Nonmagnetic State Research Articles

Length-induced large magnetoresistance in polyacene molecular spin valves

With the help of first-principles method, an unusual length effect in polyacene molecular spin valves is revealed. It is found that beyond a critical length, the magnitude of tunneling magnetoresistance is significantly enhanced from 10% to 2500%. Theoretical analysis demonstrates that such significant increase of magnetoresistance is related to the nonmagnetic-antiferromagnetic phase transition of polyacene molecules, which is determined by the relative magnetization orientation of the ferromagnetic electrodes. The molecular gap shows distinct length dependence with the change of molecular magnetic state, which keeps decreasing with length in the nonmagnetic state while a fine gap is obtained in the antiferromagnetic state. This work indicates a valid way to obtain considerable tunneling magnetoresistance in long molecular spin valves.

Void-Defect Induced Magnetism and Structure Change of Carbon Material-Ⅲ: Hydrocarbon Molecules

Void-defect induced magnetism of graphene molecule was recently reported in our previous paper of this series study. This paper investigated the case of hydrogenated graphene molecule, in chemical term, polycyclic aromatic hydrocarbon (PAH). Molecular infrared spectrum obtained by density functional theory was compared with astronomical observation. Void-defect on PAH caused serious structure change. Typical example of C23H12 had two carbon pentagon rings among hexagon networks. Stable spin state was non-magnetic singlet state. This is contrary to pure carbon case of C23, which show magnetic triplet state. It was discussed that Hydrogen played an important role to diminish magnetism by creating an SP3-bond among SP2-networks. Such a structure change affected molecular vibration and finally to photoemission spectrum in infrared region. The dication-C23H12 showed featured bands at 3.2, 6.3, 7.7, 8.6, 11.2, and 12.7 micrometer. It was surprising that those calculated bands coincided well with astronomically observed bands in many planetary nebulae. To confirm our study, large size molecule of C53H18 was studied. Calculation reproduced again similar astronomical bands. Also, small size molecule of C12H8 showed good coincidence with the spectrum observed for young stars. This paper would be the first report to indicate the specific PAH in space.

Discontinuous quantum and classical magnetic response of the pentakis dodecahedron* *Dedicated to the memory of Professor Marshall Luban.

The pentakis dodecahedron, the dual of the truncated icosahedron, consists of 60 edge-sharing triangles. It has 20 six-fold and 12 five-fold coordinated vertices, with the former forming a dodecahedron, and each of the latter connected to the vertices of one of the 12 pentagons of the dodecahedron. When spins mounted on the vertices of the pentakis dodecahedron interact according to the nearest-neighbor antiferromagnetic Heisenberg model, the two different vertex types necessitate the introduction of two exchange constants. As the relative strength of the two constants is varied the molecule interpolates between the dodecahedron and a molecule consisting only of quadrangles. The competition between the two exchange constants, frustration, and an external magnetic field results in a multitude of ground-state magnetization and susceptibility discontinuities. At the classical level the maximum is ten magnetization and one susceptibility discontinuities when the 12 five-fold vertices interact with the dodecahedron spins with approximately one-half the strength of their interaction. When the two interactions are approximately equal in strength the number of discontinuities is also maximized, with three of the magnetization and eight of the susceptibility. At the full quantum limit, where the magnitude of the spins equals , there can be up to three ground-state magnetization jumps that have the z-component of the total spin changing by ΔS z = 2, even though quantum fluctuations rarely allow discontinuities of the magnetization. The full quantum case also supports a ΔS z = 3 discontinuity. Frustration also results in nonmagnetic states inside the singlet-triplet gap. These results make the pentakis dodecahedron the molecule with the largest number of magnetization and susceptibility discontinuities from the quantum to the classical level, taking its size also into account.

