Tunneling Electroresistance Induced by Interfacial Phase Transitions in Ultrathin Oxide Heterostructures

The ferroelectric (FE) control of electronic transport is one of the emerging technologies in oxide heterostructures. Many previous studies in FE tunnel junctions (FTJs) exploited solely the differences in the electrostatic potential across the FTJs that are induced by changes in the FE polarization direction. Here, we show that in practice the junction current ratios between the two polarization states can be further enhanced by the electrostatic modification in the correlated electron oxide electrodes, and that FTJs with nanometer thin layers can effectively produce a considerably large electroresistance ratio at room temperature. To understand these surprising results, we employed an additional control parameter, which is related to the crossing of electronic and magnetic phase boundaries of the correlated electron oxide. The FE-induced phase modulation at the heterointerface ultimately results in an enhanced electroresistance effect. Our study highlights that the strong coupling between degrees of freedom across heterointerfaces could yield versatile and novel applications in oxide electronics

Observation of Ferroelectricity and Structure-Dependent Magnetic Behavior in Novel One-Dimensional Motifs of Pure, Crystalline Yttrium Manganese Oxides

Multiferroic materials, such as nanostructured <i>h</i>-YMnO₃, are expected to fulfill a crucial role as active components of technological devices, particularly for information storage. Herein, we report on the template mediated sol–gel synthesis of unique one-dimensional nanostructured motifs of hexagonal phase YMnO₃, possessing a space group of <i>P</i>6₃<i>cm</i>. We found that the inherent morphology of the as-obtained <i>h</i>-YMnO₃ nanostructures was directly impacted by the chemical composition of the employed membrane. Specifically, the use of anodic alumina and polycarbonate templates promoted nanotube and nanowire formation, respectively. Isolated polycrystalline nanotubes and single crystalline nanowires possessed diameters of 276 ± 52 nm, composed of 17 nm particulate constituent grains, and 125 ± 21 nm, respectively, with lengths of up to several microns. The structures and compositions of all our as-prepared products were probed by XRD, SEM, HRTEM, EXAFS, XANES, SAED, and far-IR spectroscopy. In the specific case of nanowires, we determined that the growth direction was mainly along the <i>c</i>-axis and that discrete, individual structures gave rise to expected ferroelectric behavior. Overall, our YMnO₃ samples evinced the onset of a spin-glass transition at 41 ± 1 K for both templateless bulk control and nanowire samples but at 26 ± 3 K for nanotubes. Interestingly, only the as-synthesized crystalline nanotubular mesh gave rise to noticeably enhanced magnetic properties (i.e., a higher magnetic moment of 3.0 μ_B/Mn) as well as a lower spin-glass transition temperature, attributable to a smaller constituent crystallite size. Therefore, this work not only demonstrates our ability to generate viable one-dimensional nanostructures of a significant and commercially relevant metal oxide but also contributes to an understanding of structure–property correlations in these systems

A Generalizable Multigram Synthesis and Mechanistic Investigation of YMnO₃ Nanoplates

The reproducible gram-scale synthesis of crystalline nanoscale multiferroics is critical for the development of the next generation of commercially relevant electronic devices. Of the subset of multiferroic materials, yttrium manganese oxide (YMnO₃) is highly attractive, because of its large magneto-electric coupling constants and the recent observation of giant polarization under pressure in these types of rare earth manganites. Utilizing a unique synthetic methodology that combines metal–oleate thermal degradation with the use of a molten salt protocol, we were able to reproducibly generate monodisperse distributions of morphologically distinctive yttrium manganese oxides. Specifically, using a molten NaCl flux, we were able to synthesize phase-pure, single-crystalline hexagonal YMnO₃ nanoplates, measuring 441 ± 241 nm in diameter and 46 ± 6 nm in height. Moreover, these nanoplates gave rise to multiferroic behavior, which was confirmed by the observation of a ferroelectric phase from a combination of high-resolution TEM (HRTEM) and selected-area electron diffraction (SAED) analysis. Magnetic measurements are consistent with the onset of a spin glass state below 5 K. To highlight the generalizability of the synthetic method we have developed herein, as a demonstration of principle, we have also successfully used the same protocol to produce nanocubes of lanthanum aluminum oxide (LaAlO₃)