Magnetic Exchange Interactions Research Articles

Thermal evolution of spin excitations in honeycomb Ising antiferromagnetic FePSe3

We use elastic and inelastic neutron scattering (INS) to study the antiferromagnetic (AF) phase transitions and spin excitations in the two-dimensional (2D) zig-zag antiferromagnet FePSe3. By determining the magnetic order parameter across the AF phase transition, we conclude that the AF phase transition in FePSe3 is first-order in nature. In addition, our INS measurements reveal that the spin waves in the AF ordered state have a large easy-axis magnetic anisotropy gap, consistent with an Ising Hamiltonian, and possible biquadratic magnetic exchange interactions. On warming across TN, we find that dispersive spin excitations associated with three-fold rotational symmetric AF fluctuations change into FM spin fluctuations above TN. These results suggest that the first-order AF phase transition in FePSe3 may arise from the competition between C3 symmetric AF and C1 symmetric FM spin fluctuations around TN, in place of a conventional second-order AF phase transition.

Investigation of magnetic exchange interactions in Cr-doped Ca0.5Sr0.5RuO3

The value of Curie temperature (T C) and resistivity for Ca0.5Sr0.5Ru1−xCrxO3 are found to increase with increase in x from 0 to 0.1. The T C increases from 38 K to 107 K. In this work, we investigate the increment using spin polarised density functional theory. The compounds exhibit Griffiths phase. Our results show that to understand the behaviour of the increase in T C , it is important to consider not only the competing super exchange and super–super exchange interactions between Ru/Cr–Ru/Cr ion pairs but also the increment in the value of the magnetic moment due to the localized character of the dopant (Cr) ion. Our results suggest that the increased resistivity with Cr doping could be due to increased scattering rate and strong on-site coulomb repulsion due to Cr doping. It is also important to note that the behaviour of T C with the Ru–O–Ru bond angle is not in line with the functional form reported for ferrites, chromites and some ferromagnetic oxides. We believe that our results will be helpful in further exploring the origin of magnetism and transport on Cr doping in perovskite ruthenates.

The Interplay of Electronic Configuration and Anion Ordering on the Magnetic Behavior of Hydroxyfluoride Diaspores.

We report a new nickel hydroxyfluoride diaspore Ni(OH)F prepared using hydrothermal synthesis from NiCl2·6H2O and NaF. Magnetic characterization reveals that, contrary to other reported transition-metal hydroxyfluoride diaspores, Ni(OH)F displays weak ferromagnetism below the magnetic ordering temperature. To understand this difference, neutron diffraction is used to determine the long-range magnetic structure. The magnetic structure is found to be distinct from those reported for other hydroxyfluoride diaspores and shows an antiferromagnetic spin ordering in which ferromagnetic canting is allowed by symmetry. Furthermore, neutron powder diffraction on a deuterated sample, Ni(OD)F, reveals partial anion ordering that is distinctive to what has previously been reported for Co(OH)F and Fe(OH)F. Density functional theory calculations show that OH/F ordering can have a directing influence on the lowest energy magnetic ground state. Our results point toward a subtle interplay between the sign of magnetic exchange interactions, the electronic configuration, and anion disordering.

Magnetic exchange interactions and non-Debye relaxation in spin-3/2 frustrated Kagomé magnet Co3V2O8

