Esr in Pulsed Fields

Very strong magnetic fields are needed to study magnetic resonances that are broadened by impurities, lattice deformation or other interactions such as the exchange energy in antiferromagnets. Certain phenomena that can be studied by means of magnetic resonances will occur only at very high magnetic fields, such as magnetic field induced phase transitions in the crystal lattice or in the arrangement of the electrons. The study of field-dependent effects over a broader field range may also call for very strong fields. It is of course most convenient to do experiments in d.c. fields where the signal-to-noise ratio can be greatly improved by lock-in techniques. Fields up to 15 T are now conveniently available from superconducting coils; for higher sweep rates water-cooled high power coils are available at the large magnet laboratories. The highest d.c. fields now available are of the order of 30 T; these are generated by combining a 10 MW watercooled magnet with a large superconducting coil. Higher fields can only be obtained in a pulsed mode, as it will be discussed in section II of this paper, together with an overview of existing pulsed field installations. For magnetic fields in the range 10 T 100 T, a typical resonance frequency such as the Zeeman doublet with a free-electron ^-factor of 2 is in the frequency range from 300 GHz to 3 THz (wavenumber range 10 cm" to 100 cm ' 1 ) , thus in the region from mm-waves into the far infrared. This frequency range is just between microwaves which can be transmitted through waveguides and light which can be focused by lenses; this requires the development of different experimental techniques. There are convenient sources of radiation such as the optically pumped far infrared laser for most of the frequency range, in particular the higher frequencies, and carcinotrons or IMPATT diodes for the lower, frequencies. Regarding sensitive detectors, this frequency range presents some difficulty. It is too low for efficient photon detectors and on the high side for coherent detection techniques (heterodyne); there is certainly still much room for further development. This will be discussed in section III.