Three-dimensional Long-range Order Research Articles

Field evolution of magnetic phases and spin dynamics in the honeycomb lattice magnet Na2Co2TeO6 : Na23 NMR study

We report on the results of 23Na NMR in the honeycomb lattice magnet Na2Co2TeO6 which has been nominated as a Kitaev material. Measurements of magnetic shift and width of the NMR line as functions of temperature and magnetic field show that a spin-disordered phase does not appear up to a field of 9 T. In the antiferromagnetic phase just below the Neel temperature TN, we find a temperature region extending down to ~TN/2 where the nuclear spin-lattice relaxation rate 1/T1 remains enhanced and is further increased by a magnetic field. This region crosses over to a low temperature region characterized by the rapidly decreasing 1/T1 which is less field-sensitive. These observations suggest incoherent spin excitations with a large spectral weight at low energies in the intermediate temperature region transforming to more conventional spin-wave excitations at low temperatures. The drastic change of the low-energy spin dynamics is likely caused by strong damping of spin waves activated only in the intermediate temperature region, which may be realized for triple-q magnetic order possessing partially-disordered moments as scattering centers of spin waves. In the paramagnetic phase near TN, dramatic field suppression of 1/T1 is observed. From analysis of the temperature dependence of 1/T1 based on the renormalized-classical description of a two-dimensional quantum antiferromagnet, we find the field-dependent spin stiffness constant that scales with TN as a function of magnetic field. This implies field suppression of the energy scale characterizing both two-dimensional spin correlations and three-dimensional long-range order, which may be associated with an increasing effect of frustration in magnetic fields.

The smectic phase in semiflexible polymer materials: A large scale molecular dynamics study

Semiflexible polymers in concentrated lyotropic solution are studied within a bead-spring model by molecular dynamics simulations, focusing on the emergence of a smectic A phase and its properties. We systematically vary the density of the monomeric units for several contour lengths that are taken smaller than the chain persistence length. The difficulties concerning the equilibration of such systems and the choice of appropriate ensemble (constant volume versus constant pressure, where all three linear dimensions of the simulation box can fluctuate independently) are carefully discussed. Using HOOMD-blue on graphics processing units, systems containing more than a million monomeric units are accessible, making it possible to distinguish the order of the phase transitions that occur. While in this model the nematic-smectic transition is continuous, the transition from the smectic phase to a related crystalline structure with true three-dimensional long-range order is clearly of first order. Further, both orientational and positional correlations of monomeric units are studied as well as the order parameters characterizing the nematic, smectic A, and crystalline phases. The analogy between smectic order and one-dimensional harmonic crystals with respect to the behavior of the structure factor is also explored. Finally, the results are put in perspective with pertinent theoretical predictions and possible experiments.

Ultrafast energy- and momentum-resolved dynamics of magnetic correlations in the photo-doped Mott insulator Sr2IrO4.

Measuring how the magnetic correlations evolve in doped Mott insulators has greatly improved our understanding of the pseudogap, non-Fermi liquids and high-temperature superconductivity. Recently, photo-excitation has been used to induce similarly exotic states transiently. However, the lack of available probes of magnetic correlations in the time domain hinders our understanding of these photo-induced states and how they could be controlled. Here, we implement magnetic resonant inelastic X-ray scattering at a free-electron laser to directly determine the magnetic dynamics after photo-doping the Mott insulator Sr2IrO4. We find that the non-equilibrium state, 2 ps after the excitation, exhibits strongly suppressed long-range magnetic order, but hosts photo-carriers that induce strong, non-thermal magnetic correlations. These two-dimensional (2D) in-plane Néel correlations recover within a few picoseconds, whereas the three-dimensional (3D) long-range magnetic order restores on a fluence-dependent timescale of a few hundred picoseconds. The marked difference in these two timescales implies that the dimensionality of magnetic correlations is vital for our understanding of ultrafast magnetic dynamics.

Zero-Field Magnetism of a Two-Dimensional Antiferromagnet, κ-(BEDT-TTF)2Cu[N(CN)2]Cl, Determined by Muon Spin Rotation and Magnetization Measurements

The zero-field magnetism of a two-dimensional noncollinear antiferromagnet, κ-(BEDT-TTF)2Cu[N(CN)2]Cl, has been investigated by magnetization and zero-field muon spin rotation (μSR) measurements. Low-field magnetization measurements enabled us to determine the magnetic transition temperature TN as 22.80 ± 0.02 K. Distinct muon spin precession signals appeared below 21.4 K. μSR spectra below 21.4 K were well described by two types of precession components and a relaxation one. The temperature dependence of internal field converted by μSR data was in good agreement with that of macroscopic residual magnetism. These results suggest that the tiny interlayer interaction, which has been suggested to be almost 106 times less than the intralayer exchange interaction, spontaneously causes the three-dimensional long-range order.

Magnetic structure of Sr2CuWO6

Magnetic structure of the double perovskite Sr2CuWO6 was determined from neutron powder diffraction data. At 3 K the material is magnetically long-range ordered into a collinear antiferromagnetic structure described by a propagation vector k = (0, 1/2, 1/2) with the CuII moments of 0.57(1) μB parallel to the a-axis. The result is in agreement with our previous prediction (Vasala et al 2014 Phys. Rev. B 89 134419) based on electronic structure calculations, showing that the three-dimensional magnetic long-range order is caused by relatively strong antiferromagnetic next-nearest-neighbor interactions.

Phase separation of bilayered perovskite manganite (La1-xGdx)4/3Sr5/3Mn2 O7 (x=0, 0.05)

La1-xGdx)4/3Sr5/3Mn2 O7 (x=0, 0.05) polycrystalline samples have been prepared by solid state reaction method, and the phase separation phenomena in this samples are investigated by measuring the magnetization-temperature (M-T) curve, electron spin resonance (ESR) curve and resistivity-temperature (-T) curve. For both samples, experimental results suggest there exists competition between ferromagnetic and antiferromagnetic interactions in low temperature range, which reflects a characteristic of cluster spin glass. A Griffiths-like phase is observed in temperature ranges 125375 K and 100375 K for x=0 sample and x=0.05 sample, respectively. It is found that doping contributes to the decrease of three-dimensional long-range ferromagnetic ordering temperature (from Tc03D 125 K for x=0 to Tc13D 100 K for x=0.05), but has no obvious effect on the Griffiths-like temperature (TG 375 K). Above TG 375 K, a pure paramagnetic phase appears in both samples. The -T curves reveal two insulator-metal transitions in the entire temperature range for x=0 sample, which is caused by coexistence of the two phases in perovskite manganese oxides. For x=0.05 sample, however, there exhibits a single insulator-metal transition, indicating that doping can hinder the coexistence phenomenon. It can be seen from the fitted -T curves that the electron conduction mechanism in high temperature range is in accordance with the three-dimensional variable range of hopping conduction.

Unique Molecular Dynamics of Structural Elements in an Asymmetric Janus Bisamide Supramolecule Characterized by Solid-State NMR

An asymmetric tapered Janus bisamide supramolecule consisting of 1,4-bis[3,4,5-tris(alkan-1-yloxy)benzamido]benzene bisamide (abbreviated as C22PhBAEO3) can possess three-dimensional (3D) long-range order under mild thermal treatment conditions. To understand its structural formation and unique phase-transition processes, the locally detailed structure and molecular dynamics of its structural elements in disordered and ordered phases of C22PhBAEO3 were investigated using various solid-state (SS) NMR techniques at the atomic level. On the basis of the determined conformations and packing structures of the alkyl chains in ordered and disordered crystalline phases, along with the geometry and kinetic parameters of the structural elements' dynamics, this study addresses the self-assembly, the phase-transition mechanisms, and the relationship between the structure and dynamics of these asymmetric Janus bisamide supramolecules.

MnIII(tetra-biphenyl-porphyrin)–TCNE Single-Chain Magnet via Suppression of the Interchain Interactions

A single-chain magnet (SCM) of [Mn(TBPP)(TCNE)]·4m-PhCl(2) (1), where TBPP(2-) = meso-tetra(4-biphenyl)porphyrinate; TCNE(•-) = tetracyanoethenide radical anion; m-PhCl(2) = meta-dichlorobenzene, was prepared via suppression of interchain interactions. 1 has a one-dimensional alternating Mn(III)(porphrin)-TCNE(•-)chain structure similar to those of a family of complexes reported by Miller and co-workers. From a comparison of the static magnetic properties of 1 with other Mn(III)(porphyrin)-TCNE(•-) chains, a magneto-structural correlation between the intrachain magnetic exchange and both the dihedral angle between the mean plane on [Mn(TBPP)(TCNE)] and Mn-N≡C was observed. The ac magnetic susceptibility data of 1 could be fit with the Arrhenius law, indicating that slow magnetic relaxation and ruling out three-dimensional long-range ordering and spin-glass-like behavior. The Cole-Cole plot for 1 was semicircular, verifying that it is an SCM. Therefore, 1 is an ideal single-chain magnet with significantly strong intrachain magnetic exchange interactions beyond the Ising limit.

