www.aspbs.com/eos Encyclopedia of SENSORS Amorphous Magnetic Materials for Sensors Julián González and Arcady Zhukov Facultad de Química, Paseo Manuel de Lardizabal 3, 20018 San Sebastián, Spain CONTENTS 1. Introduction 2. Physical Properties of Amorphous and Nanocrystalline Alloys for Sensing Applications 3. Relevant Magnetic Effects for Sensing Applications 4. Magnetic Sensors Based in Amorphous and Nanocrystalline Materials 5. Magnetoelastic Sensors 6. Sensors Based on Giant Magneto-Impedance Effect 7. Final Remarks Glossary References 1. INTRODUCTION The magnetic properties of amorphous and nanocrystalline alloys have been extensively studied for nearly a quarter of a century since their first production [1]. Several reviews of their magnetic properties have been published over this period [2–7]. Much interest in these materials has been stim- ulated by their remarkable magnetomechanical and mag- netotransport properties. Thus, magnetostriction of some tens of parts per million (ppm) combined with coercivi- ties of less than 10 A m -1 and anisotropy constants of less than 100 J m -3 give rise to many potential applications as field, stress and strain sensors, and as mechanical actua- tors. Similarly, nearly-zero magnetostrictive amorphous and nanocrystalline alloys (especially in the wire-shape) exhibit giant magnetoimpedance ratio (GMI) values as large as 300–600%, that is the magnetic-field dependence of the elec- trical impedance Z of an amorphous and nanocrystalline fer- romagnetic material (ribbon or wire) [8–11]. It is amazing to notice with respect to GMI effect that the rapid advantages in the understanding of the underlying physical mechanisms of GMI have allowed the development of practical devices and applications using this effect. Therefore, the large influ- ence of magnetic fields and mechanical stresses in determin- ing the magnetic permeability and electrical impedance of amorphous and nanocrystalline ferromagnetic alloys makes to these materials very suitable for sensing applications. Such influences are directly related with their microstruc- ture, elastic properties, magnetostriction and atomic mobil- ity which allows magnetic anisotropy to be induced in some specially chosen alloys by thermal treatments under applied magnetic fields and/or mechanical stress [12–19]. The lack of long-range periodicity, which characterizes an amorphous structure, gives rise to a vanishing magnetocrys- talline anisotropy. Consequently, magnetoelastic and shape anisotropies are the main sources that determine the mag- netization process in these kinds of materials [20]. Shape is not an intrinsic factor but a geometric one and can be conveniently chosen so that macroscopic demagnetizing field can be either neglected or have acceptable values. Nev- ertheless, magnetoelastic anisotropy originates by intrinsic stresses coupled with the magnetostriction constant. Conse- quently, it depends on the strength and local arrangement of internal stresses and on the intensity of magnetoelastic cou- pling. While the magnetostriction value can be adequately controlled according to alloy composition, internal stresses are determined by the rapid quenching process and do not depend significantly on composition. In the case of amor- phous ferromagnetic wires it is remarkable to note that these have been found to possess many of the attractive magnetic and magnetoelastic properties exhibited by amorphous rib- bons, in addition they possess interesting bistable properties associated with their cylindrical geometry [21–26]. In addi- tion to these bulk forms, amorphous materials are widely prepared in the form of glass-coated microwires [27] and deposited thin films of multilayers [28]. These are still in the early stages of development, but promise to offer magnetic properties unavailable in bulk materials [29]. 2. PHYSICAL PROPERTIES OF AMORPHOUS AND NANOCRYSTALLINE ALLOYS FOR SENSING APPLICATIONS Amorphous and nanocrystalline ferromagnetic materials can be obtained by rapid solidification of the liquid alloy. Although there are other different techniques to produce ISBN: 1-58883-056-X/$50.00 Copyright © 2006 by American Scientific Publishers All rights of reproduction in any form reserved. Encyclopedia of Sensors Edited by C. A. Grimes, E. C. Dickey, and M. V. Pishko Volume X: Pages (1–25)