OPTICAL MEASUREMENTS OF ELECTRON PARAMAGNETIC SPIN-SPIN INTERACTIONS IN CRYSTALS

The paramagnetic spin-spin interaction is investigated by means of optical probing. The Zeeman sublevel population difference in the ground state of divalent thulium ions doped in alkali-earth fluoride crystals (CaF(,2), SrF(,2) and BaF(,2)) is monitored by making use of magnetooptical activities (magnetic circular dichroism (MCD) and Faraday rotation) of the ions. Neighboring thulium spins are coupled via cross-relaxation (CR) at certain magnetic fields, and the CR signal appears as a change in population difference as the magnetic field is swept when the system is optically pumped. Several CR peaks are explained by Tm-Tm two particle process, Tm-Tm-Tm three particle process and Tm-Tm (excited state) two particle process.; Cross-relaxations between different species are investigated using CaF(,2) crystals containing divalent thulium and divalent holmium ions. For the crystal of .02% Tm and .00001% Ho, only the lowest order CR is observed. For the sample of .02% Tm and .001% Ho, a great many CR resonances are observed and they can be well interpreted by Tm-Tm-Ho three particle process and Ho (forbidden transition)-Tm two particle process. This strongly coupled system also shows a transient relaxation anomaly and an EPR quenching effect. They are thermodynamically explained by the energy flow from Tm to Ho via the CR. An anomalous heating of holium spins is also observed and this may be explained by a phonon bottleneck effect in the Orbach process.; The other aspect of the spin-spin interaction appears as a spin diffusion. The spin spatial diffusion is studied by the Bragg scattering signal from an optically induced grating in the crystal. Theoretical suggestion of the Bragg scattering experiment is made, and the numerical estimate of the signal intensity is given. The Bragg scattering signal will contain two parts. One stems from the optical population grating and the other from the Zeeman sublevel population grating. The spin diffusion constant might be obtained from the Zeeman grating. Finally the diffusion theory for a spin-phonon coupled system is given. Simultaneous diffusion equations of spins and phonons are solved for various cases. If the bottleneck factor connecting spins and phonons is big, the phonon diffusion plays an important role. If the coupling is small, the system can be solved using a perturbation method.