Defect propagation in one-, two-, and three-dimensional compounds doped by magnetic atoms

Inelastic neutron scattering experiments were performed to study manganese(II) dimer excitations in the diluted one-, two-, and three-dimensional compounds CsMn(x)Mg(1-x)Br(3), K(2)Mn(x)Zn(1-x)F(4), and KMn(x)Zn(1-x)F(3) (x<0.10), respectively. The transitions from the ground-state singlet to the excited triplet, split into a doublet and a singlet due to the single-ion anisotropy, exhibit remarkable fine structures. These unusual features are attributed to local structural inhomogeneities induced by the dopant Mn atoms which act like lattice defects. Statistical models support the theoretically predicted decay of atomic displacements according to 1/r**2, 1/r, and constant (for three-, two-, and one-dimensional compounds, respectively) where r denotes the distance of the displaced atoms from the defect. The observed fine structures allow a direct determination of the local exchange interactions J, and the local intradimer distances R can be derived through the linear law dJ/dR.Comment: 22 pages, 5 figures, 2 table

Discrete antiferromagnetic spin-wave excitations in the giant ferric wheel Fe18

The low-temperature elementary spin excitations in the AFM molecular wheel Fe18 were studied experimentally by inelastic neutron scattering and theoretically by modern numerical methods, such as dynamical density matrix renormalization group or quantum Monte Carlo techniques, and analytical spin-wave theory calculations. Fe18 involves eighteen spin-5/2 Fe(III) ions with a Hilbert space dimension of 10^14, constituting a physical system that is situated in a region between microscopic and macroscopic. The combined experimental and theoretical approach allowed us to characterize and discuss the magnetic properties of Fe18 in great detail. It is demonstrated that physical concepts such as the rotational-band or L&E-band concepts developed for smaller rings are still applicable. In particular, the higher-lying low-temperature elementary spin excitations in Fe18 or AFM wheels in general are of discrete antiferromagnetic spin-wave character.Comment: 16 pages, 10 figure

Single-ion versus two-ion anisotropy in magnetic compounds: A neutron scattering study

Anisotropy effects can significantly control or modify the ground-state properties of magnetic systems. Yet the origin and the relative importance of the possible anisotropy terms is difficult to assess experimentally and often ambiguous. Here we propose a technique which allows a very direct distinction between single-ion and two-ion anisotropy effects. The method is based on high-resolution neutron spectroscopic investigations of magnetic cluster excitations. This is exemplified for manganese dimers and tetramers in the mixed compounds CsMnxMg1-xBr3 (0.05\leqx\leq0.40). Our experiments provide evidence for a pronounced anisotropy of the order of 3% of the dominant bilinear exchange interaction, and the anisotropy is dominated by the single-ion term. The detailed characterization of magnetic cluster excitations offers a convenient way to unravel anisotropy effects in any magnetic material.Comment: 9 pages, 10 figures, 1 tabl

Coexistence of Superconductivity and Magnetism in FeSe_1-x under Pressure

An extended investigation of the electronic phase diagram of FeSe$_{1-x}$ up to pressures of $p\simeq2.4$\,GPa by means of ac and dc magnetization, zero field muon spin rotation (ZF $\mu$SR), and neutron diffraction is presented. ZF $\mu$SR indicates that at pressures $p\geq0.8$\,GPa static magnetic order occurs in FeSe$_{1-x}$ and occupies the full sample volume for $p\gtrsim 1.2$\,GPa. ac magnetization measurements reveal that the superconducting volume fraction stays close to 100% up to the highest pressure investigated. In addition, above $p\geq1.2$\,GPa both the superconducting transition temperature $T_{\rm c}$ and the magnetic ordering temperature $T_{\rm N}$ increase simultaneously, and both superconductivity and magnetism are stabilized with increasing pressure. Calculations indicate only one possible muon stopping site in FeSe$_{1-x}$, located on the line connecting the Se atoms along the $c$-direction. Different magnetic structures are proposed and checked by combining the muon stopping calculations with a symmetry analysis, leading to a similar structure as in the LaFeAsO family of Fe-based superconductors. Furthermore, it is shown that the magnetic moment is pressure dependent and with a rather small value of $\mu\approx 0.2\,\mu_B$ at $p\simeq2.4$\,GPa.Comment: 11 pages, 9 figure

