Nonmagnetic Transition Metal Research Articles

Spin and orbital Hall effect in nonmagnetic transition metals: Extrinsic versus intrinsic contributions

Spin and orbital Hall effect in nonmagnetic transition metals: Extrinsic versus intrinsic contributions

Optical properties of rutile TiO[formula omitted] with Zr, Mo, Zn, Cd impurities

To explore quasiparticle (QP) energy gaps and photoabsorption spectra of rutile TiO2 with nonmagnetic transition metal (Zr, Mo, Zn, Cd) impurities, we conducted a Γ-point only GW ＋ Bethe–Salpeter equation (BSE) calculation on a 72 (or 71) atom supercell. Our findings reveal that Zn and Cd impurities must coexist, at least partly, with oxygen vacancies to maintain charge neutrality. Among the systems considered, Mo, Zn, or Cd doped rutile TiO2 may exhibit optical absorption and catalytic activity under visible light. The resulting QP energy gaps (ΔɛQP) and photoabsorption energies (PAEs) are fairly in good agreement with both experimental and theoretical data currently available. The necessary conditions for the applicability of the Γ-point only approach in the GW ＋ BSE framework were found to be: (1) The Γ-point only GW calculation should reproduce a reasonable band gap. (2) The “superficial” exciton binding energy (the diagonal element of Wvc;vc−2Xvc;vc between v= VBM and c= CBM, where W and X are the direct and exchange terms of the BSE matrix elements, respectively) must be positive or marginally negative. (3) The “real” exciton binding energy (ΔɛQP− the lowest PAE) should be positive, even if it is exceptionally small.

Observation of a V-Shape Superconductivity Evolution on Tungsten-Intercalated 2H-Type Niobium Diselenide.

van der Waals-layered niobium diselenide (NbSe2) intercalated by d-electron transition metals is an ideal test bed for the exploration of their diversiform evolution of ground states. These intercalations are mostly viewed as ordered structures aligned with periodicities of their host materials that enable control of the electronic phases via gradually changing of intercalation ratios. Here, we present the structure and superconductivity in tungsten (W)-intercalated 2H-NbSe2 crystals, which reveals an order to disorder distribution of W atoms with increasing confined intercalating amounts, leading to an approximate V-shape suppression of superconductivity. Aided by density functional theory calculations, we demonstrate that the local magnetic moment around W intercalants induced by the charge redistribution gives rise to the quick superconductivity suppression in 2H-NbSe2 below a certain dilute amount (W% = 0.06). Simultaneously, W intercalants also induce structural aberration due to aggregation effects and inhibit the generation of an ordered structure in 2H-NbSe2, resulting in a recovery of its superconductivity. The alteration of structure and electronic phases in 2H-NbSe2 via intercalation of nonmagnetic transition metals in the van der Waals gap enables the exploration of combined magnetic quantum criticality, superconductivity, and other related electronic correlations.

Magnetic properties of binary alloys Ni[formula omitted]Mo[formula omitted] and Ni[formula omitted]Cu[formula omitted] close to critical concentrations

The search for the ferromagnetic quantum critical point (FM QCP) has always been a captivating research topic in the scientific community. In pursuit of this goal, we introduced nonmagnetic transition metals to alloy with elemental nickel, and studied the magnetic properties of nickel binary alloys Ni1−xMox and Ni1−yCuy as a function of x and y up to the critical concentrations xcr and ycr at which the FM transition TC disappears. TC−x(y) phase diagrams were constructed via the Arrott–Noakes scaling of magnetization data. An enhanced Sommerfeld coefficient (the value of C/T as T→0) is observed near xcr and ycr, manifesting the effect of quantum fluctuations. However, the spin glass behavior is identified through the ac magnetic susceptibility measurements. This observation rules out the possibility of the existence of the FM QCP in both systems.

Valley polarization in transition metal dichalcogenide layered semiconductors: Generation, relaxation, manipulation and transport

In recent years, valleytronics researches based on 2D semiconducting transition metal dichalcogenides have attracted considerable attention. On the one hand, strong spin–orbit interaction allows the presence of spin–valley coupling in this system, which provides spin addressable valley degrees of freedom for information storage and processing. On the other hand, large exciton binding energy up to hundreds of meV enables excitons to be stable carriers of valley information. Valley polarization, marked by an imbalanced exciton population in two inequivalent valleys (+K and −K), is the core of valleytronics as it can be utilized to store binary information. Motivated by the potential applications, we present a thorough overview of the recent advancements in the generation, relaxation, manipulation, and transport of the valley polarization in nonmagnetic transition metal dichalcogenide layered semiconductors. We also discuss the development of valleytronic devices and future challenges in this field.

