Possible Room-Temperature Ferromagnetic Semiconductors

Magnetic semiconductors integrate the dual characteristics of magnets and semiconductors.It is difficult to manufacture magnetic semiconductors that function at room temperature.Here,we review a series of our recent theoretical predictions on room-temperature ferromagnetic semiconductors.Since the creation of two-dimensional(2D)magnetic semiconductors in 2017,there have been numerous developments in both experimental and theoretical investigations.By density functional theory calculations and model analysis,we recently predicted several2D room-temperature magnetic semiconductors,including CrGeSe_(3)with strain,CrGeTe_(3)/PtSe_(2) heterostructure,and technetium-based semiconductors(TcSiTe_(3),TcGeSe_(3),and TcGeTe_(3)),as well as PdBr_(3)and PtBr_(3)with a potential room-temperature quantum anomalous Hall effect.Our findings demonstrated that the Curie temperature of these 2D ferromagnetic semiconductors can be dramatically enhanced by some external fields,such as strain,construction of heterostructure,and electric field.In addition,we proposed appropriate doping conditions for diluted magnetic semiconductors,and predicted the Cr doped GaSb and InSb as possible room-temperature magnetic semiconductors.

Giant Magneto-Optical Effect in van der Waals Room-Temperature Ferromagnet Fe_(3)GaTe_(2)

The discovery of ferromagnetic two-dimensional(2D)van der Waals(vdWs)materials provides an opportunity to explore intriguing physics and to develop innovative spin electronic devices.However,the main challenge for practical applications of vd Ws ferromagnetic crystals lies in the weak intrinsic ferromagnetism and small perpendicular magnetic anisotropy(PMA)above room temperature.Here,we report the intrinsic vd Ws ferromagnetic crystal Fe_(3)GaTe_(2),synthesized by the self-flux method,exhibiting a Curie temperature(TC)of 370 K,a high saturation magnetization of 33.47 emu/g,and a large PMA energy density of approximately 4.17×10^(5)J/m^(3).Furthermore,the magneto-optical effect is systematically investigated in Fe_(3)GaTe_(2).The doubly degenerate E_(2g)(Γ)mode reverses the helicity of incident photons,indicating the existence of pseudoangular-momentum(PAM)and chirality.Meanwhile,the non-degenerate non-chiral A_(1g)(Γ)phonon exhibits a significant magneto-Raman effect under an external out-of-plane magnetic field.These results lay the groundwork for studying phonon chirality and magneto-optical phenomena in 2D magnetic materials,providing the feasibility for further fundamental research and applications in spintronic devices.

Development of Intrinsic Room-Temperature 2D Ferromagnetic Crystals for 2D Spintronics

Two-dimensional(2D)ferromagnetic crystals with fascinating optical and electrical properties are crucial for nanotechnology and have a wide variety of applications in spintronics.However,low Curie temperatures of most 2D ferromagnetic crystals seriously hinder their practical applications,thus searching for intrinsic roomtemperature 2D ferromagnetic crystals is of great importance for development of information technology.Fortunately,progresses have been achieved in the last few years.Here we review recent advances in the field of intrinsic room-temperature 2D ferromagnetic crystals and introduce their applications in spintronic devices based on van der Waals heterostructures.Finally,the remaining challenge and future perspective on the development direction of intrinsic room-temperature 2D ferromagnetic crystals for 2D spintronics and van der Waals spintronics are briefly summarized.

Recent progress in synthesis and properties of 2D room-temperature ferromagnetic materials

Two-dimensional(2D)ferromagnetic materials have important applications in optics,spintronics,biomedicine,and energy conversion fields.To achieve higher integration and continue Moore's Law,it is necessary to search for 2D ferromagnetic materials.However,the low Curie temperature(Tc)of most 2D ferromagnetic materials seriously hinders their practical applications.Therefore,the search for room-temperature 2D ferromagnetic materials is of great significance for the advancement of technological applications.This article provides a systematic and comprehensive review of the synthesis methods of room-temperature 2D ferromagnetic materials that have been discovered and created to date,as well as an overview of the physical properties of the 2D ferromagnetic materials.Finally,the challenges and prospects of the room-temperature 2D ferromagnetic materials in the fields of synthesis and application are briefly summarized.

