The FEMTOSense project successfully demonstrated novel functional thin-film sensing layers with ultra-soft magnetic properties based on NiFeX and CoBX alloys, optimized for integration into TMR multilayers. Systematic structural and magnetic characterization confirmed their soft magnetic behavior, one essential requirement for achieving linear output curves and partially mitigate intrinsic magnetic noise.
The project started with the fabrication of a complete TMR multilayer exhibiting state-of-the-art magnetic performance for sensing applications. This involved fine-tuning of deposition parameters, thermal annealing protocols, and stack engineering to realize a top-pinned TMR structure. This served as the baseline for integrating advanced sensing layers.
Two novel materials were the developed for the sensing layer. Firstly, the amorphous CoBTa thin films were developed via co-sputtering, with systematic variation of growth conditions to optimize composition and magnetic properties. The optimal alloy exhibited a coercivity as low as 0.03 mT, meeting the stringent requirements for linear and high-sensitivity magnetic sensors. Secondly, the NiFe(Si) thin-films which exhibited competitive soft-magnetic properties and high thermal stability. The optimized composition showed a saturation magnetization of 525 kA/m, clear uniaxial anisotropy, and a saturation field of approximately 1.69 mT after annealing at 400ºC.
These materials were then integrated into MgO-based TMR multilayers. The final stack comprised a top-pinned synthetic-antiferromagnetic reference layer, a high-quality crystalline MgO tunnel barrier, and a composite sensing layer with either amorphous-like CoBTa or quasi-crystalline NiFeSi. The latter, showed superior soft-ferromagnetic behavior and higher temperature resilience compared to conventional materials as NiFe, with high saturation magnetization, low coercivity and intrinsic magnetic anisotropy. An optimized two-step annealing process ensured linear M(H) characteristics around zero field, critical for sensor operation.
These results push the field beyond the current state of the art. Traditional approaches rely on thick NiFe layers to achieve small magnetic anisotropy in the sensing layer. However, NiFe fcc(111) texture is incompatible with the pre-requisite bcc(001) crystalline order of CoFeB/MgO/CoFeB TMR systems, mandatory to achieve high TMR ratios. In addition, NiFe soft-magnetic properties deteriorate upon annealing above 300°C. Alternatively, amorphous CoFeBX (X = Si, Ta) alloys have been explored. Although they showed high thermal resilience, and in some cases they also displayed inferior magnetic properties compared to NiFe, such as lower saturation magnetization and higher saturation fields hindering compatibility in industry processing. FEMTOSense overcomes these limitations by delivering ultra-soft magnetic sensing layers as well as thermally robustness and compatibility with high-performance TMR multilayer.