Physical characteristics of barium based cubic perovskites

The full potential linearized augmented plan wave method (FP-LAPW) within density functional theory (DFT) is applied to examine the structural, electronic, magnetic and optical characteristic of barium based perovskitesBaXO3(X = Th, U) in cubic phase. BaThO3 and BaUO3compounds are stable in the non-magnetic and ferromagnetic ground states, respectively. The BaUO3compound showed the half metallic ferromagnetic (HMF) behavior, while BaThO3 revealed the insulating behavior with a band gap (Eg) of 3.16 eV. The spin down region has a Eg of 3.39 eV and spin up channel shows the metallic character for BaUO3 compound. The dielectric function ɛ (ω), extinction coefficient k (ω), refractive index n (ω), reflectivity R (ω), optical absorption α (ω) are also calculated. Results indicate that BaUO3 is useful for spintronics and optoelectronic devices whereas both compounds are suggested to be suitable for optoelectronic application.

Investigation of physical properties and superdislocation characters of Ni3Al under uniaxial tension/compression normal to slip plane

The <110>{111} superdislocations play a key role in the mechanical behaviors of L12 Ni3Al. Through the combination of density functional theory (DFT) calculations and Peierls-Nabarro (P–N) model, the physical properties of Ni3Al and the characters of superdislocations are systematically investigated under different uniaxial tensile/compressive loads normal to {111} slip planes. The influence of applied loads on ideal stress-strain relation, ideal strength, shear modulus along slip direction, anti-phase boundary (APB) energy, γ surface, splitting width of superdislocation (i.e., APB width), core width of super partial dislocations and Peierls stress is thoroughly discussed. The results show that the effects of uniaxial normal tensile/compressive loads on above properties are remarkable for both nonmagnetic (NM) and ferromagnetic (FM) states. The compressive loads along the <111> direction tend to expand the splitting width of superdislocations and the core width of super partial dislocations, which further induce higher Peierls stress and yield strength of Ni3Al, while the tensile loads lead to opposite effects. In addition, it is found that the stability zone of Ni3Al is in the normal applied strain range of −0.14–0.14 rather than −0.18–0.22 as suggested by previous works. For the case free of applied strain (i.e. ε = 0), both the predicted APB width and Peierls stress of the NM state are about 10% percent larger than those of the FM state. Obviously, since the physical properties of Ni3Al and characters of superdislocations are asymmetric under tensile/compressive normal loads, the tension/compression asymmetry of the mechanical behaviors for Ni3Al is inherent.

Metal–insulator transitions of fermionic mixtures with mass imbalance in disordered optical lattice

We study the metal–insulator transitions in the half-filled Anderson–Hubbard model with mass imbalance by the typical medium theory using the equation of motion method as an impurity solver. The nonmagnetic ground state phase diagram of the system with mass imbalance is constructed numerically. In addition to the three phases showed up in the balanced case, the phase diagram of the mass imbalanced case contains a spin-selective localized phase, where one spin component is metallic while the other spin component is insulating. We find that if one increases the mass imbalance the metal region in the phase diagram is reduced, while both Anderson and Mott insulator regions are enlarged.

First-principle study of structural and electronic properties of V2Se

First-principles calculation for anti-fluorite type cubic V2Se compound has been performed to study its structural, electronic and magnetic properties using WIEN2k. We used LDA and GGA as exchange-correlation energy functional for our calculation. For the calculation of ground state electronic properties, we implemented DFT + U method with the Hubbard parameter of 5.5 eV. Geometry optimization for the ferromagnetic, anti-ferromagnetic and non-magnetic states were done and results indicate that the material is more stable in ferromagnetic state. The equilibrium lattice constant and bulk modulus were found for both LDA and GGA exchange correlation energy functional which is in agreement with the experimental lattice constant. Electronic band structure and density of states have been studied. The electronic properties indicate that the material exhibit metallic behavior.

