Impact of the Synthetic Antiferromagnet in TMR Sensors for Improved Angular- Dependent Output

Tunneling magnetoresistive (TMR) sensors can precisely measure magnetic fields, being a key element in next-generation highly sensitive angular positioning systems. Still, when fields are applied in directions that differ from the easy axis, the interplay between all magnetic contributions becomes crucial. In this work, we investigate the angular-dependent output of TMR sensors, when operating under a field that saturates the free layer (FL). We have used synthetic antiferromagnets (SAFs) with tuned antiferromagnetic <inline-formula> <tex-math notation="LaTeX">$(\Delta H)$ </tex-math></inline-formula> by changing the ferromagnetic (FM) layer thicknesses. The reference layer (RL) reversal field <inline-formula> <tex-math notation="LaTeX">$\left({H}_0^{+}\right)$ </tex-math></inline-formula> was set between 57 and 260 mT. The results show that these SAF characteristics affect the angular-dependent FL saturation field, showing differences up to ≈6 mT when changing SAF magnetic moment. From <inline-formula> <tex-math notation="LaTeX">$R(H, \theta)$ </tex-math></inline-formula>, and using a macrospin model, we evaluate the magnetization misalignment in-between FM layers. For the SAF with higher H+−, deviations up to 10°, are seen when operating at 100 mT <inline-formula> <tex-math notation="LaTeX">$\ll H_0^{+}$ </tex-math></inline-formula>. Noticeably, we also observed a nonnegligible contribution of the SAF magnetization reversal to <inline-formula> <tex-math notation="LaTeX">$R(H, \theta)$ </tex-math></inline-formula> even at the low external fields of 20 mT, with the deviations of ≈1° from ideal behavior, which is often overlooked. This study links the observed resistance deviations to specific magnetic couplings and intrinsic anisotropies. With this information, we are able to design the multilayers for controlled output inaccuracies or according to the maximum allowed errors in the application. Ultimately, an optimal saturation magnetic field is obtained to deliver the best angular performance in the TMR elements.