CoFeB/MgO/CoFeB-Based Magnetic Tunnel Junction Using Soft Magnetic Composite Free Layer with TaFeB Spacer

CoFeB/MgO/CoFeB-based magnetic tunnel junctions (MTJs) with a soft magnetic composite free layer have been studied for an ultra-sensitive magnetic sensor. Further improvement to the tunnel magnetoresistance (TMR) ratio is still required, which has been hindered by a trade off between the TMR ratio and interlayer exchange coupling (IEC) depending on the spacer thickness of the composite free layer. In this study, we developed a TaFeB spacer to replace the conventional Ta spacer in the sensor-type MTJ and solve the aforementioned trade off. The TaFeB spacer had a wider thickness window of the IEC than the case of pure Ta. This allowed us to increase the TaFeB thickness up to 1.0 nm, resulting in an improved TMR ratio of 234% which is higher than the case using the Ta counterpart. The TaFeB spacer enables us to break the ceiling of the sensor-type MTJs, leading to a higher sensitivity.