POLI

POLI is a versatile two axes single crystal diffractometer mostly dedicated to investigating magnetic structures in single crystals using neutron spin polarisation. The instrument is equipped with two non-polarising variably focussing monochromators, which can be combined with optimised polarisers to allow structural investigations using polarised and non-polarised neutrons with short wavelengths and high resolution.

Three standard set-ups are currently implemented on POLI:

• Zero-field spherical neutron polarimetry (SNP) using third-generation CRYOPAD
• Polarised neutron diffraction (PND) in the magnetic field known as 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 set-ups, e.g. for high-field polarisation analysis, resonant fission experiments in heavy nuclei, irradiation or other applications, are possible on request.

The SNP method allows us precisely determine the change in the direction and value of the neutron polarisation during the scattering process in the sample by measuring the nine components of the so-called polarisation matrix for an individual Bragg reflection. The observed change in the neutron polarisation 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 magnetisation 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 polarisers and analysers, 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 polarisation is applied. An upgrade from the ³He MEOP to an in-situ polarising 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 polarisation 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. Depending on the experimental needs, a ³He spin filter cell or a dedicated supermirror bender is used as a polariser. An upgrade from the out-of-plane lifting counter detector to a small 2D PSD detector is currently ongoing.