Understanding and tailoring oxygen ion motion for ultra-low power control of magnetism
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Abstract
Digitization and artificial intelligence are increasingly shaping our everyday lives. However, their rapid adoption comes with a steep rise in energy consumption. At the same time, the climate crisis is accelerating dramatically. Consequently, it is crucial to further improve the energy efficiency of information and communication technologies, ensuring that their power demand can be met by renewable energies. A promising approach is the use of magnetic materials, which allow for non-volatile, ultra-low power switching between states. The energy consumption to manipulate the magnetic states can be minimized by using electric fields. Since no current is flowing, this avoids ohmic losses.
In this thesis, voltages were applied across an ionic liquid placed on top of the magnetic stack. This enables particularly high electric fields. Thereby, oxygen ion migration was induced which modifies the properties of the underlying magnetic layers. The resulting magnetic changes were characterized using anomalous Hall effect and superconducting quantum interference device measurements.
For Co/PtMn bilayers, this work revealed that ionic modulation can be used in such coupled ferromagnet/antiferromagnet thin films to rotate the ferromagnetic magnetization from an in-plane toward a stable out-of-plane direction while preserving the exchange bias. To achieve this rotation, a mere reduction of the demagnetization energy by decreasing the Co thickness – through oxidation or thinner growth – was shown to be insufficient. However, voltage-driven oxygen migration increases the perpendicular magnetic anisotropy via Co-3d O-2p hybridization. This was identified as the main driver of the magnetization rotation. When the order of ferromagnetic and antiferromagnetic layer is inverted, the out-of-plane magnetization of Co remains unchanged under gating, while the exchange bias can be tuned. Notably, this indicates that oxygen ions penetrated 6 nm metallic PtMn to reach the Co/PtMn interface.
To better understand voltage-induced oxide depth profiles and maximum penetration depths in metals, tantalum was investigated as a model system due to its smooth growth. It was capped with HfO₂, a good oxygen conductor. By X-ray reflectivity and X-ray photoelectron spectroscopy characterization, oxygen was shown to advance as fully oxidized layers with a sharp metal-oxide interface of a few Å. Overall, oxygen ions penetrate to depths of up to 4 nm into the Ta film at gate voltages as low as -3 V. This penetration depth can be highly relevant for magnetic applications with ultrathin functional layers, where buried layers and interfaces may be reached. To initiate the oxidation, a threshold electric field of approximately -2.8 MV/cm (corresponding to approximately -1 V gate voltage), is required, with higher fields needed as the oxide layer thickens. The primary source of the mobile oxygen ions is most likely the ionic liquid, while oxygen within the HfO₂ plays a minor role. Consistently, thinner HfO₂ capping layers increase oxygen penetration due to the enhanced electric field.
These results advance the understanding of ionic-liquid-gating-induced oxygen migration in metallic and magnetic thin films and support the design of next-generation energy-efficient magnetic devices.