Antiferromagnetic Tendencies Research Articles

Realization of two-sublattice exchange physics in the triangular lattice compound Ba3Er(BO3)3

Geometric frustration commonly occurs in materials where magnetic rare-earth ions are arranged on a two-dimensional triangular lattice. These compounds have been gaining significant attention lately, as they hold the promise of revealing unique quantum states of matter. However, little attention has been devoted to cases where spin-12\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\frac{1}{2}$$\\end{document} rare-earth ions are substituted with ions exhibiting higher spin multiplicities. Here, we successfully synthesize high-quality single crystal samples of Ba3Er(BO3)3, which is part of the family of triangular lattice compounds. In our experiments, conducted at extremely low temperatures (around 100 millikelvin), we observe two sublattice exchange interactions in Ba3Er(BO3)3, resulting in the hexagonal lattice spins exhibiting a mixture of ferromagnetic and antiferromagnetic tendencies. Our theoretical analysis suggest that this behavior may be attributed to the distinct positions of magnetic ions within the crystal lattice. However, the presence of quantum effects adds an extra layer of complexity to our findings, calling for further exploration.

Exploring high-temperature superconductivity in the extended Hubbard model with antiferromagnetic tendencies

Exploring high-temperature superconductivity in the extended Hubbard model with antiferromagnetic tendencies

Role of inter-site Coulomb interaction on the thermodynamic and ground state properties within the [formula omitted] model

In this work, we report the effect of nearest neighbor (NN) inter-site Coulomb interaction (V) with a certain finite value of on-site Coulomb repulsion (U) and NN antiferromagnetic exchange interaction (J) in the context of high-Tc cuprates. An exact diagonalization method has been applied in a 2D t−J−U model extended by NN inter-site Coulomb interaction in the tilted square cluster near optimal doping. The antiferromagnetic tendency is suppressed by the inter-site e−e repulsion. The inter-site Coulomb repulsion produces an effective on-site attraction between electrons, resulting in the creation of doublons against the on-site Coulomb repulsion. The computation of binding energy demonstrates the binding of holes for small U/t, resulting from the interplay between U and V. Superconductivity arises in a limited window of V depending on U. However, the effective dominance of either U or V results in the annihilation of these hole pairs. The presence of two peaks in specific heat is caused by two Coulomb interactions. The development of doublons as V increases leads to a more ordered state, lowering entropy. The thermodynamic properties also show that the t−J−U−V model acts as an attractive Hubbard model for U<V. However, a pure attractive t−J−U model cannot explain these phenomenologies.

Magnetic impurities in a charge-ordered background

We investigate how magnetic impurities may affect a system exhibiting a charge-density wave (CDW) in its ground state. We consider a disordered Hubbard-Holstein model with a homogeneous electron-phonon interaction but with a (randomly chosen) fraction of sites displaying a nonzero Coulomb repulsion $U$ and perform state-of-the-art finite-temperature quantum Monte Carlo simulations. For a single magnetic impurity, charge-charge correlations hamper the spin-spin ones around the repulsive site, thus requiring a strong enough value of $U$ to create nonnegligible antiferromagnetic (AFM) correlations. As the number of magnetic impurities increases, these AFM correlations become deleterious to CDW order and its features. First, the critical temperature is drastically reduced and seems to vanish $\ensuremath{\sim}40%$ of impurities (for fixed $U/\ensuremath{\lambda}=2$), which we correlate with the classical percolation threshold. We also notice that just a small amount of disorder suffices to create a bad insulating state, with the suppression of both Peierls and spin gaps, even within the charge-ordered phase. Finally, we have also found that pairing correlations are enhanced at large doping, driven by the competition between CDW and AFM tendencies.

