Magnetic order and transitions in the spin-web compound Cu3TeO6

The spin-web compound Cu3TeO6, belongs to an intriguing group of materials where magnetism is governed by 3d9 copper Cu2+ ions. This compound has been sparsely experimentally studied and we here present the first investigation of its local magnetic properties using muon-spin relaxation/rotation ({\mu}+SR). Our results show a clear long-range 3D magnetic order below TN as indicated by clear zero-field (ZF) muon-precessions. At TN = 61.7 K a very sharp transition is observed in the weak transverse-field (wTF) as well as ZF data. Contrary to suggestions by susceptibility measurements and inelastic neutron scattering, we find no evidence for either static or dynamic (on the time-scale of {\mu}+SR) spin-correlations above TN

Impact of strong disorder on the static magnetic properties of the spin-chain compound BaCu2SiGeO7

The disordered quasi-1D magnet BaCu2SiGeO7 is considered as one of the best physical realizations of the random Heisenberg chain model, which features an irregular distribution of the exchange parameters and whose ground state is predicted to be the scarcely investigated random-singlet state (RSS). Based on extensive 29Si NMR and magnetization studies of BaCu2SiGeO7, combined with numerical Quantum Monte Carlo simulations, we obtain remarkable quantitative agreement with theoretical predictions of the random Heisenberg chain model and strong indications for the formation of a random-singlet state at low temperatures in this compound. As a local probe, NMR is a well-adapted technique for studying the magnetism of disordered systems. In this case it also reveals an additional local transverse staggered field (LTSF), which affects the low-temperature properties of the RSS. The proposed model Hamiltonian satisfactorily accounts for the temperature dependence of the NMR line shapes.Comment: 10 pages, 7 figure

Note: Versatile sample stick for neutron scattering experiments in high electric fields

We present a versatile high voltage sample stick that fits into all cryomagnets and standard cryostats at the Swiss Spallation Neutron Source, Paul Scherrer Institut, and which provides a low effort route to neutron scattering experiments that combine electric field with low temperature and magnetic field. The stick allows for voltages up to 5 kV and can be easily adapted for different scattering geometries. We discuss the design consideration and thermal behavior of the stick, and give one example to showcase the abilities of the device. (C) 2014 AIP Publishing LLC

Critical scaling in the cubic helimagnet Cu2OSeO3

We present a detailed ac susceptibility investigation of the fluctuation regime in the insulating cubic helimagnet Cu2OSeO3. For magnetic fields mu H-0 >= 200 mT, and over a wide temperature (T) range, the system behaves according to the scaling relations characteristic of the classical three-dimensional Heisenberg model. For lower magnetic fields, the scaling is preserved only at higher T and becomes renormalized in a narrow-T range above the transition temperature. Contrary to the well-studied case of MnSi, where the renormalization has been interpreted within the Brazovskii theory, our analysis of the renormalization at H = 0 shows the fluctuation regime in Cu2OSeO3 to lie closer to that expected within the Wilson-Fischer scenario

Triplons, Magnons, and Spinons in a Single Quantum Spin System: SeCuO3

Quantum spin systems exhibit an enormous range of collective excitations, but their spin waves, gapped triplons, fractional spinons, or yet other modes are generally held to be mutually exclusive. Here we show by neutron spectroscopy on SeCuO$_3$ that magnons, triplons, and spinons are present simultaneously. We demonstrate that this is a consequence of a structure consisting of two coupled subsystems and identify all the interactions of a minimal magnetic model. Our results serve qualitatively to open the field of multi-excitation spin systems and quantitatively to constrain the complete theoretical description of one member of this class of materials.Comment: 8 pages, 5 figure

Magnetic nano-fluctuations in a frustrated magnet

Frustrated systems exhibit remarkable properties due to the high degeneracy of their ground states. Stabilised by competing interactions, a rich diversity of typically nanometre-sized phase structures appear in polymer and colloidal systems, while the surface of ice pre-melts due to geometrically frustrated interactions. Atomic spin systems where magnetic interactions are frustrated by lattice geometry provide a fruitful source of emergent phenomena, such as fractionalised excitations analogous to magnetic monopoles. The degeneracy inherent in frustrated systems may prevail all the way down to absolute zero temperature, or it may be lifted by small perturbations or entropic effects. In the geometrically frustrated Ising--like magnet Ca3Co2O6, we follow the temporal and spatial evolution of nanoscale magnetic fluctuations firmly embedded inside the spin--density--wave magnetic structure. These fluctuations are a signature of a competing ferrimagnetic phase with an incommensurability that is different from, but determined by the host. As the temperature is lowered, the fluctuations slow down into a super-paramagnetic regime of stable spatiotemporal nano-structures

