Potential Applications In Spintronics Research Articles

Graphene Lattices with Embedded Transition-Metal Atoms and Tunable Magnetic Anisotropy Energy: Implications for Spintronic Devices

Doping of the graphene lattice with transition metal atoms resulting in high magnetic anisotropy energy (MAE) is an important goal of materials research owing to its potential application in spintronics. In this article, by using spin-polarized density functional theory including spin-orbit coupling, we examined magnetic properties of graphene with vacancy defects, both bare and nitrogen-decorated, and doped by Cr, Mn and Fe transition metal single atom (TM-SA) and two different TM atoms simultaneously. [...] The computational findings are supplemented by an atomic-resolution characterization of an incidental Mn impurity bonded to four carbon atoms, whose localized spin matches expectations as measured using core-level electron energy-loss spectroscopy. Conducting TM-doped graphene with robust magnetic features offers prospects for the design of graphene-based spintronic devices.

Development of a half-metallic ferrimagnetic state in Lu[formula omitted]NiIrO[formula omitted] via hole-doping

Ferrimagnetic (FiM) materials have attracted much attention owing to their dynamic magnetization capacity, which has potential applications in spintronics. Herein, electronic and magnetic properties of Lu2NiIrO6 double perovskite oxide are investigated under the effect of Ca-doping at Lu site using ab-initio calculations. It is established that the undoped system is FiM Mott-insulator with an energy band gap of 0.20 eV, which is attributed to a strong antiferromagnetic (AFM) interactions between high energy half-filled Ni+23d8 (eg2↑ eg0↓) and low energy partially filled Ir+45d5 (t2g3↑ t2g2↓) ions. The structural and dynamical stability of the doped system is confirmed by computing the formation energy and phonon dispersion bands, respectively. The analysis of magnetic ordering by comparing FiM, ferromagnetic, and AFM energies of the Ca-doped Lu2NiIrO6 system reveals that FiM ordering is the ground state alike the undoped case. A half-metallic behavior is obtained for Ca-doped system, because the admixture of Ir 5d orbitals in the spin majority channel is primarily responsible for conductivity. Moreover, a substantial enhancement in the magnetic moment on one of the Ir ions is also observed due to the reduction of Ir-O hybridization.

Exchange interactions in the 1T-VSe2 monolayer and their modulation via electron doping using alkali metal adsorption and the electride substrate.

The modulation of exchange interactions in layered magnets has fundamental research value and potential applications in spintronics. Based on first-principles calculations, the exchange interactions in the experimentally controversial room-temperature ferromagnetic 1T-VSe2 monolayer are systematically studied. It is found that three shells of nearest-neighbor Heisenberg exchange interactions and higher-order interactions are crucial for an accurate description of the magnetism and its thermal stability in the 1T-VSe2 monolayer. Based on our understanding of tuning the magnetic interactions and the magnetic ground state in the 1T-VSe2 monolayer via external factors, two modulation methods, involving adsorption of the alkali metal lithium and the electride Ca2N substrate, are proposed. In both Li-VSe2 and VSe2/Ca2N systems, the strongly frustrated Heisenberg exchange interaction competes with the Dzyaloshinskii-Moriya interaction and magnetocrystalline anisotropy, leading to complex magnetic ground states, such as antiferromagnetic spin spiral and periodic antiferromagnetic cycloidal states. Moreover, the higher-order exchange interactions play a crucial role in the stabilization of long-range double-row-wise antiferromagnetic states in Li-VSe2 and VSe2/Ca2N. These results highlight the effective manipulation of exchange interactions in two-dimensional magnets.

