Thermal multiferroics in all-inorganic quasi-two-dimensional halide perovskites

Multiferroic materials, particularly those possessing simultaneous electric and magnetic orders, offer a platform for design technologies and to study modern physics. Despite the substantial progress and evolution of multiferroics, one priority in the field remains to be the discovery of unexplored materials, especially those offering different mechanisms for controlling electric and magnetic orders1. Here we demonstrate the simultaneous thermal control of electric and magnetic polarizations in quasi-two-dimensional halides (K,Rb)3Mn2Cl7, arising from a polar-antipolar transition, as evidenced using both X-ray and neutron powder diffraction data. Our density functional theory calculations indicate a possible polarization-switching path including a strong coupling between the electric and magnetic orders in our halide materials, suggesting a magnetoelectric coupling and a situation not realized in oxide analogues. We expect our findings to stimulate the exploration of non-oxide multiferroics and magnetoelectrics to open access to alternative mechanisms, beyond conventional electric and magnetic control, for coupling ferroic orders.

Calcium Tungstate Doped with Rare Earth Ions Synthesized at Low Temperatures for Photoactive Composite and Anti-Counterfeiting Applications

A precursor was prepared using a co-precipitation method to synthesize crystalline calcium tungstate. The prepared precursor was dried in an oven at 80 °C for 18 h. The dried powders, prepared without a heat treatment process, were observed in XRD analysis to be a crystalline CaWO4 phase, confirming that the synthesis of crystalline CaWO4 is possible even at low temperature. To use this crystalline CaWO4 as a light emitting material, rare earth ions were added when preparing the precursor. The CaWO4 powders doped with terbium (Tb3+) and europium (Eu3+) ions, respectively, were also observed to be crystalline in XRD analysis. The luminescence of the undoped CaWO4 sample exhibited a wide range of 300~600 nm and blue emission with a central peak of 420 nm. The Tb3+-doped sample showed green light emission at 488, 545, 585, and 620 nm, and the Eu3+-doped sample showed red light emission at 592, 614, 651, and 699 nm. Blue, green, and red CaWO4 powders with various luminescence properties were mixed with glass powder and heat-treated at 600 °C to fabricate a blue luminescent PiG disk. In addition, a flexible green and red light-emitting composite was prepared by mixing it with a silicone-based polymer. An anti-counterfeiting application was prepared by using the phosphor in an ink, which could not be identified with the naked eye but can be identified under UV light.

Emergent multiferroicity and strain-driven metal-semiconductor transitions in LaMnO3/RMnO3 superlattices (R = Pr, Pm, Sm and Gd)

It is known that rare-earth manganites LnMnO₃ with Ln = La to Gd are typical Mott insulators favoring the A-type antiferromagnetic (A-AFM) state. Certainly no ferroelectricity can be possible although the alternatively stacked LnO layers are both polar. Nevertheless, under the inspiration that one plus one is more than two, it is appreciated that by combining two components of this manganite series into a superlattice functionality is added. In this work, we construct a (001)-oriented LaMnO₃/RMnO₃ (R = Pr, Pm, Sm and Gd) superlattice and investigate the possible emergent ferroelectricity by means of first-principles calculations. It is revealed that the lattice matching in these superlattices may generate lattice distortions to each component based on the scenario of hybrid improper ferroelectricity, resulting in spontaneous ferroelectric polarization, which is larger than the traditional type II Ln'MnO₃ (Ln' radius is smaller than that of Gd) polarization. In the meantime, the A-AFM state remains the magnetic ground state of these superlattices. Furthermore, it is predicted that the externally imposed in-plane compressive strain can trigger the semiconductor to half-metal transitions accompanying the A-AFM to ferromagnetic (FM) transitions. The present work sheds light on the possibility to design multiferroic materials and functionality by tailoring artificial superlattices/heterostructures from those non-ferroelectric systems, and to design electronic devices by utilizing the electronic transport properties under epitaxial strain.

Magnetism and hybrid improper ferroelectricity in LaMO3/YMO3 superlattices

Using first-principles calculations, we investigate the structural, electronic, and magnetic properties of perovskite LaMO3/YMO3 superlattices (M = Cr, Mn, Co and Ni). It is found that ferroelectricity can emerge in LaMO3/YMO3 superlattices (M = Cr, Mn, Co), allowing them to be promising multiferroic candidates, while no ferroelectricity is found in the LaNiO3/YNiO3 superlattice. The electronic structure calculations indicate that the LaCrO3/YCrO3, LaMnO3/YMnO3, and LaCoO3/YCoO3 superlattices are insulators, and their magnetic ground states exhibit G-type antiferromagnetic (AFM), A-type AFM, and G-type AFM order, respectively, while the LaNiO3/YNiO3 superlattice is however a half-metallic ferromagnet. The electronic structure and magnetic ground state are discussed, based on the projected density of states data and Heisenberg model, respectively, and the magnetic phase transition temperature is evaluated based on mean-field theory. In the meantime, the spontaneous ferroelectric polarization of the LaMO3/YMO3 superlattices (M = Cr, Mn, Co) is determined respectively using the Born effective charge model and Berry phase method, and their hybrid improper ferroelectric character is predicted, with the net polarization mainly from the different displacements of the LaO layers and YO layers along the b-axis. It is suggested that alternative multiferroic materials can be obtained by properly designing superlattices that consist of two non-polar magnetic materials but exhibit tunable magnetic ground states and transition temperature and hybrid improper ferroelectricity.

