Antiferromagnetic Order Research Articles

Doping dependence of superconductivity on a honeycomb lattice within the framework of kinetic-energy-driven superconductivity

Unconventional superconductivity on a honeycomb lattice has received increasing interest since the discovery of graphene primarily due to the similarities between materials with a honeycomb lattice and cuprate superconductors. Many theoretical studies have been conducted on superconductivity on a honeycomb lattice, however, a consistent picture is still lacking. In this paper, we have extended the theory of kinetic-energy-driven superconductivity, which has been developed to investigate unconventional superconductivity in cuprate superconductors, to explore superconductivity on a honeycomb lattice within the t-J model. Our results demonstrate that the charge-carrier pair gap parameter with [Formula: see text]-wave symmetry exhibits a dome-like shape as a function of doping, with superconductivity emerging at a certain doping concentration and disappearing at high doping levels, similar to what has been observed in cuprate and cobaltate superconductors. Furthermore, the charge-carrier pair gap parameter decreases with increasing the value of [Formula: see text] (the ratio between the antiferromagnetic exchange coupling constant and the nearest-neighbor hopping integral), and approaches zero when [Formula: see text] reaches a sufficiently large value. This indicates that the antiferromagnetic order will suppress the superconducting state and a sufficiently strong exchange coupling will completely destroy the superconductivity. Taking into account our present results together with the corresponding results of cuprate and cobaltate superconductors, it appears that the dome-like shape of the doping dependence of the charge-carrier pair gap parameter may be a common feature in doped Mott insulators.

2D Co-Directed Metal-Organic Networks Featuring Strong Antiferromagnetism and Perpendicular Anisotropy.

Antiferromagnetic spintronics is a rapidly emerging field with the potential to revolutionize the way information is stored and processed. One of the key challenges in this field is the development of novel 2D antiferromagnetic materials. In this paper, the first on-surface synthesis of a Co-directed metal-organic network is reported in which the Co atoms are strongly antiferromagnetically coupled, while featuring a perpendicular magnetic anisotropy. This material is a promising candidate for future antiferromagnetic spintronic devices, as it combines the advantages of 2D and metal-organic chemistry with strong antiferromagnetic order and perpendicular magnetic anisotropy.

ThCr2Si2C: An Antiferromagnetic Metal with a Cr2C Square Lattice.

A quaternary compound, ThCr2Si2C, was synthesized by using the arc-melting technique. The compound adopts a tetragonal CeCr2Si2C-type crystal structure. The electronic resistivity and specific heat data exhibit metallic behavior, while the magnetic susceptibility displays a pronounced broad peak at around 370 K, indicating the antiferromagnetic phase transition. The first-principles calculations suggest A-type antiferromagnetic ordering of the Cr sublattice, which is confirmed by neutron diffraction experiments. By comparing the crystal structure of ThCr2Si2C with the isostructural Cr-based compounds, the magnetic state of Cr 3d orbital is discussed in terms of the band-filling effects and indirect spin exchange interaction.

New monoclinic ground state of FeSi

By means of ab initio techniques with the hybrid functional we show the existence of a new phase of FeSi with the monoclinic symmetry (space group P21) originated from the B20 cubic structure (space group P213) due to slight orthorhombic distortion, which is turned out to be the ground state. The monoclinic FeSi not only displays the minimum in the total energy, but it is also characterized by a phonon spectrum without imaginary frequencies and by conducting properties (contrary to semiconducting properties of cubic FeSi) with antiferromagnetic ordering and the magnetic moment of 2.3 μB for each Fe atom. These findings are supported by data of X-ray diffraction and high-resolution transmission electron microscopy of ultrathin FeSi films (∼3nm in thickness) grown on Si(111) by solid-phase epitaxy indicating the monoclinic symmetry to fit better the film structure as compared to the cubic symmetry, as well as by resistivity versus temperature measurements within a wide temperature range (2–300 K) pointing out bad metal properties. The analysis of field and temperature dependences of the magnetic moment of ultrathin FeSi films shows the presence of a ferromagnetic-antiferromagnetic two-phase state. We also discuss how our findings of the new phase of FeSi can interpret its known experimental data on electronic, transport and optical properties without involving the metal–insulator transition and Kondo-like effects.

