Capabilities of the POWDER Instrument

Due to its versatility HB-2A can be employed for a large variety of powder diffraction experiments, but it is particularly adapted for determining complex magnetic structures under extremes of temperature, pressure and magnetic field.

The following examples highlight some of the basic features and capabilities of this instrument:

  1. Studies of crystal and magnetic phase transitions in strongly correlated electron systems. Measurements performed using a 2.41 A wavelength provide an accurate determination of the magnetic commensurate or incommensurate wave vectors and allow the determination of the spin arrangement within the nuclear unit cell. Depending on the amount of sample, the collection time for solving a magnetic structure may take from 1 to 6 hours. Order parameters of the phase transition can also be measured to extract the critical exponent revealing the universality class of the studied system. For such studies, a cryostat with 3He insert capable of reaching temperatures of 275 mK is available.

    McGuire, M. A., Gout, D. J., Garlea, V. O., Sefat, A. S., Sales, B. C., and Mandrus, D., "Magnetic phase transitions in NdCoAsO," Physical Review B 81, 104405 (2010). (4 days using several sample environments).

  2. Neutron data collected at HB-2A can be used to carry out accurate crystal structure refinements to determine atomic positions, atomic displacement parameters, and site occupancies. By using a 1.54 A neutron wavelength selected by the vertically focusing Ge(115) monochromator, a “Rietveld quality” data can be obtained in about 2 hours. Collection of such data has been recently used to explore the detailed structural changes of the crystal structure of the clathrate-type thermoelectric material Ba8AlxSi46-x as a function of aluminum content. The barium atomic displacements were found to increase with increasing cage size but appear to be primarily dependent on the host framework site occupancies. Differences in the physical properties, specifically thermal conductivity, have been linked to the displacement of the atom in the large cage.

    Roudebush, J. H., de la Cruz, C., Chakoumakos, B. C., and Kauzlarich, S. M., "Neutron diffraction study of the type I clathrate Ba8AlxSi46–x: site occupancies, cage volumes, and the interaction between the guest and the host framework," Inorganic Chemistry 51, 1805-1812 (2012). (5 days of beam time to investigate 5 samples)

  3. Magnetoelastic effects in geometrically frustrated spin systems can also be investigated. Due to the good signal to noise ratio at low angles, detailed experimental studies of the development of short-range magnetic correlations can be undertaken. Scattering from two-dimensional (2D) spin ordered systems or spin glasses can be accurately measured.

    Garlea, V. O., Savici, A. T., and Jin R., "Tuning the magnetic ground state of a triangular lattice system Cu(Mn1-xCux)O2," Physical Review B 83, 172407 (2011)  (4 days).

  4. Studies of magnetic phase separation in nanocrystalline materials. Dramatic changes in the properties of spin systems are known to appear in materials reduced to the nanoscale, where finite size, grain boundary, and surface strain effects can play an important role. The HB-2A diffractometer has been used to demonstrate the strain-induced magnetic phase separation within LCMO nanocrystallites. By mechanically stressing nominally ferromagnetic La5/8Ca3/8MnO3 nanocrystallites through high-energy ball-milling techniques, the appearance of an anisotropically enhanced strain field coupled to the emergence phase-separated antiferromagnetic order has been observed.

    Dhital, C., de la Cruz, C., Opeil, C., Treat, A., Wang, K. F., Liu, J. M., Ren, Z. F., and Wilson, S. D., "Neutron scattering study of magnetic phase separation in nanocrystalline La5/8Ca3/8MnO3," Physical Review B 84, 144401 (2011). (2 days)

  5. Mapping out the Pressure-Temperature phase diagrams of hydrates and inorganic systems. In-situ neutron diffraction studies on pressure-induced structural and magnetic phases transformation are especially attractive for microscopic modeling since the chemistry of the system remains untouched. High-pressure neutron diffraction studies of neutron diffraction measurements performed at the HB-2A on the negative thermal expansion system (NTEs) ScF3 show that the stability of the cubic structure against a rhombohedral distortion becomes increasingly marginal upon cooling.

    Greve, B. K., Martin, K. L., Lee, P. L., Chupas, P. J., Chapman, K. W., and Wilkinson, A. P., "Pronounced negative thermal expansion from a simple structure: cubic ScF3," Journal of the American Chemical Society 132, 15496-15498 (2010). (3 days)

  6. Studies of magnetization density distribution in complex materials. By using an in-house–developed 3He cell, the HB-2A neutron incident beam can be polarized and used to perform polarized studies for high sensitivity to weak ferromagnetic/ferrimagnetic signals. A successful test of polarized measurements has already been performed on a thermally quenched K-Co-Fe Prussian blue analogue photomagnet. The material has been studied down to 4 K with both unpolarized and polarized neutron powder diffraction as a function of applied magnetic field, and analysis of the data have allowed the on-site coherent magnetization of the cobalt and iron spins to be established.

    Pajerowski, D. M., Garlea, V. O., Knowles, E. S., Andrus, M. J., Dumont, M. F., Calm, Y. M., Nagler, S. E., Tong, X., Talham, D. R., and Meisel, M. W. “Magnetic Neutron Scattering of Thermally Quenched K-Co-Fe Prussian Blue Analogue Photomagnet,” Physical Review B - submitted (2012) (5 days).