Detailed structure of the low-energy magnetic dispersion of the diagonal incommensurate phase in La1.975Sr0.025CuO4

Inelastic neutron scattering experiments have been performed on lightly doped La{sub 1.975}Sr{sub 0.025}CuO{sub 4}, which contains a hole concentration slightly higher than the critical concentration for three-dimensional long-range antiferromagnetic order. We previously found that the magnetic excitation spectrum in the insulating phase with a diagonal incommensurate spin modulation has similarities to that in the superconducting regime, where the spin modulation is bond parallel. In this study, we investigate the excitations in detail around E{sub cross}, at which the excitations become most nearly commensurate. It is found that both the magnitude and the anisotropy of the momentum width of the excitations change abruptly at E{sub cross}. Our experimental results suggest that the magnetic excitations rising from the pair of (diagonally) incommensurate wave vectors merge at E{sub cross} into isotropic excitations.

Magnetic correlations and the influence of atomic disorder in frustrated isosceles triangular lattice antiferromagnet CuMnO2

We have studied the magnetic correlations of the isosceles triangular lattice antiferromagnet Cu${}_{1+x}$Mn${}_{1\ensuremath{-}x}$O${}_{2}$ with $x=0.00$ and 0.04, using the magnetic susceptibility, specific heat, elastic, and inelastic neutron-scattering experiments. Above ${T}_{\mathrm{N}}$, the magnetic correlations in both samples are characterized by a two-dimensional (2D) short-range correlation, as indicated by the broad peak in the susceptibility and the asymmetric diffuse scattering observed in neutron diffraction experiments. Below ${T}_{\mathrm{N}}$, the magnetic structures, with the propagation vectors $\mathbit{Q}=(\overline{\frac{1}{2}}\frac{1}{2}\frac{1}{2})$ in $x=0.00$ and $\mathbit{Q}=(\overline{\frac{1}{2}}\frac{1}{2}0)$ in $x=0.04$, have been identified to be collinear magnetic structures with the magnetic moments almost parallel to the ${d}_{{z}^{2}}$ orbitals. Although the atomic disorder does not affect the direction and length of the magnetic moments in CuMnO${}_{2}$, the stacking sequence along the $c$ direction is changed from antiferromagnetic to ferromagnetic. Considering the interlayer exchange interactions between isosceles triangular lattice layers, we find that the second-nearest-neighbor interlayer interaction plays an important role for the magnetic stacking along the $c$ axis. The magnetic excitation below ${T}_{\mathrm{N}}$ is characterized by collective spin-wave excitation with an energy gap of 6 meV from the three-dimensional long-range magnetic order and by spin-liquid-like 2D excitation, as indicated by $C(T)\ensuremath{\propto}{T}^{2}$ in CuMnO${}_{2}$.

Long-range magnetic ordering in Na2IrO3

We report a combined experimental and theoretical investigation of the magnetic structure of the honeycomb-lattice magnet Na${}_{2}$IrO${}_{3}$, a candidate for a realization of a gapless spin liquid. Using resonant x-ray magnetic scattering at the Ir ${\mathrm{L}}_{3}$ edge, we find three-dimensional long-range antiferromagnetic order below ${T}_{N}=13.3$ K. From the azimuthal dependence of the magnetic Bragg peak, the ordered moment is determined to be predominantly along the $a$ axis. Combining the experimental data with first-principles calculations, we propose that the most likely spin structure is a zig-zag structure.

ChemInform Abstract: Spin Correlations in High-Temperature Superconductors

Abstract Recent neutron magnetic scattering experiments from single crystals of both undoped and doped cuprous “214” oxides are reviewed. The magnetic properties in the undoped crystals are essentially interpreted by the quantum two-dimensional Heisenberg antiferromagnetic model. The three-dimensional long-range order occurs because of small but finite interplanar magnetic coupling plus various anisotropies. The doping of charge modifies the spin correlations significantly as well as the spin fluctuations, which suggests close correlations between the charge and spin degrees of freedom.

Correlation Functions and Optical Effects in Surface Layers of Polymer Films

The calculated dependences of the quadrupole orientation order parameter on the Kuhn statistical segment length properly describe the experimental data, obtained by the film birefringence method for different multilayer polymer films. Transverse orientation correlations of chains segments decrease according to a power law, as in the case of infinite two-dimensional systems. The fluctuations of three-dimensional long-range orientation order in the plane of the layers are observed. These fluctuations are enhanced when the anisotropy of intra- and interchain interactions increase in the film.

