Antiferromagnetic Nearest-neighbor Research Articles

As-Se Pentagonal Linkers to Induce Chirality and Polarity in Mixed-Valent Fe-Se Tetrahedral Chains Resulting in Hidden Magnetic Ordering.

A novel mixed-valent hybrid chiral and polar compound, Fe7As3Se12(en)6(H2O), has been synthesized by a single-step solvothermal method. The crystal structure consists of 1D [Fe5Se9] chains connected via [As3Se2]-Se pentagonal linkers and charge-balancing interstitial [Fe(en)3]2+ complexes (en = ethylenediamine). Neutron powder diffraction verified that interstitial water molecules participate in the crystal packing. Magnetic polarizability of the produced compound was confirmed by X-ray magnetic circular dichroism (XMCD) spectroscopy. X-ray absorption spectroscopy (XAS) and 57Fe Mössbauer spectroscopy showed the presence of mixed-valent Fe2+/Fe3+ in the Fe-Se chains. Magnetic susceptibility measurements reveal strong antiferromagnetic nearest neighbor interactions within the chains with no apparent magnetic ordering down to 2 K. Hidden short-range magnetic ordering below 70 K was found by 57Fe Mössbauer spectroscopy, showing that a fraction of the Fe3+/Fe2+ in the chains are magnetically ordered. Nevertheless, complete magnetic ordering is not achieved even at 6 K. Analysis of XAS spectra demonstrates that the fraction of Fe3+ in the chain increases with decreasing temperature. Computational analysis points out several competing ferrimagnetic ordered models within a single chain. This competition, together with variation in the Fe oxidation state and additional weak intrachain interactions, is hypothesized to prevent long-range magnetic ordering.

Electronic and magnetic properties of metal–organic polymers with 4d and 5d-transition metal ions

The electronic and magnetic properties of one-dimensional polymers of zwitterionic quinone (ZQ) molecules with 4d and 5d transition metals (TM) are studied using density functional calculations. We present the results of first-principle calculations for the electronic and magnetic properties, including the magnetic anisotropy energy (MAE) of selected 4d- and 5d-ZQ polymers, using the Vienna Ab-initio Simulation Package (VASP). In particular, a pronounced magnetic character is found for TM-ZQ polymers with the TM ions Mo, Ru, Rh, Ag, Re, and Ir, having local magnetic moments between 1 and 4 μB and ferromagnetic or antiferromagnetic nearest neighbor exchanges. The large values of spin–orbit coupling lead to high magnetic anisotropy energies (MAE) especially for Rh-ZQ and Re-ZQ, exceeding the MAE of its 3d counterparts considerably. All investigated TM-ZQ polymer chains were found to be spin-polarized semiconductors, which make them, together with the high MAE, excellent candidate materials for spintronics.

Cluster mean-field theory studies of the frustrated two-dimensional quantum-mechanical [formula omitted] Heisenberg model

The phase diagrams of the two-dimensional quantum-mechanical Heisenberg model with antiferromagnetic nearest and next-nearest neighbor exchange interaction is investigated with the cluster mean-field theory. The four different scenarios considered are the J1−J2 model with only S=1∕2 spins, the J1−J2 model with only S=1 spins, and the mixed S=1∕2 and S=1J1−J2 model. Within the ferrimagnetic J1−J2 model, two different configurations of the spin order are assumed and investigated: checkerboard-like order of the S=1∕2 and S=1 spins, and a line order, where the S=1∕2, respectively, S=1 sublattice spins are ordered along lines which itself are ordered in a periodic sequence. The phase diagrams are characterized by calculating the energy per atom E and the magnetization m=|〈Sz〉|. In the antiferromagnetic S=1∕2J1−J2 model, the quantum states and the entanglement (von Neumann entropy) are investigated as a function of J2∕J1.

