Magnetic Atoms Research Articles

Variation of magnetism by defects and impurities in two-dimensional magnetic materials based on spin spiral method: A case of δ-(Ga, Mn)As

There is still no effective solution of first-principles calculations to the critical question of how defects and impurities affect magnetism, especially when multi-nearest-neighbor magnetic interactions are involved. In this paper, we use the Heisenberg model and the generalized Bloch condition to evaluate the magnetic interactions of magnetic atoms in various environments. By calculating the dispersion relationship E(q) between the energy E and the wave vector q of the spin spiral in δ-(Ga, Mn)As, the effective multi-nearest neighbor interaction parameters Jxi of Mnx are fitted (x means different types of Mn atoms close to or far away from defects or impurities). The Exi(q) of each kind of Mn sublattice is constructed then with Jxi, and the Curie temperature of each sublattice is obtained by mean field approximation (MFA) and random phase approximation (RPA), respectively. Defect is detrimental to the FM properties at both low and high temperature, while Ga impurity improves FM properties at low temperature by increasing the magnetic stiffness of δ-(Ga, Mn)As.

Theoretical study of the magnetic properties of the CoCu2O3 compound

In this paper we present a theoretical study of the magnetic properties of the ${\mathrm{CoCu}}_{2}{\mathrm{O}}_{3}$ compound. The magnetic effective exchange interactions and zeroth-field splitting were computed using ab initio methods, then the magnetic order and transition temperature were determined using classical Monte Carlo simulations. We showed that, unlike other members of the $A{\mathrm{Cu}}_{2}{\mathrm{O}}_{3}$ family, the presence of an additional magnetic atom, associated with a large folding of the puckered layers in the ($\stackrel{P\vec}{a}, \stackrel{P\vec}{b}$) directions, induces a magnetic pattern based on coupled three-leg ladders, quite different from the two-leg structural ladders. The propagation vector has been found to be $\stackrel{P\vec}{q}=(0,\frac{1}{2},\frac{1}{2})$. It is associated with a doubly degenerated ground state suggesting a doubling of the unit cell. The large ${\mathrm{Co}}^{2+}$-ion anisotropy was shown to be of crucial importance in the high transition temperature observed in this compound.

Static and dynamic magnetic properties in two-dimensional Lieb-like lattice

• Mixed spin Ising model was proposed to describe two-dimensional Lieb-like lattice. • Influence of the periodically oscillating magnetic field on the magnetism of two-dimensional Lieb-like lattice have been investigated. • Multiple compensation temperatures were discovered. The phase diagram and magnetization of a two-dimensional Lieb-like lattice consisting of magnetic atoms with mixed spins 5/2, 3/2, and 2 were investigated based on the effective-field theory with correlations. They exhibited a compensation point below their phase transition temperatures, reentrant behavior, and depressed saturation magnetization within typical parameters of exchange couplings and anisotropies. In addition, the influence of a periodically oscillating magnetic field on the dynamic order parameters of a two-dimensional Lieb-like lattice is discussed. These results can help further understand two-dimensional materials' static and dynamic magnetic properties. Ising model of two-dimensional Lieb lattice is constructured and the Static and dynamic magnetic properties have been investigated for the system.

The Progress and Potential Applications of Quantum Anomalous Hall Effect

Quantum Hall effect is a very common research direction in recent years, many scientists have carried out in-depth research and experiments in this field. In this paper, the advancements and applications of the quantum anomalous Hall effect(QAHE) are being presented and discussed. Primarily, the principle and development of QAHE are analyzed. The QAHE is obtained by suppressing one of the edge states based on the quantum spin Hall effect(QSHE). The main experimental method is to add hybrid magnetic atoms in topological insulators. Since first successfully observed the phenomenon in 2013 by a Chinese group, multiple studies and experiments have been carried out on this kind of theory, and many equally important results have been obtained. Moreover, many application values and potentials of QAHE are explored. The research in this paper is helpful to deepen the understanding of the principle and experiment of QAHE and offer a guideline for further exploration.

