Non-collinear magnetism in multiferroic perovskites

Lattice excitations in NdFeO3 through polarized optical spectroscopies

The possibility of inducing new polar and/or magnetic transient states through the pumping of optical phonons towards the non-linear regime has renewed the scientific interest in orthoferrites. Nonetheless, to perform these studies it is fundamental to have a deep knowledge of the lattice excitations at equilibrium conditions. In this work, we present a complete characterization of the optically-active zone-center phonons in NdFeO3 single crystals at room temperature by means of polarized Raman and infrared spectroscopies. The study is complemented with polarized infrared spectroscopy at 4 K and unpolarized Raman scattering at 10 K. The predicted polar phonons were successfully observed together with some of the crystal-field excitations. First-principles simulations further allow the eigenmode and symmetry assignments of the optical phonons. The calculated atomic motions of each mode are of significant interest, as they are common for all orthoferrites and to most of the large family of orthorhombic Pbnm perovskites.

Negative magnetization and magnetic ordering of rare earth and transition metal sublattices in NdFe0.5Cr0.5O3

We investigate the effect of alloying at the 3dtransition metal site of a rare-earth-transition metal oxide, by considering NdFe0.5Cr0.5O3mixed perovskite with two equal and random distribution of 3d ions, Cr and Fe, interacting with an early 4f rare earth ion, Nd. Employing temperature- and field- dependent magnetization measurements, temperature-dependent x-ray diffraction, neutron powder diffraction, and Raman spectroscopy, we characterize its structural and magnetic properties. Our study reveals bipolar magnetic switching (arising from negative magnetization) and magnetocaloric effect which underline the potential of the studied mixed perovskite in device application. The neutron diffraction study shows the absence of spin reorientation transition over the entire temperature range of 1.5-320 K, although both parent compounds exhibit spin orientation transition. We discuss the microscopic origin of this curious behavior. The neutron diffraction results also reveal the ordering of Nd spins at an unusually high temperature of about 40 K, which is corroborated by Raman measurements.

Ultrafast Optically Induced Perturbation of Oxygen Octahedral Rotations in Multiferroic BiFeO3 Thin Films

The functional properties of complex oxides, including magnetism and ferroelectricity, are closely linked to subtle structural distortions. Ultrafast optical excitations provide the means to manipulate structural features and ultimately to affect the functional properties of complex oxides with picosecond-scale precision. We report that the lattice expansion of multiferroic BiFeO3 following above-bandgap optical excitation leads to distortion of the oxygen octahedral rotation (OOR) pattern. The continuous coupling between OOR and strain was probed using time-resolved X-ray free-electron laser diffraction with femtosecond time resolution. Density functional theory calculations predict a relationship between the OOR and the elastic strain consistent with the experiment, demonstrating a route to employing this approach in a wider range of systems. Ultrafast control of the functional properties of BiFeO3 thin films is enabled by this approach because the OOR phenomena are related to ferroelectricity, and via the Fe-O-Fe bond angles, the superexchange interaction between Fe atoms.

Theoretical Justification of Structural, Magnetoelectronic and Optical Properties in QFeO3 (Q = Bi, P, Sb): A First-Principles Study

One of the primary objectives of scientific research is to create state-of-the-art multiferroic (MF) materials that exhibit interconnected properties, such as piezoelectricity, magnetoelectricity, and magnetostriction, and remain functional under normal ambient temperature conditions. In this study, we employed first-principles calculations to investigate how changing pnictogen elements affect the structural, electronic, magnetic, and optical characteristics of QFeO3 (Q = Bi, P, SB). Electronic band structures reveal that BiFeO3 is a semiconductor compound; however, PFeO3 and SbFeO3 are metallic. The studied compounds are promising for spintronics, as they exhibit excellent magnetic properties. The calculated magnetic moments decreased as we replaced Bi with SB and P in BiFeO3. A red shift in the values of ε2(ω) was evident from the presented spectra as we substituted Bi with Sb and P in BiFeO3. QFeO3 (Q = Bi, P, SB) showed the maximum absorption of incident photons in the visible region. The results obtained from calculating the optical parameters suggest that these materials have a strong potential to be used in photovoltaic applications.

