Magnetic Atoms Research Articles

Triplet Pairing Mechanisms from Hund's-Kondo Models: Applications to UTe_{2} and CeRh_{2}As_{2}.

Observing that several U and Ce based candidate triplet superconductors share a common structural motif, with pairs of magnetic atoms separated by an inversion center, we hypothesize a triplet pairing mechanism based on an interplay of Hund's and Kondo interactions that is unique to this structure. In the presence of Hund's interactions, valence fluctuations generate a triplet superexchange between electrons and local moments. The offset from the center of symmetry allows spin-triplet pairs formed by the resulting Kondo effect to delocalize onto the Fermi surface, precipitating superconductivity. We demonstrate this mechanism within a minimal two-channel Kondo lattice model and present support for this pairing mechanism from existing experiments.

Accuracy of quantum simulators with ultracold dipolar molecules: A quantitative comparison between continuum and lattice descriptions

With rapid progress in control and manipulation of ultracold magnetic atoms and dipolar molecules, the quantum simulation of lattice models with strongly interacting dipole-dipole interactions (DDI) and high densities is now within experimental reach. This rapid development raises the issue about the validity of quantum simulation in such regimes. In this study, we address this question by performing a full quantitative comparison between the continuum description of a one-dimensional gas of dipolar bosons in an optical lattice, and the single-band Bose-Hubbard lattice model that it quantum simulates. By comparing energies and density distributions, and by calculating direct overlaps between the continuum and lattice many-body wavefunctions, we demonstrate that in regimes of strong DDI and high densities the continuum system fails to recreate the desired lattice model. Two-band Hubbard models become necessary to reduce the discrepancy observed between continuum and lattice descriptions, but appreciable deviations in the density profile still remain. Our study elucidates the role of strong DDI in generating physics beyond lowest-band descriptions and should offer a guideline for the calibration of near-term dipolar quantum simulators.

Hydrogen‐bond Peculiarities in Nitrilotris‐(Methylenephosphonato)‐Three‐aqua‐Iron(II) from XRD Experiment and DFT Calculation

AbstractNitrilotris‐(Methylenephosphonato)‐Three‐aqua‐Iron(II) (FeNTP) is a linear organic‐inorganic coordination polymer forming on the steel surface as an anti‐corrosion coating. The compound was studied earlier experimentally and theoretically using ab‐initio DFT calculations. Here, the peculiarities of the bonds of hydrogen atoms, within and between the polymer chains, forming the three‐dimensional network of the coating, are discussed and compared to those in other compounds including water. The effect of Fe magnetic moment on the hydrogen bonds is studied. It is shown that in organometallic compounds, the magnetic state of magnetic atoms M affects the total polymer volume both through the M‐nearest neighbor distances and through the H‐bonding lengths, the change in the latter can even prevail over the former.

Site-specific atomic substitution in a giant magnetocaloric Fe2P -type system

Giant magnetocaloric (GMC) materials constitute a requirement for near room temperature magnetic refrigeration. (Fe,Mn)$_2$(P,Si) is a GMC compound with strong magnetoelastic coupling. The main hindrance towards application of this material is a comparably large temperature hysteresis, which can be reduced by metal site substitution with a nonmagnetic element. However, the (Fe,Mn)$_2$(P,Si) compound has two equally populated metal sites, the tetrahedrally coordinated $3f$ and the pyramidally coordinated $3g$ sites. The magnetic and magnetocaloric properties of such compounds are highly sensitive to the site specific occupancy of the magnetic atoms. Here we have attempted to study separately the effect of $3f$ and $3g$ site substitution with equal amounts of vanadium. Using formation energy calculations, the site preference of vanadium and its influence on the magnetic phase formation are described. A large difference in the isothermal entropy change (as high as 44\%) with substitution in the $3f$ and $3g$ sites is observed. The role of the lattice parameter change with temperature and the strength of the magnetoelastic coupling on the magnetic properties are highlighted.

