RETAINING HIGH AREAL IN-PLANE MAGNETIC ENERGY DENSITY OVER LARGE MAGNETIC THICKNESS: A PERMANENT MAGNETIC MICROLAMINATION APPROACH BASED ON SEQUENTIAL MULTILAYER ELECTROPLATING

Abstract

Many magnetic MEMS devices such as magnetic-based microscale energy harvesters rely on the availability of microscale permanent magnets capable of generating significant magnetic fluxes. Ideally, these magnets would be able to be integrated with MEMS in a batch-fabrication-compatible manner. A number of thin film permanent magnets possessing excellent intrinsic magnetic properties have been discussed in the literature. In order to achieve higher extrinsic properties such as magnetic flux, an intuitive approach is to simply increase the magnetic film thickness. However, it is observed that as the magnetic film thickness increases, the intrinsic magnetic properties (such as remanence and maximum energy product) often deteriorate, limiting the maximum achievable magnetic fluxes from these small-scale integrated magnets. In this work, we present a microfabricated permanent magnet with a multilayer structure that preserves the high magnetic energy density (and resultant magnetic flux density) of thinner magnetic films while simultaneously achieving a significant magnetic thickness and resultant extrinsic properties. The fabrication process relies on sequential multilayer electroplating: alternating layers of relatively thin (in microns) magnetic films and non-magnetic materials were electrodeposited in a multilayer fashion realizing a laminated permanent micromagnet up to a total magnetic thickness of 80µm. A maximum energy product as high as 16.2kJ/m 3 (~ 70% of the value of a 1-µm-thick thin film) was retained in the laminated permanent micromagnet, and a 30% improvement over a CoNiP nonlaminated film with the same magnetic thickness has been successfully achieved. This fabrication approach could potentially be adapted to other permanent magnet materials systems.