A device for detecting a magnetic field response or changes in a magnetic response of at least one magnetic particle in a carrier fluid, the detection comprising measuring the magnetic particles characteristic rotation period, and the measurement involving measurement of a Brownian relaxation in the carrier fluid under influence of an external pulsed magnetic field. The device includes means for generating the pulsed magnetic field, at least two substantially identical detection coils connected in gradiometer coupling to detection electronics for measuring the differential induced voltage which is dependent on the frequency dependent susceptibility or the magnetization change.