From quantum disorder to magnetic order in an s=1/2 kagome lattice: a structural and magnetic study of herbertsmithite at high pressure

The structural and magnetic properties of deuterated herbertsmithite have been studied by means of neutron powder diffraction and magnetic susceptibility measurements in a wide range of temperatures and pressures. The experimental data demonstrate that a phase transition from the quantum-disordered spin-liquid phase to the long-range ordered antiferromagnetic phase with the Néel temperature T(N)=6 K is induced at P=2.5 GPa. The observed decrease of T(N) upon compression correlates with the anomalies in pressure behavior of Cu-O bond length and Cu-O-Cu bond angles. The reasons for the observed spin-freezing transition are discussed within the framework of the available theoretical models and the recent observation of the field-induced spin freezing.     

Density-of-state oscillation of quasiparticle excitation in the spin density wave phase of (TMTSF)2ClO4

Systematic measurements of the magnetocaloric effect, heat capacity, and magnetic torque under a high magnetic field up to 35 T are performed in the spin density wave (SDW) phase of a quasi-one-dimensional organic conductor (TMTSF)2ClO4. In the SDW phase above 26 T, where the quantum Hall effect is broken, rapid oscillations (ROs) in these thermodynamic quantities are observed, which provides clear evidence of the density-of-state (DOS) oscillation near the Fermi level. The resistance is semiconducting and the heat capacity divided by temperature is extrapolated to zero at 0 K in the SDW phase, showing that all the energy bands are gapped, and there is no DOS at the Fermi level. The results show that the ROs are ascribed to the DOS oscillation of the quasiparticle excitation.     

Observation of multiferroic properties in pyroxene NaFeGe2O6

We report the observation of multiferroicity in a clinopyroxene NaFeGe(2)O(6) polycrystal from the investigation of its electrical and magnetic properties. Following the previously known first magnetic transition at T(N1) = 13 K, a second magnetic transition appears at T(N2) = 11.8 K in the temperature dependence of the magnetization. A ferroelectric polarization starts to develop clearly at T(N2) rather than T(N1) and its magnitude increases up to ~13 μC m(-2) at 5 K, supporting the idea that the ferroelectric state in NaFeGe(2)O(6) stems from a helical spin order stabilized below T(N2). When a magnetic field of 90 kOe is applied, the electric polarization decreases to 9 μC m(-2) and T(N2) slightly increases by 0.5 K. At intermediate magnetic fields, around 28 and 78 kOe, anomalies in the magnetoelectric current, magnetoelectric susceptibility, and field derivative of magnetization curves are found, indicating field-induced spin-state transitions. Based on these electrical and magnetic properties, we provide a detailed low temperature phase diagram up to 90 kOe, and discuss the nature of each phase of NaFeGe(2)O(6).     

Antiferroquadrupolar ordering in a Pr-based superconductor PrIr(2)Zn(20)

An antiferroquadrupolar ordering at T(Q)=0.11 K has been found in a Pr-based superconductor PrIr(2)Zn(20). The measurements of specific heat and magnetization revealed the non-Kramers Γ(3) doublet ground state with the quadrupolar degrees of freedom. The specific heat exhibits a sharp peak at T(Q)=0.11 K. The increment of T(Q) in magnetic fields and the anisotropic B-T phase diagram are consistent with the antiferroquadrupolar ordered state below T(Q). The entropy release at T(Q) is only 20% of Rln2, suggesting that the quadrupolar fluctuations play a role in the formation of the superconducting pairs below T(c)=0.05 K.     

Probing spin correlations with phonons in the strongly frustrated magnet ZnCr2O4

The spin-lattice coupling plays an important role in strongly frustrated magnets. In ZnCr2O4, an excellent realization of the Heisenberg antiferromagnet on the pyrochlore network, a lattice distortion relieves the geometrical frustration through a spin-Peierls-like phase transition at T(c)=12.5 K. Conversely, spin correlations strongly influence the elastic properties of a frustrated magnet. By using infrared spectroscopy and published data on magnetic specific heat, we demonstrate that the frequency of an optical phonon triplet in ZnCr2O4 tracks the nearest-neighbor spin correlations above T(c). The splitting of the phonon triplet below T(c) provides a way to measure the spin-Peierls order parameter.     

