Exchange Interaction J1 Research Articles

Sharp quantum phase transition in the frustrated spin-1/2 Ising chain antiferromagnet CaCoV2O7

We report on a quantum critical behavior in the quasi-1D spin-1/2 zigzag frustrated chain antiferromagnet CaCoV2O7, induced by an applied magnetic field. Below TN=3.3 K our zero-field neutron diffraction studies revealed the up-up-down-down spin structure, stabilized by an order-by-disorder phenomenon. At base temperature, the magnetic order is suppressed by an applied magnetic field (B), inducing a transition into a quantum paramagnetic state at Bc=3 T, as revealed by both neutron diffraction and ESR data. The transition exhibits an unusually sharp phase boundary with the critical exponent ϕ=0.164(3)≈1/6, in contrast to the earlier experimental observations for uniform spin-1/2 chain systems. Such a sharp QPT is anticipated due to a rare combination of spin-orbit coupling and competing NN and NNN exchange interactions J1 and J2 of the zigzag spin chain. Published by the American Physical Society 2024

Magnetic relaxation in a Co(II) based quasi-one dimensional Ising spin system

Single crystal study of Co(NCS)2(aniline)2 reveals that this compound orders at 6.49 K into a canted AF magnetic structure that explains the presence of a hysteresis loop along the b crystal direction. Effective s=1/2 spins of Co(II) ions within the ground state doublet create an Ising spin system that can be described by J1−J2 triangular model. Temperature dependence of specific heat yields exchange interactions J1=31.3(4) K and 2J2=−2.23(7) K. Applying magnetic field ≃6 kOe perpendicular to b compensates for the AF exchange J2 decoupling ferromagnetic spin chains. This restores the one-dimensionality of the system and allows for observation of magnetic relaxation of single chains, similar to previously studied pyridine-based Co(NCS)2L2 compounds. The analysis of the relaxation time is performed taking into account single-ion relaxation mechanisms together with the Glauber relaxation. We also show, using Monte Carlo calculations of specific heat, that for ferromagnetic J1≫|J2|, the triangular Ising model can be approximated by the rectangular Ising model for which the analytical Onsager’s solution is available.

Antiferromagnitnaya model' Pottsa na ob\"emno-tsentrirovannoy kubicheskoy reshetke

We have studied phase transitions and thermodynamic properties of the antiferromagnetic Potts model with number of spin states q = 3 on a body-centered cubic lattice by the Monte Carlo method. Investigations have been performed with account for exchange interactions J1 and J2 between the first and second nearest neighbors. The phase transition order has been analyzed using the histogram method. It is found that in this model with J2 = 0, a second-order phase transition is observed. It is concluded that the inclusion of the interaction between the second nearest neighbors changes the type of the phase transition.

Ising-like Magnetism in Quasi-Two-Dimensional Co(NO3)2·2H2O.

The appearance of electrically neutral water molecules in the structure of cobalt dinitrate dihydrate, Co(NO3)2⋅2H2O, drastically changes its magnetic properties as compared to its waterless counterpart, Co(NO3)2. The title compound shows Ising-like behavior reflected in its thermodynamic properties. It experiences long-range antiferromagnetic order at TN = 20.5 K and metamagnetic transition at µ0HC = 0.76 T. First-principles calculations produce the values of leading exchange interactions J1 ~ 10 K and J2 ~ 0.5 K and single-ion anisotropy D ~ 1 K which allows us to consider Co(NO3)2⋅2H2O as a quasi-two-dimensional magnetic system.