Diode effect in Josephson junctions with a single magnetic atom

Current flow in electronic devices can be asymmetric with bias direction, a phenomenon underlying the utility of diodes1 and known as non-reciprocal charge transport2. The promise of dissipationless electronics has recently stimulated the quest for superconducting diodes, and non-reciprocal superconducting devices have been realized in various non-centrosymmetric systems3–10. Here we investigate the ultimate limits of miniaturization by creating atomic-scale Pb–Pb Josephson junctions in a scanning tunnelling microscope. Pristine junctions stabilized by a single Pb atom exhibit hysteretic behaviour, confirming the high quality of the junctions, but no asymmetry between the bias directions. Non-reciprocal supercurrents emerge when inserting a single magnetic atom into the junction, with the preferred direction depending on the atomic species. Aided by theoretical modelling, we trace the non-reciprocity to quasiparticle currents flowing by means of electron–hole asymmetric Yu–Shiba–Rusinov states inside the superconducting energy gap and identify a new mechanism for diode behaviour in Josephson junctions. Our results open new avenues for creating atomic-scale Josephson diodes and tuning their properties through single-atom manipulation.

Unveiling the similarities and dissimilarities between dynamic and thermodynamic phase transitions in a magnetic binary alloy system: a Monte Carlo study

Being inspired by the results of recent experimental work ‘Phys. Rev. E 102 022 804’, we perform a series of Monte Carlo simulations to investigate the dynamic critical properties of a binary magnetic alloy system composed of two different components with different spin moments with the chemical formula . In the first part of the work, we concentrate our attention on the similarities between dynamic critical behavior and thermodynamic phase transition properties of the model. Finite-size scaling analyses suggest that the proposed binary alloy model exhibits 2D Ising universality class. In the second part, we examine the dynamic critical behavior in the presence of a magnetic bias field for which a metamagnetic sideband behavior originates in dynamic susceptibility curves. Our results yield that these metamagnetic anomalies survive for a wide range of magnetic atom concentration p A , and the strength of the anomalies observed in the field-dependent dynamic susceptibility varies very slowly with the T/T c ratio of the binary magnetic alloy.

Orbital selective 5f electron character, indication of Kondo effect, and subatomic features of single uranium atoms

In actinide-based materials, the multiplicity of $5f$ orbitals provides a rich setting to elucidate the large and outstanding issues of the competition and interplay among different channels of interactions. Among them, the dual character of $5f$ electrons has been studied in some $5f$ electron Kondo lattice compounds. Here, by using scanning tunneling microscopy/spectroscopy (STS), we present the indication of Kondo effect driven by $5f$ electrons and subatomic features of single U atoms on graphene/6H-SiC(0001), ${\mathrm{Cu}}_{2}\mathrm{N}/\mathrm{Cu}(100)$, and Bi(110). For all three systems, an asymmetric-dip spectral feature can be observed around the Fermi energy, which is an indication of Kondo resonance in STS. The size of the ``screening cloud'' for single U atoms on Bi(110) is much larger than that observed on graphene/6H-SiC(0001) and ${\mathrm{Cu}}_{2}\mathrm{N}/\mathrm{Cu}(100)$, indicating the $5f$ electrons of single U atoms on Bi(110) are more itinerant than that on the other two substrates. In addition to the asymmetric spectrum feature around the Fermi energy, various subatomic spectrum features are also observed from the $dI/dV$ maps of U atoms on different substrates. Further density-functional theory calculations show that these distinct subatomic features arise from the orbital selective hybridizations between U atoms and the substrates. Our results present the distinct observation of the orbital selective $5f$ electron character, indication of Kondo effect, and together with the subatomic features of individual $5f$ impurity magnetic atoms adsorbed on nonmagnetic surfaces.

Alternating magnetic field improved NiFe@MoS2 monolayer diatomic site catalyst for overall water splitting

Further uprating the catalytic activities of diatomic active sites while maintaining the atomic loading and diatomic coordination by external stimulation is a promising way to break the bottleneck in the improvement of diatomic site catalysts (DASCs). Herein, the as-prepared NiFe@MoS2 DASCs treated by external high-frequency alternating magnetic field (AMF) further expedite the alkaline water electrolysis process with a superior cell voltage of 1.576 V to afford a current density of 10 mA cm−2 than that treated without AMF (1.652 V). Theoretical simulation by COMSOL Multiphysics helps visualize the increase in temperature locally around the diatomic active sites, qualitatively revealing the magnetic heating effect that originates from the anchored magnetic Ni and Fe atoms. The selective magnetic heating of bifunctional diatomic active site proposed in this work can broaden horizons and endow another dimension in the design of highly efficient catalysts toward various complicated energy-related reactions.

