Dynamical measurement of domain-wall nucleation and annihilation in individual amorphous Co particles

We present magnetization measurements of individual amorphous Co particles (300 nm×{}200 nm×{}30 nm) patterned by electron-beam lithography. The hysteresis loops show mainly two magnetization jumps corresponding to domain-wall nucleation and annihilation. The angular dependence of these magnetization jumps is measured and the temperature dependence (0.1--6 K) of the nucleation and the annihilation processes is investigated by two independent methods: switching field and switching time measurements. Finally we compare the results obtained on one particle with those of an array of about 2×{}10⁷ particles. We show experimentally that the dynamical properties measured on individual particles are connected with an unusual 1/T relaxation of the array of particles. {} 1996 The American Physical Society.