Anomalous valley Hall effect induced by mirror symmetry breaking in transition metal dichalcogenides

The control of the valley degree of freedom in Bloch electrons has opened up new avenues for information processing. The synthesis of ferrovalley materials, however, has been limited due to the stringent requirements for breaking both time and space inversion symmetries. To address this challenge, we propose a Janus method for inducing valley polarization in nonmagnetic transition metal dichalcogenides. Our study shows that the magnetic moment in monolayer TiTeI arises from the breaking of mirror symmetry and the presence of unpaired electrons. The Stoner criterion ${I}_{ex}N({E}_{F})>1$ confirms that the band splitting originates from the $d$ orbital of titanium. Moreover, the exchange interaction combined with spin-orbit coupling opens up the valley splitting. The anomalous valley Hall effect (AVHE) can be realized by applying an in-plane electric field, as a result of the valley-contrasting Berry curvature. The modulation of valley-selective circular dichroism and AVHE by optical, electronic, and magnetic fields make TiTeI a promising material for future studies and applications in valleytronics.

Emergent magnetic-nonmagnetic transition in strain-tuned zigzag-edged PtSe2 nanoribbon

Magnetic responses in nonmagnetic transition metal dichalcogenides (TMDs) via reduced dimension and defect engineering continue to bring forth emergent properties and has revolutionized important technologies such as data storage and magnetic sensing. Using the first-principles calculation, we investigate the zigzag-edged ${\mathrm{PtSe}}_{2}$ (zz-${\mathrm{PtSe}}_{2}$) nanoribbon with a sizable magnetization solely at the zigzag edges and a remarkable magnetic-nonmagnetic switching induced by a small compressive uniaxial strain of about \ensuremath{-}2%. We further interpret the sudden disappearance of the magnetization by a simple but clear model simulation considering the localized quasi-one-dimensional magnetic channels mediated by metallic backgrounds along the edges. Such newly proposed strain-controlled magnetic transition through band structure engineering highlights zz-${\mathrm{PtSe}}_{2}$ as a unique candidate for controllable TMD-based one-dimensional magnets with promising advances in fundamental physics and technological opportunities in spintronics, computing, and sensors.

Accurate and Efficient Approximate Quasiparticle DFT‐1/2 Band Structure Calculations of Transition Metal Oxide Perovskites

The transition metal oxide perovskites are materials intensively studied due to their diversity of chemical and physical properties, as well as their potential for wide applications. From the theoretical point of view of ab initio calculations of quasiparticle (QP) energies and band structures, they remain a challenge. Herein, the performance of the low computational cost ab initio DFT‐1/2 approximate QP method is assessed for the nonmagnetic transition metal oxide perovskites, including 3d , , 4d , and 5d and . The results with experiments and a large set with other QP calculations are compared. It is demonstrated that the DFT‐1/2 method presents a remarkable accuracy for these materials, opening an avenue to study more complex systems.

Biaxial strain induced multiple magnetic phase transitions in nonmagnetic transition metal halide monolayer

Tunable multiple magnetic phases from antiferromagnetic, ferromagnetic to half metallic states are introduced by applying biaxial strains on nonmagnetic ScI2 monolayer.

Spin\u2013orbit torque engineering in \u03b2-W/CoFeB heterostructures with W\u2013Ta or W\u2013V alloy layers between \u03b2-W and CoFeB

The spin–orbit torque (SOT) resulting from a spin current generated in a nonmagnetic transition metal layer offers a promising magnetization switching mechanism for spintronic devices. To fully exploit this mechanism, in practice, materials with high SOT efficiencies are indispensable. Moreover, new materials need to be compatible with semiconductor processing. This study introduces W–Ta and W–V alloy layers between nonmagnetic β-W and ferromagnetic CoFeB layers in β-W/CoFeB/MgO/Ta heterostructures. We carry out first-principles band structure calculations for W–Ta and W–V alloy structures to estimate the spin Hall conductivity. While the predicted spin Hall conductivity values of W–Ta alloys decrease monotonically from −0.82 × 103 S/cm for W100 at% as the Ta concentration increases, those of W–V alloys increase to −1.98 × 103 S/cm for W75V25 at% and then gradually decrease. Subsequently, we measure the spin Hall conductivities of both alloys. Experimentally, when β-W is alloyed with 20 at% V, the absolute value of the spin Hall conductivity considerably increases by 36% compared to that of the pristine β-W. We confirm that the W–V alloy also improves the SOT switching efficiency by approximately 40% compared to that of pristine β-W. This study demonstrates a new material that can act as a spin current-generating layer, leading to energy-efficient spintronic devices.

