Design of multilayer microwires with controllable magnetic properties: Magnetostatic and magnetoelastic coupling J. Torrejo ´n a , G. Badini a , K. Pirota b , M. Va ´zquez a, * a Instituto de Ciencia de Materiales, CSIC, 28049 Madrid, Spain b Department of Applied Physics, Autonomous University of Madrid, 28049 Madrid, Spain Received 18 January 2007; received in revised form 26 March 2007; accepted 28 March 2007 Available online 16 May 2007 Abstract Novel two-magnetic-phase multilayer microwires with outstanding controllable behaviour are introduced. They are obtained by suit- able combined processing techniques, such as the quenching and drawing method, sputtering and/or electroplating, and consist of a mag- netic nucleus, intermediate non-magnetic layers and an outer magnetic layer. In this work, an ultrasoft CoFe-based amorphous nucleus and a magnetically harder crystalline CoNi outer layer are considered. The magnetostatic interaction between magnetic phases is proven to give rise to antiferromagnetic-like coupling, resulting in biasing of the magnetic behaviour of the soft nucleus. In addition, the effective magnetic anisotropy of the latter is tailored by the magnetoelastic coupling between the nucleus and the external layers through the stres- ses induced during the fabrication process and by their differential thermal expansion coefficients. This new family of microwires shows excellent magnetic properties which, being tailorable, make them ideal materials for novel or optimized elements in sensor devices. Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Multilayers; Magnetic anisotropy; Magnetic structure 1. Introduction The design and development of novel magnetic materi- als with optimized properties is of relevance to study new phenomena and to use them in new generation advanced sensing elements. In this regard, magnetic microwires are attracting increasing interest as they are nearly ideal mate- rials for the study of fundamental micromagnetic problems (e.g. mode of magnetization reversal [1]) and are suitable for a number of technological applications, particularly in sensor technologies [2]. Two types of soft magnetic microwire families are currently studied: water-quenched amorphous wires [3] with a diameter of around 120 lm, and quenched and drawn microwires with diameters rang- ing from 2 to 20 lm [4], covered by a protective insulating glassy coat. Recently, a new family of multilayer microwire has been introduced by Pirota et al. [5] where quenching and draw- ing, sputtering and/or electroplating techniques have been combined to prepare microwires consisting of two metallic layers: a micrometric cylindrical nucleus and a metallic outer microtube separated by an intermediate insulating microlayer. Preliminary magnetic characterization of these microwires has been introduced recently [6], and quickly applied for multifunctional sensing [7]. From the magnetic viewpoint two types of multilayer microwires can be con- sidered: (i) single-phase microwires, where just the metallic nucleus or the outer shell is magnetic; and (ii) two-phase microwires, where both the nucleus and the outer shell are magnetic. The magnetic behaviour of the single-phase multilayer microwires is controlled, apart from the shape anisotropy, by the magnetoelastic anisotropy arising from the magnetoelastic coupling between magnetostriction in the magnetic layer and the stresses induced by the other layers during fabrication or by subsequent temperature changes [8,9]. 1359-6454/$30.00 Ó 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.actamat.2007.03.023 * Corresponding author. Tel.: +34 913349051; fax: +34 913720623. E-mail address: mvazquez@icmm.csic.es (M. Va ´zquez). www.elsevier.com/locate/actamat Acta Materialia 55 (2007) 4271–4276