Exchange Splitting Research Articles

First-principles study of ferromagnetic, optical, and transport properties of double Perovskites Z2FeTiO6 (Z = Mg, Zn)

Spintronics is an emerging technology in which electron spin carries information along with its charge. The current article reveals the ferromagnetic, optical, and thermoelectric properties of Z2FeTiO6 (Z = Mg, Zn) predicted using the first-principles calculations. Heisenberg’s classical model and polarization density calculations ensure ferromagnetism above room temperature and 100% spin polarization. Besides, the nature of ferromagnetism has been addressed by s-d and p-d exchange splitting, double exchange model, exchange energies, crystal field energy, and exchange constants. Furthermore, the optical properties are elaborated by dielectric constant, refractive index, and absorption in which intra-band transitions are incorporated. Finally, the thermal transports of the materials are investigated in terms of electrical and thermal conductivities, Seebeck coefficient, and power factor.

The Theoretical Study of Unexpected Magnetism in 2D Si-Doped AlN

In this study, the structural and magnetic properties of Si-doped bulk and 2D AlN were systematically investigated by first-principles calculations. Si atoms prefer to substitute Al atoms in both bulk and 2D AlN under N-rich growth conditions. In bulk AlN, Si dopants exhibit a non-magnetic state, uniform distribution, and a strong anisotropic diffusion energy barrier. In contrast to that, Si dopants prefer to form a buckling structure and exhibit a magnetic moment of 1 μB in 2D AlN. At a low Si concentration, Si atoms tend to get together with antiferromagnetic coupling between each other. However, the magnetic coupling among Si atoms changes to ferromagnetic coupling as Si concentration increases, due to the enhanced exchange splitting and delocalized impurity states. At the extreme doping limit, monolayer SiN, along with its analogs GeN and SnN, is a ferromagnetic semiconductor with a large band gap and high Curie temperature. These results indicate that 2D AlN doped by group IV atoms has potential applications in spintronic devices.

Element-selective ultrafast magnetization dynamics of hybrid Stoner-Heisenberg magnets

Stoner and Heisenberg excitations in magnetic materials are inherently different. The first involves an effective reduction of the exchange splitting, whereas the second comprises excitation of spin waves. In this work, we test the impact of these two excitations in the hybrid Stoner-Heisenberg system of FePd. We present a microscopic picture of ultrafast demagnetization dynamics in this alloy, which represents both components of strong local exchange splitting in Fe and induced polarization in Pd. We identify the spin-orbit coupling (SOC) and the optical intersite spin transfer (OISTR) as the two dominant factors for demagnetization at ultrashort timescales. Remarkably, the drastic difference in the origin of the magnetic moment of the Fe and Pd species is not deciding the initial magnetization dynamics in this alloy. By tuning the external laser pulse, the extrinsic OISTR can be manipulated for site-selective demagnetization on femtosecond timescales providing the fastest way for optical and selective control of the magnetization dynamics in alloys. Saliently, our results signify why various experiments demonstrating OISTR might obtain conflicting results.

Strain-mediated ferromagnetism and low-field magnetic reversal in Co doped monolayer WS_2

Strain-mediated magnetism in 2D materials and dilute magnetic semiconductors hold multi-functional applications for future nano-electronics. Herein, First principles calculations are employed to study the influence of biaxial strain on the magnetic properties of Co-doped monolayer WS_2. The non-magnetic WS_2 shows ferromagnetic signature upon Co doping due to spin polarization, which is further improved at low compressive (-2 %) and tensile (+2 %) strains. From the PDOS and spin density analysis, the opposite magnetic ordering is found to be favourable under the application of compressive and tensile strains. The double exchange interaction and p-d hybridization mechanisms make Co-doped WS_2 a potential host for magnetism. More importantly, the competition between exchange and crystal field splittings, i.e. (Delta _{ex}>Delta _{cfs}), of the Co-atom play pivotal roles in deciding the values of the magnetic moments under applied strain. Micromagnetic simulation reveals, the ferromagnetic behavior calculated from DFT exhibits low-field magnetic reversal (190 Oe). Moreover, the spins of Co-doped WS_2 are slightly tilted from the easy axis orientations showing slanted ferromagnetic hysteresis loop. The ferromagnetic nature of Co-doped WS_2 suppresses beyond pm 2~% strain, which is reflected in terms of decrease in the coercivity in the micromagnetic simulation. The understanding of low-field magnetic reversal and spin orientations in Co-doped WS_2 may pave the way for next-generation spintronics and straintronics applications.

