HFM/EXED

High Magnetic Field Facility for Neutron Scattering

The HFM/EXED proposals can be submitted at any time. Only soft deadlines, after which the proposals are sent to the referees, are announced. The next soft deadline is June 1 2017.

Modes of operation available in this call

-          TOF Diffraction

-          TOF Low-Q

-          Direct TOF Spectroscopy

(for details, see the instrument section below and Refs. [2-3])

Writing a proposal

-          All the proposals have to be discussed with the local contact and contain supplementary information on bulk sample characterization in high fields, previous neutron measurements, evidences of Q-range accessibility using EXEQ-calculator and detailed measurement plan  (for details, see a short Presentation).

 Experiment planning

-        A web-based interface for calculating the instrument range and finding the optimal sample orientation is available at EXEQ. A stand-alone version is available for download.

Preparing your experiment

-        Note, since no sample rotation is available at the moment, the sample has to be properly oriented and fixed on a sample holder prior to the experiment. Information about the sample space geometry and dimensions are available in the download area. Limited capabilities for checking the sample orientation can be provided on E4 2-axis diffractometer.

-        Always check the news stream below to have the latest updates.

News:

-      March 27, 2017 - a new release of EXEQ is available for download. Version 0.16 reflects the geometry of the instrument after the installation of the new detectors, chopper and vacuum chamber. Please use the new version from now on, as the accessible scattering angles on EXED have changed.

 -     March 27, 2017 - 26 T is back! The new resistive-coil has been installed and successfully tested.

 -     March 2017 - First inelastic measurements of standard samples.

 -     Oct 2016 - Installation of the inelastic components is complete

Facility description

High Magnetic Field Facility for Neutron Scattering consists of two main components: the High Field Magnet (HFM) and the Extreme Environment Diffractometer (EXED). The former is a dedicated 26 T hybrid magnet, built by the HZB in collaboration with the National High Magnetic Field Laboratory (Florida, US). The latter is a time-of-flight instrument optimized for neutron scattering in restricted angular geometry of the magnet.

The High Field Magnet

The HFM is a "first of its kind" hybrid magnet system reaching fields as high as 26 T, making it by far the strongest continuous field available for neutron scattering experiments worldwide (see Fig. 1 below) [1]. The HFM utilizes Series Connected Hybrid System Technology where water cooled resistive insert coils are mounted in the room temperature bore of a superconducting cable-in-conduit solenoid. Operation of the magnet system requires a dedicated technical infrastructure consisting of high-pressure water cooling for the resistive coil, 4 K Helium refrigerator for cooling of the superconducting coil and 20 kA DC power supply. More information on the magnet can be found in [1] and references therein and under the following link: HFM.

As the field is horizontal, a special feature of the HFM is 30° degrees conical openings at both ends of the resistive insert envisaged for neutron-scattering access. Rotating the magnet by a maximum of 12° with respect to the incoming beam results in max≈27° in forward scattering.

Sample Environment

A dedicated 3He horizontal cryostat allows high-field experiments to be combined with temperatures down to about 0.6 K for samples with cross section <1.5x1.5 cm2.

Another cryostat enables sample rotation around vertical axis by +/- 90 deg and temperatures down to about 1.5 K (4He) for samples with cross section <1.5x1.5 cm2.

A special dilution fridge (0.1 K) is under construction in collaboration with Uni. Birmingham.

The instrument

Time-of-flight instrument EXED (Fig. 2) is equipped with a multispectral extraction system as a part of about 70 m long supermirror guide [2]. As a result, neutrons from both thermal and cold moderators with a wavelength range from 0.7 to 15 Å are available for experiments. The lower wavelength limit is given by the kink in the guide that blocks the direct view of the source and provides a sharp cut-off. Apart from the kink, the guide is essentially straight (100x60 mm2 (HxW)) and ends with a two-channel focusing section that compresses the beam spatially in both directions. For applications requiring low beam divergence, the focusing end can be replaced by a 6 m long pin-hole collimation section with variable apertures.

Flexibility of the instrument is ensured by three alternative systems that are available to create neutron pulses: a curved Fermi chopper for very high resolution (Δt ~ 6 μs), a straight Fermi chopper for high resolution (Δt ~ 15 μs) and a double disc chopper for medium to low resolution (from 115 μs up to >5000 μs). A number of single disc choppers located downstream prevents frame overlap and defines the bandwidth of interest. Due to the chopper system one can operate the instrument from narrow (~ 0.6 Å) to wide (~ 14.4 Å) wavelength band mode centered at the region of interest, and easily trade resolution for intensity. For inelastic applications there is a double disc chopper (Δt ~ 15 μs) in front of the sample. When running, it picks up a single wavelength out of the wavelength spectrum.