Electronic correlations and magnetic interactions in infinite-layer NdNiO2

The large antiferromagnetic exchange coupling in the parent high-$T_{\rm c}$ cuprate superconductors is believed to play a crucial role in pairing the superconducting carriers. The recent observation of superconductivity in hole-doped infinite-layer (IL-) NdNiO$_2$ brings to the fore the relevance of magnetic coupling in high-$T_{\rm c}$ superconductors, particularly because no magnetic ordering is observed in the undoped IL-NdNiO$_2$ unlike in parent copper oxides. Here, we investigate the electronic structure and the nature of magnetic exchange in IL-NdNiO$_2$ using state-of-the-art many-body quantum chemistry methods. From a systematic comparison of the electronic and magnetic properties with isostructural cuprate IL-CaCuO$_2$, we find that the on-site dynamical correlations are significantly stronger in IL-NdNiO$_2$ compared to the cuprate analog. These dynamical correlations play a critical role in the magnetic exchange resulting in an unexpectedly large antiferromagnetic nearest neighbor isotropic $J$ of 77 meV between the Ni$^{1+}$ ions within the $ab$-plane. While we find many similarities in the electronic structure between the nickelate and the cuprate, the role of electronic correlations is profoundly different in the two. We further discuss the implications of our findings in understanding the origin of superconductivity in nickelates.

Tuning Fe-Se Tetrahedral Frameworks by a Combination of [Fe(en)3]2+ Cations and Cl- Anions.

A one-dimensional (1D) chain compound (en = ethylenediamine), featuring tetrahedral FeSe2 chains separated by [Fe(en)3]2+ cations and Cl- anions, has been synthesized by a low temperature solvothermal method using simple starting materials. The degree of distortion in the Fe-Se backbone is similar to previously reported compounds with isolated 1D FeSe2 chains. 57Fe Mössbauer spectroscopy reveals the mixed-valent nature of [Fe(en)3]3(FeSe2)4Cl2 with Fe3+ centers in the [FeSe2]- chains and Fe2+ centers in the [Fe(en)3]2+ complexes. SQUID magnetometry indicates that is paramagnetic with a reduced average effective magnetic moment, μeff = 9.51 μB per formula unit, and a negative Weiss constant, θ = -10.9(4) K, indicating antiferromagnetic (AFM) nearest neighbor interactions within the [FeSe2]- chains. Weak antiferromagnetic coupling between chains, combined with rather strong intrachain AFM coupling, leads to spin-glass behavior at low temperatures, as indicated by a frequency shift of the peak observed at 3 K in AC magnetic measurements. A combination of [Fe(en)3]2+ and Cl- ions is also capable of stabilizing mixed-valent 2D Fe-Se puckered layers in the crystal structure of [Fe(en)3]4(Fe14Se21)Cl2, where Fe14Se21 layers have a unique topology with large open pores. Property measurements of [Fe(en)3]4(Fe14Se21)Cl2 could not be performed due to the inability to either grow large crystals or synthesize this material in single-phase form.

Haldane and dimer phases in a frustrated spin chain: an exact groundstate and associated topological phase transition

A Heisenberg spin-s chain with alternating ferromagnetic (FM) () and antiferromagnetic () nearest-neighbor (NN) interactions, exhibits the dimer and spin-2s Haldane phases in the limits and respectively. These two phases are understood to be topologically equivalent. Induction of the frustration through the next NN FM interaction () produces a very rich quantum phase diagram. With frustration, the whole phase diagram is divided into a FM and a nonmagnetic (NM) phase. For s = 1/2, the full NM phase is seen to be of Haldane–dimer type, but for s > 1/2, a spiral phase comes between the FM and the Haldane–dimer phases. The study of a suitably defined string-order parameter and spin-gap at the phase boundary indicates that the Haldane–dimer and spiral phases have different topological characters. We also find that, along the line in the NM phase, an NN dimer state is the exact groundstate, provided where κ ⩽ s + h for applied magnetic field h. Without magnetic field, the position of JC is on the FM–NM phase boundary when s = 1/2, but for s > 1/2, the location of JC is on the phase separation line between the Haldane–dimer and spiral phases.

Phase diagram of the J_{1}-J_{2} frustrated anisotropic antiferromagnet with spin S=1 on the quadratic lattice.

In the paper the phase diagram of J_{1}-J_{2} frustrated antiferromagnet with spin S=1 and single-ion anisotropy is studied on the planar quadratic lattice in the cluster approximation. The Bogolyubov inequality is adopted for the Gibbs energy calculation for the case of 2×2 and 4×4 clusters. On this basis, the ranges of existence of the antiferromagnetic, superantiferromagnetic, and paramagnetic phases are investigated for the antiferromagnetic nearest-neighbor (J_{1}<0) and next-nearest-neighbor (J_{2}<0) interactions. In particular, the occurrence of tricritical and triple points is discussed and a comparison between the results for 2×2 and 4×4 clusters is made. The results are also compared with the classical MFA method, adopted here for the model in question, as well as with selected literature results for particular choices of interaction parameters.

