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The evolution of spin dynamics from the quantum critical system YbRh2Si2 to the stable bivalent Yb system YbRh2Si2 was investigated by electron spin resonance (ESR) spectroscopy. While the Kondo temperature changes by one order of magnitude, all compositions of the single-crystalline series Yb(Rh1-xCox )2Si2 show well-defined ESR spectra with a clear Yb3+ character for temperatures below ≈20 K. With increasing Co content, the ESR g factor along the c direction strongly increases, indicating a continuous change of the ground-state wave function and, thus, a continuous change of the crystal electric field. The linewidth presents a complex dependence on the Co content and is discussed in terms of the Co-doping dependence of the Kondo interaction, the magnetic anisotropy, and the influence of ferromagnetic correlations between the 4f states. The results provide evidence that, for low Co doping, the Kondo interaction allows narrow ESR spectra despite the presence of a large magnetic anisotropy, whereas at high Co concentrations, the linewidth is controlled by ferromagnetic correlations. A pronounced broadening due to critical correlations at low temperatures is only observed at the highest Co content. This might be related to the presence of incommensurate magnetic fluctuations. [ABSTRACT FROM AUTHOR]

We present a combined experimental and theoretical study of the spin-½ compound CuP2O6 that features a network of two-dimensional (2D) antiferromagnetic (AFM) square planes, interconnected via one-dimensional (1D) AFM spin chains. Magnetic susceptibility, high-field magnetization, and electron spin resonance (ESR) data, as well as microscopic density-functional band-structure calculations and subsequent quantum Monte Carlo simulations, show that the coupling J2D ≃ 40 K in the layers is an order of magnitude larger than J1D ≃ 3 K in the chains. Below TN ≃ 8 K, CuP2O6 develops long-range order, as evidenced by a weak net moment on the 2D planes induced by anisotropic magnetic interactions of Dzyaloshinsky-Moriya type. A striking feature of this 3D ordering transition is that the 1D moments grow significantly slower than the ones on the 2D units, which is evidenced by the persistent paramagnetic ESR signal below TN. Compared to typical quasi-2D magnets, the ordering temperature of CuP2O6 TN/J2D ≃ 0.2 is unusually low, showing that weakly coupled spins sandwiched between 2D magnetic units effectively decouple these units and impede the long-range ordering. [ABSTRACT FROM AUTHOR]

Fine details of the excitation spectrum of the two-dimensional spin-gap magnet PHCC are revealed by electron spin resonance investigations. The values of anisotropy parameters and the orientations of the anisotropy axes are determined by accurate measurements of the angular, frequency-field, and temperature dependencies of the resonance absorption. The properties of a spin-gap magnet in the vicinity of critical field are discussed in terms of sublevel splittings and -factor anisotropy. [ABSTRACT FROM AUTHOR]

A microscopic magnetic model for α-Cu2P2O7 is evaluated in a combined theoretical and experimental study. Despite a dominant intradimer coupling J1, sizable interdimer couplings enforce long-range magnetic ordering at TN=27 K. The spin model for α-Cu2P2O7 is compared to the models of the isostructural β-Cu2V2O7 and α-Cu2As2O7 systems. As a surprise, coupled dimers in α-Cu2P2O7 and alternating chains in α-Cu2As2O7 contrast with a honeycomb lattice in β-Cu2V2O7. We find that the qualitative difference in the coupling regime of these isostructural compounds is governed by the nature of AO4 side groups: d elements (A=V) hybridize with nearby O atoms forming a Cu-O-A-O-Cu superexchange path, while for p elements (A= P, As) the superexchange is realized via O-O edges of the tetrahedron. Implications for a broad range of systems are discussed. [ABSTRACT FROM AUTHOR]

We report a structural-magnetic investigation by x-ray absorption spectroscopy (XAS), neutron diffraction, dc susceptibility (χdc), and electron spin resonance (ESR) of the 12R-type perovskite BaTi1/2Mn1/2O3. Our structural analysis by neutron diffraction supports the existence of structural trimers with chemically disordered occupancy of Mn4+ and Ti4+ ions, with the valence of the Mn ions confirmed by the XAS measurements. The magnetic properties are explored by combining dc-susceptibility and X-band (9.4 GHz) electron spin resonance, both in the temperature interval of 2≤T≤1000 K. A scenario is presented under which the magnetism is explained by considering magnetic dimers and trimers, with exchange constants Ja/kB=200(2) K and Jb/kB=130(10) K, and orphan spins. Thus, BaTi1/2Mn1/2O3 is proposed as a rare case of an intrinsically disordered S=32 spin gap system with a frustrated ground state. [ABSTRACT FROM AUTHOR]