Structure and cryogenic magnetocaloric effects of NaREO2 (RE = Er, Ho) compounds

Rare-earth (RE) based oxides have presented great application prospects in low-temperature magnetic refrigeration technology. Thus far, studies on magnetic and especially magnetocaloric properties of alkali-metal RE oxides are limited, largely due to the difficulty in synthesizing these oxides. In this work, NaREO2 (RE = Er, Ho) compounds with both the rhombohedral (R3‾m space group) and cubic (Fm3‾m space group) structures were prepared by a facile ball-milling-aided sintering approach. The cubic structural compounds exhibit a spherical microstructure composed of vertically-grown irregular hexahedra, which sharply contrast with a loose lamellar microstructure of the rhombohedral structural compound. The as-prepared NaHoO2 or NaErO2 compound exhibits an antiferromagnetic ordering of RE3+ moments at a temperature of 2.4 K or lower than 2 K respectively. An excellent comprehensive magnetic refrigeration performance at cryogenic temperatures in a low magnetic field change of 0–20 kOe was obtained in these compounds, including a complete reversibility of the magnetocaloric effect, a maximal magnetic-entropy change ΔSM at 2 K of 12 J/kg·K for the rhombohedral NaErO2 and a relative cooling power (RCP) of 79.7 J/kg for the cubic NaHoO2.

Magneto-electric coupling beyond van der Waals interaction in two-dimensional multiferroic heterostructures

Exploring two-dimensional (2D) multiferroic systems with strong magneto-electric coupling properties holds significant application value in nanoscale spintronics devices. However, due to the weak interlayer van der Waals interactions, strong magneto-electric coupling in 2D heterostructures is relatively rare. By using first-principles simulations, we demonstrate that in the NiPS3/Sc2CO2 heterostructure, the ferroelectric polarization switching of the Sc2CO2 layer induces a transition in the magnetic ground state of the NiPS3 layer from the ferromagnetic state to antiferromagnetic ordering, accompanied by a transformation from a semiconductor to a half-metallic state. This magnetic phase transition is caused by a novel magneto-electric coupling mechanism: the polarization switching changes the band alignment between the two materials and then induces a significant interlayer charge transfer, leading to the emergence of Stoner itinerant ferromagnetism. In addition, the polarization switching can also change the magnetic anisotropy from an easy magnetization plane to an easy magnetization axis. These results not only offer a promising multiferroic heterostructure for nonvolatile memory devices and magnetic sensors but also provide a feasible approach for designing multiferroic system with strong magneto-electric coupling.

Magnetic order in 2D antiferromagnets revealed by spontaneous anisotropic magnetostriction

The temperature dependent order parameter provides important information on the nature of magnetism. Using traditional methods to study this parameter in two-dimensional (2D) magnets remains difficult, however, particularly for insulating antiferromagnetic (AF) compounds. Here, we show that its temperature dependence in AF MPS3 (M(II) = Fe, Co, Ni) can be probed via the anisotropy in the resonance frequency of rectangular membranes, mediated by a combination of anisotropic magnetostriction and spontaneous staggered magnetization. Density functional calculations followed by a derived orbital-resolved magnetic exchange analysis confirm and unravel the microscopic origin of this magnetization-induced anisotropic strain. We further show that the temperature and thickness dependent order parameter allows to deduce the material’s critical exponents characterising magnetic order. Nanomechanical sensing of magnetic order thus provides a future platform to investigate 2D magnetism down to the single-layer limit.

Origin of the Magnetic Exciton in the van der Waals Antiferromagnet NiPS_{3}.

