Development of magnetic tunnel junctions with symmetric R-H curve for specific sensor application

Abstract

Magnetic Tunnel Junctions (MTJ) are widely used as magnetic sensors but their major disadvantage is their high noise level at low frequency. Some methods to overcome 1/f noise involve adding magnetic modulation [1] or increasing the MTJ area [2]. In order to get a linear response, in-plane MTJ are usually designed with the free layer (FL) set perpendicularly to the reference. However, for a specific application [3], a symmetric R-H characteristic is more suitable to drive a modulation and should be easily obtained by pinning the FL in the same direction as the reference.The aim of this study is to process MTJs with symmetric R-H curve and high sensitivity. The magnetic stack is composed of a pinned synthetic antiferromagnet, MgO tunnel barrier and CoFeB/NiFe free layer. After patterning micron-size junctions of different shapes, we perform a post-process annealing under magnetic field. In this work, we study different methods to achieve a coherent rotation of the FL magnetization towards the hard axis: i) applying a small external field along the easy axis; ii) using shape anisotropy; iii) using soft pinning by adding an antiferromagnetic layer on the FL separated by a thin Ta spacer [4].Our measurements highlight the importance of FL pinning on the reversibility and smoothness of the R-H curves. Nevertheless, since pinning reduces sensitivity, an optimum trade-off must be found. We observed that shape anisotropy alone is not sufficient to pin the FL (fig.1), but a coherent rotation is achieved by applying an external field. Moreover, our results show the importance of a good alignment between the reference and the easy axis. Compensating for the process misalignement by tilting the applied field allows to reduce the pinning field to 5 Oe (fig.2) with a sensitivity of 3.5%/mT. On the other hand, soft pinning can provide internal pinning ranging from 120 to 0 Oe by increasing the Ta spacer up to 0.25 nm. Choice of the optimum spacer thickness and further process optimizations should reduce hysteresis and improve the sensitivity.  Fig.1: R-H curves measured along easy axis Hy and hard axis Hx  Fig. 2: R-H curves measured with respect to the reference misalignment (1.6°) and pinning fields Hpin of 5 and -5 Oe