Spin-down States Research Articles

Silicene-Based Spin Filter With High Spin-Polarization

In this article, for the first time, we propose a silicene-based spin filter having a TFET configuration. The device portrays a voltage-dependent spin polarization and achieves a “spin polarization” as high as 98% at a minimal “operating voltage” (0.35 V). The operation of the device is based on the “spin-polarized edge states” in a silicene nanoribbon. The “spin polarization” of the output current can be tuned by modifying the bandgap of the spin-up and spin-down states via an in-plane perpendicular “electric field” generated by two “lateral gates.” The device features high spin polarization apart from its expected integration with Si-based technology. The proposed device has a simple construction, wherein a single layer of Si atoms achieves such a high value of spin polarization at a minimal operating voltage. The operation of the proposed device is demonstrated by using the first-principles quantum transport simulations.

Designing multifunctional single-molecule devices by mononuclear or binuclear manganese phthalocyanines

Molecular-scale magnetic devices have attracted great attention of research in recent years. In this work, by using the non-equilibrium Green’s function (NEGF) method in combination with density functional theory (DFT), we investigated the spin transport properties of manganese phthalocyanine based spintronic devices constructed by a mononuclear manganese phthalocyanine (MnPc) or binuclear manganese phthalocyanine (Mn2Pc2) sandwiched between two zigzag-edge graphene nanoribbon (zGNR) electrodes. The calculation results show that both MnPc and Mn2Pc2 devices are good spin filters manifesting excellent spin filtering effect, which is originated from the distinct difference in the energy alignments of the spin-up and spin-down molecular electronic states with respect to the Fermi energy (EF) of zGNR electrodes. More interestingly, the spin polarization of the tunneling electrons through the devices is intimately opposite to that of Mn atom(s). Specifically, for a spin-up polarization of Mn in the devices, almost only the spin-down electrons are allowed to go through the device, and vice versa. In addition, spin filtering efficiency (SFE) of the devices is evidently enhanced in Mn2Pc2 devices in comparison with that of the MnPc devices, of which the value of SFE can reach almost 100%. This enhancement is attributed to the fact that the spin electronic states of Mn2Pc2 devices are closer to EF and there is a weaker localization in their spatial distributions induced by the external bias voltage. Furthermore, both MnPc and Mn2Pc2 devices can act as NOT logic gates by taking the spin polarization characteristics of Mn atom(s) and current of devices as input and output signals, respectively. This work demonstrates that mononuclear MnPc and binuclear Mn2Pc2 molecules have potential applications in designing multifunctional spintronic single-molecule devices.

BASIC CONCEPTS IN PSYCHIATRIC GENETICS

We have performed spin-polarized calculation for the electronic band structure, density of states, Fermi surface and the space electronic charge density distribution for the filled-skutterudites LaFe4Pn12 (Pn=P, As and Sb) compounds. It has been noticed that for both LaFe4P12 and LaFe4As12 there are two bands cross Fermi level (EF) for the spin-up and spin-down states, while for LaFe4Sb12 there is only one band cross EF for the spin-up state and three bands cross EF for the spin-down state. As the partial DOS of La-s,d, Fe-s/p/d and Pn-s/p/d coincide Fermi level at nonzero value, it reveals that the electrons of these orbitals contribute in the conduction process. The calculated values of the density of the states at Fermi level N(EF) and the associated electronic specific heat coefficient (γ) for the spin-up/down states are decreases with substituting P→As→Sb. The bonds nature and the interactions between the atoms for the spin-up/down configurations were investigated in (1 0 0) and (1 0 1) crystallographic planes.

