphological and structural engineering of LSMO films deposited directly on Si. These results are expected to boost the realization of top-down and bottom-up LSMO device architectures on the Si platform for a variety of applications.

Tunable Surface Electron Spin Splitting with Electric Double-Layer Transistors Based on InN

Electrically manipulating electron spins based on Rashba spin-orbit coupling (SOC) is a key pathway for applications of spintronics and spin-based quantum computation. Two-dimensional electron systems (2DESs) offer a particularly important SOC platform, where spin polarization can be tuned with an electric field perpendicular to the 2DES. Here, by measuring the tunable circular photogalvanic effect (CPGE), we present a room-temperature electric-field-modulated spin splitting of surface electrons on InN epitaxial thin films that is a good candidate to realize spin injection. The surface band bending and resulting CPGE current are successfully modulated by ionic liquid gating within an electric double-layer transistor configuration. The clear gate voltage dependence of CPGE current indicates that the spin splitting of the surface electron accumulation layer is effectively tuned, providing a way to modulate the injected spin polarization in potential spintronic devices.

Magneto-optical Kerr effect studies of Cu2O/nickel heterostructures

Cuprous oxide (Cu2O) is a diamagnetic p-type semiconductor material, considered to be highly attractive for the rapidly emerging field of oxide electronics. In this work Cu2O layers with various thicknesses were produced by atomic layer deposition to form heterostructures with ferromagnetic layers of Ni as model systems for potential spintronic devices. The magnetic properties of the heterostructures were investigated by magneto-optical Kerr effect spectroscopy and magnetometry.

Structural, optical, and magnetic studies of manganese-doped zinc oxide hierarchical microspheres by self-assembly of nanoparticles

In this study, a series of manganese [Mn]-doped zinc oxide [ZnO] hierarchical microspheres [HMSs] are prepared by hydrothermal method only using zinc acetate and manganese acetate as precursors and ethylene glycol as solvent. X-ray diffraction indicates that all of the as-obtained samples including the highest Mn (7 mol%) in the crystal lattice of ZnO have a pure phase (hexagonal wurtzite structure). A broad Raman spectrum from as-synthesized doping samples ranges from 500 to 600 cm-1, revealing the successful doping of paramagnetic Mn2+ ions in the host ZnO. Optical absorption analysis of the samples exhibits a blueshift in the absorption band edge with increasing dopant concentration, and corresponding photoluminescence spectra show that Mn doping suppresses both near-band edge UV emission and defect-related blue emission. In particular, magnetic measurements confirm robust room-temperature ferromagnetic behavior with a high Curie temperature exceeding 400 K, signifying that the as-formed Mn-doped ZnO HMSs will have immense potential in spintronic devices and spin-based electronic technologies.