Machine Learning Study of the Magnetic Ordering in 2D Materials.

Magnetic materials have been applied in a large variety of technologies, from data storage to quantum devices. The development of two-dimensional (2D) materials has opened new arenas for magnetic compounds, even when classical theories discourage their examination. Here we propose a machine-learning-based strategy to predict and understand magnetic ordering in 2D materials. This strategy couples the prediction of the existence of magnetism in 2D materials using a random forest and the Shapley additive explanations method with material maps defined by atomic features predicting the magnetic ordering (ferromagnetic or antiferromagnetic). While the random forest model predicts magnetism with an accuracy of 86%, the material maps obtained by the sure independence screening and sparsifying method have an accuracy of ∼90% in predicting the magnetic ordering. Our model indicates that 3d transition metals, halides, and structural clusters with regular transition-metal sublattices have a positive contribution in the total weight deciding the existence of magnetism in 2D compounds. This behavior is associated with the competition between crystal field and exchange splitting. The machine learning model also indicates that the atomic spin orbit coupling (SOC) is a determinant feature for the identification of the patterns separating ferro- from antiferromagnetic order. The proposed strategy is used to identify novel 2D magnetic compounds that, together with the fundamental trends in the chemical and structural space, pave novel routes for experimental exploration.

Pressure‐Driven Magneto‐Topological Phase Transition in a Magnetic Weyl Semimetal

AbstractThe co‐occurrence of phase transitions with local and global order parameters, such as entangled magnetization and topological invariants, is attractive but seldom realized experimentally. In this study, a magneto‐topological phase transition (magneto‐TPT), that is, the phenomenon of magnetic materials undergoing different magnetic and topological phases during pressure loading, is investigated. By considering both out‐of‐plane ferromagnetic and in‐plane antiferromagnetic components, it is discovered that the calculated results fit well with the experimental data. The calculation results further reveal a pristine Weyl phase with four additional pairs of Weyl nodes under low pressure, and a generally defined Z2 topological insulator phase after the restoration of time‐reversal symmetry at 40.4 GPa. The transport measurements performed at 5 K reveal that the magnetic order almost vanishes at 40.3 GPa, which is consistent with the theoretical prediction. Moreover, the present magneto‐TPT involves the degeneration of a pair of crossing bands of two spin channels. Hence, all the chiral Weyl nodes annihilate with their counterparts from another spin channel, in contrast to the typical intraband annihilation of Weyl pairs in inversion‐asymmetric systems. The study reveals a method for realizing diverse topological states by modulating exchange splitting by external physical knobs such as pressure in topological magnets.

Structural stability, electronic, magnetic and thermoelectric properties for half-metallic quaternary Heusler alloys CrLaCoZ

Using first-principles calculations, we predict the CrLaCoZ (Z = Al, Ga, In, Ge, Sn, Pb) Heusler alloys to be half-metals, and their total magnetic moments follow the Slater-Pauling rule Mt = Zt − 18. Structural stability is estimated in terms of thermodynamics, dynamic, mechanical and thermal. A comparison in total energy shows that the CrLaCoZ alloys have ferrimagnetic ground states, and the competition between d-d exchange, super-exchange and RKKY exchange interactions leads to the appearance of magnetic order phase. Phase separation shows that CrLaCoAl, CrLaCoGa, CrLaCoIn and CrLaCoSn are easier to be synthesized in experiment. The formation of half-metallic gap ascribes to the d-d, p-d orbital hybridizations. Furthermore, we find that the magnitude of gap is decided by the strength of exchange splitting between eg and t2g states according the distribution and occupation of electronic states. Presence of X and DO3 disorders destroys their half-metallicity. In addition, we examine the effects of spin-orbit coupling on electronic structure, and calculate their thermoelectric properties by using Boltzmann transport theory. From the calculated exchange interactions, it is found that the Cr(A)-Cr(A), Cr(A)-La(B) and Cr(A)-Co(C) exchanges play a leading role in all interactions, and finally determine the Curie temperatures. Apart from the CrLaCoSn alloy, other alloys have evidently higher Curie temperatures than room temperature, which is favorable as an electrode materials in magnetic tunnel junction.