Solid-state room-temperature phosphorescence activated by the end-capping strategy of cyano groups

Avoiding the tedious process of crystal cultivation and directly obtaining organic crystals with desirable phosphorescent performance is of great significance for studying their structure and properties.Herein,a set of benzophenone-cored phos-phors with bright green afterglow are obtained on a large scale through in-situ generation via an end-capping strategy to suppress non-radiative triplet excitons and reinforce the intermolecular interactions.The ordered arrangement of phosphors with alkyl-cyano groups as regulators is crucial for the enhancement of room-temperature phosphorescence(RTP)emission,which has been further verified by the attenuated lifetimes in isolated states through the formation of inclusion complexes upon binding with pillar[5]arenes.Moreover,the hierarchical interactions of phos-phors,including hydrogen bonding,π-πstacking interactions,and van der Waals forces,are quantified by crystal structures and theoretical calculation to deeply inter-pret the origins of RTP emission.With this study,we provide a potential strategy for the direct acquisition of crystalline organic phosphors and modulation of RTP.

C-Type Antiferromagnetic Structure of Topological Semimetal CaMnSb_(2)

Determination of the magnetic structure and confirmation of the presence or absence of inversion(P)and time reversal(Τ)symmetry is imperative for correctly understanding the topological magnetic materials.Here highquality single crystals of the layered manganese pnictide CaMnSb_(2)are synthesized using the self-flux method.

Semiclassical approach to spin dynamics of a ferromagnetic S = 1 chain

Motivated by recent experimental progress on the quasi-one-dimensional quantum magnet Ni Nb2O6, we study the spin dynamics of an S = 1 ferromagnetic Heisenberg chain with single-ion anisotropy by using a semiclassical molecular dynamics approach. This system undergoes a quantum phase transition from a ferromagnetic to a paramagnetic state under a transverse magnetic field, and the magnetic response reflecting this transition is well described by our semiclassical method.We show that at low temperature the transverse component of the dynamical structure factor depicts clearly the magnon dispersion, and the longitudinal component exhibits two continua associated with single-and two-magnon excitations,respectively. These spin excitation spectra show interesting temperature dependence as effects of magnon interactions. Our findings shed light on the experimental detection of spin excitations in a large class of quasi-one-dimensional magnets.

Evolution of anomalous Hall effect in ferromagnetic Weyl semimetal Nb_(x)Zr_(1-x)Co_(2)Sn

Magnetic topological semimetal can host various topological non-trivial states leading to exotic novel transport properties.Here we report the systematic magneto-transport studies on the Heusler alloy Nb_(x)Zr_(1-x)Co_(2)Sn considered as a ferromagnetic(FM)Weyl semimetal.The cusp anomaly of temperature-dependent resistivity and large isotropic negative magneto-resistivity(MR)emerge around the FM transition consistent with the theoretical half-metallic predictions.The prominent anomalous Hall effect(AHE)has the same behavior with the applied field along various crystal directions.The Nb doping introduces more disorder resulting in the enhancement of the upturn for the temperature-dependent resistivity in low temperatures.With Nb doping,the AHE exhibits systemic evolution with the Fermi level lifted.At the doping level of x=0.25,the AHE mainly originates from the intrinsic contribution related to non-trivial topological Weyl states.

Influence of exchange bias on spin torque ferromagnetic resonance for quantification of spin–orbit torque efficiency

Antiferromagnet(AFM)/ferromagnet(FM)heterostructure is a popular system for studying the spin–orbit torque(SOT)of AFMs.However,the interfacial exchange bias field induces that the magnetization in FM layer is noncollinear to the external magnetic field,namely the magnetic moment drag effect,which further influences the characteristic of SOT efficiency.In this work,we study the SOT efficiencies of IrMn/NiFe bilayers with strong interfacial exchange bias by using spin-torque ferromagnetic resonance(ST-FMR)method.A full analysis on the AFM/FM systems with exchange bias is performed,and the angular dependence of magnetization on external magnetic field is determined through the minimum rule of free energy.The ST-FMR results can be well fitted by this model.We obtained the relative accurate SOT efficiencyξ_(DL)=0.058 for the IrMn film.This work provides a useful method to analyze the angular dependence of ST-FMR results and facilitates the accurate measurement of SOT efficiency for the AFM/FM heterostructures with strong exchange bias.

