AC susceptibility of Co clusters in a bias magnetic field: quantum tunneling versus nonlinear effects

Summary form only given. The authors investigate the temperature dependence of the AC susceptibility of Co clusters in the presence of a DC magnetic field of varying strength. At zero field, the real /spl chi/\´ and imaginary /spl chi/" components of the susceptibility show superparamagnetic behavior, with a maximum that shifts towards higher temperatures as the frequency increases. As the DC magnetic field is switched on, the blocking temperature T/sub B/ changes, and a remarkable fact is that the two components of the susceptibility show opposite behaviors. They show that both results can be explained in the framework of a classical model. The shift of the /spl chi/\´ peak with field is most probably due to the competition between two different classical effects caused by the magnetic field: the reduction of the relaxation time and the nonlinear decrease of the equilibrium susceptibility.