An ultrasharp photoluminescence line intimately related to antiferromagnetic order has been found in NiPS_{3}, a correlated van der Waals material, opening prospects for magneto-optical coupling schemes and spintronic applications. Here we unambiguously clarify the singlet origin of this excitation, confirming its roots in the spin structure. Based on a comprehensive investigation of the electronic structure using angle-resolved photoemission and q-dependent electron energy loss spectroscopy as experimental tools we develop, in a first step, an adequate theoretical understanding using density functional theory (DFT). In a second step the DFT is used as input for a dedicated multiplet theory by which we achieve excellent agreement with available multiplet spectroscopy. Our Letter connects the understanding of the electronic structure and of optical processes in NiPS_{3} and related materials as a prerequisite for further progress of the field.

Unveiling exotic magnetic phase diagram of a non-Heisenberg quasicrystal approximant

A magnetic phase diagram of the non-Heisenberg Tsai-type 1/1 Au-Ga-Tb approximant crystal (AC) has been established across a wide electron-per-atom (e/a) range via magnetization and powder neutron diffraction measurements. The diagram revealed exotic ferromagnetic (FM) and antiferromagnetic (AFM) orders that originate from the unique local spin icosahedron common to icosahedral quasicrystals (iQCs) and ACs; The noncoplanar whirling AFM order is stabilized as the ground state at the e/a of 1.72 or less whereas a noncoplanar whirling FM order was found at the larger e/a of 1.80, with magnetic moments tangential to the Tb icosahedron in both cases. Moreover, the FM/AFM phase selection rule was unveiled in terms of the nearest neighbour (J1) and next nearest neighbour (J2) interactions by numerical calculations on a non-Heisenberg single icosahedron. The present findings will pave the way for understanding the intriguing magnetic orders of not only non-Heisenberg FM/AFM ACs but also non-Heisenberg FM/AFM iQCs, the latter of which are yet to be discovered.

Fingerprints of Mott–Hubbard physics in optical spectra of antiferromagnetic LaTiO3

Magnetic properties of Mott–Hubbard insulators are determined by superexchange interactions mediated by the high-spin (HS) and low-spin (LS) intersite d–d charge excitations, which can be associated with the HS- and LS-Hubbard subbands in optical experiments. To explore the Mott–Hubbard physics in orthorhombic LaTiO3 crystal exhibiting the G-type antiferromagnetic order at the Néel temperature TN = 146 K, we present a comprehensive spectroscopic ellipsometry study in the spectral range 0.5–5.6 eV at temperatures 10 ⩽ T ⩽ 300 K. We found that the complex dielectric function spectra of LaTiO3 crystal are almost featureless, nearly isotropic, and weakly temperature dependent in the range of the intersite d–d optical transitions. Nonetheless, analyzing the difference spectra below the TN, we have identified the LS-state d1d1⇌d2d0 excitations at ∼3.7 and ∼5.15 eV and estimated values of the on-site Coulomb repulsion U ∼ 4.2 eV and Hund’s exchange constant JH ∼ 0.5 eV, which define the energy of the HS-state d1d1⇌d2d0 excitation at ∼2.7 eV. In addition, we discovered that the pronounced lowest-energy 1.3 eV optical band displays the critical intensity behavior and anomalous broadening with decreasing temperature below the TN. The discovered properties indicate that the 1.3 eV band in LaTiO3 can be associated with a Mott–Hubbard exciton.

