Ferromagnetic Spin Correlations Research Articles

Coexistence of spin liquid state and magnetic correlations in 3d-5d based triangular-lattice antiferromagnet Sr3CuIr2O9

Here, we report detailed lattice structure, magnetization (dc and ac) and specific heat measurements on a 3d-5d based new triple-perovskite material Sr3CuIr2O9. The Sr/Cu forms a layered structure of triangular-lattice while the Ir forms Ir2O9 dimers which lie in chain as well as simultaneously makes layered triangular-lattice with neighboring atoms. Due to random site-sharing with Sr2+, the Cu2+ (3d 9, spin-1/2) forms a diluted magnetic lattice, thus giving a disordered in-plane exchange interaction. Opposed to conventional model, the Ir5+ (5d 4, = 0) is believed to be magnetic here which participates both in-chain and in-plane magnetic interactions. This complex lattice structure driven competing exchange interaction leads the ground state to a gapless quantum-spin-liquid state which coexists with (weak) ferromagnetic spin correlations. While underling the importance of spin state (spin-1/2), we believe that the combined effect of lattice structure, geometric frustration, spin–orbit coupling and spin state has given rise this interesting ground state in this material.

Investigating the magnetoelastic properties in FeSn and Fe3Sn2 flat band metals

Topological quantum magnets FeSn and Fe$_{3}$Sn$_{2}$ were studied using neutron scattering and first-principles calculations. Both materials are metallic but host dispersionless flat bands with Dirac nodes at the $K$ point in reciprocal space. The local structure determined from the pair density function analysis of the neutron diffraction data provided no evidence for electron localization in both compounds, consistent with their metallic nature. At the same time, in FeSn, an anomalous suppression in the $c$-axis lattice constant coupled with changes in the phonon spectra were observed across T$_{N}$ indicating the presence of magneto-elastic coupling and spin-phonon interactions. In addition, it was observed that spin waves persisted well above T$_{N}$, suggesting that the in-plane ferromagnetic spin correlations survive at high temperatures. In contrast, no lattice anomaly was observed in Fe$_{3}$Sn$_{2}$. The inelastic signal could be mostly accounted for by phonons, determined from density functional theory, showing typical softening on warming.

Observation of the Sign Reversal of the Magnetic Correlation in a Driven-Dissipative Fermi Gas in Double Wells.

We report the observation of the sign reversal of the magnetic correlation from antiferromagnetic to ferromagnetic in a dissipative Fermi gas in double wells, utilizing the dissipation caused by on-site two-body losses in a controlled manner. We systematically measure dynamics of the nearest-neighbor spin correlation in an isolated double-well optical lattice, as well as a crossover from an isolated double-well lattice to a one-dimensional uniform lattice. In a wide range of lattice configurations over an isolated double-well lattice, we observe a ferromagnetic spin correlation, which is consistent with a Dicke type of correlation expected in the long-time limit. This work demonstrates the control of quantum magnetism in open quantum systems with dissipation.

Quantum Monte Carlo study of the Hubbard model with next-nearest-neighbor hopping t′: pairing and magnetism

Using the finite-temperature determinant quantum Monte Carlo (DQMC) algorithm, we study the pairing symmetries of the Hubbard Hamiltonian with next-nearest-neighbor (NNN) hopping t′ on square lattices. By varying the value of t′, we find that the d-wave pairing is suppressed by the onset of t′, while the p + ip-wave pairing tends to emerge for low electron density and t′ around −0.7. Together with the calculation of the anti-ferromagnetic and ferromagnetic spin correlation function, we explore the relationship between anti-ferromagnetic order and the d-wave pairing symmetry, and the relationship between ferromagnetic order and the p + ip-wave pairing symmetry. Our results may be useful for the exploration of the mechanism of the electron pairing symmetries, and for the realization of the exotic p + ip-wave superconductivity.