We report the experimental determination of the magnetic exchange parameter ( J/kB = 2.88 ± 0.02 K) for the Spin-3/2 ferromagnetic (FM) Kagomé lattice system: Co3V2O8 using the temperature dependence of dc-magnetic susceptibility χ(T) data by employing the fundamental Heisenberg linear chain model. Our results are quite consistent with the theoretically reported nearest neighbor dominant FM exchange coupling strength Jex−NN∼ 2.45 K. Five different magnetic phase transitions (6.2–11.2 K) and spin-flip transitions (9.6–7.7 kOe) have been probed using the ∂ ( χT )/ ∂T vs. T, heat capacity (CP -T) and differential isothermal magnetization curves. Among such sequence of transitions, the prominent ones being incommensurate antiferromagnetic (AFM) state at 11.2 K, commensurate AFM state at 8.8 K, and commensurate FM state across 6.2 K. All the successive magnetic phase transitions have been mapped onto a single H-T plane through which one can easily distinguish the above-mentioned different phases. The magnetic contribution of the CP -T near TN (11.2 K) has been analyzed using the power-law expression CM = A|T — TN|−α resulting in the critical exponent α = 0.18 ± 0.01 (0.15 ± 0.003) for T < TN (T > TN ), respectively for the Co3V2O8. It is interesting to note that non-Debye type dipole relaxation is quite prominent in Co3V2O8 and was evident from the Kohlrausch–Williams–Watts analysis of complex modulus and impedance spectra (0 ⩽β⩽ 1). Mott’s variable-range hopping of charge carriers process is evident through the resistivity analysis ( ρac -T −1/4 ) in the temperature range 275 ∘C–350 ∘C. Moreover, the frequency-dependent analysis of σac (ω) follows Jonscher’s power law yielding two distinct activation energies ( Ea∼ 0.37 and 2.29 eV) between the temperature range 39 ∘C–99 ∘C and 240 ∘C–321 ∘C.

Magnetic exchange interaction in a spin valve with a chiral spin-triplet superconductor

Magnetic exchange interaction in a spin valve with a chiral spin-triplet superconductor

Correlation between structural – Optical – Magnetic properties of cobalt based double perovskite, Sr2TiCo1-yO6-δ

The influence of charge disorders, non-stoichiometry and oxygen vacancy on the physical properties such as optical properties and magnetic properties of Sr2TiCo1-yO6-δ exhibiting simple cubic (space group Pm-3m) structure has been investigated in the article. An analysis of energy dispersive X-ray spectrum reveals a deficiency of cobalt ions. Variable range hopping conduction mechanism is observed below 100 K. X-ray photoelectron spectroscopy results indicated the presence of oxygen deficiency accompanied by multiple valent cobalt ions (Co4+ and Co3+) in the material. The optical features of the material are noteworthy, with a wide range of significant optical absorption occurring in the UV range, characterized by a band gap of ∼3.03 eV. In addition, huge photoluminescence emission in the visible range and at room temperature was observed. In this context, Sr2TiCo1-yO6-δ is a potential material for optical applications due to its high thermal stability with respect to temperature in air environment. Magnetic studies indicate the presence of mixed exchange magnetic interactions among the cobalt ions with different valence states generated during the synthesis. The weak ferromagnetic exchange interactions involved Co3+-O-Co4+ ions.

Magnetic exchange interactions at the proximity of a superconductor

Interfacing magnetism with superconductivity gives rise to a wonderful playground for intertwining key degrees of freedom: Cooper pairs, spin, charge, and spin–orbit interaction, from which emerge a wealth of exciting phenomena, fundamental in the nascent field of superconducting spinorbitronics and topological quantum technologies. Magnetic exchange interactions (MEIs), being isotropic or chiral such as the Dzyaloshinskii–Moriya interactions, are vital in establishing the magnetic behavior at these interfaces as well as in dictating not only complex transport phenomena, but also the manifestation of topologically trivial or non-trivial objects. Here, we propose a methodology enabling the extraction of the tensor of MEI from electronic structure simulations accounting for superconductivity. We apply our scheme to the case of a Mn layer deposited on Nb(110) surface and explore proximity-induced impact on the MEI. The latter are weakly modified by a realistic electron-phonon coupling. However, tuning the superconducting order parameter, we unveil potential change of the magnetic order accompanied with chirality switching, as induced by the interplay of spin-orbit interaction and Cooper pairing. Owing to its simple formulation, our methodology can be readily implemented in state-of-the-art frameworks capable of tackling superconductivity and magnetism. We thus foresee implications in the simulations and prediction of topological superconducting bits as well as of cryogenic superconducting hybrid devices involving magnetic units.

Exact Matrix Product States at the Quantum Lifshitz Tricritical Point in a Spin-1/2 Zigzag-Chain Antiferromagnet with Anisotropic Γ Term.