Evidence of a magnetic Bose glass in(CH3)2CHNH3Cu(Cl{0.95}Br{0.05})3from neutron diffraction

We report the single-crystal study of the bulk magnetization and neutron-scattering measurements on a quantum $S=1/2$ spin ladders system $\text{IPA-Cu}{({\text{Cl}}_{0.95}{\text{Br}}_{0.05})}_{3}$ with quenched disorder. In zero field, the disordered spin liquid phase is preserved as in pure ${\text{IPA-CuCl}}_{3}$. Due to the bond randomness, a different Bose glass phase was directly observed in ${H}_{c}<H<{H}^{\ensuremath{'}}$, which separates the spin liquid phase from the unconventional Bose-Einstein condensation phase. The observed finite value of boson compressibility $(dM/dH)$ and lack of field-induced three-dimensional long-range order are consistent with the theoretical prediction.

10.1007/s11498-008-3013-8

Two types of anisotropic polymer systems were studied in the spherical approximation used in the classical theory of ferromagnetism. These were a three-dimensional system composed of weakly interacting layers with isotropic interactions between chain segments in the layer planes and thin quasi-two-dimensional polymer films possessing intra-and interchain interaction anisotropy, whose behavior is close to that of two-dimensional systems. Laws that govern a change in the temperature T cr of phase transition from the long-range order state to a disordered state depending on the magnitude of anisotropy and the size of the layers were established. For systems of the former type in which interlayer interactions is weakened, T cr tends to zero, being inversely proportional to lng, where g is the ratio of the interaction constant between the layers to that of inplane interaction in a layer. For systems of the latter type, the transition temperature T cr → 0 according to the T cr ∼ √ɛ law, where ɛ is the parameter that characterizes the anisotropy of intra-and interchain interactions. The number of layers required for the systems to be considered three-dimensional was estimated. Regardless of the type of boundary conditions for finite systems, the number of layers increases with enhancement of interaction anisotropy (a decrease in g and ɛ) and an increase in the number of chains in the layers, especially for systems of the former type. Transverse orientational correlations of chain segments with respect to the arrangement of the layers decrease according to a power law, as in the case of infinite two-dimensional systems. There are fluctuations of three-dimensional long-range orientation order in the plane of the layers, the fluctuations are enhanced with an increase in the anisotropy of interactions in the system.

Antiferromagnetic ordering and structural phase transition inBa2Fe2As2with Sn incorporated from the growth flux

Antiferromagnetic ordering and structural phase transition have been investigated via comprehensive neutron and x-ray diffraction on Sn-flux-grown ${\text{BaFe}}_{2}{\text{As}}_{2}$ single crystals, the $A\text{\ensuremath{-}}122$ family of FeAs-based high-${T}_{C}$ superconductor compounds. The incorporation of Sn in the lattice resulted to an average composition of ${\text{Ba}}_{0.95}{\text{Sn}}_{0.05}{\text{Fe}}_{2}{\text{As}}_{2}$. A tetragonal-to-orthorhombic structural phase transition and a three-dimensional long-range antiferromagnetic ordering of the iron magnetic moment, with a unique magnetic propagation wave vector $\mathbf{k}=(1,0,1)$ have been found to take place at $\ensuremath{\sim}90\text{ }\text{K}$. The magnetic moments of iron are aligned along the longer $\mathbf{a}$ axis in the low-temperature orthorhombic phase ($Fmmm$ with $\mathbf{b}<\mathbf{a}<\mathbf{c}$). Our results thus demonstrate that the magnetic structure of the Sn-flux-grown ${\text{BaFe}}_{2}{\text{As}}_{2}$ single crystal is the same as those in the polycrystalline samples and in other $A\text{\ensuremath{-}}122$ iron pnictides compounds. We argue that the Sn incorporation in the lattice is responsible for a smaller orthorhombic splitting and lower N\'eel temperature ${T}_{N}$ observed in the experiments.

Heat capacities of a networked system of single-molecule magnet with three-dimensional structure

A magnetic-fields dependence of heat capacity of [Mn5(hmp)4(OH)2{N(CN)2}6]2MeCN·2THF (hmp=hydroxymethylpyridinate) is investigated by the thermal relaxation calorimetry technique. This compound is a three-dimensional system consisting of Mn4 single-molecule magnet (SMM) units and Mn2+ ions, which are linked by the dicyanamide ligands to form a coordination network structure. A sharp peak of C p being associated with the formation of three-dimensional long-range order is observed around 1.96 K. The thermodynamic discussion based on the magnetic entropy suggests that both SMMs and Mn2+ ions are involved in the formation of the anitiferromagnetic spin ordering. However, this long-range ordering is very sensitive to the external magnetic fields which work to change the magnitude of the Zeeman splitting of the SMM levels. The behavior under magnetic fields is similar to that of the two-dimensional Mn4-network system studied previously.

