Spin dynamics of the spin-1/2 kagome lattice antiferromagnet ZnCu3(OH)6Cl2

We have performed thermodynamic and neutron scattering measurements on the S=1/2 kagomé lattice antiferromagnet ZnCu3(OH)6Cl2. The susceptibility indicates a Curie-Weiss temperature of theta CW approximately = -300 K; however, no magnetic order is observed down to 50 mK. Inelastic neutron scattering reveals a spectrum of low energy spin excitations with no observable gap down to 0.1 meV. The specific heat at low-T follows a power law temperature dependence. These results suggest that an unusual spin liquid state with essentially gapless excitations is realized in this kagomé lattice system.     

The nature of the magnetism in quasi-2D layered α-Ni(OH)2

The two layered hexagonal hydroxides of Ni are β-Ni(OH)(2) and α-Ni(OH)(2); β-Ni(OH)(2) is now known to be an antiferromagnet whereas the nature of the magnetism in α-Ni(OH)(2) is not yet well established. Here, the magnetic properties of α-Ni(OH)(2) with lattice parameters a = 3.02 ? and c = 8.6 ?, and flower-like morphology with petal thickness of approximately equal to 50 ? are reported. Temperature (2-300 K) and magnetic field (up to 65 kOe) dependence of the magnetization and ac susceptibility at f = 0.1-1000 Hz were measured. Analysis of the data yields ferromagnetic ordering in the system with T(C) is approximately equal to 16 K. In addition, a nanosize related blocking temperature T(B) = 8 K and spin-glass-like ordering of the surface spins near 3.5 K are inferred from the ac frequency and dc magnetic field dependence of these transitions. Fitting to the high temperature series and quasi-2D nature of the system is used to determine J(1)/k(B) = 4.38 K (J(2)/k(B) = 0.14 K) for the intraplane (interplane) exchange coupling between the Ni(2+) ions.     

Critical spin dynamics of the 2D quantum Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2

We report a neutron scattering study of the long-wavelength dynamic spin correlations in the model two-dimensional S = 1/2 square lattice Heisenberg antiferromagnets Sr2CuO2Cl2 and Sr2Cu3O4Cl2. The characteristic energy scale, omega(0)(T/J), is determined by measuring the quasielastic peak width in the paramagnetic phase over a wide range of temperature ( 0.2 less similarT/J less similar0.7). The obtained values for omega(0)(T/J) agree quantitatively between the two compounds and also with values deduced from quantum Monte Carlo simulations. The combined data show scaling behavior, omega approximately xi(-z), over the entire temperature range with z = 1.0(1), in agreement with dynamic scaling theory.     

Magnetic ground state of an experimental S=1/2 kagome antiferromagnet

We present a detailed analysis of the heat capacity of a near-perfect S=1/2 kagome antiferromagnet, zinc paratacamite Zn(x)Cu(4-x)(OH)(6)Cl(2), as a function of stoichiometry x-->1 and for fields of up to 9 T. We obtain the heat capacity intrinsic to the kagome layers by accounting for the weak Cu2+/Zn2+ exchange between the Cu and the Zn sites, which was measured independently for x=1 using neutron diffraction. The evolution of the heat capacity for x=0.8...1 is then related to the hysteresis in the magnetic susceptibility. We conclude that for x>0.8 zinc paratacamite is a spin liquid without a spin gap, in which unpaired spins give rise to a macroscopically degenerate ground state manifold with increasingly glassy dynamics as x is lowered.     

(17) O NMR study of the intrinsic magnetic susceptibility and spin dynamics of the quantum kagome antiferromagnet ZnCu3(OH)(6)Cl(2)

We report, through 17O NMR, an unambiguous local determination of the intrinsic kagome lattice spin susceptibility as well as that created around nonmagnetic defects arising from natural Zn/Cu exchange in the S=1/2 (Cu2+) herbertsmithite ZnCu3(OH)6Cl2 compound. The issue of a singlet-triplet gap is addressed. The magnetic response around a defect is found to markedly differ from that observed in nonfrustrated antiferromagnets. Finally, we discuss our relaxation measurements in the light of Cu and Cl NMR data and suggest a flat q dependence of the excitations.     

Inhomogeneous level splitting in Pr 2-x BixRu2O7

We report that Bi doping drives Pr 2-x BixRu2O7 from an antiferromagnetic insulator (x = 0) to a metallic paramagnet (x approximately 1) with a broad low T maximum in C/T. Neutron scattering reveals local low energy spin excitations (variant Planck's omega approximately 1 meV) with a spectrum that is unaffected by heating to k(B)T > variant Planck's omega. We show that a continuous distribution of splittings of the non-Kramers Pr3+ ground-state doublet such as might result from various types of lattice strain can account for all the data.     