The effect of Cr impurity and Zn vacancy on electronic and magnetic properties of ZnSe crystal

The spin-polarized electronic energy spectra of the ZnCrSe crystal were obtained based on calculations for a supercell containing 64 atoms. First, calculation is performed with an impurity of Cr atom, replacing the Zn atom. In the second variant, the Cr impurity and the vacancy at the Zn atom site are considered simultaneously. The results obtained in the first variant are as follows. It was found that the presence of the Cr atom leads to significant changes in the electronic energy bands, showing a large difference for different spin moments. The density curves of electronic states with opposite spins show an asymmetry, the consequence of which is the existence of a nonzero magnetic moment of the supercell. It was found that in the ZnCrSe crystal electronic 3d states with spin up are present at the Fermi level, i.e. the material is a metal. For spin-down states, the material is a semiconductor in which the Fermi level is inside the band gap. The value of the direct interband gap for electronic states with spin up is equal to 1.56 eV, and the magnetic moment of the supercell is 4.00 . The results obtained by the second variant of the calculation show a significant effect of the vacancy on the zinc site on the electronic structure of the ZnCrSe crystal. The Fermi level now intersects the dispersion curves of the upper part of the valence band for both spin orientations. The magnetic moment of the supercell is 2.74 .

Magnetic properties of Mn-doped monolayer MoS2

We use first-principles method to investigate the crystal structure, electronic structure and magnetic properties of Mn-doped MoS2 monolayer (Mo0.75Mn0.25S2). The spin-up electron states exhibit metallic properties, while the spin-down electron states exhibit insulating properties, which indicates the half-metallic properties of Mo0.75Mn0.25S2 monolayer. The doping of Mn atoms leads to the strong ferromagnetism of the system with the coupling between dopant Mn 3d orbitals via Mo 4d and S 3p states. Under the generalized Bloch condition, we explore the dispersion relation of Mo0.75Mn0.25S2 between the spin-spiral energy E(q⇀) and the wave vector q⇀. The exchange coupling coefficient Ji (i=1,2,3,4) is obtained through the Heisenberg interaction (HBI) model. It is found that although the first nearest-neighbor J1 plays an important role, the second-nearest-neighbor interaction exchange coupling coefficient J2 is non-negligible.

Fe2GeS4 (010) surface oxidation mechanism and potential application of the oxidized surface in gas sensing: A first-principles study

Herein, we studied Fe2GeS4 (010) surface oxidation and its potential application in gas sensing by spin-polarized DFT+U calculations. Firstly, calculation results show that bulk Fe2GeS4 and its clean (010) surface are semiconductor and antiferromagnetic. Then calculations about interaction between O2 molecules and Fe2GeS4 (010) surface reveal that the first fifth O2 tend to dissociate and incorporate into the surface until an amorphous oxide form. The oxidized Fe2GeS4 (010) surface has decreased band gap of 0.88 eV for spin-up state and 0.32 eV for spin-down state. Finally, we investigated the adsorption behaviors of eight gas molecules (CH4, CO, CO2, NO, NO2, H2S, SO2, HCHO) towards oxidized Fe2GeS4 (010) surface. Physical adsorptions of CH4, CO, CO2 and H2S on the surface are predicted. Whereas NO, NO2, SO2 and HCHO anchor on the surface in chemical adsorption state. However, HCHO adsorption had almost no contribution to the electronic properties of the oxidized Fe2GeS4 (010) surface. While electronic properties of the surface could be effectively changed by NO, NO2 and SO2. These results indicate that the oxidized Fe2GeS4 (010) surface are more sensitive to NO, NO2 and SO2, which would facilitate the potential application of oxidized Fe2GeS4 (010) surface in NO, NO2 and SO2 gas sensing.

Magnetic single atom catalyst in C2N to induce adsorption selectivity toward oxidizing gases

Density functional theory (DFT) method is used to study the effect of single-atom catalyst (SAC) of Mn embedded in C2N nanoribbon (C2N-NR) on the adsorption properties as an attempt to achieve selectivity. Many gases (e.g., CO, CO2, H2, H2O, H2S, N2 and O2) of interest to energy and environmental applications were tested. The results show that SAC-Mn alters chemisorption processes with all gas molecules except N2. Clear adsorption selectivity is obtained towards oxidizing CO, CO2 and O2 molecules as evidenced by the enhancements in binding energy and charge transfer and the reduction in magnetization. While the SAC-Mn contributes predominantly to Fermi-energy region with spin-down states, the strong binding to oxidizing molecules introduces there more spin-up states to compromise and reduce the magnetization. Hence, C2N-NR:Mn is proposed to be used as platform for gas sensor (if combined with magnetic sensor) to yield high selectivity toward these latter gases.