Mechanism of highly sensitive strain response in antiferromagnetic chromium

We studied a possible mechanism for the highly sensitive response of electrical resistivity to strain in metal Cr by means of theoretical calculation and experimental measurement. First-principles calculations based on density functional theory were performed for antiferromagnetic Cr in the spin-density wave (SDW) state. The calculation succeeded to reproduce a significant magnetovolume effect by hydrostatic pressure observed in Cr, and the obtained result revealed that the electronic structure and magnetic properties in the SDW state are sensitive to uniaxial strain. The magnetic moment of Cr changed more than 5% with a strain of 1%. We estimated the gauge factor (GF), which denotes the sensitivity of resistance to strain, from the local density of states. The obtained GF value of Cr in the SDW state was found to be approximately 10, whereas that of Cr in the nonmagnetic state, Fe in the ferromagnetic state, and V in the nonmagnetic state was around 1. This result was consistent with our experimental measurement of the GF of Cr, Fe, and V thin films. We found that the large GF is related to a significant magnetovolume effect in Cr. The volume variation accompanying uniaxial strain influences both the magnetic state and electrical conduction of Cr through sensitive changes of the electronic structure in the SDW state.

Predicted half-metallicity and ferromagnetism in the Fe (III) doped BaZrO3 perovskite: A theoretical insight

The variance in the magnetoelectric behavior of BaZrO3 owing to the doping of Fe (III) on the Zr-site has been studied based on density functional calculations. We can predict the structural parameters, electronic structure, and ferromagnetic properties of BaFexZr1−xO3 precisely deprived of considering any strong correlation effect in the calculations. The equilibrium structural parameters are accounted to be in concurrence with the former theoretical and experimental investigations. It is observed that the structural phase remained semiconducting cubic (Pm-3m) up to 12.5% of Fe content and transformed to a conducting tetragonal phase (P4/mmm) beyond that. Spin-polarized density of states and band-structure calculations have probed that pristine BaZrO3 is a wide-bandgap semiconductor and in a nonmagnetic state. The ground-energy-state of Fe3+ doped BaZrO3 compounds varies from half-metal to metal in a ferromagnetic state. In particular, the present calculations have correctly predicted half-metallicity in BaFexZr1−xO3 when doping concentrations were 12.5% and 25%. The half-metallic gap ΔHM, which is the minimum of the lowest energy level of minority spin conduction bands relative to the Fermi level, and the absolute values of the topmost energy level of minority spin valence bands are estimated to be 0.55 eV in BaFe0.25Zr0.75O3 and 0.44 eV in BaFe0.5Zr0.5O3.

Spintronics with a Weyl point in superconducting nanostructures

We investigate transport in a superconducting nanostructure housing a Weyl point in the spectrum of Andreev bound states. A minimum magnet state is realized in the vicinity of the point. One or more normal-metal leads are tunnel-coupled to the nanostructure. We have shown that this minimum magnetic setup is suitable for realization of all common goals of spintronics: detection of a magnetic state, conversion of electric currents into spin currents, potentially reaching the absolute limit of one spin per charge transferred, detection of spin accumulation in the leads. The peculiarity and possible advantage of the setup is the ability to switch between magnetic and non-magnetic states by tiny changes of the control parameters: superconducting phase differences. We employ this property to demonstrate the feasibility of less common spintronic effects: spin on demand and alternative spin current.

Magnetic Energy Landscape of Dimolybdenum Tetraacetate on a Bulk Insulator Surface

The magnetic states and the magnetic anisotropy barrier of a transition metal molecular complex, dimolybdenum tetraacetate, are investigated via density functional theory (DFT). Calculations are performed in the gas phase and on a calcite (10.4) bulk insulating surface, using the Generalized-Gradient Approximation (GGA)-PBE and the Hubbard-corrected DFT + U and DFT + U + V functionals. The molecular complex (denoted MoMo) contains two central metallic molybdenum atoms, embedded in a square cage of acetate groups. Recently, MoMo was observed to form locally regular networks of immobile molecules on calcite (10.4), at room conditions. As this is the first example of a metal-coordinated molecule strongly anchored to an insulator surface at room temperature, we explore here its magnetic properties with the aim to understand whether the system could be assigned features of a single molecule magnet (SMM) and could represent the basis to realize stable magnetic networks on insulators. After an introductory review on SMMs, we show that, while the uncorrected GGA-PBE functional stabilizes MoMo in a nonmagnetic state, the DFT + U and DFT + U + V approaches stabilize an antiferromagnetic ground state and several meta-stable ferromagnetic and ferrimagnetic states. Importantly, the energy landscape of magnetic states remains almost unaltered on the insulating surface. Finally, via a noncollinear magnetic formalism and a newly introduced algorithm, we calculate the magnetic anisotropy barrier, whose value indicates the stability of the molecule’s magnetic moment.

