就Bethuel,Brezis和Helein提出的问题讨论了Planar Ferromagnets and Antiferromagnets泛函在H={u(x)=(sinf(r)|xx|,cosf(r))∈H1(B1,S2);f(0)=0,f(1)=2π,r=|x|}中的径向极小元的一些性质,其中包括此泛函的径向极小元的零点的分布及若...就Bethuel,Brezis和Helein提出的问题讨论了Planar Ferromagnets and Antiferromagnets泛函在H={u(x)=(sinf(r)|xx|,cosf(r))∈H1(B1,S2);f(0)=0,f(1)=2π,r=|x|}中的径向极小元的一些性质,其中包括此泛函的径向极小元的零点的分布及若干个上界估计,并给出了这一问题的肯定回答.展开更多
Berry curvature describes the intrinsic geometric property of electronic band structure,crucial for governing emergent transport phenomena.As a typical Berry-curvature-related property,the anomalous Nernst effect prob...Berry curvature describes the intrinsic geometric property of electronic band structure,crucial for governing emergent transport phenomena.As a typical Berry-curvature-related property,the anomalous Nernst effect probes local Berry curvature near the Fermi surface,whereas the anomalous Hall effect integrates contributions across all occupied states.Thus,the anomalous Nernst effect is a superior probe for detecting subtle evolution of Berry curvature near the Fermi level;however,their relation remains elusive.Here,we demonstrate giant anomalous Nernst angles induced by Berry curvature in layered itinerant ferromagnets Fe_(3)GaTe_(2)and Fe_(5)GeTe_(2).Their maximum values(≈0.33 for Fe_(3)GaTe_(2)and≈0.41 for Fe_(5)GeTe_(2))are one order of magnitude larger than those of traditional ferromagnets(θ_(AN)^(max)<0.02).Scaling analysis of anomalous Hall effect in these two systems further suggests these giant angles originate from intrinsic Berry curvature.These findings indicate Berrycurvature-dominated regimes,and establish these materials for high-performance spin-caloritronic devices.展开更多
Disordered ferromagnets with a domain structure that exhibit a hysteresis loop when driven by the external magnetic field are essential materials for modern technological applications.Therefore,the understanding and p...Disordered ferromagnets with a domain structure that exhibit a hysteresis loop when driven by the external magnetic field are essential materials for modern technological applications.Therefore,the understanding and potential for controlling the hysteresis phenomenon in thesematerials,especially concerning the disorder-induced critical behavior on the hysteresis loop,have attracted significant experimental,theoretical,and numerical research efforts.We review the challenges of the numerical modeling of physical phenomena behind the hysteresis loop critical behavior in disordered ferromagnetic systems related to the non-equilibriumstochastic dynamics of domain walls driven by external fields.Specifically,using the extended Random Field Ising Model,we present different simulation approaches and advanced numerical techniques that adequately describe the hysteresis loop shapes and the collective nature of the magnetization fluctuations associated with the criticality of the hysteresis loop for different sample shapes and varied parameters of disorder and rate of change of the external field,as well as the influence of thermal fluctuations and demagnetizing fields.The studied examples demonstrate how these numerical approaches reveal newphysical insights,providing quantitativemeasures of pertinent variables extracted from the systems’simulated or experimentally measured Barkhausen noise signals.The described computational techniques using inherent scale-invariance can be applied to the analysis of various complex systems,both quantum and classical,exhibiting non-equilibrium dynamical critical point or self-organized criticality.展开更多
On the basis of a generalized SSH model, an organic polymer ferromagnet theory is proposed at the finite temperature in the self-consistent mean field approximation, and the specific heat and charge density of the qua...On the basis of a generalized SSH model, an organic polymer ferromagnet theory is proposed at the finite temperature in the self-consistent mean field approximation, and the specific heat and charge density of the quasione-dimensional interehain coupling organic ferromagnets are presented. We find that an obvious feature is to present itself the round peak for the specific heat with the temperature. This indicates unambiguously the presence of the phase transition in the system. The transition temperature plays down with increasing of the interchain coupling t2 or decreasing of the electron repulsion u. The curves of charge density with the temperature debase monotonously. This result illustrates that the higher the temperature is, the more electrons are excited.展开更多
Several rocksalt Sr4X3N （X = O, S, Se, and Te） are predicted to be potential half-metallic ferromagnets free of transition-metal and rare-earth elements by performing the first-principles calculations. Then their ma...Several rocksalt Sr4X3N （X = O, S, Se, and Te） are predicted to be potential half-metallic ferromagnets free of transition-metal and rare-earth elements by performing the first-principles calculations. Then their magnetic properties, such as the half metallicity and the crystal-cell magnetic moments are investigated. The Sr4X3N possibly have higher Curie temperatures and have more stable half metallicity than the Sr4X3C. Their crystal-cell magnetic moments are all 1.00 μB. The crystal-cell magnetic moments and the half metallicity arise mainly from the N ions. The main mechanism is the strong covalent interaction leading to the sp2 hybridized orbitals in the Sr4X3N. Then two Sr-5s and three N-2p electrons enter into three sp2 hybridized orbitals. Among these five electrons, four electrons are paired and one is unpaired, so there are three spin-up electrons and two spin-down electrons in these sp2 hybridized orbitals.展开更多
