Implementing Room-Temperature Multiferroism by Exploiting Hexagonal-Orthorhombic Morphotropic Phase Coexistence in LuFeO3 Thin Films

Hexagonal Lu1-xInxFeO3 Room-Temperature Multiferroic Thin Films

The hexagonal rare earth ferrites h-RFeO₃(R = rare earth element) have been recognized as promising candidates for a room-temperature multiferroic system, and the primary issue for these materials is how to get a stable hexagonal structure since the centrosymmetric orthorhombic structure is generally stable for most RFeO₃ at room-temperature, while the hexagonal phase is only stable under some strict conditions. In the present work, h-Lu_1-xIn_xFeO₃ (x = 0-1) thin films were prepared on a Nb-SrTiO₃ (111) single-crystal substrate by a pulsed laser deposition (PLD) process, and the multiferroic characterization was performed at room temperature. With the combined effects of chemical pressure and epitaxial strain, the stable hexagonal structure was achieved in a wide composition range (x = 0.5-0.7), and the results of XRD (X-ray diffraction) and SAED (selected area electron diffraction) indicate the super-cell match relations between the h-Lu_0.3In_0.7FeO₃ thin film and substrate. The saturated P-E hysteresis loop was obtained at room temperature with a remanent polarization of about 4.3 μC/cm², and polarization switching was also confirmed by PFM measurement. Furthermore, a strong magnetoelectric coupling with a linear magnetoelectric coefficient of 1.9 V/cm Oe was determined, which was about three orders of magnitude larger than that of h-RFeO₃ ceramics. The present results indicate that the h-Lu_1-xIn_xFeO₃ thin films are expected to have great application potential for magnetoelectric memory and detection devices.

Polar Magnetism Above 600 K with High Adaptability in Perovskite Oxides

High magnetic order temperature, sustainable polar insulating state, and tolerance to device integrations are substantial advantages for applications in next-generation spintronics. However, engineering such functionality in a single-phase system remains a challenge owing to the contradicted chemical and electronic requirements for polar nature and magnetism, especially with an ordering state highly above room temperature. Perovskite-related oxides with unique flexibility allow electron-unpaired subsystems to merge into the polar lattice to induce magnetic interactions, combined with their inherent asymmetry, thereby promising polar magnet design. Herein, by atomic-level composition assembly, a family of Ti/Fe co-occupied perovskite oxide films Pb(Ti_1-x,Fe_x)O₃ (PFT(x)) with a Ruddlesden-Popper superstructure are successfully synthesized on several different substrates, demonstrating exceptional adaptability to different integration conditions. Furthermore, second-harmonic generation measurements convince the symmetry-breaking polar character. Notably, a ferromagnetic ground state up to 600 K and a steady insulating state far beyond room temperature were achieved simultaneously in these films. This strategy of constructing layered modular superlattices in perovskite oxides could be extended to other strongly correlated systems for triggering nontrivial quantum physical phenomena.

Flux Method Growth and Structure and Properties Characterization of Rare-Earth Iron Oxides Lu1−xScxFeO3 Single Crystals

Perovskite rare-earth ferrites (REFeO3) have attracted great attention for their high ferroelectric and magnetic transition temperatures, strong magnetoelectric coupling, and electric polarization. We report on the flux method growth of rare-earth iron oxide Lu1−xScxFeO3 single crystals through a K2CO3-B2O3-Bi2O3 mixture as a flux solution, and give a detailed characterization of the microstructure, magnetism, and ferroelectric properties. X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) measurements revealed that the obtained single crystals can be designated to three different crystal structures of different chemical compositions, that is, Lu0.96Sc0.04FeO3 (perovskite phase), Lu0.67Sc0.33FeO3 (hexagonal phase), and Lu0.2Sc0.8FeO3 (bixbyite phase), respectively. Magnetic measurements indicate that the perovskite Lu0.96Sc0.04FeO3 is an anisotropic hard ferromagnetic material with a high Curie transition temperature, the bixbyite Lu0.2Sc0.8FeO3 is a low temperature soft ferromagnetic material, and the hexagonal Lu0.67Sc0.33FeO3 exhibits multiferroic properties. Lu0.67Sc0.33FeO3 possesses a weak ferromagnetic transition at about 162 K. We further investigate the ferroelectric domain structures in hexagonal sample by scanning electron microscope and the characteristic atomic structures in ferroelectric domain walls by atomically resolved scanning transmission electron microscope. Our successful growth of perovskite Lu1−xScxFeO3 single crystals with distinct crystal structures and stochiometric Lu-Sc substitutions is anticipated to provide a useful ferrites system for furthering exploitation of their multiferroic properties and functionalities.