Influence of tantalum concentrations on the magnetic properties of amorphous CoFeTaB thin-films

The structural and magnetic properties of amorphous CoFeTaB thin-films were studied as a function of tantalum concentration. The investigations of the structural properties of amorphous CoFeTaB thin-films were undertaken to confirm layer thickness, interface roughness, and their amorphous structure. Temperature dependent magnetic characterizations were performed in order to extract Curie temperatures and their saturation magnetizations of each structure. Curie temperatures and saturation magnetizations of amorphous CoFeTaB thin-films show similar behaviour, where both quantities decrease with increasing tantalum concentration in thin-film structures. These reductions can be attributed to a result of a transfer of electrons from the metalloid atoms to the unfilled d holes of the 3d transition metals, and diffusion of tantalum atoms towards the ferromagnetic layer during thin-film deposition. Investigation of tuning both Curie temperature and magnetization by the addition of a non-magnetic transition metal is crucial for the development of spintronic applications.

Effect of Transition Metal Ion Doping on the Microstructure, Defect Evolution, and Magnetic and Magnetocaloric Properties of CuFeO2 Ceramics

The polycrystalline CuFeO2 (CFO) and CuFe1-xMxO2 (M = Ti, Hf, Zr, M-CFO) samples are synthesized by solid-state reaction method. The doping effect of tetravalent nonmagnetic transition metal M4+ ions on the microstructure, defect evolution, and magnetic and magnetocaloric properties of the CFO system are comparatively investigated. The substitution of M4+ for Fe3+ increases lattice parameters, changes the bond length of Fe/Cu-O and bond angle of Fe-O-Fe, and improves the microstructure to some extent. Positron annihilation spectroscopy results indicate that M4+ doping induces the agglomeration of small-sized vacancy defects manifested as the increase in vacancy-type defect size and the suppression of inherent defect concentration. Magnetic measurements show that the antiferromagnetic stability of CFO system is clearly affected by M4+ doping, and Ti4+ doping has a more noticeable impact than Hf4+ and Zr4+ doping. Meanwhile, the coexistence of weak ferromagnetism and antiferromagnetism is detected in all doped samples, while Ti-doped system exhibits extraordinary magnetic properties. Especially for Ti-CFO2 (x = 0.03) sample, the maximum magnetization reaches as high as 11.81 emu/g at 0.5 T, which enhanced one order of magnitude than that of undoped CFO. Isothermal M-H data at different temperatures show that entropy change (ΔSM) and refrigerant capacity (RC) for CFO and M-CFO2 samples are significantly weakening with M4+ substitution in bulk CFO. The maximum ΔSM = 4.79 J·kg−1 K−1 and RC = 12.79 J·kg−1 for undoped CFO are obtained near the transition temperature TN2 = 12 K, with the applied fields up to 6 T. The possible reasons for the above observations are discussed in detail.

Room temperature ferromagnetism in carbon doped MoO3 for spintronic applications: A DFT study

Diluted magnetic semiconductors (DMS) find applications in spintronic devices due to their excellent magnetic and magneto-transport properties. Designing room temperature DMS, using non-magnetic transition metal oxides and non-magnetic impurities (i.e., having s, p electrons), is a major challenge for researchers. Using state-of-the-art density functional theory (DFT) simulations, we have predicted for the first time, room temperature ferromagnetism (FM) in C – doped MoO3 (O substitution defect). Single C impurity in non-magnetic MoO3, which corresponds to an impurity concentration of 1.04 at.%, induces a magnetic moment of 2.0 μB per impurity with FM coupling large enough to sustain room temperature FM. DFT predictions using the generalized gradient approximation (GGA) exchange correlation functional were verified by a hybrid functional, viz., the Heyd-Scuseria-Ernzerhof (HSE06) functional. The system is stable at room temperature and the computed Curie temperature (TC) is ~565 K. From the relative formation energy it can be inferred that O substituted defect dominates over the interstitial defect; and, C doped MoO3 can indeed be synthesized with O substituted defect in experiments. The induced FM in this hole doped system is mainly contributed by C and O 2p orbitals and we have established that the Stoner criterion is satisfied to switch on FM by introducing spin polarized defect band at the top of the valence band. Stability of the system, practical feasibility to synthesize C doped MoO3 and persistence of FM at room temperature signify that C doped MoO3 can be tailored as room temperature ferromagnets for spintronic applications.