Multiple Weyl fermions in the noncentrosymmetric semimetal LaAlSi

The noncentrosymmetric $R\mathrm{Al}\mathit{Pn}$ ($R=\mathrm{rare}\phantom{\rule{0.16em}{0ex}}\mathrm{earth}, Pn=\mathrm{Si}$, Ge) family, predicted to host nonmagnetic and magnetic Weyl states, provides an excellent platform for investigating the relation between magnetism and Weyl physics. By using high-field magnetotransport measurements and first principles calculations, we have unveiled herein both type-I and type-II Weyl states in the nonmagnetic LaAlSi. By a careful comparison between experimental results and theoretical calculations, nontrivial Berry phases associated with Shubnikov--de Haas oscillations are ascribed to the electron Fermi pockets related to both types of Weyl points located \ensuremath{\sim}0.1 eV above and exactly on the Fermi level, respectively. Under high magnetic field, signatures of Zeeman splitting are also observed. These results indicate that, in addition to the importance for exploring intriguing physics of multiple Weyl fermions, LaAlSi as a comparison with magnetic Weyl semimetals in the $R\mathrm{Al}\mathit{Pn}$ family would also yield valuable insights into the correlation between magnetism and Weyl physics.

Theoretical Study of Transition Metal Dichalcogenides Compound TiS2 and Their Intercalated Compound CrTiS2 Using Density Functional Theory

The Density Functional Theory (DFT) based computational study is carried out for the transition metal dichalcogenides (TMDCs) compound TiS2 and their intercalated 3d transition metal compound CrTiS2. It is carried through Generalized Gradient Approximation (GGA) through Quantum ESPRESSO environment employing Perdew-Burke-Ernzerhof (PBE) exchange and correlation effect with an ultra-soft pseudopotential. In the present work, the structural optimization and electronic properties like energy band structure, density of states (DOS), partial or projected density of states (PDOS), total density of states (TDOS), Fermi surfaces and charge density are reported. The effect of charge transfer from guest 3d transition metal Cr-atom to self-intercalated compound TiS2 has been observed. While, the energy band structure of CrTiS2 compound is computed in the non-magnetic state. From the energy band structure of said materials, we conclude that the TiS2 compound has an indirect narrow band gap though the CrTiS2 compound has an overlapped band structure. The TiS2 shows a semiconductor or semi-metallic nature while doped compound with guest Cr-atom has a metallic material..

Europium(III), Terbium(III), and Gadolinium(III) Oxamato-Based Coordination Polymers: Visible Luminescence and Slow Magnetic Relaxation.

The reaction of aqueous solutions of EuIII, TbIII, and GdIII ions with Na2Hpcpa [H3pcpa = N-(4-carboxyphenyl)oxamic acid] afforded three new isostructural oxamate-containing lanthanide(III) coordination polymers of general formula {LnIII2(Hpcpa)3(H2O)5·H2O}n [Ln = Eu (1),Tb (2), and Gd(3)]. Their structure is made up of neutral zigzag chains running parallel to the [101] direction where double syn-syn carboxylate(oxamate)-bridged dilanthanide(III) pairs (Ln1 and Ln2) are linked by three Hpcpa2- ligands, one of them with the μ-κ2O,O':κO″ coordination mode and the other two with the μ3-κ2O,O':κO″:κO'''. Additionally, two of those chains are interlinked through hydrogen bonding and π-π type interactions, resulting in a porous structure with channels where water molecules are hosted. The emission properties of 1 and 2 are evaluated as a function of the temperature, exhibiting an emission in red and green, respectively. The external quantum yield for 2 is approximately 7 times that obtained for 1, indicating that the oxamate ligand is a better sensitizer for TbIII ions. The temperature dependence of the dc magnetic properties of 1-3 reveals a different magnetic behavior depending on the nature of the LnIII ion. A continuous decrease of χMT occurs for 1 upon cooling, and finally χMT tends to vanish, as expected for the thermal depopulation of the six magnetic 7FJ excited states (J = 1-6) of the EuIII ion with a nonmagnetic 7F0 ground state. χMT for 2 decreases sharply with decreasing the temperature due to the depopulation of the splitted mJ levels of the 7F7 ground state of the magnetically anisotropic TbIII ion. A very weak antiferromagnetic interaction between the magnetically isotropic GdIII ions across the double carboxylate(oxamate) bridge is responsible for the small decrease of χMT at low temperatures for 3. The dynamic (ac) magnetic properties of 2 and 3 reveal a slow magnetic relaxation with very incipient frequency-dependent χM″ signals below 6.0 K (2) and frequency-dependent χM″ peaks below 10.0 K (3) under nonzero applied dc magnetic fields, being thus new examples of field-induced single molecule magnets (SMMs).

