We present an inelastic neutron scattering investigation of Li2CuO2 detecting the long sought quasi-1D magnetic excitations with a large dispersion along the CuO2-chains studied up to 25 meV. The total dispersion is governed by a surprisingly large ferromagnetic (FM) nearestneighbor exchange integral J1 = −228 K. An anomalous quartic dispersion near the zone center and a pronounced minimum near (0,0.11,0.5) r.l.u. (corresponding to a spiral excitation with a pitch angle about 41 •) point to the vicinity of a 3D FM-spiral critical point. The leading exchange couplings are obtained applying standard linear spin-wave theory. The 2 nd neighbor inter-chain interaction suppresses a spiral state and drives the FM in-chain ordering below the Néel temperature. The obtained exchange parameters are in agreement with the results for a realistic five-band extended Hubbard Cu 3d O 2p model and LSDA+U predictions.

Specific heat, resistivity, magnetic susceptibility, linear thermal expansion (LTE), and highresolution synchrotron X-ray powder diffraction investigations of single crystals Fe1+yTe (0.06 ≤ y ≤ 0.15) reveal a splitting of a single, first-order transition for y ≤ 0.11 into two transitions for y ≥ 0.13. Most strikingly, all measurements on identical samples Fe1.13Te consistently indicate that, upon cooling, the magnetic transition at TN precedes the first-order structural transition at a lower temperature Ts. The structural transition in turn coincides with a change in the character of the magnetic structure. The LTE measurements along the crystallographic c-axis displays a small distortion close to TN due to a lattice striction as a consequence of magnetic ordering, and a much larger change at Ts. The lattice symmetry changes, however, only below Ts as indicated by powder X-ray diffraction. This behavior is in stark contrast to the sequence in which the phase transitions occur in Fe pnictides.

The properties of two quantum spin chain materials, K 2 CuSO 4 Cl 2 and K 2 CuSO 4 Br 2 , are studied by a variety of experimental techniques, including bulk measurements, neutron spectroscopy, and electron spin resonance. The hierarchy of relevant terms in the magnetic Hamiltonian is established. It is shown that these two compounds feature substantial Dzyaloshinskii-Moriya interactions that are uniform within each chain, but antiparallel in adjacent chains. The result is a peculiar type of frustration of interchain interactions, which leads to an unusual field-temperature phase diagram.

A thermodynamic method to extract the interchain coupling (IC) of spatially anisotropic 2D or 3D spin-1/2 systems from their empirical saturation field H(s) (T=0) is proposed. Using modern theoretical methods we study how H(s) is affected by an antiferromagnetic (AFM) IC between frustrated chains described in the J(1)-J(2)-spin model with ferromagnetic 1st and AFM 2nd neighbor in-chain exchange. A complex 3D-phase diagram has been found. For Li(2)CuO(2) and Ca(2)Y(2)Cu(5)O(10), we show that H(s) is solely determined by the IC and predict H(s)≈61 T for the latter. With H(s)≈55 T from magnetization data one reads out a weak IC for Li(2)CuO(2) close to that obtained from inelastic neutron scattering.

1Comment on "Two-spinon and fourspinon continuum in a frustrated ferromagnetic spin-1/2 chain"Recently Enderle et al. reported an inelastic neutron scattering (INS) study of the dynamic spin susceptibility Imχ(ω, k) for LiCuVO 4 [1]. Therein they claim that (i) LiCuVO 4 is well described by two interpenetrating, weakly ferromagnetically (FM) coupled Heisenberg antiferromagnetic spin-1/2 chains (HAF), (ii) the obtained exchange integrals J i (NN and NNN inchain couplings J 1 =− 1.6 meV, J 2 =3.56 meV, diagonal interchain coupling J 5 =−0.4 meV in the (ab)-plane, α=−J 2 /J 1 ∼ 2.2) agree with those from an analysis based on spin-wave theory (SWT) [2], (iii) the observed INS intensity above 10 meV belongs to a 4-spinon continuum (4SC). Applying exact diagonalization (ED) and DMRG methods to fit INS and magnetization M (H) data, supported by independent microscopic methods [3], we will show that the claims of Ref. [1] are not justified and that LiCuVO 4 exhibits α <1, i.e. strong coupling of the HAF, at odds with (i). For possible spin nematics and Bose condensation of 2-magnon bound states in LiCuVO 4 [4, 5] precise knowledge of the coupling regime is of key importance.Starting from a 2D model, the authors suggest an effective 1D-model with J eff,1 =J 1 + 2J 5 =−2.4 meV and a renormalized J eff,2 =2J 2 /π, i.e. α≃1.4. But the applied perturbative method is designed for α ≫1. Further problems occur for the dispersion of spin excitations ω(k) (dark red curve in Fig. 2 of Ref. 1). Near k=π/4 the local maximum Ω gives 4.84 meV. For α=1.42 and J eff,1 =−2.4 meV one has Ω=4.36 (4.79) meV, only, according to our dynamical DMRG (ED) calculation with L=96 (28) sites of Imχ(ω, q). Fitting our DMRG results for 0.5 ≤ α ≤ 2, the general constraint for Ω(α) reads:

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The electron spin resonance spectrum of a quasi 1D S = 1/2 antiferromagnet K2CuSO4Br2 was found to demonstrate an energy gap and a doublet of resonance lines in a wide temperature range between the Curie-Weiss and Neèl temperatures. This type of magnetic resonance absorption corresponds well to the two-spinon continuum of excitations in S = 1/2 antiferromagnetic spin chain with a uniform Dzyaloshinskii-Moriya interaction between the magnetic ions. A resonance mode of paramagnetic defects demonstrating strongly anisotropic behavior due to interaction with spinon excitations in the main matrix is also observed.

Day-Dunlop plots are widely used in paleomagnetic and environmental studies as a tool to determine the magnetic domain state of magnetite, i.e., superparamagnetic (SP), stable single-domain (SD), pseudosingle-domain (PSD), multidomain (MD), and their mixtures. The few experimental studies that have examined hysteresis properties of SD-SP mixtures of magnetite found that the ratios of saturation remanent magnetization to saturation magnetization and the coercivity of remanence to coercivity are low, when compared to expected theoretical mixing trends based on Langevin theory. This study reexamines DayDunlop plots using experimentally controlled mixtures of SD and SP magnetite grains. End-members include magnetotactic bacteria (MSR-1) as the SD source, and a commercial ferrofluid or magnetotactic bacteria (DA12) as the SP source. Each SP-component was added incrementally to a SD sample. Experimental results from these mixing series show that the magnetization and coercivity ratios are lower than the theoretical prediction for bulk SP magnetic size. Although steric repulsion was present between the particles, we cannot rule out interaction in the ferrofluid for higher concentrations. The SP bacteria are noninteracting as the magnetite was enclosed by an organic bilipid membrane. Our results demonstrate that the magnetization and coercivity ratios of SD-SP mixtures can lie in the PSD range, and that an unambiguous interpretation of particle size can only be made with information about the magnetic properties of the end-members.