Voltage Control of Perpendicular Magnetic Anisotropy of CoFeB and Co₂MnGa-based Stacks for Sensing Applications

This PhD research investigates the voltage control of magnetic anisotropy (VCMA) in CoFeB and Co₂MnGa-based thin film stacks with perpendicular magnetic anisotropy, towards a voltage-controlled magnetic sensor based on magnetic tunnel junctions. Here I explored both reversible and irreversible changes in magnetic properties under different experimental conditions. The study primarily utilises ionic liquid gating (ILG) to modulate coercivity and anisotropy in these materials.

In CoFeB stacks, two configurations of the stack with variable thicknesses of materials were tried for achieving perpendicular magnetic anisotropy (PMA). The reversible control of PMA in CoFeB was demonstrated through repeated voltage cycling, with measurable transition times. In addition, through the patterning of magnetic tunnel junctions (MTJs), I improved both their design and tunnel magnetoresistance (TMR), achieving up to ~27% TMR in CoFeB-based thin film stacks.

For PMA Co₂MnGa-based thin films, cyclic gate voltages produce distinct coercivity trends depending on the film thickness. In thinner (2 nm) Co₂MnGa layers, coercivity decreases while the thicker (3.8 nm) films exhibit increased coercivity with voltage cycling. These findings indicate irreversible coercivity changes with ILG in both thicknesses of Co₂MnGa. Through patterning of MTJs in Co₂MnGa-based films, I demonstrated the first Co₂MnGa-based MTJs made from two PMA Co₂MnGa layers and achieved ~11% TMR, highlighting the material's potential for tunnel magnetoresistance applications.

To achieve reversible control in Co₂MnGa (3.8 nm) stacks, Neon ion irradiation (30 keV, 10¹⁴ Ne⁺·cm²) was employed, resulting in reversible coercivity changes during subsequent ILG experiments. This demonstrates that ion mobility is a driver of coercivity changes with ILG and the formation of open sites through irradiation can lead to increased ion mobility, leading to more reversible anisotropy changes.

Additionally, I performed experiments to control magnetic anisotropy using two different voltage-control techniques. I demonstrated piezoelectric control of the hysteresis loop of in-plane magnetised CoFeB thin films, providing insights into tuning anisotropy through piezoelectric strain. I also explored a recently published method of using spin-orbit torque (SOT) for 3D magnetic field sensing, using Co₂MnGa-based thin films. Further optimisation of my structures is necessary for full three-axis sensing.

These findings contribute to the understanding of VCMA and ion-induced modifications in magnetic thin films, offering new perspectives on the design of structures for voltage-tunable magnetoresistive sensors and other voltage-tunable spintronic devices.