Supercurrent decay in ballistic magnetic Josephson junctions

We investigate transport properties of ballistic magnetic Josephson junctions and establish that suppression of supercurrent is an intrinsic property of the junctions, even in absence of disorder. By studying the role of ferromagnet thickness, magnetization, and crystal orientation we show how the supercurrent decays exponentially with thickness and identify two mechanisms responsible for the effect: (i) large exchange splitting may gap out minority or majority carriers leading to the suppression of Andreev reflection in the junction, (ii) loss of synchronization between different modes due to the significant dispersion of the quasiparticle velocity with the transverse momentum. Our results for Nb/Ni/Nb junctions are in good agreement with recent experimental studies. Our approach combines density functional theory and the Bogoliubov-de Gennes model and opens a path for material composition optimization in magnetic Josephson junctions and superconducting magnetic spin valves.

Magnetic and electronic properties of the RECu4Al8 (RE = Tb, Dy, Ho, and Er) intermetallic compounds

Spin density functional theory calculations were employed to study some of the magnetic and electronic properties of the RECu4Al8 (RE = Tb, Dy, Ho, and Er) intermetallic compounds in their tetragonal crystal structure and their collinear antiferromagnetic state. The exchange and correlation electronic effects were treated by both a generalized gradient approximation (GGA) in its recently revised version for solids (GGA-PBEsol), and the addition of the effective Hubbard U term (GGA-PBEsol + Ueff) in the RE 4f states. Based on these methodologies, the total magnetic moment of the RE atoms and their spin and orbital contributions were determined. The total magnetic moment calculated with GGA + PBEsol (Ueff = 0.0 eV) agreed with the experimental data, except for the TbCu4Al8 system, in which the addition of the Ueff term was needed for a better description. Using GGA + PBEsol, the magnetocrystalline anisotropy energy (MAE), easiest magnetization axis (EMA), exchange splitting of the RE 4f states (Δex), and the electronic specific heat coefficients (γ) of the four compounds were determined. The values of the MAE found for TbCu4Al8, DyCu4Al8, HoCu4Al8, and ErCu4Al8 were 3.13, 9.11, 26.93, and 17.41 meV/f.u., respectively. The EMA for TbCu4Al8, DyCu4Al8, and HoCu4Al8 was found to be [111], while for ErCu4Al8 it was [001]. The values of Δex were equal to 4.3, 3.6, 2.8, and 1.9 eV for TbCu4Al8, DyCu4Al8, HoCu4Al8, and ErCu4Al8, respectively.

Comprehensive study of ferromagnetic MgNd2X4 (X = S, Se) spinels for spintronic and solar cells device applications

Full potential LAPW + lo method is used for exploring electronic, structural and thermoelectric properties for MgNd2X4 (X = S, Se) spinels that are found to show ferromagnetic-semiconductor behaviour in the spinel structure. The investigated negative value of formation energy and positive value of phonon spectra computed using PBEsol GGA indicates the energetic and dynamical stability of the studied cubic ferromagnetic-semiconductors. We have used TB-mBJ potential functional for electronic and magnetic properties, which lead to a reliable account of electronic structure, demonstrating band occupancy in the spinels along with a clear explanation of density of states. The stability of ferromagnetic state in the studied materials is because of the exchange splitting of Nd cations based on p-d hybridization which is in accordance with the results obtained for electronic band structure and density of states. The exchange splitting of bands can be justified by the spin magnetic moment between anions and cations, and sharing of charge. The computed values of dielectric constant and their associated optical parameters are used to explain the optical active behaviour of the spinels under investigation; indicating that the two spinels studied in the present work are suitable for solar cells device applications. The calculation of thermoelectric properties is very useful for determining a material's potential use in waste energy recovery systems and many other innovative applications.

