Two-dimensional Spin Correlations Research Articles

Crystal structure and physical properties of layered Na[formula omitted]Fe[formula omitted]S[formula omitted

Layered iron oxysulfide Na2Fe2S2O serves as a cathode material for Na-ion batteries as well as a possible parent compound of novel superconductors. Here we report the crystal structure and physical properties on the basis of our successful synthesis of the high-purity sample. Na2Fe2S2O crystallizes in an anti-K2NiF4-type structure, which gives rise to the characteristic [Fe2S2O]2− block layers. The physical property measurements indicate that it is an antiferromagnetic insulator with a Néel temperature of TN=91.5 K and an energy gap no less than 0.43 eV. Evidence of two-dimensional short-range spin correlations above TN is given by the magnetic and specific-heat measurements. Our structural correlation analysis suggests that the Fe–S bond distance dominantly determines TN.

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.

Magnetic anisotropy and spin-polarized two-dimensional electron gas in the van der Waals ferromagnet Cr2Ge2Te6

We report a comprehensive experimental investigation on the magnetic anisotropy in bulk single crystals of Cr$_2$Ge$_2$Te$_6$, a quasi-two-dimensional ferromagnet belonging to the family of magnetic layered transition metal trichalcogenides that have attracted recently a big deal of interest with regard to the fundamental and applied aspects of two-dimensional magnetism. For this purpose electron spin resonance (ESR) and ferromagnetic resonance (FMR) measurements have been carried out over a wide frequency and temperature range. A gradual change in the angular dependence of the ESR linewidth at temperatures above the ferromagnetic transition temperature $T_{\rm c}$ reveals the development of two-dimensional spin correlations in the vicinity of $T_{\rm c}$ thereby proving the intrinsically low-dimensional character of spin dynamics in Cr$_2$Ge$_2$Te$_6$. Angular and frequency dependent measurements in the ferromagnetic phase clearly show an easy-axis type anisotropy of this compound. Furthermore, these experiments are compared with simulations based on a phenomenological approach, which takes into account results of static magnetization measurements as well as high temperature $g$ factors obtained from ESR spectroscopy in the paramagnetic phase. As a result the determined magnetocrystalline anisotropy energy density (MAE) $K_U$ is $(0.48 \pm 0.02) \times10^6$ erg/cm$^3$. This analysis is complemented by density functional calculations which yield the experimental MAE value for a particular value of the electronic correlation strength $U$. The analysis of the electronic structure reveals that the low-lying conduction band carries almost completely spin-polarized, quasi-homogeneous, two-dimensional states.

Bond randomness induced magnon decoherence in a spin-12ladder compound

We have used a combination of neutron resonant spin-echo and triple-axis spectroscopies to determine the energy and linewidth of the magnon resonance in IPA-Cu(Cl${}_{0.95}$Br${}_{0.05}$)${}_{3}$, a model spin-1/2 ladder antiferromagnet where Br substitution induces bond randomness. We find that the bond defects induce a blue shift, $\ensuremath{\delta}\ensuremath{\Delta}$, and broadening, $\ensuremath{\delta}\ensuremath{\Gamma}$, of the magnon gap excitation compared to the pure compound. At temperatures exceeding the energy scale of the interladder exchange interactions, $\ensuremath{\delta}\ensuremath{\Delta}$ and $\ensuremath{\delta}\ensuremath{\Gamma}$ are temperature independent within the experimental error, in agreement with Matthiessen's rule according to which magnon-defect scattering yields a temperature independent contribution to the magnon mean free path. Upon cooling, $\ensuremath{\delta}\ensuremath{\Delta}$ and $\ensuremath{\delta}\ensuremath{\Gamma}$ become temperature dependent and saturate at values lower than those observed at higher temperature, consistent with the crossover from one-dimensional to two-dimensional spin correlations with decreasing temperature previously observed in pure IPA-CuCl${}_{3}$. These results indicate limitations in the applicability of Matthiessen's rule for magnon scattering in low-dimensional magnets.

