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Abstract: Epitaxial films of the ferromagnetic manganite La0.7⁢Sr0.3⁢MnO3 on substrates of the ferroelectric perovskite BaTiO3 are known to display sharp magnetic changes and large magnetoelectric effects when the film is strained by the substrate undergoing thermally driven structural transitions and ferroelectric domain switching, respectively. However, only a single component of the in-plane magnetization has been hitherto imaged. Here we present magnetic vector maps—obtained from photoemission electron microscopy images with magnetic contrast from x-ray magnetic circular dichroism—to show that the electrically and thermally driven changes of local and global magnetization are deterministically influenced by the state of the substrate while also being complex and sample dependent. Our findings, supported by ferromagnetic resonance data and vibrating sample magnetometry, reveal that the behavior of La0.7⁢Sr0.3⁢MnO3 films on BaTiO3 substrates is not well predicted from knowledge of each system, probably due to long-range strain between BaTiO3 domains. In the future, it would be interesting to reduce complexity by patterning the film into regions between which magnetic communication is negligible.

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Abstract: BiFeO3-BaTiO3 (BF-BT) ceramics are one of the most significant multiferroic materials that attract attention for multiple applications such as piezoelectric transducers, sensors and pulse-power capacitors. Various strategies have been utilized to improve the functional properties of BF-BT ceramics, including the use of donor or acceptor dopants, additional perovskite compounds and post-sintering heat treatment procedures such as quenching and annealing. In the present study, the positive impact of donor dopant Ti4+ substituting for the B-site Fe3+ cations in the BF compound in BF-BT ceramics on ferroelectric properties and electrical resistivity were systematically investigated. Furthermore, the effects of air-quenching and annealing procedures on the structure, microstructure, dielectric, ferroelectric and piezoelectric properties of 0.7BF-0.3BT ceramics were evaluated, with particular attention to the emergence of a novel and previously unreported ferroelectric hardening mechanism induced by annealing. In the Ti-doped 0.7BF-0.3BT ceramic, characteristic BF-enriched core and BT-enriched shell features were observed, becoming more pronounced with increasing Ti concentration; the appearance of these microstructural core-shell features were associated with the development of distinct peaks in the dielectric permittivity-temperature relationship. In addition, the introduction of Ti4+ in BF-BT ceramics reduced both the dielectric loss and electrical conductivity at high temperatures, due to the suppression of the oxygen vacancy concentration and associated p-type conduction mechanism. For trace amounts of Ti doping (0.25 and 0.5 mol%), the ferroelectric polarization-electric field (P-E) hysteresis loops became more saturated, leading to enhanced maximum and remanent polarization values, together with reduced coercive field; while higher Ti concentrations were associated with slim and constricted P-E loops. In comparison with the as-sintered 0.7BF-0.3BT ceramic, the quenched materials exhibited enhanced ferroelectric properties and piezoelectric coefficient, d33, of approximately 130 pC N-1, which was attributed to the increase of rhombohedral distortion. Further annealing of the quenched materials at a temperature of 600 °C for 200 hours induced a transformation to a pseudo-cubic phase, which was attributed to the stabilization of the high temperature structure and the formation of a more distorted nano-domain configuration at room temperature. On the other hand, annealing at lower temperature (500 and 550 °C) retained the rhombohedral distortion and ordered ferroelectric domain configuration of the quenched material and increased the Curie temperature from 500 °C to 650 °C. For the ferroelectric properties, the well-saturated P-E loops of the quenched material developed a slim and constricted appearance after annealing, leading to better recoverable storage density (Wrec) and efficiency (η which refers to the ratio of recoverable energy density to the total stored energy density) of the QA550 material, measured under an electric field of 9.8 kV mm-1, and demonstrating potential for energy storage applications and temperature-independent dielectrics. Furthermore, in-situ synchrotron XRD measurements showed that the domain switching fraction in the low-temperature annealed material was negligibly small, indicating the presence of a domain pinning mechanism with extraordinary strength. Subsequent investigations into annealing pre-poled specimens of 0.7BF-0.3BT ceramics at a temperature of 500 °C for 200 hours revealed the presence of an outstanding internal bias field of 5 kV mm-1, which is 7 times larger than that in a typical ‘hard’ PZT ceramic. Compared to the quenched ceramic, Ti-doped BF-BT ceramic annealed at temperature of 500 °C for 6 hours was found to enhance the ferroelectric properties.

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Abstract: The elastic degree of freedom is widely exploited to mediate magnetoelectric coupling between ferromagnetic films and ferroelectric substrates. For epitaxial Fe films grown on clean BaTiO3 substrates, shear strain can determine the underlying magnetoelastic coupling. Here, we use PhotoEmission Electron Microscopy of ferroic Fe and BaTiO3 domains, combined with micromagnetic simulations, to directly reveal an inverted interfacial magnetoelastic coupling in the low-dimensional limit. We show that the magnetocrystalline anisotropy competes with the epitaxial shear strain to align the local magnetization of ultrathin Fe films close to the local polarization direction of the ferroelectric BaTiO3 in-plane domains. Poling the BaTiO3 substrate creates c-domains with no shear strain contribution with the local magnetization rotated by ~45°. Tuning shear strain magnetoelastic contributions suggests new routes for designing magnetoelectric devices.

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Abstract: This study investigates the intrinsic magnetism and field-driven spin alignment in NiI2 using X-ray absorption spectroscopy and X-ray magnetic circular dichroism (XMCD). NiI2, a van der Waals material, exhibits helimagnetic and type-II multiferroic behavior. This study reveals robust XMCD signals across paramagnetic, antiferromagnetic, and helimagnetic phases under applied out-of-plane fields up to 6 T, while no net moment emerges at zero field. Atomic multiplet calculations confirm a covalent Ni 3d ground state with a significantly reduced spin moment. The results establish the intrinsic nature of NiI2's magnetism and clarify its field-driven spin alignment mechanism. This comprehensive spectroscopic characterization lays the foundation for future applications of NiI2 in advanced spintronic and multiferroic devices, despite challenges posed by its low transition temperature in the monolayer limit. Future research should focus on enhancing its critical temperature through doping, strain engineering, or heterostructure fabrication.

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Abstract: BiFeO3-BaTiO3 (BF-BT) solid solutions have great potential as high-temperature piezoelectric transducers and energy storage dielectrics. However, the effects of donor doping in BF-BT on the local chemical heterogeneity and corresponding control of ferroelectric properties are not well investigated. In this study, it is shown that substitution of Nb5+ for Fe3+ at a concentration of only 0.1 at% in 0.75BF-0.25BT ceramics can induce pronounced core-shell microstructural features, which are not evident for pure BF-BT ceramics or those doped with 0.1 at% Nb5+ for Ti4+. The spatial distribution of Nb, confirmed by Nano-SIMS with exceptional resolution and sensitivity, reveals the role of Nb as an aliovalent solute that inhibits chemical homogenization, stabilizing the formation of Bi-, Fe-enriched core and Ba-, Ti-enriched shell regions at high temperatures, and reducing inter-diffusion during sintering. Electric field-induced domain switching and lattice strain measurements, obtained by in-situ high-energy synchrotron X-ray diffraction, revealed the effects of elastic constraint between the core and shell regions, which degraded the dielectric, ferroelectric, and piezoelectric properties. In contrast, substitution of 0.1 at% Nb on the Ti4+ site gave rise to more homogeneous materials and induced a softening effect with enhanced functional properties. This study provides an advanced investigation into the effects of trace amounts of donor dopant in BF-BT ceramics and offers valuable insights into optimizing doping strategy to control their microstructure and functional properties.