Nanoscale Size Effects on Push-Pull Fe-O Hybridization through the Multiferroic Transition of Perovskite ϵ-Fe2O3

Multiferroics have tremendous potential to revolutionize logic and memory devices through new functionalities and energy efficiencies. To reach their optimal capabilities will require better understanding and enhancement of the ferroic orders and couplings. Herein, we use ϵ-Fe2O3 as a model system with a simplifying single magnetic ion. Using 15, 20, and 30 nm nanoparticles, we identify that a modified and size-dependent Fe-O hybridization changes the spin-orbit coupling and strengthens it via longer octahedra chains. Fe-O hybridization is modified through the incommensurate phase, with a unique two-step rearrangement of the electronic environment through this transition with attraction and then repulsion of electrons around tetrahedral Fe. Interestingly, size effects disappear in the high-temperature phase where the strongest Fe-O hybridization occurs. By manipulating this hybridization, we tune and control the multiferroic properties.

Insight into the Temperature-Dependent Canting of 4f Magnetic Moments from 4f Photoemission

The orientation of the 4f moments offers an additional degree of freedom for engineering the spin-related properties in spintronic nanostructures of lanthanides. Yet, precise monitoring of the direction of magnetic moments remains a challenge. Here, on the example of the antiferromagnets HoRh₂Si₂ and DyRh₂Si₂, we investigate the temperature-dependent canting of the 4f moments near the surface. We demonstrate that this canting can be understood in the framework of crystal electric field theory and the exchange magnetic interaction. Using photoelectron spectroscopy, we disclose subtle but certain temperature-dependent changes in the line shape of the 4f multiplet. These changes are directly linked to the canting of the 4f moments, which is different for the individual lanthanide layers near the surface. Our results illustrate the opportunity to monitor the orientation of the 4f-moments with high precision, which is essential for development of novel lanthanide-based nanostructures, interfaces, supramolecular complexes, and single-molecule magnets for various applications.

Substitution engineering of lead-free halide perovskites for photocatalytic applications assisted by machine learning

Lead-free perovskites (A₃B₂X₉) have drawn much attention in recent years. However, a thorough understanding of these materials is still in its early stages. This is because A₃B₂X₉ perovskites have large-scale component tunability, in which the A⁺, B³⁺, and X^- ions can be replaced or partially substituted with other elements. Here, based on density functional theory and machine learning techniques we propose a data-driven method to find suitable configurations for photocatalytic water splitting. By replacing atoms in A₃B₂X₉, 3.4 million configurations are constructed and studied. Our results show that the substitutional position plays an important role in determining the photocatalytic performance. Specifically, the co-existence of Br and I elements is favorable for X-sites, while for B-site atoms, it is better to choose atoms from groups IIIB and IIIA with a period greater than 3. Considering their rarity and toxicity, we believe that In is a good choice for B-sites and propose CsRb₂BiInBr₅I₄ as a promising candidate. These results may provide guidance for the discovery of novel lead-free perovskites for photocatalytic applications.

Ferrimagnetic Ordering and Spin-Glass State in Diluted GdFeO3-Type Perovskites (Lu0.5Mn0.5)(Mn1-xTix)O3 with x = 0.25, 0.50, and 0.75

ABO₃ perovskite materials with small cations at the A site, especially those with ordered cation arrangements, have attracted a great deal of interest because they show unusual physical properties and deviations from the general characteristics of perovskites. In this work, perovskite solid solutions (Lu_0.5Mn_0.5)(Mn_1-xTi_x)O₃ with x = 0.25, 0.50, and 0.75 were synthesized by means of a high-pressure, high-temperature method at approximately 6 GPa and approximately 1550 K. All the samples crystallize in the GdFeO₃-type perovskite structure (space group Pnma) and have random distributions of the small Lu³⁺ and Mn²⁺ cations at the A site and Mn^4+/3+/2+ and Ti⁴⁺ cations at the B site, as determined by Rietveld analysis of high-quality synchrotron X-ray powder diffraction data. Lattice parameters are a = 5.4431 Å, b = 7.4358 Å, c = 5.1872 Å (for x = 0.25); a = 5.4872 Å, b = 7.4863 Å, c = 5.2027 Å (for x = 0.50); and a = 5.4772 Å, b = 7.6027 Å, c = 5.2340 Å (for x = 0.75). Despite a significant dilution of the A and B sublattices by non-magnetic Ti⁴⁺ cations, the x = 0.25 and 0.50 samples show long-range ferrimagnetic order below T_C = 89 K and 36 K, respectively. Mn cations at both A and B sublattices are involved in the long-range magnetic order. The x = 0.75 sample shows a spin-glass transition at T_SG = 6 K and a large frustration index of approximately 22. A temperature-independent dielectric constant was observed for x = 0.50 (approximately 32 between 5 and 150 K) and for x = 0.75 (approximately 50 between 5 and 250 K).

