Short-range Exchange Interactions Research Articles

Unconventional magnetism mediated by spin-phonon-photon coupling

Magnetic order typically emerges due to the short-range exchange interaction between the constituent electronic spins. Recent discoveries have found a crucial role for spin-phonon coupling in various phenomena from optical ultrafast magnetization switching to dynamical control of the magnetic state. Here, we demonstrate theoretically the emergence of a biquadratic long-range interaction between spins mediated by their coupling to phonons hybridized with vacuum photons into polaritons. The resulting ordered state enabled by the exchange of virtual polaritons between spins is reminiscent of superconductivity mediated by the exchange of virtual phonons. The biquadratic nature of the spin-spin interaction promotes ordering without favoring ferro- or antiferromagnetism. It further makes the phase transition to magnetic order a first-order transition, unlike in conventional magnets. Consequently, a large magnetization develops abruptly on lowering the temperature which could enable magnetic memories admitting ultralow-power thermally-assisted writing while maintaining a high data stability. The role of photons in the phenomenon further enables an in-situ static control over the magnetism. These unique features make our predicted spin-spin interaction and magnetism highly unconventional paving the way for novel scientific and technological opportunities.

Phase transition and magneto-caloric properties of perovskites Pr0.55Sr0.45MnO3: Modeling versus experiments

Experimental data obtained with the perovskite compounds Pr0.55Sr0.45MnO3 show that the magnetization decreases with increasing temperature T and undergoes a very sharp phase transition to the paramagnetic phase. The sharp transition in a system with a strong disorder is very rare, if not non-existent, in the theory of phase transition in systems of short-range pairwise exchange interactions. To understand this remarkable property, we introduce a model including a multispin (cluster-like) interaction between Mn ions, in addition to the usual pairwise exchange terms between these ions and the Mn–Pr interactions. We carry out Monte Carlo (MC) simulations using this model. The crystal is a body-centered tetragonal lattice where the corner sites are occupied by Mn ions and the center sites by Pr or Sr ions. Due to the doping, Mn4+ with S=3/2 has the concentration of Pr3+ (S=1) and Mn3+ with S=2 has the Sr concentration. After attempts with different spin models and various Hamiltonians, we find that the many-state Ising spin model reproduces most of the experimental results. For the Hamiltonian, we find that pairwise interactions alone between ions cannot reproduce the sharp transition and the magnetization below TC. We have to include a multispin interaction as said above. We fit the MC results with experimental data, and we estimate values of various exchange interactions in the system. These values are found to be in the range of those found in perovskite manganite compounds. We also study the applied-field effect on the magnetization in the temperature region below and above the transition temperature TC. We calculate the magnetic entropy change |ΔSm| and the Relative Cooling Power, for magnetic field from 1 to 3 T. Our simulation results are in good agreement with experiments. The role of the multispin interaction is analyzed and discussed.

Indirect Exchange Interaction Leads to Large Lattice Contribution to Magnetocaloric Entropy Change.

Materials with a large magnetocaloric response are highly desirable for magnetic cooling applications. It is suggested that a strong spin-lattice coupling tends to generate a large magnetocaloric effect, but no microscopic mechanism has been proposed. In this Letter, we use spin-lattice dynamics simulation to examine the lattice contribution to the magnetocaloric entropy change in bcc iron (Fe) and hcp gadolinium (Gd) with exchange interaction parameters determined from abinitio calculations. We find that indirect Ruderman-Kittel-Kasuya-Yosida (RKKY) exchange interaction in hcp Gd leads to longer-range spin-lattice coupling and more strongly influences the low-frequency long-wavelength phonons. This results in a higher lattice contribution toward the total magnetocaloric entropy change as compared to bcc Fe with short-range direct exchange interactions. Our analysis provides a framework for understanding the magnetocaloric effect in magnetic materials with strong spin-lattice couplings. Our finding suggests that long-range indirect RKKY-type exchange gives rise to a larger lattice contribution to the magnetocaloric entropy change and is, thus, beneficial for magnetocaloric materials.

Spin dynamics, loop formation and cooperative reversal in artificial quasicrystals with tailored exchange coupling

Aperiodicity and un-conventional rotational symmetries allow quasicrystalline structures to exhibit unusual physical and functional properties. In magnetism, artificial ferromagnetic quasicrystals exhibited knee anomalies suggesting reprogrammable magnetic properties via non-stochastic switching. However, the decisive roles of short-range exchange and long-range dipolar interactions have not yet been clarified for optimized reconfigurable functionality. We report broadband spin-wave spectroscopy and X-ray photoemission electron microscopy on different quasicrystal lattices consisting of ferromagnetic Ni81Fe19 nanobars arranged on aperiodic Penrose and Ammann tilings with different exchange and dipolar interactions. We imaged the magnetic states of partially reversed quasicrystals and analyzed their configurations in terms of the charge model, geometrical frustration and the formation of flux-closure loops. Only the exchange-coupled lattices are found to show aperiodicity-specific collective phenomena and non-stochastic switching. Both, exchange and dipolarly coupled quasicrystals show magnonic excitations with narrow linewidths in minor loop measurements. Thereby reconfigurable functionalities in spintronics and magnonics become realistic.

