On the far-infrared spectroscopic study of the electron spin resonance in lanthanum manganate

This dissertation presents the far-infrared spectroscopic investigation of electron spin resonance (FSR) in LaMnO3. The first two chapters of the dissertation describe the infrared spectroscopy as a tool for measuring the optical properties of solids, the characterization of the U12IR far-infrared beamline of the National Synchrotron Light Source, Brookhaven National Laboratory.; In Chapter 3, we introduce the main concepts of the antiferromagnetic resonance (AFMR) theory. We summarize the properties of the LaMnO3 crystal in Chapter 4. LaMnO3 is an antiferromagnet in which magnetic moments of the Mn ions form ferromagnetic planes, their magnetization direction alternating between the planes. We discuss how single-ion anisotropy and the rotation of oxygen octahedra lead to the canted antiferromagnetic structure.; The results of our own ESR measurements on LaMnO3 are presented in Chapter 5. We have recorded the far-infrared transmission spectra at various temperatures, magnetic fields, and crystal orientations. Field-frequency maps illustrate the field dependence of AFMR frequencies. In antiferromagnetic LaMnO3 without canting, the role of the easy axis would have been played by the orthorhombic b axis. When the static field is along the b axis, the measured frequencies follow closely the predictions of Kittel's low-temperature theory of AFMR. When the static field is perpendicular to b, we find significant discrepancies between Kittel's theory and the measured frequencies. A mean-field model of a canted antiferromagnet accounts for the discrepancy and allows us to calculate the parameters of the staggered single-ion anisotropy and those of the Dzyaloshinski-Moriya interaction.; Chapter 6 proposes to use high-power half-optical-cycle terahertz (THz) pulses as an excitation for the electronic spins in pulsed spin resonance experiments. We have discussed the existing THz sources and modeled the dynamics of the classical spin system in the high-power THz pulses.