Coexistence and Coupling of Spin-Induced Ferroelectricity and Ferromagnetism in Perovskites

Spin-induced ferroelectricity usually does not occur in perovskites with simple collinear magnetic structures. Here, we demonstrate that in even-layer perovskite systems, some common distortion modes involving octahedral rotation and Jahn-Teller distortion can break the inversion symmetry, allowing the emergence of spin-dependent out-of-plane polarization in a simple magnetic structure. Such spin-induced ferroelectricity is very common in double-perovskite systems and can coexist with ferromagnetism or ferrimagnetism above room temperature. We explain its origin by modifying the spin-dependent p-d hybridization mechanism. Our Letter provides a universal design for two-dimensional multiferroics and enables the control of polarization by means of a magnetic field.

High-Magnetic-Sensitivity Magnetoelectric Coupling Origins in a Combination of Anisotropy and Exchange Striction

Magnetoelectric (ME) coupling is highly desirable for sensors and memory devices. Herein, the polarization (P) and magnetization (M) of the DyFeO₃ single crystal were measured in pulsed magnetic fields, in which the ME behavior is modulated by multi-magnetic order parameters and has high magnetic-field sensitivity. Below the ordering temperature of the Dy³⁺-sublattice, when the magnetic field is along the c-axis, the P (corresponding to a large critical field of 3 T) is generated due to the exchange striction mechanism. Interestingly, when the magnetic field is in the ab-plane, ME coupling with smaller critical fields of 0.8 T (a-axis) and 0.5 T (b-axis) is triggered. We assume that the high magnetic-field sensitivity results from the combination of the magnetic anisotropy of the Dy³⁺ spin and the exchange striction between the Fe³⁺ and Dy³⁺ spins. This work may help to search for single-phase multiferroic materials with high magnetic-field sensitivity.

Magnetostructural coupling in RFeO3 (R = Nd, Tb, Eu and Gd)

We investigate the interplay of magnetization and lattice vibrations in rare-earth orthoferrites RFeO₃, with a specific focus on non-symmetry-breaking anomalies. To do so, we study the magnetization, magnon excitations and lattice dynamics as a function of temperature in NdFeO₃, TbFeO₃, EuFeO₃ and GdFeO₃. The magnetization shows distinct temperature anomalous behavior for all investigated rare-earth orthoferrites, even in the compounds with no phase transitions occurring at those temperatures. Through spin-phonon coupling, these magnetic changes are mirrored by the FeO₆ rotation mode for all the studied RFeO₃, revealing a common magnetostructural effect associated with the octahedra rotations. The R³⁺ oscillation modes evidence a Fe³⁺/R³⁺ spins cross-talk for the NdFeO₃ and TbFeO₃ cases. Our work sheds light into the common magnetostructural coupling in rare-earth orthoferrites, and the important role of magnetic anisotropy and spin-orbit coupling strength of the R-Fe interactions on the spin-reorientation transition at high temperatures.

Emerging spin-phonon coupling through cross-talk of two magnetic sublattices

Many material properties such as superconductivity, magnetoresistance or magnetoelectricity emerge from the non-linear interactions of spins and lattice/phonons. Hence, an in-depth understanding of spin-phonon coupling is at the heart of these properties. While most examples deal with one magnetic lattice only, the simultaneous presence of multiple magnetic orderings yield potentially unknown properties. We demonstrate a strong spin-phonon coupling in SmFeO3 that emerges from the interaction of both, iron and samarium spins. We probe this coupling as a remarkably large shift of phonon frequencies and the appearance of new phonons. The spin-phonon coupling is absent for the magnetic ordering of iron alone but emerges with the additional ordering of the samarium spins. Intriguingly, this ordering is not spontaneous but induced by the iron magnetism. Our findings show an emergent phenomenon from the non-linear interaction by multiple orders, which do not need to occur spontaneously. This allows for a conceptually different approach in the search for yet unknown properties.

