Planar Alignment of Isolated Magnetic Disks in Newtonian Fluids by a Rotating Field

Magnetic anisotropy can be induced in soft magnetic composites by aligning constituent anisotropic magnetic particles. While the uniaxial alignment by a constant field has been extensively investigated, reported studies on alignment of magnetic particles in a rotating field lack experimental validation. In this effort, we present a systematic experimental and theoretical investigation of the alignment dynamics of isolated disk-shaped magnetic particles in a planar rotating magnetic field in order to fabricate composites with planar anisotropy. The theoretical model is developed assuming: 1) quiescent flow field, 2) low-Reynolds-number Stokes flow, 3) negligible Brownian motion, and 4) oblate-ellipsoid approximation for the magnetic disks. The model is experimentally verified by optical microscopy of disk suspensions for varied rotating field strength and frequency, fluid viscosity, and disk size. Good agreement is obtained between the predicted and measured results, yielding insight to parameters important for the control of anisotropy of composite magnetic materials.