Control of multiferroic domains by external electric fields in TbMnO3

The control of multiferroic domains through external electric fields has been studied by dielectric measurements and by polarized neutron diffraction on single-crystalline TbMnO$_3$. Full hysteresis cycles were recorded by varying an external field of the order of several kV/mm and by recording the chiral magnetic scattering as well as the charge in a sample capacitor. Both methods yield comparable coercive fields that increase upon cooling.Comment: 12 pages, 6 figure

Moderator-Reflector Assembly for HBS

In a compact accelerator driven neutron source, the design of the moderator/reflector system is crucial to convert the fast neutrons produced at the compact target into a thermal or cold neutron cloud that can be extracted with a high brilliance. A quick moderation together with a small diffusion length in the thermal moderator is suitable to confine the thermal neutrons in a small space, yielding a high phase space density for extraction into a small space, e.g. to feed a neutron guide or a 1-dimensional cold neutron source. On the other hand, a strong confinement of the neutron cloud inside the thermal moderator has an influence to the neutron pulse length, which is important for the resolution of time-of-flight instruments. Increased absorption or leakage may result into shorter pulses, while longer pulses intended to accumulate more thermal neutrons in a larger volume can be achieved by diluting the hydrogen rich thermal moderator material. Therefore, the moderator/reflector system has to be adapted to the desired pulse length and will be different at the different target stations of the HBS facility to serve the proper resolution for different instrument types

Combined Arrhenius-Merz Law Describing Domain Relaxation in Type-II Multiferroics

Electric fields were applied to multiferroic TbMnO3 single crystals to control the chiral domains, and the domain relaxation was studied over 8 decades in time by means of polarized neutron scattering. A surprisingly simple combination of an activation law and the Merz law describes the relaxation times in a wide range of electric field and temperature with just two parameters, an activation-field constant and a characteristic time representing the fastest possible inversion. Over the large part of field and temperature values corresponding to almost 6 orders of magnitude in time, multiferroic domain inversion is thus dominated by a single process, the domain wall motion. Only when approaching the multiferroic transition other mechanisms yield an accelerated inversion

Ortho-Parahydrogen Mixer, Catalysts, Measurement Devices and their Application

For the majority of low temperature hydrogen applications the ortho- to parahydrogen (o-p-H2) spin configuration plays a vital role. Examples are liquid storage (heat of conversion) or neutron moderation (unequal cross section). Decreasing temperatures below ambient imply higher equilibrium p-H2 ratios. Often, rapid conversion close to the equilibrium is beneficial. The rather slow natural conversion can be sped up for example by catalytic contact to paramagnetic substances. For this, the paper gives an overview on published isothermal conversion catalyst data at LN2 temperature. Furthermore, an enhanced Catalyst Test Cryostat (CTC) facilitating new adiabatic and isothermal o-p-H2 catalyst conversion measurements is presented. Precise concentration determination is decisive. Hence, two commercial analyzers, essential for isothermal conversion measurements, using speed of sound or thermal conductivity will be compared to the performance of the independent adiabatic CTC results. Obtained activity data on an industrial catalyst at ≈77.3 K will be provided. Finally, a novel process, batch and closed cycle, providing mixtures with p-H2 contents ranging from 0.25 to ≈1.00 is introduced. The latter enables e.g. neutron moderation at stable, non-equilibrium o-p-H2 ratios

Cryostat for the provision of liquid hydrogen with variable ortho-para-ratio for a low-dimensional cold neutron moderator

A significant contribution to the enhancement of the neutron brilliance achievable with Compact Accelerator-driven Neutron Sources (CANS) can be made by an optimized cold moderator design. When using liquid para-H2 as the moderating medium, the concept of low-dimensional cold moderators can be employed to increase the neutron brightness (as currently foreseen at the European Spallation Source ESS). Para-H2 shows a drop in the scattering cross section by about one order of magnitude around 15 meV, resulting in large deviation between the mean free paths of thermal and cold neutrons. Taking advantage of this effect, the cold moderator geometry can be optimized to allow the intake of thermal neutrons through a relatively large envelope surface and then extracting them in an efficient way towards the neutron guides. One drawback of this solution is the lack of thermalization of the cold neutrons. In the context of the HBS (High Brilliance Source) project, efforts are made to overcome this problem by increasing the scattering cross section of the H2 in a defined way. The idea is to admix small amounts of ortho-H2, which maintains its large scattering cross section in the region below 15 meV. Like this, the neutron spectrum can be shifted towards lower energies and adjusted for the needs of the respective instrument. In a cooperation between TU Dresden and FZ Jülich, an experimental setup has been created to proof the feasibility of this concept. The main component of the experimental setup is a LHe-cooled flow cryostat that enables the separate condensation of a para-H2 and a normal-H2 flow and a subsequent mixing of the two in precise proportions. The resulting LH2 mixture at 17 – 20 K is fed into a small cold moderator vessel (approx. 200 ml). In this work, the current status of the setup is presented. The construction and commissioning of the mixing cryostat have been completed and extensive test runs show that different, stable ortho-para-H2 mixtures can be produced and monitored. Currently, preparations for first measurements of the resulting neutron spectra are being prepared at Forschungszentrum Jülich