Periodic Reporting for period 1 - FemtoSense (Pushing the limits of detection in spintronic sensors into the femtoTesla range)
Periodo di rendicontazione: 2024-04-01 al 2026-03-31
The development of innovative and compact magnetic field sensors with high sensitivity and ultra-low detection levels is a key ingredient for advancing the next generation precision tools in healthcare and more energy-efficient sensing technologies. This project addressed spintronic sensor technologies, with a focus on tunnel magnetoresistance (TMR) devices. These are promising candidates for pushing beyond picoTesla range detectivities in the low-frequency regime (<100 Hz) and at room temperature. This provides the base specifications for applications in biomagnetic signal detection, autonomous driving, electrical vehicles sensorization, even opening doors to novel quantum metrology systems.
The fundamental TMR sensors structures are composed of nanometer scale multilayer thin films, where the basic blocks include two ferromagnetic layers (e.g. Fe, Co, Ni, or their alloys) separated by a tunnel oxide barrier (e.g. AlO or MgO). TMR devices offer advantages such as elements with reduced dimensions, tunable sensitivity, compatibility with CMOS fabrication processes, and a broad range of operation temperatures.
The FEMTOSense project aimed at delivering novel material based solutions to meet the stringent performance requirements of ultra-sensitive TMR sensors. The project proposed to bridge fundamental physical knowledge of the mechanisms that govern intrinsic noise contributions by using materials science and device engineering to manipulate the physical properties of thin films. Specifically, the chosen approaches focused on the precise tuning of the compositional, structural and magnetic properties of the main building blocks in TMR multilayers, with particular emphasis on the sensing and barrier layers.
The fundamental TMR sensors structures are composed of nanometer scale multilayer thin films, where the basic blocks include two ferromagnetic layers (e.g. Fe, Co, Ni, or their alloys) separated by a tunnel oxide barrier (e.g. AlO or MgO). TMR devices offer advantages such as elements with reduced dimensions, tunable sensitivity, compatibility with CMOS fabrication processes, and a broad range of operation temperatures.
The FEMTOSense project aimed at delivering novel material based solutions to meet the stringent performance requirements of ultra-sensitive TMR sensors. The project proposed to bridge fundamental physical knowledge of the mechanisms that govern intrinsic noise contributions by using materials science and device engineering to manipulate the physical properties of thin films. Specifically, the chosen approaches focused on the precise tuning of the compositional, structural and magnetic properties of the main building blocks in TMR multilayers, with particular emphasis on the sensing and barrier layers.
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.
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.
FEMTOSense has successfully produced high-quality TMR multilayered structures with enhanced magnetic and structural properties. These structures meet the magnetic requirements for highly sensitive field detection, such as those necessary for portable biomagnetic sensors operating at room temperature.
This project has established a strong foundation for future developments aimed at addressing key challenges in pushing the detection limits of TMR sensors. Further studies will focus on the performance of these materials under harsh environmental conditions, including long term thermal stress and others, to have a complete picture of the commercial viability of the developed materials. Additionally, the compatibility of these new soft ferromagnetic electrodes with various tunnel barriers will be investigated.
These efforts will also contribute to refining the noise models currently used to describe these sensing devices, helping to identify where the research community should focus next. Ultimately, this work will enable future researchers to fully explore the performance of these devices and advance their integration into complete commercial systems with improved performance.
This project has established a strong foundation for future developments aimed at addressing key challenges in pushing the detection limits of TMR sensors. Further studies will focus on the performance of these materials under harsh environmental conditions, including long term thermal stress and others, to have a complete picture of the commercial viability of the developed materials. Additionally, the compatibility of these new soft ferromagnetic electrodes with various tunnel barriers will be investigated.
These efforts will also contribute to refining the noise models currently used to describe these sensing devices, helping to identify where the research community should focus next. Ultimately, this work will enable future researchers to fully explore the performance of these devices and advance their integration into complete commercial systems with improved performance.