Quantum anisotropic exchange interactions in magnets can induce competitions between phases in a different manner from those typically driven by geometrically frustrated interactions. We study a one-dimensional spin-1/2 zigzag chain with such an interaction, Γ term, in conjunction with the Heisenberg interactions. We find a ground state phase diagram featuring a multicritical point where five phases converge: a uniform ferromagnet, two antiferromagnets, Tomonaga-Luttinger liquid, and a dimer-singlet coexisting with nematic order. This multicritical point is simultaneously quantum tricritical and Lifshitz, and most remarkably, it hosts multidegenerate ground state wave functions with the degeneracy increasing in squares of system size. The exact ground states are obtained in the matrix product form opening wide applications to frustration-free models.

First-principles study of electronic and magnetic properties in site dependent Fe-doped chalcopyrite ZnSnAs2semiconductors: Comparison with those of Mn- and Cr-doped ZnSnAs2

The electronic and magnetic properties of Fe-doped ZnSnAs2 within chalcopyrite structures have been studied by first-principles calculations using the density functional theory (DFT) full-potential linearized augmented-plane-wave method and the plane-wave pseudopotential methods. The band gap of ZnSnAs2 was determined to be 0.816 eV using the modified Beck-Jonson (mBJ) exchange potential. When Fe dopants substitutionally replace Sn atoms, half-metallic ferromagnetism is observed in ZnSnAs2 where in the spin-down band at the Fermi level is entirely unoccupied. The magnetic exchange interaction between two Fe atoms substituting Zn and Sn sites in the supercell with different geometries has been studied. The total energy difference (ΔE) between the antiferromagnetic (AFM) and ferromagnetic (FM) configurations suggests that Fe atom occupation of Zn sites in ZnSnAs2 leads to a predominantly AFM state, while the Fe atom occupation of Sn sites prefers the FM state.

Noncentrosymmetric Triangular Magnet CaMnTeO6: Strong Quantum Fluctuations and Role of s0 versus s2 Electronic States in Competing Exchange Interactions.

Noncentrosymmetric triangular magnets offer a unique platform for realizing strong quantum fluctuations. However, designing these quantum materials remains an open challenge attributable to a knowledge gap in the tunability of competing exchange interactions at the atomic level. Here, a new noncentrosymmetric triangular S=3/2 magnet CaMnTeO6 is created based on careful chemical and physical considerations. The model material displays competing magnetic interactions and features nonlinear optical responses with the capability of generating coherent photons. The incommensurate magnetic ground state of CaMnTeO6 with an unusually large spin rotation angle of 127°(1) indicates that the anisotropic interlayer exchange is strong and competing with the isotropic interlayer Heisenberg interaction. The moment of 1.39(1)µB, extracted from low-temperature heat capacity and neutron diffraction measurements, is only 46% of the expected value of the static moment 3µB. This reduction indicates the presence of strong quantum fluctuations in the half-integer spin S=3/2 CaMnTeO6 magnet, which is rare. By comparing the spin-polarized band structure, chemical bonding, and physical properties of AMnTeO6 (A=Ca, Sr, Pb), how quantum-chemical interpretation can illuminate insights into the fundamentals of magnetic exchange interactions, providing a powerful tool for modulating spin dynamics with atomically precise control is demonstrated.

Critical behavior in the ferromagnet

High-Curie-temperature ferromagnets are promising candidates for designing new spintronic devices. Here we have successfully synthesized the single crystal of the itinerant ferromagnet Mn5Ge3 using flux method. Its critical properties were investigated by means of bulk dc magnetization at the boundary of the paramagnetic (PM) and ferromagnetic (FM) phase to determine intrinsic magnetic interactions. Critical exponents with a critical temperature K and with K are acquired by the modified Arrott plot, whereas is deduced by a critical isotherm analysis at K. The self-consistency and reliability of these critical exponents are verified by the Widom scaling law and the scaling equations. Further analysis reveals that the spin interaction in Mn5Ge3 exhibits three-dimensional Ising-like behavior. The magnetic exchange interaction is found to decay as , meaning that the spin interactions exceed the nearest neighbors, which may be related to the different Mn-Mn interactions with inequal exchange strengths.