Direct observation of local Mn-Mn distances in the paramagnetic compound CsMnxMg1-xBr3

We introduce a novel method for local structure determination with a spatial resolution of the order of 0.01 Angstroem. It can be applied to materials containing clusters of exchange-coupled magnetic atoms. We use neutron spectroscopy to probe the energies of the cluster excitations which are determined by the interatomic coupling strength J. Since for most materials J is related to the interatomic distance R through a linear relation dJ/dR={\alpha} (for dR/R<<1), we can directly derive the local distance R from the observed excitation energies. This is exemplified for the mixed one-dimensional paramagnetic compound CsMnxMg1 xBr3 (x=0.05, 0.10) containing manganese dimers oriented along the hexagonal c-axis. Surprisingly, the resulting Mn-Mn distances R do not vary continuously with increasing internal pressure, but lock in at some discrete values.Comment: 16 pages, 2 tables, 3 figure

New structural and magnetic aspects of the nanotube system Na2V3O7

We present new experimental results of low temperature x-ray synchrotron diffraction, neutron scattering and very low temperature (mK-range) bulk measurements on the nanotube system {\tube}. The crystal structure determined from our data is similar to the previously proposed model (P. Millet {\it et al.} J. Solid State Chem. $\bf{147}$, 676 (1999)), but also deviates from it in significant details. The structure comprises nanotubes along the c-axis formed by stacking units of two V-rings buckled in the $ab$-plane. The space group is P$\bar{3}$ and the composition is nonstoichiometric, Na(2-x)V3O7, x=0.17. The thermal evolution of the lattice parameters reveals anisotropic lattice compression on cooling. Neutron scattering experiments monitor a very weak magnetic signal at energies from -20 to 9 meV. New magnetic susceptibility, specific heat measurements and decay of remanent magnetization in the 30 mK - 300 mK range reveal that the previously observed transition at ~76 mK is spin-glass like with no long-range order. Presented experimental observations do not support models of isolated clusters, but are compatible with a model of odd-legged S=1/2 spin tubes possibly segmented into fragments with different lengths

Oxygen disorder in ice probed by X-ray Compton scattering

We use electron momentum density in ice as a tool to quantify order-disorder transitions by comparing Compton profiles differences of ice VI, VII, VIII and XII with respect to ice Ih. Quantitative agreement is found between theory and experiment for ice VIII, which is the most ordered phase. Robust signatures of the oxygen disorder are identified in the momentum density for the VIII-VII ice phase transition. The unique aspect of this work is the determination of the fraction n_e of electron directly involved in phase transitions as well as the use of position space signatures for quantifying oxygen site disorder.Comment: 3 figures, 2 tables. Accepted for publication in Phys. Rev.

Spin-state polaron in lightly hole-doped LaCoO_3

Inelastic neutron scattering (INS), electron spin (ESR) and nuclear magnetic resonance (NMR) measurements were employed to establish the origin of the strong magnetic signal in lightly hole-doped La_{1-x}Sr_xCoO_3, x=0.002. Both, INS and ESR low temperature spectra show intense excitations with large effective g-factors ~10-18. NMR data indicate the creation of extended magnetic clusters. From the Q-dependence of the INS magnetic intensity we conclude that the observed anomalies are caused by the formation of octahedrally shaped spin-state polarons comprising seven Co ions.Comment: 10 pages, 3 figure

Tetrahedra system Cudaca: high-temperature manifold of molecular configurations governing low-temperature properties

The Cudaca system composed of isolated Cu2+ S=1/2 tetrahedra with antiferromagnetic exchange should exhibit properties of a frustrated quantum spin system. ab initio density functional theory calculations for electronic structure and molecular dynamics computations suggest a complex interplay between magnetic exchange, electron delocalization and molecular vibrations. Yet, extensive experimental characterization of Cudaca by means of synchrotron x-ray diffraction, magnetization, specific heat and inelastic neutron scattering reveal that properties of the real material can be only partly explained by proposed theoretical models as the low temperature properties seem to be governed by a manifold of molecular configurations coexisting at high temperatures.Comment: 15 figure