Magnetic Exchange Interactions in Geometrically Frustrated Antiferromagnet of ZnCr2−xGaxO4

In this work, the structural and magnetic properties in ZnCr2−xGaxO4 were investigated. In ZnCr2−xGaxO4, Ga3+ ions substitute partial Cr3+ ions and occupy the B sites. ZnCr2O4 undergoes a paramagnetic-antiferromagnetic transition at 12 K. In ZnCr2−xGaxO4, the antiferromagnetic short-range correlation occurs below 60 K above $T_{\mathrm {N}}$ . The Ga3+ doping weakens the antiferromagnetic nearest-neighbor exchange interaction (J1) and induces ferromagnetic next-nearest-neighbor exchange interaction (J2) among the B site magnetic ions. The generation of the ferromagnetic J2 is ascribed to structure distortion induced by the Ga3+ doping.

Low dimensional mixed-spin Ising model with next-nearest neighbor interaction

In this study, the effects of next-nearest neighbor interaction on a one-dimensional mixed spin 3 - spin 3/2 Ising model was investigated using Cellular Automaton (CA). The Ising model hamiltonian contains the antiferromagnetic nearest neighbor interaction (J1), the ferromagnetic next- nearest neighbor interaction (J2) and the external magnetic field (h = H/J1). Magnetization (M) of the mixed spin system was obtained in the interval 0 ≤ R ≤ 1 of the interaction ratio (R = J2/J1) using field cooling (FC) and zero-field cooling (ZFC) processes. Hysteresis curves were drawn for several R values in the interval −0.1 ≤ h ≤ 0.1 using FC results. The functional behavior for coercive field (HC) was determined depending on R. Thus, the mixed spin system became a hard magnetic material and the lattice geometry also changed from one dimensional linear chain to triangular chain with increasing R value.

The large-m limit, and spin liquid correlations in kagome-like spin models

It is noted that the pair correlation matrix $\hat{\chi}$ of the nearest neighbor Ising model on periodic three-dimensional ($d=3$) kagome-like lattices of corner-sharing triangles can be calculated partially exactly. Specifically, a macroscopic number $1/3 \, N+1$ out of $N$ eigenvalues of $\hat{\chi}$ are degenerate at all temperatures $T$, and correspond to an eigenspace $\mathbb{L}_{-}$ of $\hat{\chi}$, independent of $T$. Degeneracy of the eigenvalues, and $\mathbb{L}_{-}$ are an exact result for a complex $d=3$ statistical physical model. It is further noted that the eigenvalue degeneracy describing the same $\mathbb{L}_{-}$ is exact at all $T$ in an infinite spin dimensionality $m$ limit of the isotropic $m$-vector approximation to the Ising models. A peculiar match of the opposite $m=1$ and $m\rightarrow \infty$ limits can be interpreted that the $m\rightarrow\infty$ considerations are exact for $m=1$. It is not clear whether the match is coincidental. It is then speculated that the exact eigenvalues degeneracy in $\mathbb{L}_{-}$ in the opposite limits of $m$ can imply their quasi-degeneracy for intermediate $1 \leqslant m < \infty$. For an anti-ferromagnetic nearest neighbor coupling, that renders kagome-like models highly geometrically frustrated, these are spin states largely from $\mathbb{L}_{-}$ that for $m\geqslant 2$ contribute to $\hat{\chi}$ at low $T$. The $m\rightarrow\infty$ formulae can be thus quantitatively correct in description of $\hat{\chi}$ and clarifying the role of perturbations in kagome-like systems deep in the collective paramagnetic regime. An exception may be an interval of $T$, where the order-by-disorder mechanisms select sub-manifolds of $\mathbb{L}_{-}$.

Magnetic structure of theS=12quasi-two-dimensional square-lattice Heisenberg antiferromagnetSr2CuTeO6

The magnetic structure of the double perovskite compound Sr$_2$CuTeO$_6$ was determined from neutron powder diffraction data. This material is magnetically described as an $S\,{=}\,1/2$ quasi-two-dimensional square-lattice Heisenberg model with antiferromagnetic nearest-neighbor and next-nearest-neighbor interactions. Sr$_2$CuTeO$_6$ undergoes a magnetic phase transition at $T_{\rm N}\,{\simeq}\,29\,$ K. The spin structure below $T_{\rm N}$ is N\'{e}el antiferromagnetic on the square lattice, which means that the nearest-neighbor interaction ($J_1$) is stronger than the next-nearest-neighbor interaction ($J_2$), in contrast to other isostructural compounds such as Ba$_2$CuWO$_6$ and Sr$_2$CuWO$_6$, for which $|J_1|\,{<}\,|J_2|$ is realized.