CoN4C2 : Two-dimensional cobalt carbonitride with a flat-band feature

Based on the first-principles simulations within the framework of density functional theory, we predict that ${\mathrm{CoN}}_{4}{\mathrm{C}}_{2}$ is a single-atom-thick two-dimensional material, which is composed of ${\mathrm{CoN}}_{4}$ unit and ${\mathrm{C}}_{2}$ dimer. The cohesive energy and phonon dispersion calculations of ${\mathrm{CoN}}_{4}{\mathrm{C}}_{2}$ monolayer, as well as molecular dynamics simulation, have been carried out and its stability is confirmed. The electronic states near the Fermi energy are dominated by Co $3d$ orbital states and the $2p$ orbitals of C and N atoms make a small contribution. The unusual electronic feature of ${\mathrm{CoN}}_{4}{\mathrm{C}}_{2}$ is the flat band state close to the Fermi energy, which is derived from Co ${\mathrm{d}}_{{z}^{2}}$ partial orbital. With the tensile strain increasing, a phase transition from nonmagnetic state to magnetic state is induced. At the biaxial tensile strain of 8.6%, the flat band is pushed to the Fermi energy. Moreover, the effect of electron correlation correction on the electronic structure of ${\mathrm{CoN}}_{4}{\mathrm{C}}_{2}$ is also discussed by the SCAN meta-GGA method and $\mathrm{GGA}+U$ method. Our results demonstrate that tensile strain is another knob to realize the flat band state at the Fermi level, similar to the twist angle in twisted bilayer graphene reported recently.

Thermally driven state in a spin-1 model with competing interactions

We study a recently proposed spin-1 model with competing antiferromagnetic first-neighbor interaction and a third-neighbor coupling mediated by nonmagnetic states, which reproduces topological features of the phase diagrams of high-T_{c} superconductors [S. A. Cannas and D. A. Stariolo, Phys. Rev. E 99, 042137 (2019)2470-004510.1103/PhysRevE.99.042137]. We employ a cluster mean-field approach to investigate effects of crystal field anisotropy on the phase transitions hosted by this model. At low temperatures, the temperature-crystal field phase diagram exhibits superantiferromagnetic (SAF), antiferromagnetic (AF), and paramagnetic (PM) phases. In addition, we found a thermally driven state between SAF and PM phases. This thermally driven state and the SAF phase appears in the phase diagram as a domelike structure. Our calculations indicate that only second-order phase transitions occur in the PM-AF phase boundary, as suggested by previous Monte Carlo simulations.

Imprinting Tunable π-Magnetism in Graphene Nanoribbons via Edge Extensions.

Magnetic carbon nanostructures are currently under scrutiny for a wide spectrum of applications. Here, we theoretically investigate armchair graphene nanoribbons patterned with asymmetric edge extensions consisting of laterally fused naphtho groups, as recently fabricated via on-surface synthesis. We show that an individual edge extension acts as a spin- center and develops a sizable spin-polarization of the conductance around the band edges. The Heisenberg exchange coupling between a pair of edge extensions is dictated by the position of the second naphtho group in the carbon backbone, thus enabling ferromagnetic, antiferromagnetic, or nonmagnetic states. The periodic arrangement of edge extensions yields full spin-polarization at the band extrema, and the accompanying ferromagnetic ground state can be driven into nonmagnetic or antiferromagnetic phases through external stimuli. Overall, our work reveals a precise tunability of the π-magnetism in graphene nanoribbons induced by naphtho groups, thereby establishing these one-dimensional architectures as suitable platforms for logic spintronics.