Robust quantum entanglement and teleportation in the tetrapartite spin-1/2 square clusters: Theoretical study on the effect of a cyclic four-spin exchange

The whole entanglement measure so-called geometric Π4 average of tangles and bipartite entanglement of the antiferromagnetic spin-1/2 XXX Heisenberg model on a tetranuclear square cluster with cyclic four-spin interaction are rigorously examined by the help of thermal negativities. The model comprises two nearest-neighbor exchange couplings J1 and J2 such that J1≫J2. When the cyclic four-spin exchange is zero, the maximum value of whole entanglement Π4 is achieved at low enough temperatures and relatively high magnetic fields (B≈J1). Also, maximum bipartite entanglement between pair spins with exchange coupling J1 is achievable at high temperature and high magnetic field. This quantity remains alive for sufficiently high temperature and high magnetic field values comparable with the relevant exchange coupling J1. A nonzero value of the cyclic four-spin exchange notably enhances the degree of the whole entanglement, while it weakens the bipartite entanglement degree. We demonstrate that the whole entanglement reaches an unconventional minimum at a special parameter region of cyclic four-spin exchange almost ten order of magnitude smaller than J1, where the system is in a quantum antiferromagnetic state. The real complex [Cu4L4(H2O)4](ClO4)4 as a strong antiferromagnetic tetranuclear square compound provides us an experimental representative to estimate the strength of the whole and bipartite entanglements at high enough temperature. It is demonstrated that the entanglement negativities of this complex are yet depend on the considered cyclic four-spin interaction even though its magnitude is significantly smaller than J1. We realize that the possibility of quantum teleportation through a couple of discrete Cu4II complexes would solely happen for the magnetic fields lower than a critical value. It is also demonstrated that enhancing the cyclic four-spin exchange results in diminishing the average fidelity. Moreover, we conclude that the quantum Fisher information investigated for the pair of spins with exchange interaction J1 reaches its minimum close to the critical magnetic field at which a ground-state phase transition occurs from highly entangled state to separable one.

Strain effect on magnetic-exchange-induced phonon splitting in NiO films

NiO thin films with various strains were grown on SrTiO3 (STO) and MgO substrates using a pulsed laser deposition technique. The films were characterized using an x-ray diffraction, atomic force microscopy, and infrared reflectance spectroscopy. The films grown on STO (001) substrate show a compressive in-plane strain which increases as the film thickness is reduced resulting in an increase of the NiO phonon frequency. On the other hand, a tensile strain was detected in the NiO film grown on MgO (001) substrate which induces a softening of the phonon frequency. Overall, the variation of in-plane strain from −0.36% (compressive) to 0.48% (tensile) yields the decrease of the phonon frequency from 409.6 cm−1 to 377.5 cm−1 which occurs due to the ∼1% change of interatomic distances. The magnetic exchange-driven phonon splitting Δω in three different samples, with relaxed (i.e. zero) strain, 0.36% compressive strain and 0.48% tensile strain, was measured as a function of temperature. The Δω increases on cooling in NiO relaxed film as in the previously published work on a bulk crystal. The splitting increases on cooling also in 0.48% tensile strained film, but Δω is systematically 3–4 cm−1 smaller than in relaxed film. Since the phonon splitting is proportional to the non-dominant magnetic exchange interaction J1, the reduction of phonon splitting in tensile-strained film was explained by a diminishing of J1 with lattice expansion. Increase of Δω on cooling can be also explained by rising of J1 with reduced temperature.

Magnetic and electronic properties of layered Sr2NiO2Cl2 with square planar coordination

Local coordination geometry is crucial in determining the physical properties of transition metal compounds. In this work, we report a detailed density functional theory study on the magnetic and electronic properties of layered Sr2NiO2Cl2, which possesses an unusual square planar coordination. Due to the crystal field and spin splitting, Ni 3d electrons fully occupy the dxz, dyz, and dxy orbitals, while partially occupy the dz2 and dx2-y2 states, leading to the high spin state of Ni2+ ions and strong antiferromagnetic in-plane nearest-neighbor exchange interaction J1. Thus, Sr2NiO2Cl2 is predicted to exhibit a G-type magnetic ground state, which all nearest-neighbor magnetic moments are antiparallel to each other in the NiO2 plane. A band gap is opened irrespective of the specific magnetic state since the unique square planar crystal field splitting. Finally, large orbital moments and giant easy-plane magnetocrystalline anisotropy energy are obtained, which is again related to the unique coordination environment.