Pressure-induced incommensurate antiferromagnetic order in a ferromagnetic B -site ordered double-perovskite Lu2NiMnO6

We have investigated the pressure effect on magnetic ordering of the ferromagnetic double perovskite ${\mathrm{Lu}}_{2}{\mathrm{NiMnO}}_{6}$ by magnetization, ac magnetic susceptibility, and neutron diffraction experiments up to 8.0 GPa in order to understand the ferromagnetic-to-antiferromagnetic phase transition by substitution of the $A$ sites in ${A}_{2}{\mathrm{NiMnO}}_{6}$ from rare-earth to indium or scandium ions. Strong ferromagnetic spin correlation seen in the susceptibility at low pressure is significantly suppressed by increasing pressure. In a neutron diffraction experiment, the magnetic Bragg reflections associated with ferromagnetic ordering disappear above 4.5 GPa. For the high-pressure region above 4.5 GPa, an antiferromagnetic ordering with long-period incommensurate modulation appears, which is coexistent with ferromagnetic short-range order. From mean-field calculations, we infer that pressure modification of the delicate balance of the competing next-nearest-neighbor exchange interactions between Ni and Ni, or Mn and Mn, plays an important role in the phase transition from ferromagnetic to antiferromagnetic ordering in ${A}_{2}{\mathrm{NiMnO}}_{6}$.

Ferromagnetic spin correlations in the two-dimensional Hubbard model

We analyze the dynamical nearest-neighbor and next-nearest-neighbor spin correlations in the 4-site and 8-site dynamical cluster approximation to the two-dimensional Hubbard model. Focusing on the robustness of these correlations at long imaginary times, we reveal enhanced ferromagnetic correlations on the lattice diagonal, consistent with the emergence of composite spin-1 moments at a temperature scale that essentially coincides with the pseudo-gap temperature $T^*$. We discuss these results in the context of the spin-freezing theory of unconventional superconductivity.

Suppression of ferromagnetic spin fluctuations in the filled skutterudite superconductor SrOs4As12 revealed by As75 NMR-NQR measurements

Motivated by the recent observation of ferromagnetic spin correlations in the filled skutterudite SrFe$_4$As$_{12}$ [Ding et al., Phys. Rev. B 98, 155149 (2018)], we have carried out $^{75}$As nuclear magnetic resonance (NMR) and nuclear quadrupole resonance (NQR) measurements to investigate the role of magnetic fluctuations in a newly discovered isostructural superconductor SrOs$_4$As$_{12}$ with a superconducting transition temperature of $T_{\rm c}$ $\sim$ 4.8 K. Knight shift $K$ determined by the NQR spectrum under a small magnetic field ($\le$ 0.5 T) is nearly independent of temperature, consistent with the temperature dependence of the magnetic susceptibility. The nuclear spin-lattice relaxation rate divided by temperature, 1/$T_1T$, is nearly independent of temperature above $\sim$ 50 K and increases slightly with decreasing temperature below the temperature. The temperature dependence is reasonably explained by a simple model where a flat band structure with a small ledge near the Fermi energy is assumed. By comparing the present NMR data with those in SrFe$_4$As$_{12}$, we found that the values of $|K|$ and $1/T_1T$ in SrOs$_4$As$_{12}$ are smaller than those in SrFe$_4$As$_{12}$, indicating no obvious ferromagnetic spin correlations in SrOs$_4$As$_{12}$. From the temperature dependence of 1/$T_1$ in the superconducting state, an $s$-wave superconductivity is realized.