Spin gap in the zigzag spin-1/2 chain cuprate Sr(0.9)Ca(0.1)CuO(2)

We report a comparative study of (63)Cu nuclear magnetic resonance spin lattice relaxation rates T(1)(-1) on undoped SrCuO(2) and Ca-doped Sr(0.9)Ca(0.1)CuO(2) spin chain compounds. A temperature independent T(1)(-1) is observed for SrCuO(2) as expected for an S=1/2 Heisenberg chain. Surprisingly, we observe an exponential decrease of T(1)(-1) for T<90 K in the Ca-doped sample evidencing the opening of a spin gap. The data analysis within the J(1)-J(2) Heisenberg model employing density-matrix renormalization group calculations suggests an impurity driven small alternation of the J(2)-exchange coupling as a possible cause of the spin gap.     

Muon spin relaxation and susceptibility studies of the pure and diluted spin 1/2 kagomé-like lattice system (CuxZn(1-x))3V2O7(OH2) 2H2O

Muon spin relaxation and magnetic susceptibility measurements have been performed on the pure and diluted spin 1/2 kagomé system (CuxZn(1-x))3V2O7(OH)2 2H2O. In the pure x=1 system we found a slowing down of Cu spin fluctuations with decreasing temperature towards T approximately 1 K, followed by slow and nearly temperature-independent spin fluctuations persisting down to T=50 mK, indicative of quantum fluctuations. No indication of static spin freezing was detected in either of the pure (x=1.0) or diluted samples. The observed magnitude of fluctuating fields indicates that the slow spin fluctuations represent an intrinsic property of kagomé network rather than impurity spins.     

Low-temperature broken-symmetry phases of spiral antiferromagnets

We study Heisenberg antiferromagnets with nearest- (J1) and third- (J3) neighbor exchange on the square lattice. In the limit of spin S-->infinity, there is a zero temperature (T) Lifshitz point at J(3)=1/4J(1), with long-range spiral spin order at T=0 for J3>1/4J(1). We present classical Monte Carlo simulations and a theory for T>0 crossovers near the Lifshitz point: spin rotation symmetry is restored at any T>0, but there is a broken lattice reflection symmetry for 0< or =T相似文献   

Spin liquid state in an organic Mott insulator with a triangular lattice

1H NMR and static susceptibility measurements have been performed in an organic Mott insulator with a nearly isotropic triangular lattice, kappa-(BEDT-TTF)2Cu2(CN)(3), which is a model system of frustrated quantum spins. The static susceptibility is described by the spin S=1/2 antiferromagnetic triangular-lattice Heisenberg model with the exchange constant J approximately 250 K. Regardless of the large magnetic interactions, the 1H NMR spectra show no indication of long-range magnetic ordering down to 32 mK, which is 4 orders of magnitude smaller than J. These results suggest that a quantum spin liquid state is realized in the close proximity of the superconducting state appearing under pressure.     

Magnetic susceptibility of the Kagome antiferromagnet ZnCu3(OH)6Cl2

We analyze the experimental data for the magnetic susceptibility of the material ZnCu3(OH)6Cl2 in terms of the Kagome Lattice Heisenberg model (KLHM), discussing possible role of impurity spins, dilution, exchange anisotropy, and both out-of-plane and in-plane Dzyloshinski-Moriya (DM) anisotropies, with explicit theoretical calculations using the numerical linked cluster method and exact diagonalization. The high-temperature experimental data are well described by the pure Heisenberg model with J=170 K. We show that the sudden upturn in the susceptibility around T=75 K is due to DM interactions. We also observe that at intermediate temperatures, below T=J, our calculated susceptibility for KLHM fits well with a power law T(-0.25).     

Frustrated cuprate route from antiferromagnetic to ferromagnetic spin-1/2 Heisenberg chains: Li2ZrCuO4 as a missing link near the quantum critical point

From thermodynamics, local spin density approximation+Hubbard U studies and exact diagonalizations of a five-band Hubbard model on CuO2 stripes we find that Li2ZrCuO4 (Li2CuZrO4 in traditional notation) is close to a ferromagnetic critical point. Analyzing its susceptibility chi(T) and specific heat cp(T,H) within a Heisenberg model, we show that the ratio of the 2nd to the 1st neighbor exchange integrals alpha=-J2/J1 approximately 0.3 is close to the critical value alphac=1/4. Comparing with related chain cuprates we explain the rather strong field dependence of cp, the monotonic downshift of the peak of chi(T), and its increase for alpha-->alphac+0.     