Ferromagnetically Coupled Chromium(III) Dimer Shows Luminescence and Sensitizes Photon Upconversion.

There has been much progress on mononuclear chromium(III) complexes featuring luminescence and photoredox activity, but dinuclear chromium(III) complexes have remained underexplored in these contexts until now. We identified a tridentate chelate ligand able to accommodate both meridional and facial coordination of chromium(III), to either access a mono- or a dinuclear chromium(III) complex depending on reaction conditions. This chelate ligand causes tetragonally distorted primary coordination spheres around chromium(III) in both complexes, entailing comparatively short excited-state lifetimes in the range of 400 to 800 ns in solution at room temperature and making photoluminescence essentially oxygen insensitive. The two chromium(III) ions in the dimer experience ferromagnetic exchange interactions that result in a high spin (S=3) ground state with a coupling constant of +9.3 cm-1. Photoinduced energy transfer from the luminescent ferromagnetically coupled dimer to an anthracene derivative results in sensitized triplet-triplet annihilation upconversion. Based on these proof-of-principle studies, dinuclear chromium(III) complexes seem attractive for the development of fundamentally new types of photophysics and photochemistry enabled by magnetic exchange interactions.

Metal-Ligand Exchange Coupling Alters the Open-Shell Ligand Electronic Structure in a Bis(semiquinone) Complex.

The electronic structure of the bis(dioxolene) bridging ligand -SQ2Th2- is responsive to metal-ligand magnetic exchange coupling. Comparison of the crystal structure of (NiSQ)2Th2 to that of (ZnSQ)2Th2 indicates an open-shell biradical ground state for the dinuclear Ni(II) complex compared to the closed-shell quinoidal character found in the dinuclear Zn(II) complex. Consistent with a comparison of bond lengths obtained by X-ray diffraction, the analysis of the variable-temperature magnetic susceptibility data for crystalline (NiSQ)2Th2 yields reduced SQ-SQ radical-radical magnetic exchange coupling (JSQ-SQ = -203 cm-1) compared to that of (ZnSQ)2Th2 (JSQ-SQ = -321 cm-1). The reduced SQ-SQ exchange coupling in (NiSQ)2Th2 derives from an attenuation of the SQ spin densities, which in turn is derived from the Ni-SQ antiferromagnetic exchange interactions. This reduction in SQ--SQ exchange that we observe for (NiSQ)2Th2 correlates with an effective lengthening of the bridge unit by ∼2.1 Å relative to that of (ZnSQ)2Th2. This magnitude of the effective increase in the bridge distance is consistent with the (NiSQ)2Th2 JSQ-SQ value lying between those of (ZnSQ)2Th2 and (ZnSQ)2Th3. The ability to modulate spin populations on an organic radical via pairwise Ni-SQ magnetic exchange interactions is a general way to affect electronic coupling in the Th-Th bridge. Our results suggest that metal-radical exchange coupling represents a powerful mechanism for tuning organic molecular electronic structure, with important implications for molecular electronics and molecular electron transport.

Theory for Charge Density Wave and Orbital-Flux State in Antiferromagnetic Kagome Metal FeGe

We theoretically study the charge order and orbital magnetic properties of a new type of antiferromagnetic kagome metal FeGe. Based on first-principles density functional theory calculations, we study the electronic structures, Fermi-surface quantum fluctuations, as well as phonon properties of the antiferromagnetic kagome metal FeGe. It is found that charge density wave emerges in such a system due to a subtle cooperation between electron–electron interactions and electron–phonon couplings, which gives rise to an unusual scenario of interaction-triggered phonon instabilities, and eventually yields a charge density wave (CDW) state. We further show that, in the CDW phase, the ground-state current density distribution exhibits an intriguing star-of-David pattern, leading to flux density modulation. The orbital fluxes (or current loops) in this system emerge as a result of the subtle interplay between magnetism, lattice geometries, charge order, and spin-orbit coupling (SOC), which can be described by a simple, yet universal, tight-binding theory including a Kane–Mele-type SOC term and a magnetic exchange interaction. We further study the origin of the peculiar step-edge states in FeGe, which sheds light on the topological properties and correlation effects in this new type of kagome antiferromagnetic material.

