Amanda Haglund

Influence of Co-doping on the Crystal Structure, Magnetocaloric Properties and Elastic Moduli of the La(Fe, Si)13 Compound

The La(Fe, Si)13 compound shows a large magnetocaloric effect near room temperature. Partial substitution of Co for Fe can raise its Curie temperature and reduce the thermal hysteresis of its ferromagnetic transition. However, the influence of Co substitution on the crystal structure and magnetocaloric properties of La(Fe, Si)13 is not well understood yet. In this work, we report a comparative study of the crystal structure, magnetocaloric effects and elastic moduli of polycrystalline LaFe11.5Si1.5 and LaCoFe10.5Si1.5 samples using synchrotron radiation X-ray diffraction, magnetic measurements and resonant ultrasound spectroscopy. Compared to the Co-free sample, the Co-doped sample shows a sluggish ferromagnetic transition and reduced volumetric expansion as well as softer elastic moduli at room temperature. The Co-doped sample also shows stronger temperature dependence of Fe-Fe distances in its ferromagnetic state. Such differences between the samples are explained by considering the influence of Co doping on ferromagnetic interactions and lattice entropy.

Ion Migration Studies in Exfoliated 2D Molybdenum Oxide via Ionic Liquid Gating for Neuromorphic Device Applications

The formation of an electric double layer in ionic liquid (IL) can electrostatically induce charge carriers and/or intercalate ions in and out of the lattice which can trigger a large change of the electronic, optical, and magnetic properties of materials and even modify the crystal structure. We present a systematic study of ionic liquid gating of exfoliated 2D molybdenum trioxide (MoO3) devices and correlate the resultant electrical properties to the electrochemical doping via ion migration during the IL biasing process. A nearly 9 orders of magnitude modulation of the MoO3 conductivity is obtained for the two types of ionic liquids that are investigated. In addition, notably rapid on/off switching was realized through a lithium-containing ionic liquid whereas much slower modulation was induced via oxygen extraction/intercalation. Time of flight-secondary ion mass spectrometry confirms the Li intercalation. Density functional theory (DFT) calculations have been carried out to examine the underlying metallization mechanism. Results of short-pulse tests show the potential of these MoO3 devices as neuromorphic computing elements due to their synaptic plasticity.

Impact of gate geometry on ionic liquid gated ionotronic systems

Ionic liquid electrolytes are gaining widespread application as a gate dielectric used to control ion transport in functional materials. This letter systematically examines the important influence that device geometry in standard “side gate” 3-terminal geometries plays in device performance of a well-known oxygen ion conductor. We show that the most influential component of device design is the ratio between the area of the gate electrode and the active channel, while the spacing between these components and their individual shapes has a negligible contribution. These findings provide much needed guidance in device design intended for ionotronic gating with ionic liquids.