Symmetry Analysis of Magnetoelectric Effects in Perovskite-Based Multiferroics

In this article, we performed symmetry analysis of perovskite-based multiferroics: bismuth ferrite (BiFeO₃)-like, orthochromites (RCrO₃), and Ruddlesden–Popper perovskites (Ca₃Mn₂O₇-like), being the typical representatives of multiferroics of the trigonal, orthorhombic, and tetragonal crystal families, and we explored the effect of crystallographic distortions on magnetoelectric properties. We determined the principal order parameters for each of the considered structures and obtained their invariant combinations consistent with the particular symmetry. This approach allowed us to analyze the features of the magnetoelectric effect observed during structural phase transitions in Bi_xR_1−xFeO₃ compounds and to show that the rare-earth sublattice has an impact on the linear magnetoelectric effect allowed by the symmetry of the new structure. It was shown that the magnetoelectric properties of orthochromites are attributed to the couplings between the magnetic and electric dipole moments arising near Cr³⁺ ions due to distortions linked with rotations and deformations of the CrO₆ octahedra. For the first time, such a symmetry consideration was implemented in the analysis of the Ruddlesden–Popper structures, which demonstrates the possibility of realizing the magnetoelectric effect in the Ruddlesden–Popper phases containing magnetically active cations, and allows the estimation of the conditions required for its optimization.

Evolution from sinusoidal to collinear A-type antiferromagnetic spin-ordered magnetic phase transition in Tb1-xPrxMnO3solid solution

The present study reports on the structural and magnetic phase transitions in Pr-doped polycrystalline Tb_0.6Pr_0.4MnO₃, using high-resolution neutron powder diffraction (NPD) collected at SINQ spallation source, to emphasize the suppression of the sinusoidal magnetic structure of pure TbMnO₃and the evolution to a collinear A-type antiferromagnetic ordering. The phase purity, Jahn-Teller distortion, and one-electron bandwidth for e_gorbital of Mn³⁺cation have been calculated for polycrystalline Tb_0.6Pr_0.4MnO_3,in comparison to the parent materials TbMnO₃and PrMnO₃, through the Rietveld refinement study from x-ray diffraction data at room temperature, which reveals the GdFeO₃type orthorhombic structure of Tb_0.6Pr_0.4MnO₃havingPnmaspace group symmetry. The temperature-dependent zero field-cooled and field-cooled dc magnetization study at low temperature down to 5 K reveals a variation in the magnetic phase transition due to the effect of Pr³⁺substitution at the Tb³⁺site, which gives the signature of the antiferromagnetic nature of the sample, with a weak ferromagnetic component at low temperature-induced by an external magnetic field. The field-dependent magnetization study at low temperatures gives the weak coercivity having the order of 2 kOe, which is expected due to the canted-spin arrangement or ferromagnetic nature of Terbium ordering. The NPD data for Tb_0.6Pr_0.4MnO₃confirms that the nuclear structure of the synthesized sample maintains its orthorhombic symmetry down to 1.5 K. Also, the magnetic structures have been solved at 50 K, 25 K, and 1.5 K through the NPD study, which shows an A-type antiferromagnetic spin arrangement having the magnetic space groupPn'ma'.

Dzyaloshinskii-Moriya-like interaction in ferroelectrics and antiferroelectrics

The Dzyaloshinskii-Moriya interaction (DMI) between two magnetic moments m_i and m_j is of the form [Formula: see text]. It originates from spin-orbit coupling, and is at the heart of fascinating phenomena involving non-collinear magnetism, such as magnetic topological defects (for example, skyrmions) as well as spin-orbit torques and magnetically driven ferroelectricity, that are of significant fundamental and technological interest. In sharp contrast, its electric counterpart, which is an electric DMI characterized by its [Formula: see text] strength and describing an interaction between two polar displacements u_i and u_j, has rarely been considered, despite the striking possibility that it could also generate new features associated with non-collinear patterns of electric dipoles. Here we report first-principles simulations combined with group theoretical symmetry analysis which not only demonstrate that electric DMI does exist and has a one-to-one correspondence with its magnetic analogue, but also reveals a physical source for it. These findings can be used to explain and/or design phenomena of possible technological importance in ferroelectrics and multiferroics.

