Polarization enhancement and ferroelectric switching enabled by interacting magnetic structures in DyMnO₃ thin films

Role of oxygen vacancies in colossal polarization in SmFeO3-δ thin films

The orthorhombic rare-earth manganates and ferrites multiferroics are promising candidates for the next generation multistate spintronic devices. However, their ferroelectric polarization is small, and transition temperature is far below room temperature (RT). The improvement of ferroelectricity remains challenging. Here, through the subtle strain and defect engineering, an RT colossal polarization of 4.14 μC/cm2 is achieved in SmFeO3-δ films, which is two orders of magnitude larger than its bulk and is also the largest one among the orthorhombic rare-earth manganite and ferrite family. Meanwhile, its RT magnetism is uniformly distributed in the film. Combining the integrated differential phase-contrast imaging and density functional theory calculations, we reveal the origin of this superior ferroelectricity in which the purposely introduced oxygen vacancies in the Fe-O layer distorts the FeO6 octahedral cage and drives the Fe ion away from its high-symmetry position. The present approach can be applied to improve ferroelectric properties for multiferroics.

Investigation of Stereometric and Fractal Patterns of Spin-Coated LuMnO3 Thin Films

In this paper, we have performed qualitative and quantitative analysis of LuMnO3 thin films surfaces, deposited by spin coating over Pt(111)/TiO2/SiO2/Si substrates, to evaluate their spatial patterns as a function of the film’s sintering temperature. Atomic force microscopy was employed to obtain topographic maps that were extensively analyzed via image processing techniques and mathematical tools. 3D (three-dimensional) topographical images revealed that films sintered at 650°C and 750°C presented the formation of smoother surfaces, while the film sintered at 850°C displayed a rougher surface with a root mean square roughness of ∼2.5 nm. On the other direction, the height distribution of the surface for all films has similar asymmetries and shape, although the film sintered using the highest temperature showed the lower density of rough peaks and a sharper peak shape. The advanced fractal parameters revealed that the film sintered at 850°C is dominated by low spatial frequencies, showing less spatial complexity, higher microtexture homogeneity, and uniform height distribution. These results suggest that the combination of stereometric and fractal parameters can be especially useful for identification of unique topographic spatial patterns in LuMnO3 thin films, helping in their implementation in technological applications, such as photovoltaic solar cells and information magnetic date storage and spintronic devices.

Non-collinear magnetism & multiferroicity: the perovskite case

The most important types of non-collinear magnetic orders that are realized in simple perovskite oxides are outlined in relation to multiferroicity. These orders are classified and rationalized in terms of a mimimal spin Hamiltonian, based on which the notion of spin-driven ferroelectricity is illustrated. These concepts find direct application in reference materials such as BiFeO3, GdFeO3 and TbMnO3 whose multiferroic properties are briefly reviewed.

Single-phase multiferroics: new materials, phenomena, and physics

Multiferroics, where multiple ferroic orders coexist and are intimately coupled, promise novel applications in conceptually new devices on one hand, and on the other hand provide fascinating physics that is distinctly different from the physics of high-T_C superconductors and colossal magnetoresistance manganites. In this mini-review, we highlight the recent progress of single-phase multiferroics in the exploration of new materials, efficient roadmaps for functionality enhancement, new phenomena beyond magnetoelectric coupling, and underlying novel physics. In the meantime, a slightly more detailed description is given of several multiferroics with ferrimagnetic orders and double-layered perovskite structure and also of recently emerging 2D multiferroics. Some emergent phenomena such as topological vortex domain structure, non-reciprocal response, and hybrid mechanisms for multiferroicity engineering and magnetoelectric coupling in various types of multiferroics will be briefly reviewed.

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.

Solution-processed resistive switching memory devices based on hybrid organic–inorganic materials and composites

We review emerging low-cost solution-processed resistive random-access memory (ReRAM) made of either hybrid nanocomposites or hybrid organo-lead halide perovskites.

The crucial role of Mn spiral spin order in stabilizing the Dy–Mn exchange striction in multiferroic DyMnO3

Mn spiral spin ordering can be a prerequisite for the symmetric Dy–Mn exchange striction in DyMnO₃.

Influence of Fe substitution on the Jahn-Teller distortion and orbital anisotropy in orthorhombic Y(Mn1-xFex)O3 epitaxial films

Multiferroic YMn1-xFexO3(020) (x = 0.125, 0.25, 0.50) epitaxial thin films with an orthorhombic structure (space group Pbnm) were prepared on a YAlO3(010) substrate by pulsed-laser deposition. Upon Fe substitution, the b-axis was clearly shortened, whereas the a- and c-axes were slightly lengthened based on XRD analysis. To understand the influence of orbital polarization and the Jahn-Teller effect of Mn(3+) on Fe substitution and also the local octahedral-site distortion of Fe(3+) in an environment of Jahn-Teller-active Mn(3+) ions in YMn1-xFexO3 films, we measured the polarization-dependent X-ray absorption spectra at the Mn-L2,3 and Fe-L2,3 edges, and also simulated the experimental spectra using configuration-interaction multiplet calculations. Although Δeg for the Mn(3+) ion decreased from 0.9 eV in pure YMnO3 to 0.6 eV in the half-Fe-substituted sample, a single eg electron was still strongly constrained to the d3y(2)-r(2) orbital for all the Fe concentrations tested. The largest Δeg, 0.5 eV, for the Fe(3+) ion was derived for a sample with 12.5% Fe substitution, and gradually decreased to 0.15 eV for the half-Fe-substituted sample. The local octahedral-site distortion of the Fe(3+) ion inside the YMnO3 lattice was similar to that of the Mn(3+) ion, whereas the Jahn-Teller distortion and GdFeO3-type distortion of the Mn(3+) ion were decreased by the spherical high-spin Fe(3+) ions. The combination of the experimental and theoretical data provides both profound insight into the variation of the Jahn-Teller distortion and orbital anisotropy and instructive information about the magnetic structures in these orthorhombic YMn1-xFexO3 thin films.