Delving into the anisotropic interlayer exchange in bilayer CrI3

Bilayer CrI3 attracted much attention due to stacking-induced switching between the layered ferromagnetic and antiferromagnetic order. This discovery brought under the spotlight the interlayer Cr–Cr exchange interaction, which despite being much weaker than the intralayer exchange, plays an important role in shaping the magnetic properties of bilayer CrI3. In this work we delve into the anisotropic part of the interlayer exchange with the aim to separate the contributions from the Dzyaloshinskii–Moriya (DMI) and the Kitaev interactions (KI). We leverage the density functional theory calculations with spin Hamiltonian modeling and develop an energy mapping procedure to assess these anisotropic interactions with accuracy. After inspecting the rhombohedral and monoclinic stacking sequences of bilayer CrI3, we reveal a considerable DMI and a weak interlayer KI between the sublattices of a monoclinic bilayer. We explain the dependence of DMI and KI on the interlayer distance, stacking sequence, and the spin–orbit coupling strength, and we suggest the dominant superexchange processes at play. In addition, we demonstrate that the single-ion anisotropy in bilayer CrI3 is highly stacking-dependent, increasing by 50% from monoclinic to rhombohedral bilayer. Remarkably, our findings prove that iodines are highly efficient in mediating the DMI across the van der Waals gap, much owing to spatially extended 5p orbitals which feature strong spin–orbit coupling. Our study gives promise that the interlayer chiral control of spin textures, demonstrated in thin metallic films where the DMI is with a much longer range, can be achieved with similar efficiency in semiconducting two-dimensional van der Waals magnets.

Otklik zaryadovoy podsistemy na fazovye perekhody v dvoynykh manganitakh LnBaMn2O6

We study the structural, magnetic, and optical properties of double manganites LnBaMn2O6 with Ln = Pr, Nd, Sm, Nd1 – xSmx (x = 0.25, 0.5, 0.75). Analysis of the temperature dependences of transmission in the near IR range has shown the difference in the responses of the charge subsystem for different types of charge/orbital ordering in the system. In PrBaMn2O6 manganite, the orbital ordering of the dx2−y2 type leads to an insulator state at low temperatures. The charge subsystem of manganites NdBaMn2O6, Nd0.75Sm0.25BaMn2O6, and Nd0.5Sm0.5BaMn2O6 is sensitive to the orbital ordering type: in the temperature interval TCO2 T TCO1, where pairwise alignment of layers with d3x2−r2/d3y2−r2 ordered orbitals is realized, the semiconductor character of charge carriers is observed, while upon a transition to the layer-by-layer alignment of orbitally ordered layers for T TCO2, charge carriers are of the metal character. In manganites Nd0.25Sm0.75BaMn2O6 and SmBaMn2O6, the absence of clearly manifested metal nature of the charge subsystem at T TCO2 is associated with the formation of the antiferromagnetic ordering of the CE type.

Antiferromagnitnyy eksitonnyy dielektrik

The effective the two-band Hamiltonian is obtained for iridium oxides with account for strong electron correlations (SEC) and the spin–orbit interaction. The intraatomic electron correlations in iridium ions induce the formation of Hubbard fermions (HF) filling the states in the valence band. Another consequence of SEC is associated with the emergence of the antiferromagnetic (AFM) exchange interaction between HF in accordance with the Anderson mechanism. As a result, a long-range antiferromagnetic order is established in the system, and in the conditions of band overlapping, the intersite Coulomb interaction induces a phase transition to the excitonic insulator (EI) state with a long-range AFM order. The system of integral self-consistent equations, the solution to which determines the excitonic order parameter components Δi, j(k), sublattice magnetization M, Hubbard fermion concentration nd, and chemical potential μ, is obtained using the atomic representation, the method of two-time temperature Green’s functions, and the Zwanzig–Mori projection technique. The symmetry classification of AFM EI phases is performed, and it is shown that in the nearest neighbor approximation, state Δi, j(k) with the s-type symmetry corresponds to the ground state, while the phases with the d- and p-symmetries are metastable.

Thermodynamic Properties and DFT Study on Highly Frustrated Cr3BO6: Coexistence of Spin-Singlets with Long-Range Magnetic Order.