Фотогальванический эффект в ферромагнетике со спин-орбитальным взаимодействием

The effect of the appearance of an electric current induced by the electromagnetic radiation at the interface of a ferromagnet and a non-magnetic material is calculated theoretically, taking into account the Rashba spin-orbit coupling. It is shown that the electric dipole transitions between the spin subbands of the conduction electrons of a ferromagnet due to the Rashba interaction lead to a photocurrent. This current has a resonance at a frequency corresponding to the energy of the exchange splitting of spin subbands. The resonance width is determined by the spin-orbit interaction constant. The estimates made show the possibility of experimental observation of this effect in specially prepared multilayer systems.

Large perpendicular magnetic anisotropy of transition metal dimers driven by polarization switching of a two-dimensional ferroelectric In2Se3 substrate.

Large perpendicular magnetic anisotropy (MA) is highly desirable for realizing atomic-scale magnetic data storage which represents the ultimate limit of the density of magnetic recording. In this work, we study the MA of transition metal dimers Co-Os, Co-Co and Os-Os adsorbed on two-dimensional ferroelectric In2Se3 (In2Se3-CoOs, In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs) using first-principles calculations. We find that the Co-Os dimer in In2Se3-CoOs has a total magnetic anisotropy energy (MAE) of ∼40 meV. The MAE arising from the Os atom in In2Se3-CoOs is up to ∼60 meV. Such large MAE is attributed to the high spin-orbit coupling constant and the onefold coordination of the Os atom. In addition, perpendicular MA can be enhanced in In2Se3-CoOs and induced in In2Se3-OsCo, In2Se3-CoCo and In2Se3-OsOs by the ferroelectric polarization reversal of In2Se3. We demonstrate that the enlargement of exchange splitting of dxy/dx2-y2 and dxz/dyz orbitals for Os atoms in In2Se3-OsOs, Co atom in In2Se3-CoOs and Os and Co atoms in In2Se3-OsCo is responsible for the increase of MAE; while, for the upper Co atom in In2Se3-CoCo and the Os atom in In2Se3-CoOs, the energy rise of the dz2 orbital owing to the change of the crystal field effect by the reversal of ferroelectric polarization results in the increase of MAE.

Photovoltaic effect in a ferromagnet with spin-orbit coupling

The effect of the appearance of an electric current induced by the electromagnetic radiation at the interface of a ferromagnet and a non-magnetic material is calculated theoretically, taking into account the Rashba spin-orbit coupling. It is shown that the electric dipole transitions between the spin subbands of the conduction electrons of a ferromagnet due to the Rashba interaction lead to a photocurrent. This current has a resonance at a frequency corresponding to the energy of the exchange splitting of spin subbands. The resonance width is determined by the spin-orbit interaction constant. The estimates show the possibility of experimental observation of this effect in specially prepared multilayer systems. Keywords: Ferromagnet, exchange coupling, Rashba spin-orbit coupling, photovoltaic effect.

XPS on Mn0.95Dy0.05NiSb Heusler Alloy

"XPS results on Mn0.95Dy0.05NiSb Heusler alloy are presented. The compound is single phase with a C1b type cubic structure. All the experimental results suggest that the rare earth atoms entered in the lattice and they are not forming second phases. An exchange splitting of Mn 3s core-level spectrum of about 4.6 eV was shown, giving a direct evidence of the local magnetic moments on Mn sites. The Ni 3d states are giving the major contribution to the XPS valence band spectra. The Ni 3d satellite peak situated around 6 eV was attributed to the fact that the Ni 3d states in the valence band are not fully occupied. However, the Ni 3d band is almost full in this alloy and the Stoner criterion for the existence of a magnetic moment on Ni sites is no longer fulfilled. Keywords: Heusler alloys, X-ray photoelectron spectroscopy, valence band spectra. "