The geometrical structures of wurtzite CrX （X=As, Sb, O, Se, and Te） were optimized, then their electric and magnetic properties were investigated by the first-principle calculations within the generalized gradient ...The geometrical structures of wurtzite CrX （X=As, Sb, O, Se, and Te） were optimized, then their electric and magnetic properties were investigated by the first-principle calculations within the generalized gradient approximation for the exchange-correlation functional based on the density functional theory. These Cr-phosphides and Cr-sulphides were predicted to be half-metallic ferromagnets whose spin-polarization at the Fermi level is absolutely 100%. The molecular magnetic moments of Cr-phosphides and Cr-sulphides are 3.00 and 4.00 μB, which arise mainly from Cr-ions, respectively. There is ferromagnetic coupling in both Cr- phosphides and Cr-sulphides. The Curie temperatures of Cr-sulphides and Cr-phosphides are high. The electronic structures of Cr-ions are a1g^2↑↓t1u^4↑↓t1u^1↑↓eg^2↑↓in Cr-phosphides and a1g^2↑↓t1u^4↑↓t1u^1↑t2g^3↑in Cr-sulphides, respectively.展开更多
Considering the attractive interaction between ferromagnet, we propose the model of magnon-pairs, which two magnons with opposite wave vectors in a Heisenberg is suitable for low-temperature environment. A dressed mag...Considering the attractive interaction between ferromagnet, we propose the model of magnon-pairs, which two magnons with opposite wave vectors in a Heisenberg is suitable for low-temperature environment. A dressed magnon is an energy quantum of the magnon-pairs whose energy is a monotonically increasing function of absolute temperature. Based on the model, we re-investigate the excitation mechanism and thermodynamic properties of the Heisenberg ferromagnet. The correction factor e（O） plays an important role in studying the low-temperature properties of a ferromagnet.展开更多
Owing to their charge-free property,magnons are highly promising for achieving dissipationless transport without Joule heating,and are thus potentially applicable to energy-efficient devices.Here,we investigate valley...Owing to their charge-free property,magnons are highly promising for achieving dissipationless transport without Joule heating,and are thus potentially applicable to energy-efficient devices.Here,we investigate valley magnons and associated valley modulations in a kagome ferromagnetic lattice with staggered exchange interaction and Dzyaloshinskii-Moriya interaction.The staggered exchange interaction breaks the spatial inversion symmetry,leading to a valley magnon Hall effect.With nonzero Dzyaloshinskii-Moriya interaction in a staggered kagome lattice,the magnon Hall effect can be observed from only one valley.Moreover,reversing the Dzyaloshinskii-Moriya interaction(D→-D)and exchanging J_(1)and J_(2)(J_(1)■J_(2))can also regulate the position of the unequal valleys.With increasing Dzyaloshinskii-Moriya interaction,a series of topological phase transitions appear when two bands come to touch and split at the valleys.The valley Hall effect and topological phase transitions observed in kagome magnon lattices can be realized in thin films of insulating ferromagnets such as Lu_(2)V_(2)O_(7),and will extend the basis for magnonics applications in the future.展开更多
Two kinds of topological soliton （skyrmion and magnetic vortex ring） in ferromagnets are studied. They have the common topological origin, a tensor Hαβ = n→·（δαn→×βδn→）, which describes the non-...Two kinds of topological soliton （skyrmion and magnetic vortex ring） in ferromagnets are studied. They have the common topological origin, a tensor Hαβ = n→·（δαn→×βδn→）, which describes the non-trivial distribution of local orientation of magnetization n→ at large distances in space. The topological stability of skyrmion is protected by the winding number. Knot-like topological defect as magnetic vortex rings is also studied. On the assumption that magnetic vortex rings are geometric lines, we present their δ-function distribution in ferromagnetic materials. Furthermore, it is briefly shown that Hopf invariant is a proper topological invariant to describe the topology of magnetic vortex rings.展开更多
A micromechanics-based finite element model for the constitutive behavior of polycrystalline ferromagnets is developed. In the model, the polycrystalline solid is assumed to comprise numerous single crystals with rand...A micromechanics-based finite element model for the constitutive behavior of polycrystalline ferromagnets is developed. In the model, the polycrystalline solid is assumed to comprise numerous single crystals with randomly distributed crystallographic orientations, and the single crystals, in turn, consist of ferromagnetic domains, each of which is represented by a cubic element. The dipole directions of the domains are randomly assigned to simulate the crystallographic nature of ferromagnetic polycrystals. A switching criterion for the domains is specified at the microscopic level. The macroscopic constitutive behavior is obtained by averaging the microscopic/local behavior of each domain. The developed model has been applied to the simulation of a ferromagnetic material. With appropriate material parameters adopted, hysteresis loops of the predicted magnetic induction versus magnetic field and those of the strain versus magnetic field are shown to agree well with experimental observations.展开更多