Stacking-dependent exchange bias in two-dimensional ferromagnetic/antiferromagnetic bilayers

A clear microscopic understanding of exchange bias is crucial for its application in magnetic recording, and further progress in this area is desired. Based on the results of our first-principles calculations and Monte Carlo simulations,we present a theoretical proposal for a stacking-dependent exchange bias in two-dimensional compensated van der Waals ferromagnetic/antiferromagnetic bilayer heterostructures. The exchange bias effect emerges in stacking registries that accommodate inhomogeneous interlayer magnetic interactions between the ferromagnetic layer and different spin sublattices of the antiferromagnetic layer. Moreover, the on/off switching and polarity reversal of the exchange bias can be achieved by interlayer sliding, and the strength can be modulated using an external electric field. Our findings push the limits of exchange bias systems to extreme bilayer thickness in two-dimensional van der Waals heterostructures, potentially stimulating new experimental investigations and applications.

Ab initio nonadiabatic molecular dynamics study on spin–orbit coupling induced spin dynamics in ferromagnetic metals

Understanding the photoexcitation induced spin dynamics in ferromagnetic metals is important for the design of photo-controlled ultrafast spintronic device.In this work,by the ab initio nonadiabatic molecular dynamics simulation,we have studied the spin dynamics induced by spin–orbit coupling(SOC)in Co and Fe using both spin-diabatic and spin-adiabatic representations.In Co system,it is found that the Fermi surface(E_(F))is predominantly contributed by the spin-minority states.The SOC induced spin flip will occur for the photo-excited spin-majority electrons as they relax to the E_(F),and the spin-minority electrons tend to relax to the EFwith the same spin through the electron–phonon coupling(EPC).The reduction of spin-majority electrons and the increase of spin-minority electrons lead to demagnetization of Co within100 fs.By contrast,in Fe system,the E_(F) is dominated by the spin-majority states.In this case,the SOC induced spin flip occurs for the photo-excited spin-minority electrons,which leads to a magnetization enhancement.If we move the E_(F) of Fe to higher energy by 0.6eV,the E_(F) will be contributed by the spin-minority states and the demagnetization will be observed again.This work provides a new perspective for understanding the SOC induced spin dynamics mechanism in magnetic metal systems.

Half-metallic ferromagneticWeyl fermions related to dynamic correlations in the zinc-blende compound VAs

The realization of 100%polarized topologicalWeyl fermions in half-metallic ferromagnets is of particular importance for fundamental research and spintronic applications.Here,we theoretically investigate the electronic and topological properties of the zinc-blende compound VAs,which was deemed as a half-metallic ferromagnet related to dynamic correlations.Based on the combination of density functional theory and dynamical mean field theory,we uncover that the half-metallic ferromagnet VAs exhibits attractive Weyl semimetallic behaviors which are very close to the Fermi level in the DFT+U regime with effect U values ranging from 1.5 eV to 2.5 eV.Meanwhile,we also investigate the magnetization-dependent topological properties;the results show that the change of magnetization directions only slightly affects the positions of Weyl points,which is attributed to the weak spin–orbital coupling effects.The topological surface states of VAs projected on semi-infinite(001)and(111)surfaces are investigated.The Fermi arcs of all Weyl points are clearly visible on the projected Fermi surfaces.Our findings suggest that VAs is a fully spin-polarized Weyl semimetal with many-body correlated effects in the effective U values range from 1.5 eV to 2.5 eV.

The atlas of ferroicity in two-dimensional MGeX_(3) family:Room-temperature ferromagnetic half metals and unexpected ferroelectricity and ferroelasticity

Two-dimensional(2D)ferromagnetic and ferroelectric materials attract unprecedented attention due to the spontaneous-symmetry-breaking induced novel properties and multifarious potential applications.Here we systematically investigate a large family(148)of 2D MGeX3(M=metal elements,X=O/S/Se/Te)by means of the high-throughput first-principles calculations,and focus on their possible ferroic properties including ferromagnetism,ferroelectricity,and ferroelasticity.We discover eight stable 2D ferromagnets including five semiconductors and three half-metals,212D antiferromagnets,and 11 stable 2D ferroelectric semiconductors including two multiferroic materials.Particularly,MnGeSe3 and MnGeTe3 are predicted to be room-temperature 2D ferromagnetic half metals with Tc of 490 and 308 K,respectively.It is probably for the first time that ferroelectricity is uncovered in 2D MGeX3 family,which derives from the spontaneous symmetry breaking induced by unexpected displacements of Ge-Ge atomic pairs,and we also reveal that the electric polarizations are in proportion to the ratio of electronegativity of X and M atoms,and IVB group metal elements are highly favored for 2D ferroelectricity.Magnetic tunnel junction and water-splitting photocatalyst based on 2D ferroic MGeX3 are proposed as examples of wide potential applications.The atlas of ferroicity in 2D MGeX3 materials will spur great interest in experimental studies and would lead to diverse applications.