Large easy-plane magnetic anisotropy in square planar coordinate Sr2CoO2X2 (X = Cl, Br)

Large magnetocrystalline anisotropy energy (MAE) is an important requirement for applications in spintronic devices. Here, we focus on layered cobalt oxyhalides Sr2CoO2X2 (X = Cl, Br) with unusual square planar coordination, which are naturally expected to possess a strong MAE. Based on density functional calculations, we find Co ions adopt a high spin state due to the large spin-exchange splitting and exhibit a sizable orbital moment of about 0.6 μB for both compounds. The in-plane nearest-neighbor exchange interaction J1 is antiferromagnetic coupling, and at least one order of magnitude stronger than other exchange interactions, which results in the observed G-type magnetic ground state. As expected, Sr2CoO2X2 have a giant MAE of about 13 meV per Co atom, and the magnetic moments prefer to lie in the ab plane, which is largely originated from the unique crystal field environment and could be explained by perturbation theory.

Magnetic exchange interactions and band gap bowing in NixMg1−xO (0.0 ≤ x ≤ 1.0): A GGA+U density functional study

We report variations in magnetic exchange interactions and energy bandgap of the NixMg1−xO (0.0≤x≤1.0) system with a change in x using the first principle density functional theory calculations employing the generalized gradient approximation + Coulomb interaction approach. We consider large supercell fcc structures (2×2×2) for different Ni molar fractions (x). Our results demonstrate that the type-II antiferromagnetic ordering is energetically more favorable for all the compositions, which exhibits a small local moment (about 2μB) due to high-spin divalent Ni. The nearest neighbor (NN) and the next nearest neighbor (NNN) exchange interactions J1 and J2, respectively, are evaluated by considering different Hund's coupling parameters (0≤J≤4) for a constant Columbic interaction (U=6.3eV). Below a critical composition, xC<0.25, NN interaction dominates over NNN resulting in |J2J1|<1 and for moderate compositions (0.25≤x≤0.75), J2 decides the global magnetic ordering of NixMg1−xO, making the system strongly antiferromagnetic for |J2J1|>1. For a higher concentration (x>0.75), the ratio (|J2J1|) remains constant suggesting that both J1 and J2 play important roles. For x=1, calculated values of J1(=1.52meV) and J2(=−17.14meV) are consistent with the experimental results (J1=1.4meV and J2=−19.1meV) obtained from spin-wave dispersion measurements. Our calculations and analysis of the electronic structure reveal the evidence for energy bandgap (Eg) bowing in NixMg1−xO with two different charge transfer transitions: (i) O−2p→Ni−4s/Mg−3s (Eg↓) and (ii) O−2p→Ni−3d−eg states (Eg↑). For x>xc, additional O−2p states are generated near the Fermi level in the valence band (∼−0.87eV), which contributes to the bowing of Eg.

Spin transport in quantum d-dimensional XY model

The interplay between quantum phase transition and spin transport is analyzed in the spin-1 d-dimensional (d = 1, 2) XY chain bilayer and with a single ion anisotropy at T = 0. The model presents exchange interactions J1, J2 and J⊥. We have calculated the AC spin conductivity for this model and verified its behavior with the critical anisotropy Dc and parameter α = J2∕J1. The effect of the sudden change in the graphic Dc vs. α when Dc → 0 on spin conductivity is verified. Since our results show a sudden variation of spin conductivity in region where Dc → 0, we have gotten an influence of the quantum phase transition on the spin transport. We also calculate the quadrupole structure factor for the model that can be directly measured by polarized inelastic light scattering experiments.

Discovery of coexisting Dirac and triply degenerate magnons in a three-dimensional antiferromagnet

Topological magnons are emergent quantum spin excitations featured by magnon bands crossing linearly at the points dubbed nodes, analogous to fermions in topological electronic systems. Experimental realisation of topological magnons in three dimensions has not been reported so far. Here, by measuring spin excitations (magnons) of a three-dimensional antiferromagnet Cu3TeO6 with inelastic neutron scattering, we provide direct spectroscopic evidence for the coexistence of symmetry-protected Dirac and triply degenerate nodes, the latter involving three-component magnons beyond the Dirac–Weyl framework. Our theoretical calculations show that the observed topological magnon band structure can be well described by the linear-spin-wave theory based on a Hamiltonian dominated by the nearest-neighbour exchange interaction J1. As such, we showcase Cu3TeO6 as an example system where Dirac and triply degenerate magnonic nodal excitations coexist, demonstrate an exotic topological state of matter, and provide a fresh ground to explore the topological properties in quantum materials.