Long-range coupling interaction between a non-magnetic transition metal capping layer and a neighboring magnetic layer

In nanoscale magnetic multilayers, capping layers are often used to protect the underlying magnetic layers from oxidation. However, little research has investigated possible long-range coupling interactions between nonmagnetic transition metal (TM) capping layers and neighboring magnetic layers. In this paper, the temperature (T) dependence of the magnetic moment of different thicknesses of cobalt (Co) was studied in a tantalum (Ta)/Co/TM trilayer structure with four TM capping layers, where the TMs were Ta, Chromium (Cr), titanium (Ti), and zirconium (Zr), respectively. It was found that the capping layer had a large effect on the phase-transition behavior and thermal stability of the Co layer. In the Ta and Cr layers, the T-dependence of Co magnetic moment showed nonmonotonic behavior, and in the Ti and Zr layers, the Co M-T curve exhibited very few effects of the capping layer. We attribute this phenomenon to the long-range coupling between the Co and TM layers. Furthermore, the coupling mechanism was linked to the indirect magnetic exchange coupling in Co/TM multilayers, similar to the Ruderman-Kittel-Kasuya-Yoshida coupling. The results of this work will support further development of the understanding of the coupling between the 3d ferromagnetic (FM) metal and nonmagnetic TM at nanoscales. Relative to potential applications, it will inspire us to rediscover the role of both the TM capping layer and buffer nonmagnetic layer in FM/TM multilayers, especially for nanoscale magnetic multilayers with spin-dependent effects, such as spin valves, spin halls, spin transfer torque, and spin–orbit coupling, which are in widespread use in the manufacture of various spintronics devices.

Magnetic properties of transition metal dichalcogenides-Fe/Ir(111) interfaces from first principles

In our density functional theory study we systematically explored how the interaction between magnetic and nonmagnetic transition metal dichalcogenide (TMD) monolayers and a magnetic Fe/Ir(111) electrode reciprocally modifies their electronic and magnetic properties. From an electronic point of view, all investigated semiconductor TMDs become metallic upon adsorption on the Fe/Ir(111) substrate related to a rather strong TMD-Fe/Ir(111) hybridization. On the other hand, from a magnetic point of view, the TMD-surface hybridization leads to a spin-imbalanced electronic structure of such hybrid systems with system-specific features. Additionally, the calculated exchange coupling constants for the Fe surface atoms evidenced a stronger magnetic interaction upon TMD adsorption as compared to the case of the clean surface. This observation indicates the presence of a magnetic hardening of the exchange coupling constants similarly to the case of molecular systems on magnetic surfaces, which emphasizes the generality of this adsorbate-induced effect on magnetic substrates.

Elastic fields, dipole tensors, and interaction between self-interstitial atom defects in bcc transition metals

Density functional theory (DFT) calculations show that self-interstitial atom (SIA) defects in nonmagnetic body-centered-cubic (bcc) metals adopt strongly anisotropic configurations, elongated in the $\ensuremath{\langle}111\ensuremath{\rangle}$ direction [S. Han et al., Phys. Rev. B 66, 220101 (2002); D. Nguyen-Manh et al., Phys. Rev. B 73, 020101 (2006); P. M. Derlet et al., Phys. Rev. B 76, 054107 (2007); S. L. Dudarev, Annu. Rev. Mater. Res. 43, 35 (2013)]. Elastic distortions, associated with such anisotropic atomic structures, appear similar to distortions around small prismatic dislocation loops, although the extent of this similarity has never been quantified. We derive analytical formulas for the dipole tensors of SIA defects, which show that, in addition to the prismatic dislocation looplike character, the elastic field of a SIA defect also has a significant isotropic dilatation component. Using empirical potentials and DFT calculations, we parametrize dipole tensors of $\ensuremath{\langle}111\ensuremath{\rangle}$ defects for all the nonmagnetic bcc transition metals. This enables a quantitative evaluation of the energy of elastic interaction between the defects, which also shows that in a periodic three-dimensional simple cubic arrangement of crowdions, long-range elastic interactions between a defect and all its images favor a $\ensuremath{\langle}111\ensuremath{\rangle}$ orientation of the defect.

Magnetically Ordered Transition-Metal-Intercalated WSe2.