Ferromagnetic single-atom spin catalyst for boosting water splitting.

Heterogeneous single-atom spin catalysts combined with magnetic fields provide a powerful means for accelerating chemical reactions with enhanced metal utilization and reaction efficiency. However, designing these catalysts remains challenging due to the need for a high density of atomically dispersed active sites with a short-range quantum spin exchange interaction and long-range ferromagnetic ordering. Here, we devised a scalable hydrothermal approach involving an operando acidic environment for synthesizing various single-atom spin catalysts with widely tunable substitutional magnetic atoms (M1) in a MoS2 host. Among all the M1/MoS2 species, Ni1/MoS2 adopts a distorted tetragonal structure that prompts both ferromagnetic coupling to nearby S atoms as well as adjacent Ni1 sites, resulting in global room-temperature ferromagnetism. Such coupling benefits spin-selective charge transfer in oxygen evolution reactions to produce triplet O2. Furthermore, a mild magnetic field of ~0.5 T enhances the oxygen evolution reaction magnetocurrent by ~2,880% over Ni1/MoS2, leading to excellent activity and stability in both seawater and pure water splitting cells. As supported by operando characterizations and theoretical calculations, a great magnetic-field-enhanced oxygen evolution reaction performance over Ni1/MoS2 is attributed to a field-induced spin alignment and spin density optimization over S active sites arising from field-regulated S(p)-Ni(d) hybridization, which in turn optimizes the adsorption energies for radical intermediates to reduce overall reaction barriers.

Strong ferromagnetism of g-C3N4 achieved by atomic manipulation

Two-dimensional (2D) metal-free ferromagnetic materials are ideal candidates to fabricate next-generation memory and logic devices, but optimization of their ferromagnetism at atomic-scale remains challenging. Theoretically, optimization of ferromagnetism could be achieved by inducing long-range magnetic sequence, which requires short-range exchange interactions. In this work, we propose a strategy to enhance the ferromagnetism of 2D graphite carbon nitride (g-C3N4), which is facilitating the short-range exchange interaction by introducing in-planar boron bridges. As expected, the ferromagnetism of g-C3N4 was significantly enhanced after the introduction of boron bridges, consistent with theoretical calculations. Overall, boosting ferromagnetism of 2D materials by introducing bridging groups is emphasized, which could be applied to manipulate the magnetism of other materials.

Thermodynamics of Light-Induced Nanoscale Polar Structures in Ferroelectric Superlattices.

We study the thermodynamics of nanoscale polar structures in PbTiO3/SrTiO3 ferroelectric superlattices induced by above-bandgap optical excitation using a phase-field model explicitly considering both structural and electronic processes. We demonstrate that the light-excited carriers provide the charge compensation of polarization bound charges and the lattice thermal energy, both of which are key to the thermodynamic stabilization of a previously observed supercrystal, a three-dimensionally periodic nanostructure, within a window of substrate strains, while different mechanical and electrical boundary conditions can stabilize a number of other nanoscale polar structures by balancing the competing short-range exchange interactions responsible for the domain wall energy and long-range electrostatic and elastic interactions. The insights into the light-induced formation and richness of nanoscale structures from this work offer theoretical guidance for exploring and manipulating the thermodynamic stability of nanoscale polar structures employing a combination of thermal, mechanical, and electrical stimuli as well as light.

Theoretical study on the magnetic properties of cathode materials in the lithium-ion battery.

