The research and development in the field of magnetoresistive sensors has played an important role in the last few decades. Here, the authors give an introduction to the fundamentals of the anisotropic magnetoresistive (AMR) and the giant magnetoresistive (GMR) effect as well as an overview of various types of sensors in industrial applications. In addition, the authors present their recent work in this field, ranging from sensor systems fabricated on traditional substrate materials like silicon (Si), over new fabrication techniques for magnetoresistive sensors on flexible substrates for special applications, e.g., a flexible write head for component integrated data storage, micro-stamping of sensors on arbitrary surfaces or three dimensional sensing under extreme conditions (restricted mounting space in motor air gap, high temperatures during geothermal drilling).

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A notable advantage of electroplated binary Co-Fe alloys in MEMS applications like sensors and actuators is the high saturation flux density BS, which may reach values of 2.4 T in a composition range of 50 – 70 wt% Fe. In contrast, there are certain disadvantages limiting the use of these alloys, e. g. high film stress, high corrosion sensitivity and high magnetostriction. The mechanical and magnetic properties of Co-Fe films influence each other strongly. They are determined by the films’ microstructure, given by film texture, grain size and morphology, structural defects and impurities. In this study, the effect of process conditions like current density, pulse duration, duty cycle, pH-value of the plating bath and application of a constant magnetic field parallel to the electrode surface on film stress, microstructure and magnetic properties are examined.

As shown in previous investigations, a correlation between a NiMn-based spin valve's thermal stability and its inherent exchange bias exists, even if the blocking temperature of the antiferromagnet is clearly above the heating temperature and the reason for thermal degradation is mainly diffusion and not the loss of exchange bias. Samples with high exchange bias are thermally more stable than samples with low exchange bias. Those structures promoting a high exchange bias are seemingly the same suppressing thermally induced diffusion processes (A. Wienecke and L. Rissing, “Relationship between thermal stability and layer-stack/structure of NiMn-based GMR systems,” in IEEE Transaction on Magnetic Conference (EMSA 2014)). Many investigations were carried out on the influence of the sputtering parameters as well as the layer thickness on the magnetoresistive effect. The influence of these parameters on the exchange bias and the sample's thermal stability, respectively, was hardly taken into account. The investigation described here concentrates on the last named issue. The focus lies on the influence of the sputtering parameters and layer thickness of the “starting layers” in the stack and the layers forming the (synthetic) antiferromagnet. This paper includes a guideline for the evaluated sputtering conditions and layer thicknesses to realize a high exchange bias and presumably good thermal stability for NiMn-based spin valves with a synthetic antiferromagnet.

The most notable advantage electroplated binary Co-Fe alloys offer for MEMS applications like sensors and actuators is the high saturation flux density, which may reach values of 2.4 T in a composition range of 50 – 70 % Fe. In contrast, there are certain disadvantages limiting the use of these alloys, e. g. high film stress, high corrosion sensitivity and high magnetostriction. This triggered a lot of research aiming at the minimization of these unfavorable properties (1-4, 6). In previous studies (5), we examined very high (20 µm) ring-shaped Co-Fe flux guide structures for a 3D, ultra-thin magnetic field sensor. Due to the GMR sensors underneath the flux guides, a thermal treatment at temperatures above 150°C to reduce film stress was not possible. Other measures were taken instead, like the use of saccharin, reverse pulse plating and the application of a magnetic field during deposition (2, 6). Thus, saturation flux densities above 2.1 T with drastically lowered film stress and good corrosion behavior were achieved, which may be attributed for a great part to the magnetohydrodynamic (MHD)-effect generated by the applied magnetic field. The gained results still necessitate further investigation of the connection between the process conditions and the mechanical and magnetic properties of the films. The goal is to maximize BS and the squareness of the hysteresis loop γ as well as to further reduce film stress σ and the coercivity Hc. The mechanical and magnetic properties of Co-Fe films influence each other strongly. They are determined by the films’ microstructure, given by the film texture, grain size and morphology, structural defects and impurities. In this study, the effect of process conditions like current density, pulse duration, duty cycle, pH of the plating bath and application of a constant magnetic field parallel to the electrode surface on film stress, microstructure and magnetic properties are examined. The data for film stress, calculated on the basis of profilometry and x-ray diffraction (XRD) measurements, and the data for magnetic properties (saturation flux density, coercivity, hysteresis loop squareness) are linked together and will be referenced with the associated parameters for film texture, grain size and morphology – determined with XRD, scanning electron microscopy (SEM) and atomic force microscopy (AFM) – as well as magnetic domain structure, observed with Kerr microscopy. Figure 1: a) Top view of electroplated flux guide b) B-H-loops of flux guides c) domain pattern of flux guide E. I. Cooper, C.Bonhote, J. Heidemann et. al., IBM J. Res. Dev., 49, p. 103 (2005) 2005 J. A. Koza, M. Uhlemann, A. Gebert et. al., Electrochim. Acta, 53, p. 5344 (2008) W. Szmaja, W. Kozłowski, K. Polański et. al., Mat. Chem. Phys., 132, p. 1060 (2012) M. Zieliński, Int. J. Electrochem. Sci., 8, p. 12192 (2013) R. Kruppe, A. Wienecke and L. Rissing, Trans. Mag, 50 (4), (2013) S. Riemer, J. Gong, M. Sun et. al., J. Electrochem. Soc., 156 (10), D439 (2009)