Two kinds of alignment of two neighboring organic ferromagnetic chains are studied with Su-Schrieffer-Heeger Hamiltonian added by Hubbard repulsion and antiferromagnetic correlation between side radicals and main chai...Two kinds of alignment of two neighboring organic ferromagnetic chains are studied with Su-Schrieffer-Heeger Hamiltonian added by Hubbard repulsion and antiferromagnetic correlation between side radicals and main chain.The results show that interchain coupling makes the energy gap decreasing.The out-of^phase alignment of two chains is energetically preferred to the in-phase alignment and it is a more possible ferromagnetic structure if the oscillatory part of interchain coupling of the in-phase is small.The spin density and dimerization along the main chain are also discussed.展开更多
Nowadays the studies of organize ferromagnets have been becoming a very attractive area of material science in both theoretical and experimental aspects. A major theoretical problem on design of OFM is to estimate the...Nowadays the studies of organize ferromagnets have been becoming a very attractive area of material science in both theoretical and experimental aspects. A major theoretical problem on design of OFM is to estimate the effective exchange integrals(J) between radical sites. However, most of theoretical work was on hypothetical hydrocarbons, which were impracticable in the synthesis. The recent breakthrough in the synthesis of OFM turns the theorist’s attention to some more realistic models such as those展开更多
By making use of the C-mapping topological current theory and the decomposition of gauge potential theory, we investigate the （2＋1）-dimensional skyrmion excitations in ferromagnets. We also discuss the branch proce...By making use of the C-mapping topological current theory and the decomposition of gauge potential theory, we investigate the （2＋1）-dimensional skyrmion excitations in ferromagnets. We also discuss the branch processes of these skyrmions and the generation and annihilation of skyrmion-antiskyrmion pairs.展开更多
The role of magnetoelastic coupling effects in nanocrystalline ferromagnets is investigated by means of high-field magnetization and Doppler-broadening spectrum measurements. For the nanocrystalline Fe73.5 Cu1Nb3-Si13...The role of magnetoelastic coupling effects in nanocrystalline ferromagnets is investigated by means of high-field magnetization and Doppler-broadening spectrum measurements. For the nanocrystalline Fe73.5 Cu1Nb3-Si13 5B9 alloy, the results show that the pinning effects resulting from the quasidislocation dipole intensely influence the movement of domain wall; by coupling with the magnetostriction the defects-induced stress fields determine the magnetic properties at the early stage of crystallization. In view of the effective anisotropy and magnetoelastic coupling energy the optimal annealing conditions of alloys are discussed.展开更多
The role of effective anisotropy in nanocrystalline ferromagnets is investigated. These alloys are prepared by annealing amorphous ribbons and have excellent soft magnetic properties. A two-phase model is established ...The role of effective anisotropy in nanocrystalline ferromagnets is investigated. These alloys are prepared by annealing amorphous ribbons and have excellent soft magnetic properties. A two-phase model is established considering the role of the mtergranular amorphous phase. The results indicate a strong dependence of effective anisotropy on the structure and magnetic parameters of the amorphous phase as well as on the size of a grains. In view of the new model, the magnetic hardening beyond the optimally annealing temperature seems to be ascribed to the de-terioration in magnetic properties of interfacial amorphous phase.展开更多
Optical control of magnons in two-dimensional(2D)materials promises new functionalities for spintronics and magnonics in atomically thin devices.Here,we report control of magnon dynamics,using laser polarization,in a ...Optical control of magnons in two-dimensional(2D)materials promises new functionalities for spintronics and magnonics in atomically thin devices.Here,we report control of magnon dynamics,using laser polarization,in a ferromagnetic van der Waals(vdW)material,Fe3.6Co1.4GeTe2.The magnon amplitude,frequency,and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy.We show substantial(over 25%)and continuous modulation of magnon dynamics as a function of incident laser polarization.Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption.This implies that pump laser pulses modify the local spin environment,which enables the launch of magnons with tunable dynamics.Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations.Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices.展开更多