First-principles investigations on the magnetic structure of α-NaMnO2

α-NaMnO2 contains layers of high spin Mn3+ (d4) ions parallel to the ab plane, and Mn3+ layers alternate with Na+ ions layers along the c direction. To explore the interesting two-dimensional spin correlations and one-dimensional magnetic excitations, we carefully studied the electronic and magnetic properties of α-NaMnO2 via density functional theory calculations. By evaluating the intra-layer interactions J1–J4 as well as the inter-layer exchanges J5 and J6 through performing mapping analysis, we found that α-NaMnO2 displays typical two-dimensional spin correlations, which are dominated by the intra-chain interaction J1 and the inter-chain interaction J3 (J3/J1 ≈ 0.21). Interestingly, both the strong spin exchange interactions J1 and J3 come from supersuperexchange (SSE), other than superexchange (SE). Also, due to the topology of the lattices, the magnetic excitation is dominated by the intra-chain interaction J1, leading to the one-dimensional model.

Tuning the Magnetic Dimensionality by Charge Ordering in the Molecular TMTTF Salts

We theoretically investigate the interplay between charge ordering and magnetic states in quasi-one-dimensional molecular conductors TMTTF(2)X, motivated by the observation of a complex variation of competing and/or coexisting phases. We show that the ferroelectric-type charge order increases two-dimensional antiferromagnetic spin correlation, whereas in the one-dimensional regime two different spin-Peierls states are stabilized. By using first-principles band calculations for the estimation for the transfer integrals and comparing our results with the experiments, we identify the controlling parameters in the experimental phase diagram to be not only the interchain transfer integrals but also the amplitude of the charge order.

Synthesis, structure, and magnetic properties of the layered iron oxychalcogenide Na2Fe2Se2O

A new layered iron-oxychalcogenide Na2Fe2OSe2 has been synthesized and structurally characterized by powder X-ray diffraction. The structure is formed by alternate stacking of the newly discovered [Fe2OSe2] blocks and double layers of Na. Conductivity study shows that Na2Fe2OSe2 is a semiconductor with activation energy of 0.26 eV. Magnetic susceptibility and heat capacity measurements reveal an antiferromagnetic phase transition occurs at TN=73 K. A broad maximum of magnetic susceptibility and a slow decay of the specific heat above TN, arises as a result of two-dimensional short-range spin correlation.

Low-temperature 31P NMR study of the two-dimensional frustrated square lattice compound BaCdVO(PO4)2

We report the 31P NMR spectrum and spin-lattice relaxation rate 1/T1 as a function of temperature in the two-dimensional frustrated square lattice compound BaCdVO(PO4)2. The temperature dependence of 1/T1 indicates the existence of antiferromagnetic ordering at TN ∼ 1.05 K which is also evidenced by the broadening of the NMR spectrum below that temperature. The temperature dependences of 1/T1 and 1/T1Tχ just above TN demonstrate the importance of two-dimensional antiferromagnetic spin correlations for critical fluctuations.

One- and two-dimensional spin correlation of complex fluids and the relation to fluid composition

Many natural fluids, including crude oils, are complex mixtures of molecules with a broad range of sizes and chemical properties. In such systems, the molecular dynamics is complex and the relaxation and diffusion behavior cannot be described by a single relaxation time or diffusion coefficient. Nevertheless, low field NMR relaxation and diffusion measurements have become powerful tools for the study of such fluids. The complexity can be overcome by analyzing the measurements in terms of one- and two-dimensional distribution functions. The one-dimensional relaxation and diffusion distribution functions are directly related to the composition of the fluids. We also show that from the analysis of the shape of the relaxation and diffusion decay, it is possible to predict the temperature dependence of viscosity. Two-dimensional distribution functions probe further aspects of the molecular dynamics. The correlation between T1 and T2 relaxation behavior can be used to detect the presence of slow motion and to probe the aggregation of molecules into supermolecular structures. Diffusion–relaxation distribution functions give information about the chemical composition of the fluid. Beaucoup de fluides naturels, en particulier les huiles lourdes, sont des mélanges complexes de molécules possédant une large distribution de tailles et de propriétés chimiques. Dans ces systèmes, la dynamique des molécules est complexe et le comportement de relaxation et de diffusion ne peut pas se décrire par un simple temps de relaxation ou coefficient de diffusion. Néanmoins, la relaxation RMN à bas champ et les mesures de diffusion sont devenus des outils puissants pour étudier de tels fluides. On peut contourner la complexité en analysant les mesures en termes de fonctions de distributions à une et deux dimensions. Les fonctions de distribution de relaxation et de diffusion à une dimension sont directement reliées à la composition des fluides. On montre également que l'analyse de la forme des déclins de relaxation et de diffusion permet de prédire la dépendance en température de la viscosité. Les fonctions de distribution à deux dimensions permettent de suivre d'autres aspects de la dynamique moléculaire. Les corrélations entre les comportements de relaxation T1 et T2 peuvent être utilisés pour détecter des mouvements lents et mettre en évidence l'agrégation des molécules en structures supramoléculaires. Les corrélations entre les fonctions de distribution de diffusion et de relaxation donnent enfin des informations sur la composition chimique des fluides étudiés.