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.

A soft chemistry approach to the synthesis of single crystalline and highly pure (NH4)CoF3 for optical and magnetic investigations

A new ionothermal synthesis utilizing 1-alkyl-pyridinium hexafluorophosphates [C_xPy][PF₆] (x = 2, 4, 6) led to the formation of highly crystalline single-phase ammonium cobalt trifluoride, (NH₄)CoF₃. Although ammonium transition-metal fluorides have been extensively studied with respect to their structural and magnetic properties, multiple aspects remain unclear. For that reason, the obtained (NH₄)CoF₃ has been investigated over a broad temperature range by means of single-crystal and powder x-ray diffraction as well as magnetization and specific heat measurements. In addition, energy-dispersive x-ray and vibrational spectroscopy as well as thermal analysis measurements were undertaken. (NH₄)CoF₃ crystallizes in the cubic perovskite structure and undergoes a structural distortion to a tetragonal phase at 127.7 K, which also is observable in the magnetic susceptibility measurements, which has not been observed before. A second magnetic phase transition occurring at 116.9 K is of second-order character. The bifurcation of the susceptibility curves indicates a canted antiferromagnetic ordering. At 2.5 K, susceptibility measurements point to a third phase change for (NH₄)CoF₃.

Single-ion anisotropy driven splitting of spin wave resonances in BiFeO3at low temperature

The spin wave resonances of BiFeO3ceramics have been followed at low temperature through far-infrared reflectance measurements. Following the scheme of Fishmanet al(2015Phys. Rev.B92094422) we have been able to assign all the spin wave modes observed. A complete lifting of the degeneracies of all these modes is seen at 250 K concomitant with the increase in single-ion anisotropy. For the first time, all the spin wave modes have been observed in the infrared spectra of BiFeO3. Correlated changes in the strength and frequencies of spin wave excitations with the reported magnetic transitions at low temperature are observed. A simultaneous increase in anharmonicity of the magnetic cycloid and single-ion anisotropy with decreasing temperature results in a partial suppression of the spin wave excitations. An increase in the magnetoelectric coupling is also observed below 150 K.

Coupling Lattice Instabilities Across the Interface in Ultrathin Oxide Heterostructures

Oxide heterointerfaces constitute a rich platform for realizing novel functionalities in condensed matter. A key aspect is the strong link between structural and electronic properties, which can be modified by interfacing materials with distinct lattice symmetries. Here, we determine the effect of the cubic-tetragonal distortion of SrTiO3 on the electronic properties of thin films of SrIrO3, a topological crystalline metal hosting a delicate interplay between spin-orbit coupling and electronic correlations. We demonstrate that below the transition temperature at 105 K, SrIrO3 orthorhombic domains couple directly to tetragonal domains in SrTiO3. This forces the in-phase rotational axis to lie in-plane and creates a binary domain structure in the SrIrO3 film. The close proximity to the metal-insulator transition in ultrathin SrIrO3 causes the individual domains to have strongly anisotropic transport properties, driven by a reduction of bandwidth along the in-phase axis. The strong structure-property relationships in perovskites make these compounds particularly suitable for static and dynamic coupling at interfaces, providing a promising route towards realizing novel functionalities in oxide heterostructures.

Magneto-electric multiferroics: designing new materials from first-principles calculations

In parallel with the revival of interest for magneto-electric multiferroic materials in the beginning of the century, first-principles simulations have grown incredibly in efficiency during the last two decades. Density functional theory calculations, in particular, have so become a must-have tool for physicists and chemists in the multiferroic community. While these calculations were originally used to support and explain experimental behaviour, their interest has progressively moved to the design of novel magneto-electric multiferroic materials. In this article, we mainly focus on oxide perovskites, an important class of multifunctional material, and review some significant advances to which contributed first-principles calculations. We also briefly introduce the various theoretical developments that were at the core of all these advances.

