Status of Experiments on Far-Infrared Absorption Spectroscopy in Magnetic Material

Current experimental techniques in far-infrared spectroscopy applicable to the study of magnetic materials are surveyed. A comparison between interferometric and diffraction methods of wavelength isolation illustrates the use of weak incoherent radiation sources. Observation of electric and magnetic dipole transitions between the Stark levels of the ground state manifold in rare earth ethyl-sulfates demonstrates the indispensable value of magnetic field and radiation polarization techniques. Experiments on antiferromagnetic resonance modes can provide information concerning the type of antiferromagnetic ordering as well as numerical values of exchange and anisotropy constants. Absorption spectra attributed to the creation of k=0 magnons via magnetic dipole transitions and to the creation of zone-edge magnons via two-magnon electric dipole transitions have been observed in 3d ion compounds. Recent results on the 5d ion compound K2IrCl6 allow some interesting comparisons.