The triangle-based magnetic subsystem of borates with the mineral norbergite structure M3BO6 (M = Fe, Cr, V) makes these compounds unique to investigate rare quantum ground states influenced by strong magnetic frustration. In this work, we investigated the thermal and magnetic properties of Cr3BO6 to find that despite very large negative Weiss temperature Θ = -160.7 K, it orders only at TN = 4.5 K and experiences a spin-flop transition at µ0H = 5 T. Density functional theory (DFT) calculations of exchange interaction parameters allow for suggesting the model of magnetic subsystem in chromium borate Cr3BO6. The results prove the decisive role of magnetic frustration on the formation of long-range order, providing therefore a basis for future study. Both experimental data and first-principles calculations point to the coexistence of chromium spin-singlets with long-range antiferromagnetic order.

Fe doping induced cluster/spin glass state and metamagnetic transition in phase separated La0.5Sr0.5Mn1−xFexO3

We report the impurity doping effects in manganites La0.5Sr0.5Mn1−xFexO3 (0 ≤ x ≤ 0.2) and the field-induced metamagnetic transition in which the antiferromagnetic insulating phase transforms into ferromagnetic metallic phase. The Rietveld refined results of XRD data display no obvious structure change with a significant degree of Fe3+ doping on Mn sites. Significant ferromagnetism suppression and magnetic inhomogeneity tendency upon Fe3+-doping are evidenced, phase separated and spin-glass like states are identified. With applying a pulsed high magnetic field up to 50 T, a metamagnetic transition was induced, especially clear for x ≥ 0.1 compounds, which exhibited an irreversible phase transition from antiferromagnetic to ferromagnetic phase, and a saturation magnetization was obtained. The replacement of Mn3+ by Fe3+ ions eliminates the effects of lattice structure change on the magnetic and electronic properties because of the very similar ionic size of Mn3+ and Fe3+, but reduces the Mn3+/Mn4+ ratio and depopulates the number of available hopping sites and the hopping electrons, thus weakens the double exchange interaction. The occurrence of the AFM superexchange interaction via Fe3+–O2−–Fe3+ and Fe3+–O2−–Mn3+ bonds may bring about the competition between the antiferromagnetic ordering and the ferromagnetic ordering, leading to a cluster/spin-glass behavior in the samples with high Fe dopants concentration.

Quantum spin nematic phase in a square-lattice iridate.

Spin nematic is a magnetic analogue of classical liquid crystals, a fourth state of matter exhibiting characteristics of both liquid and solid1,2. Particularly intriguing is a valence-bond spin nematic3-5, in which spins are quantum entangled to form a multipolar order without breaking time-reversal symmetry, but its unambiguous experimental realization remains elusive. Here we establish a spin nematic phase in the square-lattice iridate Sr2IrO4, which approximately realizes a pseudospin one-half Heisenberg antiferromagnet in the strong spin-orbit coupling limit6-9. Upon cooling, the transition into the spin nematic phase at TC ≈ 263 K is marked by a divergence in the static spin quadrupole susceptibility extracted from our Raman spectra and concomitant emergence of a collective mode associated with the spontaneous breaking of rotational symmetries. The quadrupolar order persists in the antiferromagnetic phase below TN ≈ 230 K and becomes directly observable through its interference with the antiferromagnetic order in resonant X-ray diffraction, which allows us to uniquely determine its spatial structure. Further, we find using resonant inelastic X-ray scattering a complete breakdown of coherent magnon excitations at short-wavelength scales, suggesting a many-body quantum entanglement in the antiferromagnetic state10,11. Taken together, our results reveal a quantum order underlying the Néel antiferromagnet that is widely believed to be intimately connected to the mechanism of high-temperature superconductivity12,13.

Raman study of the spin–lattice excitations of the layered antiferromagnets MPSe3 (M = Fe, Mn)

Magnetism in van der Waals (vdW) materials has recently garnered significant attention due to their potential for advanced applications. In this study, we investigate the magnetic transition in layered MPSe3 (M = Fe, Mn) bulk flakes utilizing temperature-dependent Raman spectroscopy. Our findings reveal unique features in the Raman spectrum that are strongly correlated with the antiferromagnetic ordering in MPSe3. We analyze the spin-dependent Raman scattering modes in MPSe3 by applying the Suzuki–Kamimura formalism. Consequently, our comparative analyses of the spin correlation function and the reduced magnetization provide compelling evidence of the 2D Ising characteristic especially in FePSe3. Our results clearly show the potential of MPSe3 as a prominent platform for exploring fundamental aspects of low-dimensional magnetism.

