Abstract
The microstructure of suspensions of ferromagnetic particles with subwavelength size can be controlled by an external field, making it possible to develop novel broadband acoustic materials with anisotropic and tunable acoustic properties. In this study we experimentally show that dilute suspensions of nickel micro-flakes exhibit a greater than 20% change in attenuation coefficient at MHz frequencies upon changing the direction of an external magnetic field, at particle volume fractions of only 0.5%. Optical transmission measurements and analysis of the characteristic timescales of particle alignment and chaining are used to study the mechanisms behind this acoustic anisotropy. By making comparison to suspensions of spherical particles, we show that the shape and orientation of the nickel micro-flakes play important roles in the tunable acoustic attenuation of these suspensions.