Effect of transition metals on the crystal field in CeCo0.4Fe0.6Ge3

We present an investigation of the influence of the 3d elements, Co and Fe, on the interactions between magnetic Ce atoms in the CeCo0.4Fe0.6Ge3 alloy, the member of the CeCo1−xFexGe3 series. The investigated alloy crystallizes in the noncentrosymmetric tetragonal BaNiSn3-type structure (space group I4mm, No. 107), in which the Co and Fe atoms occupy the same 2a position. The close vicinity to the quantum critical point for this alloy was indicated based on previous reports and confirmed by our low temperature resistivity measurements presented in this article. The role of Co and Fe atoms in this alloy was deduced from the scheme of the crystal field levels employing inelastic neutron scattering, specific heat, and magnetic susceptibility. For the analysis, samples LaCoGe3, LaFeGe3, and LaCo0.4Fe0.6Ge3 were used as non-magnetic analogs. The obtained results for CeCo0.4Fe0.6Ge3 alloy were compared with those for CeCoGe3 and CeFeGe3 compounds to emphasize the observed changes in physical properties. The specific heat results are supported by the first-principles calculations. The analysis indicated that the excitation levels of Ce ions are different for the Co- and Fe-rich neighborhood. Based on this information, we can conclude that the 3d elements provide the same energies of RKKY and Kondo interactions for the nearest Ce ions, but their energies for the further ions are lowered due to the presence of other 3d elements.

Single Metal Atoms on Oxide Surfaces: Assessing the Chemical Bond through 17O Electron Paramagnetic Resonance.

ConspectusEven in the gas phase single atoms possess catalytic properties, which can be crucially enhanced and modulated by the chemical interaction with a solid support. This effect, known as electronic metal-support interaction, encompasses charge transfer, orbital overlap, coordination structure, etc., in other words, all the crucial features of the chemical bond. These very features are the object of this Account, with specific reference to open-shell (paramagnetic) single metal atoms or ions on oxide supports. Such atomically dispersed species are part of the emerging class of heterogeneous catalysts known as single-atom catalysts (SACs). In these materials, atomic dispersion ensures maximum atom utilization and uniform active sites, whereby the nature of the chemical interaction between the metal and the oxide surface modulates the catalytic activity of the metal active site by tuning the energy of the frontier orbitals. A comprehensive set of examples includes fourth period metal atoms and ions in zeolites on insulating (e.g., MgO) or reducible (e.g., TiO2) oxides and are among the most relevant catalysts for a wealth of key processes of industrial and environmental relevance, from the abatement of NOx to the selective oxidation of hydrocarbons and the conversion of methane to methanol.There exist several spectroscopic techniques able to inform on the geometric and electronic structure of isolated single metal ion sites, but either they yield information averaged over the bulk or they lack description of the intimate features of chemical bonding, which include covalency, ionicity, electron and spin delocalization. All of these can be recovered at once by measuring the magnetic interactions between open-shell metals and the surrounding nuclei with Electron Paramagnetic Resonance (EPR) spectroscopy. In the case of oxides, this entails the synthesis of 17O isotopically enriched materials. We have established 17O EPR as a unique source of information about the local binding environment around oxygen of magnetic atoms or ions on different oxidic supports to rationalize structure-property relationships. Here, we will describe strategies for 17O surface enrichments and approaches to monitor the state of charge and spin delocalization of atoms or ions from K to Zn dispersed on oxide surfaces characterized by different chemical properties (i.e., basicity or reducibility). Emphasis is placed on chemical insight at the atomic-scale level achieved by 17O EPR, which is a crucial step in understanding the structure-property relationships of single metal atom catalysts and in enabling efficient design of future materials for a range of end uses.

Ferromagnetic Layers in a Topological Insulator (Bi,Sb)2Te3 Crystal Doped with Mn.