Neutron scattering study of the field-dependent ground state and the spin dynamics in spin-one-half NH4CuCl3

Elastic and inelastic neutron scattering experiments have been performed on the dimer spin system NH4CuCl3, which shows plateaus in the magnetization curve at m=1/4 and m=3/4 of the saturation value. Two structural phase transitions at T1 approximately 156 K and at T(2)=70 K lead to a doubling of the crystallographic unit cell along the b direction and as a consequence a segregation into different dimer subsystems. Long-range magnetic ordering is reported below T(N)=1.3 K. The magnetic field dependence of the excitation spectrum identifies successive quantum phase transitions of the dimer subsystems as the driving mechanism for the unconventional magnetization process in agreement with a recent theoretical model.     

Commensurate spin density wave in LaFeAsO: a local probe study

We present a detailed study on the magnetic order in the undoped mother compound LaFeAsO of the recently discovered Fe-based superconductor LaFeAsO1-xFx. In particular, we present local probe measurements of the magnetic properties of LaFeAsO by means of 57Fe M?ssbauer spectroscopy and muon-spin relaxation in zero external field along with magnetization and resistivity studies. These experiments prove a commensurate static magnetic order with a strongly reduced ordered moment of 0.25(5)muB at the iron site below T(N)=138 K, well separated from a structural phase transition at T(S)=156 K. The temperature dependence of the sublattice magnetization is determined and compared to theory. Using a four-band spin density wave model both, the size of the order parameter and the quick saturation below T(N) are reproduced.     

Electron spin relaxation in pseudo-Jahn-Teller low-symmetry Cu(II) complexes in diaqua(L-aspartate)Zn(II).H(2)O crystals.

Low-temperature (4-55 K) pulsed EPR measurements were performed with the magnetic field directed along the z-axis of the g-factor of the low-symmetry octahedral complex [(63)Cu(L-aspartate)(2)(H2O)2] undergoing dynamic Jahn-Teller effect in diaqua(L-aspartate)Zn(II) hydrate single crystals. Spin-lattice relaxation time T(1) and phase memory time T(M) were determined by the electron spin echo (ESE) method. The relaxation rate 1/T(1) increases strongly over 5 decades in the temperature range 4-55 K. Various processes and mechanisms of T(1)-relaxation are discussed, and it is shown that the relaxation is governed mainly by Raman relaxation processes with the Debye temperature Theta(D)=204 K, with a detectable contribution from disorder in the doped Cu(2+) ions system below 12 K. An analytical approximation of the transport integral I(8) is given in temperature range T=0.025-10Theta(D) and applied for computer fitting procedures. Since the Jahn-Teller distorted configurations differ strongly in energy (delta(12)=240 cm(-1)), there is no influence of the classical vibronic dynamics mechanism on T(1). Dephasing of the ESE (phase relaxation) is governed by instantaneous diffusion and spectral diffusion below 20 K with resulting rigid lattice value 1/T(0)(M)=1.88 MHz. Above this temperature the relaxation rate 1/T(M) increases upon heating due to two mechanisms. The first is the phonon-controlled excitation to the first excited vibronic level of energy Delta=243 cm(-1), with subsequent tunneling to the neighbor potential well. This vibronic-type dynamics also produces a temperature-dependent broadening of lines in the ESEEM spectra. The second mechanism is produced by the spin-lattice relaxation. The increase in T(M) is described in terms of the spin packets forming inhomogeneously broadened EPR lines.     

Magnetism and superconductivity in Eu0.2Sr0.8(Fe0.86Co0.14)2As2 probed by 75As NMR

We report bulk superconductivity (SC) in Eu(0.2)Sr(0.8)(Fe(0.86)Co(0.14))(2)As(2) single crystals by means of electrical resistivity, magnetic susceptibility and specific heat measurements with T(c) is approximately equal to 20 K and an antiferromagnetic (AFM) ordering of Eu(2+) moments at T(N) is approximately equal to 2.0 K in zero field. (75)As NMR experiments have been performed in the two external field directions (H is parallel to ab) and (H is parallel to c). (75)As-NMR spectra are analysed in terms of first-order quadrupolar interaction. Spin-lattice relaxation rates (1/T(1)) follow a T(3) law in the temperature range 4.2-15 K. There is no signature of a Hebel-Slichter coherence peak just below the SC transition, indicating a non-s-wave or s(±) type of superconductivity. In the temperature range 160-18 K 1/T(1)T follows the C/(T+θ) law reflecting 2D AFM spin fluctuations.     

