POLI

POLI is a versatile two axes single crystal diffractometer mostly dedicated to the investigation of magnetic structures in single crystals using neutron spin polarisation. The using of the separate non-polarising variably focussing monochromators and optimised polarisers allows the structural investigations using polarised and non-polarised neutrons with short wave length and high resolution. Two standard options are currently implemented on POLI:

• Zero-field spherical neutron polarimetry (SNP) using third generation polarimeter CRYOPAD;
• Non-polarised diffraction under special conditions e.g. very low temperatures (< 3 K), magnetic and electric fields, high pressures, high temperatures, etc. using dedicated sample environments and out-of-plane lifting counter.

A third measurement method named flipping-ratio is under development and planned to be operational using a new 8 T magnet in 2016.

SNP allows distinguishing between the depolarisation and polarisation rotation occurring during the scattering in the sample. X, Y, Z components of the scattered polarisation are measured for each orientation of the incoming polarisation and hence a polarisation matrix of 9 elements for an individual Bragg reflection can be found. Determining the relationship between the directions of incident and scattered polarisations gives access to the 16 independent correlation functions involved in the most general nuclear and magnetic scattering processes. Generally, this leads to the direction’s determination of the magnetic interaction vector of magnetic structures. For those structures coinciding nuclear and magnetic reflections in reciprocal space, SNP leads to the amplitude’s determination of the magnetic interaction vectors, and hence to the magnetisation distribution. SNP could also be employed to study the magnetic domain distribution.

Actually, polarised ³He spin filters are used on POLI in order to produce and to analyse neutron polarisation. This method offers best performance in the polarising of hot neutrons. Tuning the pressure of ³He in the filter cell it is possible to optimise the polariser’s performance depending on wave length or experimental needs. The automatic correction of the time-dependent polarisation is applied. Because of the ³He filters’ sensitivity about non-homogeneous magnetic fields, a dedicated solid state supermirror bender will be used in the future for experiments with high magnetic fields.