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The neutron time-of-flight spectrometer NEAT has a long history of successful applications and is best suited to probe dynamic phenomena directly in the large time domain 10−14 – 10−10 s and on the length scale ranging from 0.05 to up to about 5 nm. To address user community needs for more powerful instrumental capabilities, a concept of the full upgrade of NEAT has been proposed. The upgrade started in 2010 after a rigorous internal and external selection process and resulted in 300-fold neutron count rate increase compared to NEAT′1995. Combined with new instrumental and sample environmental capabilities the upgrade allows NEAT to maintain itself at the best world class level and provide an outstanding experimental tool for a broad range of scientific applications. The advanced features of the new instrument include an integrated guide-chopper system that delivers neutrons with flexible beam properties: either highly homogeneous beam with low divergence suitable for single crystals studies or “hot-spot” neutron distribution serving best small samples. Substantial increase of the detector angle coverage is achieved by using 416 3He position sensitive detectors. Placed at 3 m from the sample, the detectors cover 20 m2 area and are equipped with modern electronics and DAQ using event recording techniques. The installation of hardware has been completed in June 2016 and on January 23, 2017 NEAT has welcomed its first regular users who took advantage of the high counting rate, broad available range of incoming neutron wavelengths and high flexibility of NEAT. Here we present details of NEAT upgrade, measured instrument characteristics and show first experimental results.

Fe3 N and Fe3 C nanocomposites have a wide range of applications thanks to their ceramic nature, magnetic properties, conductivity and catalytic activity, just to cite some. In many fields optimal performances are ensured by crystallinity, homogeneity and hierarchical organization. In the present paper, crystalline, magnetic and well-defined nanofibres of iron nitride and iron carbide/carbon nanocomposite with tunable composition and size were prepared via electrospinning. The starting polymeric material was directly electrospun into fibres and then calcined, leading to a highly homogeneous final product of nanoparticles along the fibres (both outside and inside). A mechanistic study was undertaken and here discussed. The magnetic properties of the as-prepared nanofibres were also studied. The as-prepared final fibre mat composite material can serve as active catalyst, for example, in oxygen reduction reaction (where nanofibres outperformed mere nanoparticles), it can serve as functional support for classical catalytic processes or, thanks to its magnetic properties, can be applied in magnetic-field assisted separation or as magneto-active membranes.

Doping of Li/MgO with additional metal oxides is suggested leading to an improved system with respect to the catalytic performance and stability when used for oxidative coupling of methane. We used Gd and Fe as dopants and characterized the resulting materials, showing that Fe seems to be completely and Gd partly incorporated into the MgO lattice. The catalytic performance is improved in most cases, but all materials still suffer from severe deactivation. A loss of Li is observed when being used under reaction conditions, but this Li loss is retarded for Fe-Li/MgO as compared to undoped Li/MgO.

The layered $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{NaMn}{\mathrm{O}}_{2}$, a promising Na-ion energy-storage material, has been investigated for its triangular lattice capability to promote complex magnetic configurations that may release symmetry restrictions for the coexistence of ferroelectric and magnetic orders. The complexity of the neutron powder diffraction patterns underlines that the routinely adopted commensurate structural models are inadequate. Instead, a single-phase superspace symmetry description is necessary, demonstrating that the material crystallizes in a compositionally modulated $\mathbf{q}=(0.077(1)$, 0, 0) structure. Here $\mathrm{M}{\mathrm{n}}^{3+}$ Jahn-Teller distorted $\mathrm{Mn}{\mathrm{O}}_{6}$ octahedra form corrugated layer stacking sequences of the $\ensuremath{\beta}\text{\ensuremath{-}}\mathrm{NaMn}{\mathrm{O}}_{2}$ type, which are interrupted by flat sheets of the \ensuremath{\alpha}-like oxygen topology. Spontaneous long-range collinear antiferromagnetic order, defined by the propagation vector $\mathbf{k}=(1/2,1/2,1/2)$, appears below ${T}_{\mathrm{N}1}=200\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. Moreover, a second transition into a spatially modulated proper-screw magnetic state (k \ifmmode\pm\else\textpm\fi{} q) is established at ${T}_{\mathrm{N}2}=95\phantom{\rule{0.16em}{0ex}}\mathrm{K}$, with an antiferromagnetic order parameter resembling that of a two-dimensional (2D) system. The evolution of $^{23}\mathrm{Na}$ NMR spin-lattice relaxation identifies a magnetically inhomogeneous state in the intermediate $T$ region $({T}_{\mathrm{N}2}lTl{T}_{\mathrm{N}1})$, while its strong suppression below ${T}_{\mathrm{N}2}$ indicates that a spin gap opens in the excitation spectrum. High-resolution neutron inelastic scattering confirms that the magnetic dynamics are indeed gapped (\ensuremath{\Delta} \ensuremath{\sim} 5 meV) in the low-temperature magnetic phase, while simulations on the basis of the single-mode approximation suggest that Mn spins residing on adjacent antiferromagnetic chains, establish sizable 2D correlations. Our analysis points out that novel structural degrees of freedom promote cooperative magnetism and emerging dielectric properties in this nonperovskite type of manganite.