Spin-orbital liquids and insulator-metal transitions on the pyrochlore lattice

The two orbital Hubbard model, with the electrons additionally coupled to a complex magnetic background, arises in the pyrochlore molybdates. The background involves local moments Hund's coupled to the electrons, driving double exchange ferromagnetism, and antiferromagnetic (AF) tendency arising from competing superexchange. The key scales include the Hubbard repulsion and the superexchange, both of which can be tuned in these materials. They control the phase transition from a ferromagnetic metal to a spin-glass metal and then to a spin-glass (Mott) insulator. We provide a comprehensive description of the ground state of this model using an unrestricted Hartree-Fock scheme implemented via a simulated annealing procedure and establish the metal-insulator transition line for varying Hubbard interaction and superexchange. The electrons see an effective disorder, due to orbital frustration, already in the ferromagnetic phase. The disorder is further enhanced by antiferromagnetic coupling and the resulting magnetic disorder. As a result, increasing AF coupling shifts the metal-insulator transition to lower Hubbard interaction and gives it an additional ``Anderson'' character. We provide detailed results on the magnetic and orbital correlations, the density of states, and the optical conductivity.

Electronic, magnetic and magnetotransport properties of Mn-doped Dirac semimetal Cd[formula omitted]As[formula omitted

Doping by Mn breaks the time-reversal and crystal symmetry of Cd3As2, splits Dirac cones and changes band structure. We study these changes using both DFT calculations and magnetotransport measurements. Our band structure of Cd3As2 is confirmed by calculations of dielectric tensor and reflectivity consistent with an experiment. Calculations take into account the n-carrier density of Cd3As2 samples ∼1·1018 cm−3 which shifts the Fermi level by εF∼50 meV. The Mn-doping of Cd3As2 is simulated with the Cd46Mn2As32 cell for 9 configurations of Mn atoms. Our simulation shows the preference for antiferromagnetic spin order and tendency towards Mn clustering. Doping by Mn causes transition of Cd3As2 into a trivial topological state with a gap of Eg≈40 meV. Our experimental studies of (Cd1−xMnx)3As2 show that a negative sign of magneto-conductivity is observed at x=0 and 0.017, but changes to a positive one at x=0.033. We argue, this sign change is closely connected with the gap opening and is basically controlled by the ratio Eg/εF.

Origin of Insulating Ferromagnetism in Iron Oxychalcogenide Ce_{2}O_{2}FeSe_{2}.

An insulating ferromagnetic (FM) phase exists in the quasi-one-dimensional iron oxychalcogenide Ce_{2}O_{2}FeSe_{2}, but its origin is unknown. To understand the FM mechanism, here a systematic investigation of this material is provided, analyzing the competition between ferromagnetic and antiferromagnetic tendencies and the interplay of hoppings, Coulomb interactions, Hund's coupling, and crystal-field splittings. Our intuitive analysis based on second-order perturbation theory shows that large entanglements between doubly occupied and half filled orbitals play a key role in stabilizing the FM order in Ce_{2}O_{2}FeSe_{2}. In addition, via many-body computational techniques applied to a multiorbital Hubbard model, the phase diagram confirms the proposed FM mechanism.

Frustration and Glasslike Character in RIn1- xMn xO3 (R = Tb, Dy, Gd).

We bring together ac susceptibility and dc magnetization to uncover the rich magnetic field-temperature behavior of a series of rare earth indium oxides, RInO3 (R = Tb, Dy, and Gd). The degree of frustration is much larger than expected, particularly in TbInO3, and the ground states are glasslike with antiferromagnetic tendencies. The activation energy for spin reorientation is low. Chemical substitution with Mn3+ ions to form TbIn1- xMn xO3 ( x ≤ 0.01) relieves much of the frustration that characterizes the parent compound and slightly enhances the short-range antiferromagnetic order. The phase diagrams developed from this work reveal the rich competition between spin orders and provide an opportunity to compare the dynamics in the RInO3 and Mn-substituted systems. These structure-property relations may be useful for understanding magnetism in other geometrically frustrated multiferroics.