Topological Phase Transitions Driven by Sn Doping in (Mn1−xSnx)Bi2Te4

The antiferromagnetic ordering that MnBi2Te4 shows makes it invariant with respect to the combination of the time-reversal and primitive-lattice translation symmetries, giving rise to its topologically nontrivial nature and a number of fundamental phenomena. At the same time, the possibility to control the electronic and magnetic properties of this system can provide new effective ways for its application in devices. One of the approaches to manipulate MnBi2Te4 properties is the partial substitution of magnetic atoms in the compound with atoms of non-magnetic elements, which inevitably affect the interplay of magnetism and band topology in the system. In this work, we have carried out theoretical modelling of changes in the electronic structure that occur as a result of increasing the concentration of Sn atoms at Mn positions in the (Mn1−xSnx)Bi2Te4 compound both using Korringa–Kohn–Rostoker (KKR) Green’s function method as well as the widespread approach of using supercells with impurity in DFT methods. The calculated band structures were also compared with those experimentally measured by angle-resolved photoelectron spectroscopy (ARPES) for samples with x values of 0, 0.19, 0.36, 0.52 and 0.86. We assume that the complex hybridization of Te-pz and Bi-pz orbitals with Sn and Mn ones leads to a non-linear dependence of band gap on Sn content in Mn positions, which is characterized by a plateau with a zero energy gap at some concentration values, suggesting possible topological phase transitions in the system.

Topological Dirac semimetal BaAuSb

Dirac semimetals feature Dirac cones with topologically nontrivial states in the bulk that are protected by crystalline symmetries. They attract considerable attention due to rich quantum states and properties of interest, such as high Fermi velocities, mobility, and small effective masses. Here, we show that a ternary intermetallic BaAuSb crystal hosts both trivial and nontrivial topological Dirac states in the bulk. The nontrivial Fermi-surface pocket at the Brillouin zone center is characterized with only a few hundredths of a bare electron mass and with very high mobility. All parts of the Fermi surface detected by quantum oscillations, topologically trivial or nontrivial, have carriers with unusually high Fermi velocities and small masses. These conducting states may be coupled with magnetic moments in materials where Ba is replaced by magnetic rare-earth atoms.

Collective Magnetic Behavior in Vanadium Telluride Induced by Self-Intercalation

Self-intercalation of native magnetic atoms within the van der Waals (vdW) gap of layered two-dimensional (2D) materials provides a degree of freedom to manipulate magnetism in low-dimensional systems. Among various vdW magnets, the vanadium telluride is an interesting system to explore the interlayer order-disorder transition of magnetic impurities due to its flexibility in taking nonstoichiometric compositions. In this work, we combine high-resolution scanning transmission electron microscopy (STEM) analysis with density functional theory (DFT) calculations and magnetometry measurements, to unveil the local atomic structure and magnetic behavior of V-rich V1+xTe2 nanoplates with embedded V3Te4 nanoclusters grown by chemical vapor deposition (CVD). The segregation of V intercalations locally stabilizes the self-intercalated V3Te4 magnetic phase, which possesses a distorted 1T'-like monoclinic structure. This phase transition is controlled by the electron doping from the intercalant V ions. The magnetic hysteresis loops show that the nanoplates exhibit superparamagnetism, while the temperature-dependent magnetization curves evidence a collective superspin-glass magnetic behavior of the nanoclusters at low temperature. Using four-dimensional (4D) STEM diffraction imaging, we reveal the formation of collective diffuse magnetic domain structures within the sample under the high magnetic fields inside the electron microscope. Our results shed light on the studies of dilute magnetism at the 2D limit and on strategies for the manipulation of magnetism for spintronic applications.

Spontaneous Anomalous Hall Effect Arising from an Unconventional Compensated Magnetic Phase in a Semiconductor.

