Mission Statement

Many materials have magnetic and electrical properties. The Polarized Triple-Axis Spectrometer (PTAX) is used to study these properties in crystalline materials using polarized and un-polarized beams of neutrons. This instrument can be used to examine these properties in topical materials, such as multiferroic materials, thermoelectric materials, high-temperature superconductors, geometrically frustrated magnets, and quantum magnets. Research on PTAX enables scientists to better understand the microscopic origin of the unique properties in these materials.

Instrument Description

Of the four triple-axis spectrometers installed at HFIR, the HB-1 PTAX instrument is specifically designed for polarized beam measurements, but it is also a highly efficient general-purpose unpolarized neutron spectrometer. Upgrades of the beam tube and monochromator drum enabled vertical focusing of the incident neutron beam, yielding an almost 3-fold increase of neutron flux at sample position. The high flux and a wide range of incident energies, 5–120 meV, make the instrument particularly well-suited for polarized studies of magnetic excitations in colossal magnetoresistance (CMR) and high-temperature superconductivity (HTSC) materials, “bad” metals, quantum magnets, magnetic multilayers, and commercial magnetic alloys. A good example is the study of the famous “resonance” excitations in certain HTSC compounds thought to play a crucial role in electron pairing. In the unpolarized mode, HB-1 becomes a state-of-the-art tool for studying various types of excitations and critical phenomena in a broad variety of materials, from shape memory and superelastic alloys to thin films and heavy fermion systems. The choice of two monochromator plugs gives this instrument great versatility. The polarizing plug accommodates the world’s largest 18-cm tall and 25-cm wide fixed vertical focusing Heusler alloy monochromator with a vertical guide field. The unpolarized plug accommodates a pyrolitic graphite (PG) monochromator with variable vertical focusing. The instrument offers a choice of Heusler alloy (for polarized experiments) and PG (unpolarized or half-polarized) analyzers. The current design allows for future installation of a multi-blade horizontally focusing analyzer system. Polarized beam experiments can use a set of Mezei-type “flipper” devices that are fully controlled by the instrument software. The magnetic guide field at sample position is generated by a Helmholz-type coil system. The field can be rotated in three dimensions electronically by adjusting the current distribution in the coils, which is also fully automated. The polarized setup is designed to be compact enough to accommodate all available sample environment equipments, including a 4He/3He dilution refrigerator, standard 4He flow cryostats, a 4He cryofurnace, and a variety of closed-cycle refrigerators.

Applications

  • Spin waves in ordered magnetic materials
  • Exotic excitations in low-dimensional, molecular, itinerate, and other "quantum" magnets
  • Spin and lattice excitations in HTCS, CMR materials, and multiferroic systems
  • Spin density distributions in magnetic compounds
  • Phonon dispersion curves in alloys and phonon-driven phase transitions

Specifications

Beam SpectrumThermal
MonochromatorsUnpolarized PG(002) (variable vertically focused)
Polarized Heusler(111) (fixed veritcally focused)
Analzyers
Unpolarized PG(002), Be(101), Be(002), Si(111) (fixed veritcally focused)
Polarized Heusler (111) (flat)
Monochromator angle2Θ = 14 – 75°
Sample angle+/- 180°
Scattering angle–90 – 120°
Analyzer angle–40 – 140°
Collimations (FWHM)Premonochromator: 15', 30', 48'
Monochromator-sample: 20', 40', 60', 80'
Sample-analyzer: 20', 40', 60', 80'
Analyzer-detector: 20', 70', 90', 120', 210', 240'