The first experimental characterization of a multiple energy analysis wide angle backend for a cold triple-axis spectrometer is reported. The multi-analyzer module MultiFLEXX employs 155 detection channels which simultaneously probe an extensive range in wavevector and energy transfer. Successful mapping of magnetic excitations in MnF2 and Ho demonstrate order of magnitude gains in data collection efficiency using this novel type backend. MultiFLEXX is competitive to standard triple-axis spectroscopy in terms of energy resolution and signal-to-noise ratio. A minority of the detector channels is affected by spurious signals inherent to this multiplexing concept. The characteristic signature of these spurious signals easily allows for their discrimination. The instrument concept focuses on detection efficiency in the horizontal scattering plane which makes it an ideal technique for fast mapping and parametric studies including extreme sample environment.

Magnetic insulators have proven to be usable as quantum simulators for itinerant interacting quantum systems. In particular the compound (C$_{5}$H$_{12}$N)$_{2}$CuBr$_{4}$ (short (Hpip)$_{2}$CuBr$_{4}$) was shown to be a remarkable realization of a Tomonaga-Luttinger liquid (TLL) and allowed to quantitatively test the TLL theory. Substitution weakly disorders this class of compounds and allows thus to use them to tackle questions pertaining to the effect of disorder in TLL as well, such as the formation of the Bose glass. As a first step in this direction we present in this paper a study of the properties of the related (Hpip)$_{2}$CuCl$_{4}$ compound. We determine the exchange couplings and compute the temperature and magnetic field dependence of the specific heat, using a finite temperature Density Matrix Renormalization group (DMRG) procedure. Comparison with the measured specific heat at zero magnetic field confirms the exchange parameters and Hamiltonian for the (Hpip)$_{2}$CuCl$_{4}$ compound, giving the basis needed to start studying the disorder effects.

Weperformedmagneticsusceptibility,dielectricconstant,neutron diffraction,andsynchrotronradiationx raydiffractionmeasurementsonaspin latticecouplingsystemCuFeO2underapplieduniaxialpressurepupto600MPa.Wefoundthatthephase transitiontemperatureTN1fromtheparamagneticphasetothepartiallydisorderedphaseincreasesbyasmuchas5KfromtheoriginalTN1 14Kunderappliedpof600MPa,andthatthelatticeconstantbmofCuFeO2changess

A low-temperature magnetic phase diagram under magnetic fields along the orthorhombic $a$ axis of an isosceles-triangular-lattice antiferromagnet ${\mathrm{CoNb}}_{2}{\mathrm{O}}_{6}$ was investigated through single-crystal neutron diffraction measurements made at temperatures down to $T=0.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. We produced a phase diagram that consists of three magnetically ordered phases, i.e., the antiferromagnetic (AF), the incommensurate sinusoidal magnetic (IC), and the induced ferromagnetic (IFM) phases, which were characterized by the propagation wave vectors $\mathbf{Q}=(0\phantom{\rule{0.16em}{0ex}}\frac{1}{2}\phantom{\rule{0.16em}{0ex}}0)$, (0 $q$ 0), and (0 0 0), respectively. We found that a field-induced ferrimagnetic phase with $\mathbf{Q}=(0\phantom{\rule{0.16em}{0ex}}\frac{1}{4}\phantom{\rule{0.16em}{0ex}}0)$ that had been observed by previous neutron diffraction studies down to $T=1.8\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ [H. Weitzel et al., Phys. Rev. B 62, 12146 (2000)] does not exist as a single equilibrium phase, but rather it always coexists with the other ordered phases near the triple point where the AF, IC, and IFM phases meet. We also found that the relaxation time of the system becomes extremely long below $T=0.6\phantom{\rule{0.16em}{0ex}}\mathrm{K}$ in comparison with our observation time; this was considered to possibly be the reason for magnetization plateaus appearing at $T=0.5\phantom{\rule{0.16em}{0ex}}\mathrm{K}$. These plateaus have a half-saturation magnetization, from which another field-induced state was inferred in previous magnetization measurements [J. Phys. Soc. Jpn. 63, 2706 (1994)].