Formation of an Antiferromagnetic Metal Phase in the Electron-Doped Sr0.98La0.02MnO3 Oxide According to 17O NMR Data

The spin density distribution of itinerant electrons, neg, and their effect on pairwise correlations of localized spins S(t2g) of Mn4+ ions in cubic Sr1 − xLaxMnO3 antiferromagnet (x = 0.02, TN = 230 K, G type magnetic structure) is studied experimentally by nuclear magnetic resonance on 17O nuclei. The regions with neg > x, in which the local spin susceptibility of pairs of Mn atoms follows the dependence χ ∼ (T − Θ)−1 with Θ = 20(5) K, indicating the growth of ferromagnetic spin correlations of neighboring magnetic ions in these domains, are found in the paramagnetic phase. The fraction of Mn-O-Mn bonds (neg > x) increases with the decrease in the temperature. The interpenetrating meshes of Mn-O-Mn bonds with different densities of itinerant eg electrons form an antiferromagnetic metal phase below TN. The role of weak localization effects is discussed as the main origin of the magnetic inhomogeneity of the antiferromagnetic metal phase in Sr0.98La0.02MnO3.

Impurity-Induced Disordering and Suppressed Ferromagnetic Spin Correlations in Ir Doped Sr2RuO4

Impurity-Induced Disordering and Suppressed Ferromagnetic Spin Correlations in Ir Doped Sr₂RuO₄

Breakdown of the Nagaoka phase at finite doping

The Nagaoka ($U=\infty$) limit of the Hubbard model on a square lattice is mapped onto the itinerant-localized Kondo model at infinitely strong coupling. Such a model is well suited to perform quantum Monte Carlo (QMC) simulations to compute spin correlation functions. This model is shown to exhibit no short-range ferromagnetic (FM) spin correlations at any doping $\delta\ge 0.01$ and finite temperature, $T=0.1t$. Our simulations give no indication that there is a tendency towards ferromagnetic ordering in the ground state.

Coexistence of antiferromagnetic and ferromagnetic spin correlations in Ca(Fe1−xCox)2As2 revealed by As75 nuclear magnetic resonance

Recent nuclear magnetic resonance (NMR) measurements revealed the coexistence of stripe-type antiferromagnetic (AFM) and ferromagnetic (FM) spin correlations in both the hole- and electron-doped BaFe$_2$As$_2$ families of iron-pnictide superconductors by a Korringa ratio analysis. Motivated by the NMR work, we investigate the possible existence of FM fluctuations in another iron pnictide superconducting family, Ca(Fe$_{1-x}$Co$_x$)$_2$As$_2$. We re-analyzed our previously reported data in terms of the Korringa ratio and found clear evidence for the coexistence of stripe-type AFM and FM spin correlations in the electron-doped CaFe$_2$As$_2$ system. These NMR data indicate that FM fluctuations exist in general in iron-pnictide superconducting families and thus must be included to capture the phenomenology of the iron pnictides.

Evidence of emerging Griffiths singularity in La0.5 Sr0.5 MnO3 nanocrystalline probed by magnetization and electron paramagnetic resonance

We present an investigation of Griffiths singularity in La0.5 Sr0.5 MnO3 nanocrystalline by means of magnetic susceptibility and electron paramagnetic resonance (EPR). An unusual platform was found in paramagnetic region. Based on the analysis of EPR spectrum and magnetization variation across the whole temperature range of phase transition, we confirm it is due to the presence of Griffiths singularity rather than a superparamagnetic state in the nanocrystalline system. Such a singularity phase is constituted with some correlated ferromagnetic clusters which embed in paramagnetic matrix. Although they form ferromagnetic spin correlation, the system do not yield any spontaneous magnetization. According to core–shell model, the emergence of Griffiths singularity can be considered due to the presence of local ferromagnetic fluctuations originated from surface spin disorder as the sample size is confined to nanoscale.

Magnetism in Strongly Interacting One-Dimensional Quantum Mixtures.

We consider two species of bosons in one dimension near the Tonks-Girardeau limit of infinite interactions. For the case of equal masses and equal intraspecies interactions, the system can be mapped to a S=1/2 XXZ Heisenberg spin chain, thus allowing one to access different magnetic phases. Using a powerful ansatz developed for the two-component Fermi system, we elucidate the evolution from few to many particles for the experimentally relevant case of an external harmonic confinement. In the few-body limit, we already find clear evidence of both ferromagnetic and antiferromagnetic spin correlations as the ratio of intraspecies and interspecies interactions is varied. Furthermore, we observe the rapid emergence of symmetry-broken magnetic ground states as the particle number is increased. We therefore demonstrate that systems containing only a few bosons are an ideal setting in which to realize the highly sought-after itinerant ferromagnetic phase.