There is growing interest in revealing exotic properties of collective spin excitations in kagome-lattice ferromagnets such as magnon Hall effects,topological magnon insulators,and flat magnon bands.Using the well-est...There is growing interest in revealing exotic properties of collective spin excitations in kagome-lattice ferromagnets such as magnon Hall effects,topological magnon insulators,and flat magnon bands.Using the well-established nearest-neighbor Heisenberg ferromagnet model with Dzyaloshinskii-Moriya interaction(DMI),in this study we uncover intriguing new aspects in the selectivity and topology of flat magnon bands.Among the three magnon bands(except for the top one,which is flat in the absence of DMI),we observe that each of the three bands can be selectively flattened at the critical DMI of D=±√3 J/3 and D=±√3 J.With a general DMI,the magnon bands become non-flat;however,there are nested lines that create a David star pattern for all three magnon bands whose flatness is robust during changing exchange coupling or DMIs.Contrary to prevailing belief,we show that each of the three flat bands is actually topologically trivial at critical DMIs.Furthermore,we show that while the middle band remains topologically trivial,for the other two bands,D=0 corresponds to the topological phase transition where their Chern numbers get interchanged;when D=±√3 J,the system undergoes a phase transition to the nonferromagnetic state.These central findings increase our understanding of spin excitations for future magnonics applications.展开更多
The magnetic coupling between the unpaired spin of nitroxyl group (>N--O) and itsneighbor sp^2 carbon atom has been studied in detail by UHF/6--31G and MP4/6-31G methods.and a number of conjugate nitroxyl diradical...The magnetic coupling between the unpaired spin of nitroxyl group (>N--O) and itsneighbor sp^2 carbon atom has been studied in detail by UHF/6--31G and MP4/6-31G methods.and a number of conjugate nitroxyl diradicals have also been studied by ab initio UHFmethod. A simple topological rule about the ferromagnetic coupling between two nitroxylgroups has been revealed by our calculation. Based on this rule as well as ab initio cal-culation, some nitroxyl radical models of organic ferromagnets (OFM) have been proposed,and the possible way of synthesis has also been suggested. Moreover, the ab initio UHF-crystal orbital (UHF--CO) method has been used to characterize the electronic band struc-ture of one of the models. The ab initio UHF--CO results show that this model is a verypromising candidate of OFM,展开更多
Two-dimension(2D)magnets have recently developed into a class of stoichiometric materials with prospective applications in ultra-compact spintronics and quantum computing.Their functionality is particularly rich when ...Two-dimension(2D)magnets have recently developed into a class of stoichiometric materials with prospective applications in ultra-compact spintronics and quantum computing.Their functionality is particularly rich when different magnetic orders are competing in the same material.Metalloxenes REX2(RE=Eu,Gd;X=Si,Ge),silicene or germanene—heavy counterparts of graphene—coupled with a layer of rare-earth metals,evolve from three-dimension(3D)antiferromagnets in multilayer structures to 2D ferromagnets in a few monolayers.This evolution,however,does not lead to fully saturated 2D ferromagnetism,pointing at a possibility of coexisting/competing magnetic states.Here,REX2 magnetism is explored with element-selective X-ray magnetic circular dichroism(XMCD).The measurements are carried out for GdSi2,EuSi2,GdGe2,and EuGe2 of different thicknesses down to 1 monolayer employing K absorption edges of Si and Ge as well as M and L edges of the rare-earths.They access the magnetic state in REX2 and determine the seat of magnetism,orbital,and spin contributions to the magnetic moment.High-field measurements probe remnants of the bulk antiferromagnetism in 2D REX2.The results provide a new platform for studies of complex magnetic structures in 2D materials.展开更多
Realizing ferromagnetic semiconductors with high Curie temperature TC is still a challenge in spintronics.Recent experiments have reported two-dimensional(2D)room temperature ferromagnetic metals,such as monolayer Cr_...Realizing ferromagnetic semiconductors with high Curie temperature TC is still a challenge in spintronics.Recent experiments have reported two-dimensional(2D)room temperature ferromagnetic metals,such as monolayer Cr_(3)Te_(6).In this paper,through density functional theory(DFT)calculations,we propose a method to obtain 2D high TC ferromagnetic semiconductors through element replacement in these ferromagnetic metals.We predict that monolayer(Cr_(4/6),Mo_(2/6))_(3)Te_(6),created via element replacement in monolayer Cr_(3)Te_(6),is a room-temperature ferromagnetic semiconductor exhibiting a band gap of 0.34 eV and a TC of 384 K.Our analysis reveals that the metal-to-semiconductor transition stems from the synergistic interplay of Mo-induced lattice distortion,which resolves band overlap,and the electronic contributions of Mo dopants,which further drive the formation of a distinct band gap.The origin of the high TC is traced to strong superexchange coupling between magnetic ions,analyzed via the superexchange model with DFT and Wannier function calculations.Considering the fast developments in fabrication and manipulation of 2D materials,our theoretical results propose an approach to explore high-temperature ferromagnetic semiconductors derived from experimentally obtained 2D high-temperature ferromagnetic metals through element replacement.展开更多
文摘就Bethuel,Brezis和Helein提出的问题讨论了Planar Ferromagnets and Antiferromagnets泛函在H={u(x)=(sinf(r)|xx|,cosf(r))∈H1(B1,S2);f(0)=0,f(1)=2π,r=|x|}中的径向极小元的一些性质,其中包括此泛函的径向极小元的零点的分布及若干个上界估计,并给出了这一问题的肯定回答.
