High Field Millimeter and Submillimeter Wave Facility
The Magnet Lab provides millimeter and submillimeter (sub/mm) wave facility for resonance and optical properties measurements in high magnetic fields. The unique feature of the facility is a combination of the tunable radiation sources, Backward Wave Oscillators, (quasi-continuously covering frequency range of 140-700 GHz), and extremely homogenous high magnetic field, provided by our 25 T resistive magnet. The facility has been in operation since April 2002.
Jurek Krzystek and Sergei Zvyagin testing the facility in 2002.
Main Parameters and Characteristics of the Facility
- Radiation sources: Backward Wave Oscillators (BWOs)
- Frequency range: 140-700 GHz /4.7-23.3 cm-1/ (140 - 260 GHz /OB-24/, 200 - 380 GHz /OB-30/, 320 - 550 GHz /OB-32/, 450 - 700 GHz /OB-80)
- Average output power: 80 mW /OB-24/, 25 mW /OB-30/, 10 mW /OB-32/, 5 mW /OB-80/
- Frequency resolution: better than 0.05 GHz
- Magnetic fields available: up to 25 T (resistive magnet), up to 17.5/19.5 T (superconducting magnet)
- Magnetic field homogeneity: 10-5/cm (resistive magnet), 10-3/cm (superconducting magnet)
- Sample temperature range: 1.6 - 330 K
- Probe configurations: Faraday, Voigt (magnetic field B is perpendicular to or in the plane of the magnetic component h of the radiation, respectively)
- Spectra are recorded during the magnetic field sweeping
- Type of modulation: magnetic field modulation, radiation power modulation
- Sample size: OD < 9 mm (Faraday probe), 3 mm (Voigt probe)
More about Sub/mm Wave Spectroscopy at the Mag Lab
Sub/mm wave spectroscopy plays an important role connecting far-infrared and conventional microwave spectral methods. It covers the wave length range of ~0.1-10 mm, which spans the frequency, energy, temperature and magnetic field scales relevant to numerous fascinating phenomena in condensed matter science, chemistry and biology.
A high field sub/mm wave facility was developed at the DC Field Facility in Tallahassee and offered to users in April 2002. The key feature of the facility is a set of easily-tunable, highly-monochromatic, stable and relatively powerful radiation sources, Backward Wave Oscillators (BWOs). These radiation sources allow for performing various experiments over the wide frequency range of 140 - 700 GHz (which corresponds to the wave length range of ~2.1 - 0.43 mm) with remarkable frequency resolution — better than 0.05 GHz in the entire frequency range. High-quality magnetic field (the homogeneity of the magnetic field is better than 10-5/cm) is produced by the 25 T resistive magnet (built with support from the W. M. Keck Foundation). The facility is an extremely powerful tool to study magnetic excitation spectra in solids, liquid and gases, providing very valuable information concerning magnetic structure and interactions in many substances.
Jim Brooks adjusts the microwave tract. The black box is the Backward Wave Oscillator.
Examples of measurements that may be done include:
- Study of the elementary excitation spectra in highly-correlated electron systems
- Spin dynamics in quantum low-dimensional and spin-ordered materials
- Single-molecule magnetism
- Electron and magnetic structure of solids
- Ferromagnetic (FMR), antiferromagnetic (AFMR) and cyclotron (CR) resonance phenomena
- Field-induced and spontaneous phase transitions
- High-resolution electron spin resonance (ESR) spectroscopy of transition metal ions (which is of great importance in chemistry, biochemistry and structural biology)
- ESR on paramagnetic ions with large zero-field splitting
Recent applications of the facility include:
- Investigation of the spin-Peierls materials1
- High-field ESR of organic molecular compounds2
- Study of quantum spin-ladder systems3
- ESR of high-spin Co(II) compounds.4
1 S. Zvyagin, et al., High field ESR of the spin-Peierls materials: mapping structural phase transition, to be published.
2 I. Rutel, et al., High-field ESR and NMR of the organic molecular materials, to be published.
3 C. Landee, et al., Resonance properties of the quantum spin-ladders, to be published.
4 J. Krzystek, et al., Wide frequency range ESR on the high-spin Co(II) (S=3/2) complexes, to be published.
More about Backward Wave Oscillators
Jim Brooks, Jurek Krzystek and Sergei Zvyagin discuss an experiment.
A BWO is a classic vacuum-tube microwave device. Unlike solid-state microwave sources (IMPATT and Gann diodes), the BWO possesses an important distinguishing characteristic: It is quasi-continuously tunable over a very wide frequency range — up to ± 30% from its central frequency. The main part of a BWO is a corrugated comb-like electrode called a slowing system. Interaction between the electron beam and the variable potential of the slowing system results in velocity/phase modulation of the electrons. The periodically grouped electron bunches continue to interact with the variable potential, producing an electromagnetic wave traveling in the opposite direction (backward wave). The velocity of the electrons, and thus the radiation frequency, are determined by the magnitude of the accelerating field. The BWO needs to be adjusted in the high magnetic field by rotating it around two axes. A BWO is a highly sophisticated device, working in an extremely intensive mode (at high voltage — up to 5-6 kV; high temperature of cathode — up to 1200ºC; with high electron beam current density — up to 150 A/cm2). The BWO output power is a rather complex function of the anode voltage. An example of the output power pattern and a typical calibration curve are shown in the figures.
For more information about BWO techniques, see: G.V. Kozlov and V.V. Volkov, in Millimeter and Submillimeter Wave Spectroscopy of Solids, edited by G.Grüner (Springer, Berlin, 1998).
For more information about this facility, contact Dmitry Smirnov.
For information about scheduling magnet time and submitting facility requests, contact DC Program Director Eric Palm.