Iron nitride and carbide nanoparticles were synthesized using iron oxide particles as template. They were furthermore dispersed in aqueous solution via stabilization with a poly(ionic liquid). They provide a great potential combining a high saturation magnetization with low toxicity compared to the iron based compounds that are currently used in several applications such as cell-sorting and hyperthermia or as contrast enhancers for magnetic resonance imaging. We here present a sustainable and green procedure to synthesize iron nitride and carbide by resorting to the variety of iron oxide template nanoparticles. In this way the shape and the size can be precisely controlled and tuned within the nanometer range. During calcination, urea enables to control the composition of the product material, whereas a biopolymer agar protects the particles from agglomeration. We dispersed the particles in water by using poly(1-ethyl-3-vinylimidazolium bromide) as stabilizing agent. Magnetic measurements of the converted...

Dropping a Dimension? In most magnetic materials, the exchange interaction causes the spins on the neighboring sites of a crystal lattice to align. In the absence of exchange interactions, dipolar interactions, which are highly orientation dependent, are also expected to be able to cause magnetism. Kraemer et al. (p. 1416 ) present evidence for antiferromagnetism in a dipolar-coupled material, LiErF 4 . Although a three-dimensional system, its critical behavior was more reminiscent of a two-dimensional material.

A resolution model, which takes into account a violation of the spin-echo conditions for inelastic scattering as it is appropriate for high resolution spin-echo measurements of mode multiplets, is experimentally tested. Phase-sensitive measurements were performed while a dispersion surface is moved through the resolution ellipsoid of a triple-axis background spectrometer. The results are found to be in agreement with the model predictions. A tunable, artificially split dispersion was realized with a double crystal setup mounted on a piezoelectric device allowing for rotation and tilt of the first crystal, while the second one was rigidly fixed. Elastic measurements with the instrument configured in Larmor diffraction geometry were used to probe the crystal orientation with high sensitivity. First inelastic measurements are in agreement with a simplified model indicating persistence of the echo modulation over the entire spin-echo time range probed by the experiment.

The thermal properties of single crystalline Dy2Ti2O7 have been studied in a temperature range from 0.3 K to 30 K and magnetic fields applied along [110] direction up to 1.5 T. Based on a thermodynamic field theory various heat relaxation and thermal transport measurements were analysed. So we were able to present not only the heat capacity of Dy2Ti2O7 in the whole temperature and magnetic field range, but also the different contributions of the magnetic excitations and their corresponding relaxation times in the spin ice phase. In addition, the thermal conductivity and the shortest relaxation time were determined by thermodynamic analysis of steady state heat transport measurements. Finally, we were able to reproduce the temperature profiles recorded in heat pulse experiments on the basis of the thermodynamic field theory using the previously determined heat capacity and thermal conductivity data without additional parameters. Thus, the thermodynamic field theory has been proved to be thermodynamically consistent in describing three thermal transport experiments on different time scales. The observed temperature and field dependencies of heat capacity contributions and relaxation times indicate the magnetic excitations in the spin ice material Dy2Ti2O7 as thermally activated monopole-antimonopole defects.

In a ferroelectric helimagnetic phase of a spin driven multiferroic, CuFeO2, we find irreversibly additive evolution of electric polarization P induced by sweeping magnetic field H under an applied electric field E, despite a large coercive electric field in the phase. From the unpolarized neutron diffraction experiments with in situ P measurements under applied E, we reveal that increment of P is achieved by the variation of an incommensurate magnetic modulation wave number q of the helical magnetic ordering in H sweeping regardless of increasing or decreasing H. Combining this result with the H dependence of the magnetic diffraction intensity and a result of off bench P measurements, we conclude that the H evolution of P is caused by a change in a ferroelectric helicity domain volume fraction by driving the helicity domain wall DW . Taking into account the results of further detailed P measurements, we provide a speculation for microscopic helicity DW motion. The present study demonstrates the magnetoelectric cross correlation in driving a multiferroic DW we can activate the frozen ferroelectric DWbymeans of H sweeping. This is also an achievement of driving an antiferromagnetic DW, which is difficult in conventional antiferromagnets in principle