The layered LiMO2 (M = Co, Ni, and Mn) materials are commonly used as the cathode materials in the lithium-ion battery due to the distinctive layer structure for lithium extraction and insertion. Although their electrochemical properties have been extensively studied, the structural and magnetic properties of LiNiO2 are still under considerable debate, and the magnetic properties of monoclinic LiMnO2 are seldom reported. In this work, a detailed study of LiNiO2, LiMnO2, and a half-doped material LiNi0.5Mn0.5O2 is performed via both first-principles calculations and Monte Carlo simulations based on the effective spin Hamiltonian model. Through considering different structures, it is verified that a structure with a zigzag-type pattern is the most stable one of LiNiO2. Moreover, in order to figure out the magnetic properties, the spin exchange interactions are calculated, and then magnetic ground states are predicted in these three systems. The results show that LiNiO2 forms a spiral order that is caused by the competition from both the short-range and long-range spin exchange interactions, whereas the magnetic ground state of LiMnO2 is collinearly antiferromagnetic due to its nearest and next-nearest neighbor antiferromagnetic spin exchange interactions. However, LiNi0.5Mn0.5O2 is collinearly ferrimagnetic because of the ferromagnetic nearest neighbor Ni-Ni and Mn-Mn exchange interactions. Our work demonstrates the competition between the different exchange interactions in these cathode materials, which may be relevant to the performance of the lithium-ion battery.

Cyanido-Bridged Heterobimetallic Molecular Squares: Low-Dimensional Models of Prussian Blue Analogues and Beyond

The cyanido-bridged square-like molecules with d/f metal ions occupying the corners of the square are considered molecular models of the Prussian Blue Analogues (PBAs) because they reproduce one face of their 3D face-centered cubic structure, being called low-dimensional PBAs. Such highly regular complexes provide unique insights into the short-range magnetic exchange interaction between the metal ions of PBAs and exhibit magnetic properties which are not found for PBAs. This review focuses on the preparative routes and magneto-structural characterization of d-d(d′)/d-f tetranuclears with a square-like topology generating rich families of molecular switches and nanomagnets. Relevant examples that provided a better comprehension of the magnetic behavior of PBAs, including subtle chemical factors that interfere in the photo- and thermally induced electron transfer are reviewed herein. A special attention is paid to the design of d-f cyanido-bridged squares, less numerous compared to the d-d/d′ ones, that proved to be suitable model compounds for the theoretical treatment of the magnetic exchange interaction between d and f spin carriers across the cyanide ligand. The square-shaped PBAs represent a very active area of research that is enriched yearly with new examples with potential for the development of molecular-scaled electronic devices.

First-principles study of the magnetic exchange forces between the RuO2 (110) surface and Fe tip.

Magnetic exchange force microscopy (MExFM) is an important experimental technique for mapping the magnetic structure of surfaces with atomic resolution relying on the spin-dependent short-range exchange interaction between a magnetic tip and a magnetic surface. RuO2 is a significant compound with applications in heterogeneous catalysis and electrocatalysis. It has been characterized recently as an antiferromagnetic (AFM) material, and its magnetism has been predicted somewhat surprisingly to play an important role in its catalytic properties. In the current study, we explore theoretically whether MExFM can visualize the magnetic surface structure of RuO2 . We use density functional theory (DFT) calculations to extract the exchange interactions between a ferromagnetic Fe tip interacting with an AFM RuO2 (110) surface, as a function of tip-surface distance and the position of the tip over the surface. Mimicking the MExFM experiment, these data are then used to calculate the normalized frequency shift of an oscillating cantilever tip versus the minimum tip-surface distance, and construct corrugation height line profiles. It is found that the exchange interaction between tip and surface is strongest for a parallel configuration of the spins of the tip and of the surface; it is weakest for an anti-parallel orientation. In a corrugation profile, this gives rise to a sizable height difference of 25 pm between the spin-up and spin-down Ru atoms in the RuO2 (110) surface at a normalized frequency shift =-10.12 fNm1/2 . The O atoms in the surface are not or hardly visible in the corrugation profile.

Unveiling Phase Transitions in 1D Systems with Short-Range Interactions

The statement that any phase transition is related to the appearance or disappearance of long-range spatial correlations and precludes a finite transition temperature in one-dimensional (1D) systems is rooted in condensed matter theory. In this paper, we demonstrate that the 1D Ising model with short-range exchange interactions exhibits a second-order phase transition at a finite temperature relying on the proper choice of the order parameter. We combined analytical calculations and high-precision entropic sampling simulations and chose a slightly different order parameter, namely, the modulus of the magnetization. Notably, we detected a phase transition with a corresponding critical temperature around 15 K, which is in excellent agreement with experimental results. Our study indicates that the incorrect choice of the order parameter may mask phase transitions in 1D systems.

Intervalley electron-hole exchange interaction and impurity-assisted recombination of indirect excitons in WS2 and WSe2 monolayers

The variety of excitonic states in tungsten-based dichalcogenide monolayers stems from unique interplay between the spin and valley degrees of freedom. One of the exciton species is the indirect exciton (momentum or valley dark), which is responsible for a series of resonances when the monolayer is charge neutral. We investigate the short-range electron-hole exchange interaction of the indirect exciton, as well as its recombination mechanism mediated by impurities. The analysis provides thorough understanding of the energy and polarization of the zero-phonon indirect exciton resonance in the emission spectrum.

Retaining the Curie temperature of Ce substituted Nd-Fe-B nano-ribbons by Alnico elements substitution

A balanced consumption of the rare earth resources can be realized by increasing the content of Ce in Nd-Fe-B magnetic alloys. However, the substitution of Ce for Nd reduces the magnetic properties and the Curie temperature of the 2:14:1 phase resulting in limited application fields. In this manuscript, targeting to increase the Curie temperature by maintaining the magnetic properties at acceptably high level, Fe in 25% Ce substituted Nd-Ce-Fe-B alloys has been substituted by Alnico constituent elements. The Curie temperature of the main phase for 25% Ce substitution (544 K) is enhanced to 589 K by 20% Alnico substitution which is higher than the Curie temperature of the pure Nd2Fe14B compound (⁓585 K). The magnetic properties initially increased as a result of Alnico element substitution, and then slightly decreased. Maximum energy density of 100 kJ/m3 and Curie temperature of 582 K is obtained by substituting 15% Alnico. These properties are comparable to Ce-free Nd-Fe-B melt spun alloys. The working temperature range increased by the increase of Curie temperature and the decrease of spin reorientation temperature. Due to the accumulation of ferromagnetic element, Ni, at the grain boundary and triple junction phases and the refined grains, the short-range exchange interactions among the nanograins increased which helped to obtain high energy density in the alloys.

Magnetostructural transition, magnetocaloric effect and critical exponent analysis in Nd(Co0.8Fe0.2)2 alloy

In the present study, the magnetostructural transition, magnetocaloric effect (MCE) and critical behavior of Nd(Co0.8Fe0.2)2 alloy in the vicinity of ferromagnetic transition were studied. The Rietveld refinement analysis of in-situ synchrotron XRD patterns reveals that the present alloy crystallizes in the cubic phase (space group Fd3̅m) at room temperature and undergoes a structural transition to the tetragonal phase (space group I41/amd) below its Curie temperature. Temperature dependent heat capacity measurements under different applied magnetic fields were carried out to investigate the magnetocaloric properties. The maximum magnetic entropy change (ΔSM) of 5.6 J/kgK and relative cooling power (RCP) of 445.7 J/kg are obtained over the wide working temperature range of 82 K under magnetic field change of 5 T. The corresponding adiabatic temperature change (ΔTad) is evaluated to be 2.9 K under the same magnetic field change. The sample shows metallic behavior with its significant magnetoresistance value of −41.2% around phase transition at magnetic field change of 5 T. These results may make Nd(Co0.8Fe0.2)2 alloy as one of the promising material for refrigeration applications. Moreover, the critical parameters (β, γ, and δ) were deduced independently from the magnetization data by using different techniques such as modified Arrott plot, Kouvel–Fisher method and critical magnetization isotherms. The derived β=0.319, γ =1.214, and δ=4.78 values are found to be consistent with the 3D-Ising class signifying the presence of short-range exchange interactions in this alloy. We have further shown that these exponents follow the M(H, ε)=|(T − TC)/TC |βf±H/|(T − TC)/TC|β+γ scaling equation very well for both the above and below Curie temperature demonstrating that the estimated exponents are unambiguous and are intrinsic to this alloy system.

Vortex structure in magnetic nanodots: Dipolar interaction, mobile spin model, phase transition and melting

We study in this article properties of a nanodot embedded in a support by Monte Carlo simulation. The nanodot is a piece of simple cubic lattice where each site is occupied by a mobile Heisenberg spin which can move from one lattice site to another under the effect of the temperature and its interaction with neighbors. We take into account a short-range exchange interaction between spins and a long-range dipolar interaction. We show that the ground-state configuration is a vortex around the dot central axis: the spins on the dot boundary lie in the xy plane but go out of plane with a net perpendicular magnetization at the dot center. Possible applications are discussed. Finite-temperature properties are studied. We show the characteristics of the surface melting and determine the energy, the diffusion coefficient and the layer magnetizations as functions of temperature.

Investigation of domain structure in ferroelectric thin films by means of the Ising model

The domain structure in thin ferroelectric films was investigated in the framework of the modified Ising model with short-range exchange and long-range dipole-dipole interactions. The depolarizing field was also taken into account for the study of size effects in thin films. This field is the dominant factor influencing on the existence of phase transitions in the films. We used Monte Carlo method with the standard Metropolis algorithm to generate configurations on three-dimensional cubic lattices. The domains sizes were calculated in the longitudinal and transverse directions relatively to the spontaneous polarization direction. Temperature dependences of the domain sizes are obtained for different values of interaction constants. It was proved that the introduction of the depolarizing field leads to the appearance of a dead layer on the film boundaries. The dead layer thickness increases with increasing temperature. At a certain temperature (not equal to the Curie temperature), the dead layer is destroyed. The temperature dependences of the dielectric susceptibility at different film thicknesses have been calculated. It was shown that these dependencies have two maxima. The first maximum corresponds to a phase transition, the second one exists only for only thin films and corresponds to the destruction of the dead layer.