Introducing magnetic behavior in nonmagnetic transition metal dichalcogenides is essential to broaden their applications in spintronic and nanomagnetic devices. In this article, we investigate the electronic and magnetic properties of transition-metal-intercalated tungsten diselenide (WSe2) using density functional theory. We find that intercalation compounds with composition of T1/4WSe2 (T is an iron-series transition-metal atom) exhibit substantial magnetic moments and pronounced ferromagnetic order for late transition metals. The densities of states of the T atoms and the magnetic moments on the W sites indicate that the moments of the intercalated atoms become more localized with increasing atomic number. A large perpendicular magnetocrystalline anisotropy of about 9 meV per supercell has been found for Fe1/4WSe2. Furthermore, using mean field theory, we estimated high Curie temperatures of 660, 475, and 379 K for Cr, Mn, and Fe, respectively. The predicted magnetic properties suggest that WSe2 may have applications in spin electronics and nanomagnetic devices.

Role of Metal Lattice Expansion and Molecular π-Conjugation for the Magnetic Hardening at Cu-Organics Interfaces.

Magnetic hardening and generation of room-temperature ferromagnetism at the interface between originally nonmagnetic transition metals and π-conjugated organics is understood to be promoted by interplay between interfacial charge transfer and relaxation-induced distortion of the metal lattice. The relative importance of the two contributions for magnetic hardening of the metal remains unquantified. Here, we disentangle their role via density functional theory simulation of several models of interfaces between Cu and polymers of different steric hindrance, π-conjugation, and electron-accepting properties: polyethylene, polyacetylene, polyethylene terephthalate, and polyurethane. In the absence of charge transfer, expansion and compression of the Cu face-centered cubic lattice is computed to lead to magnetic hardening and softening, respectively. Contrary to expectations based on the extent of π-conjugation on the organic and resulting charge transfer, the computed magnetic hardening is largest for Cu interfaced with polyethylene and smallest for the Cu–polyacetylene systems as a result of a differently favorable rehybridization leading to different enhancement of exchange interactions and density of states at the Fermi level. It thus transpires that neither the presence of molecular π-conjugation nor substantial charge transfer may be strictly needed for magnetic hardening of Cu–substrates, widening the range of organics of potential interest for enhancement of emergent magnetism at metal–organic interfaces.

First principles investigation of carbon-screw dislocation interactions in body-centered cubic metals

Using ab initio density functional theory calculations, we investigate the effect of interstitial carbon solutes on screw dislocations in non-magnetic body-centered cubic transition metals from group 5 (V, Nb, Ta) and group 6 (Mo, W). The two groups are found to display different solute–dislocation interaction behaviors. Group 6 shows a core reconstruction similar to that previously reported in Fe(C): the dislocation adopts a hard-core configuration with the carbon atoms at the center of regular trigonal prisms formed by the metal atoms. The solute–dislocation interaction energies are strongly attractive, ranging from −1.3 to −1.9 eV depending on the metal and the carbon–carbon distance. By way of contrast, the configuration of lowest energy in group 5 consists of the dislocation in its easy core and the carbon atom in a fifth nearest neighbor octahedral site. The configuration is attractive, but less than in group 6. We show that this group dependence is consistent with the carbon local environment in the stable stoichiometric carbide structures, namely cubic NaCl-type for group 5 and hexagonal WC-type for group 6: in both cases the carbon atoms are at the center of octahedra and prisms respectively.

Finite-temperature properties of nonmagnetic transition metals: Comparison of the performance of constraint-based semilocal and nonlocal functionals

We assess the performance of nonempirical, truly nonlocal and semi-local functionals with regard to structural and thermal properties of $3d$, $4d$, and $5d$ non-magnetic transition metals. We focus on constraint-based functionals and consider the new consistent-exchange van der Waals density functional version vdW-DF-cx [Phys. Rev. B 89, 035412 (2014)], the semi-local PBE [Phys. Rev. Lett. 77, 3865 (1996)] and PBEsol functionals [Phys. Rev. Lett. 100, 136406 (2008)] as well as the AM05 meta-functional [Phys. Rev. B 72, 085108 (2005)]. Using the quasi-harmonic approximation structural parameters, elastic response, and thermal expansion at finite temperatures are computed and compared to experimental data. We also compute cohesive energies explicitly including zero-point vibrations. It is shown that overall vdW-DF-cx provides an accurate description of thermal properties and retains a level of transferability and accuracy that is comparable to or better than some of the best constraint-based semi-local functionals. Especially, with regard to the cohesive energies the consistent inclusion of spin polarization effects in the atoms turns out to be crucial and it is important to use the rigorous spin-vdW-DF-cx formulation [Phys. Rev. Lett. 115, 136402 (2015)]. This demonstrates that vdW-DF-cx has general-purpose character and can be used to study systems that have both sparse and dense electron distributions.