Spiral spin structures and skyrmions in multiferroics

In this article, we focus on (1) type-II multiferroics driven by spiral spin orderings and (2) magnetoelectric couplings in multiferroic skyrmion-hosting materials. We present both phenomenological understanding and microscopic mechanisms for spiral spin state, which is one of the essential starting points for type-II multiferroics and magnetic skyrmions. Two distinct mechanisms of spiral spin states (frustration and Dzyaloshinskii–Moriya [DM] interaction) are discussed in the context of the lattice symmetry. We also discuss the spin-induced ferroelectricity on the basis of the symmetry and microscopic atomic configurations. We compare two well-known microscopic models: the generalized inverse DM mechanism and the metal-ligand d-p hybridization mechanism. As a test for these models, we summarize the multiferroic properties of a family of triangular-lattice antiferromagnets. We also give a brief review of the magnetic skyrmions. Three types of known skyrmion-hosting materials with multiferroicity are discussed from the view point of crystal structure, magnetism, and origins of the magnetoelectric couplings. For exploration of new skyrmion-hosting materials, we also discuss the theoretical models for stabilizing skyrmions by magnetic frustration in centrosymmetric system. Several basic ideas for material design are given, which are successfully demonstrated by the recent experimental evidences for the skyrmion formation in centrosymmetric frustrated magnets.

Mild Hydrothermal Crystallization of Heavy Rare-Earth Chromite RECrO3 (RE = Er, Tm, Yb, Lu) Perovskites and Magnetic Properties

Crystallization of perovskite structure chromites (ACrO₃) in aqueous conditions is difficult owing to the amphoteric nature of the Cr³⁺ in the alkaline-mediated reaction conditions. This is especially true for the small metal cations at the A-site with large distortion angle of CrO₆ octahedral and small Goldschmidt tolerance factors. Here, we performed a progressive dehydration crystallization strategy to synthesize four RECrO₃ with the smallest radii of rare earth elements (Er, Tm, Yb, Lu) in mild hydrothermal conditions. Profile refinement of the high-resolution powder X-ray diffraction results indicated slightly longer unit cell parameters of  a and c in our samples with a higher distorted angle of CrO₆ octahedral units along ⟨010⟩ direction. All of the samples show rounded rectangle plate morphology with uniform distribution of particle sizes. These four RECrO₃ crystals can only form in a very narrow mineralization temperature range, i.e., 260-280 °C and 4.45-6.24 M of KOH. HRTEM results indicated that the normal crystallographic direction is ⟨001⟩, and the lattice of steps at the edge of elliptic rounded crystal is consistent with the bulk, which demonstrated single crystalline nature of the as-obtained crystals. Room-temperature Raman and FT-IR spectra reveal a continuous symmetry mode shift-dependent on the radii of A-site rare-earth cations. Temperature-dependent magnetization curves of RECrO₃ show typical antiferromagnetism to paramagnetism transition with Néel temperature of 93, 90, 86, and 83 K for ErCrO₃, TmCrO₃, YbCrO₃, and LuCrO₃, respectively. Samples of YbCrO₃ and LuCrO₃ show clear magnetization reversal and exchange bias phenomena below their Néel points. This paper indicates that the coupling of magnetic exchange in perovskite structure oxides could be tailorable in mild hydrothermal condition, towards the exploration of new magnetic and other physical properties.

Theoretical investigation of multiferroic metal-organic framework magnet [CH3NH3][Co(HCOO)3]: Green's function method

For the first presented magnetic ordering-induced multiferroics with a metal-organic framework (MOF) of formula [CH₃NH₃][Co(HCOO)₃], we theoretically investigate its multiple ferroics. It is found that Dzyaloshinskii-Moriya interaction is a main cause that leads to non-zero magnetization, and electric polarization, and the induced electric polarization can be regulated by magnetic fields. As an assistant mechanism, magnon-magnon interaction and quantum fluctuation play an important role on ferroelectrics and magnetism. Our methods are based on the double-time Green's function and Holstein-Primakoff transformation. Theoretical results can be compared with experiments, though there are some discrepancies.

