Planar Alignment of Magnetic Microdisks in Composites Using Rotating Fields

Soft magnetic composites with aligned anisotropic magnetic particles can have properties not achievable in traditional single-phase materials. These materials are promising for high-frequency inductor and antenna applications. Composites with uniaxial anisotropy, achieved by aligning rod-shaped particles in a constant magnetic field, have previously been reported. In this paper, we discuss the advantages and conditions for realizing composites with planar anisotropy, by aligning disk-shaped particles in a rotating magnetic field. We use Ni-Fe microdisks in a UV-curable matrix as the study system, and present an experimental and theoretical investigation of their alignment under a rotating field in a viscous fluid. The theoretical model is based on Stokes flow of an isolated magnetic oblate ellipsoidal particle in a rotating magnetic field and enables an understanding of the hydrodynamics of the microdisk alignment process. Alignment rate under varying aligning field strength, field rotation frequency, and fluid viscosity is observed using optical microscopy, and characterized by the magnetic properties resultant in the composite. Good agreement is found between measured results and the model. Our work demonstrates for the first time that planar anisotropy in a magnetic composite can be tuned by precise control of aligning field strength and duration, and provides the insight necessary to engineer the alignment dynamics for achieving high-frequency operation.