From a technological point of view one of the most important characteristics of magnetorheological fluids and ferrofluids is their fast (on the s time scale) response to external magnetic fields. We report response time measurements based on the time domain magnetic susceptibility response of commercially available fluids. The dynamic susceptibility is measured over time in the presence of external magnetic field by a method using an inductive resonant circuit. The response time of the fluids is extracted as the characteristic time constant from the response to ramp excitation. We investigated the influence of the slope of the ramp excitation on the response time. The macroscopic response times were correlated with the characteristic time scales of the microscopic processes behind the susceptibility response. In case of the magnetorheological fluids the overall response time have been associated with the characteristic times of structure formation. The relatively slow response of the ferrofluid corresponded to a relaxation dominated by Brownian mechanism, which indicated the presence of multi-core clusters.
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