Magnetic Field Dependence of Ni Nanorod Brownian Relaxation

AC magnetic susceptibility and optical transmission measurements were performed on Ni nanorod suspensions for ac and superimposed dc magnetic fields up to 9 mT. With the comparably large magnetic moment per nanorod, these field values correspond to Langevin parameters up to about 80. These wide range of ξ values, the narrow size distribution, and the absence of Néel relaxation make these single-domain ferromagnetic nanorods ideally suited for the experimental verification of the dependence of the Brownian relaxation time on ac and dc magnetic fields. Consistent results for the magnetic moment m and the zero-field Brownian relaxation time τ
_B,0
were obtained for all measurement modalities and the valid field range of alternative models for the ac susceptibility was evaluated.