Neutron Resonance Spin Echo Research Articles

Correcting aberrations of a transverse-field neutron resonance spin echo instrument.

Neutron resonance spin echo (NRSE) technique has the potential to increase the Fourier time and energy resolution in neutron scattering by using radio frequency (rf) neutron spin-flippers. However, aberrations arising from variations in the neutron path length between the rf flippers reduce the polarization. Here, we develop and test a transverse static-field magnet, a series of which are placed between the rf flippers, to correct for these aberrations. The prototype correction magnet was both simulated in an NRSE beamline using McStas, a Monte Carlo neutron ray-tracing software package, and measured using neutrons. The results from the prototype demonstrate that this static-field design corrects for transverse-field NRSE aberrations.

Time-of-flight modulated intensity small-angle neutron scattering measurement of the self-diffusion constant of water

The modulated intensity by zero effort small-angle neutron scattering (MI-SANS) technique is used to measure scattering with a high energy resolution on samples normally ill-suited for neutron resonance spin echo. The self-diffusion constant of water is measured over a q–t range of 0.01–0.2 Å−1 and 70–500 ps. In addition to demonstrating the methodology of using time-of-flight MI-SANS instruments to observe diffusion in liquids, the results support previous measurements on water performed with different methods. This polarized neutron technique simultaneously measures the intermediate scattering function for a wide range of time and length scales. Two radio frequency flippers were used in a spin-echo setup with a 100 kHz frequency difference in order to create a high-resolution time measurement. The results are compared with self-diffusion measurements made by other techniques and the general applicability of MI-SANS at a pulsed source is assessed.

Tuning Neutron Resonance Spin-Echo Spectrometers with Pulsed Beams

The neutron spin-echo spectroscopy technique involving pulsed beams can be used to effectively access a wide range of space-time correlations of condensed matter. In this study, the features of this technique, in particular, the modulation of the intensity with zero effort (MIEZE) by using pulsed beams, which is based on the quantum-state manipulation of the neutron spin and energy, are comprehensively examined. A formulation of the MIEZE combined with the time of flight method (TOF MIEZE) is established by considering the characteristics of the pulsed neutron beams. Moreover, a parameter, namely, the detuning parameter, is introduced as a measure of the magnitude of detuning from the optimized instrumental state, known as the spin-echo condition. The phase and frequency shifts of the neutron intensity signals resulting from the TOF MIEZE under various configurations are investigated systematically. It is found that the detuning parameter equals the derivative of phase with respect to the TOF, whose zero-point corresponds to the spin-echo condition. The theoretical predictions on phase and frequency shifts by the established formulation are well validated by the experiments using an intense pulsed neutron source. The detuning parameter helps clarify the principle of the TOF MIEZE technique and can provide practical guidance regarding the implementation and optimization of spectrometers.

The resolution of the MIEZE setup on the longitudinal neutron resonance spin-echo spectrometer at CMRR

A longitudinal neutron resonance spin echo (L-NRSE) spectrometer combined with a MIEZE (modulated intensity with zero effort) configuration will be constructed at the C33 beam port at China Mianyang Research Reactor (CMRR) . In the MIEZE mode, the analyzer is placed in front of the sample, and between the sample and the detector there is no other optical device. The MIEZE signal is sensitive to the neutron path length differences from the sample to the detector. In this work, we have analytically studied the effects of sample geometry, detector thickness, and experimental setup on the resolution function.

Design and performance of a superconducting neutron resonance spin flipper.

Despite the challenges, neutron resonance spin echo still holds the promise to improve upon neutron spin echo for the measurement of slow dynamics in materials. We present a bootstrap, radio frequency neutron spin flipper using high temperature superconducting technology capable of flipping neutron spin with either nonadiabatic or adiabatic modes. A frequency of 2 MHz has been achieved, which would achieve an effective field integral of 0.35 T m for a meter of separation in a neutron resonance spin echo spectrometer at the current device specifications. In bootstrap mode, the self-cancellation of Larmor phase aberrations can be achieved with the appropriate selection of the polarity of the gradient coils.

Weak Crystallization of Fluctuating Skyrmion Textures in MnSi

We report an experimental study of the emergence of non-trivial topological winding and long-range order across the paramagnetic to skyrmion lattice transition in the transition metal helimagnet MnSi. Combining measurements of the susceptibility with small angle neutron scattering, neutron resonance spin echo spectroscopy and all-electrical microwave spectroscopy, we find evidence of skyrmion textures in the paramagnetic state exceeding $10^3$\AA with lifetimes above several 10$^{-9}$s. Our experimental findings establish that the paramagnetic to skyrmion lattice transition in MnSi is well-described by the Landau soft-mode mechanism of weak crystallization, originally proposed in the context of the liquid to crystal transition. As a key aspect of this theoretical model, the modulation-vectors of periodic small amplitude components of the magnetization form triangles that add to zero. In excellent agreement with our experimental findings, these triangles of the modulation-vectors entail the presence of the non-trivial topological winding of skyrmions already in the paramagnetic state of MnSi when approaching the skyrmion lattice transition.