LiVGe2O6, an anomalous quasi-1D, S = 1 system, as revealed by NMR

We report the results of 7Li nuclear magnetic resonance (NMR) studies of LiVGe2O6, a quasi-one-dimensional spin S = 1 model system, at low temperatures. Our data, including NMR spectra and the temperature dependence of the spin-lattice relaxation rate T-11, indicate that a first-order phase transition occurs at T(c) approximately 23 K. The NMR response of LiVGe2O6 below T(c) suggests that the ordered phase is antiferromagnetic and has unusual features. Possible reasons for this unexpected behavior are discussed.     

Tomonaga-Luttinger liquid in a quasi-one-dimensional S = 1 antiferromagnet observed by specific heat measurements

Specific-heat experiments on single crystals of the S = 1 quasi-one-dimensional bond-alternating antiferromagnet Ni(C9H24N4)(NO2)ClO2 (NTENP) have been performed in magnetic fields applied both parallel and perpendicular to the spin chains. We have found for the parallel field configuration that the magnetic specific heat (C(mag)) is proportional to temperature (T) above a critical field H(c), at which the energy gap vanishes, in a temperature region above that of the long-range ordered state. The ratio C(mag)/T increases as the magnetic field approaches H(c) from above. The data are in good quantitative agreement with the prediction of the c= 1 conformal field theory in conjunction with the velocity of the excitations calculated by a numerical diagonalization, providing conclusive evidence for a Tomonaga-Luttinger liquid.     

Bose-Einstein condensation of magnons in Cs2CuCl4

We report on results of specific heat measurements on single crystals of the frustrated quasi-2D spin-1/2 antiferromagnet Cs2CuCl4 (T(N)=0.595 K) in external magnetic fields B<12 T and for temperatures T>30 mK. Decreasing B from high fields leads to the closure of the field-induced gap in the magnon spectrum at a critical field Bc approximately = 8.51 T and a magnetic phase transition is clearly seen below Bc. In the vicinity of Bc, the phase transition boundary is well described by the power law Tc(B) proportional, variant (Bc-B)(1/phi), with the measured critical exponent phi approximately =1.5. These findings are interpreted as a Bose-Einstein condensation of magnons.     

Magnetic behavior of Eu(3)Ni(4)Ga(4): antiferromagnetic order and large magnetoresistance

The results of the magnetic susceptibility, isothermal magnetization, heat capacity, electrical resistivity and magnetoresistance measurements on polycrystalline Eu(3)Ni(4)Ga(4) are presented. Eu(3)Ni(4)Ga(4) forms in Na(3)Pt(4)Ge(4)-type cubic crystal structure (space group [Formula: see text]). The temperature dependence of the magnetic susceptibility of Eu(3)Ni(4)Ga(4) confirms the divalent state (Eu(2+)) of Eu ions with an effective magnetic moment μ(eff)?=?7.98?μ(B). At low fields, e.g.?at 0.01?T, a magnetic phase transition to an antiferromagnetically ordered state occurs at T(N)?=?10.9?K, which is further confirmed by the temperature dependence of the heat capacity and electrical resistivity. The field dependence of isothermal magnetization at 2?K reveals the presence of two field induced metamagnetic transitions at H(c1) and H(c2)?=?0.55 and 1.2?T, respectively and a polarized phase above H(PO)?=?1.7?T. The reduced jump in the heat capacity at the transition temperature, ΔC|(T(N))?=?13.48?J/mol-Eu?K would indicate an amplitude modulated (AM) antiferromagnetic structure. An interesting feature is that a large negative magnetoresistance, MR?=?[ρ(H)?-?ρ(0)]/ρ(0), is observed in the vicinity of magnetic transition even up to 2T(N). Similar large magnetoresistance has been observed in the paramagnetic state in some Gd and Eu based alloys and has been attributed to the magneto-polaronic effect.     

Magnetic phase transition of Mn1.88Cr0.12Sb

The nature of the first order phase transition in Mn_1.88Cr_0.12Sb crystal is discussed. According to the magnetic susceptibility χ_⊥ and the specific heat measurements, there are two phase transitions, at 280 K and 315 K, respectively. They correspond to the vanishing of the “magnetic soft modes” for FIM and AF phases [4]. No change of the energy level positions was detected using the ESCA method.     

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.     