Bose-Einstein condensation of S = 1 nickel spin degrees of freedom in NiCl2-4SC(NH2)2

It has recently been suggested that the organic compound NiCl2-4SC(NH2)2 (DTN) undergoes field-induced Bose-Einstein condensation (BEC) of the Ni spin degrees of freedom. The Ni S = 1 spins exhibit three-dimensional XY antiferromagnetism above a critical field H(c1) approximately 2 T. The spin fluid can be described as a gas of hard-core bosons where the field-induced antiferromagnetic transition corresponds to Bose-Einstein condensation. We have determined the spin Hamiltonian of DTN using inelastic neutron diffraction measurements, and we have studied the high-field phase diagram by means of specific heat and magnetocaloric effect measurements. Our results show that the field-temperature phase boundary approaches a power-law H - H(c1) proportional variant T(alpha)(c) near the quantum critical point, with an exponent that is consistent with the 3D BEC universal value of alpha = 1.5.     

Quest for frustration driven distortion in Y2(Mo2)O(7)

We investigated the nature of the freezing in the geometrically frustrated Heisenberg spin glass Y2(Mo2)O(7) by measuring the temperature dependence of the static internal magnetic field distribution above the spin-glass temperature, Tg, using the muon spin relaxation technique. The evolution of the field distribution cannot be explained by changes in the spin susceptibility alone and suggests a lattice deformation. This possibility is addressed by numerical simulations of the Heisenberg Hamiltonian with magnetoelastic coupling at T > 0.     

Nuclear spin resonance of (129)Xe doped with O(2)

Spin-lattice relaxation of (129)Xe nuclei in solid natural xenon has been investigated in detail over a large range of paramagnetic O(2) impurity concentrations. Direct measurements of the ground state magnetic properties of the O(2) are difficult because the ESR (electron spin resonance) lines of O(2) are rather unstructured, but NMR measurements in the liquid helium temperature region (1.4-4 K) are very sensitive to the effective magnetic moments associated with the spin 1 Zeeman levels of the O(2) molecules and to the O(2) magnetic relaxation. From these measurements, the value of the D[Sz(2)-(1/3)S(2)] spin-Hamiltonian term of the triplet spin ground state of O(2) can be determined. The temperature and magnetic field dependence of the measured paramagnetic O(2)-induced excess line width of the (129)Xe NMR signal agree well with the theoretical model with the spin-Hamiltonian D=0.19 meV (2.3 K), and with the reasonable assumption that the E[S(x)(2)-S(y)(2)] spin-Hamiltonian term is close to 0 meV. An anomalous temperature dependence between 1.4 K and 4.2K of the (129)Xe spin-lattice relaxation rate, T(1n)(-1)(T), is also accounted for by our model. Using an independent determination of the true O(2) concentration in the Xe-O(2) solid, the effective spin lattice relaxation time (which will be seen to be transition dependent) of the O(2) at 2.3 K and 0.96 T is determined to be approximately 1.4 x 10(-8)s. The experimental results, taken together with the relaxation model, suggest routes for bringing highly spin-polarized (129)Xe from the low temperature condensed phase to higher temperatures without excessive depolarization.     

Dzyaloshinsky-Moriya anisotropy in the spin-1/2 kagome compound ZnCu3(OH)6Cl2

We report the determination of the Dzyaloshinsky-Moriya interaction, the dominant magnetic anisotropy term in the kagome spin-1/2 compound ZnCu3(OH)6Cl2. Based on the analysis of the high-temperature electron spin resonance (ESR) spectra, we find its main component |Dz|=15(1) K to be perpendicular to the kagome planes. Through the temperature dependent ESR linewidth, we observe a building up of nearest-neighbor spin-spin correlations below approximately 150 K.     

Exchange bias in bulk layered hydroxylammonium fluorocobaltate (NH?OH)?CoF?