We determine the phase diagram of copper nitrate Cu(NO$_3$)$_2\cdot$2.5D$_2$O in the context of quantum phase transitions and novel states of matter. We establish this compound as an ideal candidate to study quasi-1D Luttinger liquids, 3D Bose-Einstein-Condensation of triplons, and the crossover between 1D and 3D physics. Magnetocaloric effect, magnetization, and neutron scattering data provide clear evidence for transitions into a Luttinger liquid regime and a 3D long-range ordered phase as function of field and temperature. Theoretical simulations of this model material allow us to fully establish the phase diagram and to discuss it in the context of dimerized spin systems., Comment: 5 pages, 4 figures

A collaboration of authors from Switzerland, Estonia, and Germany present the results of neutron diffraction experiments, AC magnetometry and specific heat measurements for an organic quantum magnet NiCl${}_{2}\ensuremath{\cdot}$4SC(NH${}_{2}$)${}_{2}$ (often abbreviated as DTN).For more than a decade, DTN has been considered as a prominent candidate for BEC realization in quantum magnets. However, the presented results suggest that previous interpretations of data were strongly affected by distribution of transition magnetic fields in the samples, and that the critical exponents describing the transition are incompatible with the BEC universality class.

Magnetic oxide CuFeO 2 is a magnetoelectric multiferroic with new type of spin–polarization coupling different from that in the spin-current mechanism, where magnetic field-induced or nonmagnetic impurity-induced proper helical magnetic ordering generates a spontaneous electric polarization parallel to the helical axis. Using a CuFe 1− x Ga x O 2 sample with x =0.035, in which the single ferroelectric phase is realized below T N ∼8 K in zero magnetic field unlike CuFe 1− x Al x O 2 , we have performed pyroelectric current, thermally stimulated current (TSC) and polarized neutron diffraction measurements to study a memory effect that the electric polarization is partially preserved even for 2nd-cooling from above T N without poling electric field. It was found that the charge trapped during 1st-cooling with poling electric field, which is released as TSC on heating, plays an important role in the memory effect.

In 1998, prompted by the pioneering efforts of Prof Michael Meiβner, sample environment teams from different facilities started a series of biennial international workshops with the aim to discuss ...

We present a comprehensive macroscopic thermodynamic study of the quasi-one-dimensional (1D) $s = \tfrac{1}{2}$ frustrated spin-chain system linarite. Susceptibility, magnetization, specific heat, magnetocaloric effect, magnetostriction, and thermal-expansion measurements were performed to characterize the magnetic phase diagram. In particular, for magnetic fields along the b axis five different magnetic regions have been detected, some of them exhibiting short-range-order effects. The experimental magnetic entropy and magnetization are compared to a theoretical modelling of these quantities using DMRG and TMRG approaches. Within the framework of a purely 1D isotropic model Hamiltonian, only a qualitative agreement between theory and the experimental data can be achieved. Instead, it is demonstrated that a significant symmetric anisotropic exchange of about 10% is necessary to account for the basic experimental observations, including the 3D saturation field, and which in turn might stabilize a triatic (three-magnon) multipolar phase.

We study the strongly anisotropic quasi-one-dimensional S = 1 quantum magnet NiCl2·4SC(NH2)2 using elastic and inelastic neutron scattering. We demonstrate that a magnetic field splits the excited doublet state and drives the lower doublet state to zero energy at a critical field Hc1. For Hc1HHc2, where Hc2 indicates the transition to a fully magnetized state, three-dimensional magnetic order is established with the AF moment perpendicular to the magnetic field. We mapped the temperature/magnetic field phase diagram, and we find that the total ordered magnetic moment reaches m(tot) = 2.1 μB at the field μ(0)H = 6 T and is thus close to the saturation value of the fully ordered moment. We study the magnetic spin dynamics in the fully magnetized state for HHc2, and we demonstrate the presence of an AF interaction between Ni(2+) on the two interpenetrating sublattices. In the antiferromagnetically ordered phase, the spin-waves that develop from the lower-energy doublet are split into two modes. This is most likely the result of the presence of the AF interaction between the interpenetrating lattices.

