Quantum tunneling of magnetization in mesoscopic systems

Mesoscopic systems provide a unique opportunity for the study of the phenomenon of quantum tunneling of magnetization (QTM). This phenomenon is a macroscopic quantum tunneling effect in which the magnetization vector undergoes quantum underbarrier transitions which manifest themselves as temperature independent magnetic relaxation. The study of this relaxation at very low temperatures affords the opportunity to study the phenomenon experimentally. Low temperature relaxation will be discussed in a variety of mesoscopic systems. Data will be presented for logarithmic and exponential relaxation to demonstrate the occurrence of QTM. At high temperatures the relaxation shows the temperature dependence. Below a certain temperature, called the crossover temperature, T_c, the relaxation becomes independent of temperature. These data agree qualitatively with the recent theoretical work on QTM. We will conclude the paper with the recent work on polynuclear high spin molecules where we have observed thermally assisted resonant tunneling processes between degenerate spin levels of opposite orientation.