Enhanced sensitivity and thermal tolerance in tunnel magnetoresistance sensor using Ta-doped CoFeSiB soft magnetic layer

Abstract

We developed a tunnel magnetoresistance (TMR) sensor consisting of a CoFeB/MgO/CoFeB magnetic tunnel junction (MTJ) and a CoFeSiB amorphous soft magnetic layer. This multilayer structure is promising for a high-sensitivity sensor because a giant TMR ratio of the MTJ and a small anisotropy field Hk of the free layer can be obtained simultaneously. However, the soft magnetic properties of the CoFeSiB layer disappear when it is annealed at above the crystallization temperature (around 300 °C), which determines the thermal tolerance of the TMR sensor and limits improvements to the sensor's sensitivity and applications. In this study, we doped the CoFeSiB layer with various amounts of Ta to raise its crystallization temperature. TMR sensors using the Ta-doped CoFeSiB layers showed thermal tolerance to annealing temperatures above 425 °C, whereas the sensor with the undoped CoFeSiB layer was tolerant to annealing temperatures up to 325 °C. As well, the Ta doping effectively reduced Hk of the CoFeSiB layer, which resulted in a sensitivity of 50%/Oe, over three times higher than the sensor with the undoped CoFeSiB layer. These results pave the way toward next-generation TMR sensors having higher sensitivity and wider applicability.