Magnon Dynamics and Transport in Antiferromagnetic Manganese Fluoride.

This thesis is concerned with the transport of energy by large wavevector magnons in the antiferromagnet insulator MnF_2. In as much as the various decay and scattering rates determine the nature and degree of energy transport by magnons, the dynamics of magnons is also addressed. In order to distinguish magnon from phonon transport, phonon dynamics and transport are also of interest and receive some attention. The two-magnon absorption band in the far infrared (FIR) provides a means of generating monochromatic magnons without generating phonons. The magnon sidebands in the optical region may also be used to generate large wavevector monochromatic magnons, but, phonons generated by decay of the excitons to traps are also produced. A FIR quantum counter scheme is described which uses the first excited Stark level of Er^ {3+} dopant ions as a detector of 36 cm ^{-1} phonons and magnons. Resonant pumping of the Stark level gave a 36 cm^ {-1} phonon anharmonic lifetime of 40 +/- 20 ns. Nonresonant pumping in the two-magnon band gave a zone edge magnon lifetime of 2.9 +/- 0.6 mus which was attributed to thermal magnon - magnon scattering. In addition, a lower limit on the magnon-phonon decay time in the prescence of 0.1% impurities was determined to be on the order of 30 mus. Magnon transport was studied in a time of flight experiment using monochromatic magnon generation in the FIR two-magnon band and bolometer detection. A magnon quasidiffusion model was developed which used a frequency dependent diffusion constant resulting from nuclear magnon - magnon scattering, and the thermal magnon - magnon lifetime determined by the quantum counter experiments. The lifetime due to nuclear magnon - magnon scattering at the zone edge was determined to be on the order of 40 ns. The role of magnon-phonon decay is discussed as well as the relationship of these results to thermal conductivity and transient magnetization experiments. Overall consistency is maintained if it is assumed that the magnon-phonon decay rate increases in the presence of impurities or other spatial inhomogeneities due to a relaxation of quasimomentum conservation.