have been identi ed as bulktransitions. As well, the muon precession frequency doessuggest the presence of a fairly large ordered magneticmoment in both phases. On the other hand, in neutronpowder di raction no additional intensity from scatteringin the magnetically ordered phase has been observed. Asyet, this failure to detect magnetic intensity in neutronscattering is not understood.Given that CePt

The complex interplay between the $3d$ and $4f$ moments in hexagonal ErMnO${}_{3}$ is investigated by magnetization, optical second harmonic generation, and neutron-diffraction measurements. We revise the phase diagram and provide a microscopic model for the emergent spin structures, with a special focus on the intermediary phase transitions. Our measurements reveal that the $3d$ exchange between Mn${}^{3+}$ ions dominates the magnetic symmetry at $10\phantom{\rule{4.pt}{0ex}}\text{K}lTl{T}_{\mathrm{N}}$ with Mn${}^{3+}$ order according to the ${\ensuremath{\Gamma}}_{4}$ representation triggering $4f$ ordering according to the same representation on the Er${}^{3+}$(4b) site. Below 10 K the magnetic order is governed by $4f$ exchange interactions of Er${}^{3+}$ ions on the 2a site. The magnetic Er${}^{3+}(2a)$ order according to the representation ${\ensuremath{\Gamma}}_{2}$ induces a magnetic reorientation (${\ensuremath{\Gamma}}_{4}\ensuremath{\rightarrow}{\ensuremath{\Gamma}}_{2}$) at the Er${}^{3+}(4b)$ and the Mn${}^{3+}$ sites. Our findings highlight the fundamentally different roles that Mn${}^{3+}$, ${R}^{3+}$(2a), and ${R}^{3+}$(4b) magnetism play in establishing the magnetic phase diagram of the hexagonal RMnO${}_{3}$ system.

Iron nitride and carbide nanoparticles are obtained from an aqueous slurry of iron oxide particles, urea, and agar (N2 atmosphere, 800 °C, 1 h).

In this paper we investigate the complex transformation of the magnetoelectric properties from the collinear A-type phase to the multiferroic noncollinear spin cycloid in the solid solution Nd${}_{1\ensuremath{-}x}$Y${}_{x}$MnO${}_{3}$ from compositions of $x=0.3$ to 0.55 in single crystal samples. Neutron diffraction experiments at the structural phase transition ($x=0.35$ to 0.45) reveal the coexistence of the A-type order and several incommensurate orders, varying with composition and temperature.

Magnetism has been predicted to occur in systems in which dipolar interactions dominate exchange. We present neutron scattering, specific heat, and magnetic susceptibility data for LiErF(4), establishing it as a model dipolar-coupled antiferromagnet with planar spin-anisotropy and a quantum phase transition in applied field H(c|| = 4.0 ± 0.1 kilo-oersteds. We discovered non-mean-field critical scaling for the classical phase transition at the antiferromagnetic transition temperature that is consistent with the two-dimensional XY/h(4) universality class; in accord with this, the quantum phase transition at H(c) exhibits three-dimensional classical behavior. The effective dimensional reduction may be a consequence of the intrinsic frustrated nature of the dipolar interaction, which strengthens the role of fluctuations.

We report the discovery of ferroelectricity below 4.5 K in highly underdoped La2CuO4+x accompanied by slow charge dynamics which develop below T similar to 40 K. An anisotropic magnetoelectric response has also been observed, indicating considerable spin-charge coupling in this lightly doped parent high-temperature copper-oxide superconductor. The ferroelectric state is proposed to develop from polar nanoregions, in which spatial inversion symmetry is locally broken due to nonstoichiometric carrier doping.

We present a combined neutron diffraction and bulk thermodynamic study of the natural mineral linarite ${\mathrm{PbCuSO}}_{4}(\mathrm{OH}{)}_{2}$, this way establishing the nature of the ground-state magnetic order. An incommensurate magnetic ordering with a propagation vector $\mathbit{k}=(0,0.186,\frac{1}{2})$ was found below ${T}_{N}=2.8\text{ }\text{ }\mathrm{K}$ in a zero magnetic field. The analysis of the neutron diffraction data yields an elliptical helical structure, where one component ($0.638{\ensuremath{\mu}}_{B}$) is in the monoclinic $ac$ plane forming an angle with the $a$ axis of 27(2)\ifmmode^\circ\else\textdegree\fi{}, while the other component ($0.833{\ensuremath{\mu}}_{B}$) points along the $b$ axis. From a detailed thermodynamic study of bulk linarite in magnetic fields up to 12 T, applied along the chain direction, a very rich magnetic phase diagram is established, with multiple field-induced phases, and possibly short-range-order effects occurring in high fields. Our data establish linarite as a model compound of the frustrated one-dimensional spin chain, with ferromagnetic nearest-neighbor and antiferromagnetic next-nearest-neighbor interactions. Long-range magnetic order is brought about by interchain coupling 1 order of magnitude smaller than the intrachain coupling.

