Magnetic Exchange Interactions Research Articles

Chemical ordering and magnetism in face-centered cubic CrCoNi alloy

The impact of magnetism on chemical ordering in face-centered cubic CrCoNi medium entropy alloy is studied by a combination of ab initio simulations, machine learning potentials, and Monte Carlo simulations. Large magnetic energies are revealed for some mixed L12/L10 type ordered configurations, which are rooted in strong nearest-neighbor magnetic exchange interactions and chemical bonding among the constituent elements. There is a delicate interplay between magnetism and stability of MoPt2 and L12/L10 type of order, which may explain opposing experimental and theoretical findings.

Ferromagnetic Exchange and Slow Magnetic Relaxation in Cobalt Bis(1,2-dithiolene)-Bridged Dilanthanide Complexes.

The construction of multinuclear lanthanide-based molecules with significant magnetic exchange interactions represents a key challenge in the realization of single-molecule magnets with high operating temperatures. Here, we report the synthesis and magnetic characterization of two series of heterobimetallic compounds, (Cp*2Ln)2(μ-Co(pdt)2) (Ln = Y3+, Gd3+, Dy3+; pdt2- = 1,2-diphenylethylenedithiolate) and [K(18-crown-6)][(Cp*2Ln)2(μ-Co(pdt)2)] (Ln = Y3+, Gd3+), featuring two lanthanide centers bridged by a cobalt bis(1,2-dithiolene) complex. Dc magnetic susceptibility data collected for the Gd congeners indicate significant Gd-Co ferromagnetic exchange interactions with fits affording J = +11.5 and +7.33 cm-1, respectively. Magnetization decay and ac magnetic susceptibility measurements carried out on the single-molecule magnet (Cp*2Dy)2(μ-Co(pdt)2) reveal full suppression of quantum tunneling and open-loop hysteresis persisting up to 3.5 K. These results, along with those of high-field EPR spectroscopy, suggest that transition metalloligands can enforce strong exchange interactions with adjacent lanthanide centers while maintaining a geometry that preserves molecular anisotropy. Furthermore, the magnetic properties of [K(18-crown-6)][(Cp*2Gd)2(μ-Co(pdt)2)] show that increasing the spin of the ground state of the bridging complex may be a viable alternative to increasing J in obtaining well-isolated, strongly coupled magnetic ground states.

Spin-orbit entangled moments and magnetic exchange interactions in cobalt-based honeycomb magnets BaCo2(XO4)2 (X = P, As, Sb)

Co-based honeycomb magnets have been actively studied recently for the potential realization of emergent quantum magnetism therein such as the Kitaev spin liquid. Here we employ density functional and dynamical mean-field theory methods to examine a family of the Kitaev magnet candidates BaCo2(XO4)2 (X = P, As, Sb), where the compound with X = Sb being not synthesized yet. Our study confirms the formation of Mott insulating phase and the Jeff = 1/2 spin moments at Co2+ sites despite the presence of a sizable amount of trigonal crystal field in all three compounds. The pnictogen substitution from phosphorus to antimony significantly changes the in-plane lattice parameters and direct overlap integral between the neighboring Co ions, leading to the suppression of the Heisenberg interaction. More interestingly, the marginal antiferromagnetic nearest-neighbor Kitaev term changes sign into a ferromagnetic one and becomes sizable at the X = Sb limit. Our study suggests that the pnictogen substitution can be a viable route to continuously tune magnetic exchange interactions and to promote magnetic frustration for the realization of potential spin liquid phases in BaCo2(XO4)2.

Above-Room-Temperature Ferromagnetism Regulation in Two-Dimensional Heterostructures by van der Waals Interfacial Magnetochemistry.