Investigations Into the Role of Native Defects on Photovoltaic and Spintronic Properties in Copper Oxide

CuO is a promising contender for photovoltaics, photodetection, photocatalysis, and spintronics in theory, but experimental success in terms of device performance has been limited. We used experimental and theoretical techniques to examine the fascinating optoelectronic and spintronic features of a p-type semiconductor; i.e. copper oxide. Absorption spectra of CuO have revealed intriguing properties such as defect-induced strong absorption in the visible and near-infrared (NIR) regions, making it an attractive candidate for NIR and broadband detection. Additionally, due to its antiferromagnetic characteristics, CuO has potential applications in spintronics. Clearly, these applicability ranges are greatly dependent on the intrinsic material qualities and defects. To gain a better understanding of CuO band structures, defect dynamics, charge distribution, and absorption properties; ab-initio calculations were conducted in a systematic manner. Additionally, the stability of several types of defects has been investigated theoretically in Cu and O-rich environments. The literature is ambiguous about the stability of several defects in CuO, including copper vacancies, oxygen vacancies, and interstitials. Interestingly, it is discovered that V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> -V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> di-vacancies and V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">O</sub> are extremely stable in O-deficient environments, whereas V <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Cu</sub> is highly stable in O-rich environments. Numerous defects such as copper vacancies, oxygen vacancies, and di-vacancies all contribute significantly to the photovoltaic features such as quantum-efficiency. Furthermore, unlike pristine CuO, defect assisted CuO has a significant magnetic moment as shown by first-principle calculations, making it a suitable option for spintronics. The work will open several features of CuO for next generation devices.

Above-room Curie temperature and barrier-layer-dependent tunneling magnetoresistance in 1T-CrO2 monolayer based magnetic tunnel junctions.

van der Waals (vdW) heterostructures based on two-dimensional (2D) ferromagnetic materials hold great potential applications in spintronics. Using the density functional theory (DFT) method and first-principles quantum transport simulation, we studied the structures, magnetic properties and spin-resolved transport of 1T-CrO2 monolayer (ML) based vdW magnetic tunnel junctions (MTJs). Owing to a high Curie temperature (TC) of 392 K and a moderate magnetic anisotropy energy (MAE) of 94 μeV of the ferromagnetic 1T-CrO2 monolayer, Cu(111)|CrO2|nML-Gr|CrO2|Cu(111) MTJs were built. Our results reveal that their tunneling magnetoresistance (TMR) ratios are dependent on the number of Gr barrier layers within a working bias voltage of 1 V. For the thin barrier layers (n = 1-2), the maintained TMR ratios can reach a giant value of about 1 × 104%, while there appears a decreasing trend with the increasing bias voltage for thick Gr layers (n = 3-5). The barrier-layer-dependent phenomenon is attributed to the decreasing transmission magnitude with increasing bias voltage in a parallel configuration (PC), which is as small as that in an anti-parallel configuration (APC) eventually. Our results would provide some guidance for future experimental fabrications of these 2D materials based MTJs.

Spin engineering of triangulenes and application for nano nonlinear optical materials design.

The recently synthesized triangulenes with non-bonding edge states could have broad potential applications in magnetics, spintronics and electro-optics if they have appropriate electronic structure modulation. In the present work, strategies based on molecular orbital theory through heteroatom doping are proposed to redistribute, reduce or eliminate the spin of triangulenes for novel functional materials design, and the role of B, N, NBN, and BNB in such intended electronic structure manipulation is scrutinized. π-Extended triangulenes with tunable electronic properties could be potential nonlinear optical (NLO) materials with appropriate inhibition of their polyradical nature. The elimination of spin is achieved by B, N, NBN, and BNB doping with the intended geometric arrangement for enhanced polarity. Intended doping of BNB results in an optimal structure with large static first hyperpolarizability (〈β0〉) as well as strong Hyper-Rayleigh scattering (HRS) βHRS(-2ω; ω, ω) (ω = 1064.0 nm), TG7-BNB-ba with a large 〈β0〉 (18.85 × 10-30 esu per heavy atom) and βHRS (1.15 × 10-28 esu per heavy atom) much larger than that of a synthesized triangular molecule (1.12 × 10-30 esu of 〈β0〉 per heavy atom and 5.04 × 10-30 esu of βHRS per heavy atom). The strong second order NLO responses in the near-infrared and visible regions, particularly the strong sum frequency generation, make these B or (and) N doped triangulenes promising candidates for the fabrication of novel carbon-based optoelectronic devices and micro-NLO devices.