Equilibrium, metastability, and hysteresis in a model spin-crossover material with nearest-neighbor antiferromagnetic-like and long-range ferromagnetic-like interactions

Phase diagrams and hysteresis loops were obtained by Monte Carlo simulations and a mean-field method for a simplified model of a spin-crossover material with a two-step transition between the high-spin and low-spin states. This model is a mapping onto a square-lattice $S=1/2$ Ising model with antiferromagnetic nearest-neighbor and ferromagnetic Husimi-Temperley (equivalent-neighbor) long-range interactions. Phase diagrams obtained by the two methods for weak and strong long-range interactions are found to be similar. However, for intermediate-strength long-range interactions, the Monte Carlo simulations show that tricritical points decompose into pairs of critical endpoints and mean-field critical points surrounded by horn-shaped regions of metastability. Hysteresis loops along paths traversing the horn regions are strongly reminiscent of thermal two-step transition loops with hysteresis, recently observed experimentally in several spin-crossover materials. We believe analogous phenomena should be observable in experiments and simulations for many systems that exhibit competition between local antiferromagnetic-like interactions and long-range ferromagnetic-like interactions caused by elastic distortions.

Dipolar needles in the microcanonical ensemble: Evidence of spontaneous magnetization and ergodicity breaking

We have studied needle-shaped three-dimensional classical spin systems with purely dipolar interactions in the microcanonical ensemble, using both numerical simulations and analytical approximations. We have observed spontaneous magnetization for different finite cubic lattices. The transition from the paramagnetic to the ferromagnetic phase is shown to be of first order. For two lattice types we have observed magnetization flips in the phase transition region. In some cases, gaps in the accessible values of magnetization appear, a signature of the ergodicity breaking found for systems with long-range interactions. We analytically explain these effects by performing a nontrivial mapping of the model Hamiltonian onto a one-dimensional Ising model with competing antiferromagnetic nearest-neighbor and ferromagnetic mean-field interactions. These results hint at performing experiments on isolated dipolar needles in order to verify some of the exotic properties of systems with long-range interactions in the microcanonical ensemble.

Antiferromagnetic ordering on the frustrated fcc lattice in the intermetallic compound GdPtBi

The type of antiferromagnetic magnetic order in the half-Heusler intermetallic compound GdPtBi, where the Gd atoms form a magnetically frustrated fcc lattice, has been investigated using exchange constants calculated from first principles within the framework of the Green's function based magnetic force theorem. It is found that the degeneracy of the magnetic states on the fcc lattice with antiferromagnetic nearest neighbor interaction is removed by strong fourth-nearest neighbor interaction along the cube diagonal. We show that an antiferromagnetic ordering of so-called ``third kind'' occurs at low temperatures. The Monte Carlo simulations using our derived exchange constants predict an experimental N\'eel temperature (${T}_{N}$) with good accuracy. The estimated temperature dependence of the correlation functions in the paramagnetic region exhibit a typical behavior for a frustrated system---a massive short-range order is retained for temperatures which are one order of magnitude higher than ${T}_{N}$.

Crystal and Electronic Structure and Magnetic Properties of Divalent Europium Perovskite Oxides EuMO3 (M = Ti, Zr, and Hf): Experimental and First-Principles Approaches

A comparative study of the crystal and electronic structure and magnetism of divalent europium perovskite oxides EuMO(3) (M = Ti, Zr, and Hf) has been performed on the basis of both experimental and theoretical approaches playing complementary roles. The compounds were synthesized via solid-state reactions. EuZrO(3) and EuHfO(3) have an orthorhombic structure with a space group Pbnm at room temperature contrary to EuTiO(3), which is cubic at room temperature. The optical band gaps of EuZrO(3) and EuHfO(3) are found to be about 2.4 and 2.7 eV, respectively, much larger than that of EuTiO(3) (0.8 eV). On the other hand, the present compounds exhibit similar magnetic properties characterized by paramagnetic-antiferromagnetic transitions at around 5 K, spin flop at moderate magnetic fields lower than 1 T, and the antiferromagnetic nearest-neighbor and ferromagnetic next-nearest-neighbor exchange interactions. First-principles calculations based on a hybrid Hartree-Fock density functional approach yield lattice constants, band gaps, and magnetic interactions in good agreement with those obtained experimentally. The band gap excitations are assigned to electronic transitions from the Eu 4f to Mnd states for EuMO(3) (M = Ti, Zr, and Hf and n = 3, 4, and 5, respectively).