Most methods for regulating physical and chemical properties of materials involve the breaking and formation of chemical bonds, which inevitably change local structures. Two-dimensional (2D) ferromagnets are critical for spintronic memory and quantum devices, but most of them maintain ferromagnetism at low temperature, and multiaspect control of 2D ferromagnetism at room temperature or above is still missing. Here, we report a nondestructive, van der Waals (vdW) interfacial magnetochemistry strategy for above-room-temperature, multiaspect 2D ferromagnetism regulation. By vdW coupling nonmagnetic MoS2, WSe2, or Bi1.5Sb0.5Te1.7Se1.3 with 2D vdW ferromagnet Fe3GaTe2, the Curie temperature is enhanced up to 400 K, best for 2D ferromagnets, with 26.8% tuning of room-temperature perpendicular magnetic anisotropy and an unconventional anomalous Hall effect up to 340 K. These phenomena originate from changes in magnetic exchange interactions and magnetic anisotropy energy by interfacial charge transfer and spin-orbit coupling. This work opens a pathway for engineering multifunctional 2D heterostructures by vdW interfacial magnetochemistry.

Dirac fermions and topological phases in magnetic topological insulator films

We develop a Dirac fermion theory for topological phases in magnetic topological insulator films. The theory is based on exact solutions of the energies and the wave functions for an effective model of the three-dimensional topological insulator (TI) film. It is found that the TI film consists of a pair of massless or massive Dirac fermions for the surface states, and a series of massive Dirac fermions for the bulk states. The massive Dirac fermion always carries zero or integer quantum Hall conductance when the valence band is fully occupied while the massless Dirac fermion carries a one-half quantum Hall conductance when the chemical potential is located around the Dirac point for a finite range. The magnetic exchange interaction in the magnetic layers in the film can be used to manipulate either the masses or chirality of the Dirac fermions and gives rise to distinct topological phases, which cover the known topological insulating phases, such as the quantum anomalous Hall effect, quantum spin Hall effect and axion effect, and also the novel topological metallic phases, such as the half-quantized Hall effect, half quantum mirror Hall effect, and metallic quantum anomalous Hall effect.

Separate magnetic and structural phase transitions in Mn50−xFexRh50 films grown on MgO

Abstract Investigations of the magnetic and structural characteristics of Mn50−x Fe x Rh50 alloys are important due to their notable phase transition behavior. In this study, a series of highly ordered epitaxial films with varying Fe concentrations are grown on MgO (001) substrate. At low Fe concentrations (x = 0, 2, 6), a separation between the structural phase transition and the magnetic phase transition is observed. Unlike structural phase transitions, temperature-dependent magnetization exhibits fairly large temperature hysteresis. In addition, the structural transition induces further tetragonal distortion, resulting in an intermediate phase between the B2 and L10 structures. This separated magnetic and structural phase transitions have been further validated through x-ray diffraction, anisotropic magnetoresistance and spin-pumping measurements. Moreover, as the Fe concentration is increased, the Mn50−x Fe x Rh50 films exhibit ferromagnetic behavior due to competitive magnetic exchange interactions, while the structural phase transition is suppressed.

Magnetic properties and magnetocaloric effect of chromium-based high-entropy perovskite oxides

High-entropy oxides (HEOs) represent a significant advancement in the field of high-entropy ceramics and have received considerable attention in magnetism research. This study systematically investigated the magnetic and magnetocaloric properties of HEOs at low temperatures. To this end, three high-entropy perovskites metal oxides were designed using rare-earth transition metals ((R0.2Gd0.2Eu0.2Dy0.2Tb0.2)CrO3, R = Pr, Nd and Sm) because of their remarkable performance in magnetic refrigeration. The samples were prepared using solid-state sintering method and had a single-phase orthogonal distortion structure. All prepared samples exhibited weak antiferromagnetic properties and an antiferromagnetic–paramagnetic transition at lower temperatures. Further, the samples exhibited maximum magnetic entropy changes of 12.8, 13.1, and 13.6 J/kg K under a magnetic field intensity of 50 kOe with relative cooling powers of 153.05, 143.71, and 149.15 J/kg, respectively. An Arrott diagram indicated that all samples exhibited second-order phase transitions. The changes in the degree of spin ordering and the transition trends of the magnetic exchange interaction for each sample were analyzed at the phase transition temperature. The calculated magnetocaloric performance data indicate that these compounds exhibit excellent magnetocaloric properties and can be used as low-temperature magnetic refrigeration materials.