Monolayer gadolinium halides, GdX2 (X = F, Cl, Br): intrinsic ferrovalley materials with spontaneous spin and valley polarizations.

Two-dimensional (2D) intrinsic ferrovalley semiconductors provide unprecedented opportunities to investigate valley physics as well as providing promising device applications due to their exceptional combination of spontaneous spin and valley polarizations. Here, we have predicted from first-principles calculations and Monte Carlo simulations that monolayers (MLs) GdX2 are such extremely rare excellent materials. Apart from their robust stabilities energetically, dynamically, thermally, and mechanically, these 2D materials are found to be semiconducting intrinsic ferromagnets where the magnetic coupling is ascribed to 5d-electron-mediated 4f-4f exchange interactions. Moreover, MLs GdX2 (X = F, Cl, Br) not only exhibit significant magnetic anisotropy energy of 351, 268, and 30 μeV per Gd, but also have a high Curie temperature of 300, 245, and 225 K, respectively. In particular, spontaneous valley polarization in three systems occurs due to the cooperative interplay between the spin-orbit coupling and magnetic exchange interactions, whose magnitude is as sizable as 55, 38, and 82 meV for MLs GdF2, GdCl2, and GdBr2, respectively. Under the action of an in-plane longitudinal electrical field, the valley-contrasting Berry curvatures arising from the broken space-inversion and time-reversal symmetries in MLs GdX2 could yield opposite transverse velocities of the carriers, giving rise to the occurrence of a spin-polarized anomalous valley Hall effect. Overall, these findings render 2D GdX2 a class of promising candidate materials for experimental studies and practical spintronics and valleytronics applications.

Two-dimensional magnetic transition metal halides: molecular beam epitaxy growth and physical property modulation

The spontaneous magnetization of two-dimensional (2D) magnetic materials can be maintained down to the monolayer limit, providing an ideal platform for understanding and manipulating magnetic-related properties on a 2D scale, and making it important for potential applications in optoelectronics and spintronics. Transition metal halides (TMHs) are suitable 2D magnetic candidates due to partially filled d orbitals and weak interlayer van der Waals interactions. As a sophisticated thin film growth technique, molecular beam epitaxy (MBE) can precisely tune the growth of 2D magnetic materials reaching the monolayer limit. Moreover, combining with the advanced experimental techniques such as scanning tunneling microscopy, the physical properties of 2D magnetic materials can be characterized and manipulated on an atomic scale. Herein, we introduce the crystalline and magnetic structures of 2D magnetic TMHs, and show the 2D magnetic TMHs grown by MBE and their electronic and magnetic characterizations. Then, the MBE-based methods for tuning the physical property of 2D magnetic TMHs, including tuning interlayer stacking, defect engineering, and constructing heterostructures, are discussed. Finally, the future development opportunities and challenges in the field of the research of 2D magnetic TMHs are summarized and prospected.

The Density Functional Theory Account of Interplaying Long-Range Exchange and Dispersion Effects in Supramolecular Assemblies of Aromatic Hydrocarbons with Spin

Aromatic hydrocarbons with fused benzene rings and regular triangular shapes, called n-triangulenes according to the number of rings on one edge, form groundstates with n-1 unpaired spins because of topological reasons. Here, we focus on methodological aspects emerging from the density functional theory (DFT) treatments of dimer models of the n = 2 triangulene (called also phenalenyl), observing that it poses interesting new problems to the issue of long-range corrections. Namely, the interaction comprises simultaneous spincoupling and van der Waals effects, i.e., a technical conjuncture not considered explicitly in the benchmarks calibrating long-range corrections for the DFT account of supramolecular systems. The academic side of considering dimer models for calculations and related analysis is well mirrored in experimental aspects, and synthetic literature revealed many compounds consisting of stacked phenalenyl cores, with intriguing properties, assignable to their long-range spin coupling. Thus, one may speculate that a thorough study assessing the performance of state-of-the-art DFT procedures has relevance for potential applications in spintronics based on organic compounds.