Optimization of a superconducting adiabatic radio frequency neutron resonant spin flipper

The neutron radio frequency (RF) spin flipper is one of the key components to implement neutron resonance spin echo (NRSE) for high resolution measurements of dynamics in materials. Various types of RF flippers have been built and implemented including adiabatic and non-adiabatic RF flippers. For a pulsed neutron source, adiabatic RF flipper is more favorable due to its wide operation bandwidth. However, the optimization criteria of an adiabatic RF flipper is still lacking. Also they are difficult to be operated in bootstrap mode, which can be use to minimize the phase aberration and increase the achievable resolution. In the report by Dadisman et al. (2019), an adiabatic RF flipper has been built for the first time with superconducting technologies, which allows bootstrap mode. In this article, the method of optimizing the device and the detailed design will be discussed for each component of the device.

Longitudinal Neutron Resonance Spin Echo Spectroscopy under Large Energy Transfers

Longitudinal neutron resonance spin echo (LNRSE) spectroscopy offers very high energy resolution due to the self correction of the resonant spin flippers, and a wide dynamical range nominally exceeding six orders of magnitude in resolution down to sub-ps range. In this paper, the technical realisation how to achieve such low Fourier times at the spectrometer RESEDA at the MLZ Garching is described. The requirements of data collection and data analysis in the limit of very low Fourier times, notably those related to the breakdown of the spin echo approximation, are discussed. A method to infer the scattering function from the experimental data under large energy tansfers beyond the spin echo approximation is presented.

Current status of the neutron resonance spin echo spectrometer on BL06 “VIN ROSE” at MLF, J-PARC

Neutron spin echo technique is essential for high resolution inelastic and quasi-elastic neutron scattering experiments. We have developed a neutron resonance spin echo (NRSE) spectrometer at BL06 in Materials and Life Science Experimental Facility (MLF), Japan Proton Accelerator Research Complex (J-PARC), and installed a pair of 900 mm long ellipsoidal focusing m = 5 supermirrors with 2.5 m focal length. In this manuscript, we report the first result of time-of-flight neutron spin echo signals by the NRSE instrument.

Influence of Concentration Fluctuations on Relaxation Processes in Spin Glasses

Using the unique combination of atomically resolved atom probe tomography (APT) and volume averaged neutron (resonance) spin echo (NRSE and NSE) experiments, the influence of nano-scaled clusters on the spin relaxation in spin glasses was studied. For this purpose, the phase transition from the paramagnetic phase to the spin glass phase in an Fe-Cr spin glass with a composition of Fe 17 . 8 Cr 82 . 2 was studied in detail by means of NRSE. The microstructure was characterised by APT measurements, which show local concentration fluctuations of Fe and Cr on a length scale of 2 to 5 nm, which lead (i) to the coexistence of ferro- and anti-ferromagnetic clusters and (ii) a change of the magnetic properties of the whole sample, even in the spin glass phase, where spins are supposed to be randomly frozen. We show that a generalized spin glass relaxation function, which was successfully used to describe the phase transition in diluted spin glasses, can also be used for fitting the spin dynamics in spin glasses with significant concentration fluctuations.

Supermirror neutron guide system for neutron resonance spin echo spectrometers at a pulsed neutron source

ABSTRACTA neutron guide system for neutron resonance spin echo spectrometers has been constructed at BL06 of the Japan Proton Accelerator Research Complex, Materials and Life Science Experimental Facility. The spectrometers consist of two types of neutron spin echo instruments, a modulated intensity by zero effort instrument (MIEZE) and a neutron resonance spin echo instrument (NRSE), to cover a wide energy range for various sample environments. A neutron beam from the moderator is deflected by supermirror neutron guides, split, and separately guided into the MIEZE and NRSE. The characteristic wavelengths of the neutron guide tube for the MIEZE and NRSE are 2.9 and 4.9 Å, respectively. The cross sections of the exit of the MIEZE and NRSE guides are 15 mm × 50 mm and 30 mm × 50 mm, respectively. The neutronics and shielding design were optimized by using the heavy ion transport code system (PHITS), and the absolute average neutron fluxes at the exits of the MIEZE and NRSE guides are estimated to be 2.7 × 108 and 6.9 × 108 n/cm2/s/MW, respectively. The measured fluxes of the MIEZE and NRSE neutron guides are 0.56 and 0.95 times the calculated values, respectively.

