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MLZ (eng)

Lichtenbergstr.1
85748 Garching

POLI

Polarized hot neutron diffractometer

Insrumentscheme POLI Insrumentscheme POLI

POLI is a versatile two axes single crystal diffractometer mostly dedicated to the investigation of magnetic structures in single crystals using neutron spin polarization. The instrument is equipped with two non-polarizing variably focusing monochromators which can be combined with optimised polarisers to allow structural investigations using polarised and non-polarised neutrons with short wavelength and high resolution. Three standard setups are currently implemented on POLI:
  • Zero-field spherical neutron polarimetry (SNP) using third generation CRYOPAD
  • Polarised neutron diffraction (PND) in magnetic field named also Flipping-Ratio method
  • Non-polarised diffraction under special conditions (very low temperatures, magnetic and electric fields, high pressures, high temperatures and their combinations) using dedicated sample environments and out-of-plane lifting counter

Also non-standard setups e.g. for high field polarization analysis, resonant fission experiments in heavy nuclei, irradiation or other applications are possible on request.

The SNP method allows to precisely determine the change in the direction and value of the neutron polarization during the scattering process in the sample by measuring the nine components of the so called polarization matrix for an individual Bragg reflection. The observed change in the neutron polarization gives access to the 16 independent correlation functions involved in the most general nuclear and magnetic scattering processes and thus, allows to determine the direction of the magnetic interaction vector of even complex magnetic structures. For those structures with nuclear and magnetic reflections coinciding in the reciprocal space, the amplitude of the magnetic interaction vectors, and hence the magnetization distribution, can additionally be determined from SNP. Moreover, the technique can be employed to study magnetic domain distributions. For SNP, polarised 3He spin filters are used as polarizser and analyser, as they offer the best performance for hot neutrons. By tuning the pressure of 3He in the filter cell, they can be optimised for the used wavelength or the experimental needs. An automatic correction for the time-dependent neutron polarization is applied. An upgrade from the 3He MEOP to an in-situ polarizing SEOP system is currently ongoing.

The PND method is another powerful tool for the investigation of magnetic structures. With the sample situated in strong magnetic fields and a polarised incoming neutron beam, two scattering cross-sections are measured for each Bragg reflection for the two polarization directions parallel and antiparallel to the magnetic field, and the asymmetry between them is built. Compared to the non-polarised neutron diffraction, this asymmetry gives additional access to contributions from the magnetic chirality and the interference between the nuclear and magnetic scattering. This interference term is especially powerful as it depends linearly, and not quadratic, on the magnetic structure factor which increases the precision in the determination of small ordered magnetic moments by at least one order of magnitude and provides unique access to phase information. Dependent on the experimental needs, a 3He spin filter cell or a dedicated supermirror bender is used as polariser. An upgrade from the out-of-plane lifting counter detector to a 2D PSD detector is currently ongoing.

Typical Applications
  • Complex commensurate and incommensurate magnetic structures studied in the ground state (zero-field), which is especially useful for superconductors
  • Studies of magnetic and magneto-electric domains using SNP on samples cooled in zero-field as well as in external magnetic or electric fields.
  • Mapping out of the magnetic phase diagrams by field, temperature, pressure etc.
  • Studies on multiferroic materials by simultaneous application of magnetic and electric fields
  • Determination of magnetic form factors, magnetization density distribution maps, local susceptibility tensors (also in certain antiferromagnets).
  • Absolute sign determination of the Dzyaloshinskii-Moriya interaction vector in e.g. weak ferromagnets or chiral structures
  • Crystal structure determination under special conditions
  • Search for the T-odd violation in the fission reactions of the heavy nuclei
Sample Environment
  • Standard closed-cycle cryostat (CCR) (3.7 – 425 K)
  • Variox LHe cryostat (1.5 – 300 K)
  • Lower temperature inserts for CCR or Variox on request:
    • 3He (0.45 – 4 K)
    • 3He/4He dilution (0.05 – 1 K)
  • 2.2 T HTS magnet in combination with standard CCR and/or low temperature insert
  • 8 T dedicated magnet (1.7 – 800 K); in combination with Kelvinox dilution insert (0.05 K) on request
  • Electric field up to 10 kV
  • High pressure cells on request
Technical Data
  • Primary beam: SR-9a on hot source
Focussing monochromators
CrystalλFlussλFluss
Ån cm-2 s-1 Ån cm-2 s-1
Cu (220)0.555,5 · 1060.91.4 · 107
Si (311)0.77 · 1061.151.8 · 107
  • Er filter for suppression of λ/2- or λ/3-contamination of the monochromatised beam
Neutron polarisation:
  • 3He MEOP spin filter cell:
    • Diameter: 60 mm, length: 130, 160 mm
    • Neutron polarization: 0.8 – 0.93
    • Neutron transmission: 0.2 – 0.27
    • Cell holding time: 2 days
    • Cells replacement time: 10 min
  • 3He SEOP in-situ polarizer:
    • Diameter: 70 mm, length: 150 mm
    • Neutron polarisation: 0.93 – 0.98
    • Neutron transmission: 0.22 – 0.28
    • Under development (2022)
  • Supermirror bender polarizer:
    • Fe/Si m = 3 supermirror
    • Beam cross section H x W: 130 × 42 mm2
    • Neutron polarisation: > 0.99
    • Total transmission: 0.07 – 0.09

Diffractometer angelal

Non-polarizedPND (8T Magnet)SNP (Cryopad)
-20° < 2θ < 130°-20° < 2θ < 130°-15° < 2θ < 120°
-180° < ω < 180°-180° < ω < 180°-180° < ω < 180°
-5° < ξ1 < 5°-4° <ξ1 = 0°-4° < ξ1 < 4°
-5° < ξ2 < 5°-4° < ξ2 = 0°-4° < ξ2 < 4°
-4.2° < v < 30°-4.2° < v < 25°v = 0°
Cryopad (zero-field polarimeter):
  • Precision in polarization control: better 1°
  • Low background / low absorption
  • LHe autonomy: 10 days
  • LN2 refill (automatic): daily
  • Space for closed cycle cryostat or orange type cryostat
Detectors:
  • 2-inch end window 3He single counter tube optimized for hot neutrons.
  • 2D PSD with 1-2 mm resolution (under development 2022).

Instrument Scientists

Dr. Jianhui Xu
Phone: +49 (0)89 158860-825
E-Mail: jianhui.xu@frm2.tum.de

Dr. Rajesh Dutta
Phone: +49 (0)89 158860-811
E-Mail: Rajesh.Dutta@frm2.tum.de

POLI
Phone: +49 (0)89 158860-518

Operated by

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Publications

Find the latest publications regarding POLI in our publication database iMPULSE:

impulse.mlz-garching.de

Citation of the instrument

Heinz Maier-Leibnitz Zentrum. (2015). POLI: Polarised hot neutron diffractometer. Journal of large-scale research facilities, 1, A16. http://dx.doi.org/10.17815/jlsrf-1-22

For citation please always include the DOI.

Gallery

PND setup at instrument POLI using 8 T magnet und supermirror bender.

© Bernhard Ludewig, FRM II / TUM

MLZ is a cooperation between:

Technische Universität München> Technische Universität MünchenHelmholtz-Zentrum Hereon> Helmholtz-Zentrum Hereon
Forschungszentrum Jülich> Forschungszentrum Jülich

MLZ is a member of:

LENS> LENSERF-AISBL> ERF-AISBL

MLZ on social media: