Spin valves with conetic based free layer

Abstract

Giant magneto-resistance (GMR) sensors have been widely applied in various industries, such as contactless position monitoring in automotive sensing and detecting magnetic nanoparticles in bio-sensing. GMR sensors with high sensitivity and low coercivity are required to facilitate the detection of small magnetic field variations. Good thermal stability of the sensor up to 125 ˚C is also necessary in automotive applications. The Conetic alloy (NiFeCuMo) is a promising soft magnetic material for use in spin valves as the free layer to reduce the hysteresis effect in the spin valve. Using a synthetic ferrimagnet (SF; consist of a ferromagnet/Ru/ferromagnet trilayer system) free layer can also help to minimize the hysteresis effect of the spin valve. By adapting a SF free layer, the effective thickness of the free layer in the spin valve is reduced while retaining its physical thickness. This modification can decrease free layer coercivity in a spin valve without significant signal loss. To enhance the magnetic and thermal stability of spin valves, a synthetic antiferromagnetic (SAF; consist of a SF stack exchange biased by an antiferromagnetic layer) pinned layer is incorporated. The pinning field of a spin valve can be increased due to strong Ruderman-Kittel-Kasuya-Yosida coupling in the SAF structure. The large pinning field can be retained even at high temperatures. In this thesis, we describe techniques to fabricate a thermally stable, low-hysteresis GMR sensor with a Conetic-based free layer and SAF pinned layer. Two main experimental results are presented. In the first part, spin valves with a Conetic-based SF free layer and CoFeB-based SAF pinned layer were developed. Small coercivity was achieved by introducing SAF into the pinned layer. In the SF free layer, the thickness of the Conetic alloy and Ru spacer was optimized. A sensitivity of 0.27%/Oe and a small coercivity of 0.3 Oe are attained by adopting a CoFeB/SF free layer, which holds promise for small magnetic field detecting applications. In the second part, thermally stable GMR spin valves with a Conetic-alloy-based free layer and CoFe-based SAF pinned layer were developed. The pinning effect of the spin valves was enhanced by magnetic annealing. The temperature-dependent GMR properties of two spin valves with conventional and SAF pinned layers were comparatively studied over a thermal cycle ranging from 25 to 130 ˚C. After adopting a SAF pinned layer, the sensitivity of the spin valve increased twofold. The Conetic-based spin valve with a SAF pinned layer also exhibited much better thermal stability and reversibility. The good thermal stability, small coercivity (≤1.1 Oe), and stable sensitivity (0.19–0.27%/Oe) over broad temperature range of the spin valve sensor shows promise for low field sensing at elevated temperatures. (437 words)