Coupling ferromagnetic ordering electron transfer channels and surface reconstructed active species for spintronic electrocatalysis of water oxidation

Sluggish reaction kinetics of oxygen evolution reaction(OER), resulting from multistep proton-coupled electron transfer and spin constriction, limits overall efficiency for most reported catalysts. Herein, using modeled ZnFe_(2-x)Ni_xO_(4)(0 ≤ x ≤ 0.4) spinel oxides, we aim to develop better OER electrocatalyst through combining the construction of ferromagnetic(FM) ordering channels and generation of highly active reconstructed species. The number of symmetry-breaking Fe–O–Ni structure links to the formation of FM ordering electron transfer channels. Meanwhile, as the number of Ni^(3+)increases, more ligand holes are formed, beneficial for redirecting surface reconstruction. The electro-activated ZnFe_(1.6)Ni_(0.4)O_(4) shows the highest specific activity, which is 13 and 2.5 times higher than that of ZnFe_(2)O_(4) and unactivated ZnFe_(1.6)Ni_(0.4)O_(4), and even superior to the benchmark IrO_(2) under the overpotential of 350 mV. Applying external magnetic field can make electron spin more aligned, and the activity can be further improved to 39 times of ZnFe_(2)O_(4). We propose that intriguing FM exchange-field interaction at FM/paramagnetic interfaces can penetrate FM ordering channels into reconstructed oxyhydroxide layers, thereby activating oxyhydroxide layers as spin-filter to accelerate spin-selective electron transfer. This work provides a new guideline to develop highly efficient spintronic catalysts for water oxidation and other spin-forbidden reactions.

Simultaneous resonance of an axially moving ferromagnetic thin plate under a line load in a time-varying magnetic field

In this paper,the simultaneous resonance of a ferromagnetic thin plate in a time-varying magnetic field,having axial speed and being subjected to a periodic line load,is studied.Based on the large deflection theory of thin plates and electromagnetic field theory,the nonlinear vibration differential equation of the plate is obtained by using the Hamilton′s principle and the Galerkin method.Then the boundary condition in which the longer opposite sides are clamped and hinged is considered.The dimensionless nonlinear differential equations are solved by using the method of multiple scales,and the analytical solution is given.In addition,the stability analysis is also carried out by using Lyapunov stability theory.Through numerical analysis,the variation curves of system resonance amplitude with frequency tuning parameter,magnetic field strength and external excitation amplitude are obtained.Different parameters that have significant effects on the response of the system,such as the thickness,the axial velocity,the magnetic field intensity,the position,and the frequency of external excitation,are considered and analyzed.The results show that the system has multiple solution regions and obvious nonlinear coupled characteristics.

Li_(2)NiSe_(2):A new-type intrinsic two-dimensional ferromagnetic semiconductor above 200 K

By using first-principles electronic structure calculations,we propose a two-dimensional ferromagnetic semiconductor Li_(2)NiSe_(2)with a Curie temperature above 200 K.The structure of monolayer Li_(2)NiSe_(2)is dynamically stable,which is derived from the synthesized prototype compound Li_(2)Ni O_(2)and can be denoted as Li-decorated 1T-type NiSe_(2).The Ni–Se–Ni ferromagnetic superexchange dominates the magnetic couplings between the Ni atoms,which can be understood in the frame of the Goodenough–Kanamori–Anderson(GKA)rules.Our systematic study of monolayer Li_(2)NiSe_(2)enables its promising applications in spintronics and suggests a new choice to design two-dimensional ferromagnetic semiconductors.

