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Voltage-tunable magnetic hysteresis and domain patterns by electrochemical reactions

Subject Area Experimental Condensed Matter Physics
Physical Chemistry of Solids and Surfaces, Material Characterisation
Term from 2018 to 2023
Project identifier Deutsche Forschungsgemeinschaft (DFG) - Project number 400178764
 
Final Report Year 2023

Final Report Abstract

Magneto-ionic control of magnetism relies on voltage-triggered ion migration and electrochemical reactions to modulate magnetic properties. A key benefit of magneto-ionic approaches (compared to conventional magnetoelectrics) is the room-temperature device operation a low voltage, which presents a route to energy-efficient computing or sensing devices. In the project, we investigated the electrochemical control of the magnetic hysteresis and domains for Fe- and Co-based thin films and heterostructures. To probe the magnetic domain changes during polarization of the films in liquid electrolyte, an electrochemical cell compatible with Kerr microscopy was developed. In FeOx/Fe films with uniaxial in-plane anisotropy, we achieved a robust and reversible on-off switching of magnetic hysteresis by electrochemical surface oxidation and reduction. In-depth analysis revealed that this “magnetic de-blocking” is associated with an increase in both the anisotropy and the domain size and relies on changes in the domain wall interactions with other walls, and with grain boundary oxides. With this approach, we demonstrated voltage controlled 180° magnetization switching with high energy-efficiency at a low (constant) magnetic field. In patterned FeOx/Fe thin film stripes, we revealed voltage-induced changes in magnetoresistance due to local oxidation/reduction at the grain boundaries. In a next step, we combined the voltage-tunable FeOx/Fe layer with an antiferromagnetic IrMn underlayer. This enabled non-volatile magneto-ionic control of in-plane exchange bias. The proposed mechanism of a redox-induced ferromagnetic-layer thickness change is consistent with a simple model for the exchange bias and has been proven by surface analysis. In collaboration with the Beach group (MIT, Cambridge, USA), we also demonstrated magneto-ionic control of perpendicular exchange bias with Co and GdCo as voltage-tunable ferro- and ferrimagnetic layers. Besides sputtered thin films, we investigated iron-based electrodeposited structures with a high surface area for magneto-ionics. We achieved large magneto-ionic effects and on-offswitching of magnetization in FeOOH nanoplatelets and FeOx/Fe nanoislands, respectively. In iron nanocuboids, we identified the critical size for the transition between single domain and vortex state, which become interesting for future magneto-ionic control of 3D nanomagnets. Overall, the project results demonstrate reversible, low-voltage control of magnetic properties such as magnetization, magnetic domains, hysteresis, magnetoresistance and exchange bias of metal oxide/metal films and nanostructures by electrochemical reactions. We provided an in-depth understanding of the magneto-ionic mechanisms by using complementary and newly developed analysis techniques. The grain boundaries and the surface-to-volume ratio were identified as key factor for magneto-ionic control and will be studied further in the future.

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