Electric-field based modulation of exchange bias effect in epitaxial oxide heterostructures

Abstract

The control of magnetism by electrical means is of importance for both fundamental science and technological applications. Among these methods, electric-field regulation has motivated numerous explorations due to reduced power consumption in devices. Exchange bias (EB) effect has been widely used in commercial electronic devices like spin valve read heads and magnetic random access memories (MRAM). However, the underlying physical mechanism for EB is still not completely understood. Attempts of electric field tuning the EB have been made in the last few decades to explore the physical origins of EB and to functionalize EB-based devices. In this thesis, I studied electric-field based regulation of EB in antiferromagnetic La0.35Sr0.65MnO₃ (AF-LSMO)/ferromagnetic La0.7Sr0.3MnO₃ (FM-LSMO) bilayer structures and single-layer LaMnO₃ (LMO) films. Before investigating the electric-field based modulation of EB in AF-LSMO/FM-LSMO, the impact of AF layer growth temperature on the EB behaviour in AF-LSMO/FM-LSMO bilayers and low-voltage pulsing effect on FM-LSMO and AF-LSMO single layers were studied. It was demonstrated that the best modulation was obtained in AF-LSMO (15 nm)/FM-LSMO (5 nm) structure. The EB effect was suppressed by positive pulses but only slightly enhanced by negative pulses, suggesting a weak tunability of negative pulses. By analysing high angle annular dark field (HAADF) images and electron energy loss spectroscopy (EELS) for different gating states, oxygen vacancies in AF-LSMO layer created by +10 V pulses were proved to exist from the top surface to the interface area while -10 V pulses can only slightly deplete these oxygen vacancies at the top surface of AF-LSMO layer, which elaborated the strong and weak modulation of EB effect in AF-LSMO/FM-LSMO by positive and negative pulses, respectively. The final study focused on the regulation of EB in single-layer LMO epitaxial thin films via ionic liquid (IL) gating. It was demonstrated that +2 V IL gating had a more significant impact on the structure and magnetic properties of LMO films. The X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements suggested that +2 V IL gating enlarged the out-of-plane lattice constant while keeping the surface morphology and fully strained state, while -2 V IL gating seemed to have little effect. This was consistent with influence of IL gating on the magnetic properties, where +2 V IL gating enhanced the EB while -2 V IL gating showed a weak modulation. EELS measurement suggested that +2 V IL gating reduced oxidation states of Mn ions while -2 V IL gating increased oxidation states of Mn ions. In pristine state, Mn²⁺ and Mn³⁺ coexisted in LMO film. For -2V IL gated state, Mn³⁺ and Mn⁴⁺ dominated, suggesting ferromagnetism in LMO film. This might be the reason why -2V IL gating showed little effect on EB in LMO films. When +2V IL gating was performed, the Mn²⁺ ions were increased at the expense of Mn³⁺ ions. Considering the AF phase comes from superexchange interactions between Mn3⁺ ions, AF phase should be suppressed after +2V IL gating, which is in contrary with the experimental observations. I speculated that the structural transition (from pseudocubic symmetry to orthorhombic symmetry) due to increased oxygen vacancies is possibly linked to the enhanced EB effect in LMO films.