2472 IEEE TRANSACTIONS ON MAGNETICS, VOL. 44, NO. 11, NOVEMBER 2008 High Sensitivity Spin Valve Sensors With AF Coupled Flux Guides I. G. Trindade , J. Teixeira , R. Fermento , J. B. Sousa , S. Cardoso , R. C. Chaves , and P. P. Freitas Physics Department, Faculdade de Ciências da Universidade do Porto, 4169-007 Oporto, Portugal IFIMUP and IN, 4169-007 Oporto, Portugal INESC-MN and IN, 1000 Lisbon, Portugal Physics Department, Instituto Superior Têcnico, 1000 Lisbon, Portugal Giant magnetoresistive (GMR) sensors can have their field sensitivity enhanced by many fold if located in the gap of two magnetically soft flux-guides (FG). In this paper, we present spin valve (SV) sensors, with saturation fields of less than 3 Oe and high linearity char- acterized by coercive forces of less than 0.5 Oe. FG require magnetically soft thin films with thicknesses in the range of 100–500 nm. In previous work, we prepared single-layer (SL) films of amorphous Co (Zr-Nb) that exhibited stripe domains (SD) when patterned into FG, causing Barkhausen noise and complete lost of linearity in the SV sensors response. In this article, single layer films of an amorphous alloy of Co Zr Nb , patterned into flux guides, do not exhibit SD but well-behaved closure domains. Nevertheless, these induce hysteresis in the sensors response, characterized by a coercive force of 0.7 Oe. This hypothesis is corroborated by focused beam magneto-optic Kerr effect (MOKE) magnetometry, performed in the poles region the CZN FG. By contrast, FG integrating in- stead multilayer (ML) thin films consisting of ferromagnetic layers of permalloy weakly anti-ferromagnetically (AF) coupled through Ru interlayers cause a strong reduction of hysteresis in the SV sensors response. The sensors in the gap of AF coupled (NiFe/Ru)xn FG, exhibit saturation fields of about 2 Oe and coercive forces of 0.3 Oe, despite the fact that the isolated sensors exhibit coercive forces of 2 Oe. Index Terms—Antiferromagnetic coupling, flux-fuides, multilayer thin films. I. INTRODUCTION G IANT magnetoresistive (GMR) sensors can have their field sensitivity enhanced by many fold if located in the gap of two magnetically soft flux-guides [1]. These can sense magnetic fields in the pico-Tesla range [2], [3] and nano-size magnetic beads, labeling biological molecules of medical interest [4]. Schematics of the device are shown in Fig. 1. The magnetoresistive (MR) sensor is located in the center of the gap of the flux-guides (FG), consisting of magnetically soft poles, whose distance defines the FG gap-width, and of yokes at the backside of the poles. Often, single layer soft thin films with a thickness in the range of 100–500 nm, may exhibit stripe domains (SD) when patterned into tenth of millimeter planar structures [5]–[7]. SD can have either magnetoelastic or magne- tocrystalline origin and are nucleated by a small perpendicular to the plane of the substrate magnetization component. The best single-layer (SL) soft thin films, patterned into FG, exhibit a low density of closure domains [8] [see Fig. 1(a)], nucleated in part by magnetostatic energy and by other part by roughness at the edges of the structures produced by the patterning process (lithography and etching or lift-off). The closure domains at the edges of the poles, produce nonnegligible in-plane fringing fields, inducing an hysteretic behavior in the sensor. By con- trast, ML thin films exhibit null remanent magnetic field when integrating suitable thin spacers, between physically identical ferromagnetic layers, providing a long range interlayer anti-fer- romagnetic (AF) coupling between the two ferromagnetic layers [9], [10], thus forming a synthetic anti-ferromagnet [11]. Digital Object Identifier 10.1109/TMAG.2008.2002602 Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org Fig. 1. Schematics of FGs with MR sensor in the center of their gap: (a) FGs have closure-domain states; (b) FGs have single-domain AF coupled ferromag- netic layers. Magnetization vector is represented by an arrow. If an even number of magnetic layers is used the net magnetic moment is null [see Fig. 1(b)]. ML films of (Ni Fe /Ru)xn prepared by ion beam deposition (IBD) exhibit maximum AF coupling energy strength, erg/cm with an interlayer thickness of only 3 Å [12]. The AF coupled ML sheet films saturation field, , depends upon and thickness of the ferromagnetic layer as , with , for two ferromagnetic layers and , for ( , spontaneous magnetic moment). The total magnetic thickness and saturation field amplitude can then be adjusted by the Ru thickness first, then by the number of laminations and thickness of the ferromagnetic layers. II. DEVICE FABRICATION Spin valve (SV) films, consisting of Ta20 Å/Ni Fe 25 Å/Co Fe 25 Å/Cu20 Å/Co Fe 25 Å/Mn Ir 60 Å/Ta20 Å/Ti W (N )70 Å are prepared by IBD with a Nordiko 3000 [13]. The substrate table of the system uses a permanent magnet to apply a magnetic field to the substrate, during film growth, to induce a uniaxial anisotropy field in the soft magnetic layers and to set the pinning field orientation of the top CoFe layer, exchange-coupled to an antiferromagnet, MnIr. The as deposited films exhibit a GMR effect of 7%, an easy axis coercivity of 2 Oe, an interlayer coupling field (through the 0018-9464/$25.00 © 2008 IEEE