Sensors and Actuators A 109 (2003) 166–173 Magnetic MEMS: key issues and some applications  D. Niarchos Institute of Materials Science, NCSR “Demokritos”, 153 10 Athens, Greece Abstract Magnetic micro-electro-mechanical-systems (Magnetic MEMS) present a new class of conventional MEMS devices with great potential for science and applications. Using the same technology as for MEMS and incorporating magnetic materials as the sensing or active element offer new capabilities and open new markets within the information technology, automotive, biomedical, space and instrumentation. Magnetic MEMS are based on electromagnetic interactions between magnetic materials and active (coils) or passive magnetic field sources (permanent magnets). At the micrometer scale Magnetic MEMS offer distinct advantages as compared with electrostatic and piezoelectric actuators in strength, polarity and distance of actuation to name a few. The compatibility of magnetic materials with MEMS technology is a key issue, which is addressed by a number of groups worldwide. In this article, we will present an overview of the Magnetic MEMS technology and we will present some of the existing and future applications. © 2003 Published by Elsevier B.V. 1. Introduction The Microworld is a charming and a challenging area of the 21st century, owing to the fact that mechanics (sensing-actuation-IT) can be quite different at the micro- scopic scale from what we experience at the macroscopic scale everyday. The main difference is the surface to vol- ume ratio, which leads to new phenomena and the scaling of various properties is not linear. An interesting property, which is different between macroscopic and microscopic world is the nature of the forces that move things around. We do know that electromagnetic forces dominate in the macroworld and electrostatic forces appear only in very few cases, like sticking a balloon in a wall [1]. Scaling down a crossover from electromagnetic to electrostatic interactions occurs. Using simple arguments [2] it is possible to esti- mate the dimensions where electromagnetic forces are still dominant, yet the dimensions are in the micrometer range. For the case of an electrostatic actuator the field energy density is: U electrostatic = 1 2 εE 2 with ε the permitivity and E the electric field. The maximum electrostatic energy is limited by the maximum electric field that can be applied before electric breakdown occurs, which is approximately 3 MV/m, giving an energy density in vac-  doi of original article 10.1016/S0924-4247(03)00179-1. E-mail address: dniarchos@ims.demokritos.gr (D. Niarchos). uum of the order of 40 J/m 3 . In the case of an equivalent magnetostatic actuator the energy density is given by: U magnetostatic =  1 2  B µ  2 where B is the flux density and µ the magnetic permeability. The maximum energy that can be produced is determined essentially by the maximum value of the flux density of the existing materials. For iron the flux density is equal to 2.15 T, which gives a magnetostatic energy of 1.84 × 10 6 J/m 3 . The ration of the magnetostatic to electrostatic energy density is then: U magnetostatics U electrostatic ≈ 4.6 × 10 4 which shows why magnetostatic interactions dominate in macroworld over electrostatic ones. Scaling of the dimen- sions leads to new phenomena. Although not so apparent for the magnetic materials, in the case of electrostatics this situation is more pronounced. This breakdown voltage as a function of electrode gap, known as the Paschen curve [3] is given in Fig. 1. A careful analysis and considering that the most com- mon magnetic materials used for magnetic actuators are iron or nickel based alloys, the cross-over point (Fig. 2) from magnetic actuation to electrostatic actuation is ∼2 m. If for some cases, the maximum voltages are not the voltages for breakdown but moderate values, these values are much lower (e.g. <1 m for voltages of the order of 100 V). 0924-4247/$ – see front matter © 2003 Published by Elsevier B.V. doi:10.1016/j.sna.2003.11.007