Optimization of asymmetric reference structures through non-evenly layered synthetic antiferromagnet for full bridge magnetic sensors based on CoFeB/MgO/CoFeB

Abstract

Industrial sensor applications rely on the implementation of full Wheatstone bridge architectures, demanding the development of low-cost and mass production methods of magnetic tunnel junctions (MTJ) based on CoFeB/MgO/CoFeB. In particular, monolithic bridge microfabrication has been demonstrated through the double deposition of MTJ stacks engineered by asymmetric reference layers with non-evenly layered synthetic antiferromagnet (SAF) structures. However, extending the standard double magnetic layered SAF into a triple magnetic multilayer system brings critical changes in the overall performance of the reference structure, which directly influences the magnetic stability of the device. Consequently, a theoretical model of a triple magnetic layered AF/SAF structure was developed to support the understanding of the magnetic response of the reference layers, aiming to improve the magnetic stability around zero field. A full MTJ Wheatstone bridge incorporating the optimized double and triple reference structures was microfabricated with a linear and hysteresis-free response. Furthermore, a high thermal endurance of both structures was verified through the measurement of the magnetotransport behavior of each type of MTJ structure within a reversible magnetic field range of ±2 kOe and a temperature sweep from room temperature up to 200 °C.