Polymer Bonded Anisotropic Thick Hard Films for Micromotors/Microgenerators

Abstract

Hard magnetic films are very promising elements for the production of MEMS, due to the optimum scaling properties of magnetic interactions. These elements are specially well suited for the development of micromotors and microgenerators. A magnetization multipolar structure is required on the material to be used in such applications. In order to generate this multipolar structure, a well defined magnetic anisotropy is required, this anisotropy being directed in many cases in the direction perpendicular to the film plane. The maximum torque obtainable from a micromotor depends mainly on the mass of magnetic material (i.e. the film thickness). Anisotropic hard magnetic films with thicknesses on the order of tenths of millimetres can not be prepared by any of the common thin film deposition techniques available nowadays. Though some preparation techniques have been developed that can be used to prepare hard magnetic films with the desired thicknesses, they are not able to produce materials with a well defined anisotropy along the direction perpendicular to the film plane. In this work, we describe a method to prepare anisotropic hard magnetic films with thicknesses of tenths of millimetres by dispersing NdFeB anisotropic microparticles on a polymethyl methacrylate (PMMA) solution at room temperature under low intensity magnetic fields. Large macrospic anisotropics have been reached for magnetic fields as low as 150 Oe applied during the deposition, with room temperature coercivities in excess of 1.2 T and maximum energy products up to 9 MGOe. The analysis of the temperature dependence of the magnetic properties indicates that a columnar arrangement of the particles on the direction parallel to the field applied during the preparation is the reason of the observed macroscopic anisotropy. The films can be easily deposited onto different substrates and mechanised in order to obtain a smooth surface with the desired shape. Their use in micromotors/microgenerators has been tested.