We have grown several single crystals of the highly frustrated $S$ = 1 spinel MgV${}_{2}$O${}_{4}$ using different starting compositions and growth conditions. From our study of their physical properties by magnetic susceptibility, heat capacity, and single-crystal neutron and powder-diffraction measurements, we observe that a minute amount of disorder suppresses the structural and magnetic phase transitions. As little as 3$%$ disorder in the octahedral site introduced random strain, which was enough to completely suppress the transitions and induce a spin-glass phase at low temperatures. We believe that the reason is spin-exchange disorder rather than disorder in the weakly diluted magnetic lattice. Our results also show that the MgO-V${}_{2}$O${}_{3}$ system is a solid solution that melts slightly incongruently, and we demonstrate that by using an optimized solvent in the traveling-solvent floating-zone configuration a large single crystal ($l$ = 25 mm, $d$ = 6 mm) with a homogeneous composition, free of site disorders can be grown.

Dieser Beitrag ist mit Zustimmung des Rechteinhabers aufgrund einer (DFG geförderten) Allianz- bzw. Nationallizenz frei zugänglich. This publication is with permission of the rights owner freely accessible due to an Alliance licence and a national licence (funded by the DFG, German Research Foundation) respectively. Microemulsions with magnetic properties were formed by employing a magnetic room temperature ionic liquid (MRTIL) as polar phase, cyclohexane as oil, and an appropriate mixture of ionic surfactant and decanol as a cosurfactant. By means of small-angle neutron scattering (SANS) and electric conductivity the microemulsion structure could be confirmed, where the classical structural sequence of oil-continuous–bicontinuous–polar phase continuous is observed with increasing ratio [polar phase]/[oil]. Accordingly a maximum of the structural size is observed at about equal volumes of oil and MRTIL contained. Therefore this system is structurally the same as normal microemulsions but with the magnetic properties added to it by the incorporation into the systems formulation.

{\alpha}-CaCr2O4 is a distorted triangular antiferromagnet. The magnetic Cr3+ ions which have spin-3/2 and interact with their nearest neighbors via Heisenberg direct exchange interactions, develop long-range magnetic order below T_N=42.6 K. Powder and single-crystal neutron diffraction reveal a helical magnetic structure with ordering wavevector k=(0,~1/3,0) and angles close to 120{\deg} between neighboring spins. Spherical neutron polarimetry unambiguously proves that the spins lie in the ac plane perpendicular to k. The magnetic structure is therefore that expected for an ideal triangular antiferromagnet where all nearest neighbor interactions are equal, in spite of the fact that {\alpha}-CaCr2O4 is distorted with two inequivalent Cr3+ ions and four different nearest neighbor interactions. By simulating the magnetic order as a function of these four interactions it is found that the special pattern of interactions in {\alpha}-CaCr2O4 stabilizes 120{\deg} helical order for a large range of exchange interactions.

The control of domains in ferroic devices lies at the heart of their potential for technological applications. Multiferroic materials offer another level of complexity as domains can be either or both of a ferroelectric and magnetic nature. Here we report the discovery of a novel magnetic state in the orthoferrite TbFeO3 using neutron diffraction under an applied magnetic field. This state has a very long incommensurate period ranging from 340 Angstrom at 3K to 2700 Angstrom at the lowest temperatures and exhibits an anomalously large number of higher-order harmonics, allowing us to identify it with the periodic array of sharp domain walls of Tb spins separated by many lattice constants. The Tb domain walls interact by exchanging spin waves propagating through the Fe magnetic sublattice. The resulting Yukawa-like force, familiar from particle physics, has a finite range that determines the period of the incommensurate state., Comment: 11 pages 14 figures