Room-Temperature Magnetoelectric Coupling in Electronic Ferroelectric Film based on [(n-C3H7)4N][FeIIIFeII(dto)3] (dto = C2O2S2)

Great importance has been attached to magnetoelectric coupling in multiferroic thin films owing to their extremely practical use in a new generation of devices. Here, a film of [(n-C₃H₇)₄N][Fe^IIIFe^II(dto)₃] (1; dto = C₂O₂S₂) was fabricated using a simple stamping process. As was revealed by our experimental results, in-plane ferroelectricity over a wide temperature range from 50 to 300 K was induced by electron hopping between Fe^II and Fe^III sites. This mechanism was further confirmed by the ferroelectric observation of the compound [(n-C₃H₇)₄N][Fe^IIIZn^II(dto)₃] (2; dto = C₂O₂S₂), in which Fe^II ions were replaced by nonmagnetic metal Zn^II ions, resulting in no obvious ferroelectric polarization. However, both ferroelectricity and magnetism are related to the magnetic Fe ions, implying a strong magnetoelectric coupling in 1. Through piezoresponse force microscopy (PFM), the observation of magnetoelectric coupling was achieved by manipulating ferroelectric domains under an in-plane magnetic field. The present work not only provides new insight into the design of molecular-based electronic ferroelectric/magnetoelectric materials but also paves the way for practical applications in a new generation of electronic devices.

Interplay of Magnetic Properties and Doping in Epitaxial Films of h-REFeO3 Multiferroic Oxides

Multiferroic materials demonstrating coexistence of magnetic and ferroelectric orders are promising candidates for magnetoelectric devices. While understanding the underlying mechanism of interplaying of ferroic properties is important, tailoring their properties to make them potential candidates for magnetoelectric devices is challenging. Here, the antiferromagnetic Neel ordering temperature above 200 K is realized in successfully stabilized epitaxial films of (Lu,Sc)FeO₃ multiferroic oxide. The first-principles calculations show the shrinkage of in-plane lattice constants of the unit cells of the films on different substrates which corroborates well the enhancement of the Neel ordering temperature (T_N ). The profound effect of lattice strain/stress at the interface due to differences of in-plane lattice constants on out of plane magnetic properties and on spin reorientation temperature in the antiferromagnetic region is further elucidated in the epitaxial films with and without buffer layer of Mn-doped LuFeO₃ . Writing and reading ferroelectric domains reveal the ferroelectric response of the films at room temperature. Detailed electron microscopy shows the presence of lattice defects in atomic scale. First-principles calculations show that orbital rehybridization of rare-earth ions and oxygen is one of the main driving force of ferroelectricity along c-axis in thin films of hexagonal ferrites.

Structure Quality of LuFeO3 Epitaxial Layers Grown by Pulsed-Laser Deposition on Sapphire/Pt

Structural quality of LuFeO3 epitaxial layers grown by pulsed-laser deposition on sapphire substrates with and without platinum Pt interlayers has been investigated by in situ high-resolution X-ray diffraction (reciprocal-space mapping). The parameters of the structure such as size and misorientation of mosaic blocks have been determined as functions of the thickness of LuFeO3 during growth and for different thicknesses of platinum interlayers up to 40 nm. By means of fitting of the time-resolved X-ray reflectivity curves and by in situ X-ray diffraction measurement, we demonstrate that the LuFeO3 growth rate as well as the out-of-plane lattice parameter are almost independent from Pt interlayer thickness, while the in-plane LuFeO3 lattice parameter decreases. We reveal that, despite the different morphologies of the Pt interlayers with different thickness, LuFeO3 was growing as a continuous mosaic layer and the misorientation of the mosaic blocks decreases with increasing Pt thickness. The X-ray diffraction results combined with ex situ scanning electron microscopy and high-resolution transmission electron microscopy demonstrate that the Pt interlayer significantly improves the structure of LuFeO3 by reducing the misfit of the LuFeO3 lattice with respect to the material underneath.