Chemical Control of Spin-Orbit Coupling and Charge Transfer in Vacancy-Ordered Ruthenium(IV) Halide Perovskites.

Vacancy-ordered double perovskites are attracting significant attention due to their chemical diversity and interesting optoelectronic properties. With a view to understanding both the optical and magnetic properties of these compounds, two series of RuIV halides are presented; A2 RuCl6 and A2 RuBr6 , where A is K, NH4 , Rb or Cs. We show that the optical properties and spin-orbit coupling (SOC) behavior can be tuned through changing the A cation and the halide. Within a series, the energy of the ligand-to-metal charge transfer increases as the unit cell expands with the larger A cation, and the band gaps are higher for the respective chlorides than for the bromides. The magnetic moments of the systems are temperature dependent due to a non-magnetic ground state with Jeff =0 caused by SOC. Ru-X covalency, and consequently, the delocalization of metal d-electrons, result in systematic trends of the SOC constants due to variations in the A cation and the halide anion.

Half metallicity in Cr substituted Fe2TiSn

Band structure tailoring has been a great avenue to achieve the half-metallic electronic ground state in materials. Applying this approach to the full Heusler alloy Fe2TiSn, Cr is introduced systematically at Ti site that conforms to the chemical formula {text{Fe}}_{2} {text{Ti}}_{{1 - x}} {text{Cr}}_{x}Sn. Compositions so obtained have been investigated for its electronic, magnetic, and electrical transport properties with an aim to observe the half-metallic ferromagnetic ground state, anticipated theoretically for Fe2CrSn. Our experimental study using synchrotron X-ray diffraction reveals that only compositions with x le 0.25 yield phase pure L2_1 cubic structures. The non-magnetic ground state of Fe2TiSn gets dramatically affected upon inclusion of Cr giving rise to a localized magnetic moment in the background of Ruderman–Kittel–Kasuya–Yosida (RKKY) correlations. The ferromagnetic interactions begin to dominate for x = 0.25 composition. Results of its resistivity and magnetoresistance (MR) measurement point towards a half-metallic ground state. The calculation of exchange coupling parameter, hbox {J}_{{ij}}, and orbital projected density of states that indicate a change in hybridization between 3d and 5p orbital, support the observations made from the study of local crystal structure made using the extended X-ray absorption fine structure spectroscopy. Our findings here highlight an interesting prospect of finding half-metallicity via band structure tailoring for wide application in spintronics devices.

First principle computation of half metallicity and mechanical properties of a new series of half Heusler alloys KMnZ (Z = B, Si, Ge, As) for spintronics

In the framework of density functional theory, the first principle calculations are performed to investigate various physical properties of new series of half Heusler alloys KMnZ (Z = B, Si, Ge, As) by using WIEN2k code. The results of structural parameters show that ferromagnetic state is more stable, with Type 3, configuration than the non-magnetic and antiferromagnetic states for all KMnZ (Z = B, Si, Ge, As) compounds. All considered alloys KMnZ (Z = B, Si, Ge, As) are completely spin polarized and show half-metallic character because of the presence of band gap (BG) in minority-spin channels. The calculated magnetic moments are 3 µB, 4 µB, and 5 µB for KMnB, KMnSi (Ge), and KMnAs, respectively. The values of minority BG for KMnB, KMnSi, KMnGe, and KMnAs are 0.60 eV, 0.85 eV, 1.00 eV, and 1.20 eV, respectively. The origin of their HM gaps is also examined in addition to density of states and band structures. The Curie temperature (TC), within mean field approximation, is also calculated. The calculations for elastic constants and other mechanical parameters are computed to check the mechanical stability which revealed that all other alloys are stable with ductile nature except KMnB. From the results of several physical properties, it can be envisaged that these materials can be used for spintronic devices.