Transition from two-to three-dimensional behavior in anisotropic polymer layers

Two types of anisotropic polymer systems were studied in the spherical approximation used in the classical theory of ferromagnetism. These were a three-dimensional system composed of weakly interacting layers with isotropic interactions between chain segments in the layer planes and thin quasi-two-dimensional polymer films possessing intra-and interchain interaction anisotropy, whose behavior is close to that of two-dimensional systems. Laws that govern a change in the temperature Tcr of phase transition from the long-range order state to a disordered state depending on the magnitude of anisotropy and the size of the layers were established. For systems of the former type in which interlayer interactions is weakened, Tcr tends to zero, being inversely proportional to lng, where g is the ratio of the interaction constant between the layers to that of inplane interaction in a layer. For systems of the latter type, the transition temperature Tcr → 0 according to the Tcr ∼ √ɛ law, where ɛ is the parameter that characterizes the anisotropy of intra-and interchain interactions. The number of layers required for the systems to be considered three-dimensional was estimated. Regardless of the type of boundary conditions for finite systems, the number of layers increases with enhancement of interaction anisotropy (a decrease in g and ɛ) and an increase in the number of chains in the layers, especially for systems of the former type. Transverse orientational correlations of chain segments with respect to the arrangement of the layers decrease according to a power law, as in the case of infinite two-dimensional systems. There are fluctuations of three-dimensional long-range orientation order in the plane of the layers, the fluctuations are enhanced with an increase in the anisotropy of interactions in the system.

Subtle competition between ferromagnetic and antiferromagnetic order in a Mn(II)-free radical ferrimagnetic chain

The macroscopic magnetic characterization of the Mn(II) nitronyl nitroxide free radical chain $[\mathrm{Mn}{(\mathrm{hfac})}_{2}(R)\text{\ensuremath{-}}3\mathrm{MLNN}]$ evidenced its transition from a one-dimensional behavior of ferrimagnetic chains to a three-dimensional ferromagnetic long-range order below $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Neutron-diffraction experiments, performed on a single crystal around the transition temperature, led to a different conclusion: the magnetic Bragg reflections detected below $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ correspond to a canted antiferromagnet, where the magnetic moments are mainly oriented along the chain axis. Surprisingly, in the context of other compounds in this family of magnets, the interchain coupling is antiferromagnetic. This state is shown to be very fragile, since a ferromagnetic interchain arrangement is recovered in a weak magnetic field. This peculiar behavior might be explained by the competition between dipolar interaction, shown to be responsible for the antiferromagnetic long-range order below $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and exchange interaction, the balance between these interactions being driven by the strong intrachain spin correlations. More generally, this study underlines the need, in this kind of molecular compounds, to go beyond macroscopic magnetization measurements.

Magnetic order in theS=1∕2two-dimensional molecular antiferromagnet copper pyrazine perchlorateCu(Pz)2(ClO4)2

We present an investigation of magnetic ordering in the two-dimensional $S=1∕2$ quantum magnet $\mathrm{Cu}{(\mathrm{Pz})}_{2}{(\mathrm{Cl}{\mathrm{O}}_{4})}_{2}$ using specific heat and zero-field muon-spin relaxation $({\ensuremath{\mu}}^{+}\mathrm{SR})$. The magnetic contribution to the specific heat is consistent with an exchange strength of $17.7(3)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We find unambiguous evidence for a transition to a state of three-dimensional long-range order below a critical temperature ${T}_{N}=4.21(1)\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ using ${\ensuremath{\mu}}^{+}\mathrm{SR}$ even though there is no feature in the specific heat at that temperature. The absence of a specific heat anomaly at ${T}_{N}$ is consistent with recent theoretical predictions. The ratio of ${T}_{N}∕J=0.24$ corresponds to a ratio of intralayer to interlayer exchange constants of $\ensuremath{\mid}{J}^{\ensuremath{'}}∕J\ensuremath{\mid}=6.8\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}4}$, indicative of excellent two-dimensional isolation. The scaled magnetic specific heat of $[\mathrm{Cu}{(\mathrm{Pz})}_{2}(\mathrm{H}{\mathrm{F}}_{2})]\mathrm{B}{\mathrm{F}}_{4}$, a compound with an analogous structure, is very similar to that of $\mathrm{Cu}{(\mathrm{Pz})}_{2}{(\mathrm{Cl}{\mathrm{O}}_{4})}_{2}$ although both differ slightly from the predicted value for an ideal 2D $S=1∕2$ Heisenberg antiferromagnet.