Two-Dimensional Ferroics and Multiferroics: Platforms for New Physics and Applications

Two-dimensional (2D) ferroics, including ferromagnets, ferroelectrics, ferroelastics, and multiferroics, recently have been theoretically proposed or experimentally revealed. The research has attracted tremendous attention because of the novel physics and promising applications for nanoelectronics, revealing ferroics in the 2D limit. In the present Perspective, we comprehensively review the recent research progress and also the proposed applications of 2D ferromagnetic, ferroelectric, and ferroelastic materials from theoretical and experimental viewpoints. We then introduce the coupling between ferroic orders and highlight the latest research on 2D multiferroic materials. The promising research directions and outlooks are discussed at the end of the Perspective. It is expected that the comprehensive overview of 2D ferroic materials can provide guidelines for researchers in the area and inspire further explorations of new physics and ferroic devices.

Lattice strain-enhanced exsolution of nanoparticles in thin films

Nanoparticles formed on oxide surfaces are of key importance in many fields such as catalysis and renewable energy. Here, we control B-site exsolution via lattice strain to achieve a high degree of exsolution of nanoparticles in perovskite thin films: more than 1100 particles μm⁻² with a particle size as small as ~5 nm can be achieved via strain control. Compressive-strained films show a larger number of exsolved particles as compared with tensile-strained films. Moreover, the strain-enhanced in situ growth of nanoparticles offers high thermal stability and coking resistance, a low reduction temperature (550 °C), rapid release of particles, and wide tunability. The mechanism of lattice strain-enhanced exsolution is illuminated by thermodynamic and kinetic aspects, emphasizing the unique role of the misfit-strain relaxation energy. This study provides critical insights not only into the design of new forms of nanostructures but also to applications ranging from catalysis, energy conversion/storage, nano-composites, nano-magnetism, to nano-optics.

Dispersion of metallic nanoparticles is promising for energy conversion and storage, but gaining control of size and distribution is not trivial. Here the authors use lattice mismatch to manipulate exsolution of nanoparticles, achieving a high population of small nanoparticles in perovskite thin films.

A breakthrough in the intrinsic multiferroic temperature region in Prussian blue analogues

Thin films of [(Fe^II_xCr^II_1−x)]_1.5[Cr^III(CN)₆]·yH₂O (x ≈ 0.30–0.35, y ≈ 1.77) (1) on FTO substrates (namely film 1) were synthesized with an electrochemical method. Investigation of the ferroelectricity of film 1 at different temperatures reveals that it exhibits ferroelectric behaviour in the temperature range from 10 K to 310 K. Study of the X-ray absorption (XAS) of the crushed film 1 and simulation of the structure of film 1 and crushed film 1 by using the Materials Studio software indicate that the vacancy defects and interactions between the film and FTO substrate make a key contribution to the ferroelectricity of film 1. Owing to the magnetic phase transition point being up to 210 K, film 1 is a multiferroic material and its magneto/electric coexistence temperature can be as high as 210 K.

Prussian blue analogue film exhibits ferroelectric from 10 to 310 K and works up to 210 K as a molecular-based multiferroic material.

Nanoscale LuFeO3: shape dependent ortho/hexa-phase constitution and nanogenerator application

In multiferroic LuFeO₃ the hexagonal (-h) phase is an intermediate metastable phase encountered during the amorphous to orthorhombic (-o) transformation and is ferroelectric in nature. Thus far it has only been stabilized in a substrate-supported few layered ultrathin film form. Herein we show that the surface-induced strain field intrinsically present in nano-systems can self-stabilize this phase and the hexagonal to orthorhombic phase constitution ratio depends on the shape of the nanomaterial. Thus, nanoparticles (nanofibres) strain-stabilize the o : h ratio of about 75 : 25 (23 : 77). The inclusion of nano-LuFeO₃ into PDMS renders impressive nanogenerator performance, consistent with the ferroelectric phase content.