Magnetic topological insulators (MTIs) have recently become a subject of poignant interest; among them, Z2 topological insulators with magnetic moment ordering caused by embedded magnetic atoms attract special attention. In such systems, the case of magnetic anisotropy perpendicular to the surface that holds a topologically nontrivial surface state is the most intriguing one. Such materials demonstrate the quantum anomalous Hall effect, which manifests itself as chiral edge conduction channels that can be manipulated by switching the polarization of magnetic domains. In the present paper, we uncover the atomic structure of the bulk and the surface of Mn0.06Sb1.22Bi0.78Te3.06 in conjunction with its electronic and magnetic properties; this material is characterized by naturally formed ferromagnetic layers inside the insulating matrix, where the Fermi level is tuned to the bulk band gap. We found that in such mixed crystals septuple layers (SLs) of Mn(Bi,Sb)2Te4 form structures that feature three SLs, each of which is separated by two or three (Bi,Sb)2Te3 quintuple layers (QLs); such a structure possesses ferromagnetic properties. The surface obtained by cleavage includes terraces with different terminations. Manganese atoms preferentially occupy the central positions in the SLs and in a very small proportion can appear in the QLs, as indirectly indicated by a reshaped Dirac cone.

Tracing magnetic atom diffusion with annealing at the interface between CoMn alloy and MnGa layer by X-ray magnetic circular dichroism

The magnetic atom diffusion at the interface between CoMn alloy and MnGa layer with annealing is studied using x-ray magnetic circular dichroism (XMCD) analysis. We found that the spins in bcc CoMn are coupled parallel to those in perpendicularly magnetized MnGa layer under the as-grown conditions, while the post-annealing modulates the interfacial magnetic coupling to antiferromagnetic in Co. The elementspecific hysteresis curves at each absorption edge revealed the large coercive fields in Mn and Co through the exchange coupling with MnGa. After the annealing process, the changes of XMCD spectral line shapes are related to the interfacial reactions promoting the formation of Mn and Co2MnGa layers, which is deduced from the analysis of transmission electron microscopy. The interfacial diffusion of Mn atoms modulates the magnetic exchange coupling between Mn and Co sites and reverses the direction of perpendicular magnetization.

Magnetic-atom strategy enables unilamellar MoS2-C interoverlapped superstructure with ultrahigh capacity and ultrafast ion transfer capability in Li/Na/K-ion batteries

Constructing a unilamellar MoS2-C interoverlapped superstructure (UIS) is the most promising way to improve electrical conductivity and alleviate volume expansion of MoS2 electrodes during Li+/Na+/K+ storage due to the maximized atomic interface contact. However, the interlayer distance of ∼ 0.48 nm between MoS2 and C (lower than 0.62 nm of pristine MoS2), the unconspicuous enhancement of intrinsic conductivity of MoS2, and the inevitable decrease in capacity due to the introduced low-capacity C undoubtedly hamper ion transport and storage, thus resulting in limited enhancement of capacity and fast-charging performances of UIS. Herein, we propose a magnetic-atom strategy for UIS via a one-step high-pressure vapor-phase synthesis method, during which the interlayer electrostatic repulsion is in-situ constructed by magnetic-atom Fe and/or Co doping to adjust the interlayer distance from 0.48 to 0.64 nm. In addition, the doped magnetic atom can regulate the electronic structure of UIS to obtain the bandgap of 0 eV to enhance electron transfer. Importantly, the doped magnetic atom can be reduced to superparamagnetic metallic nanoparticles during conversion reactions, which can store abundant spin-polarized electrons to induce strong surface-capacitance effects, thus boosting ion transport and storage. Consequently, the magnetic-atom strategy endows the UIS with ultrahigh reversible capacities of 1572.1/738.5/542.3 mAh/g at 0.1C, 971.2/383.5/209.8 mAh/g at 5C after 3000 cycles, and 761.5/340.8/204.5 mAh/g at 20C as Li/Na/K-ion-battery anodes, respectively. This work verifies the efficiency of magnetic-atom strategy and paves a way for the design of other transition metal dichalcogenides for electrochemical energy storage.