Local environment dependence of Mn magnetism in bcc iron-manganese alloys: A first-principles study

Magnetic behavior of a $3d$ solute in a ferromagnetic lattice can be very sensitive to local environment, which is the case of manganese in bcc Fe. Body-centered cubic iron-manganese alloys are studied by means of density functional theory in order to elucidate properties of the lowest-energy magnetic states. Multiple magnetic minima are determined even for the simplest case of an isolated Mn and a Mn dimer in bcc iron. The magnetoenergetic landscape is analyzed within and beyond the collinear magnetic approximation. A direct correlation is identified between the local electronic charge and the local magnetic moment of a Mn solute, being either isolated or forming a small cluster. In particular, the presence of a vacancy near the Mn atom, inducing a local charge decrease, tends to favor the antiferromagnetic Fe-Mn interaction while the presence of an interstitial impurity with a strong electronic hybridization with Mn can favor a ferromagnetic Fe-Mn interaction. Energetic and magnetic properties of Fe-Mn alloys are systematically investigated for a large range of Mn concentrations. An unmixing tendency is clearly noted. A detailed magnetic analysis suggests the Mn-Mn magnetic interactions to be generally dominant over the Fe-Mn interactions, both exhibiting an antiferromagnetic tendency. The average magnetic moment of the Mn atoms in locally random alloys tends to be antiparallel (parallel) to lattice Fe moments for Mn concentrations smaller (larger) than approximately 6 at. %. The transition concentration is shown to be lowered if considering Mn clustering which is energetically favorable. The unsolved discrepancies between experimental and theoretical predictions on the critical concentration for the Mn magnetic behavior change in Fe-Mn solid solution are discussed in the light of the obtained results.

Interfacial phase competition induced Kondo-like effect in manganite-insulator composites

Kondo-like effect, namely the upturn of resistivity at low temperatures, has been observed in perovskite manganite when nonmagnetic insulators were doped as the secondary phase. In this paper, the low-temperature resistivity upturn effect was argued to be originated from the interfacial magnetic phase reconstruction. Heisenberg spin lattices have been simulated using Monte Carlo method to reveal the phase competition around the secondary phase boundary, namely the manganite-insulator boundary behaves a weak antiferromagnetic tendency. And the resistor-network model based on the double-exchange conductive mechanism indeed reproduces the low-temperature resistivity upturn effect. Our work provide a reasonable physical mechanism to understand novel transport behaviors in micro-structures of correlated electron systems.

Electronic and magnetic properties of superconducting LnO1−xFxBiS2 (Ln = La, Ce, Pr, and Nd) from first principles

A density functional theory study of the BiS2 superconductors containing rare-earths: LnO1−xFxBiS2 (Ln = La, Ce, Pr, and Nd) is presented. We find that CeO0.5F0.5BiS2 has competing ferromagnetic and weak antiferromagnetic tendencies, the first one corresponding to experimental results. We show that PrO0.5F0.5BiS2 has a strong tendency for magnetic order, which can be ferromagnetic or antiferromagnetic depending on subtle differences in 4f orbital occupations. We demonstrate that NdO0.5F0.5BiS2 has a stable magnetic ground state with weak tendency to order. Finally, we show that the change of rare earth does not affect the Fermi surface, and predict that CeOBiS2 should display a pressure induced phase transition to a metallic, if not superconducting, phase under pressure.

Magnetoelectric coupling at the interface of BiFeO3/La0.7Sr0.3MnO3multilayers

Electric-field controlled exchange bias in a heterostructure composed of the ferromagnetic manganite La0.7Sr0.3MO3 and the ferroelectric antiferromagnetic BiFeO3 has recently been demonstrated experimentally. By means of a microscopic model Hamiltonian we provide a possible explanation of the origin of this magnetoelectric coupling. We find, in agreement with experimental results, a net ferromagnetic moment at the BiFeO3 interface. The induced ferromagnetic moment is the result of the competition between the e_g-electrons double exchange and the t_2g-spins antiferromagnetic superexchange that dominate in bulk BiFeO3. The balance of these simultaneous ferromagnetic and antiferromagnetic tendencies is strongly affected by the interfacial electronic charge density which, in turn, can be controlled by the BiFeO3 ferroelectric polarization.

Electronic structure of nickelates: From two-dimensional heterostructures to three-dimensional bulk materials

Reduced dimensionality and strong electronic correlations, which are among the most important ingredients for cupratelike high-${T}_{\text{c}}$ superconductivity, characterize also the physics of nickelate-based heterostructures. Starting from the local-density approximation we arrive at a simple two-band model for quasi-two-dimensional (2D) ${\text{LaNiO}}_{3}/{\text{LaAlO}}_{3}$ heterostructures and extend it by introducing an appropriate hopping in the $z$ direction to describe the dimensional crossover to three dimensions (3D). Using dynamical mean-field theory, we study the effects of electronic correlations with increasing interaction strength along the crossover from 2D to 3D. Qualitatively, the effects of electronic correlations are surprisingly similar, albeit quantitatively larger interaction strengths are required in three dimensions for getting a Mott-Hubbard insulating state. The exchange parameters of an effective Kugel-Khomskii-type spin-orbital model are also derived and reveal strong antiferromagnetic tendencies.

Absence of magnetic ordering in low dimensional mixed magnetic Mn 1− xNi xCl 2·H 2O

A new mixed magnet, Mn 1− x Ni x Cl 2·H 2O, is examined by dc magnetization and susceptibility measurements across the entire composition range. The pure components are quasi-one-dimensional Heisenberg antiferromagnets ordering at 2.17 K (Mn) and 5.6 5 K (Ni) due to weaker interchain exchange supplementing the dominant exchange along MCl 2MCl 2M… chemical and structural chains. High temperature magnetic susceptibilities yield Curie and Weiss constants in χ M= C/( T− θ). C( x) is linear but θ( x) displays curvature, which is analyzed to show that unlike-ion exchange is ferromagnetic and similar in size to like-ion. Most notable is the absence of antiferromagnetic susceptibility maxima down to 1.6 K from x=0.10 to 0.95. For x=0.05 a susceptibility maximum appears, with T max almost 20% lower than in the pure Mn component but T c reduced by 2%. The size of the susceptibility is enhanced by admixture, the effect of disrupted antiferromagnetic tendencies. Magnetization isotherms evolve with composition. Larger values of magnetization, under the same measuring conditions, occur for mixtures than for pure components, consistent with frustration, which weakens antiferromagnetic alignment tendencies. The competing ferromagnetic (Ni) and antiferromagnetic (Mn) intrachain interactions, along with disorder and low dimensional characteristics, presumably lead to the absence of magnetic order over a remarkably broad composition range.

Spin-Unrestricted Calculations of Bare-Edged Nanographenes Using DFT and Many-Body Perturbation Theory.

The ability of Density Functional Theory to predict the electronic and magnetic properties of semi-infinite graphene with a single bare edge has been probed. In order to improve the accuracy of spin-unrestricted calculations performed with semilocal density functionals, higher-level methods including double hybrid density functionals and many-body perturbation theory have been applied to the polycyclic aromatic hydrocarbons model systems. We show that the antiferromagnetic or ferromagnetic tendencies of the corresponding electronic ground states strongly depend on the choice of the density functional. In addition the relative stability of the armchair and zigzag edges has been investigated, emphasizing the importance of using methods beyond semilocals density functionals.

Correlation Effects on the Doped Triangular Lattice in View of the Physics of Sodium-RichNaxCoO2

The peculiar correlation effects on the triangular lattice are studied by means of the rotationally invariant slave boson method in a cellular cluster approach. Hence nonlocal correlations are included in a short-range regime. Their impact for the single-band Hubbard model is studied at half filling, i.e., on the Mott transition, and with doping. Using the realistic band structure of NaxCoO2, we may also shed light on the cobaltate physics for x > or = 1/3, with the in-plane transition from antiferromagnetic tendencies towards the onset of ferromagnetism for a finite Hubbard U.