Direct Magnetization-Polarization Coupling in BaCuF_{4}

Herewith, first-principles calculations based on density functional theory are used to describe the ideal magnetization reversal through polarization switching in BaCuF_{4} which, according to our results, could be accomplished close to room temperature. We also show that this ideal coupling is driven by a single soft mode that combines both polarization, and octahedral rotation. The later being directly coupled to the weak ferromagnetism of BaCuF_{4}. This, added to its strong Jahn-Teller distortion and its orbital ordering, makes this material a very appealing prototype for crystals in the ABX_{4} family for multifunctional applications. The described mechanism behaves ideally as it couples the ferroelectric and the magnetic properties naturally and it has not been reported previously.

High-Pressure Synthesis, Structures, and Properties of Trivalent A-Site-Ordered Quadruple Perovskites RMn7O12 (R = Sm, Eu, Gd, and Tb)

A-site-ordered quadruple perovskites RMn₇O₁₂ with R = Sm, Eu, Gd, and Tb were synthesized at high pressure and high temperature (6 GPa and ∼1570 K), and their structural, magnetic, and dielectric properties are reported. They crystallize in space group I2/ m at room temperature. All four compounds exhibit a high-temperature phase transition to the cubic Im3̅ structure at ∼664 K (Sm), 663 K (Eu), 657 K (Gd), and 630 K (Tb). They all show one magnetic transition at T_N1 ≈ 82-87 K at zero magnetic field, but additional magnetic transitions below T_N2 ≈ 12 K were observed in SmMn₇O₁₂ and EuMn₇O₁₂ at high magnetic fields. Very weak kinklike dielectric anomalies were observed at T_N1 in all compounds. We also observed pyroelectric current peaks near 14 K and frequency-dependent sharp steps in dielectric constant (near 18-35 K)-these anomalies are probably caused by dielectric relaxation, and they are not related to any ferroelectric transitions. TbMn₇O₁₂ shows signs of nonstoichiometry expressed as (Tb_{1- x}Mn _x)Mn₇O₁₂, and these samples exhibit negative magnetization or magnetization reversal effects of an extrinsic origin on zero-field-cooled curves in intermediate temperature ranges. The crystal structures of SmMn₇O₁₂ and EuMn₇O₁₂ were refined from neutron powder diffraction data at 100 K, and the crystal structures of GdMn₇O₁₂ and (Tb_0.88Mn_0.12)Mn₇O₁₂ were studied by synchrotron X-ray powder diffraction at 295 K.

Pressure-induced insulator-metal transition in EuMnO3

We study the influence of external pressure on the electronic and magnetic structure of EuMnO₃ from first-principles calculations. We find a pressure-induced insulator-metal transition at which the magnetic order changes from A-type antiferromagnetic to ferromagnetic with a strong interplay with Jahn-Teller distortions. In addition, we find that the non-centrosymmetric E ^*-type antiferromagnetic order can become nearly degenerate with the ferromagnetic ground state in the high-pressure metallic state. This situation can be exploited to promote a magnetically-driven realization of a non-centrosymmetric (ferroelectric-like) metal.

Improper electric polarization in simple perovskite oxides with two magnetic sublattices

ABO₃ perovskite oxides with magnetic A and B cations offer a unique playground to explore interactions involving two spin sublattices and the emergent effects they may drive. Of particular interest is the possibility of having magnetically driven improper ferroelectricity, as in the much studied families of rare-earth orthoferrites and orthochromites; yet, the mechanisms behind such effects remain to be understood in detail. Here we show that the strongest polar order corresponds to collinear spin configurations and is driven by non-relativistic exchange-strictive mechanisms. Our first-principles simulations reveal the dominant magnetostructural couplings underlying the observed ferroelectricity, including a striking magnetically driven piezoelectric effect. Further, we derive phenomenological and atomistic theories that describe such couplings in a generic perovskite lattice. This allows us to predict how the observed effects can be enhanced, and even how similar ones can be obtained in other perovskite families.

Magnetically-driven ferroelectricity holds the key for novel multiferroic effects in perovskite oxides, but it remains poorly understood. Here, Zhao et al. determine the dominant magnetostructural couplings that yield improper ferroelectricity in a generic perovskite with two spin sublattices.