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.
Antennas typically have emission/radiation efficiencies bounded by A/λ2(A < λ2) where A is the emitting area and λ is the emitted wavelength. That makes it challenging to miniaturize antennas to extreme subwavelength dimensions without severely compromising their efficiencies. To overcome this challenge, an electromagnetic (EM) antenna is actuated with a surface acoustic wave (SAW) whose wavelength is about five orders of magnitude smaller than the EM wavelength at the same frequency. This allows to implement an extreme subwavelength EM antenna, radiating an EM wave of wavelength λ = 2 m, whose emitting area is ≈10−8 m2 (A/λ2 = 2.5 × 10−9), and whose measured radiation efficiency exceeds the A/λ2 limit by over 105. The antenna consists of magnetostrictive nanomagnets deposited on a piezoelectric substrate. A SAW launched in the substrate with an alternating electrical voltage periodically strains the nanomagnets and rotates their magnetizations owing to the Villari effect. The oscillating magnetizations emit EM waves at the frequency of the SAW. These extreme subwavelength antennas that radiate with efficiencies a few orders of magnitude larger than the A/λ2 limit allow drastic miniaturization of communication systems.
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.
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.
Ferromagnetic nanostripes have gained massive attention due to their intriguing magnetic properties associated with dimensional confinements and shape anisotropy leading toward potential applications in magnetic storage, memory, and spin‐wave‐based devices. Consequently, reconfiguration of their static and dynamic magnetic properties by the geometric parameters and external field is imperative. Here, a combined experimental and numerical study of the reconfigurable spin‐wave dynamics in arrays of ferromagnetic nanostripes by the stripe thickness and external magnetic field strength and orientation is presented. Different uniform, localized, and standing spin waves in the nanostripes and their monotonic and nonmonotonic variation, including mode merging with these parameters, are observed. The observed variations are interpreted with the aid of simulated spin configurations, magnetostatic field maps, and spin‐wave mode profiles. Further numerical study reveals anisotropic spin‐wave propagation in nanostripes for different thicknesses and in different bias‐field geometry opening potential applications in magnonic circuit components such as reconfigurable magnonic waveguides and omnidirectional spin‐wave emitters.
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