Magnetic properties of the quasi-two-dimensionalS=12Heisenberg antiferromagnet[Cu(pyz)2(HF2)]PF6

We report on high-field magnetization, specific-heat and electron spin resonance (ESR) studies of the quasi-two-dimensional spin-1/2 Heisenberg antiferromagnet [Cu(pyz)2(HF2)]PF6. The frequency-field diagram of ESR modes below TN = 4.38 K is described in the frame of the meanfield theory, confirming a collinear magnetic structure with an easy-plane anisotropy. The obtained results allowed us to determine the anisotropy/exchange interaction ratio, A/J = 0.003, and the upper limit for the inter/intra-plane exchange-interaction ratio, J'/J = 1/16. It is argued that despite the onset of 3D long-range magnetic ordering the magnetic properties of this material (including high-magnetic-field magnetization and non-monotonic field dependence of the Neel temperature) are strongly affected by two-dimensional spin correlations.

Spin-singlet superconductivity and antiferromagnetic correlations for the field-aligned powder of the triangular lattice Na xCoO 2 · yH 2O

Nuclear magnetic resonance and relaxation measurements of the 59Co nuclei for the well-defined field-aligned powder of the triangular lattice superconductor Na x CoO 2 · yH 2O with a nearly optimal composition for the transition temperature T c have been performed. Detailed analyses indicate that the Knight shifts for the directions parallel and perpendicular to the CoO 2 plane and the spin-lattice relaxation rates, taken by making an inevitable deterioration of specimen minimal, are significantly smaller than most of the data reported to date, and that this compound is classified into one of the unconventional spin-singlet superconductors as suggested by recent works for single crystals with a lower T c. A small enhancement of two-dimensional antiferromagnetic spin correlations at some nonzero wave vector may emerge near T c.

Magnetoelastic Coupling and Symmetry Breaking in the Frustrated Antiferromagnetα−NaMnO2

The magnetic and crystal structures of the alpha-NaMnO2 have been determined by high-resolution neutron powder diffraction. The system maps out a frustrated triangular spin lattice with anisotropic interactions that displays two-dimensional spin correlations below 200 K. Magnetic frustration is lifted through magneto-elastic coupling, evidenced by strong anisotropic broadening of the diffraction profiles at high temperature and ultimately by a structural phase transition at 45 K. In this low-temperature regime a three-dimensional antiferromagnetic state is observed with a propagation vector k=(1/2,1/2,0).

Unconventional spin order in the triangular lattice system NaCrO 2: A neutron scattering study

We report high resolution neutron scattering measurements on the rhombohedrally stacked triangular antiferromagnet NaCrO 2 which has recently been shown to exhibit an unusually broad fluctuating cross-over regime extending far below the onset of spin freezing at T c . Our results show that at T c purely two-dimensional quasi-static spin correlations of the 120° type exist. Below some cross-over temperature ( T ∼ 0.75 T c ) a small incommensuration develops which helps resolve the inter-layer spin frustration and drives short-range three-dimensional magnetic order. This incommensuration assisted dimensional cross-over suggests that inter-layer frustration is responsible for stabilizing the rare 2D correlated phase above 0.75 T c .