Spin Wave Theory in Two-Dimensional Coupled Antiferromagnets

We apply spin wave theory to two-dimensional coupled antiferromagnets. In particular, we primarily examine a system that consists of small spins coupled by a strong exchange interaction J1, large spins coupled by a weak exchange interaction J2, and an anisotropic exchange interaction J12 between the small and large spins. This system is an effective model of the organic antiferromagnet λ-(BETS)2FeCl4 in its insulating phase, in which intriguing magnetic phenomena have been observed, where the small and large spins correspond to π electrons and 3d spins, respectively. BETS stands for bis(ethylenedithio)tetraselenafulvalene. We obtain the antiferromagnetic transition temperature TN and the sublattice magnetizations m(T) and M(T) of the small and large spins, respectively, as functions of the temperature T. When T increases, m(T) is constant with a slight decrease below TN, even where M(T) decreases significantly. When J1 ≫ J12 and J2 = 0, an analytical expression for TN is derived. The estimated value of TN...

Meissner mechanism for the spin supercurrent and interplay between quantum phase transition and spin transport in the frustrated Heisenberg model

We have propose the Meissner mechanism for the spin supercurrent in quantum spin systems. Besides, we study the behavior of the AC spin conductivity in neighborhood of quantum phase transition in a frustrated spin model such as the antiferromagnet in the union jack lattice with single ion anisotropy at T=0. We investigate the spin conductivity for this model that presents exchange interactions J1 and J2. Our results show a single peak for the conductivity with the height varying with the behavior of critical anisotropy Dc with J2. We obtain the conductivity tending to zero in the limit ω→0.

Distant exchange interactions in Cd1−xMnxS from magnetization steps method

Three new distant neighbor (DN) antiferromagnetic exchange constants have been measured in wurtzite Cd1–xMnxS (x = 0.0087 and x = 0.0278) using the magnetization steps method in the millikelvin regime. The second (J(2)), third (J(3)), and fourth (J(4)) largest exchange constants (after the two first neighbor exchange interactions J1 and J1′) have been measured. The results are J(2) = 300 ± 10 mK, J(3) = 173 ± 10 mK, and J(4) = 55 ± 10 mK. No significant change of the exchange constant values has been observed as a function of the Mn concentration. The mapping of the DN exchange interactions has been investigated within a sphere of radius 2 times the nearest neighbors distance. The only way to identify the J constants was using the number coordination Zn of the different DN classes: J(2) is associated to DN classes with Zn = 6, J(3) with Zn = 12, and J(4) is ascribed to a group of three DN classes with J values around J(4). The magnitude of the J constants is not in agreement with theoretical predictions of the monotonic decrease of the exchange constant with increasing distance. The present results are similar to those obtained for Cd1–xMnxSe but are different from the ones obtained for Zn1–xMnxO. The present work confirms the long-range character of the DN exchange interactions in the wurtzite type Mn-based II-VI diluted magnetic semiconductors.

Influence of quantum phase transition on spin transport in the quantum antiferromagnet in the honeycomb lattice

We use the SU(3) Schwinger boson theory to study the spin transport properties of the two-dimensional anisotropic frustrated Heisenberg model in a honeycomb lattice at T=0 with single ion anisotropy and third neighbor interactions. We have investigated the behavior of the spin conductivity for this model that presents exchange interactions J1, J2 and J3. We study the spin transport in the Bose–Einstein condensation regime where the bosons tz are condensed. Our results show an influence of the quantum phase transition point on the spin conductivity behavior. We also have made a diagrammatic expansion for the Green-function and did not obtain any significant change of the results.