We have investigated single crystals and polycrystals from the series Nb1-yFe2+y,-0.004≤ y ≤0 018, by electron spin resonance, muon spin relaxation, and Mössbauer spectroscopy. Our data establish that at lowest temperatures all samples exhibit bulk magnetic order. Slight Fe excess induces low-moment ferromagnetism, consistent with bulk magnetometry, while Nb-rich and stoichiometric NbFe2 display spin density wave order with small magnetic moment amplitudes of order ~0.001-0.01μB/Fe. This provides microscopic evidence for a modulated magnetic state on the border of ferromagnetism in NbFe2. [ABSTRACT FROM AUTHOR]

We report static and dynamic properties of the antiferromagnetic compound Zn2(VO)(PO4)2, and the consequences of nonmagnetic Ti4+ doping at the V4+ site. 31P nuclear magnetic resonance (NMR) spectra and spin-lattice relaxation rate (1/T1) consistently show the formation of the long-range antiferromagnetic order below TN = 3.8-3.9 K. The critical exponent β= 0.33 ± 0.02 estimated from the temperature dependence of the sublattice magnetization measured by 31P NMR at 9.4 MHz is consistent with universality classes of three-dimensional spin models. The isotropic and axial hyperfine couplings between the 31P nuclei and V4+ spins are Aisohf = (9221 ± 100) Oe/μB and Aaxhf = (1010 ± 50) Oe/μBrespectively. Magnetic susceptibility data above 6.5 K and heat capacity data above 4.5 K are well described by quantum Monte Carlo simulations for the Heisenberg model on the square lattice with J ∼ 7.7 K. This value of J is consistent with the values obtained from the NMR shift, 1/T1), and electron spin resonance intensity analysis. Doping Zn2VO(P04)2 with nonmagnetic Ti4+ leads to a marginal increase in the J value and the overall dilution of the spin lattice. In contrast to the recent ab initio results, we find neither evidence for the monoclinic structural distortion nor signatures of the magnetic one-dimensionality for doped samples with up to 15% of Ti4+. The Neel temperature TN decreases linearly with increasing the amount of the nonmagnetic dopant. [ABSTRACT FROM AUTHOR]

The evolution of the electronic density of states at the Fermi level of intermetallic EuFe2-xRuxAs2 in the whole concentration range (0⩽x⩽2) is reported. Systematic electron spin resonance investigations of the Korringa-type Eu spin relaxation in combination with the results of band-structure calculations and magnetic susceptibility measurements can provide a clear picture of how the density of states is reduced by substituting Fe with Ru in EuFe2As2. Moreover, the strength of the exchange coupling between the local Eu and the itinerant spin systems can be estimated consistently with the weak coupling expected for this class of layered compounds. [ABSTRACT FROM AUTHOR]

The spin dynamics of the layered square-lattice vanadate Pb2VO(PO4)2 is investigated by electron spin resonance (ESR) at various magnetic fields and at temperatures above magnetic ordering. The linewidth divergence towards low temperatures seems to agree with isotropic Heisenberg-type spin exchange suggesting that the spin relaxation in this quasi-two-dimensional compound is governed by low-dimensional quantum fluctuations. However, a weak easy-plane anisotropy of the g factor points to the presence of a planar XY type of exchange. Indeed, we found that the linewidth divergence is described best by XY-like spin fluctuations which requires a single parameter only. Therefore, ESR-probed spin dynamics could establish Pb2VO(PO4)2 as the first frustrated square-lattice system with XY-inherent spin topological fluctuations. [ABSTRACT FROM AUTHOR]

Kondo insulators and in particular their noncubic representatives have remained poorly understood. Here we report on the development of an anisotropic energy pseudogap in the tetragonal compound CeRu4Sn6 employing optical reflectivity measurements in broad frequency and temperature ranges. The low-energy optical conductivity shows semiconductor-like features within the a-a plane but a Drude-like response along the c axis, signaling weak metallicity. Local density approximation plus dynamical mean-field theory calculations reproduce these observations qualitatively and help to identify their origin. For large parts of the Brillouin zone, the strongly correlated band structure presents a narrow direct gap within the Kondo resonance. Only for the c direction, due to anisotropy in the hybridization, there is a pronounced band crossing at the Fermi level. [ABSTRACT FROM AUTHOR]