High resolution neutron Larmor diffraction using superconducting magnetic Wollaston prisms

The neutron Larmor diffraction technique has been implemented using superconducting magnetic Wollaston prisms in both single-arm and double-arm configurations. Successful measurements of the coefficient of thermal expansion of a single-crystal copper sample demonstrates that the method works as expected. The experiment involves a new method of tuning by varying the magnetic field configurations in the device and the tuning results agree well with previous measurements. The difference between single-arm and double-arm configurations has been investigated experimentally. We conclude that this measurement benchmarks the applications of magnetic Wollaston prisms in Larmor diffraction and shows in principle that the setup can be used for inelastic phonon line-width measurements. The achievable resolution for Larmor diffraction is comparable to that using Neutron Resonance Spin Echo (NRSE) coils. The use of superconducting materials in the prisms allows high neutron polarization and transmission efficiency to be achieved.

Towards a high-resolution TOF-MIEZE spectrometer with very cold neutrons

We report the first experimental test of TOF-MIEZE technique using very cold neutrons (VCNs) towards high-resolution quasi-elastic neutron scattering spectroscopy. TOF-MIEZE is a type of neutron resonance spin echo spectroscopy with a combination of the time-of-flight (TOF) method and modulation of intensity by zero effort (MIEZE). A compact MIEZE instrument was constructed at the VCN beam port at the High Flux Reactor at the Institut Laue Langevin. By accumulating individual oscillations of raw data, we observed a TOF-MIEZE signal with an effective frequency of 50kHz in a wavelength band of 4–6nm. The signal contrasts were 0.59±0.04 and 0.29±0.03 for wavelengths of 4nm and 6nm, respectively. The Fourier time was estimated to be 70ns with 6nm VCNs in the experimental set-up.

Neutron resonance spin echo with longitudinal DC fields.

We report on the design, construction, and performance of a neutron resonance spin echo (NRSE) instrument employing radio frequency (RF) spin flippers combining RF fields with DC fields, the latter oriented parallel (longitudinal) to the neutron propagation direction (longitudinal NRSE (LNRSE)). The advantage of the longitudinal configuration is the inherent homogeneity of the effective magnetic path integrals. In the center of the RF coils, the sign of the spin precession phase is inverted by a π flip of the neutron spins, such that non-uniform spin precession at the boundaries of the RF flippers is canceled. The residual inhomogeneity can be reduced by Fresnel- or Pythagoras-coils as in the case of conventional spin echo instruments (neutron spin echo (NSE)). Due to the good intrinsic homogeneity of the B0 coils, the current densities required for the correction coils are at least a factor of three less than in conventional NSE. As the precision and the current density of the correction coils are the limiting factors for the resolution of both NSE and LNRSE, the latter has the intrinsic potential to surpass the energy resolution of present NSE instruments. Our prototype LNRSE spectrometer described here was implemented at the resonance spin echo for diverse applications (RESEDA) beamline at the MLZ in Garching, Germany. The DC fields are generated by B0 coils, based on resistive split-pair solenoids with an active shielding for low stray fields along the beam path. One pair of RF flippers at a distance of 2 m generates a field integral of ∼0.5 Tm. The LNRSE technique is a future alternative for high-resolution spectroscopy of quasi-elastic excitations. In addition, it also incorporates the MIEZE technique, which allows to achieve spin echo resolution for spin depolarizing samples and sample environments. Here we present the results of numerical optimization of the coil geometry and first data from the prototype instrument.

Impact of uniaxial pressure on structural and magnetic phase transitions in electron-doped iron pnictides

We use neutron resonance spin echo and Larmor diffraction to study the effect of uniaxial pressure on the tetragonal-to-orthorhombic structural ($T_s$) and antiferromagnetic (AF) phase transitions in iron pnictides BaFe$_{2-x}$Ni$_{x}$As$_{2}$ ($x=0,0.03,0.12$), SrFe$_{1.97}$Ni$_{0.03}$As$_2$, and BaFe$_2$(As$_{0.7}$P$_{0.3}$)$_2$. In antiferromagnetically ordered BaFe$_{2-x}$Ni$_{x}$As$_{2}$ and SrFe$_{1.97}$Ni$_{0.03}$As$_2$ with $T_N$ and $T_s$ ($T_N\leq T_s$), a uniaxial pressure necessary to detwin the sample also increases $T_N$, smears out the structural transition, and induces an orthorhombic lattice distortion at all temperatures. By comparing temperature and doping dependence of the pressure induced lattice parameter changes with the elastoresistance and nematic susceptibility obtained from transport and ultrasonic measurements, we conclude that the in-plane resistivity anisotropy found in the paramagnetic state of electron underdoped iron pnictides depends sensitively on the nature of the magnetic phase transition and a strong coupling between the uniaxial pressure induced lattice distortion and electronic nematic susceptibility.