Quasi-one-dimensional antiferromagnetism and multiferroicity in CuCrO4

The bulk magnetic properties of the new quasi-one-dimensional Heisenberg antiferromagnet, CuCrO${}_{4}$, were characterized by magnetic susceptibility, heat capacity, optical spectroscopy, electron paramagnetic resonance and dielectric capacitance measurements, and density functional evaluations of the intrachain and interchain spin-exchange interactions. We found type-II multiferroicity below the N\'eel temperature of 8.2(5) K, arising from competing antiferromagnetic nearest-neighbor (${J}_{\mathrm{nn}}$) and next-nearest-neighbor (${J}_{\mathrm{nnn}}$) intrachain spin-exchange interactions. Experimental and theoretical results indicate that the ratio ${J}_{\mathrm{nn}}$/${J}_{\mathrm{nnn}}$ is close to 2, putting CuCrO${}_{4}$ in the vicinity of the Majumdar-Ghosh point.

Half magnetization plateau of a frustrated S = 1 antiferromagnetic chain

We numerically investigate the magnetization process of a frustrated S=1 chain with antiferromagnetic nearest-neighbor (nn) and next-nearest-neighbor (nnn) exchange interactions, which compete with each other, and also with uniaxial single-ion-type anisotropy energies. Special attention is paid to the half magnetization plateau which appears at the half of the saturation magnetization in the ground-state magnetization curve. We determine the parameter region where this plateau appears by employing the level spectroscopy method, and find that the plateau appears even in the absence of the single-ion-type anisotropy, if the ratio of the nnn to the nn interaction constant is in a specified region.

Phase diagram and critical behavior of the square-lattice Ising model with competing nearest-neighbor and next-nearest-neighbor interactions

Using the parallel tempering algorithm and graphics processing unit accelerated techniques, we have performed large-scale Monte Carlo simulations of the Ising model on a square lattice with antiferromagnetic (repulsive) nearest-neighbor and next-nearest-neighbor interactions of the same strength and subject to a uniform magnetic field. Both transitions from the (2x1) and row-shifted (2x2) ordered phases to the paramagnetic phase are continuous. From our data analysis, re-entrance behavior of the (2x1) critical line and a bicritical point which separates the two ordered phases at T=0 are confirmed. Based on the critical exponents we obtained along the phase boundary, Suzuki's weak universality seems to hold.

Edge effects in the two-dimensional spin-12Heisenberg antiferromagnet

We use quantum Monte Carlo simulations to study effects of free edges in the two-dimensional spin-1/2 Heisenberg antiferromagnet. We find that the magnetic response of an edge is smaller than the bulk susceptibility. This counter-intuitive quantum effect can be traced to enhanced antiferromagnetic nearest-neighbor spin correlations, i.e., tendency to local singlet formation, at and close to the edge. These correlations form a comb-like pattern, which can be reproduced with a simple variational valence-bond state. We also study rough edges, and find that these instead significantly enhance the susceptibility, due to local sublattice imbalance impeding singlet formation.

First-principles calculations of spin interactions and the magnetic ground states of Cr trimers on Au(111)

We present calculations of the magnetic ground states of Cr trimers in different geometries on top of a Au(111) surface. By using a least squares fit method based on results obtained by means of a fully relativistic embedded-cluster Green's function method, first we determined the parameters of a classical spin vector model containing second- and fourth-order interactions. The developed method requires no a priori assumed symmetry constraints; therefore, it is applicable throughout for small nanoparticles of arbitrary geometry. The magnetic ground states were then found by solving the Landau-Lifshitz-Gilbert equations. In all cases considered, the configurational energy of the Cr trimers is dominated by large antiferromagnetic nearest neighbor interactions, while the biquadratic spin interactions provide the second largest contributions to the energy. We find that an equilateral Cr trimer exhibits a frustrated $120\ifmmode^\circ\else\textdegree\fi{}$ N\'eel type of ground state with a small out-of-plane component of the magnetization. Furthermore, we show that the Dzyaloshinsky-Moriya interactions determine the chirality of the magnetic ground state. In cases of a linear chain and an isosceles trimer, collinear antiferromagnetic ground states are obtained with the magnetization lying parallel to the surface.