Effect of Oriented External Electric Field in Altering Magnetic Exchange and Magnetic Anisotropy in Lanthanide-Radical Complexes.

Magnetic exchange coupling (J) is one of the important spin Hamiltonian parameters that control the magnetic characteristics of single-molecule magnets (SMMs). While numerous chemical methodologies have been proposed to modify ligands and control the J value, and magneto-structural correlations have been developed accordingly, altering this parameter through non-chemical means remains a challenging task. This study explores the impact of an Oriented-External Electric Field (OEEF) on over twenty lanthanide-radical complexes using Density Functional Theory (DFT) and ab initio Complete Active Space Self-Consistent Field (CASSCF) methods. Five complexes - [{(Me3Si)2N]2Gd(THF)}2(μ-η2:η2-N2)] (1), [Gd(Hbpz3)2(dtbsq)] (2), [Gd(hfac)3(IM-2py)] (3), [Gd(hfac)3(NITBzImH)] (4), and [Gd(hfac)3{2Py-NO}(H2O)] (5) - were selected for detailed analysis, revealing significant OEEF effects on magnetic exchange interactions and structural parameters. Various parameters such as bond distances, bond angles, and torsional angles were examined as a function of OEEF to establish guiding principles for molecule selection. In complexes 1, 2, and 3, OEEF influenced torsional angles and altered exchange interactions. Complex 4 demonstrated enhanced ferromagnetic coupling under OEEF, reaching a maximum J value of +5.3 cm-1. Complex 5 reveals switching the sign of JGd-rad exchange interaction from antiferromagnetic to ferromagnetic under OEEF, highlighting the potential of electric fields in designing materials with tuneable magnetic properties. These findings offer valuable insights for future research and applications in advanced materials and molecular electronics.

First-principles studies on the magnetic properties, exchange interactions and spin Hall conductivity of the half-Heusler alloy MgMnGe

This study presents first-principles estimates of the magnetic characteristics, exchange interactions and the spin Hall conductivity (SHC) of the half -Heusler alloy MgMnGe. Various configurations of spin-ordering are taken into consideration. It is discovered that MgMnGe has an antiferromagnetic ground state, with a degenerate in plane Néel vector. The Korringa-Kohn-Rostoker method has been used to calculate the Néel temperature and exchange interactions. The Néel temperature of this material obtained theoretically using mean field approximation is 447.2 K while the experimental reported value is largely higher than 300 K. The exchange interaction has been shown to be mostly mediated by the Mn1-Mn2 exchange. The spin Hall conductivity of MgMnGe is obtained by mapping the electronic structures to the Wannier basis via linear response theory. The spin Hall conductivity as a function of the Fermi level has been determined.

Tuning the Magnetism in Ultrathin CrxTey Films by Lattice Dimensionality

Abstract2D magnetic crystals with atomic thickness exhibit intriguing physical properties, which have attracted considerable research interest in the related materials’ family, both in fundamental research and in developing spintronic devices. The recent discovery of some non‐van der Waals 2D magnetic crystals expands the systems. Nevertheless, the relationship between the dimensionality of microscopic magnetic exchange interactions and macroscopic magnetic properties at the 2D limit remains to be fully elucidated. Here, we have fabricated mono‐phased continuous ultrathin CrTe2 and Cr3Te4 films by molecular beam epitaxy and elucidated the diverse magnetism tuned by the dimensionality of exchange interactions by a joint study of spin‐polarized scanning tunneling microscopy, magnetization, magneto‐transport measurements, and density functional theory calculations. The transition from a zigzag‐antiferromagnetic order in the monolayer CrTe2 to a ferromagnetic (FM) order in the second‐layer CrTe2 is confirmed, which is driven by their varied in‐plane lattice constants induced change of 2D exchange interactions. A robust FM state with large perpendicular magnetic anisotropy in Cr3Te4 is observed, originating from its strong 3D exchange interactions. The observed evolution of magnetism demonstrates that the dimensionality of magnetic exchange interactions strongly influences magnetism even at the 2D limit.

Perturbational Analysis of Magnetic Force Theorem for Magnetic Exchange Interactions in Molecules and Solids.