Non-collinear magnetism in multiferroic perovskites

We present an overview of the current interest in non-collinear magnetism in multiferroic perovskite crystals. We first describe the different microscopic mechanisms giving rise to the non-collinearity of spins in this class of materials. We discuss, in particular, the interplay between non-collinear magnetism and ferroelectric and antiferrodistortive distortions of the perovskite structure, and how this can promote magnetoelectric responses. We then provide a literature survey on non-collinear multiferroic perovskites. We discuss numerous examples of spin cantings driving weak ferromagnetism in transition metal perovskites, and of spin-induced ferroelectricity as observed in the rare-earth based perovskites. These examples are chosen to best illustrate the fundamental role of non-collinear magnetism in the design of multiferroicity.

Continuous Magnetoelectric Control in Multiferroic DyMnO3 Films with Twin-like Domains

The magnetic control of ferroelectric polarization is currently a central topic in the multiferroic researches, owing to the related gigantic magnetoelectric coupling and fascinating physics. Although a bunch of novel magnetoelectric effect have been discovered in multiferroics of magnetic origin, the manipulation of polarization was found to be fundamentally determined by the microscopic origin in a certain multiferroic phase, hindering the development of unusual magnetoelectric control. Here, we report emergent magnetoelectric control in DyMnO₃/Nb:SrTiO₃ (001) films showing twin-like domain structure. Our results demonstrate interesting magnetically induced partial switch of polarization due to the coexistence of polarizations along both the a-axis and c-axis enabled by the twin-like domain structure in DyMnO₃ films, despite the polarization-switch was conventionally believed to be a one-step event in the bulk counterpart. Moreover, a continuous and periodic control of macroscopic polarization by an in-plane rotating magnetic field is evidenced in the thin films. This distinctive magnetic manipulation of polarization is the consequence of the cooperative action of the twin-like domains and the dual magnetic origin of polarization, which promises additional applications using the magnetic control of ferroelectricity.

Multiferroicity and Magnetoelectric Coupling in TbMnO3 Thin Films

In this work, we report the growth and functional characterizations of multiferroic TbMnO3 thin films grown on Nb-doped SrTiO3 (001) substrates using pulsed laser deposition. By performing detailed magnetic and ferroelectric properties measurements, we demonstrate that the multiferroicity of spin origin known in the bulk crystals can be successfully transferred to TbMnO3 thin films. Meanwhile, anomalous magnetic transition and unusual magnetoelectric coupling related to Tb moments are observed, suggesting a modified magnetic configuration of Tb in the films as compared to the bulk counterpart. In addition, it is found that the magnetoelectric coupling enabled by Tb moments can even be seen far above the Tb spin ordering temperature, which provides a larger temperature range for the magnetoelectric control involving Tb moments.

Novel multiferroicity in GdMnO3 thin films with self-assembled nano-twinned domains

There have been many interests in exploring multiferroic materials with superior ferroelectric and magnetic properties for the purpose of developing multifunctional devices. Fabrication of thin films plays an important role in achieving this purpose, since the multiferroicity can be tuned via strain, dimensionality, and size effect, without varying the chemical composition. Here, we report exotic multiferroic behaviors, including high-T_C (~75 K) ferroelectric state, a large spontaneous polarization (~4900 μC/m²) and relatively strong ferromagnetism emerging at ~105 K, in orthorhombic GdMnO₃/SrTiO₃ (001) thin films with self-assembled nano-scale twin-like domains. We propose a possible ab-plane spiral-spin-order phase to be responsible for the large spontaneous polarization in the films, which can only be stabilized by relatively high magnetic field H > 6 T in the bulk crystals. It is suggested that the nano-scale twin-like domain structure is essential for the high temperature ferroelectricity and ferromagnetism of the thin films.

Collagen nanofibers facilitated presynaptic maturation in differentiated neurons from spinal-cord-derived neural stem cells through MAPK/ERK1/2-Synapsin I signaling pathway

Neural stem cells (NSCs) are deemed to be a potential cell therapy for brain and spinal cord reconstruction and regeneration following injury. In this study, we investigated the role of nanofibrous scaffolds on NSCs-derived neurons in the formation of neural networks. Miniature excitatory postsynaptic currents (mEPSCs) were recorded using the whole-cell patch clamp recording method after the spinal cord-derived NSCs were differentiated into neurons and cultured in vitro for 10-14 days. It was observed that the frequency of mEPSCs in the differentiated neurons cultured on both randomly oriented and aligned collagen nanofibrous scaffolds was higher than that on the collagen-coated control and can be inhibited by an ERK inhibitor (PD98059), indicating that the collagen nanofibers affected the maturation of the synapses from presynaptic sites via the MAPK/ERK1/2 pathway. In addition, both of the collagen nanofibers increased the phosphorylation of Synapsin I and facilitated the interaction of p-ERK1/2 and p-Synapsin I. All these results suggested that the collagen nanofibrous scaffolds contributed to the presynaptic maturation via the ERK1/2-Synapsin I signaling pathway.