Magnetism and chemical ordering in binary transition metal clusters

Binary $XY$ clusters, with $X,Y∊{\text{Mn},\text{Fe},\text{Co},\text{Ni}}$, are investigated within first-principles simulations in the framework of density-functional theory. In the case of Co-Mn, the increase in magnetic moment with Mn concentration, which is in contrast to the bulk behavior and was also observed experimentally, is explained on the basis of the electronic structure of the clusters. For Co-Fe, Ni-Mn, Ni-Fe, and Ni-Co, due to the ferromagnetic couplings between the local moments, the same dependence is found, i.e., moments increasing with the concentration of the element with the lower value of atomic number. For Fe-Mn (as well as Co-Mn and Ni-Mn clusters containing many Mn atoms), the situation appears to be more complex with antiferromagnetic tendencies resulting in a nonmonotonic change of the cluster moment with Mn concentration.

Ferromagnetic tendency at the surface of CE-type charge-ordered manganites

Most previous investigations have shown that the surface of a ferromagnetic material may have antiferromagnetic tendencies. However, experimentally, the opposite effect has been recently observed-ferromagnetism appears in some nanosized manganites with a composition such that the antiferromagnetic charge-ordered CE state is observed in the bulk. A possible origin is the development of ferromagnetic correlations at the surface of these small systems. To clarify these puzzling experimental observations, we have studied the two-orbital double-exchange model near half doping, n = 0.5, using open boundary conditions to simulate the surface of either bulk or nanosized manganites. Considering the enhancement of surface charge density due to a possible AO termination (A = trivalent/divalent ion composite, O = oxygen), an unexpected surface phase-separated state emerges when the model is studied using Monte Carlo techniques on small clusters. This tendency suppresses the CE charge ordering and produces a weak ferromagnetic signal that could explain the experimental observations.

Construction of a microscopic model forf-electron systems on the basis of aj−jcoupling scheme

We construct a microscopic model for f-electron systems, composed of f-electron hopping, Coulomb interaction, and crystalline electric field (CEF) terms. In order to clarify the meaning of one f-electron state, here the j-j coupling scheme is considered, since the spin-orbit interaction is generally large in f-electron systems. Thus, the f-electron state at each site is labelled by $\mu$, namely, the z-component of total angular momentum j. By paying due attention to f-orbital symmetry, the hopping amplitudes between f-electron states are expressed using Slater's integrals. The Coulomb interaction terms among the $\mu$-states are written by Slater-Condon or Racah parameters. Finally, the CEF terms are obtained from the table of Hutchings. The constructed Hamiltonian is regarded as an orbital degenerate Hubbard model, since it includes two pseudo-spin and three pseudo-orbital degrees of freedom. For practical purposes, it is further simplified into a couple of two-orbital models by discarding one of the three orbitals. One of those simplified models is here analyzed using the exact diagonalization method to clarify ground-state properties by evaluating several kinds of correlation functions. Especially, the superconducting pair correlation function in orbital degenerate systems is carefully calculated based on the concept of off-diagonal long-range order. We attempt to discuss a possible relation of the present results with experimental observations for recently discovered heavy fermion superconductors CeMIn$_5$ (M=Ir, Co, and Rh), and a comprehensive scenario to understand superconducting and antiferromagnetic tendencies in the so-called ``115'' materials such as CeMIn$_5$, UMGa$_5$, and PuCoGa$_5$ from the microscopic viewpoint.

Magnetochemical origin for Invar anomalies in iron-nickel alloys

Zero- and finite-temperature (T) first-principles calculations versus composition (c) show that magnetochemical effects lead to Invar anomalies in Fe-(Ni, Co, Pt) alloys. Chemical short- or long-range order and negative interatomic exchange interaction of electrons in antibonding majority-spin states force the face-centered-cubic lattice to compete simultaneously for a smaller volume (from antiferromagnetic tendencies) and a larger volume (from Stoner ferromagnetic tendencies). The resulting additional negative lattice anharmonicity is very large for Fe-(Ni, Co) while absent for Fe-Pt. Our results explain the T- and c-dependent behavior of Invar properties, including the lattice softening and thermal expansion of Fe-Ni. In addition, the occurrence of a noncollinear spin structure at $T=0 \mathrm{K}$ near Invar can be understood on the basis of our results.