Influence of quantum phase transition on spin conductivity in the anisotropic three-dimensional ferromagnetic model

We use the SU(3) Schwinger boson formalism to study the spin transport in the three-dimensional S=1 Heisenberg ferromagnet in the cubic lattice with an easy plane crystal field, considering first-, second- and third-neighbor interactions. We have got one single peak for the spin conductivity for this system at ω=ωk and a variation of the height of the peak with the parameters Dc and η, and hence an influence of the quantum phase transition, between the disordered paramagnetic phase and the ordered ones, on the spin conductivity of this system. We have considered the exchange interaction J1 as ferromagnetic and the interactions J2 and J3 as antiferromagnetic.

Monte Carlo simulations of the spin-2 Blume-Emery-Griffiths model with four-spin interactions

The magnetic properties of a spin S = 2 Ising system with bilinear exchange interaction J1, the biquadratic exchange interaction K, four-spin exchange interactions J4 and crystal field Δ are discussed using the Monte Carlo simulation. The lattice is divided into two sublattices: A and B, for which we compute the magnetizations mA and mB. The phase obtained diagrams of this system are deduced in the planes: (T, Δ/J1), (K/J1, Δ/J1), (Δ/J1, J4/J1) and (J4/J1, K/J1). In addition to the usual phases, we found a new phase called nonmagnetic quadratic, for which the magnetizations are mA ≠ mB and the quadrupolar moments are so that are qA = qB. Furthermore, the behavior of the magnetizations as a function of temperature, crystal field, four-spin exchange interactions and biquadratic exchange interaction are deduced.

Spin transport in the frustrated anisotropic three-dimensional XY model

Abstract We use the SU(3) Schwinger's boson theory to study the spin transport in the frustrated anisotropic three-dimensional XY model at T=0 with single ion anisotropy. We have investigated the behavior of the spin conductivity for this model that presents exchange interactions J1, J2 and J ′ . We study the spin transport in the Bose–Einstein regime where we have that the tz bosons are condensed i.e. 〈 t z 〉 = 〈 t z † 〉 = t . Our results show a metallic spin transport for ω > 0 and a superconductor spin transport in the limit of DC conductivity.

Spin-Flop Transition and a Tilted Canted Spin Structure in a Coupled Antiferromagnet

We study a uniaxial coupled Heisenberg antiferromagnet that consists of two subsystems of classical spins with small and large lengths and spin-flop transitions in a magnetic field parallel to the magnetic easy axis. It is proved that the anisotropy of inter-subsystem coupling stabilizes an asymmetric canted antiferromagnetic phase with a tilted direction of antiferromagnetism that is not perpendicular to the magnetic field. In contrast to the conventional first-order spin-flop transition, the spin-flop transition from the Neel phase to such a tilted canted antiferromagnetic (TCAF) phase is of the second order in the absence of simple anisotropic energies in the subsystems. The transition from the TCAF phase to the high-field saturated spin phase is of the second order in the strong coupling limit of the exchange interactions J1 between the small spins, whereas when J1 is finite, it becomes first-order. Therefore, in the former case, the TCAF phase converts the Neel phase continuously into the saturated p...

Stepwise magnetic behavior of the liquid crystal iron(III) complex

EPR and Mossbauer spectroscopy is used to study a new liquid crystal complex of iron(III) with a Schiff base: 4,4′-dodecyloxybenzoyloxybenzoyl-4-oxysalicylidene-2-aminopyridine with a PF6− counterion. It is shown that Fe(III) ions exist only in the high-spin (HS, S = 5/2) state. It is found that under the influence of temperature the system demonstrates the stepwise behavior of the product of the integrated intensity of EPR lines (I) and temperature (proportional to χT, where χ is the magnetic susceptibility) with an inflection point at ∼80 K. Above 80 K a new EPR spectrum is detected due to the excited S = 2 state and the formation of dimeric molecules (through oxygen bridges) with a strong intramolecular antiferromagnetic exchange interaction J1 = 162.1 cm−1. Below 80 K iron(III) complexes are organized in 1D chains where the exchange value J2 = 2.1 cm−1. At 80 K there is a structural phase transition in the system: the transition from a 1D chain organization of HS Fe(III) centers to dimeric molecules. Based on quantum chemical calculations a model of the binuclear iron(III) complex is proposed.