The anomalous Hall effect, commonly observed in metallic magnets, has been established to originate from the time-reversal symmetry breaking by an internal macroscopic magnetization in ferromagnets or by a noncollinear magnetic order. Here we observe a spontaneous anomalous Hall signal in the absence of an external magnetic field in an epitaxial film of MnTe, which is a semiconductor with a collinear antiparallel magnetic ordering of Mn moments and a vanishing net magnetization. The anomalous Hall effect arises from an unconventional phase with strong time-reversal symmetry breaking and alternating spin polarization in real-space crystal structure and momentum-space electronic structure. The anisotropic crystal environment of magnetic Mn atoms due to the nonmagnetic Te atoms is essential for establishing the unconventional phase and generating the anomalous Hall effect.

Magnetic susceptibility La<sub><sub>1-0.397</sub><sub><i>x</i></sub></sub>Gd<sub><sub>0.067</sub><sub><i>x</i></sub></sub>Ca<sub><sub>0.33</sub><sub><i>x</i></sub></sub>Mn<sub><i>х</i></sub>Al<sub><sub>1-</sub><sub><i>х</i></sub></sub>O<sub>3 </sub>solid solutions

Solid solutions of lanthanum manganites with perovskite structure containing calcium and gadolinium atoms were studied by the method of magnetic dilution. The obtained magnetic characteristics and the character of their temperature changes point to a clustering of magnetic atoms in the perovskite structure. A comparison of the solid solutions under study with La1- y Gd y AlO3 testifies for gadolinium atoms taking part in the exchange interactions in the La1-0.397 х Gd0.067 х Ca0.33 х Mn х Al1- х O3 solid solutions.

Prediction of monolayer FeP4 with intrinsic half-metal ferrimagnetism above room temperature

The design of high-temperature ferrimagnetic materials is highly demanded for next-generation functional spintronic devices. Here, we propose that the combination of nonmetallic structural units and magnetic atoms is an effective way to achieve high-temperature magnetism in two-dimensional (2D) materials. The predicted $\mathrm{Fe}{\mathrm{P}}_{4}$ monolayer, consisting of quasisquare ${\mathrm{P}}_{4}$ units, shows intrinsic half-metal ferrimagnetism above room temperature. Each Fe atom is coordinated with four P atoms associated with the surrounding four quasisquare ${\mathrm{P}}_{4}$ units. First-principles calculations suggest that the $\mathrm{Fe}{\mathrm{P}}_{4}$ monolayer presents a Curie temperature of 460 K. More interestingly, the itinerant electrons and the unique quasisquare ${\mathrm{P}}_{4}$ units act as intermediaries and play an important role in promoting the Ruderman-Kittel-Kasuya-Yosida and superexchange interactions, respectively, which induces a robust ferrimagnetism. Our findings not only shed light on the promising future of 2D magnetic materials, but also are of interest for high-temperature spintronic applications.

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 (Δs) 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 Δs 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.

Phase transition regulation, magnetocaloric effect, and abnormal thermal expansion

As a common phenomenon in nature, phase transition has received much attention for a long time. It has been applied to various fields, such as refrigeration, information and energy storage, and negative thermal expansion. Solid refrigeration technology based on magnetocaloric effect, electrocaloric effect, and mechanocaloric effect has the advantages of environmental protection, high efficiency, no noise, and easy miniaturization, and is expected to replace vapor compression technology. Among them, the magnetocaloric effect has the longest research history. However, the shortcomings of magnetocaloric effect driven by a single magnetic field limit its solid-state refrigeration application, such as insufficient amplitude of caloric effect, large hysteresis loss, and narrow refrigeration temperature span. To solve these problems, multifield tuning and multicaloric effect have come into people's sight. This paper introduces our recent research on improving the caloric effect by applying multifield, such as increasing entropy change, expanding transition temperature range, adjusting transition temperature, and reducing hysteresis losses. The thermodynamics of multifield and coupled-caloric effect are presented in the meantime. On the other hand, materials with abnormal thermal expansion (zero thermal expansion, negative thermal expansion) have important applications in precision manufacturing. The phase transition and lattice effect dominated by magnetic atoms in the giant magnetocaloric materials with strong magnetic-crystal coupling provide an ideal platform for exploring abnormal thermal expansion. This paper also introduces our recent research on abnormal thermal expansion in magnetocaloric materials, and looks forward to future relevant research.