Effect of vacancy defects in 2D vdW graphene/h-BN heterostructure: First-principles study

Graphene (G) and hexagonal Boron Nitride (h-BN) are structurally similar materials but have very different electronic and magnetic properties. Heterostructures formed by the combination of these materials are of great research interest. To assess the role played by the crystalline defects in such heterostructures is also of crucial importance owing to their novel properties. In the present work, we study the structural, electronic, and magnetic properties of the G/h-BN heterostructure and the different possible point defects of B and N atoms in it by using first-principles calculations based on the spin-polarized density functional theory (DFT) method within the van der Waals correction DFT-D2 approach. The structural analysis of these systems shows that they are stable two dimensional van der Waals heterostructure materials. Band structure calculations of these materials reveal their semimetallic nature. On the basis of density of states and partial density of states calculations, the defective systems are magnetic materials. The magnetic moment obtained in these defective systems is due to the unpaired up-spin and down-spin states in the orbitals of C, B, and N atoms created by the vacancy defects. On the other hand, the G/h-BN heterostructure has an approving condition for ferromagnetism due to the presence of flat bands in the neighborhood of the Fermi energy.

Structural, Electronic and Magnetic Properties of Impurities Defected Graphene/MoS2 Van Der Waals Heterostructure: First-principles Study

Two-dimensional (2D) pristine and defected van der Waals (vdW) heterostructure (HS) materials open up fortune in nanoelectronic and optoelectronic devices. So, they are compatible for designing in the fields of device applications. In the present work, we studied structural, electronic and magnetic properties of vdW (HS) graphene/MoS2 ((HS)G/MoS2), Nb impurity defect in vdW (HS) graphene/MoS2 (Nb-(HS)G/MoS2), and Tc impurity defect in vdW (HS) graphene/MoS2 (Tc-(HS)G/MoS2) materials by using spin-polarized DFT-D2 method. We examined the structure of these materials, and found that they are stable. Based on band structure analysis, we found that (HS)G/MoS2, Nb-(HS)G/MoS2 and Tc-(HS)G/MoS2 have metallic characteristics. Also, (HS)G/MoS2 and Tc-(HS)G/MoS2 materials have n-type Schottky contact, while Nb-(HS)G/MoS2 material has p-type Schottky contact. To understand the magnetic properties of materials, we have used DoS, IDoS and PDoS calculations. We found that (HS)G/MoS2 is a non-magnetic material, but Nb-(HS)G/MoS2 and Tc-(HS)G/MoS2 are magnetic materials. Magnetic moment of Nb-(HS)G/MoS2 and Tc-(HS)G/MoS2 materials are -0.24 μB/cell and +0.07μB/cell values respectively from DoS/PDoS calculations, and 0.26 μB/cell and 0.08μB/cell values respectively from IDoS calculations. Up-spin and down-spin states of electrons in 2p orbital of C atoms, 3p orbital of S atoms, 4d orbital of Mo atoms, 4d orbital of Tc atom in Tc-(HS)G/MoS2, and 2p orbital of C atoms, 3p orbital of S atoms, 4p & 4d orbitals of Mo atoms, 4p & 4d orbitals of Nb atom in Nb-(HS)G/MoS2 have major contribution for the development of magnetic moment.