There have been increasing efforts to compute magnetic exchange coupling constants for transition metal complexes and magnetic insulators using the magnetic force theorem and Green's function-based linear response methods. These were originally conceived for magnetic metals, yet it has not been clear how these methods fare conceptually with the conventional models based on electron-correlation interactions among so-called magnetic orbitals. We present a spinor-based theoretical analysis pertinent to the magnetic force theorem and linear response theory using Brillouin-Wigner perturbation method and Green's function perturbation method, and we shed light on the conceptual nature of the Lichtenstein formula in its applications for calculations of the total energy and magnetic exchange coupling constants for both molecules and solids. Derivation of the magnetic force theorem in this perturbational analysis identifies the first-order energy correction terms, which are considered as the ferromagnetic component for the magnetic exchange interactions of transition metal compounds but are not included in the Lichtenstein formula. Detailed perturbational analysis of the energy components involved in the magnetic force theorem identifies the energy components that are missing in the Lichtenstein formula but are critical in the Anderson's model for transition metal complexes and magnetic insulators where magnetic orbitals can overlap.

The Role of Rare-Earth Atoms in the Anisotropy and Antiferromagnetic Exchange Coupling at a Hybrid Metal-Organic Interface.

Magnetic anisotropy and magnetic exchange interactions are crucial parameters that characterize the hybrid metal-organic interface, a key component of an organic spintronic device. It is shown that the incorporation of 4f RE atoms to hybrid metal-organic interfaces of CuPc/REAu2 type (RE = Gd, Ho) constitutes a feasible approach toward on-demand magnetic properties and functionalities. The GdAu2 and HoAu2 substrates differ in their magnetic anisotropy behavior. Remarkably, the HoAu2 surface promotes the inherent out-of-plane anisotropy of CuPc, owing to the match between the anisotropy axis of substrate and molecule. Furthermore, the presence of RE atoms leads to a spontaneous antiferromagnetic exchange coupling at the interface, induced by the 3d-4f superexchange interaction between the unpaired 3d electron of CuPc and the 4f electrons of the RE atoms. It is shown that 4f RE atoms with unquenched quantum orbital momentum ( ), as it is the case of Ho, induce an anisotropic interfacial exchangecoupling.

Structural Manipulation of Spin Excitations in a Molecular Junction.

Single metallocene molecules act as sensitive spin detectors when decorating the probe of a scanning tunneling microscope (STM). However, the impact of the atomic-scale electrode details on the molecular spin state has remained elusive to date. Here, a nickelocene (Nc) STM junction is manipulated in an atomwise manner showing clearly the dependence of the spin excitation spectrum on the anchoring of Nc to Cu(111), a Cu monomer, and trimer. Moreover, while the spin state of the same Nc tip is a triplet with tunable spin excitation energies upon contacting the surface, it transitions to a Kondo-screened doublet on a Cu atom. Notably, the nontrivial magnetic exchange interaction of the molecular spin with the electron continuum of the substrate determines the spectral line shape of the spin excitations.

Synthesis, magnetic and structural properties of (1-x)LiFe5O8–(x)LiZn2.5Ti2.5O8 spinel solid solutions

Compounds with the spinel structure present a technologically important class of materials. Spinels show a great variety of magnetic and magnetoelectric phenomena owing to the facile entrance of transition metals in the cationic sublattices. One of such examples is lithium ferrite LiFe5O8 with high ferrimagnetic phase transition temperature and cationic order-disorder transformations. The latter are important as they influence the crystal symmetry, which is crucial for physical properties. In this work we synthesize a series of (1-x)LiFe5O8–(x)LiZn2.5Ti2.5O8 (0≤x≤1) solid solutions and characterize them with various experimental and theoretical methods. The solid solutions experience a series of concentrational phase transformations between atomically ordered (P43(1)32) and disordered (Fd3¯m) phases. The variation with concetration x of Fe3+ occupancies in cationic sublattices is studied using Mössbauer spectroscopy. The Mössbauer and magnetic measurements reveal sharp suppression of magnetic properties at x≥0.5 when the number of Fe3+ ions in the A-sublattice vanishes. The magnetic behaviour in the studied series of solid solutions is confirmed by Monte Carlo calculations with magnetic exchange interactions determined using the density functional theory.

