Resonance modes of magnetically generated surface waves in acoustic wave guide systems G. Scheerschmidt a,Ã , K.J. Kirk a , G. McRobbie b a Microscale Sensors, School of Engineering and Science, University of the West of Scotland, UK b Microscale Sensors, School of Computing, University of the West of Scotland, UK article info Available online 8 July 2009 Keywords: Magnetostrictive thin film Acoustic resonance FEM abstract Acoustic resonances were modelled in magnetostrictively activated surface acoustic wave devices. The devices consisted of a substrate with a thin magnetic film and patterned electrodes. The finite element model was verified against experimental results of two different device length scales. Optical and electrical measurements showed resonances at the same frequencies. One group of devices had an electrode centre to centre distance of 300 mm and showed resonances in the range 10–30 MHz in contrast to the other group with a 10th of that electrode feature. Acoustic modes were identified and Rayleigh velocities were calculated. The model agrees well with devices on glass substrates, whereas devices on silicon substrates require careful consideration of material parameters due to the anisotropic mechanical characteristics of silicon. & 2009 Elsevier B.V. All rights reserved. 1. Introduction During the rise of the semiconductor era and as a result of the need for acousto-electronic devices for signal processing, ferro- magnetic materials were one of the favorites to be used in thin film technology. Acousto-electronic components comprise surface acoustic wave devices (SAW), bulk acoustic wave (BAW) resona- tors and film bulk acoustic resonators (FBAR). Piezoelectric materials dominate the acousto-electronic market with annual volume of billions of pieces used in consumer electronics [1]. However, there has always been a steady interest in ferromagnetic materials in acousto-electronic devices. Therefore it is no wonder that Voltmer and White not only developed the inter-digital transducer for piezoelectric materials [2] but also designed an electrode shape for a magnetostrictive substrate [3]. Theoretical analysis for such magneto-acoustic surface waves (MASW) in bulk magnetostrictive materials such as YIG was presented in 1977 [4]. Magnetostrictive material has been deployed in tunable SAW devices [5,6]. Here the acoustic waves are excited in a piezo- electric material, and an overlayer of magnetostrictive material in combination with a magnetic field is used to change the elasticity and hence the velocity of the propagating wave. Ellis et al. [7] then developed a mechanism for exciting a wave using a thin magnetostrictive film on top of a commonly available substrate such as glass or silicon. The magneto-mechanical interaction excited by a harmonic current applied to an array of strip electrodes is coupled into the substrate and gives rise to surface or plate waves. From this point of view the substrate can be considered as a two-dimensional acoustic waveguide. If a qualitative prediction of the substrate’s acoustic resonance characteristic is required then the structure can be modelled in two dimensions as a mechanically vibrating plate subject to spatially periodic excitation. Other structures that can be found in optical isolators and magnetooptic spatial light modulators MOSLM [8–11] also comply with the condition of guiding acoustic waves. Therefore, discussion that follows might be also of benefit for designers of these optical systems. 2. Computational methods In a SAW device plate modes and surface modes coexist. Therefore, dispersion and finite element modelling are required to investigate the acoustic properties. The dispersion curves of the device were computed with Disperse [13] analysis software to identify the Lamb modes of either symmetric or antisymmetric nature. The dispersion model was one-dimensional with mechanical parameters satisfying an isotropic media. The material parameters were set as follows: for silicon density r ¼ 2300 kg m 3 , Young’s modulus E ¼ 131 GPa and Poisson’s ratio u ¼ 0:27 and for borosilicate glass density r ¼ 2230 kg m 3 , Young’s modulus E ¼ 63 GPa and Poisson’s ratio u ¼ 0:20. Finite element modelling with Comsol Multiphysics [12] was used to find the eigenvalues and mode shapes of the system. ARTICLE IN PRESS Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2009.06.084 Ã Corresponding author. E-mail address: Guido.Scheerschmidt@uws.ac.uk (G. Scheerschmidt). Journal of Magnetism and Magnetic Materials 322 (2010) 1628–1630