Enhanced Magnetic Interaction between Ga and Fe in Two-Dimensional van der Waals Ferromagnetic Crystal Fe_(3)GaTe_(2)

Fe_(3)GaTe_(2),a recently discovered van der Waals ferromagnetic crystal with the highest Curie temperature and strong perpendicular magnetic anisotropy among two-dimensional(2D)magnetic materials,has attracted significant attention and makes it a promising candidate for next-generation spintronic applications.Compared with Fe_(3)GeTe_(2),which has the similar crystal structure,the mechanism of the enhanced ferromagnetic properties in Fe_(3)GaTe_(2)is still unclear and needs to be investigated.Here,by using x-ray magnetic circular dichroism measurements,we find that both Ga and Te atoms contribute to the total magnetic moment of the system with antiferromagnetic coupling to Fe atoms.Our first-principles calculations reveal that Fe_(3)GaTe_(2)has van Hove singularities at the Fermi level in nonmagnetic state,resulting in the magnetic instability of the system and susceptibility to magnetic phase transitions.In addition,the calculation results about the density of states in ferromagnetic states of two materials suggest that the exchange interaction between Fe atoms is strengthened by replacing Ge atoms with Ga atoms.These findings indicate the increase of both the itinerate and local moments in Fe_(3)GaTe_(2)in view of Stoner and exchange interaction models,which results in the enhancement of the overall magnetism and a higher Curie temperature.Our work provides insight into the underlying mechanism of Fe_(3)GaTe_(2)’s remarkable magnetic properties and has important implications for searching 2D materials with expected magnetic properties in the future.

Recent Advances in Pure-Organic Host-Guest Room-Temperature Phosphorescence Systems Toward Bioimaging

Organic room-temperature phosphorescence(RTP)materials have garnered considerable attention in the fields of biosensing,optoelectronic devices,and anticounterfeiting because of their substantial Stokes shifts,tunable emission wavelengths,and prolonged lifetimes.These materials offer remarkable advantages for biological imaging applications by effectively reducing environmental autofluorescence and enhancing imaging resolution.Recently,host-guest systems have been employed as efficient approaches to fabricate pure-organic RTP materials for bioimaging,providing benefits such as controllable preparation and flexible modulation.Consequently,an increasing number of corresponding studies are being reported;however,a comprehensive systematic review is still lacking.Therefore,we summarize recent advances in the development of pureorganic RTP materials using host-guest systems with regard to bioimaging,including rigid matrices and sensitization.The challenge and potential of RTP for biological imaging are also proposed to promote the biomedical applications of organic RTP materials with excellent optical properties.

Lattice thermal conductivity switching via structural phase transition in ferromagnetic VI_(3)

The realization of reversible thermal conductivity through ferromagnetic ordering can improve the heat management and energy efficiency in magnetic materials-based devices.VI_(3),as a new layered ferromagnetic semiconductor,exhibits a structural phase transition from monoclinic(C2/m)to rhombohedral(R3^(-))phase as temperature decreases,making it a suitable platform to investigate thermal switching in magnetic phase transition materials.This work reveals that the thermal switching ratio of VI_(3)can reach 3.9 along the a-axis.Mechanical properties analysis indicates that the C2/m structure is stiffer than the R^(-)one,causing the larger phonon velocity in C2/m phase.Moreover,due to the fewer phonon branches in C2/m phase,the number of phonon–phonon scattering channels in C2/m phase is smaller compared to that of R^(-)phase.Both the larger phonon velocity and the longer phonon lifetime lead to larger lattice thermal conductivity in C2/m phase.This study uncovers the mechanical and thermal properties of VI_(3),which provides useful guides for designing magnetic materials-based devices such as thermal switch.

Prediction of one-dimensional CrN nanostructure as a promising ferromagnetic half-metal

Searching for one-dimensional(1D)nanostructure with ferromagnetic(FM)half-metallicity is of significance for the development of miniature spintronic devices.Here,based on the first-principles calculations,we propose that the 1D CrN nanostructure is a FM half-metal,which can generate the fully spin-polarized current.The ab initio molecular dynamic simulation and the phonon spectrum calculation demonstrate that the 1D CrN nanostructure is thermodynamically stable.The partially occupied Cr-d orbitals endow the nanostructure with FM half-metallicity,in which the half-metallic gap(?s)reaches up to 1.58 eV.The ferromagnetism in the nanostructure is attributed to the superexchange interaction between the magnetic Cr atoms,and a sizable magnetocrystalline anisotropy energy(MAE)is obtained.Moreover,the transverse stretching of nanostructure can effectively modulate?s and MAE,accompanied by the preservation of half-metallicity.A nanocable is designed by encapsulating the CrN nanostructure with a BN nanotube,and the intriguing magnetic and electronic properties of the nanostructure are retained.These novel characteristics render the 1D CrN nanostructure as a compelling candidate for exploiting high-performance spintronic devices.