Spectroscopic properties and photoluminescence of the Li2B4O7:Mn,Sm glass

The Li2B4O7:Mn,Sm glass containing 1.0 mol.% MnO2 and Sm2O3 were researched by X-ray diffraction (XRD), electron paramagnetic resonance (EPR), and optical spectroscopy methods. The studied glass of high optical quality was obtained by high temperature melting technique. Parameters of the local structure (interatomic distances and coordination numbers) of Li2B4O7:Mn,Sm glass were derived from XRD data analysis. The EPR and optical spectroscopy (absorption, luminescence excitation, emission, decay kinetics) show the presence of Mn2+(3d5) and Mn3+(3d4) impurity ions in the Li2B4O7:Mn,Sm glass. By EPR spectroscopy in the studied glass were identified three types of Mn2+ centres: single Mn2+ (1) centres in the strongly distorted sites (ratio of rhombic and axial constants, |E/D| ≤ 1/3), single Mn2+ (2) centres in the sites with almost cubic symmetry (D ≅ 0, E ≅ 0) as well as Mn2+ pairs and small clusters coupled by magnetic dipolar and exchange interaction, respectively. The Mn2+ EPR spectra parameters in the Li2B4O7:Mn,Sm glass have been determined at T = 295 K. Optical absorption spectrum of the Li2B4O7:Mn,Sm glass contains a very broad intense band peaked at 467 nm belonging to the 5Eg(D) → 5T2g(D) transition of Mn3+ ions and several weak narrow lines corresponding to the 6H5/2 → 6P3/2, 6H5/2 → 6F9/2, 6F7/2, 6F5/2 transitions of Sm3+ (4f5, 6H5/2) ions. Emission spectrum of the Li2B4O7:Mn,Sm glass exhibits a broad band corresponding to the 4T1g(G) → 6A1g(S) transition of Mn2+ ions and three characteristic bands in the yellow-orange-red range belonging to the 4G5/2 → 6H5/2, 6H7/2, 6H9/2 transitions of Sm3+ ions. Photoluminescence spectra (excitation and emission) and decay kinetics of the Mn2+ and Sm3+ ions in the Li2B4O7:Mn,Sm glass are interpreted in comparison with corresponding results for Li2B4O7:Mn and Li2B4O7:Sm glasses. Energy transfer processes Sm3+→Mn2+, Mn3+, and Mn2+→Mn3+ in the studied glass are considered.

A Route to Two-Dimensional Room-Temperature Organometallic Multiferroics: The Marriage of d-p Spin Coupling and Structural Inversion Symmetry Breaking.

Two-dimensional (2D) room-temperature multiferroic materials are highly desirable but still very limited. Herein, we propose a potential strategy to obtain such materials in 2D metal-organic frameworks (MOFs) by utilizing the d-p direct spin coupling in conjunction with center-symmetry-breaking six-membered heterocyclic rings. Based on this strategy, a screening of 128 2D MOFs results in the identification of three multiferroics, that is, Cr(1,2-oxazine)2, Cr(1,2,4-triazine)2, and Cr(1,2,3,4-trazine)2, simultaneously exhibiting room-temperature ferrimagnetism and ferroelectricity/antiferroelectricity. The room-temperature ferrimagnetic order (306-495 K) in these MOFs originates from the strong d-p direct magnetic exchange interaction between Cr cations and ligand anions. Specifically, Cr(1,2-oxazine)2 exhibits ferroelectric behavior with an out-of-plane polarization of 4.24 pC/m, whereas the other two manifest antiferroelectric characteristics. Notably, all three materials present suitable polarization switching barriers (0.18-0.31 eV). Furthermore, these MOFs are all bipolar magnetic semiconductors with moderate band gaps, in which the spin direction of carriers can be manipulated by electrical gating.