Effect of manganese on electronic and optical properties of Ba2ZnS3: A DFT study

The structural, electronic, and optical properties of pristine Ba2ZnS3 and Mn4+ doped Ba2ZnS3 at Ba-site have been carried out by employing the first-principles calculations based on density functional theory (DFT). For the calculation of exchange and correlation interactions, the modified Becke Johnson (TB-mBJ) potential and generalized gradient approximation with the Hubbard parameter (GGA ​+ ​U) were used for pristine and Mn4+ doped Ba2ZnS3 respectively. The defective states due to Mn4+ doping are introduced below the conduction band which produces n-type conductivity in both spin-up and spin-down states. The direct band gaps are observed for Ba2ZnS3 and Ba2ZnS3:Mn4+ (spin up and spin down) with bandgap values 3.157 ​eV, 1.582 ​eV, and 2.176 ​eV respectively. For spin-up states, the intermediate states are induced due to the majority contribution of s-orbital of Mn atom, while minority contribution of p-orbital of Mn atom. For spin-down states, the defective states are produced as a result of the prominent contribution of d-orbital of Mn atom and minor contribution of s-orbital of Mn atom. The optical properties such as the real and imaginary parts of the dielectric function, reflectivity, electron energy loss spectrum, extinction coefficient, refractive index, absorption coefficient, and optical conductivity as a function of photon energy are calculated. Our results revealed that Mn4+ doped Barium zinc sulfide emerged as a highly efficient luminescent phosphor material, and is more suitable for optical devices due to its direct bandgap nature and small bandgap value (1.582eV).

Robust ferromagnetism in single-atom-thick ternary chromium carbonitride CrN4C2

The design and exploration of ferromagnetic two-dimensional materials is an important research topic due to their potential applications in spintronic devices. By the first-principles calculations, we examine the structural stability and the electronic properties of transition metal carbonitrides MN4C2 (M = 3d transition metals) and find out that chromium carbonitride CrN4C2 is a ferromagnetic two-dimensional material. CrN4C2 is made up of CrN4 unit and C2 dimer, stacked alternatively to form a planar single-atom-thick sheet. For the Cr 3d electronic states, there is a complete spin splitting between Cr 3d spin-up and spin-down states, leading to a large moment up to 3.76 μB around Cr atoms. The magnetic interaction among Cr atoms can be described by Ising model, and the ferromagnetic couplings are energetically favorable, which indicates that there must be a ferromagnetic phase transition and a ferromagnetic ground state. The Curie temperature of CrN4C2 is evaluated to be 226.6 K in terms of the tensor renormalization group method. With Hubbard U of 3 eV and 5 eV being considered, the Curie temperatures reach up to 331.1 and 283.6 K. Consequently, our results demonstrate that CrN4C2 is a graphene-like two-dimensional material with robust ferromagnetism.

Ion-acoustic solitary, breathers, and freak waves inadegenerate quantum plasma

In this investigation, the characteristics of ion acoustic (IA) solitary, breathers, and freak waves have been examined by using spin evolution quantum hydrodynamic model in a degenerate quantum plasma consisting of inertial non-degenerate ions and degenerate electrons with spin-up and spin-down states. The Korteweg–de Vries (KdV) equation is derived by employing reductive perturbation technique and its solution is determined to see the existence of IA solitary waves. Only positive potential solitary structures are formed. Further, using appropriate transformation, the KdV equation is transformed into nonlinear Schrödinger equation (NLSE) and its different order solutions are used to study the characteristics of breathers such as the Akhmediev breather, the Kuznetsov–Ma breather and freak waves. Further, the formation of super order of freak waves has also been explored. The influence of different plasma parameters including density polarization ratio has been investigated on the characteristics of IA solitary waves and different types of breathers. It is observed that the density polarization ratio has a strong influence on the characteristics of different types of nonlinear structures. The present investigation may be of great importance to provide physical insight to understand nonlinear dynamical processes in dense astrophysical regions such as white dwarfs and neutron stars.