Single crystal growth,structural and transport properties of bad metal RhSb_2

We have successfully grown an arsenopyrite marcasite type RhSb2 single crystal, and systematically investigated its crystal structure, electrical transport, magnetic susceptibility, heat capacity, and thermodynamic properties. We found that the temperature-dependent resistivity exhibits a bad metal behavior with a board peak around 200 K. The magnetic susceptibility of RhSb2 shows diamagnetism from 300 K to 2 K. The low-temperature specific heat shows a metallic behavior with a quite small electronic specific-heat coefficient. No phase transition is observed in both specific heat and magnetic susceptibility data. The Hall resistivity measurements show that the conduction carriers are dominated by electrons with ne = 8.62 × 10¹⁸ cm^-3 at 2 K, and the electron carrier density increases rapidly above 200 K without change sign. Combining with ab-initio band structure calculations, we showed that the unusual peak around 200 K in resistivity is related to the distinct electronic structure of RhSb_2. In addition, a large thermopower S(T) about -140 μV/K is observed around 200 K, which might be useful for future thermoelectric applications.     

Unconventional magnetic correlations in DyB2C and HoB2C

Layered borocarbides RB2C (R=Dy, Ho, and Er) have been studied by powder neutron diffraction at 2-30 K. ErB2C has two-sublattice antiferromagnetic order below T(N)=16.3 K, but DyB2C and HoB2C show a coexistence of a conventional canted k=(000) ferromagnetic structure and unconventional magnetic correlations. The k=(000) phase orders at T(c)=8.5 K (DyB2C) and 7.1 K (HoB2C), but low-Q diffraction peaks from the unconventional correlations appear above T(c) with different critical temperatures for different peaks: at 8, 10.5, and 15.7 K for HoB2C. This scattering is fitted as diffraction from a Warren-type random magnetic layer lattice and may result from quadrupolar interactions between R3+ spins.     

Heat transport as a probe of electron scattering by spin fluctuations: the case of antiferromagnetic CeRhIn(5)

Heat and charge conduction were measured in the heavy-fermion metal CeRhIn(5), an antiferromagnet with T(N)=3.8 K. The thermal resistivity is found to be proportional to the magnetic entropy, revealing that spin fluctuations are as effective in scattering electrons as they are in disordering local moments. The electrical resistivity, governed by a q(2) weighting of fluctuations, increases monotonically with temperature. In contrast, the difference between thermal and electrical resistivities, characterized by a omega(2) weighting, peaks sharply at T(N) and eventually goes to zero at a temperature T(*) approximately = 8 K. T(*) thus emerges as a measure of the characteristic energy of magnetic fluctuations.     

Unusual single-ion non-fermi-liquid behavior in Ce(1-x)LaxNi9Ge4

We report on specific heat, magnetic susceptibility, and resistivity measurements on the compound Ce(1-x)LaxNi9Ge4 for various concentrations ranging from the stoichiometric system with x = 0 to the dilute limit x = 0.95. Our data reveal single-ion scaling with the Ce concentration and the largest ever recorded value of the electronic specific heat Deltac/T approximately 5.5 J K-2 mol(-1) at T = 0.08 K for the stoichiometric compound x = 0 without any trace of magnetic order. While in the doped samples Deltac/T increases logarithmically below 3 K down to 50 mK, their magnetic susceptibility behaves Fermi-liquid-like below 1 K. These properties make the compound Ce(1-x)LaxNi9Ge4 a unique system on the borderline between Fermi-liquid and non-Fermi-liquid physics.     

Magnetic field effect on the static antiferromagnetism of the electron-doped superconductor Pr1-xLaCexCuO4 (x=0.11 and 0.15)

Effects of magnetic fields (applied along the c axis) on static spin correlation were studied for the electron-doped superconductors Pr1-xLaCexCuO4 with x=0.11 (T(c)=25 K) and x=0.15 (T(c)=16 K) by neutron-scattering measurements. In the x=0.11 sample, which is located near the antiferromagnetic (AF) and superconducting phase boundary, a commensurate magnetic order develops below around T(c) at zero field. Upon applying a magnetic field up to 9 T both the magnetic intensity and the onset temperature of the order increase with the maximum field effect at approximately 5 T. In contrast, in the overdoped x=0.15 sample any static AF order is neither observed at zero field nor induced by the field up to 8.5 T. Difference and similarity of the field effect between the hole- and electron-doped high-T(c) cuprates are discussed.