G. Brandl

Turn-key module for neutron scattering with sub-micro-eV resolution

We report the development of a compact turn-key module that boosts the resolution in quasielastic neutron scattering by several orders of magnitude down to the low sub-micro-eV range. It is based on a pair of neutron resonance spin flippers that generate a well defined temporal intensity modulation, also known as Modulation of IntEnsity by Zero Effort (MIEZE). The module may be used under versatile conditions, in particular, in applied magnetic fields and for depolarizing and incoherently scattering samples. We demonstrate the power of MIEZE in studies of the helimagnetic order in MnSi under applied magnetic fields.

Large Scales - Long Times: Adding High Energy Resolution to SANS

AbstractThe Neutron Spin Echo (NSE) variant MIEZE (Modulation of IntEnsity by Zero Effort), where all beam manipulations are per-formed before the sample position, offers the possibility to perform low backgroundSANS measurements in strong magnetic fieldsand depolarising samples. However, MIEZE is sensitive to differences ∆L in the length of neutron flight paths through the instru-ment and the sample. In this article, we discuss the major infl uence of ∆L on contrast reduction of MIEZE measurements and itsminimisation. Finally we present a design case for enhancing a small-angle neutron scattering (SANS) instrument at theplannedEuropean Spallation Source (ESS) in Lund, Sweden, using a combination of MIEZE and other TOF options, such as TISANEoffering time windows from ns to minutes. The proposed instrument would allow obtaining an excellent energy- andQ-resolutionstraightforward to µs for 0.01A −1 , even in magnetic fields, depolarising samples as they occur in soft matter and magnetism whilekeeping the instrumental effort and costs low.Keywords: MIEZE, NSE, Resolution function, ESS1. IntroductionThe Neutron Spin Echo technique (NSE) [1] in its differentvariants is a unique method for measuring dynamic processesin soft matter [2] and spin excitations in magnetic systems [3].As it allows for the decoupling of the incoming wavelengthdistribution and the energy resolution, typically values in theneV to µeV regime can be reached. Contrary to backscatter-ing NSE provides an excellent Q-resolution. There exist dif-ferent methods for neutron spin echo measurements, namelyclassical neutron spin echo (NSE) [1] and neutron resonancespin echo (NRSE) [4]. The application of NSE and NRSE iscurrently limited to measurements with dedicated instruments,where neither the samples nor the sample environment may de-polarise the beam.A methodsimilar toNRSE is theMIEZE(ModulationofIn-tEnsity by Zero Effort)[5] technique. As in MIEZE all spin ma-nipulations are performed before the sample position it avoidsthe complications operating with polarised neutrons in strongmagnetic fields or in a depolarising environment, like hydro gencontaining samples, which introduce spin-flips. It can easilyoperate in any neutronscattering experimentwith enoughspacebefore the sample and a detector with nanosecond time resolu-tion. This method invented by Gahler and Golub [5] has alsobeen demonstratedexperimentallyin the nineties forsoft mattersamples [6, 7, 8, 9]. Recently we published MIEZE measure-ments on the itinerant magnet MnSi in a magnetic field [10]demonstrating the power of the method for magnetic applica-tions.

Monte-Carlo simulations for the optimisation of a TOF-MIEZE instrument

Abstract The MIEZE (Modulation of Intensity with Zero Effort) technique is a variant of neutron resonance spin echo (NRSE), which has proven to be a unique neutron scattering technique for measuring with high energy resolution in magnetic fields. Its limitations in terms of flight path differences have already been investigated analytically for neutron beams with vanishing divergence. In the present work Monte-Carlo simulations for quasi-elastic MIEZE experiments taking into account beam divergence as well as the sample dimensions are presented. One application of the MIEZE technique could be a dedicated NRSE-MIEZE instrument at the European Spallation Source (ESS) in Sweden. The optimisation of a particular design based on Montel mirror optics with the help of Monte Carlo simulations will be discussed here in detail.

Optical floating zone growth of high-quality Cu2MnAl single crystals

We report the growth of large single-crystals of Cu2MnAl, a ferromagnetic Heusler compound suitable for polarizing neutron monochromators, by means of optical floating zone under ultra-high vacuum compatible conditions. Unlike Bridgman or Czochralsky grown Cu2MnAl, our floating zone grown single-crystals show highly reproducible magnetic properties and an excellent crystal quality with a narrow and homogeneous mosaic spread as examined by neutron diffraction. An investigation of the polarizing properties in neutron scattering suggests a high polarization efficiency, limited by the relatively small sample dimensions studied. Our study identifies optical floating zone under ultra-high vacuum compatible conditions as a highly reproducible method to grow high-quality single-crystals of Cu2MnAl.

The multi-purpose three-axis spectrometer (TAS) MIRA at FRM II

Abstract The cold-neutron three-axis spectrometer MIRA is an instrument optimized for low-energy excitations. Its excellent intrinsic Q -resolution makes it ideal for studying incommensurate magnetic systems (elastic and inelastic). MIRA is at the forefront of using advanced neutron focusing optics such as elliptic guides, which enable the investigation of small samples under extreme conditions. Another advantage of MIRA is the modular assembly allowing for instrumental adaption to the needs of the experiment within a few hours. The development of new methods such as the spin-echo technique MIEZE is another important application at MIRA. Scientific topics include the investigation of complex inter-metallic alloys and spectroscopy on incommensurate magnetic structures.

Versatile module for experiments with focussing neutron guides

We report the development of a versatile module that permits fast and reliable use of focussing neutron guides under varying scattering angles. A simple procedure for setting up the module and neutron guides is illustrated by typical intensity patterns to highlight operational aspects as well as typical parasitic artefacts. Combining a high-precision alignment table with separate housings for the neutron guides on kinematic mounts, the change-over between neutron guides with different focussing characteristics requires no readjustments of the experimental set-up. Exploiting substantial gain factors, we demonstrate the performance of this versatile neutron scattering module in a study of the effects of uniaxial stress on the domain populations in the transverse spin density wave phase of single crystal Cr.

Compact turnkey focussing neutron guide system for inelastic scattering investigations

We demonstrate the performance of a compact neutron guide module which boosts the intensity in inelastic neutron scattering experiments by approximately a factor of 40. The module consists of two housings containing truly curved elliptic focussing guide elements, positioned before and after the sample. The advantage of the module lies in the ease with which it may be reproducibly mounted on a spectrometer within a few hours, on the same timescale as conventional sample environments. It is particularly well suited for samples with a volume of a few mm3, thus enabling the investigation of materials which to date would have been considered prohibitively small or samples exposed to extreme environments, where there are space constraints. We benchmark the excellent performance of the module by measurements of the structural and magnetic excitations in single crystals of model systems. In particular, we report the phonon dispersion in the simple element lead. We also determine the magnon dispersion in the spine...

Tailoring phase-space in neutron beam extraction

Abstract In view of the trend towards smaller samples and experiments under extreme conditions it is important to deliver small and homogeneous neutron beams to the sample area. For this purpose, elliptic and/or Montel mirrors are ideally suited as the phase space of the neutrons can be defined far away from the sample. Therefore, only the useful neutrons will arrive at the sample position leading to a very low background. We demonstrate the ease of designing neutron transport systems using simple numeric tools, which are verified using Monte-Carlo simulations that allow taking into account effects of gravity and finite beam size. It is shown that a significant part of the brilliance can be transferred from the moderator to the sample. Our results may have a serious impact on the design of instruments at spallation sources such as the European Spallation Source (ESS) in Lund, Sweden.