Stress Evaluation of Ferromagnetic Materials Based on a New Barkhausen Noise Sensor Composed by High Entropy Alloy Magnetic Core

Abstract During the service of ferromagnetic structural steel materials, stress should be evaluated accurately. Although the magnetic Barkhausen noise (MBN) testing has the ability to sense stress, it can be easily interfered with by environment. In this paper, a new MBN sensor is fabricated by selecting FeCoNi(AlMn)0.25 high entropy alloy (HEA) as the case of magnetic core to improve the accuracy of stress evaluation. The process optimization results show that the stability of MBN signal characteristics is the largest when the excitation frequency is 4 Hz and the voltage is 6 V. The signal-to-noise ratio of MBN indicates that the HEA and Ni-Zn ferrite probes have better anti-interference capability. The MBN signal characteristic values peak voltage and root mean square measured by the HEA probe can linearly quantify the stress level with higher efficiency, stability, and accuracy. The underlying reason of high sensitivity of HEA probe to the variation of MBN signals is revealed based on the magnetic properties. The microstructure and the thermodynamic parameters are analyzed to clarify whether the additions of Al and Mn atoms can affect the short-ranged magnetic exchange interaction and lattice distortion, which affects the magnetization behavior of HEA. Finally, the availability of MBN sensor with HEA magnetic core to the stress evaluation on the retired slide rails of car seats is conducted, which demonstrates its great application value.

On the study of proximity magnetism in van der Waals graphene/CuCrP2S6 heterostructure via the anomalous Hall effect

Recently, several experimental works have appeared in the literature where induced magnetism in single- and few-layer graphene (SL-gr and FL-gr) interfaced with layered van der Waals materials was investigated via the application of the anomalous Hall effect (AHE). In most of these works, it is suggested that the observation of the AHE in such systems can be explained by a magnetic exchange interaction appearing at the interface between graphene and the underlying magnetic insulator. Considering the recently studied FL-graphene/bulk-CuCrP2S6 system as an example, our careful and rigorous analysis of recent experimental and theoretical data presented in the literature shows that the claimed observation of the AHE and magnetic proximity effect in this system is not supported. Moreover, the theoretically calculated electronic structures of the studied system contain serious errors and flaws that cannot be considered as an accurate description of such an interface and cannot be taken as solid support for the proposed proximity effect.

Strong Magnetic Exchange Interactions and Delocalized Mn-O States Enable High-Voltage Capacity in the Na-Ion Cathode P2-Na0.67[Mg0.28Mn0.72]O2.

The increased capacity offered by oxygen-redox active cathode materials for rechargeable lithium- and sodium-ion batteries (LIBs and NIBs, respectively) offers a pathway to the next generation of high-gravimetric-capacity cathodes for use in devices, transportation and on the grid. Many of these materials, however, are plagued with voltage fade, voltage hysteresis and O2 loss, the origins of which can be traced back to changes in their electronic and chemical structures on cycling. Developing a detailed understanding of these changes is critical to mitigating these cathodes' poor performance. In this work, we present an analysis of the redox mechanism of P2-Na0.67[Mg0.28Mn0.72]O2, a layered NIB cathode whose high capacity has previously been attributed to trapped O2 molecules. We examine a variety of charge compensation scenarios, calculate their corresponding densities of states and spectroscopic properties, and systematically compare the results to experimental data: 25Mg and 17O nuclear magnetic resonance (NMR) spectroscopy, operando X-band and ex situ high-frequency electron paramagnetic resonance (EPR), ex situ magnetometry, and O and Mn K-edge X-ray Absorption Spectroscopy (XAS) and X-ray Absorption Near Edge Spectroscopy (XANES). Via a process of elimination, we suggest that the mechanism for O redox in this material is dominated by a process that involves the formation of strongly antiferromagnetic, delocalized Mn-O states which form after Mg2+ migration at high voltages. Our results primarily rely on noninvasive techniques that are vital to understanding the electronic structure of metastable cycled cathode samples.

