Electric field induced modification of magnetic and structural properties in thin films

This experimental work focuses on modifying the characteristics of ultra-thin magnetic multilayers through electric field (E-field) driven migration of oxygen. We investigated thin film multilayers composed of Ta/Pt/CoB/Ir or Cu/Pt/HfO2. In these systems, metal layers such as Ir and Cu are utilized to induce structural inversion asymmetry. The Pt gives rise to a strong perpendicular magnetic anisotropy (PMA), spin-orbit coupling (SOC), and the Dzyaloshinskii-Moriya Interaction (DMI). We conducted measurements of magnetic properties such as effective anisotropy and DMI, which are relevant for devices based on skyrmions or domain walls.

The modification of magnetic properties by ionic liquid gating (ILG) is explored as a promising technique in spintronics due to its room temperature application, non-volatility, energy efficiency, and reversibility. We have investigated the oxygen migration via ILG of structures within metallic layers, which provide flexibility in terms of material choice. With an applied negative voltage, oxygen migrates from the HfO2 to the magnetic layer, changing the interfacial chemistry of the magnetic system. As a result, changes in coercive field, domain nucleation field and effective anisotropy. The oxygen migration process is almost fully reversible, and the migrated oxygen produces a nonvolatile effect for up to fifteen weeks.

The magneto-ionic modification of structural and interfacial properties was studied using X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy (STEM), energy dispersive X-ray spectroscopy (EDX), and electron energy loss spectroscopy (EELS). The O-K edge and Co-L2,3 edges of XPS study predict the oxidization of the system during two growths. The XAS and XMCD studies shows the oxidization states are changing with applied voltage and it is therefore changing the magnetic moments. This data is also consistent with SQUID-VSM data. The Pt -M3 edge also confirms the oxidation of the Pt top layer in the as-grown sample, which changes with the applied voltage.

Additionally, interfacial magnetic properties were examined using Brillouin light scattering (BLS). This experimental work aims to understand the origin of DMI in the studied magnetic system. We found that the DMI contribution is significantly coming from top CoB/Heavy metal interface. Applied voltage change the interface chemistry of this interface and changes the DMI, particularly for the Cu/Pt samples.

The magneto-ionic modulation in all metallic structure might be useful in spintronics device application, emphasizing domain wall-based and skyrmion-based spintronics devices. The modulated magnetic properties, in combination with nonvolatility and reversibility, allow for the possibility of field-programmable domain wall and skyrmion devices.