Prototype of a unidirectional magnetic field-applied fast atom beam source for surface-activated bonding

Prototype of a unidirectional magnetic field-applied fast atom beam source for surface-activated bonding

Electronic properties, skyrmions and bimerons in Janus CrXY (X, Y = S, Se, Te, Cl, Br, I, and X ≠ Y) monolayers.

Using first-principles calculations, we systematically investigate the electronic properties, chiral skyrmions and bimerons in two-dimensional (2D) Janus CrXY (X, Y = S, Se, Te, Cl, Br, I, and X ≠ Y) monolayers. We found that the categories of nonmagnetic atoms (X and Y in CrXY) determine whether CrXY is a ferromagnetic metal or a semiconductor. Unexpectedly, the CrBrS monolayer of these CrXY materials is a room temperature ferromagnetic semiconductor with a Curie temperature of 303 K, and it possesses an off-plane magnetic anisotropy energy of 0.06 meV. Besides, a strong Dzyaloshinskii-Moriya interaction (DMI) of 3.10 meV is found in CrTeI and is mainly induced by the strong spin-orbit coupling of the nonmagnetic atoms Te(I) rather than that of the magnetic Cr atoms. Furthermore, using micromagnetic simulations, skyrmions can be stabilized in CrSeBr without external magnetic fields. More importantly, the bimerons in CrSeCl with in-plane magnetic anisotropy can be transformed into skyrmions or a ferromagnetic state by controlling the direction of external magnetic fields. Our work investigates fourteen kinds of Janus monolayers, serving as guidelines for materials research on DMI, skyrmions and bimerons.

Electric-field induced half-metallic properties in an experimentally synthesized CrSBr monolayer.

Two-dimensional (2D) half-metallic materials are highly desirable for nanoscale spintronic applications. Here, we propose a new mechanism that can achieve half-metallicity in 2D ferromagnetic (FM) materials with two-layer magnetic atoms by electric field tuning. We use a concrete example of an experimentally synthesized CrSBr monolayer to illustrate our proposal through first-principles calculations. It is found that half-metallic properties can be achieved in CrSBr within an appropriate electric field range, and the corresponding amplitude of electric field intensity can be realized experimentally. Janus monolayer Cr2S2BrI is constructed, which possesses a built-in electric field due to broken horizontal mirror symmetry. However, Cr2S2BrI without and with an applied external electric field is always a FM semiconductor. A possible memory device is also proposed based on the CrSBr monolayer. Our work will stimulate the application of 2D FM CrSBr in future spintronic nanodevices.

Effects of adding B element on amorphous forming ability, magnetic properties, and mechanical properties of FePBCCu alloy

Fe-based amorphous alloys are widely used in power electronics fields such as transformers and reactors due to their low coercivity, high permeability and low loss. However, the relatively low saturation magnetization (<i>B</i><sub>s</sub>) limits their further applications. Generally speaking, the adjustable magnetic Fe content as an effective strategy can ameliorate the magnetic properties, and the higher the Fe content, the higher the obtained <i>B</i><sub>s</sub> is, but the decrease of the corresponding non-magnetic element content will result in the drop of the ability of alloys to form amorphous phase, leading to the deterioration of the magnetic softness and bending ductility of nanocrystalline alloys. To address this critical issue, in this work, based on the metal-metalloid hybridization, the FePBCCu amorphous ribbons, each with a thickness of ~25 μm, are prepared by the single-roller melt spinning method via 7% (atomic percent) B substitution for P, and the effects of B element addition on the ability to form amorphous phase, magnetic properties and mechanical properties of ribbons are investigated. Thermodynamic behavior shows that the addition of small quantities of B element can reduce the structural heterogeneity of alloy and the crystallization driving force as well, thus effectively improving the thermal stability of the amorphous matrix. The melting and solidification curves show that the addition of B can promote alloy to approach to the eutectic composition, and there is a large degree of undercooling. As a result, the critical thickness of ribbons increases from ~21 μm for B-free alloy to ~30 μm for B-added alloy due to the micro-alloying effect. The addition of B increases the effective magnetic moment of magnetic atoms in alloy, resulting in the increase of the saturation magnetization. Furthermore, the results of nanoindentation tests show that the modulus value of the B-added alloy decreases greatlyr and fluctuates in a smaller range than that of the B-free alloy, which is closely associated with the structural uniformity of the alloy.