Spin model for the honeycomb NiPS3

In the Van der Waals material NiPS3, Ni atoms have spin S = 1 and realize a honeycomb lattice. Six sulfur atoms surround each Ni and split their d manifold into three filled and two unfilled bands. Aimed to determine the spin Hamiltonian of NiPS3, we study its exchange mechanisms using a two-band half-filled Hubbard model. Hopping between d-orbitals is mediated by p orbitals of sulfur and gives rise to bilinear and biquadratic spin couplings in the limit of strong electronic correlations. The microscopic model exposed a ferromagnetic biquadratic spin interaction K1, allowing the completion of a minimal J1−J3−K1 spin Hamiltonian for NiPS3. In bulk, a ferromagnetic first nearest neighbor J1 and a more significant antiferromagnetic third nearest neighbor spin coupling J3 agreed with the literature, while in monolayer, J1 is positive and very small in comparison. Using a variational scheme, we found that a zig–zag antiferromagnetic order is the ground state of bulk samples. The zig–zag pattern is adjacent to commensurate and incommensurate spin spirals, which could hint at the puzzling results reported in NiPS3 monolayers.

Surface‐Sensitive Detection of Magnetic Phase Transition in Van Der Waals Magnet CrSBr

Abstract2D magnets have recently drawn enormous interest. As an air‐stable A‐type van der Waals antiferromagnet (AFM), CrSBr has attracted great attention but has also led to controversies about its large‐span ordering temperatures. Herein, a systematic study of the magnetic phase transition in single‐crystalline CrSBr with ultrahigh‐quality through surface‐sensitive X‐ray magnetic linear dichroism and X‐ray magnetic circular dichroism measurements combined with vibrating sample magnetometry for characterization of bulk magnetization is reported. The interlayer AFM order of both surface and bulk CrSBr is revealed to maintain a similar Néel temperature within the range of 132–142 K. However, the intralayer ferromagnetic (FM) order of surface CrSBr is found to sustain up to ≈238 K, 70 K higher than the value obtained from the bulk CrSBr, demonstrating a dramatically different surface and bulk Curie temperature in CrSBr. Moreover, a half‐filled t2g electronic state for Cr3+ ions with magnetic moment of ≈3 µB/Cr in CrSBr is clearly identified. These results enrich the understanding of the electronic structure and magnetism in CrSBr, providing this material as a promising building block for future spintronic devices.

Large off-diagonal magnetoelectricity in a triangular Co2+-based collinear antiferromagnet

Magnetic toroidicity is an uncommon type of magnetic structure in solid-state materials. Here, we experimentally demonstrate that collinear spins in a material with R-3 lattice symmetry can host a significant magnetic toroidicity, even parallel to the ordered spins. Taking advantage of a single crystal sample of CoTe6O13 with an R-3 space group and a Co2+ triangular sublattice, temperature-dependent magnetic, thermodynamic, and neutron diffraction results reveal A-type antiferromagnetic order below 19.5 K, with magnetic point group -3′ and k = (0,0,0). Our symmetry analysis suggests that the missing mirror symmetry in the lattice could lead to the local spin canting for a toroidal moment along the c axis. Experimentally, we observe a large off-diagonal magnetoelectric coefficient of 41.2 ps/m that evidences the magnetic toroidicity. In addition, the paramagnetic state exhibits a large effective moment per Co2+, indicating that the magnetic moment in CoTe6O13 has a significant orbital contribution. CoTe6O13 embodies an excellent opportunity for the study of next-generation functional magnetoelectric materials.