Chirality of structure and magnetism in the magnetoelectric compound Cu2OSeO3

Small-angle diffraction of polarized neutrons and resonant contribution to diffraction of synchrotron radiation have been applied to prove chirality of the crystal and magnetic structures of the magnetoelectric insulator Cu2OSeO3. Similarly to other chiral magnets with P2(1)3 crystal structure the corresponding chiralities are linked to each other via the phenomenological Dzyaloshinskii-Moriya interaction. The crystal and magnetic structures for Cu2OSeO3 have the same chirality as is observed for MnSi, Mn1-x Fe-x Si, and MnGe. However, the combination of chiralities is opposite to that proposed from a recent theoretical consideration. The chiral link between structure and magnetism previously found for metallic compounds is also proved to exist for an insulator (Cu2OSeO3), allowing us to conclude that the conducting electrons play no role in a possible common microscopic mechanism of this specific magnetolattice interaction

Exploration of the helimagnetic and skyrmion lattice phase diagram in Cu2OSeO3 using magnetoelectric susceptibility

Using superconducting quantum interference device magnetometry techniques, we have studied the change in magnetization versus applied ac electric field, i.e. the magnetoelectric (ME) susceptibility dM/dE, in the chiral-lattice ME insulator Cu2OSeO3. Measurements of the dM/dE response provide a sensitive and efficient probe of the magnetic phase diagram, and we observe clearly distinct responses for the different magnetic phases, including the skyrmion lattice phase. By combining our results with theoretical calculation, we estimate quantitatively the ME coupling strength as lambda = 0.0146 meV/(V/nm) in the conical phase. Our study demonstrates the ME susceptibility to be a powerful, sensitive, and efficient technique for both characterizing and discovering new multiferroic materials and phases

Intrachain antiferromagnetic exchange in a 1D branched-chain built of two different copper(II) centres interlinked by end-on azido and phenoxo bridges: electron density map, electrochemical and magnetic properties

A 1D, end-on azido- and phenoxo-bridged, coordination polymer, [Cu-2(mu-NAPPR)(mu(1,1)-N-3)(2)](n) (1), (NAPPR(2-) = 1,3-bis(naphthylideneimino) propane dianion), was synthesized and characterized. The 1D branched-chain is built of two types of slightly different, square-pyramidal, Cu(II) ions with different sets of donor atoms. Temperature-and field-dependent magnetic analyses reveal an antiferromagnetic interaction between copper centres in the main chain (Cu-2) mediated by EO-azide bridges along the chain. The interaction between the copper centres Cu1 (in the branches of the main chain) and Cu-2 is very weak and is included by means of an own Weiss constant theta'. The electron density map of 1 from synchrotron data shows that the d(x2-y2) orbitals of Cu-2 are not well described by the procrystal model, a difference which might be meaningful for the magnetic properties. Cyclic voltammetry studies show two different redox waves with a large difference between the redox potentials of the two types of copper centres (1.21 V). The resulting data are consistent with the crystal structure of this complex determined by single crystal X-ray diffraction

Neutron Scattering Investigations of Weakly Coupled and Weakly Connected Antiferromagnets

Antiferromagnetic insulators at low temperatures offer a clean arena to study non-semiclassical phenomena because the character of interactions is usually known. Macroscopic properties of the system can then be calculated by more or less sophisticated approximations and compared to experimental results. Generally, quantum effects are expected to increase by lowering the couplings or connectivity between the magnetic moments. Magnetic excitations, which can be easily measured by the neutron spectroscopy technique, are particularly sensitive to quantum effects. Answers to questions related to the ground state and the excitation spectra of quantum magnets are of primary importance to understand magnetism. We here study the coupled tetrahedra system Cu2Te2O5X2 (X=Cl, Br) and weakly connected Cu3TeO6 system. Both systems are complex magnetic insulators, with relatively low antiferromagnetic transition temperatures. In the Cu2Te2O5X2 system we observed an unusual situation where an anomalously weak low-energy Goldstone-like mode is accompanied by a strong higher energy gapped mode. When compared to a random phase approximation theory, there is a striking difference in the intensities, but also in the gap size. We propose that the origin of the discrepancy lies in the quantum fluctuations originating between the tetrahedra, which were not taken into account by the theory. In the "spin-web" lattice system Cu3TeO6, no reasonable fit of the semiclassical theory based on the Heisenberg Hamiltonian to the dispersion of excitations has been possible. Here, however, no quantum effects have been explicitly demonstrated. By doing additional polarized neutron spectroscopy experiment, we proved that a strong magnon-phonon coupling in this system significantly changes the properties of the spectrum