Mission Statement

Exploring nuclear and magnetic structures as a function of temperature, pressure, magnetic field, and electric field.

Instrument Description

The Dimensional Extreme Magnetic Neutron Diffractometer (DEMAND) has two modes of operation. Four-circle mode with the χ circle goniometer can operate with a closed-cycle-refrigerator (4–800 K) that allows a pressure cell (0-10 GPa), or a permanent magnet (0-0.9 T), and electric field. Two-axis mode allows extreme sample equipment cryomagnet (0-6 T), 3He insert and dilution refrigerators (0.05-300 K). Both modes can have an unpolarized and polarized neutron beam that can be switched by automated elevator device. The current detector has three Anger camera modules stacked vertically to cover 16° (horizontally) x 48° (vertically). The detector sits on the 2Θ rotation arm to cover the scattering angle range of 3° < 2Θ < 155°. A multilayer-[110]-wafer silicon monochromator with the reflection from HHL planes ensures sharp diffraction peaks in specified ranges of detector angles by control of the horizontal radius of curvature. Any HHL planes can be set in Bragg position, but only the (331), (220) with (440), and (111) with (333) reflections are of practical interest. For the fixed monochromator angle of 47.5°, these reflections provide principal incident wavelengths of 1.005 Å, 1.546 Å, and 2.541 Å, respectively. A PC-based LabView system provides user-friendly diffractometer control and data acquisition. The beam size is 6.3 mm in diameter, and the minimum measured crystal size is 0.02 mm3. The maximum crystal dimension is usually limited to 5 mm. The flux on the sample can be up to 2.2 × 107 n/cm2/s. The horizontal bending of the monochromator can be changed to optimize the Q-resolution or flux depending upon the needs of the measurement. The longer wavelength of 2.541 Å has ~5% λ/3 contamination and is mainly used for polarized neutron diffraction (S-bender will filter out the high-order contamination). The 1.546 Å-wavelength has the highest flux but with ~1.4% λ/2 contamination (PG filter is available to reduce the contamination below 10-4), is mainly used for determining magnetic structures. The 1.005 Å wavelength is monochromatic and is good for precisely determining both nuclear and magnetic structures although the flux is 8 times lower than the highest flux at 1.546 Å.

Applications

The HB-3A DEMAND has the mission to explore nuclear and magnetic structures as a function of temperature, pressure, magnetic field, and electric field. The instrument is particularly suitable for studying phase transitions and accompanying structure changes, as well as measuring order parameters and exploring the phase diagram. It is also suitable for a wide range of small-unit-cell crystallography studies, including superlattice structures and atomic anharmonicity. Users have researched problems in physics, materials science, chemistry, and mineralogy. Recent specific areas of study include magnetic structure and nuclear superstructure in iron pnictide superconductors, magnetic and nuclear structures of possible quantum magnets, Weyl semimetals, topological insulators, multiferroic oxide phase transitions and diagrams, magnetic/orbital frustration in spinels, new permanent magnets, temperature dependence of atomic displacement parameters in battery and thermoelectric materials, structural phase transitions of photo-voltaic hybrid perovskites, hydrogen bonding in rock-forming minerals, crystallography of novel scintillators, and diffuse scattering.

These materials have a wide range of contemporary and prospective applications, such as terahertz equipment, sensors, high temperature power harvesting, high-efficient power transmission, green refrigeration, wireless communication, bolometers in space investigation, data storage and “qubit” in quantum computation.

Specifications

Beam Spectrum: Thermal
Monochromators: Double focusing silicon
Monochromator angle: 47.5°
Incident Wavelengths: 1.005 Å (331), 1.546 Å (220), 2.541 Å (111)
Four-circle mode: Huber, half full χ circle, with 4 - 800 K CCR, 2Θ<155°
Two-axis mode: -181°< omega <181°, out-of-scattering plane coverage -8°< γ < 40°, 2Θ < 155°
Detector: 2D Anger camera
Crystal size requirement: > 0.1 mm3, maximum crystal dimension 5mm
Flux at sample: 2.2 × 107 n/cm2/s
Polarized neutron diffraction:

Polarizer: In situ pumped 3He filter
S-bender supermirror