Absence of Spontaneous Spin Symmetry Breaking in 1D and 2D Quantum Ferromagnetic Systems with Bilinear and Biquadratic Exchange Interactions

Some measurements have shown that the second-order exchange interaction is non-negligible in ferromagnetic compounds whose microscopic interactions are described by means of half-odd integer quantum spins. In these spin systems the ground state is either ferromagnetic or antiferromagnetic when the bilinear exchange interaction is dominant. Instead, in ferromagnetic systems characterized by bilinear and biquadratic exchange interactions of comparable magnitude, the energy minimum occurs when spins are in a canting ground-state. To this aim, a one-dimensional (1D) quantum spin chain and a two-dimensional (2D) lattice of quantum spins subjected to periodic boundary conditions are modeled via the generalized quantum Heisenberg Hamiltonian containing, in addition to the isotropic and short-range bilinear exchange interaction of the Heisenberg type, a second-order interaction, the isotropic and short-range biquadratic exchange interaction between nearest-neighbors quantum spins. For these 1D and 2D quantum systems a generalization of the Mermin–Wagner–Hohenberg theorem (also known as Mermin–Wagner–Berezinksii or Coleman theorem) is given. It is demonstrated, by means of quantum statistical arguments, based on Bogoliubov’s inequality, that, at any finite temperature, (1) there is absence of long-range order and that (2) the law governing the vanishing of the order parameter is the same as in the bilinear case for both 1D and 2D quantum ferromagnetic systems. The physical implications of the absence of a spontaneous spin symmetry breaking in 1D spin chains and 2D spin lattices modeled via a generalized quantum Heisenberg Hamiltonian are discussed.

Non-local effect of impurity states on the exchange coupling mechanism in magnetic topological insulators

Since the discovery of the quantum anomalous Hall (QAH) effect in the magnetically doped topological insulators (MTI) Cr:(Bi,Sb)2Te3 and V:(Bi,Sb)2Te3, the search for the magnetic coupling mechanisms underlying the onset of ferromagnetism has been a central issue, and a variety of different scenarios have been put forward. By combining resonant photoemission, X-ray magnetic circular dichroism and density functional theory, we determine the local electronic and magnetic configurations of V and Cr impurities in (Bi,Sb)2Te3. State-of-the-art first-principles calculations find pronounced differences in their 3d densities of states, and show how these impurity states mediate characteristic short-range pd exchange interactions, whose strength sensitively varies with the position of the 3d states relative to the Fermi level. Measurements on films with varying host stoichiometry support this trend. Our results explain, in an unified picture, the origins of the observed magnetic properties, and establish the essential role of impurity-state-mediated exchange interactions in the magnetism of MTI.

Long-range electron-hole exchange interaction in aluminum nitride

To resolve the discrepancies in the exciton fine structure of aluminum nitride (AlN), polarization- and angle-resolved photoluminescence (PL) spectroscopies are performed. The excitonic PL spectra strongly depend on the optical polarization and detection angle. We propose that both the long-range and short-range electron-hole exchange interaction should be used to interpret the luminescence spectra. The theoretical framework fully explains the present and previous experimental results. The large longitudinal-transverse splitting energy obtained in this study suggests that AlN has strong light-matter coupling without quantum-confined structures.

Fragment Exchange Potential for Realizing Pauli Deformation of Interfragment Interactions.

In fragment-based methods, the lack of explicit short-range exchange interactions between monomers can result in unphysical deformation in charge density. In this study, we describe a fragment exchange potential (XFP) to explicitly account for interfragmental Pauli deformation. In our implementation, a Kohn-Sham exchange potential is adopted along with the Yukawa potential. The method has been validated by comparison of the computed exchange energies using the XFP potential with results obtained from antisymmetrized fragmental orbitals on the S66×8 data set containing 528 bimolecular interactions of equilibrium and arbitrary geometries. It was also found that it is only necessary to deploy numerical grids on atoms within their van der Waals contacts, significantly reducing the small, albeit extra, computational cost. We anticipate that the XFP presented here may be applied to molecular dynamics simulations of macromolecules using a fragment-based quantum mechanical potential with improved SCF convergence and computational accuracy.