Vladimir Hutanu

Development of compact magnetostatic cavities for 3He spin filter cells

In order to preserve the 3He polarization in a spin filter cell used as a polarizer or analyser on a neutron beam line, the cell should be situated in a homogeneous magnetic guide field. Mu-metal magnetic shields are used to assure the stability of the guide field in the presence of moderate magnetic stray fields. Very compact (shoe box dimension) magnetostatic cavities made of mu-metal and permanent magnets have recently been constructed and optimized for rather large (13 cm long, 6 cm diameter) spin filter cells for use with hot neutrons. The relaxation time due to the magnetic gradients measured in the cells, situated in these compact boxes, is more than 500 h at 1 bar 3He pressure. Here we present the construction of such cavities and discuss the possibilities of further improvement in performance.

Control of multiferroic domains by external electric fields in TbMnO3

The control of multiferroic domains through external electric fields has been studied by dielectric measurements and by polarized neutron diffraction on single-crystalline TbMnO3. Full hysteresis cycles were recorded by varying an external field of the order of several kV mm(-1) and by recording the chiral magnetic scattering as well as the charge in a sample capacitor. Both methods yield comparable coercive fields that increase upon cooling.

The low-temperature crystal structure of the multiferroic melilite Ca2CoSi2O7.

In the antiferromagnetic ground state, below TN ≃ 5.7 K, Ca2CoSi2O7 exhibits strong magnetoelectric coupling. For a symmetry-consistent theoretical description of this multiferroic phase, precise knowledge of its crystal structure is a prerequisite. Here we report the results of single-crystal neutron diffraction on Ca2CoSi2O7 at temperatures between 10 and 250 K. The low-temperature structure at 10 K was refined assuming twinning in the orthorhombic space group P2(1)2(1)2 with a 3 × 3 × 1 supercell [a = 23.52 (1), b = 23.52 (1), c = 5.030 (3) Å] compared with the high-temperature normal state [tetragonal space group P42(1)m, a = b ≃ 7.86, c ≃ 5.03 Å]. The precise structural parameters of Ca2CoSi2O7 at 10 K are presented and compared with the literature X-ray diffraction results at 130 and 170 K (low-temperature commensurate phase), as well as at ∼ 500 K (high-temperature normal phase).

3He spin filters for spherical neutron polarimetry at the hot neutrons single crystal diffractometer POLI-HEiDi

3D vector polarisation analysis called also SNP (Spherical Neutron Polarimetry) is a powerful method for the detailed investigation of complex magnetic structures. The precise control of the incoming and scattered neutron polarisations is essential for this technique. Here we show an instrumental setup, that was recently implemented on the new single crystal diffractometer POLI-HEiDi at the FRM II for performing SNP experiments using two 3He spin filters for the production and for the analysis of the neutron polarisation. The design and optimisation procedure for the used spin filter cells are presented. Methods for in-situ measurements of the incoming polarisation as well as the particularities of the using two spin filters and corrections for the time dependent relaxation are discussed. Statistical precision of 1% has been achieved for the measurements of the polarisation matrix under the real experimental conditions using described cells and applying proposed correction method for the data.

Low temperature magnetic structure of CeRhIn5 by neutron diffraction on absorption-optimized samples

Two aspects of the ambient pressure magnetic structure of heavy fermion material CeRhIn5 have remained under some debate since its discovery: whether the structure is indeed an incommensurate helix or a spin density wave, and what is the precise magnitude of the ordered magnetic moment. By using a single crystal sample optimized for hot neutrons to minimize neutron absorption by Rh and In, here we report an ordered moment of [Formula: see text]. In addition, by using spherical neutron polarimetry measurements on a similar single crystal sample, we have confirmed the helical nature of the magnetic structure, and identified a single chiral domain.

Implementation of a new Cryopad on the diffractometer POLI at MLZ

A new polarized neutron single crystal diffractometer POLI (Polarization Investigator) has been developed at the Maier-Leibnitz Zentrum, Garching, Germany. After reviewing existing devices, spherical neutron polarimetry has been implemented on POLI as a main experimental technique using a third-generation cryogenic polarization analysis device (Cryopad) built in cooperation between RWTH University and Institut Laue-Langevin. In this report we describe the realization and present the performance of the new Cryopad on POLI. Some improvements in the construction as well as details regarding calibrations of Cryopad and its practical use are discussed. The reliable operation of the new Cryopad on POLI is also demonstrated.

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-Tc 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 A, 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.

Origin of forbidden reflections in multiferroic Ba2CoGe2O7 by neutron diffraction: Symmetry lowering or Renninger effect?

For a symmetry consistent theoretical description of the multiferroic phase of Ba

Tunable emergent heterostructures in a prototypical correlated metal

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Crystal Structure of Magnetoelectric Ba2MnGe2O7 at Room and Low Temperatures by Neutron Diffraction

CoGe