Competing Magnetic Fluctuations in Iron Pnictide Superconductors: Role of Ferromagnetic Spin Correlations Revealed by NMR.

In the iron pnictide superconductors, theoretical calculations have consistently shown enhancements of the static magnetic susceptibility at both the stripe-type antiferromagnetic and in-plane ferromagnetic (FM) wave vectors. However, the possible existence of FM fluctuations has not yet been examined from a microscopic point of view. Here, using ^{75}As NMR data, we provide clear evidence for the existence of FM spin correlations in both the hole- and electron-doped BaFe_{2}As_{2} families of iron-pnictide superconductors. These FM fluctuations appear to compete with superconductivity and are thus a crucial ingredient to understanding the variability of T_{c} and the shape of the superconducting dome in these and other iron-pnictide families.

Coexistence of antiferromagnetic and ferromagnetic spin correlations inSrCo2As2revealed byCo59andAs75NMR

In nonsuperconducting, metallic paramagnetic ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$, inelastic neutron scattering measurements have revealed strong stripe-type $\mathbf{q}=(\ensuremath{\pi},0)$ antiferromagnetic (AFM) spin correlations. Here, using nuclear magnetic resonance (NMR) measurements on $^{59}\mathrm{Co}$ and $^{75}\mathrm{As}$ nuclei, we demonstrate that stronger ferromagnetic (FM) spin correlations coexist in ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$. Our NMR data are consistent with density functional theory (DFT) calculations which show enhancements at both $\mathbf{q}=(\ensuremath{\pi},0)$ and the in-plane FM $\mathbf{q}=0$ wave vectors in static magnetic susceptibility $\ensuremath{\chi}(\mathbf{q})$. We suggest that the strong FM fluctuations prevent superconductivity in ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$, despite the presence of stripe-type AFM fluctuations. Furthermore, since DFT calculations have consistently revealed similar enhancements of the $\ensuremath{\chi}(\mathbf{q})$ at both $\mathbf{q}=(\ensuremath{\pi},0)$ and $\mathbf{q}=0$ in the iron-based superconductors and parent compounds, our observation of FM correlations in ${\mathrm{SrCo}}_{2}{\mathrm{As}}_{2}$ calls for detailed studies of FM correlations in the iron-based superconductors.

Reconstruction of Chiral Edge States in a Magnetic Chern Insulator

We study reconstruction of the chiral edge states in a magnetic Chern insulator in the Kondo lattice model on a triangular lattice at 1/4 filling. In this state, the spin scalar chirality associated with a four-sublattice noncoplanar magnetic order assures the topological nature characterized by a nonzero Chern number. Performing a Langevin-based numerical simulation for finite-size clusters with the open boundary condition in one direction, we clarify how the magnetic and electronic properties are modulated near the edges of the system. As a result, we find that the magnetic state near the edges is reconstructed to develop ferromagnetic spin correlations. At the same time, the electronic state is also modified and the chiral edge current is enhanced. We discuss this enhancement from the viewpoint of the electronic band structure for the gapless edge states.

Co-existence of Griffiths phase and ferromagnetic insulating state in electron doped nanomanganite

Detailed temperature dependent magnetic and transport properties of electron doped Sm0.09Ca0.91MnO3 nanomanganite exhibiting Griffiths phase (GP) have been investigated. A GP-like behavior having ferromagnetic spin correlation above ferromagnetic transition temperature (TC) without any spontaneous magnetization has been observed in this nanomanganite. Moreover, in this electron doped nanomanganite, the experimental large effective magnetic moment and deviation of the inverse susceptibility from Curie–Weiss law clearly indicate the possible existence of GP. The complex non-linear ac susceptibilities (2nd and 3rd harmonics) have also been studied to explore the role of nature of disorder in the GP phenomenon. The transport measurements yield a ferromagnetic insulating ground state exhibiting colossal magneto resistance (CMR) near the ferromagnetic transition temperature. It has also been reported that the temperature dependent average spin moment at the hopping site and the ferromagnetic phase fraction bear the signature of the onset of GP.

