Tunable acoustic attenuation in dilute suspensions of subwavelength, non-spherical magnetic particles

The microstructure (e.g., particle orientation and chaining) of suspensions of non-spherical ferromagnetic particles can be controlled by an external field, potentially making it possible to tune the acoustic properties of the suspension. Here, we experimentally demonstrate that dilute suspensions of subwavelength-sized oblate-spheroidal nickel particles exhibit up to a 35% change in attenuation coefficient at MHz frequencies upon changing the direction of an external magnetic field, for particle volume fractions of only 0.5%. Comparison is made to suspensions of spherical particles, in which the attenuation is smaller and nearly isotropic. Optical transmission measurements and analysis of the characteristic timescales of particle alignment and chaining are also performed to investigate the reasons for this acoustic anisotropy. The alignment of the oblate-spheroidal particles is found to be the dominant mechanism for the anisotropic and tunable acoustic attenuation of these suspensions.