Defect Engineering Of Perpendicular Magnetic Anisotropy In Cobalt-Based Thin Films

Magnetic sensors have widespread applications in industrial, security, space, and biomedical fields, offering precise measurement of magnetic fields using principles like the Hall effect and magnetoresistance. Magnetic tunnel junctions (MTJs) have played a pivotal role in advancing magnetic sensing through tunnelling magnetoresistance (TMR), contributing to spintronic devices, memory systems, and magnetic sensors. The magnetic anisotropy of the reference ferromagnetic layer in MTJ sensors is a critical factor, influencing sensitivity, accuracy, and sensing direction. Perpendicular magnetic anisotropy (PMA) is particularly advantageous for detecting magnetic fields perpendicular to the sensor plane, opening possibilities in non-destructive testing and magnetic read heads. However, conventional sensors are restricted to fixed axes, hindering their ability to measure fields in three dimensions. To address this limitation, this thesis proposes a magnetic sensor with a tuneable sensing axis, enabling comprehensive field detection in all axes. As part of this exploration, the thesis investigates ion irradiation as a post-processing technique to modify the PMA of ferromagnetic layers, offering versatility for various applications.

Cobalt-based materials, especially CoFeB, are well-known for their suitability in MTJs due to high TMR with MgO and PMA at low thicknesses. Half-metallic Heusler alloys, like Co2MnGa, have gained attention for their theoretically infinite TMR, high spin polarization, and high PMA. This research assesses the effects of ion irradiation on ultra-thin Co2MnGa and CoFeB stacks with PMA.

Ion irradiation is a versatile tool for precisely tailoring magnetic stack properties with uniformity and reproducibility. This thesis investigates the impact of 30 keV Ar and Ne ion irradiation on CoFeB and Co2MnGa thin film stacks at low fluences (1 × 1013 to 1 × 1015 ion·cm−2), introducing displacement per atom (DPA) as a predictive parameter for ion-induced magnetic property changes. Results show a consistent reduction in PMA for both materials in half-MTJ stacks after irradiation, which is linked to intermixing of the magnetic layer with its adjacent MgO and heavy metal layers. Ar irradiation has a more pronounced effect due to higher DPA, while Ne irradiation has a milder impact. Co2MnGa displays relatively more resilience to irradiation due to reduced intermixing with adjacent layers, compared to CoFeB. Furthermore, the thesis explores the dynamic tuning of magnetic properties through ionic liquid gating, allowing anisotropy and coercivity manipulation via voltage control. It also delves into ion irradiation's potential to enhance the magneto-ionic effect. Lastly, a novel magnetic sensor design is introduced featuring a tuneable sensing axis, enabling three-dimensional detection capabilities.