Modulation instability of ion-acoustic waves and exchange interaction in magnetized quantum plasma with relative density effects of spin-up and spin-down electrons

The separate-spin evolution quantum hydrodynamic (SSE-QHD) model is used to study Ion-Acoustic Waves (IAWs) in a magnetized quantum plasma having electrons with different spin populations in the presence of the spin particle exchange interactions (Coulomb type) in spin-down electrons only. The plasma consists of inertia-less separated spin-up and spin-down electrons states, while ions are inertial and classical. Besides, a uniform magnetic field is assumed to be directed in the Z-direction i.e. . An interesting interplay between exchange interaction and density polarization ratio κ has been observed on dispersive properties, effective Debye screening length and effective ion-acoustic speed. The non-linear Schrödinger equation is derived through a reductive perturbation approach. Moreover, the modulation instability condition and resulting envelope waves are analyzed. The parametric effects of density polarization ratio κ and Coulomb exchange interaction on dispersion, effective screening length, effective ion-acoustic speed, instability growth rate, and on the solitons profile have been discussed in detail. This model can be helpful to study properties of the dilute magnetic semiconductors and degenerate 2D & 3D Fermi electron gas, where exchange interactions are significant.

First principles study on electronic structure, ferromagnetism and dielectric properties of α-Fe, FeSiAl and Fe3Si

Based on the plane-wave pseudo-potential theory, the first principle calculation was used to study the electronic structure, ferromagnetism and dielectric properties of α-Fe, FeSiAl and Fe3Si alloys. Results indicate that the Fe3Si has the strongest hybridization and covalent bond. And α-Fe has the most stable system structure. The PDOS shown that the peak for the imaginary part of dielectric constant in the low-energy region is due to the electronic transitions of the hybridized 3d spin-down Fe states above and below the Fermi level, and the high-energy peak of it is formed by the transitions from the broad 3d spin-down band in the valence band to the unoccupied 3d spin-down states. Compared with α-Fe and FeSiAl, the imaginary part of the dielectric constant of Fe3Si is larger, and the magnetic moment of Fe3Si alloy is the largest according to the electronic structure and magnetic analysis. Therefore, Fe3Si has better absorbing properties.

First-principles study on magneto-optical effects in the ferromagnetic semiconductorsY3Fe5O12andBi3Fe5O12

The magneto-optical (MO) effects not only are a powerful probe of magnetism and electronic structure of magnetic solids but also have valuable applications in high-density data-storage technology. Yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$) (YIG) and bismuth iron garnet (Bi$_3$Fe$_5$O$_{12}$) (BIG) are two widely used magnetic semiconductors with strong magneto-optical effects and have also attracted the attention for fundamental physics studies. In particular, YIG has been routinely used as a spin current injector. In this paper, we present a thorough theoretical investigation on magnetism, electronic, optical and MO properties of YIG and BIG, based on the density functional theory with the generalized gradient approximation plus onsite Coulomb repulsion. We find that both semiconductors exhibit large MO effects with their Kerr and Faraday rotation angles being comparable to that of best-known MO materials such as MnBi. Especially, the MO Kerr rotation angle for bulk BIG reaches -1.2$ ^{\circ}$ at photon energy $\sim2.4$ eV, and the MO Faraday rotation angle for BIG film reaches -74.6 $ ^{\circ}/\mu m$ at photon energy $\sim2.7$ eV. Furthermore, we also find that both valence and conduction bands across the MO band gap in BIG are purely spin-down states, i.e., BIG is a single spin semiconductor. These interesting findings suggest that the iron garnets will find valuable applications in semiconductor MO and spintronic nanodevices. The calculated optical conductivity spectra, MO Kerr and Faraday rotation angles agree well with the available experimental data. The main features in the optical and MO spectra of both systems are analyzed in terms of the calculated band structures especially by determining the band state symmetries and the main optical transitions at the $\Gamma$ point in the Brillouin zone.

Deterministic Approach to Achieve Full-Polarization Cloak.