Magnetic properties of the spin-3/2 Heisenberg system Cr2[BP3O12] are investigated by magnetic susceptibility X(T) measurements, electron spin resonance, neutron diffraction, and density functional theory (DFT) calculations, as well as classical and quantum Monte Carlo (MC) simulations. The broad maximum of x(T) at 85 K and the antiferromagnetic Weiss temperature of 139 K indicate low-dimensional magnetic behavior. Below TN = 28 K, Cr2[BP3O12] is antiferromagnetically ordered with the k = 0 propagation vector and an ordered moment of 2.5μB/Cr. DFT calculations, including DFT + U and hybrid functional, yield a microscopic model of spin chains with alternating nearest-neighbor couplings J1 and J'1. The chains are coupled by two nonequivalent interchain exchanges of similar strength (~l-2 K), but different sign (antiferromagnetic and ferromagnetic). The resulting spin lattice is quasi-one-dimensional and not frustrated. Quantum MC simulations show excellent agreement with the experimental data for the parameters J1≃ 50 K and J'1/J1 ≃ 0.5. Therefore, Cr2[BP3O12] is close to the gapless critical point ( J1/J1 = 0.41) of the spin-3/2 bond-altemating Heisenberg chain. The applicability limits of the classical approximation are addressed by quantum and classical MC simulations. Implications for a wide range of low-dimensional S = 3/2 materials are discussed. [ABSTRACT FROM AUTHOR]

We report on superconductivity in CeFeAs1-xPxO and the possible coexistence with Ce ferromagnetism (FM) in a small homogeneity range around x = 30% with ordering temperatures of TSC ≅ TC ≅ 4 K. The antiferromagnetic (AFM) ordering temperature of Fe at this critical concentration is suppressed to TNFe ≈ 40 K and does not shift to lower temperatures with a further increase of the P concentration. Therefore, a quantum-critical-point scenario with TNFe → 0 K which is widely discussed for the iron based superconductors can be excluded for this alloy series. Surprisingly, thermal expansion and x-ray powder diffraction indicate the absence of an orthorhombic distortion despite clear evidence for short-range AFM Fe ordering from muon-spin-rotation measurements. Furthermore, we discovered the formation of a sharp electron spin resonance signal unambiguously connected with the emergence of FM ordering. [ABSTRACT FROM AUTHOR]

A detailed study of the spin dynamics of the S-state ions Gd3+ and Eu2+ in the filled skutterudites Lai-jAjPtGe (R = Gd, Eu) is reported. The spin dynamics is investigated directly by means of Gd3+ and Eu2+electron spin resonance, performed at X-band (&ap;9.4 GHz) and g-band (&ap;34 GHz) frequencies in the temperature intervals 8 < T < 300 K and 1.5 < T < 300 K, respectively. The ESR parameters provide direct evidence for the vibrational behavior of the Gd3+ ions but not for the Eu2+ ions. These results are interpreted in light of recent discussions about the spin relaxation in cage systems. In particular, the Gd3+ spin relaxation in La0.9Gd0.1Pt4Ge12 provides evidence for the existence of an extra phonon mode with an Einstein temperature θE &ap; 24 K in this system. The work suggests that the so-called "rattling" modes have a general and important role that should be taken into account in the study of spin dynamics in cage systems. [ABSTRACT FROM AUTHOR]

We report magnetization measurements, full-potential band-structure calculations, and microscopic modeling for the spin-1/2 Heisenberg magnets A2CuP2O7 (A = Na, Li) involving complex Cu-O-O-Cu superexchange pathways. Based on a quantitative evaluation of the leading exchange integrals and the subsequent quantum Monte Carlo simulations, we propose a quasi-one-dimensional magnetic model for both compounds, in contrast to earlier studies that conjectured on a two-dimensional scenario. The one-dimensional nature of A2CuP2O7 is unambiguously verified by magnetization isotherms measured in fields up to 50 T. The saturation fields of about 40 T for both Li and Na compounds are in excellent agreement with the intrachain exchange J1 ≃ 27 K extracted from the magnetic susceptibility data. The proposed magnetic structure entails spin chains with the dominating antiferromagnetic nearest-neighbor interaction J1 and two inequivalent, nonfrustrated antiferromagnetic interchain couplings of about 0.01J1 each. A possible long-range magnetic ordering is discussed in comparison with the available experimental information. [ABSTRACT FROM AUTHOR]

A correction to the article "Magnetic model for A2CuP2O7 (A = Na, Li): One-dimensional versus two-dimensional behavior," which was published in the December 5, 2011 issue is presented.