Antiphase boundary in antiferromagnetic multiferroic LuMn0.5Fe0.5O3: anomalous ferromagnetism, exchange bias effect and large vertical hysteretic shift

The emergence of exchange bias effect in Fe₃O₄ thin films has been since attributed to the presence of anti phase boundary (APB) growth defects despite lack of direct experimental evidence. In the present report, APB induced anomalous weak ferromagnetism and exchange bias property of single-phase antiferromagnetic (AFM) system LuMn_0.5Fe_0.5O₃ (LMFO) is discussed and ⁵⁷Fe Mössbauer spectroscopy and high resolution transmission electron microscopy (HRTEM) measurements are used to probe the origin of the observed effect. In addition to the sextet component corresponding to the long range AFM ordering, the measured Mössbauer spectra reveal the presence of a small component (10%-12%) near zero velocity with unusually small internal field. This indicates the presence of APB defects. From micro structural investigations using HRTEM, presence of APB type defects and dislocations are confirmed. In addition to the exchange bias effect, upon field cooling, hysteresis loop exhibits large vertical shift due to strong pinning effect of the APB. Finally we further annealed the optimally sintered sample LMFO and studied the evolution of defects, and their influence on weak ferromagnetism and exchange bias properties. Our present experimental findings may pave the way in creating new functionalities in materials using APB-type growth defects.

Switchable ferroelectric photovoltaic effects in epitaxial h-RFeO3 thin films

Ferroelectric photovoltaics (FPVs) have drawn much attention owing to their high stability, environmental safety, and anomalously high photovoltages, coupled with reversibly switchable photovoltaic responses. However, FPVs suffer from extremely low photocurrents, which is primarily due to their wide band gaps. Here, we present a new class of FPVs by demonstrating switchable ferroelectric photovoltaic effects and narrow band-gap properties using hexagonal ferrite (h-RFeO3) thin films, where R denotes rare-earth ions. FPVs with narrow band gaps suggest their potential applicability as photovoltaic and optoelectronic devices. The h-RFeO3 films further exhibit reasonably large ferroelectric polarizations (4.7-8.5 μC cm-2), which possibly reduces a rapid recombination rate of the photo-generated electron-hole pairs. The power conversion efficiency (PCE) of h-RFeO3 thin-film devices is sensitive to the magnitude of polarization. In the case of the h-TmFeO3 (h-TFO) thin film, the measured PCE is twice as large as that of the BiFeO3 thin film, a prototypic FPV. The effect of electrical fatigue on FPV responses has been further investigated. This work thus demonstrates a new class of FPVs towards high-efficiency solar cell and optoelectronic applications.

Enhanced Switchable Ferroelectric Photovoltaic Effects in Hexagonal Ferrite Thin Films via Strain Engineering

Ferroelectric photovoltaics (FPVs) are being extensively investigated by virtue of switchable photovoltaic responses and anomalously high photovoltages of ∼10⁴ V. However, FPVs suffer from extremely low photocurrents due to their wide band gaps (E_g). Here, we present a promising FPV based on hexagonal YbFeO₃ (h-YbFO) thin-film heterostructure by exploiting its narrow E_g. More importantly, we demonstrate enhanced FPV effects by suitably exploiting the substrate-induced film strain in these h-YbFO-based photovoltaics. A compressive-strained h-YbFO/Pt/MgO heterojunction device shows ∼3 times enhanced photovoltaic efficiency than that of a tensile-strained h-YbFO/Pt/Al₂O₃ device. We have shown that the enhanced photovoltaic efficiency mainly stems from the enhanced photon absorption over a wide range of the photon energy, coupled with the enhanced polarization under a compressive strain. Density functional theory studies indicate that the compressive strain reduces E_g substantially and enhances the strength of d-d transitions. This study will set a new standard for determining substrates toward thin-film photovoltaics and optoelectronic devices.