Spin correlations in the electron-doped high-transition-temperature superconductor Nd2-xCexCuO4±δ

High-transition-temperature (high-T(c)) superconductivity develops near antiferromagnetic phases, and it is possible that magnetic excitations contribute to the superconducting pairing mechanism. To assess the role of antiferromagnetism, it is essential to understand the doping and temperature dependence of the two-dimensional antiferromagnetic spin correlations. The phase diagram is asymmetric with respect to electron and hole doping, and for the comparatively less-studied electron-doped materials, the antiferromagnetic phase extends much further with doping and appears to overlap with the superconducting phase. The archetypal electron-doped compound Nd2-xCexCuO4+/-delta (NCCO) shows bulk superconductivity above x approximately 0.13 (refs 3, 4), while evidence for antiferromagnetic order has been found up to x approximately 0.17 (refs 2, 5, 6). Here we report inelastic magnetic neutron-scattering measurements that point to the distinct possibility that genuine long-range antiferromagnetism and superconductivity do not coexist. The data reveal a magnetic quantum critical point where superconductivity first appears, consistent with an exotic quantum phase transition between the two phases. We also demonstrate that the pseudogap phenomenon in the electron-doped materials, which is associated with pronounced charge anomalies, arises from a build-up of spin correlations, in agreement with recent theoretical proposals.

Structural and magnetic properties of the single-layer manganese oxideLa1−xSr1+xMnO4

Using x-ray and neutron scattering, we have studied the structural and magnetic properties of the single-layer manganite La{1-x}Sr{1+x}MnO4 (0 < x < 0.7). Single crystals were grown by the traveling-solvent floating-zone method at 18 La/Sr concentrations. The low-temperature phase diagram can be understood by considering the strong coupling of the magnetic and orbital degrees of freedom, and it can be divided into three distinct regions: low (x < 0.12), intermediate (0.12< x < 0.45), and high (x > 0.45) doping. LaSrMnO_4 (x=0) is an antiferromagnetic Mott insulator,and its spin-wave spectrum is well-described by linear spin-wave theory for the spin-2 square-lattice Heisenberg Hamiltonian with Ising anisotropy. Upon doping, as the e_g electron concentration (1-x) decreases, both the two-dimensional antiferromagnetic spin correlations in the paramagnetic phase and the low-temperature ordered moment decrease due to an increase of frustrating interactions, and Neel order disappears above xc = 0.115(10). In the intermediate region, there exists neither long-range magnetic nor superstructural order. Short-range-correlated structural "nanopatches" begin to form above x ~ 0.25. At high doping (x > 0.45), the ground state of La{1-x}Sr{1+x}MnO4 exhibits long-range superstructural order and a complex (CE-type) antiferromagnetic order which differs from that at low doping. The superstructural order is thought to arise from charge and orbital ordering on the Mn sites. For x > 0.50, the superstructural order becomes incommensurate with the lattice, with a modulation wavevector that depends linearly on the e_g electron concentration. On the other hand, the magnetic order remains commensurate, but loses its long-range coherence upon doping beyond x = 0.50.

Unconventional superconductivity and electron correlations in the cobalt oxyhydrate Na0.35CoO2.yH2O from nuclear quadrupole resonance.

We report a careful 59Co nuclear quadrupolar resonance measurement on the recently discovered cobalt oxyhydrate Na0.35CoO2.yH(2)O superconductor from T=40 K down to 0.2 K. We find that in the normal state the spin-lattice relaxation rate 1/T(1) follows a Curie-Weiss type temperature (T) variation, 1/T(1)T=C/(T-theta), with theta=-42 K, suggesting two-dimensional antiferromagnetic spin correlations. Below T(c)=3.9 K, 1/T(1) decreases with no coherence peak and follows a T(n) dependence with n approximately 2.2 down to approximately 2.0 K but crosses over to a 1/T(1) proportional to T variation below T=1.4 K, which suggests non-s-wave superconductivity. The data in the superconducting state are most consistent with the existence of line nodes in the gap function.

Neutron scattering, magnetometry, and quantum Monte Carlo study of the randomly diluted spin-1/2 square-lattice Heisenberg antiferromagnet