Quantum phase transitions take place between distinct phases of matter at zero temperature. Near the transition point, exotic quantum symmetries can emerge that govern the excitation spectrum of the system. A symmetry described by the E8 Lie group with a spectrum of 8 particles was long predicted to appear near the critical point of an Ising chain. We realize this system experimentally by tuning the quasi-one-dimensional Ising ferromagnet CoNb2O6 through its critical point using strong transverse magnetic fields. The spin excitations are observed to change character from pairs of kinks in the ordered phase to spin-flips in the paramagnetic phase. Just below the critical field, the spin dynamics shows a fine structure with two sharp modes at low energies, in a ratio that approaches the golden mean as predicted for the first two meson particles of the E8 spectrum. Our results demonstrate the power of symmetry to describe complex quantum behaviours., 18 pages with embedded figures

The apparently inimical relationship between magnetism and superconductivity has come under increasing scrutiny in a wide range of material classes, where the free energy landscape conspires to bring them in close proximity to each other. This is particularly the case when these phases microscopically interpenetrate, though the manner in which this can be accomplished remains to be fully comprehended. Here, we present combined measurements of elastic neutron scattering, magnetotransport, and heat capacity on a prototypical heavy fermion system, in which antiferromagnetism and superconductivity are observed. Monitoring the response of these states to the presence of the other, as well as to external thermal and magnetic perturbations, points to the possibility that they emerge from different parts of the Fermi surface. This enables a single 4$f$ state to be both localized and itinerant, thus accounting for the coexistence of magnetism and superconductivity., Comment: 4 pages, 4 figures

Spin dynamics of the square lattice Heisenberg antiferromagnet, \BaMnGeO, is studied by a combination of bulk measurements, neutron diffraction, and inelastic neutron scattering techniques. Easy plane type antiferromagnetic order is identified at $T \le 4.0$ K. The exchange interactions are estimated as $J_1$ = 27.8(3)${\mu}$eV and $J_2$ = 1.0(1) ${\mu}$eV, and the saturation field $H_{\rm C}$ is 9.75 T. Magnetic excitation measurements with high experimental resolution setup by triple axis neutron spectrometer reveals the instability of one magnon excitation in the field range of $0.7H_{\rm C} \lesssim H \lesssim 0.85H_{\rm C}$., Comment: 5 pgase, 5 figuers, to be published in PRB RC

${\text{Sr}}_{3}{\text{Cr}}_{2}{\text{O}}_{8}$ consist of a lattice of spin-1/2 ${\text{Cr}}^{5+}$ ions, which form hexagonal bilayers and which are paired into dimers by the dominant antiferromagnetic intrabilayer coupling. The dimers are coupled three dimensionally by frustrated interdimer interactions. A structural distortion from hexagonal to monoclinic leads to orbital order and lifts the frustration giving rise to spatially anisotropic exchange interactions. We have grown large single crystals of ${\text{Sr}}_{3}{\text{Cr}}_{2}{\text{O}}_{8}$ and have performed DC susceptibility, high-field magnetization and inelastic neutron scattering measurements. The neutron scattering experiments reveal three gapped and dispersive singlet to triplet modes arising from the three twinned domains that form below the transition thus confirming the picture of orbital ordering. The exchange interactions are extracted by comparing the data to a random phase approximation model and the dimer coupling is found to be ${J}_{0}=5.551(9)\text{ }\text{meV}$, while the ratio of interdimer to intradimer exchange constants is ${J}^{\ensuremath{'}}/{J}_{0}=0.64(2)$. The results are compared to those for other gapped magnets.

We study excitations in weakly interacting pairs of quantum spin ladders coupled through geometrically frustrated bonds. The ground state is a disordered spin liquid, that at high fields is replaced by an ordered chiral helimagnetic phase. The spectra observed by high-field inelastic neutron scattering experiments on the prototype compound Sul-Cu2Cl4 are qualitatively different from those in the previously studied frustration-free spin liquids. Beyond the critical field Hc=3.7 T, the soft mode that drives the quantum phase transition spawns two separate excitations: a gapless Goldstone mode and a massive magnon. Additional massive quasiparticles are clearly visible below Hc, but are destroyed in the ordered phase. In their place one observes a sharply bound excitation continuum., Comment: 6 pages, 4 figures