Achieving full-polarization (σ) invisibility on an arbitrary three-dimensional (3D) platform is a long-held knotty issue yet extremely promising in real-world stealth applications. However, state-of-the-art invisibility cloaks typically work under a specific polarization because the anisotropy and orientation-selective resonant nature of artificial materials made the σ-immune operation elusive and terribly challenging. Here, we report a deterministic approach to engineer a metasurface skin cloak working under an arbitrary polarization state by theoretically synergizing two cloaking phase patterns required, respectively, at spin-up (σ+) and spin-down (σ−) states. Therein, the wavefront of any light impinging on the cloak can be well preserved since it is a superposition of σ+ and σ− wave. To demonstrate the effectiveness and applicability, several proof-of-concept metasurface cloaks are designed to wrap over a 3D triangle platform at microwave frequency. Results show that our cloaks are essentially capable of restoring the amplitude and phase of reflected beams as if light was incident on a flat mirror or an arbitrarily predesigned shape under full polarization states with a desirable bandwidth of ~17.9%, conceiving or deceiving an arbitrary object placed inside. Our approach, deterministic and robust in terms of accurate theoretical design, reconciles the milestone dilemma in stealth discipline and opens up an avenue for the extreme capability of ultrathin 3D cloaking of an arbitrary shape, paving up the road for real-world applications.

Circularly-polarized light controlled thermal spin transport in stanene nanoribbon

The major challenge of spintronics lies in how to generate, manipulate, and detect spin current. Multiple methods, such as using magnetic materials, magnetic field, and polarized light field to manipulate the spin of electrons, have been proposed. Owing to the possible applications in spintronic devices, there is currently great interest in the field of spin caloritronics, which focuses on the interplay of spin and heat currents. Stanene is a type of two-dimensional topological insulator consisting of a single layer of Sn atoms arranged in a hexagonal lattice. In this paper, the effects of light and electric fields on the spin-dependent thermoelectric effect of the stanene nanoribbon are studied theoretically based on the non-equilibrium Green’s function method. The results show that the properties and intensity of the thermoelectric current can be effectively controlled by the intensity and the polarization direction of the circularly polarized light field. Under the joint action of a strong circularly-polarized light field and an electric field, the stanene can transform from a quantum spin-Hall insulator into a spin-polarized quantum Hall insulator. When the left-circularly-polarized light field is applied, the spin-down edge states of stanene undergo a phase transition to form a bandgap, and a 100% spin-polarized spin-down current driven by temperature gradient can be obtained. When the right-circularly-polarized light is applied, the edge states of spin-up electrons are destroyed, and a completely polarized spin-up thermal current can be generated. In the weak external field, the properties of the edge state do not change, and the system does not output a thermoelectric current. In addition, the study shows that the intensity of the thermal spin current is related to the width of the bandgap, and a moderate increase in temperature can significantly increase the peak value of the current, but the higher equilibrium temperature and temperature gradient will restrain the spin thermoelectric effect.

Tuning MoSO monolayer properties for optoelectronic and spintronic applications: effect of external strain, vacancies and doping.

Since the successful synthesis of the MoSSe monolayer, two-dimensional (2D) Janus materials have attracted huge attention from researchers. In this work, the MoSO monolayer with tunable electronic and magnetic properties is comprehensively investigated using first-principles calculations based on density functional theory (DFT). The pristine MoSO single layer is an indirect gap semiconductor with energy gap of 1.02(1.64) eV as predicted by the PBE(HSE06) functional. This gap feature can be efficiently modified by applying external strain presenting a decrease in its value upon switching the strain from compressive to tensile. In addition, the effects of vacancies and doping at Mo, S, and O sites on the electronic structure and magnetic properties are examined. Results reveal that Mo vacancies, and Al and Ga doping yield magnetic semiconductor 2D materials, where both spin states are semiconductors with significant spin-polarization at the vicinity of the Fermi level. In contrast, single S and O vacancies induce a considerable gap reduction of 52.89% and 58.78%, respectively. Doping the MoSO single layer with F and Cl at both S and O sites will form half-metallic 2D materials, whose band structures are generated by a metallic spin-up state and direct gap semiconductor spin-down state. Consequently, MoV, MoAl, MoGa, SF, SCl, OF, and OCl are magnetic systems, and the magnetism is produced mainly by the Mo transition metal that exhibits either ferromagnetic or antiferromagnetic coupling. Our work may suggest the MoSO Janus monolayer as a prospective candidate for optoelectronic applications, as well as proposing an efficient approach to functionalize it to be employed in optoelectronic and spintronic devices.

