Simultaneous co-existence of room-temperature(T) ferromagnetism and ferroelectricity in Fe doped BaTiO3 (BTO) is intriguing, as such Fe doping into tetragonal BTO, a room-T ferroelectric (FE), results in the stabilization of its hexagonal polymorph which is FE only below ∼80K. Here, we investigate its origin and show that Fe-doped BTO has a mixed-phase room-temperature multiferroicity, where the ferromagnetism comes from the majority hexagonal phase and a minority tetragonal phase gives rise to the observed weak ferroelectricity. In order to achieve majority tetragonal phase (responsible for room-T ferroelectricity) in Fe-doped BTO, we investigate the role of different parameters which primarily control the PE hexagonal phase stability over the FE tetragonal one and identify three major factors namely, the effect of ionic size, Jahn-Teller (J-T) distortions and oxygen vacancies (OVs), to be primarily responsible. The effect of ionic size which can be qualitatively represented using the Goldschmidt's tolerance (GT) factor seems to be the major dictating factor for the hexagonal phase stability. The understanding of these factors not only enables us to control them but also, achieve suitable co-doped BTO compound with enhanced room-T multiferroic properties.
Resistive switching (RS) in metal oxides, which offers self‐compliance and multiple resistance states without the requirement of any high voltage forming step, holds the potential of application in selector less high density resistive random access memory (RRAM) devices. Typically, operation of metal oxide‐based RS devices requires the integration of additional oxide layers or circuit elements to achieve current compliance and complicated device architecture for high‐density memory applications. In this study, a self‐compliance, and multi‐level RS is demonstrated that does not require high voltage forming in a single layer non‐stoichiometric WO3–x. This study suggests that high oxygen vacancy (VO) concentration in the pristine WO3–x layer leads to its forming‐free filamentary switching characteristics, whereas reversible formation and annihilation of an oxygen‐rich region in the filament at the WO3–x/Pt junction has been envisaged to be responsible for self‐compliance set and voltage controlled multiple reset resistance states. The results demonstrate non‐stoichiometric WO3–x with an active metal/oxide interface permeable to reversible oxygen migration can pave the way for producing high density, reliable RRAM devices.
Ferroelectric (FE) materials usually possess very high band gap (∼3–4 eV) and extremely poor electrical conductivity, which renders them unsuitable for photovoltaic applications. Here, we demonstrate that a carefully designed Bi–Fe codoped BaTiO3 (BTO) system (Ba1–x Bi x Ti0.9Fe0.1O3−δ, 0 ≤ x ≤ 0.10) provides a unique platform with the simultaneous optimization of low band gap, high FE polarization, and reasonable conductivity. We, thereby, find that the Jahn–Teller distortion associated with the doped transition metal ions, tetragonality (c/a), and oxygen vacancy content lead to such a controlled tuning of optical band gap, FE polarization, and electrical conductivity, respectively, over a wide range. While x = 0.00 (only Fe-doped) stabilizes in the undesirable paraelectric-hexagonal phase, x = 0.02 (Bi–Fe codoped) is engineered to possess a low band gap (∼1.55 eV), high FE polarization (∼5.2 μC/cm2) due to significant recovery of the FE tetragonal phase by more than 60%, and reasonably high electrical conductivity compared to BaTiO3, which cause it to exhibit the largest photovoltaic response within the series. Such an approach of optimizing the desired physical properties in a closely related mixed phase material where the ferroelectricity is engineered in the majority tetragonal BTO phase, while the minority hexagonal BTO phase aids in the reasonable conductivity (a combination that is not realizable in usual single phase FE materials), along with an optimum band gap, is promising in the realization of many more potential FE-based photovoltaic materials.
A great deal of interest has grown in both academia and industry toward flexible multiferroics in the recent years. The coupling of ferromagnetic properties with ferroelectric properties in multiferroic materials opens up many opportunities in applications such as magnetoelectric random access memories, magnetic field sensors, and energy harvesters. Multiferroic materials on a flexible platform bring an exciting opportunity for the next generation of consumer electronics owing to their unique characteristics of wearability, portability, and weight reduction. However, the fabrication of flexible multiferroic devices is still a great challenge due to various technical difficulties, including the requirement of high growth temperature of the oxide-based multiferroic materials, their lattice mismatch with the flexible substrates, and the brittleness of the functional layers. In this review article, we will discuss different methods of fabricating flexible or even freestanding oxide films to achieve flexible electronics. This article will address the benefits and challenges of each synthesis method in terms of interlayer interactions and growth parameters. Furthermore, the article will include an account of the possible bending limits of different flexible substrates without degrading the properties of the functional layer. Finally, we will address the challenges, opportunities, and future research directions in flexible multiferroics.
Fe doping into BaTiO3 stabilizes the paraelectric hexagonal phase in place of the ferroelectric tetragonal one. We show that simultaneous doping of Bi along with Fe into BaTiO3 effectively enhances the magnetoelectric (ME) multiferroic response (both ferromagnetism and ferroelectricity) at room temperature, through careful tuning of Fe valency along with the controlled recovery of the ferroelectric-tetragonal phase. We also report a systematic increase in large dielectric constant values as well as reduction in loss tangent values with relatively moderate temperature variation of the dielectric constant around room temperature with increasing Bi doping content in Ba1−xBixTi0.90Fe0.10O3 (0 ≤ x ≤ 0.10), which makes the higher Bi–Fe codoped sample (x = 0.08) promising for use as a room-temperature high-κ dielectric material. Interestingly, the x = 0.08 (Bi–Fe codoped) sample is not only found to be ferroelectrically (∼20 times) and ferromagnetically (∼6 times) stronger than x = 0 (only Fe-doped) at room temperature, but also observed to be better insulating (larger bandgap) with indirect signatures of larger ME coupling as indicated from anomalous reduction of the magnetic coercive field with decreasing temperature. Thus, room-temperature ME multiferroicity has been engineered in Bi and Fe codoped BTO (BaTiO3) compounds.
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