Observation of room temperature d0 ferromagnetism, band-gap widening, zero dielectric loss and conductivity enhancement in Mg doped TiO2 (rutile + anatase) compounds for spintronics applications

Recently, non-magnetic element doped d0 ferromagnetic oxides have been widely studied for their potential spintronics applications. Crystal Structure, elemental color mapping, magnetic, electrical and optical properties of Ti1-xMgxO2 (x ​= ​0, 0.03, 0.06, 0.09 and 0.12) compounds, synthesized by solid-state route were systematically investigated in this paper. X-ray diffraction patterns of these compounds reveal that all Mg-doped TiO2 have been formed in mixed phase of rutile (predominant) and anatase. The anatase phase increases with the increase of the Mg doping concentration. The elemental color mapping using energy dispersive spectroscopy indicates that all elements such as Ti and Mg are distributed uniformly throughout the sample. Ferromagnetism with hysteresis loops is seen in all the compounds, with Curie temperatures above 400 ​K and coercivity values in the range 380–790 ​Oe. The maximum value of saturation magnetization was exhibited by the 6% doped sample. Zero field cooled and field cooled magnetization curves show a bifurcation, which pertains to magnetic irreversibility. The optical band gap of these compounds widens up to 9% of Mg doping and thereafter narrows on further doping. Moreover, these materials show zero loss tangent and an enhancement of dielectric constant with Mg doping. These materials may be useful in magnetic storage, spintronics and optoelectronics applications at high frequency.

Charged vacancy defects in monolayer phosphorene

Two-dimensional semiconductor phosphorene has attracted extensive research interests for potential applications in optoelectronics, spintronics, catalysis, sensors, and energy conversion. To harness phosphorene's potential requires a better understanding of how intrinsic defects control carrier concentration, character, and mobility. Using density functional theory and a charge correction scheme to account for the appropriate boundary conditions, we conduct a comprehensive study of the effect of structure on the formation energy, electronic structure, and charge transition level of the charged vacancy point defects in phosphorene. We predict that the neutral vacancy exhibits a 9-5 ring structure with a formation energy of 1.7 eV and transitions to a negatively charged state at a Fermi level 1.04 eV above the valence band maximum. The corresponding optical charge transitions display sizable Frank-Condon shifts with a large Stokes shift of 0.3 eV. Phosphorene vacancies should become negatively charged in $n$-doped phosphorene, which would passivate the dopants and reduce the charge carrier concentration and mobility.

2D growth of high-quality magnetic CuCr2Se4 crystals with a high Curie temperature

Recently, two-dimensional (2D) magnetic materials have attracted broad attention due to their fascinating magnetic behavior and potential application in spintronics. However, most of exfoliated magnetic materials usually show random thickness and size, and the stability in the air is still a challenge. Hence, the controllable synthesis of 2D magnetic materials with good quality should be addressed. Here we report the controllable growth of 2D CuCr2Se4 crystals on mica substrates with large domain size. Most importantly, the as-grown CuCr2Se4 crystals demonstrate obvious ferromagnetic properties, showing a high Curie temperature of up to 436 K. Our discovery of 2D room-temperature ferromagnet CuCr2Se4 provides paves the way for the epitaxial growth of many more 2D magnets and provides a new platform for the spintronic applications.

Geometric wavefront dislocations of RKKY interaction in graphene

Magnetically doped graphene has great potential for applications in spintronics, in which local magnetic moments are coupled by the Rudermann-Kittel-Kasuya-Yosida (RKKY) interaction. RKKY interaction in graphene has attracted intensive interest. However, previous studies only show the features of the RKKY interaction dictated by energy dispersion, leaving the effect of the geometric nature of electronic structures unexplored. Here, we focus on the short-range oscillation behaviors of RKKY interaction contributed by intervalley scattering. Due to the unique geometric nature of electronic structures, two wavefront dislocations emerge in the RKKY interaction corresponding to winding number 1 of graphene. We further demonstrate the robustness of wavefront dislocations against the doping, trigonal warping, and gap opening of energy bands. This study reveals a unique feature for the RKKY interaction and implies the importance of the geometric nature of electronic structures for magnetism.

