A method to measure magnetic quantities of ferrimagnetic and ferromagnetic samples under the effect of time‐dependent fields [Ha(t)] is presented. The method is based on the alternating field method together with the powerful technique of synchronous detection. Interesting quantities such as the total Bloch wall area present during the magnetization reversal, the reversible permeability, and its first derivative with respect to the magnetic field at any point of the hysteresis loop are plotted rapidly, even in the case of very small samples (for instance, iron whiskers). Reversible permeability is obtained without need of extapolation. The switching time and the dynamic coercive field are easily measured as well. The sensitivity of the method is very high, and the noise is widely rejected. The technique is applied to iron whiskers and polycrystalline YIG.