The magnetic properties of layered hydroxylammonium fluorocobaltate (NH(3)OH)(2)CoF(4) were investigated by measuring its dc magnetic susceptibility in zero-field-cooled (ZFC) and field-cooled (FC) regimes, its frequency dependent ac susceptibility, its isothermal magnetization curves after ZFC and FC regimes, and its heat capacity. Effects of pressure and magnetic field on magnetic phase transitions were studied by susceptibility and heat capacity measurements, respectively. The system undergoes a magnetic phase transition from a paramagnetic state to a canted antiferromagnetic state exhibiting a weak ferromagnetic behavior at T(C) = 46.5 K and an antiferromagnetic transition at T(N) = 2.9 K. The most spectacular manifestation of the complex magnetic behavior in this system is a shift of the isothermal magnetization hysteresis loop in a temperature range below 20 K after the FC regime-an exchange bias phenomenon. We investigated the exchange bias as a function of the magnetic field during cooling and as a function of temperature. The observed exchange bias was attributed to the large exchange anisotropy which exists due to the quasi-2D structure of the layered (NH(3)OH)(2)CoF(4) material.     

Spin dynamics of the S = 5/2 2D triangular antiferromagnet Ba3NbFe3Si2O14

We report pulse-field magnetization, ac susceptibility, and 100 GHz electron spin resonance (ESR) measurements on the S = 5/2 two-dimensional triangular compound Ba3NbFe3Si2O14 with the Néel temperature T(N) = 26 K. The magnetization curve shows an almost linear increase up to 60 T with no indication of a one-third magnetization plateau. An unusually large frequency dependence of the ac susceptibility in the temperature range of T = 20-100 K reveals a spin-glass behavior or superparamagnetism, signaling the presence of frustration-related slow magnetic fluctuations. The temperature dependence of the ESR linewidth exhibits two distinct critical regimes; (i) ΔH(pp)(T) is proportional to (T-T(N))(-p) with the exponent p = 0.2(1)-0.2(3) for temperatures above 27 K, and (ii) ΔH(pp)(T) is proportional to (T-T*)(-p) with T* = 12 K and p = 0.8(1)-0.8(4) for temperatures between 12 and 27 K. This is interpreted as indicating a dimensional crossover of magnetic interactions and the persistence of short-range correlations with a helically ordered state.     

Electron spin resonance in the spin-1/2 quasi-one-dimensional antiferromagnet with Dzyaloshinskii-Moriya interaction BaCu2Ge2O7

We have investigated the electron spin resonance (ESR) on single crystals of BaCu2Ge2O7 at temperatures between 300 and 2 K and in a large frequency band, 9.6-134 GHz, in order to test the predictions of a recent theory, proposed by Oshikawa and Affleck (OA) [Phys. Rev. Lett. 82, 5136 (1999)]], which describes the ESR in a spin-1/2 Heisenberg chain with the Dzyaloshinskii-Moriya interaction. We find, in particular, that the ESR linewidth, Delta H, displays a rich temperature behavior. As the temperature decreases from T(max)/2 approximately 170 to 50 K, Delta H shows a rapid and linear decrease, Delta H approximately T. At low temperatures, below 50 K, Delta H acquires a strong dependence on the magnetic field orientation and for H axially c it shows a (h/T)(2) behavior which is due to an induced staggered field h, according to OA's prediction.     

Doping dependent evolution of magnetism and superconductivity in Eu(1-x)K(x)Fe2As2 (x = 0-1) and temperature dependence of the lower critical field H(c1)

We have synthesized polycrystalline samples of Eu(1-x)K(x)Fe2As2 (x = 0-1) and carried out systematic characterization using x-ray diffraction, ac and dc magnetic susceptibility, and electrical resistivity measurements. A clear signature of the coexistence of a superconducting transition (T(c) = 5.5 K) with spin density wave (SDW) ordering is observed in our underdoped sample with x = 0.15. The SDW transition disappears completely for the x = 0.3 sample and superconductivity arises below 20 K. The superconducting transition temperature Tc increases with increase in the K content and a maximum Tc = 33 K is reached for x = 0.5, beyond which it decreases again. The doping dependent Tx phase diagram is extracted from the magnetic and electrical transport data. It is found that magnetic ordering of Eu moments coexists with the superconductivity up to x = 0.6. The isothermal magnetization data taken at 2 K for the doped samples suggest the 2+ valence state of the Eu ions. We also present the temperature dependence of the lower critical field H(c1) of the superconducting polycrystalline samples. The values of H(c1)(0) obtained for x = 0.3, 0.5, and 0.7 after taking the demagnetization factor into account are 202, 330, and 212 Oe, respectively. The London penetration depth λ(T) calculated from the lower critical field does not show exponential dependence at low temperature, as would be expected for a fully gapped clean s-wave superconductor. In contrast, it shows a T2 power law feature up to T = 0.3Tc, as observed in Ba(1-x)K(x)Fe2As2 and BaFe(2-x)Co(x)As2.