Tuning the magnetic anisotropy of transition-metal atoms on two-dimensional In2Se3 substrate via ferroelectric polarization switching

Exploring large and tunable magnetic anisotropy (MA) in a single magnetic atom is of merit for information storage due to the highest information data density. In this work, we studied the MA of transition-metal atoms X (X = Co, Fe, Ir, Mo, Nb, Ta, Ti, V, W) adsorbed on the surface of two-dimensional (2D) ferroelectric (FE) In2Se3 by first-principles calculations. We find that when the ferroelectric polarization of the In2Se3 is switched, the strengths of the MA for Co, Ir, Nb and Ti single atoms can be changed, while the directions of the MA for Fe, Mo, Ta, V and W single atoms are reversed. Calculated electronic structure and orbital-resolved magnetic anisotropy energy suggest that the variations in the occupation of spin-minority state and the exchange splitting of d orbitals at Fermi level by switching FE polarization are responsible for the change of the strength and direction of the MA on X atoms.

Predicting ferromagnetism and thermoelectric characteristics in bulk spinels ZnCr2X4 (X = S, Se) using density functional theory

To have control over the properties of electronic devices with the help of the spin of electrons is considered an amalgamation field of innovative technology. The thermoelectric and ferro-magnetic characteristics of bulk ZnCr2X4 (X = S, Se) spinels have been investigated by the BoltzTraP and WIEN2k codes. The optimization of the fully relaxed structures has been performed by PBE generalized gradient approximation (GGA) for the exposure of ground state parameters and our calculated results of lattice constant represent a reasonable agreement with experimental values. The comparative analysis of the energies that emerged fromnonmagnetic and ferromagnetic states using PBE-GGA shows that the state is the ferromagnetic. The modified Becke-Johnson local density approximation (mBJLDA) has been brought into use for the computation of the density of states (DOS) and precise band structures (BS), which authenticates the ferromagnetic semiconducting behavior. Further, the calculation of exchange splitting energies, John-Teller energy, and crystal field energy explored the origin of ferromagnetism. The strong hybridization resulting in decomposition in Cr, the magnetic moment and creates the magnetic moments at the nonmagnetic sites. Consequently, the thermoelectric characteristichas been explored by the BoltzTraP code that reveals that the increasing temperature increases the power factor, the thermal conductivity, and the electrical conductivity whereas the Seebeck coefficient reduces with it. However, the compounds in our study prove to be suitable for being used in thermoelectric devices for alternative energy resources.

Binder-free trimetallic phosphate nanosheets as an electrode: Theoretical and experimental investigation

Transition metal phosphides and phosphates are newly emerging electrode material candidates in energy storage devices. For the first time, we report a uniformly distributed, interconnected, and well-aligned two-dimensional nanosheets made from trimetallic Zn-Co-Ga phosphate (ZCGP) electrode materials with preserved crystal phase. It is found that the ZCGP electrode material exhibits about 2.85 and 1.66 times higher specific capacity than mono- (Co-phosphate) and bimetallic phosphate (Zn-Co phosphate) electrode materials at the same current density. The trimetallic ZCGP electrode exhibits superior conductivity, lower internal resistance (IR) drop, and high coulombic efficiency compared to mono- and bimetallic phosphate. By means of density functional theory (DFT) calculations, ZCGP shows superior metallic conductivity due to the modified exchange splitting originating from 3d-orbitals of Co atoms in the presence of Zn and Ga. Moreover, hybrid supercapacitor (ZCGP//rGO) device is engineered which delivered a high energy density of 40 W h kg −1 and a high-power density of 7745 W kg −1 , lighting 5 different colors of light emitting diodes (LEDs). These outstanding results confirm the promising battery-type electrode materials for energy storage applications. • Novel trimetallic Zn-Co-Ga phosphate (ZCGP) with nanosheets-like morphology. • Preserved crystalline phase for mono-, bi-, and trimetallic phosphates electrodes. • The charge storage mechanism for electrodes exhibit diffusion-dominated response. • The superior properties of ZCGP has been verified experimentally and theoretically. • Hybrid supercapacitor device delivered excellent energy density.