The secondary instrument is equipped with position-sensitive 3He detector tubes combined in 6 detector panels and positioned in forward (FWD) and backward (BWD) scattering to reflect the geometry of the magnet (Fig. 2). The backscattering panels are stationary, while the forward ones rotate together with the magnet.

Operation Modes of HFM/EXED

In order to enable a broad range of scientific applications using a unique combination of neutron scattering and high magnetic fields, EXED combines several scattering techniques in one instrument. The operation modes of the instrument are described below.

Diffraction mode is used for studying single crystalline and powder samples in high magnetic fields [2]. The accessible Q-range is from 0.1 up to 3 Å-1 in FWD (mainly in direction perpendicular to the field) and from 1 up to 12 Å-1 in BWD (mainly along the field). The precise Q-maps can be obtained by means of EXEQ-software.

Low-Q mode enables studies of mesoscopic entities in high magnetic fields such as e.g. vortex state in type-two superconductors [2]. Its momentum transfer range extends down to 10-2 Å achieved by a 6m-long collimation section which replaces the focusing guide.

Spectroscopy mode has been built to study magnetic fluctuations and excitations as function of high magnetic field, and is best  suited for non-dispersive modes along the field (e.g. 1D- and 2D-sytems, isotropic systems with respect to the field, etc). EXED enables energy-resolved measurements over a limited Q-range < 3.25/λ (Å-1) for the incoming energies below 25 meV (>1.8 Å) and energy resolution of a few percent [3]. The precise (Q,E)-maps can be obtained by means of InEXEQ-software for inelastic mode (to be available for download soon).

Experiment planning

We provide a web-based interface for calculating the instrument range and finding the optimal sample orientation. It is available at:

www.helmholtz-berlin.de/pubbin/exeq.pl

An interactive version of EXEQ software is available for download (link on the right). The code is written in Python 2.7.x, and installation notes are included in the archive. Please keep in mind that this is a beta-release and correct operation of the software may not always be guaranteed. Also, this is free software, and therefore all the code is available for reading. The current version available for download is 0.16, which is the same version as the one accessible via the web interface. This new version describes the instrument geometry correctly, including the changes related to the introduction of the inelastic mode on EXED.

Please note that the code has not been optimised for speed as yet. If the web-based version of the code (which performs the calculations on the server side) is not performing up to your expectations, you may want to try downloading the interactive version and running it on your own computer. The downloadable version writes processed detector definitions to hard disk, therefore its start-up speed should improve after the first time it has been used. The parameter files, where the sample definition and orientation are saved, can be transferred between the web interface and the stand-alone version of EXEQ.

Applications

  • Quantum magnets and quantum phase transitions
  • Superconductivity
  • Correlated electrons in 3d, 4f and 5f metal compounds
  • Spin, charge and lattice degrees of freedom in transition metal
    oxides
  • Frustrated magnets
  • Novel states of matter

Fig. 1: HFM and its technical characteristics

Fig. 1: HFM and its technical characteristics

Fig. 2: Schematic layout of HFM/EXED

Fig. 2: Schematic layout of HFM/EXED


Liste von Publikationen
Instrument Data
Beam tube NL 4A, 75 m long ballistic multispectral guide
60 x 100 mm2 (straight section)
elliptically tapered down to 30 x 50 mm2 (12 m long focusing section)
Collimation i) None;
ii) 6 m long pin-hole collimation section can replace focusing section
Wave length 0.7 < λ < 15 A
Flux ~1.5·109 n/cm2/s - continuous flux
Range of scattering angles Elastic      0–30°, 150–170°
Inelastic    0–30°
Range of lattice spacing Forward scattering: 2 < d < 1000 A
Backward scattering: 0.5 A < d < 7 A
d resolution Forward scattering: Δd/d>2·10-2
Backscattering: Δd/d>1·10-3
       (using full beam divergence)
Sample size <1.5x1.5 cm2
Detector 204 3He linear position sensitive detectors combined in 6 sections(2x48 tubes with L=1 m and 0.5" diam. in BWD; 2x23 tubes with L=2 m and 0.75" diam. and 2x31 tubes with L=2.4 m and 0.75" diam. in FWD
Instrument options 1 - Powder and Single Crystal Diffraction;
2 - Low Q
3 - Direct TOF spectroscopy
Sample environment T > 0.5 K .. RT, B=26 T
Software egraph (event recording data reduction), Mantid (data reduction)
Chopper speed range 5 – 600 Hz (Fermi chopper)
5 – 215 Hz (double disc choppers)
5 – 120 Hz (single disc choppers)
5 – 250 Hz (monochromating double disc chopper)
Sample-detector distance 2.5, 4.5 m
Sample rotation Maximum sample cross section in the cryostat 1.5x1.5 cm2