Updating the phase diagram of the archetypal frustrated magnet Gd3Ga5O12

The applied magnetic field and temperature phase diagram of the archetypal frustrated magnet, Gd3Ga5O12, has been reinvestigated using single crystal magnetometry and polarised neutron diffraction. The updated phase diagram is substantially more complicated than previously reported and can be understood in terms of competing interactions with loops of spins, trimers and decagons, in addition to competition and interplay between antiferromagnetic, incommensurate and ferromagnetic order. Several additional distinct phase boundaries are presented. The phase diagram centers around a multiphase convergence to a single point at 0.9 T and ~ 0.35 K, below which, in temperature, a very narrow magnetically disordered region exists. These data illustrate the richness and diversity that arises from frustrated exchange on the 3 dimensional hyperkagome lattice

Contact Changes of Sheared Systems: Scaling, Correlations, and Mechanisms

We probe the onset and effect of contact changes in 2D soft harmonic particle packings which are sheared quasistatically under controlled strain. First, we show that in the majority of cases, the first contact changes correspond to the creation or breaking of contacts on a single particle, with contact breaking overwhelmingly likely for low pressures and/or small systems, and contact making and breaking equally likely for large pressures and in the thermodynamic limit. The statistics of the corresponding strains are near-Poissonian. The mean characteristic strains exhibit scaling with the number of particles N and pressure P, and reveal the existence of finite size effects akin to those seen for linear response quantities. Second, we show that linear response accurately predicts the strains of the first contact changes, which allows us to study the scaling of the characteristic strains of making and breaking contacts separately. Both of these show finite size scaling, and we formulate scaling arguments that are consistent with the observed behavior. Third, we probe the effect of the first contact change on the shear modulus G, and show in detail how the variation of G remains smooth and bounded in the large system size limit: even though contact changes occur then at vanishingly small strains, their cumulative effect, even at a fixed value of the strain, are limited, so that effectively, linear response remains well-defined. Fourth, we explore multiple contact changes under shear, and find strong and surprising correlations between alternating making and breaking events. Fifth, we show that by making a link with extremal statistics, our data is consistent with a very slow crossover to self averaging with system size, so that the thermodynamic limit is reached much more slowly than expected based on finite size scaling of elastic quantities or contact breaking strains

Dispersionless spin waves and underlying field-induced magnetic order in gadolinium gallium garnet

We report the results of neutron diffraction and inelastic neutron scattering on a powder sample of Gd3Ga5O12 at high magnetic fields. Analysis of the diffraction data shows that in high fields (B ≳ 1.8 T) the spins are not fully aligned, but are canted slightly as a result of the dipolar interaction. The magnetic phase for fields ≲1.8 T is characterized by antiferromagnetic peaks at (210) and an incommensurate wave vector. The dominant contribution to inelastic scattering at large momentum transfers is from a band of almost dispersionless excitations. We show that these correspond to the spin waves localized on ten site rings, expected on the basis of nearest neighbor exchange interaction, and that the spectrum at high fields B ≳ 1.8 T is well described by a spin wave theory

Supercritical fluid coating of API on excipient enhances drug release

A process to coat particles of active pharmaceutical ingredient (API) onto microcrystalline cellulose (MCC) excipient shows promise as a new way to dosage forms showing enhanced drug release. The process consists of a fluidized bed operated at elevated pressure in which API particles are precipitated from a Supercritical Anti-Solvent process (SAS). MCC particles were used as an excipient in the fluidized bed and collect the SAS-generated API particles. Naringin was selected as the model API to coat onto MCC. A number of operational parameters of the process were investigated: fluidization velocity, coating pressure, temperature, concentration of drug solution, drug solution flow rate, drug mass, organic solvent, MCC mass and size and CO2-to-organic solution ratio. SEM and SPM analyses showed that the MCC particle surfaces were covered with near-spherical nanoparticles with a diameter of approximately 100–200 nm, substantially smaller than the as-received API material. XRD showed that naringin changed from crystalline to amorphous during processing. The coated particles resulting from the SAS fluidized bed process have a higher loading of API, gave faster release rates and higher release ratios in comparison with those produced using a conventional fluidized bed coating process. The approach could be transferred to other industries where release is important such as agrochemical, cosmetic and food

Detailed investigation of granulation processes using a fibre-optical probe and discrete element simulations

Spout fluidized beds are frequently used for the production of granules or particles through granulation, which are widely applied for example in the production of detergents, pharmaceuticals, food and fertilizers. Spout fluidized beds have a number of advantageous properties, such as high mobility of the particles preventing undesired agglomeration and enabling excellent heat transfer control. The particle growth mechanism in a spout fluidized bed as function of the particle-droplet interaction has a profound influence on the particle morphology and thus on the product quality. Nevertheless, little is known about the details of the granulation process. This is mainly due to the fact that it is not visually accessible. In this work we use fundamental, deterministic models to enable the detailed investigation of granulation behavior in a spout fluidized bed

Pair Correlations, Short Range Order and Dispersive Excitations in the Quasi-Kagome Quantum Magnet Volborthite

We present spatial and dynamic information on the s=1/2 distorted kagome antiferromagnet volborthite, Cu3V2O7(OD)2.2D2O, obtained by polarized and inelastic neutron scattering. The instantaneous structure factor, S(Q), is dominated by nearest neighbor pair correlations, with short range order at wave vectors Q1=0.65(3) {\AA}^-1 and Q2=1.15(5) {\AA}^-1 emerging below 5 K. The excitation spectrum, S(Q,{\omega}), reveals two steep branches dispersing from Q1 and Q2, and a flat mode at {\omega}=5.0(2) meV. The results allow us to identify the cross-over at T*=1 K in 51V NMR and specific heat measurements as the build-up of correlations at Q_1. We compare our data to theoretical models proposed for volborthite, and demonstrate that the excitation spectrum can be explained by spin-wave-like excitations with anisotropic exchange parameters, as also suggested by recent local density calculations.Comment: Rewritten article resubmitted to Phys. Rev. Lett. 021

Contact Changes near Jamming

We probe the onset and effect of contact changes in soft harmonic particle packings which are sheared quasistatically. We find that the first contact changes are the creation or breaking of contacts on a single particle. We characterize the critical strain, statistics of breaking versus making a contact, and ratio of shear modulus before and after such events, and explain their finite size scaling relations. For large systems at finite pressure, the critical strain vanishes but the ratio of shear modulus before and after a contact change approaches one: linear response remains relevant in large systems. For finite systems close to jamming the critical strain also vanishes, but here linear response already breaks down after a single contact change.Comment: 5 pages, 4 figure