The role of spin-lattice coupling for ultrafast changes of the magnetic order in rare earth metals

By comparing femtosecond laser-pulse-induced spin dynamics in the surface state of the rare earth metals Gd and Tb, we show that the spin polarization of valence states in both materials decays with significantly different time constants of 15 ps and 400 fs, respectively. The distinct spin polarization dynamics in Gd and Tb are opposed by similar exchange splitting dynamics in the two materials. The different time scales observed in our experiment can be attributed to weak and strong 4f spin to lattice coupling in Gd and Tb, suggesting an intimate coupling of spin polarization and 4f magnetic moment. While in Gd the lattice mainly acts as a heat sink, it contributes significantly to ultrafast demagnetization of Tb. This helps explain why all optical switching is observed in FeGd—but rarely in FeTb-based compounds.

Comprehensive DFT investigation of Cd-based spinel chalcogenides for spintronic and solar cells devices

In light of existing literature of stable ferromagnetic phase, the thermoelectric and optical behavior of the Cd-based spinel chalcogenides i.e. CdCr2X4, (X = S, Se, Te) were inspected and their structural and electronic characteristics were examined in terms of density functional theory calculations. The negative values of formation energy of said spinels were calculated to confirm their ferromagnetic phase stability. Optimization of band structure and calculations of density of states confirmed their ferromagnetic nature. For both channel, the band structure exhibited the semiconducting behavior. The exchange constants (N0α and N0β) were investigated with the help of exchange splitting energies, which were explored from the density of states. The splitting of 3d-states of Cr were explained on the basis of larger value of N0β as compared with N0α. Owing to the strong p-d hybridization indicated by N0β = −0.11, −0.12 and −0.15 and N0α = 0.14, 0.32 and 0.44 for CdCr2S4, CdCr2Se4 and CdCr2Te4, respectively, the necessary condition for ferromagnetic system is satisfied showing that exchange field dominates over the crystal field that induces the ferromagnetism in these spinels. Moreover the p-d hybridization was used to decrease the magnetic moment at Cr ions, by using a fraction of magnetic moment at non-magnetic sites. Further, optical characteristics were investigated in terms of photon energy 0–6 eV showing less dispersion of light and refractive index ranges up to 1–1.5 in visible region, thus suggesting spinels as potential candidates for opto-electronic applications. The thermoelectric performance observed in the temperature range of 200–600K which indicate positive Seebeck coefficients of ∼250 CdCr2S4, CdCr2Se4 while a negative Seebeck coefficients of −112 for CdCr2Te4 at room temperature. Further the power factors increased from S to Te as well as with increase of temperature which shows the potential of these chalcogens for thermoelectric power generators.

Effects of the Addition of Co or Ni Atoms on Structure and Magnetism of FeB Amorphous Alloy: Ab Initio Molecular Dynamics Simulation.

The effects of the substitution of Fe by Co or Ni on both the structure and the magnetic properties of FeB amorphous alloy were investigated using first-principle molecular dynamics. The pair distribution function, Voronoi polyhedra, and density of states of Fe80−xTMxB20 (x = 0, 10, 20, 30, and 40 at.%, TM(Transition Metal): Co, Ni) amorphous alloys were calculated. The results show that with the increase in Co content, the saturation magnetization of Fe80−xCoxB20 (x = 0, 10, 20, 30, and 40 at.%) amorphous alloys initially increases and then decreases upon reaching the maximum at x = 10 at.%, while for Fe80−xNixB20 (x = 0, 10, 20, 30, and 40 at.%), the saturation magnetization decreases monotonously with the increase in Ni content. Accordingly, for the two kinds of amorphous alloys, the obtained simulation results on the variation trends of the saturation magnetization with the change in alloy composition are in good agreement with the experimental observation. Furthermore, the relative maximum magnetic moment was recorded for Fe70Co10B20 amorphous alloy, due to the induced increased magnetic moments of the Fe atoms surrounding the Co atom in the case of low Co dopant, as well as the increase in the exchange splitting energy caused by the enhancement of local atomic symmetry.