Effect of transition metal (TM) doping on structural and magnetic properties in hexagonal YMn0.917TM0.083O3 systems

Suitable TM doping at the Mn site is an important access to manipulate magnetic properties of hexagonal YMnO₃, however, it has not yet been systematically explored how the strength of antiferromagnetic interactions and the magnetic transition temperatures (T_N) are modified in the doping YMn_0.917TM_0.083O₃ systems. In the work, we have performed first-principles calculations to study the effect of TM doping on the structural and magnetic properties of hexagonal YMn_0.917TM_0.083O₃ bulks; the results are combined with the available experimental results. The calculated results reveal that the planar TM-O bonds and O-TM-O angles of TMO₅ bipyramid are both prominent structural features for the transformations of magnetic properties. We have also predicted the Ti, V, Cr and Fe doping effects on magnetic properties and further analyzed the TM electronic structures of TMO₅ bipyramid in the YMn_0.917TM_0.083O₃(001)/MgO(001) film configurations, which could provide more understanding towards the designing of new generation multifunctional devices.

Atomic scale imaging of competing polar states in a Ruddlesden-Popper layered oxide

Layered complex oxides offer an unusually rich materials platform for emergent phenomena through many built-in design knobs such as varied topologies, chemical ordering schemes and geometric tuning of the structure. A multitude of polar phases are predicted to compete in Ruddlesden–Popper (RP), A_n+1B_nO_3n+1, thin films by tuning layer dimension (n) and strain; however, direct atomic-scale evidence for such competing states is currently absent. Using aberration-corrected scanning transmission electron microscopy with sub-Ångstrom resolution in Sr_n+1Ti_nO_3n+1 thin films, we demonstrate the coexistence of antiferroelectric, ferroelectric and new ordered and low-symmetry phases. We also directly image the atomic rumpling of the rock salt layer, a critical feature in RP structures that is responsible for the competing phases; exceptional quantitative agreement between electron microscopy and density functional theory is demonstrated. The study shows that layered topologies can enable multifunctionality through highly competitive phases exhibiting diverse phenomena in a single structure.

Competing phases in layered complex oxides are believed to be relevant for emergent phenomena, which still await to be witnessed. Here, Stone et al. report direct atomic-scale imaging of a multitude of polar phases in Ruddlesden-Popper oxide thin films, exhibiting diverse phenomena in a single structure.

Genetic engineering of inorganic functional modular materials

Since the launch of the Materials Genome Initiative by the US government in 2011, many computer techniques have been developed to predict the structures and properties of advanced materials, providing important guidance for laboratory experimentation and a promising new direction for future materials innovation. However, lots of inorganic materials are difficult for computers to process because of their complex three-dimensionally extended structures. Fortunately, many of these materials are built from well-defined stacking layer modules, and the stacking sequences of their layer modules unambiguously determine their three-dimensional structures. Such one-dimensional stacking sequence representation is naturally accessible for computer processing, easing the problems not only of structure elucidation, but also in the enumeration, evaluation, and screening of a large number of unknown materials with desired properties. More importantly, with the aid of various computational methods, we may reveal the relationship between the stacking sequences and the properties of these materials, which is a key prerequisite for function-led targeted synthesis. This Minireview covers the most recent progress in this emerging area.

Effect of divalent Ba cation substitution with Sr on coupled 'multiglass' state in the magnetoelectric multiferroic compound Ba3NbFe3Si2O14

(Ba/Sr)₃NbFe₃Si₂O₁₄ is a magneto-electric multiferroic with an incommensurate antiferromagnetic spiral magnetic structure which induces electric polarization at 26 K. Structural studies show that both the compounds have similar crystal structure down to 6 K. They exhibit a transition, T_N at 26 K and 25 K respectively, as indicated by heat capacity and magnetization, into an antiferromagnetic state. Although Ba and Sr are isovalent, they exhibit very different static and dynamic magnetic behaviors. The Ba-compound exhibits a glassy behavior with critical slowing dynamics with a freezing temperature of ~35 K and a critical exponent of 3.9, a value close to the 3-D Ising model above T_N, in addition to the invariant transition into an antiferromagnetic state. The Sr-compound however does not exhibit any dispersive behavior except for the invariant transition at T_N. The dielectric constant reflects magnetic behavior of the two compounds: the Ba-compound has two distinct dispersive peaks while the Sr-compound has a single dispersive peak. Thus the compounds exhibit coupled ‘multiglass’ behavior. The difference in magnetic properties between the two compounds is found to be due to modifications to super exchange path angle and length as well as anti-site defects which stabilize either ferromagnetic or antiferromagnetic interactions.

Magnetic materials. Tilt engineering of spontaneous polarization and magnetization above 300 K in a bulk layered perovskite

Crystalline materials that combine electrical polarization and magnetization could be advantageous in applications such as information storage, but these properties are usually considered to have incompatible chemical bonding and electronic requirements. Recent theoretical work on perovskite materials suggested a route for combining both properties. We used crystal chemistry to engineer specific atomic displacements in a layered perovskite, (Ca(y)Sr(1- y))(1.15)Tb(1.85)Fe2O7, that change its symmetry and simultaneously generate electrical polarization and magnetization above room temperature. The two resulting properties are magnetoelectrically coupled as they arise from the same displacements.

Understanding ferroelectricity in layered perovskites: new ideas and insights from theory and experiments

Recent theoretical and experimental studies showing how polar structures or ferroelectricity arise in layered perovskites are highlighted.