Single and Double Polarized Neutron Diffraction Options of POLI at MLZ

Polarized neutron diffraction (PND) is a powerful method to investigate magnetic structures. It gives access to unique information, which cannot be determined by non-polarized neutron diffraction or with x-rays. Thus, it is a valuable tool to untangle complex (e.g. chiral) magnetic structures. Three different PND methods are implemented at the hot neutron single crystal diffractometer POLI [1] at the Heinz Maier-Leibnitz Zentrum (MLZ) in Garching, Germany. The first technique, the so called flipping ratio (FR) method, uses a single ³He spin filter cell (SFC) [2] to polarize the incoming neutron beam. With a Mezei type double-coil spin flipper between the polarizer and the high Tc superconducting magnet at the sample position providing fields up to 2.2 T, the ratio between the scattered intensity with and without activated spin flipper is build. All components of the setup were optimized for short-wavelength neutrons and its performance successfully tested [3]. Due to lifting counter mechanics, out of plane Bragg reflections can be accessed and a large q-space covered. Thus, this setup is well suited for the high-quality determination of magnetic form factors, to refine the local anisotropy in the magnetic susceptibility tensor at the unit cell level and to reconstruct magnetization density distribution maps.The second technique, the so called uniaxial polarization analysis (PA), is very similar to the FR method, expect the lifting counter is replaced by the Decpol, containing a second ³He SFC for polarization analysis after the scattering process. Although the Bragg reflection access for this setup is limited to the horizontal plane, valuable information about the movement of magnetic domains or the magnetic moment orientation can be collected.The third technique is the so called spherical neutron polarimetry (SNP). Whereas the sample was situated in a strong magnetic field for the previous two PND options, the SNP method provides a zero field at the sample position by using the Cryopad [4]. Thus, the incoming polarization direction and the analysis axis can be chosen arbitrary, giving precise access to information about the phase difference between the nuclear and magnetic structure and the magnetic moment values. This is especially helpful for chiral and non-centrosymmetric structures. Selected examples for each option are provided and show the high performance of the PND setup of POLI.[1] V. Hutanu, Heinz Maier-Leibnitz Zentrum, Journal of large-scale research facilities, 1, A16 (2015)[2] V. Hutanu, M. Meven, S. Masalovich et al., J. Phys.: Conf. Ser., 294, 012012 (2011)[3] H. Thoma, W. Luberstetter, J. Peters, and V. Hutanu, J. Appl. Cryst. 51, 17-26 (2018)[4] V. Hutanu, W. Luberstetter, E. Bourgeat-Lami, et al., Rev. Sci. Instrum. 87, 105108 (2016