Visualizing band selective enhancement of quasiparticle lifetime in a metallic ferromagnet

Electrons navigate more easily in a background of ordered magnetic moments than around randomly oriented ones. This fundamental quantum mechanical principle is due to their Bloch wave nature and also underlies ballistic electronic motion in a perfect crystal. As a result, a paramagnetic metal that develops ferromagnetic order often experiences a sharp drop in the resistivity. Despite the universality of this phenomenon, a direct observation of the impact of ferromagnetic order on the electronic quasiparticles in a magnetic metal is still lacking. Here we demonstrate that quasiparticles experience a significant enhancement of their lifetime in the ferromagnetic state of the low-density magnetic semimetal EuCd2As2, but this occurs only in selected bands and specific energy ranges. This is a direct consequence of the magnetically induced band splitting and the multi-orbital nature of the material. Our detailed study allows to disentangle different electronic scattering mechanisms due to non-magnetic disorder and magnon exchange. Such high momentum and energy dependence quasiparticle lifetime enhancement can lead to spin selective transport and potential spintronic applications.

Antiferromagnetic topological crystalline insulator and mixed Weyl semimetal in two-dimensional NpAs monolayer

Magnetic topological states have attracted significant attentions due to their intriguing quantum phenomena and potential applications in topological spintronic devices. Here, we propose a two-dimensional material NpAs monolayer as a candidate for multiple topological states accompanied with the changes of magnetic structures. Under the antiferromagnetic configuration, the long-awaited topological crystalline insulator (TCI) emerges with a nonzero mirror Chern number and a giant band gap of 630 meV, and remarkably a pair of gapless edge states can be tailored by rotating the magnetization directions while the TCI phase survives. Moreover, we establish the existence of quantum anomalous Hall effect and nontrivial nodal points under the ferromagnetic configuration, thereby giving rise to the mixed Weyl semimetal after adding the magnetization direction to topological classification. Our findings not only provide an ideal candidate for uncovering exotic topological characters with magnetism but also put forward potential applications in topological spintronics.

Exotic Spintronic Properties of Transition‐Metal Monolayers on Graphyne

AbstractThe recent discovery of 2D magnetic materials, which are compounds of transition metals (TM) with other elements, has opened new avenues for basic research on low‐dimensional magnetism and potential applications in spintronics. To further explore new 2D magnets of pure TM is an interesting topic. Based on the first‐principles calculations, here a strategy of obtaining monolayer TM magnets, i.e., depositing TM atoms on graphyne (Gy), which has proper hexagonal hollow geometry, is proposed. It is found that a TM monolayer with perfect hexagonal geometry can be formed on Gy. The TM monolayer exhibits a wealth of physical properties in dependence of TM species, such as ferromagnetic and anti‐ferromagnetic ground states, as well as intriguing semimetal and half‐metal characteristics. It is also found that the half‐metal characteristics make the monolayer TM have great potential applications in horizontal magnetic tunnel junction (MTJ) devices, where the tunneling magnetoresistance can reach as high as 850 000%. These results provide a new framework for obtaining 2D magnets with outstanding spintronic properties.

Switchable valley polarization and quantum anomalous Hall state in the VN2X2Y2 nanosheets (X = group-III and Y = group-VI elements)

Motivated by the recent synthesis of MA2Z4 family materials, we perform a first-principles investigation on the structural stability and electronic properties of VN2X2Y2 nanosheets (X = B-Ga, Y = O-Te), which are the isostructural and isoelectronic analogues of the VSi2N4 system. We find that among the 16 possible III–VI combinations, there are four XY compositions, i.e., XY = BO, BS, AlO, and GaO, that can form stable VN2X2Y2 nanosheets with robust dynamic and thermal stabilities. Intrinsic ferromagnetism appears in these VN2X2Y2 nanosheets, among which the VN2B2S2 and VN2Ga2O2 ones possess an easy out-of-plane magnetization. Thus, unlike the VSi2N4 system, the VN2B2S2 and VN2Ga2O2 nanosheets exhibit spontaneous valley polarization with a sizeable polarization value in the bottom conduction band. Through the strain modulation, the valley polarization can be switched from the bottom conduction band to the top valence band, which is closely related to the strain-induced band inversion. Due to the existence of valley polarization, the band inversion occurs at different strains for the two valleys. Therefore, in the critical strain region, an intriguing quantum anomalous Hall state is present in the strained VN2X2Y2 nanosheets, which is characterized by a nonzero Chern number of C = 1. Such nontrivial topology is further confirmed by a quantized Hall conductance and a single gapless edge state in the bulk gap. Our study demonstrates as the cousin of MA2Z4 materials, the III–VI VN2X2Y2 nanosheets possess robust structural stability, peculiar electronic and topological properties, which have potential applications in electronics, spintronics, and valleytronics.

Inducing Half-Metallicity in Monolayer MoSi2N4.

First-principles calculations are performed for the recently synthesized monolayer MoSi2N4 [Science 369, 670–674 (2020)]. We show that N vacancies are energetically favorable over Si vacancies, except for Fermi energies close to the conduction band edge in the N-rich environment, and induce half-metallicity. N and Si vacancies generate magnetic moments of 1.0 and 2.0 μB, respectively, with potential applications in spintronics. We also demonstrate that N and Si vacancies can be used to effectively engineer the work function.

Front Cover: Lifting the Spin‐Momentum Locking in Ultra‐Thin Topological Insulator Films (Adv. Quantum Technol. 11/2021)

In article number 2100083, Arthur Leis, Bert Voigtländer and co-workers demonstrate nanoscale four-probe measurements on atomically thin topological insulator films. For this purpose, four individual scanning tunneling microscope tips are directly contacting individual terraces on the sample surface to quantify the thickness-dependent film conductivity. This enables the authors to detect a breakdown of spin-momentum locking in the topological surface state as a function of the film thickness. In the few-layer limit, this effect can realize helical edge modes along step edges with potential applications in spintronics and quantum computing.

Electronic and magnetic properties of one‐dimensional sandwich transition metal‐anthracene molecular wires

AbstractOrganometallic sandwich complexes have been attracting interest for their potential applications in electronics and spintronics. Here, we systematically studied the structures, electronic and magnetic properties of one‐dimensional (1D) transition metal (TM)‐anthracene (Ant) sandwich molecular wires (SMWs), [TM2Ant]∞, and [TM3Ant]∞ (TM = Ti, V, Cr, Mn), based on density functional theory calculations. Our results show that all the 1D SMWs display normal sandwich configurations with their binding energies closely related to the choice of TM atoms. Excepting 1D [Mn2Ant]∞ and [Fe3Ant]∞ favoring antiferromagnetic ordering, most 1D [TM2Ant]∞ and [TM3Ant]∞ SMWs display robust ferromagnetic (FM) features. Particularly, 1D [Cr3Ant]∞ SMW is revealed to be FM half‐metal (HM), and 1D [Cr2Ant]∞, [Fe2Ant]∞ and [Mn3Ant]∞ SMWs are FM quasi HMs. Of which, high magnetic transform temperature up to 310 and 220 K are identified for 1D [V2Ant]∞ and [Fe2Ant]∞, respectively. Further spintransport calculations demonstrated that 1D Cr2‐Ant and Cr3‐Ant molecule lines are good spintransport devices. Our findings shed light on the properties of 1D Ant based SMWs and propose a new way to design potential electronic and spintronic devices.