Magnetic and magnetocaloric effect of Ba[formula omitted]FeReO[formula omitted] double perovskite: Ab-initio and Monte Carlo studies

In this paper, the double perovskite Ba2FeReO6 was studied using ab initio calculations and Monte Carlo simulation within the Ising model framework. The band structure and density of states of the compound were investigated. Moreover, energy calculations were employed to determine the magnetic anisotropy and exchange interactions for conducting a Monte Carlo study. The calculated curie temperature Tc of the compound is found 296 K, which agrees with the experimental value (Tc = 303 K). We have studied also the magnetic and magnetocaloric properties, the maximum entropy change (ΔSmax) and the relative cooling power (RCP) at a magnetic field of h=60 kOe are 4.28 J/kg K and 255 J/kg, respectively. Furthermore, the energy product was calculated using hysteresis loops. These findings suggest that Ba2FeReO6 is a competitive candidate material for magnetic refrigeration applications.

Non-volatile voltage-controlled magnetization in single-phase multiferroic ceramics at room temperature

Single-phase multiferroics (MFs) exhibiting ferroelectricity and ferromagnetism and the strong magnetoelectric (ME) coupling effect at room temperature are seen as key to the development of the next-generation of spintronic devices, multi-state memories, logic devices and sensors. Herein, the single-tetragonal phase (1–x) (Sr0·3Bi0·35Na0·329Li0.021)TiO3-xBiFeO3 (x = 0.2 or 0.4) system was designed to study the intrinsic ME coupling effect at room temperature and high frequencies. The polarization arises from the cooperative displacement of both Fe3+ and Ti4+ relative to the oxygen sublattice in the tetragonally distorted perovskite structure, and the magnetization stems from indirect exchange magnetic interaction between adjacent iron ions. A switchable voltage-controlled magnetization was confirmed by a change of the coercive magnetic field, Hc, and remnant magnetization, Mr, in the x = 0.4 component subjected to an external electric field at room temperature and was possibly attributed to a strain-mediated ME coupling effect. In addition, resonance behaviours of the complex magnetic permeability and complex dielectric permittivity in the GHz band indicate that this ME effect is intrinsic in nature and could broaden the applications of multiferroics to devices operating at microwave frequencies.

Homochiral antiferromagnetic merons, antimerons and bimerons realized in synthetic antiferromagnets

The ever-growing demand for device miniaturization and energy efficiency in data storage and computing technology has prompted a shift towards antiferromagnetic topological spin textures as information carriers. This shift is primarily owing to their negligible stray fields, leading to higher possible device density and potentially ultrafast dynamics. We realize in this work such chiral in-plane topological antiferromagnetic spin textures namely merons, antimerons, and bimerons in synthetic antiferromagnets by concurrently engineering the effective perpendicular magnetic anisotropy, the interlayer exchange coupling, and the magnetic compensation ratio. We demonstrate multimodal vector imaging of the three-dimensional Néel order parameter, revealing the topology of those spin textures and a globally well-defined chirality, which is a crucial requirement for controlled current-induced dynamics. Our analysis reveals that the interplay between interlayer exchange and interlayer magnetic dipolar interactions plays a key role to significantly reduce the critical strength of the Dzyaloshinskii-Moriya interaction required to stabilize topological spin textures, such as antiferromagnetic merons, in synthetic antiferromagnets, making them a promising platform for next-generation spintronics applications.

La(Fe,Si/Al)13-based materials with exceptional magnetic functionalities: a review

The field of magnetic functional materials continues to garner significant attention due to its research and diverse applications, such as magnetic storage and spintronics. Among these, La(Fe,Si/Al)13-based materials exhibit abundant magnetic properties and emerge as highly captivating subjects with immense potential. This review provides an overview of the diverse magnetic structures and itinerant electron metamagnetic transition observed in La(Fe,Si/Al)13-based materials. The transformation of different magnetic configurations elicits the phenomena such as negative thermal expansion, magnetostriction, magnetocaloric effect, and barocaloric effect. In addition, the pivotal role of spin and lattice coupling in these phenomena is revealed. The magnetic functionalities of La(Fe,Si/Al)13-based materials can be controlled through adjustments of magnetic exchange interactions. Key methods, including chemical substitution, external field application, and interstitial atom insertion, enable precise modulation of these functionalities. This review not only provides valuable insights into the design and development of magnetic functional materials but also offers significant contributions to our understanding of the underlying mechanisms governing their magnetic behaviors.