Peierls distortion, magnetism, and high hardness of manganese tetraboride

We report crystal structure, electronic structure, and magnetism of manganese tetraboride, ${\text{MnB}}_{4}$, synthesized under high-pressure, high-temperature conditions. In contrast to superconducting ${\text{FeB}}_{4}$ and metallic ${\text{CrB}}_{4}$, which are both orthorhombic, ${\text{MnB}}_{4}$ features a monoclinic crystal structure. Its lower symmetry originates from a Peierls distortion of the Mn chains. This distortion nearly opens the gap at the Fermi level, but despite the strong dimerization and the proximity of ${\text{MnB}}_{4}$ to the insulating state, we find indications for a sizable paramagnetic effective moment of about 1.7 \ensuremath{\mu}${}_{B}$/f.u., ferromagnetic spin correlations, and, even more surprisingly, a prominent electronic contribution to the specific heat. However, no magnetic order has been observed in standard thermodynamic measurements down to 2 K. Altogether, this renders ${\text{MnB}}_{4}$ a structurally simple but microscopically enigmatic material; we argue that its properties may be influenced by electronic correlations.

Crystallographic, electronic, thermal, and magnetic properties of single-crystal SrCo2As2

In tetragonal SrCo2As2 single crystals, inelastic neutron scattering measurements demonstrated that strong stripe-type antiferromagnetic (AFM) correlations occur at a temperature T = 5 K [W. Jayasekara et al., arXiv:1306.5174] that are the same as in the isostructural AFe2As2 (A = Ca, Sr, Ba) parent compounds of high-Tc superconductors. This surprising discovery suggests that SrCo2As2 may also be a good parent compound for high-Tc superconductivity. Here, structural and thermal expansion, electrical resistivity rho, angle-resolved photoemission spectroscopy (ARPES), heat capacity Cp, magnetic susceptibility chi, 75As NMR and neutron diffraction measurements of SrCo2As2 crystals are reported together with LDA band structure calculations that shed further light on this fascinating material. The c-axis thermal expansion coefficient alpha_c is negative from 7 to 300 K, whereas alpha_a is positive over this T range. The rho(T) shows metallic character. The ARPES measurements and band theory confirm the metallic character and in addition show the presence of a flat band near the Fermi energy E_F. The band calculations exhibit an extremely sharp peak in the density of states D(E_F) arising from a flat d_{x^2 - y^2} band. A comparison of the Sommerfeld coefficient of the electronic specific heat with chi(T = 0) suggests the presence of strong ferromagnetic itinerant spin correlations which on the basis of the Stoner criterion predicts that SrCo2As2 should be an itinerant ferromagnet, in conflict with the magnetization data. The chi(T) does have a large magnitude, but also exhibits a broad maximum at 115 K suggestive of dynamic short-range AFM spin correlations, in agreement with the neutron scattering data. The measurements show no evidence for any type of phase transition between 1.3 and 300 K and we propose that metallic SrCo2As2 has a gapless quantum spin-liquid ground state.

Ferromagnetic spin correlations in a few-fermion system

We study the spin correlations of a few fermions in a quasi-one-dimensional trap. Exact diagonalization calculations demonstrate that repulsive interactions between the two species drives ferromagnetic correlations. The ejection probability of an atom provides an experimental probe of the spin correlations. With more than five atoms trapped, the system approaches the itinerant Stoner limit. Losses to Feshbach molecules are suppressed by the discretization of energy levels when fewer than seven atoms are trapped.