March 2005

High Resolution Terahertz Spectroscopy in Magnetic Field

Over the past few years a new magneto-optical facility has been developed at the U12IR beamline at the NSLS. The main components are an Oxford Instrument superconducting magnet, and a Bruker IFS 125HR high-resolution spectrometer. The principal reason for using the synchrotron source in far-IR spectroscopy is the brightness advantage over a conventional light source -- depending on the frequency range and on the sample geometry, the synchrotron results in a factor of 50 - 200 gain in the intensity, making a vast array of new measurements possible.

The Oxford Instruments magnet, installed next to Bruker IFS 125HR spectrometer. The stainless steel tubes contain the optical coupling to the VUV-IR ring and between the spectrometer and the magnet.

The Bruker spectrometer is essentially an interferometer where spectral resolution is proportional to the available path difference (between the two interferometer "arms"). This particular spectrometer has an extremely long path difference, yielding a 0.001 cm^-1 (0.125 eV) resolution. The available spectral range is from 5 cm^-1 (0.63 eV) to over 7000 cm^-1. The magnet can produce fields up to 16 Tesla and its 20 liter He reservoir has a hold time of nearly 1 week. A set of three wedged single crystal quartz windows at the bottom provide optical access to the sample from below, along the vertical axis of the magnet (see figure). The sample temperature can be varied between 1.8 K and room temperature.

The facility was tested extensively and the first results have been published on LaMnO₃, a well-known antiferromagnet and the parent compound of the so-called colossal magnetoresistance materials. Other projects currently in progress include: the study of the single molecular magnet Mn₁₂-acetate (in collaboration with Myriam Sarachik, City College of New York), spin resonance on NaNiO₂ (with Sophie De Brion, Grenoble HMFL), the investigation of correlated magnetic systems, including LiCu₂O₂ and others (with Laszlo Forro, EPFL, Lausanne), and magneto-optical studies on superconductors, including carbon-doped MgB₂.