Dynamic magnetization in ferrofluids

A pulsed-field (1 μsec-3 msec) inductive magnetometer has been constructed that measures the magnetic moment of a ferrofluid as a function of time. After the applied field is removed, the magnetization is characterized by a fast decay (<1 μsec) and a gradual decay that can be as long as 4 msec. The magnitude of the magnetization for each component versus applied field can be fit to a Langevin function, and the calculated particle sizes agree well with those measured in the electron microscope. The slow component of magnetization may be due to particle rotation; however, a simple model predicts particle sizes that are not consistent with other measurements. Extending the model to include dimers and trimers in the form of aggregates with an increased local viscosity may explain the slow relaxation.