基金supported by the National Key Research and Development Program of China(Grant Nos.2024YFA1408104,2021YFA1202901,and 2022YFA1204001)the National Natural Science Foundation of China(Grant Nos.92365203,U24A6002,52302180,and U21A2086)+1 种基金the Natural Science Foundation of Jiangsu Province(Grant No.BK20243011)the Hebei Natural Science Foundation No.E2023203002).
文摘Berry curvature describes the intrinsic geometric property of electronic band structure,crucial for governing emergent transport phenomena.As a typical Berry-curvature-related property,the anomalous Nernst effect probes local Berry curvature near the Fermi surface,whereas the anomalous Hall effect integrates contributions across all occupied states.Thus,the anomalous Nernst effect is a superior probe for detecting subtle evolution of Berry curvature near the Fermi level;however,their relation remains elusive.Here,we demonstrate giant anomalous Nernst angles induced by Berry curvature in layered itinerant ferromagnets Fe_(3)GaTe_(2)and Fe_(5)GeTe_(2).Their maximum values(≈0.33 for Fe_(3)GaTe_(2)and≈0.41 for Fe_(5)GeTe_(2))are one order of magnitude larger than those of traditional ferromagnets(θ_(AN)^(max)<0.02).Scaling analysis of anomalous Hall effect in these two systems further suggests these giant angles originate from intrinsic Berry curvature.These findings indicate Berrycurvature-dominated regimes,and establish these materials for high-performance spin-caloritronic devices.
基金Djordje Spasojevic and Svetislav Mijatovic acknowledge the support from the Ministry of Science,TechnologicalDevelopment and Innovation of the Republic of Serbia(Agreement No.451-03-65/2024-03/200162)S.J.ibid.(Agreement No.451-03-65/2024-03/200122)Bosiljka Tadic from the Slovenian Research Agency(program P1-0044).
文摘Disordered ferromagnets with a domain structure that exhibit a hysteresis loop when driven by the external magnetic field are essential materials for modern technological applications.Therefore,the understanding and potential for controlling the hysteresis phenomenon in thesematerials,especially concerning the disorder-induced critical behavior on the hysteresis loop,have attracted significant experimental,theoretical,and numerical research efforts.We review the challenges of the numerical modeling of physical phenomena behind the hysteresis loop critical behavior in disordered ferromagnetic systems related to the non-equilibriumstochastic dynamics of domain walls driven by external fields.Specifically,using the extended Random Field Ising Model,we present different simulation approaches and advanced numerical techniques that adequately describe the hysteresis loop shapes and the collective nature of the magnetization fluctuations associated with the criticality of the hysteresis loop for different sample shapes and varied parameters of disorder and rate of change of the external field,as well as the influence of thermal fluctuations and demagnetizing fields.The studied examples demonstrate how these numerical approaches reveal newphysical insights,providing quantitativemeasures of pertinent variables extracted from the systems’simulated or experimentally measured Barkhausen noise signals.The described computational techniques using inherent scale-invariance can be applied to the analysis of various complex systems,both quantum and classical,exhibiting non-equilibrium dynamical critical point or self-organized criticality.
基金National Natural Science Foundation of China under Grant Nos.10574047 and 20490210
文摘On the basis of a generalized SSH model, an organic polymer ferromagnet theory is proposed at the finite temperature in the self-consistent mean field approximation, and the specific heat and charge density of the quasione-dimensional interehain coupling organic ferromagnets are presented. We find that an obvious feature is to present itself the round peak for the specific heat with the temperature. This indicates unambiguously the presence of the phase transition in the system. The transition temperature plays down with increasing of the interchain coupling t2 or decreasing of the electron repulsion u. The curves of charge density with the temperature debase monotonously. This result illustrates that the higher the temperature is, the more electrons are excited.
基金Project supported by Chongqing Natural Science Foundation,China (Grant Nos.CSCT2010BB4405 and CSTC2008BB4083)the Doctoral Foundation of Chongqing University of Posts and Telecommunications,China(Grant No.A2008-63)
文摘Several rocksalt Sr4X3N （X = O, S, Se, and Te） are predicted to be potential half-metallic ferromagnets free of transition-metal and rare-earth elements by performing the first-principles calculations. Then their magnetic properties, such as the half metallicity and the crystal-cell magnetic moments are investigated. The Sr4X3N possibly have higher Curie temperatures and have more stable half metallicity than the Sr4X3C. Their crystal-cell magnetic moments are all 1.00 μB. The crystal-cell magnetic moments and the half metallicity arise mainly from the N ions. The main mechanism is the strong covalent interaction leading to the sp2 hybridized orbitals in the Sr4X3N. Then two Sr-5s and three N-2p electrons enter into three sp2 hybridized orbitals. Among these five electrons, four electrons are paired and one is unpaired, so there are three spin-up electrons and two spin-down electrons in these sp2 hybridized orbitals.
基金ACKNOWLEDGMENTS This work was supported by the Chongqing Natural Science Foundation （No.CSTC2007BB4391 and No.CSTC2008BB4083） and the Chongqing Science and Technology Foundation （No.kj060515 and No.kj080518）
文摘The geometrical structures of wurtzite CrX （X=As, Sb, O, Se, and Te） were optimized, then their electric and magnetic properties were investigated by the first-principle calculations within the generalized gradient approximation for the exchange-correlation functional based on the density functional theory. These Cr-phosphides and Cr-sulphides were predicted to be half-metallic ferromagnets whose spin-polarization at the Fermi level is absolutely 100%. The molecular magnetic moments of Cr-phosphides and Cr-sulphides are 3.00 and 4.00 μB, which arise mainly from Cr-ions, respectively. There is ferromagnetic coupling in both Cr- phosphides and Cr-sulphides. The Curie temperatures of Cr-sulphides and Cr-phosphides are high. The electronic structures of Cr-ions are a1g^2↑↓t1u^4↑↓t1u^1↑↓eg^2↑↓in Cr-phosphides and a1g^2↑↓t1u^4↑↓t1u^1↑t2g^3↑in Cr-sulphides, respectively.
基金Supported by the National Natural Science Foundation of China under Grant Nos. 10174024 and 10474025
文摘Considering the attractive interaction between ferromagnet, we propose the model of magnon-pairs, which two magnons with opposite wave vectors in a Heisenberg is suitable for low-temperature environment. A dressed magnon is an energy quantum of the magnon-pairs whose energy is a monotonically increasing function of absolute temperature. Based on the model, we re-investigate the excitation mechanism and thermodynamic properties of the Heisenberg ferromagnet. The correction factor e（O） plays an important role in studying the low-temperature properties of a ferromagnet.
基金support from the Funding for School-level Research Projects of Yancheng Institute of Technology(Grant Nos.xjr2020038,xjr2022039,and xjr2022040)。
文摘Owing to their charge-free property,magnons are highly promising for achieving dissipationless transport without Joule heating,and are thus potentially applicable to energy-efficient devices.Here,we investigate valley magnons and associated valley modulations in a kagome ferromagnetic lattice with staggered exchange interaction and Dzyaloshinskii-Moriya interaction.The staggered exchange interaction breaks the spatial inversion symmetry,leading to a valley magnon Hall effect.With nonzero Dzyaloshinskii-Moriya interaction in a staggered kagome lattice,the magnon Hall effect can be observed from only one valley.Moreover,reversing the Dzyaloshinskii-Moriya interaction(D→-D)and exchanging J_(1)and J_(2)(J_(1)■J_(2))can also regulate the position of the unequal valleys.With increasing Dzyaloshinskii-Moriya interaction,a series of topological phase transitions appear when two bands come to touch and split at the valleys.The valley Hall effect and topological phase transitions observed in kagome magnon lattices can be realized in thin films of insulating ferromagnets such as Lu_(2)V_(2)O_(7),and will extend the basis for magnonics applications in the future.
基金National Natural Science Foundation of China under Grant No.10275030
文摘Two kinds of topological soliton （skyrmion and magnetic vortex ring） in ferromagnets are studied. They have the common topological origin, a tensor Hαβ = n→·（δαn→×βδn→）, which describes the non-trivial distribution of local orientation of magnetization n→ at large distances in space. The topological stability of skyrmion is protected by the winding number. Knot-like topological defect as magnetic vortex rings is also studied. On the assumption that magnetic vortex rings are geometric lines, we present their δ-function distribution in ferromagnetic materials. Furthermore, it is briefly shown that Hopf invariant is a proper topological invariant to describe the topology of magnetic vortex rings.
基金The project supported by the National Natural Science Foundation of China (90205030. 10472088, 10425210), the National Basic Research Program of China (2006CB601202) and the State Administration of the Foreign Experts Affairs Through the "111" Project (
文摘A micromechanics-based finite element model for the constitutive behavior of polycrystalline ferromagnets is developed. In the model, the polycrystalline solid is assumed to comprise numerous single crystals with randomly distributed crystallographic orientations, and the single crystals, in turn, consist of ferromagnetic domains, each of which is represented by a cubic element. The dipole directions of the domains are randomly assigned to simulate the crystallographic nature of ferromagnetic polycrystals. A switching criterion for the domains is specified at the microscopic level. The macroscopic constitutive behavior is obtained by averaging the microscopic/local behavior of each domain. The developed model has been applied to the simulation of a ferromagnetic material. With appropriate material parameters adopted, hysteresis loops of the predicted magnetic induction versus magnetic field and those of the strain versus magnetic field are shown to agree well with experimental observations.
