No abstract

Complex oxide heterointerfaces, which play host to an incredible variety of interface physical phenomena, are of great current interest in introducing new functionalities to systems. Here, coherent super‐tetragonal BiFeO3/LaAlO3 and rhombohedral BiFeO3/LaAlO3 heterointerfaces are investigated by using a combination of high‐angle annular dark‐field (HAADF) imaging and annular bright‐field (ABF) imaging in a spherical aberration (Cs) corrected scanning transmission electron microscope (STEM), and first‐principles calculations. The complicated ferroelectric polarization pinning and relaxation that occurs at both interfaces is revealed with atomic resolution, with a dramatic change in structure of BiFeO3, from cubic to super‐tetragonal‐like. The results enable a detailed explanation to be given of how non‐bulk phase structures are stabilized in thin films of this material.

We have deposited self-assembled BiFeO3-CoFe2O4 (BFO-CFO) thin films on (100)-oriented SrRuO3-buffered Pb(Mg1/3Nb2/3)0.62Ti0.38O3 (PMN-38PT) single crystal substrates. These heterostructures were used for the study of real-time changes in the magnetization with applied DC electric field (EDC). With increasing EDC, a giant magnetization change was observed along the out-of-plane (easy) axis. The induced magnetization changes of the CFO nanopillars in the BFO/CFO layer were about ΔM/MrDC = 93% at EDC = −3 kv/cm. A giant converse magnetoelectric (CME) coefficient of 1.3 × 10−7 s/m was estimated from the data. By changing EDC, we found multiple(N ≥ 4) unique possible values of a stable magnetization with memory on the removal of the field.

(Pb0.98, La0.02)(Zr0.95, Ti0.05)O3 (PLZT) thin films of 300 nm thickness were epitaxially deposited on (100), (110), and (111) SrTiO3 single crystal substrates by pulsed laser deposition. X‐ray diffraction line and reciprocal space mapping scans were used to determine the crystal structure. Tetragonal ((001) PLZT) and monoclinic MA ((011) and (111) PLZT) structures were found, which influenced the stored energy density. Electric field‐induced antiferroelectric to ferroelectric (AFE→FE) phase transitions were found to have a large reversible energy density of up to 30 J/cm3. With increasing temperature, an AFE to relaxor ferroelectric (AFE→RFE) transition was found. The RFE phase exhibited lower energy loss, and an improved energy storage efficiency. The results are discussed from the perspective of crystal structure, dielectric phase transitions, and energy storage characteristics. Besides, unipolar drive was also performed, providing notably higher energy storage efficiency values due to low energy losses.

Utilizing temperature dependent dielectric/impedance spectroscopy, multi-dielectric responses involving two dielectric relaxations (DRs) and two magnetic-order-associated dielectric anomalies were observed in polycrystalline DyMnO3. It is elucidated that both DRs’ dynamics, established in terms of equivalent circuit model and small polaron (SP) theories, are closely linked with localized SP migration features. Namely, low-temperature relaxation process can be attributed to short range polaronic variable-range-hopping induced dipolar-type relaxation in grains, whereas the higher-temperature one is due to Maxwell-Wagner relaxation at grain/grain boundary interfaces, which are governed by SP nearest-neighbor-hopping conduction. Additionally, magnetic-orders-associated dielectric anomalies may be assigned to strong spin-lattice couplings by magnetoelasticity-aroused lattice deformation.

Highly ordered Bi 2 FeMnO 6 epitaxial thin films have been successfully grown on SrTiO 3 substrate.Both high-flux synchrotron X-ray diffraction reciprocal space mapping and high resolution transmission electron microscopy confirmed the alternative alignment of Fe and Mn along [111] direction of Bi 2 FeMnO 6 films. Magnetic and ferroelectric properties of Bi 2 FeMnO 6 films are characterized and analyzed. The room-temperature ferroelectricity is well kept in Bi 2 FeMnO 6 film as expected.However, it is very interesting that Bi 2 FeMnO 6 film exhibits a typical spin-glass behavior and very weak magnetism rather than a ferri/ferromagnetism as generally believed. Our first-principles calculations suggest a spin frustration model for Bi 2 FeMnO 6 , which can well explain the intriguing magnetic property of Bi 2 FeMnO 6 film.

Different (1–3) heterostructures, such as BiFeO3-CoFe2O4 and BiFeO3-CuFe2O4 on Pb(Mg1/3Nb2/3)0.74Ti0.26O3 (PMN-26PT), were selected for study as possible materials for magnetoelectric (ME) random access memory. The (1–3) heterostructures were deposited, and multimagnetic states were found under different E-field (E) conditions. Upon removal of E, two possible remnant magnetization states remained stable. If an H-field (H) was also applied, two additional stable remnant magnetization states were found. Our investigations demonstrate (1–3) heterostructures with nonvolatility even though the individual phases/substrates had only volatile properties. This simplifies materials selection for multistate systems based on these heterostructures, averting difficulties with compositional nonuniformity and property repeatability, in particular, with regard to PMN-xPT crystal substrates. With such N≥4 magnetic state systems, a multilevel-cell memory device could readily be built with high ME coupling and numerous accessible magnetic states.

We report a systematic theoretical study on the ferroelectric behavior of ultrathin three-component ferroelectric films, e.g., CaTiO3-BaTiO3-SrTiO3, sandwiched between electrodes. Using first-principles calculations we demonstrate that such structures have intrinsic asymmetric ferroelectricity which is robust even at the nanoscale. In addition, there exists a certain relationship between the polarization directions and geometric stacking sequences of the superlattices. Specifically, the lowest energy states always have polarizations pointing from CaTiO3 via BaTiO3 to SrTiO3, while the sequence in the metastable states is SrTiO3-BaTiO3-CaTiO3. Therefore we were able to distinguish one ferroelectric state from its opposite state by means of their geometric stackings along the polarization directions. Besides this, band alignment analysis reveals that such structures are well behaved at the metal/ferroelectric interface, confirming the credibility and reliability of our first-principles calculation. Our finding may suggest a controllable and unambiguous way to build ferroelectric and multiferroic tunnel junctions.