Rare Earth Transition Metal Thin Films and Devices

Abstract

This article reviews the fabrication, characterisation methods and applications of rare earth transition metal films with giant magnetostrictive and soft magnetic properties. Optimised materials could be the base for various microactuators such as microfluidic devices, micromotors, micromirrors or -sensors e.g. for detecting mechanical stresses or magnetic fields. One of the key advantages of magnetostrictive microactuators is seen in the possibility of a remote operation. Examples of possible materials are amorphous Tb₄₀Fe₆₀ and Sm₄₀Fe₆₀ films and (Tb₄₀Fe₆₀ / Co₅₀Fe₅₀) multilayer films, having amorphous TbFe layers and crystalline FeCo layers. The most common deposition method is magnetron sputtering, the influences of the deposition parameters on the film quality will be addressed. Special post deposition treatments like magnetic field annealing are discussed with respect to an achievement of a well defined magnetic anisotropy. The magnetostrictive characterisation of the films is performed using an optical beam deflection method. This method derives the magnetoelastic coupling coefficient which is independent of the Young’s modulus of the film. The dependencies of the magnetic and magnetostrictive properties on the multilayer compositions will be explained as well as the temperature dependencies of the magnetostriction of these materials. Optimised (7 nm Tb₄₀Fe₆₀ / 9 nm Co₅₀Fe₅₀) multilayer films show a magnetoelastic coupling coefficient of 27 MPa at an external magnetic field of 20 mT and a coercive field of about 2 mT. Further effects like magnetoresistance and the ΔE-effect are investigated and the correlation to the magnetostrictive effect are presented. The results of the materials development are discussed in view of possible applications which are mainly in the area of microelectromechanical systems (MEMS).