Polarized neutron diffraction using a novel high-T_c superconducting magnet on the single-crystal diffractometer POLI at MLZ

The polarized single-crystal diffractometer POLI is the first neutron scattering instrument routinely using ³He spin filters both to produce and to analyse neutron polarization. The instrument, with a non-magnetic goniometer, was designed to perform two types of polarized neutron diffraction experiment: spherical neutron polarimetry, also known as full three-dimensional polarization analysis in zero magnetic field, and classical polarized neutron diffraction, also called the flipping-ratio (FR) method, in high applied magnetic fields. Reported here is the implementation of the FR setup for short-wavelength neutrons on POLI using a new high-T_c superconducting magnet with a maximal field of 2.2 T. The complete setup consists of a ³He polarizer, a nutator, a Mezei-type flipper, guide fields and dedicated pole pieces, together with the magnet. Each component, as well as the whole setup, was numerically simulated, optimized, built and finally successfully tested under real experimental conditions on POLI. The measured polarized neutron spin transport efficiency is about 99% at different wavelengths, e.g. as short as 0.7 Å, and up to the maximal available field of the magnet. No further depolarization of the ³He cells due to stray fields of the magnet occurs. The additional use of the available ³He analyser allows uniaxial polarization analysis experiments in fields up to 1.2 T. The results of the first experiment on the field-dependent distribution of the trigonal antiferromagnetic domains in haematite (α-Fe₂O₃) are presented and compared with the literature data.