Observable tests for the light-sail scenario of interstellar objects

Context.Enigmatic dynamical and spectral properties of the first interstellar object (ISO), 1I/2017 U1 (Oumuamua), led to many hypotheses, including a suggestion that it may be an “artificial” spacecraft with a thin radiation-pressure-driven light sail. Since similar discoveries by forthcoming instruments, such as theVera C. Rubintelescope and the Chinese Space Station Telescope (CSST), are anticipated, a critical identification of key observable tests is warranted for the quantitative distinctions between various scenarios.Aims.We scrutinize the light-sail scenario by making comparisons between physical models and observational constraints. These analyses can be generalized for future surveys of ‘Oumuamua-like objects.Methods.The light sail goes through a drift in interstellar space due to the magnetic field and gas atoms, which poses challenges to the navigation system. When the light sail enters the inner Solar System, the sideways radiation pressure leads to a considerable non-radial displacement. The immensely high dimensional ratio and the tumbling motion could cause a light curve with an extremely large amplitude and could even make the light sail invisible from time to time. These observational features allow us to examine the light-sail scenario of interstellar objects.Results.The drift of the freely rotating light sail in the interstellar medium is ~100 au even if the travel distance is only 1 pc. The probability of the expected brightness modulation of the light sail matching with ‘Oumuamua’s observed variation amplitude (~2.5–3) is <1.5%. In addition, the probability of the tumbling light sail being visible (brighter thanV= 27) in all 55 observations spread over two months after discovery is 0.4%. Radiation pressure could cause a larger displacement normal to the orbital plane for a light sail than that for ‘Oumuamua. Also, the ratio of antisolar to sideways acceleration of ‘Oumuamua deviates from that of the light sail by ~1.5σ.Conclusions.We suggest that ‘Oumuamua is unlikely to be a light sail. The dynamics of an intruding light sail, if it exists, has distinct observational signatures, which can be quantitatively identified and analyzed with our methods in future surveys.

Two ultracold highly magnetic atoms in a one-dimensional harmonic trap

We theoretically investigate the properties of two interacting ultracold highly magnetic atoms trapped in a one-dimensional harmonic potential. The atoms interact via an anisotropic long-range dipole-dipole interaction, which in one dimension effectively can be modeled by the contact interaction. We investigate the interplay of the external magnetic field, the spin-spin interaction, and the trapping potential and how they affect the magnetization of the system. We show the role of indistinguishability and symmetries in the dynamics by studying the time evolution of the observables that could be measured experimentally. The presented model may depict the on-site interaction of the extended Hubbard models, therefore giving a better understanding of the fundamental building block of the respective many-body quantum simulators.

Experimental Determination of a Single Atom Ground State Orbital through Hyperfine Anisotropy.

Historically, electron spin resonance (ESR) has provided excellent insight into the electronic, magnetic, and chemical structure of samples hosting spin centers. In particular, the hyperfine interaction between the electron and the nuclear spins yields valuable structural information about these centers. In recent years, the combination of ESR and scanning tunneling microscopy (ESR-STM) has allowed to acquire such information about individual spin centers of magnetic atoms bound atop a surface, while additionally providing spatial information about the binding site. Here, we conduct a full angle-dependent investigation of the hyperfine splitting for individual hydrogenated titanium atoms on MgO/Ag(001) by measurements in a vector magnetic field. We observe strong anisotropy in both the g factor and the hyperfine tensor. Combining the results of the hyperfine splitting with the symmetry properties of the binding site obtained from STM images and a basic point charge model allows us to predict the shape of the electronic ground state configuration of the titanium atom. Relying on experimental values only, this method paves the way for a new protocol for electronic structure analysis for spin centers on surfaces.