Anomalous thermal decoherence in a quantum magnet measured with neutron spin echo spectroscopy

The effect of temperature dependent asymmetric line broadening is investigated in Cu(NO$_3$)$_2\cdot$2.5D$_2$O, a model material for a 1-D bond alternating Heisenberg chain, using the high resolution neutron-resonance spin-echo (NRSE) technique. Inelastic neutron scattering experiments on dispersive excitations including phase sensitive measurements demonstrate the potential of NRSE to resolve line shapes, which are non-Lorentzian, opening up a new and hitherto unexplored class of experiments for the NRSE method beyond standard line width measurements. The particular advantage of NRSE is its direct access to the correlations in the time domain without convolution with the resolution function of the background spectrometer. This novel application of NRSE is very promising and establishes a basis for further experiments on different systems, since the results for Cu(NO$_3$)$_2\cdot$2.5D$_2$O are applicable to a broad range of quantum systems.

Neutron spin echo spectroscopy under 17 T magnetic field at RESEDA

We report proof-of-principle measurements at the neutron resonance spin echo spectrometer RESEDA (MLZ) under large magnetic fields by means of Modulation of IntEnsity with Zero Effort (MIEZE). Our study demonstrates the feasibility of applying strong magnetic fields up to 17\,T at the sample while maintaining unchanged sub-$\mu$eV resolution. We find that the MIEZE-spin-echo resolution curve remains essentially unchanged as a function of magnetic field up to the highest fields available, promising access to high fields without need for additional fine-tuning of the instrument. This sets the stage for the experimental investigation of subtle field dependent phenomena, such as magnetic field-driven phase transitions in hard and soft condensed matter physics.

The upgraded cold neutron triple-axis spectrometer FLEXX – enhanced capabilities by new instrumental options

The upgrade of the cold neutron triple axis spectrometer FLEXX, a work-horse instrument for inelastic neutron scattering matching the sample environment capabilities at Helmholtz-Zentrum Berlin, has been successfully accomplished. Experiments confirmed an order of magnitude gain in flux now allowing for intensity demanding options to be fully exploited at FLEXX. In this article, we describe the layout and design of two newly available FLEXX instrument options in detail. The new Heusler analyzer gives an increase of the detected polarized neutron flux due to its superior focusing properties, significantly improving the feasibility of future polarized and neutron resonance spin echo experiments. The MultiFLEXX option provides simultaneous access to large regions in wavevector and energy space for inelastic excitations thus adding mapping capabilities to the spectrometer.

A novel neutron spin echo technique for measuring phonon linewidths using magnetic Wollaston prisms

A new method of implementing neutron spin echo measurement of phonon linewidths on a triple-axis neutron spectrometer is introduced, based on recently developed superconducting magnetic Wollaston prisms. Each arm of the spectrometer is composed of two Wollaston prisms with a rectangular field region between them. By introducing triangular and rectangular field regions, loci of constant spin echo phase can be manipulated easily to achieve the so-called phonon focusing condition. Unlike the neutron resonance spin echo method, which is tuned by physically tilting the field boundaries, the new device can be tuned electromagnetically to achieve the phonon focusing condition. By adjusting the field configurations, the linewidths of phonon excitations with high energy and large group velocity can be measured. By employing superconducting films to define the various field regions, high neutron transmission and good neutron polarization efficiency can be obtained.

Neutron resonance spin flippers: static coils manufactured by electrical discharge machining.

Radiofrequency spin flippers (RFSF) are key elements of Neutron Resonance Spin Echo (NRSE) spectrometers, which allow performing controlled manipulations of the beam polarization. We report on the design and test of a new type of RFSF which originality lies in the new manufacturing technique for the static coil. The largely automated procedure ensures reproducible construction as well as an excellent homogeneity of the neutron magnetic resonance condition over the coil volume. Two salient features of this concept are the large neutron window and the closure of the coil by a μ-metal yoke which prevents field leakage outside of the coil volume. These properties are essential for working with large beams and enable new applications with coils tilted with respect to the beam axis such as neutron Larmor diffraction or the study of dispersive excitations by inelastic NRSE.