VP16-01 Magnetotransport Properties of Tunneling Magnetoresistance Devices with CoFeBSi, CoFeB/CoFeBSi and CoFeB/Ru/CoFeBSi Free Layers
Creators: Patryk Łaś
Francisco Matos
Susana Freitas
Paulo Freitas
Rita Macedo
Piotr Wiśniowski
Publisher: name: IEEE
Year: 2025

The ongoing development of TMR devices for detecting ultralow magnetic fields at low frequencies focuses on increasing sensitivity and lowering noise. Sensitivity improvement research aims to find free layer materials and structures with a low saturation field (anisotropy) and a high tunneling magnetoresistance ratio. Several studies [1,2.3] showed that the most promising free layer is based on CoFeB coupled to CoFeBSI. The devices with that free layer showed up to 50% Oe sensitivity and a hysteresis of 4 Oe [3]. However, the devices' field detection depends on both sensitivity and noise. The noise is typically of magnetic origin, resulting from magnetization fluctuations in the free (sensing) layer. Noise is strongly dependent on the free layer's magnetization process and correlates with sensitivity. Thus, investigating the free layer response to magnetic field for the devices is equally important as improving sensitivity. We studied the response to magnetic field of devices with CoFeBSi, CoFeB/CoFeBSi, and CoFeB/Ru/CoFeBSi free layers. We evaluated the sensitivity, saturation (switch) field, coercivity, and coupling field for the devices. We use a two-step annealing process to induce cross anisotropy between the reference and free layer in order to linearize device responses to magnetic fields (transfer curves). Shape anisotropy was also used on some devices to influence their magnetization process. The device without shape anisotropy demonstrated a switching of magnetization for all free layers and high sensitivity (Fig. 1). While the device with shape anisotropy showed rotation of the magnetization and lower sensitivity. These results demonstrated that a two-step annealing process alone is insufficient to achieve linear response in the device. Furthermore, using high aspect ratio devices does not eliminate hysteresis in devices with those types of free layers.References: [1] M.Kato et al. ,Fabrication of Magnetic Tunnel Junctions with Amorphous CoFeSiB Ferromagnetic Electrode for Magnetic Field Sensor Devices Appl. Phys. Express 6, 103004 (2013). [2] R. Rasly,et al. Magnetic, magnetoresistive and low-frequency noise properties of tunnelmagnetoresistance sensor devices with amorphous CoFeBTa soft magnetic layersJ. Phys. D.Appl. Phys. 54, 095002 (2021) [3] M. Nakano Enhanced sensitivity and thermal tolerance in tunnel magnetoresistance sensor using Ta-doped CoFeSiB soft magnetic layer Appl. Phys. Lett. 123, 072404 (2023).
The ongoing development of TMR devices for detecting ultralow magnetic fields at low frequencies focuses on increasing sensitivity and lowering noise. Sensitivity improvement research aims to find free layer materials and structures with a low saturation field (anisotropy) and a high tunneling magnetoresistance ratio. Several studies [1,2.3] showed that the most promising free layer is based on CoFeB coupled to CoFeBSI. The devices with that free layer showed up to 50% Oe sensitivity and a hysteresis of 4 Oe [3]. However, the devices' field detection depends on both sensitivity and noise. The noise is typically of magnetic origin, resulting from magnetization fluctuations in the free (sensing) layer. Noise is strongly dependent on the free layer's magnetization process and correlates with sensitivity. Thus, investigating the free layer response to magnetic field for the devices is equally important as improving sensitivity. We studied the response to magnetic field of devices with CoFeBSi, CoFeB/CoFeBSi, and CoFeB/Ru/CoFeBSi free layers. We evaluated the sensitivity, saturation (switch) field, coercivity, and coupling field for the devices. We use a two-step annealing process to induce cross anisotropy between the reference and free layer in order to linearize device responses to magnetic fields (transfer curves). Shape anisotropy was also used on some devices to influence their magnetization process. The device without shape anisotropy demonstrated a switching of magnetization for all free layers and high sensitivity (Fig. 1). While the device with shape anisotropy showed rotation of the magnetization and lower sensitivity. These results demonstrated that a two-step annealing process alone is insufficient to achieve linear response in the device. Furthermore, using high aspect ratio devices does not eliminate hysteresis in devices with those types of free layers.References: [1] M.Kato et al. ,Fabrication of Magnetic Tunnel Junctions with Amorphous CoFeSiB Ferromagnetic Electrode for Magnetic Field Sensor Devices Appl. Phys. Express 6, 103004 (2013). [2] R. Rasly,et al. Magnetic, magnetoresistive and low-frequency noise properties of tunnelmagnetoresistance sensor devices with amorphous CoFeBTa soft magnetic layersJ. Phys. D.Appl. Phys. 54, 095002 (2021) [3] M. Nakano Enhanced sensitivity and thermal tolerance in tunnel magnetoresistance sensor using Ta-doped CoFeSiB soft magnetic layer Appl. Phys. Lett. 123, 072404 (2023).