Routes for the topological surface state energy gap modulation in antiferromagnetic MnBi2Te4

We have analyzed different factors responsible for changes in the Dirac gap in MnBi2Te4 and the routes that determine the possibilities of the purposeful gap modulation. It was shown that upon changing the surface van der Waals interval and surface spin–orbit coupling strength the topological surface states localization shifts between the surface septuple layers with opposite magnetizations, which leads to a nonmonotonic change in the Dirac gap size. The minimum in the Dirac gap corresponds to the point of changing the sign of the emerging exchange field. Moreover, we have shown that the Dirac gap can be effectively modulated by replacing magnetic Mn atoms in the surface layer with nonmagnetic ones or Bi and Te atoms with atoms of elements with a lower spin–orbit coupling that makes it possible to create synthetic layered topological systems with purposeful modification of the surface properties.

Second-order topological corner states in zigzag graphene nanoflake with different types of edge magnetic configurations

We study the energy spectrum and energy levels of the extended Kane-Mele model with magnetic atoms on their zigzag edges. It is demonstrated that the edges of ferromagnetism or antiferromagnetism are enough to break the time-reversal symmetry and host one-dimensional gapped edge states. Thus, a second-order topological phase transition could happen, which leads to the emergence of topological in-gap zero-dimensional corner states. We also prove that the in-gap corner states are robust against corner defects and magnetic disorder. Our proposal based on the edge antiferromagnetism shows that the high-order topological states can be realized, although the net magnetization of the material is zero. In addition, we discuss the influence of spin magnetization orientation on the degeneracy and energy of in-gap corner states.

The half-metallicity induced by non-magnetic adatoms on phosphorene nanoribbons

Phosphorene has attracted great interest in spintronics. Here, the electronic structures of monolayer zigzag black phosphorene nanoribbons(ZBPNRs) are researched. We found that the magnetic ground state of ZBPNRs is dependent on nanoribbon widths. In addition, we examine the role of the surface adatoms including Li, Na, K, Mg, Ca and Al in modifying the electronic properties of ZBPNRs. Our calculations show that the adsorption systems with Li, Na and K are half-metals due to the localization of transferred charge. On the other hand, Mg, Ca and Al, which provide more valence electrons than Li, Na and K, result in a smaller net magnetic moment and prefer semiconductor or metal. Our findings propose a new approach without magnetic atoms, magnetic field and strong electric field to realize half-metal in phosphorene, which overcome the drawbacks of graphene/silicene with negligible band-gap as well as transition metal sulfide with low carrier mobility.

Tuning the topological state of a helical atom chain via a Josephson phase

By solving the Bogoliubov-de Gennes equations, we study the quasiparticle spectrum of a magnetic atom chain placed inside a short constriction-type Josephson junction. A helical magnetic order of the atomic spins is assumed, so that a topologically nontrivial state with Majorana edge modes at the ends of the chain can appear. It is found that in the presence of a nonzero Josephson phase the subgap spectrum of an infinite chain consists of four branches (Shiba bands). This spectrum is almost certainly gapped if the atomic spins form a coplanar spiral. The Majorana number of the given system is calculated analytically. It is demonstrated that a Josephson phase shift can be used to drive the system into a topologically nontrivial state and to tune the size of the topological gap. The spatial structure of Majorana edge modes is studied as well. We generalize the effective model based on discrete Bogoliubov-de Gennes equations developed by Pientka et al. for a bulk superconductor to the case of a Josephson junction. Using these discrete equations, the wave functions of Majorana zero modes are calculated analytically for a coplanar atomic spin spiral with arbitrary pitch. The wave functions exhibit an intermediate asymptotic behavior with a nonexponential fall-off with distance from the edge of the atomic chain.

Role of the Magnetic Anisotropy in Atomic-Spin Sensing of 1D Molecular Chains.