基金Supported by the National Natural Science Foundation of China under Grant No.19775016the Direct Grant for Research from the Research Grants Council of the Hong Kong Goverment.
文摘Two kinds of alignment of two neighboring organic ferromagnetic chains are studied with Su-Schrieffer-Heeger Hamiltonian added by Hubbard repulsion and antiferromagnetic correlation between side radicals and main chain.The results show that interchain coupling makes the energy gap decreasing.The out-of^phase alignment of two chains is energetically preferred to the in-phase alignment and it is a more possible ferromagnetic structure if the oscillatory part of interchain coupling of the in-phase is small.The spin density and dimerization along the main chain are also discussed.
文摘Nowadays the studies of organize ferromagnets have been becoming a very attractive area of material science in both theoretical and experimental aspects. A major theoretical problem on design of OFM is to estimate the effective exchange integrals(J) between radical sites. However, most of theoretical work was on hypothetical hydrocarbons, which were impracticable in the synthesis. The recent breakthrough in the synthesis of OFM turns the theorist’s attention to some more realistic models such as those
基金supported by National Natural Science Foundation of Chinathe Cuiying Programme of Lanzhou University
文摘By making use of the C-mapping topological current theory and the decomposition of gauge potential theory, we investigate the （2＋1）-dimensional skyrmion excitations in ferromagnets. We also discuss the branch processes of these skyrmions and the generation and annihilation of skyrmion-antiskyrmion pairs.
基金Project supported by the Special Item Foundation for Doctor of Advanced College,Laboratory of Nuclear Analysis Technique,Chinese Academy of Sciences and Beijing Synchrotron Radiation Facility,National Laboratory.
文摘The role of magnetoelastic coupling effects in nanocrystalline ferromagnets is investigated by means of high-field magnetization and Doppler-broadening spectrum measurements. For the nanocrystalline Fe73.5 Cu1Nb3-Si13 5B9 alloy, the results show that the pinning effects resulting from the quasidislocation dipole intensely influence the movement of domain wall; by coupling with the magnetostriction the defects-induced stress fields determine the magnetic properties at the early stage of crystallization. In view of the effective anisotropy and magnetoelastic coupling energy the optimal annealing conditions of alloys are discussed.
基金Project supported by the Special Item Foundation for Doctor of Advanced College,the Chinese Academy of Sciences and Beijing Synchroiron Radiation Facility,National Laboratory
文摘The role of effective anisotropy in nanocrystalline ferromagnets is investigated. These alloys are prepared by annealing amorphous ribbons and have excellent soft magnetic properties. A two-phase model is established considering the role of the mtergranular amorphous phase. The results indicate a strong dependence of effective anisotropy on the structure and magnetic parameters of the amorphous phase as well as on the size of a grains. In view of the new model, the magnetic hardening beyond the optimally annealing temperature seems to be ascribed to the de-terioration in magnetic properties of interfacial amorphous phase.
基金Research reported in this publication was supported in part by the NSF and SC EPSCoR/IDeA Program under NSF Award#OIA-1655740(GEAR CRP 20-GC02,23-GC01)and NSF Award No.2030128,2110033supported in part by the US Department of Energy,Office of Science,Office of Workforce Development for Teachers and Scientists(WDTS)under the Visiting Faculty Program(VFP)+1 种基金support from the Air Force Office of Scientific Research under Award No.FA9550-22-1-0349 and National Science Foundation under Award No.DMR-2326944 and No.DMR-2340773the support from the program of Educational Department of Liaoning Province(grant no.LQGD2020008).
文摘Optical control of magnons in two-dimensional(2D)materials promises new functionalities for spintronics and magnonics in atomically thin devices.Here,we report control of magnon dynamics,using laser polarization,in a ferromagnetic van der Waals(vdW)material,Fe3.6Co1.4GeTe2.The magnon amplitude,frequency,and lifetime are controlled and monitored by time-resolved pump-probe spectroscopy.We show substantial(over 25%)and continuous modulation of magnon dynamics as a function of incident laser polarization.Our results suggest that the modification of the effective demagnetization field and magnetic anisotropy by the pump laser pulses with different polarizations is due to anisotropic optical absorption.This implies that pump laser pulses modify the local spin environment,which enables the launch of magnons with tunable dynamics.Our first-principles calculations confirm the anisotropic optical absorption of different crystal orientations.Our findings suggest a new route for the development of opto-spintronic or opto-magnonic devices.
