Optimization of spin-valve parameters for magnetic bead trapping and manipulation y Wendy R. Altman a,b,n , John Moreland b , Stephen E. Russek b , Victor M. Bright a a Mechanical Engineering, University of Colorado, Boulder, CO 80309, USA b National Institute of Standards and Technology, Boulder, CO 80305, USA article info Article history: Received 6 February 2010 Available online 27 May 2010 Keywords: Spin-valve Magnetic bead Microfluidic Magnetic manipulation Switchable magnetic trap abstract Magneto-optic Kerr effect (MOKE) and magnetoresistance (MR) measurements were used to measure the switching characteristics of spin-valve (SV) arrays currently being developed to trap and release superparamagnetic beads within a fluid medium. The effect of SV size on switching observed by MOKE showed that a 1 mm  8 mm SV element was found to have optimal switching characteristics. MR measurements on a single 1 mm  8 mm SV switched with either an external applied magnetic field or a local magnetic field generated by an integrated write wire (current density ranging from 10 6 to 10 7 A/cm 2 ) confirmed the MOKE findings. The 1 mm  8 mm SV low field switching was observed to be + 8 and À 2 mT with two stable states at zero field; the high field switching was observed to be À18 mT. The low switching fields and the large magnetic moment of the SV trap along with our observation of minimal magnetostatic effects for dense arrays are necessary design characteristics for high-force, ‘‘switchable-magnet,’’ microfluidic bead trap applications. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Magnetic beads are commonly used in biological assays to purify, manipulate, and transport biomolecules [1–3]. In general, target biomolecules are bound to the surfaces of magnetic beads and then the beads are manipulated with magnetic field gradients. This method is routinely used for bulk biomolecular manipulation applications and more recently developed for single-bead manipulation applications. Additionally, magnetic devices offer many advantages for lab-on-a-chip (LOC) systems because they are typically not hindered by surface charge, pH, ionic concentrations, and temperature [4]. Even though numerous microsystems have been developed to transport magnetic beads [5–9], none of these systems involve non-volatile trapping and release of a single bead by an individually addressable magnetic device. Mirowski et al. [10] demonstrated that a 1.2 mm  3.6 mm  30 nm Permalloy (Ni 80 Fe 20 ) element could trap 2–3 mm diameter beads in the presence of an external applied field with forces as large as 97 715 pN [10]. They also demonstrated that a spin-valve (SV) element can serve as a non-volatile bistable magnetic structure that can confine a magnetic bead [11]. The purpose of this paper is to optimize the sizing and array density of SV traps so that they have low switching fields, large magnetic moments, and minimal magnetostatic effects in dense arrays, all of which are necessary design characteristics for high-force, ‘‘switchable-magnet,’’ microfluidic bead trap applications. By arranging these individually addressable SVs in series, we can develop a matrix addressable array to transport beads within a microfluidic platform. SVs are commonly used to sense beads [12–15], but sensing is not the focus of this study. Here we focus on an optimized SV design to trap and release individual superparamagnetic micro beads. SVs are giant magnetoresistance (GMR) devices consisting of two magnetic layers separated by a spacer layer and they are commonly used in read heads of high-density disk drives. An antiferromagnet thin-film pins the magnetization of the bottom layer in one direction while the top magnetic layer remains free to rotate between stable magnetization states. The two layers can be oriented either parallel (open flux) or antiparallel (closed flux). If both orientations are stable at zero applied field, then the SVs can be used as non-volatile magnetic bead traps that can be turned ‘‘on’’ and ‘‘off’’ as shown in Fig. 1(a). With the SV in the ‘‘on’’ state (moments of the two layers are parallel), a bead will be trapped near points of high magnetic field gradients. When in the ‘‘off’’ state (moments of the two layers are antiparallel), the bead will be released. Because no external magnetic field or current is required to maintain the state of the SV, this ‘‘switchable permanent magnet’’ offers a low power alternative to other microfluidic transport devices. Several design requirements are considered here to optimize the magnetic properties of SVs for bead trapping: (1) magnetic Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.05.043 y Contribution of the National Institute of Standards and Technology, not subject to copyright. n Corresponding author at: NIST MS 818.03, 325 Broadway, Boulder, CO 80305, USA. Tel.: + 1 303 497 4350; fax: + 1 303 497 3725. E-mail address: wendy.krauser@colorado.edu (W.R. Altman). Journal of Magnetism and Magnetic Materials 322 (2010) 3236–3239