Evaluation of the optical properties of the lead-free mixed-halide iron perovskite CH3NH3FeI2Br for application in solar cells: A computational study

Performance of the proposed lead-free mixed halide iron perovskite CH3NH3FeI2Br as the light absorber layer of the perovskite solar cells is evaluated based on the analysis of the calculated optical properties. Crystal structure of this mixed halide iron perovskite is optimized at spin-polarized DFT GGA-PBE/FP-LAPW+lo level of theory. Structural analysis shows that the optimized unit cell parameters of this CH3NH3FeI2Br perovskite are all smaller than their corresponding values for the pure iodide iron perovskite CH3NH3FeI3. Thermodynamic analysis based on the total SCF energies calculated for the CH3NH3FeI2Br, CH3NH3FeI3, I2 and Br2 lattices shows also that the mixed halide iron perovskite CH3NH3FeI2Br is more stable than the pure iodide iron perovskite CH3NH3FeI3 by 1477.5 kJmol−1. Band structure, density of states and band gaps are calculated with the GGA-PBE(mBJ)/FP-LAPW+lo method. Analysis of the results show that the mixed halide iron perovskite CH3NH3FeI2Br is a direct band gap semiconductor having band gaps of 3.041 and 0.689 eV for its spin-up and spin-down states, respectively. The absorption spectrum calculated for the mixed halide iron perovskite MAFeI2Br and its corresponding Brewster angles calculated for the TiO2/perovskite and perovskite/spiro interfaces, the proposed mixed halide iron perovskite MAFeI2Br can be regarded as a promised candidate for application as light-harvesting layer of perovskite solar cells due to its good optical performance in the 500–630 nm range of wavelengths of the sunlight spectrum. Stability of the proposed CH3NH3FeI2Br perovskite is also validated based on the analysis of the calculated octahedral and Goldschmidt tolerance factors (0.465 and 1.009, respectively).

Structural Evolution and Magnetic Properties of Gd2Hf2O7 Nanocrystals: Computational and Experimental Investigations.

Structural evolution in functional materials is a physicochemical phenomenon, which is important from a fundamental study point of view and for its applications in magnetism, catalysis, and nuclear waste immobilization. In this study, we used x-ray diffraction and Raman spectroscopy to examine the Gd2Hf2O7 (GHO) pyrochlore, and we showed that it underwent a thermally induced crystalline phase evolution. Superconducting quantum interference device measurements were carried out on both the weakly ordered pyrochlore and the fully ordered phases. These measurements suggest a weak magnetism for both pyrochlore phases. Spin density calculations showed that the Gd3+ ion has a major contribution to the fully ordered pyrochlore magnetic behavior and its cation antisite. The origin of the Gd magnetism is due to the concomitant shift of its spin-up 4f orbital states above the Fermi energy and its spin-down states below the Fermi energy. This picture is in contrast to the familiar Stoner model used in magnetism. The ordered pyrochlore GHO is antiferromagnetic, whereas its antisite is ferromagnetic. The localization of the Gd-4f orbitals is also indicative of weak magnetism. Chemical bonding was analyzed via overlap population calculations: These analyses indicate that Hf-Gd and Gd-O covalent interactions are destabilizing, and thus, the stabilities of these bonds are due to ionic interactions. Our combined experimental and computational analyses on the technologically important pyrochlore materials provide a basic understanding of their structure, bonding properties, and magnetic behaviors.