One-dimensional metal-organic chains often possess a complex magnetic structure susceptible to modification by alteration of their chemical composition. The possibility to tune their magnetic properties provides an interesting playground to explore quasi-particle interactions in low-dimensional systems. Despite the great effort invested so far, a detailed understanding of the interactions governing the electronic and magnetic properties of the low-dimensional systems is still incomplete. One of the reasons is the limited ability to characterize their magnetic properties at the atomic scale. Here, we provide a comprehensive study of the magnetic properties of metal-organic one-dimensional (1D) coordination polymers consisting of 2,5-diamino-1,4-benzoquinonediimine ligands coordinated with Co or Cr atoms synthesized under ultrahigh-vacuum conditions on a Au(111) surface. A combination of integral X-ray spectroscopy with local-probe inelastic electron tunneling spectroscopy corroborated by multiplet analysis, density functional theory, and inelastic electron tunneling simulations enables us to obtain essential information about their magnetic structures, including the spin magnitude and orientation at the magnetic atoms, as well as the magnetic anisotropy.

Odd-frequency Cooper pair around a magnetic impurity

The Yu-Shiba-Rusinov (YSR) state appears as a bound state of a quasiparticle at a magnetic atom embedded in a superconductor. We discuss why the YSR state has energy below the superconducting gap and why the pair potential changes the sign at the magnetic atom. Although a magnetic atom in a superconductor has been considered as a pair breaker since 1960s, we propose an alternative physical picture to explain these reasons. We show that a magnetic atom converts a spin-singlet s-wave Cooper pair into an odd-frequency pair rather than breaking it. The odd-frequency pairing correlations always coexist with the quasiparticle states below the gap. The YSR state is an example of such a subgap quasiparticle state. The paramagnetic property of an odd-frequency pair explains the sign change of the pair potential at a magnetic atom and the decrease of superconducting transition temperature in the presence of many magnetic impurities.

Evidence for a spinon Kondo effect in cobalt atoms on single-layer 1T-TaSe2

Quantum spin liquids (QSLs) are highly entangled, disordered magnetic states that arise in frustrated Mott insulators and host exotic fractional excitations such as spinons and chargons. Despite being charge insulators some QSLs are predicted to exhibit gapless itinerant spinons that yield metallic behavior in the spin channel. We have deposited isolated magnetic atoms onto single-layer (SL) 1T-TaSe$_2$, a gapless QSL candidate, to experimentally probe how itinerant spinons couple to impurity spin centers. Using scanning tunneling spectroscopy we observe the emergence of new, impurity-induced resonance peaks at the 1T-TaSe$_2$ Hubbard band edges when cobalt adatoms are positioned to have maximal spatial overlap with the Hubbard band charge distribution. These resonance peaks disappear when the spatial overlap is reduced or when the magnetic impurities are replaced with non-magnetic impurities. Theoretical simulations using a modified Anderson impurity model integrated with a gapless quantum spin liquid show that these resonance peaks are consistent with a Kondo resonance induced by spinons combined with spinon-chargon binding effects that arise due to QSL gauge-field fluctuations.

Atomic occupation and role of Cr atoms in Cr-doped Ni43Co7Mn39Sn11 magnetic shape memory alloys

The atomic occupation, magnetic properties and the relationship between them of Ni43Co7Mn39-xCrxSn11(x = 0, 0.4, 0.6, 0.8, 1.0, 1.2 at%) alloys have been systematically investigated. To elucidate the occupancy of Cr atoms, extended X-ray absorption fine structure (EXAFS) is performed and Cr K-edge EXAFS analysis determines that Cr atoms occupy Y site of the Ni43Co7Mn38Cr1Sn11 sample. Ferromagnetic resonance shows the evolution of the microscopic magnetic properties of the magnetic atoms with the increase of Cr doping content, which further confirms the Y-site occupation of Cr atoms. The experimental results show that in the Ni43Co7Mn39Sn11 system, Cr doping replaces Mn at the Y site, breaking the symmetry of the crystal field of Mn atoms at the Z site, reducing the atomic displacement potential. The doping of Cr atoms significantly enhances the driving force of its magnetic field-induced martensitic transformation.