Development of Spin Tunnel Magnetoresistive Read Head

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Spin tunnel junction is one of the potential candidate for highly sensitive magnetic read head. However, in practical use, there are some difficulties to be overcome : large noise, low data transfer rate, and difficulty in applying magnetic bias fields. The first and the second problerns are due to large junction resistance and the third one is due to the characteristic junction geometry which is much different from metallic GMR head geometry. In this research project, the research group in Graduate School of Engineering, Tohoku university, together with the group in SONY Corporation have made experimental research to overcome these difficulties.The Tohoku University group engaged in the fabrication of low tunnel resistance junctions by thinning insulating films in the junctions. Local transport properties of the insulating layers were examined using conductive atomic force microscopy (c-AFM). It has been concluded that the insulating layers less than 0.8 nm contain defects, which caus … More e the reduction of tunnel magnetoresistance (TMR) ratios. It has been also made clear that oxygen content in the insulating layer becomes homogeneous in the annealing process, resulted in the increase of the TMR ratios. In order to reduce the number of defects in the insulating layer, various kinds of materials were investigated as buffer layers. In conclusion, a low resistance value (about 80 Ω・um^2) and a high TMR (30%) were obtained using Cu buffer layer and 0.6 nm thick Al in the insulating layer. In case of Ni-Fe buffer layer, it is possible to reduce the resistance further keeping high TMR ratios.The SONY group engaged in the development of read head design for the spin tunnel junctions. Flux guide was adopted in the head to avoid the mechanical damage on the junction and also to increase the junction area. The permanent magnet bias method, which is commonly used in GMR read head, causes electrical short circuiting in spin tunnel magnetoresistive read head. Therefore, a long range exchange coupling biasing scheme was employed to apply a bias field to the flux-guide. The reading performance of a test head was measured in a stationary head/metal tape configuration using helical scanning system. The output signal increased up to 1.75 mV_<p-p>, which was corresponding to +2.4 dB output of a conventional read head. In conclusion it has been demonstrated that the flux-guide type structure combined with a long-range exchange bias method is feasible for spin tunnel magnetoresistive read head. Less