Giant Magnetostrictive Multilayer Thin Film Transducers
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ABSTRACT Magnetostrictive multilayer films which combine exchange coupled giant magnetostrictive materials (amorphous Tbo.4Feo.6) and materials with large polarizations (Fe or Feo.5Coo.5) were prepared by dc or rf magnetron sputtering using a rotary turn-table technique in a stop-and-go mode. The magnetic properties of TbFe/Fe and TbFe/FeCo multilayers were investigated in relation to the layer thicknesses and the annealing temperatures. Giant magnetoelastic coupling coefficients (or magnetostrictions) are achieved at low fields, due to the magnetic polarization enhancement in such multilayers. Saturation magnetoelastic coupling coefficients of 20 MPa at 20 mT in the case of TbFe/Fe and of 28 MPa at 20 mT in the case of ThFe/FeCo were achieved. These high low-field magnetoelastic coupling coeffients and the possibility to engineer the material's properties by layer thickness variation are considered to be important features for applications of these films as thin film transducers in microsystems. INTRODUCTION Interest in giant magnetostrictive materials in thin film form has rapidly grown over the past few years due to their potential as a powerful transducer system for the realization of microactuators as they can easily be scaled down to small lateral dimensions. Special features of magnetostrictive in comparison to piezoelectric and shape memory thin films are their remote
control and high frequency operation, their simple actuator lay-out, and the low process temperatures being compatible to microsystems and microelectronics fabrication technologies. The development of magnetostrictive thin films is based on the rare earth - Fe 2 Laves phases which are well known to exhibit giant magnetostriction [1]. As the maximum applied magnetic fields in microsystems are limited, research on these materials concentrates in developing low field giant magnetostrictive properties. Attempts to reduce the magnetic saturation field are normally based around techniques for reducing the macroscopic anisotropy by using amorphous materials, either binary rare earth transition metals like Tb-Fe [e.g. 2] or for further anisotropy compensation even ternary alloys like Th-Dy-Fe [e.g. 3]. Additionally, it was found to be extremely important to adjust an inplane magnetic easy axis [4], whereas the easy axis orientation is dependent on the film stress, itself being controlable by the fabrication conditions, especially by the applied rf bias voltage [5] or by stress annealing after deposition. An alternative, very promising new approach uses the combination of two materials in a multilayer arrangement, which consists of an amorphous giant magnetostrictive and a soft magnetic material with very high magnetic polarization [6]. In order to achieve the required properties, the layers have to be magnetically coupled, i.e. they have to be thinner than the magnetic exchange length to avoid domain wall formation at the interfaces. In this case the magnetic properties of the multilayer will be defined by the mean value of the individual layers, lea