基金supported by the National Natural Science Foundation of China(Grant Nos.11890703,11574154,61434002,11704191,and11634011)the National Program on Key Basic Research Project(Grant No.2014CB921103)the National Key Research and Development Program of China(Grant No.2017YFA0303500)。
文摘There is growing interest in revealing exotic properties of collective spin excitations in kagome-lattice ferromagnets such as magnon Hall effects,topological magnon insulators,and flat magnon bands.Using the well-established nearest-neighbor Heisenberg ferromagnet model with Dzyaloshinskii-Moriya interaction(DMI),in this study we uncover intriguing new aspects in the selectivity and topology of flat magnon bands.Among the three magnon bands(except for the top one,which is flat in the absence of DMI),we observe that each of the three bands can be selectively flattened at the critical DMI of D=±√3 J/3 and D=±√3 J.With a general DMI,the magnon bands become non-flat;however,there are nested lines that create a David star pattern for all three magnon bands whose flatness is robust during changing exchange coupling or DMIs.Contrary to prevailing belief,we show that each of the three flat bands is actually topologically trivial at critical DMIs.Furthermore,we show that while the middle band remains topologically trivial,for the other two bands,D=0 corresponds to the topological phase transition where their Chern numbers get interchanged;when D=±√3 J,the system undergoes a phase transition to the nonferromagnetic state.These central findings increase our understanding of spin excitations for future magnonics applications.
文摘The magnetic coupling between the unpaired spin of nitroxyl group (>N--O) and itsneighbor sp^2 carbon atom has been studied in detail by UHF/6--31G and MP4/6-31G methods.and a number of conjugate nitroxyl diradicals have also been studied by ab initio UHFmethod. A simple topological rule about the ferromagnetic coupling between two nitroxylgroups has been revealed by our calculation. Based on this rule as well as ab initio cal-culation, some nitroxyl radical models of organic ferromagnets (OFM) have been proposed,and the possible way of synthesis has also been suggested. Moreover, the ab initio UHF-crystal orbital (UHF--CO) method has been used to characterize the electronic band struc-ture of one of the models. The ab initio UHF--CO results show that this model is a verypromising candidate of OFM,
基金This work was supported by National Research Center(NRC)“Kurchatov Institute”(No.1359,characterization)the Russian Science Foundation(No.19-19-00009(synthesis)and No.20-79-10028(magnetization measurements))。
文摘Two-dimension(2D)magnets have recently developed into a class of stoichiometric materials with prospective applications in ultra-compact spintronics and quantum computing.Their functionality is particularly rich when different magnetic orders are competing in the same material.Metalloxenes REX2(RE=Eu,Gd;X=Si,Ge),silicene or germanene—heavy counterparts of graphene—coupled with a layer of rare-earth metals,evolve from three-dimension(3D)antiferromagnets in multilayer structures to 2D ferromagnets in a few monolayers.This evolution,however,does not lead to fully saturated 2D ferromagnetism,pointing at a possibility of coexisting/competing magnetic states.Here,REX2 magnetism is explored with element-selective X-ray magnetic circular dichroism(XMCD).The measurements are carried out for GdSi2,EuSi2,GdGe2,and EuGe2 of different thicknesses down to 1 monolayer employing K absorption edges of Si and Ge as well as M and L edges of the rare-earths.They access the magnetic state in REX2 and determine the seat of magnetism,orbital,and spin contributions to the magnetic moment.High-field measurements probe remnants of the bulk antiferromagnetism in 2D REX2.The results provide a new platform for studies of complex magnetic structures in 2D materials.
基金supported by the National Key R&D Program of China(Grant No.2022YFA1405100)Chinese Academy of Sciences Project for Young Scientists in Basic Research(Grant No.YSBR-030)+3 种基金the Basic Research Program of the Chinese Academy of Sciences Based on Major Scientific Infrastructures(Grant No.JZHKYPT-2021-08)GS was supported in part by the Innovation Program for Quantum Science and Technology(Grant No.2024ZD03005)the National Natural Science Foundation of China(Grant No.12447101)Chinese Academy of Sciences.
文摘Realizing ferromagnetic semiconductors with high Curie temperature TC is still a challenge in spintronics.Recent experiments have reported two-dimensional(2D)room temperature ferromagnetic metals,such as monolayer Cr_(3)Te_(6).In this paper,through density functional theory(DFT)calculations,we propose a method to obtain 2D high TC ferromagnetic semiconductors through element replacement in these ferromagnetic metals.We predict that monolayer(Cr_(4/6),Mo_(2/6))_(3)Te_(6),created via element replacement in monolayer Cr_(3)Te_(6),is a room-temperature ferromagnetic semiconductor exhibiting a band gap of 0.34 eV and a TC of 384 K.Our analysis reveals that the metal-to-semiconductor transition stems from the synergistic interplay of Mo-induced lattice distortion,which resolves band overlap,and the electronic contributions of Mo dopants,which further drive the formation of a distinct band gap.The origin of the high TC is traced to strong superexchange coupling between magnetic ions,analyzed via the superexchange model with DFT and Wannier function calculations.Considering the fast developments in fabrication and manipulation of 2D materials,our theoretical results propose an approach to explore high-temperature ferromagnetic semiconductors derived from experimentally obtained 2D high-temperature ferromagnetic metals through element replacement.