We have successfully grown sizable single crystals of La 2Cu 1− z (Zn,Mg) z O 4 with up to nearly half of the magnetic Cu sites replaced by non-magnetic Zn and Mg. Neutron scattering, SQUID magnetometry, and complementary quantum Monte Carlo (QMC) simulations demonstrate that this material is an excellent model system for the study of site percolation of the square-lattice Heisenberg antiferromagnet (SLHAF) in the quantum-spin limit S=1/2. Carefully oxygen-reduced samples exhibit Néel order up to the percolation threshold for site dilution, z p≈40.7%. For z>10%, the material exhibits a low-temperature tetragonal (LTT) structural phase, with a transition temperature that increases linearly with doping. Above z≈25%, Néel order occurs in the LTT phase. Up to at least z=35%, the Néel temperature T N( z) of the experimental system corresponds to the temperature at which QMC indicates that the spin correlations for the nearest-neighbor S=1/2 SLHAF have grown to approximately 100 lattice constants. Neutron scattering measurements of the static structure factor in the paramagnetic regime allow the determination of the two-dimensional spin correlations, which are found to be in excellent quantitative agreement with QMC over a wide common temperature and doping range. Neutron scattering and QMC results for the temperature dependence of the static structure factor amplitude S(π,π) are in good agreement as well. As the concentration of non-magnetic sites is increased, the magnetic correlation length ξ( T, z) crosses over from an exponential dependence on ρ s/ T to power-law behavior in the temperature regime studied. Fits to a heuristic cross-over form for ξ( T, z) allow an estimate of the spin stiffness, ρ s= ρ s( z), which approaches zero at z= z p. The combined experimental and numerical data presented here provide valuable quantitative information for tests of theories of the randomly diluted S=1/2 SLHAF.

Raman Scattering of the Charge-Spin Stripes inLa1.67Sr0.33NiO4

Raman scattering spectra have been investigated for a single crystal of ${\mathrm{La}}_{1.67}{\mathrm{Sr}}_{0.33}{\mathrm{NiO}}_{4}$ which shows stripe ordering of charge ( ${T}_{\mathrm{c}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}230\mathrm{K}$) and spin ( ${T}_{\mathrm{s}}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}180\mathrm{K}$). Upon the charge ordering, the activated phonon peaks grow with the evolution of the charge modulation amplitude, while the electronic Raman continuum of the metallic state abruptly disappears. The two broad peaks with the ${B}_{1g}$ symmetry arising from the two-magnon excitations emerge immediately below ${T}_{\mathrm{c}}$, indicating the evolution of the two-dimensional antiferromagnetic spin correlation in the charge-ordered state even above ${T}_{\mathrm{s}}$.

Stripe correlations of spins and holes in copper oxide superconductors

Abstract Neutron scattering has played a crucial role in elucidating the phase diagram of the prototypical high-tem-perature superconductor La2-xSrxCuO4 (see Figure 1). The commensurate antiferromagnetic order that is present in the undoped compound is destroyed by a hole concentration (given by x) of 0.02 [l]. At higher hole concentrations (x > 0.05), where high-temperature superconductivity occurs, dynamical two-dimensional spin correlations characterized by an incommensurate wave vector (see Figure 2a) have been detected [2–4]. Further studies have clarified the momentum, energy and temperature dependencies of these excitations [5–8]; nevertheless, their underlying nature and the cause of the incommensurability have not been firmly established.

Magnetism and magnetic fluctuations in La 1 − xSr xCuO 4 for x = 0 (2D antiferromagnet), 0.04 (3D spin glass) and x = 0.15 (superconductor)

We review recent studies of the static and dynamic magnetic fluctuations in La 2 − x Sr x CuO 4. In La 2 CuO 4(T N = 325K) the instantaneous two-dimensional (2D) spin correlations have been studied over the temperature range 340 K < T ≤ 820 K. We find quantitative agreement for the correlation length over the complete temperature range with no adjustable parameters with predictions for the 2D quantum non-linear sigma model in the renormalized classical regime. We have carried out bulk magnetization measurements in La 1.96Sr 0.04CuO 4 which has neither antiferromagnetic long range order nor superconductivity. All of the features that characterize a canonical spin glass transition have been seen: irreversibility, remnant magnetization and scaling behavior. Finally, neutron inelastic scattering experiments have been performed on homogeneous single crystals of La 1.85Sr 0.15CuO 4 with T c = 37.3K (at onset), higher than any previously studied single crystals. The temperature dependence of the low energy incommensurate peak intensity at ( π(1 ± δ), π) and ( π, π(1 ± δ)) exhibits a pronounced maximum near 3T c . In contrast to the results reported on lower T c crystals, the intensity for energies below 3.5 meV dramatically decreases as the temperature decreases below T c , vanishing into the background below ~ 15K. The behavior is consistent with predictions based on a d x 2 − y 2 superconducting order parameter.