Microwave-Assisted Magnetization Reversal in Individual Isolated Clusters of Cobalt

At the nanometric range, the superparamagnetic state has become a crucial issue in fundamental research as well as practical applications. In both cases, technological improvements require understanding dynamical magnetization reversal processes at the nanosecond time scales. In this paper, we present the first magnetization reversal measurements performed on a single cobalt cluster (counting only 1000 spins) using the micro-SQUID technique by applying a constant magnetic field combined with a radio-frequency field pulse. First of all, we present the different technical steps necessary to detect the magnetic reversals at low temperature (T=35 mK) of a well-defined nanoparticle prepared by low-energy clusters beam deposition. We previously showed that the 3-D switching Stoner-Wohlfarth astroid represents the magnetic anisotropy of the nanoparticle. Then, an improved device, coupled with a gold stripe line, allow us to reverse such a macrospin using an RF pulse. A qualitative understanding of the magnetization reversal by nonlinear resonance has been obtained with the Landau-Lifschitz-Gilbert equation.