QUANTUM TUNNELING IN MAGNETIC SYSTEMS OF VARIOUS SIZES (INVITED)

Magnetic relaxation experiments constitute a unique method of determin ing the nature of fluctuations in dissipative magnetic systems. At hig h temperatures these fluctuations are thermal and strongly temperature dependent. At low temperatures, where quantum fluctuations dominate, magnetic relaxation becomes independent of temperature. Such behavior has been observed in many systems. In this review we emphasize the stu dy of low temperature relaxation in ferromagnetic nanoparticles, layer s, and multilayers (including ''domain wall junctions''), and large si ngle crystals. The results of magnetic relaxation experiments are show n to agree with theoretical predictions of quantum tunneling of the ma gnetization. When dissipation becomes important, in large and complex systems, a time dependent WKB exponent needs to be introduced.