While sources of magnetic fields - magnetic monopoles - have so far proven elusive as elementary particles, several scenarios have been proposed recently in condensed matter physics of emergent quasiparticles resembling monopoles. A particularly simple proposition pertains to spin ice on the highly frustrated pyrochlore lattice. The spin ice state is argued to be well-described by networks of aligned dipoles resembling solenoidal tubes - classical, and observable, versions of a Dirac string. Where these tubes end, the resulting defect looks like a magnetic monopole. We demonstrate, by diffuse neutron scattering, the presence of such strings in the spin-ice Dy2Ti2O7. This is achieved by applying a symmetry-breaking magnetic field with which we can manipulate density and orientation of the strings. In turn, heat capacity is described by a gas of magnetic monopoles interacting via a magnetic Coulomb interaction., Comment: 32 pages (19 pages of article, 13 pages of supporting online material)

We refined the magnetic structure of a ferroelectric (FE) phase of multiferroic ${\text{CuFe}}_{1\ensuremath{-}x}{\text{Ga}}_{x}{\text{O}}_{2}$ with $x=0.035$ by complementary use of spherical neutron polarimetry and a four-circle neutron-diffraction measurement, revealing that the proper-screw-type magnetic structure in the ferroelectric phase has a finite ellipticity of $\ensuremath{\sim}0.9$. By means of polarized neutron-diffraction and in-situ pyroelectric measurements, we also investigated the quantitative relationship between the macroscopic ferroelectric polarization $(P)$ and the asymmetry in volume fractions with left-handed and right-handed helical magnetic order in ${\text{CuFe}}_{1\ensuremath{-}x}{\text{Al}}_{x}{\text{O}}_{2}$ with $x=0.0155$ and ${\text{CuFe}}_{1\ensuremath{-}x}{\text{Ga}}_{x}{\text{O}}_{2}$ with $x=0.035$. These measurements revealed that the substitution of a small amount of nonmagnetic ${\text{Ga}}^{3+}$ or ${\text{Al}}^{3+}$ ions does not significantly change the magnitude of the local ferroelectric polarization but does reduce the sensitivity of $P$ to the poling electric field $({E}_{p})$. This implies that the mobility of the magnetic domain walls, which is sensitive to magnetic defects due to nonmagnetic substitution, determines the sensitivity of $P$ to ${E}_{p}$ because of a one-to-one correspondence between the magnetic and ferroelectric domains.

We measure by inelastic neutron scattering the spin excitation spectra as a function of applied magnetic field in the quantum spin-ladder material (C5H12N)2CuBr4. Discrete magnon modes at low fields in the quantum disordered phase and at high fields in the saturated phase contrast sharply with a spinon continuum at intermediate fields characteristic of the Luttinger-liquid phase. By tuning the magnetic field, we drive the fractionalization of magnons into spinons and, in this deconfined regime, observe both commensurate and incommensurate continua., Comment: 4 pages, 4 figures

Neutron diffraction is used to investigate the field-induced, antiferromagnetically ordered state in the two-leg spin-ladder material (Hpip)2CuBr4. This “classical” phase, a consequence of weak interladder coupling, is nevertheless highly unconventional: its properties are influenced strongly by the spin Luttinger-liquid state of the ladder subunits. We determine directly the order parameter (transverse magnetization), the ordering temperature, the spin structure, and the critical exponents around the transition. We introduce a minimal microscopic model for the interladder coupling and calculate the quantum fluctuation corrections to the mean-field interaction.

Ga for Mn substitution in multiferroic TbMnO$_{3}$ has been performed in order to study the influence of Mn-magnetic ordering on the Tb-magnetic sublattice. Complete characterization of TbMn$_{1-x}$Ga$_x$O$_{3}$ ($x$ = 0, 0.04, 0.1) samples, including magnetization, impedance spectroscopy, and x-ray resonant scattering and neutron diffraction on powder and single crystals has been carried out. We found that keeping the same crystal structure for all compositions, Ga for Mn substitution leads to the linear decrease of $T_{\rm N}^{\rm Mn}$ and $\tau^{\rm Mn}$, reflecting the reduction of the exchange interactions strength $J_{\rm Mn-Mn}$ and the change of the Mn-O-Mn bond angles. At the same time, a strong suppression of both the induced and the separate Tb-magnetic ordering has been observed. This behavior unambiguously prove that the exchange fields $J_{\rm Mn-Tb}$ have a strong influence on the Tb-magnetic ordering in the full temperature range below $T_{\rm N}^{\rm Mn}$ and actually stabilize the Tb-magnetic ground state., Comment: 9 pages, 8 figures