Proximity coupling induced two dimensional magnetic order in EuO-based synthetic ferrimagnets

Proximity effects allow for the adjustment of magnetic properties in a physically elegant way. If two thin ferromagnetic (FM) films are brought into contact, electronic coupling alters their magnetic exchange interaction at their interface. For a low-TC\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_C$$\\end{document} rare-earth FM coupled to a 3d transition metal FM, even room temperature magnetism is within reach. In addition, magnetic proximity coupling is particularly promising for increasing the magnetic order of metastable materials such as europium monoxide (EuO) beyond their bulk TC\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_C$$\\end{document}, since neither the stoichiometry nor the insulating properties are modified. We investigate the magnetic proximity effect at Fe/EuO and Co/EuO interfaces using hard X-ray photoelectron spectroscopy. By exciting the FM layers with circularly polarized light, magnetic dichroism is observed in angular dependence on the photoemission geometry. In this way, the depth-dependence of the magnetic signal is determined element-specifically for the EuO and 3d FM parts of the bilayers. In connection with atomistic spin dynamics simulations, the thickness of the EuO layer is found to be crucial, indicating that the observed antiferromagnetic proximity coupling is a short-ranged and genuine interface phenomenon. This fact turns the bilayer into a strong synthetic ferrimagnet. The increase in magnetic order in EuO occurs in a finite spatial range and is therefore particularly strong in the 2D limit—a counterintuitive but very useful phenomenon for spin-based device applications.

Zero Thermal Expansion Behavior in High‐Entropy Anti‐Perovskite Mn3Fe0.2Co0.2Ni0.2Mn0.2Cu0.2N

AbstractThe exploration of the non‐collinear antiferromagnetic (AFM) phase holds promise for the discovery of zero thermal expansion (ZTE) materials, which is of great significance to resist the temperature effect in aerospace and precision engineering fields. Currently, there is still a lack of effective approaches to regulate this special AFM phase. In this work, a non‐collinear AFM phase has been obtained in the anti‐perovskite compound Mn3Fe0.2Co0.2Ni0.2Mn0.2Cu0.2N proposed by high‐entropy engineering. Utilizing neutron powder diffraction (NPD) analysis, the magnetic structure is resolved to be a triangular AFM phase with a k = [0, 0, 0] and a ferromagnetic (FM) component located at the corner of the cubic structure, which belongs to the R‐3 space group. Particularly, it presents ZTE behavior in a wide temperature range from 10 to 180 K. In‐situ NPD analysis reveals that the negative thermal expansion attributed to magnetic evolution almost offsets the normal positive thermal expansion quantified by the Debye formula. Further first principles calculations reveal that the specific AFM phase derives from the AFM‐type nearest neighboring magnetic exchange interactions and the easy‐axis‐type magnetic anisotropy. This demonstration offers an efficient strategy for designing magnetic structures and achieving ZTE over a wide temperature range.

Copper Intercalation Induces Amorphization of 2D Cu/WO3 for Room‐Temperature Ferromagnetism

AbstractFerromagnetism in the two‐dimensional limit has become an intriguing topic for exploring new physical phenomena and potential applications. To induce ferromagnetism in 2D materials, intercalation has been proposed to be an effective strategy, which could introduce lattice distortion and unpaired spin into the material to modulate the magnetocrystalline anisotropy and magnetic exchange interactions. To strengthen the understanding of the magnetic origin of 2D material, Cu was introduced into a 2D WO3 through chemical intercalation in this work (2D Cu/WO3). In contrast to the diamagnetic nature of Cu and WO3, room‐temperature ferromagnetism was characterized for 2D Cu/WO3. Experimental and theoretical results attribute the ferromagnetism to the bound magnetic polaron in 2D Cu/WO3, which is consist of unpaired spins from W5+/W4+ with localized carriers from oxygen vacancies. Overall, this work provides a novel approach to introduce ferromagnetism into diamagnetic WO3, which could be applied for a wider scope of 2D materials.