Magnetostriction-polarization coupling in multiferroic Mn2MnWO6

Magnetoelectric coupling in multiferroics probed by optical second harmonic generation

Magnetoelectric coupling, as a fundamental physical nature and with the potential to add functionality to devices while also reducing energy consumption, has been challenging to be probed in freestanding membranes or two-dimensional materials due to their instability and fragility. In this paper, we report a magnetoelectric coupling probed by optical second harmonic generation with external magnetic field, and show the manipulation of the ferroelectric and antiferromagnetic orders by the magnetic and thermal fields in BiFeO₃ films epitaxially grown on the substrates and in the freestanding ones. Here we define an optical magnetoelectric-coupling constant, denoting the ability of controlling light-induced nonlinear polarization by the magnetic field, and found the magnetoelectric-coupling was suppressed by strain releasing but remain robust against thermal fluctuation for freestanding BiFeO₃.

γ-BaFe2O4: a fresh playground for room temperature multiferroicity

Multiferroics, showing the coexistence of two or more ferroic orderings at room temperature, could harness a revolution in multifunctional devices. However, most of the multiferroic compounds known to date are not magnetically and electrically ordered at ambient conditions, so the discovery of new materials is pivotal to allow the development of the field. In this work, we show that BaFe2O4 is a previously unrecognized room temperature multiferroic. X-ray and neutron diffraction allowed to reveal the polar crystal structure of the compound as well as its antiferromagnetic behavior, confirmed by bulk magnetometry characterizations. Piezo force microscopy and electrical measurements show the polarization to be switchable by the application of an external field, while symmetry analysis and calculations based on density functional theory reveal the improper nature of the ferroelectric component. Considering the present findings, we propose BaFe2O4 as a Bi- and Pb-free model for the search of new advanced multiferroic materials.

Topochemical Synthesis of LiCoF3 with a High-Temperature LiNbO3-Type Structure

A novel perovskite fluoride, Li_xCoF₃, which has an exceptionally low tolerance factor (0.81), has been synthesized via low-temperature lithium intercalation into a distorted ReO₃-type fluoride CoF₃ using organolithium reagents. Interestingly, this reaction is completed within 15 min at room temperature. Synchrotron X-ray diffractometry and optical second harmonic generation at room temperature have revealed that this compound shows a high-temperature LiNbO₃-type structure (space group: R3̅c) involving Li-Co antisite defects and A-site splitting along the c direction. A-site splitting is consistent with the prediction based on hybrid Hartree-Fock density functional theory calculations. Co-L_2,3 edge X-ray absorption spectroscopy, as well as bond valence sum analysis, has verified the divalent oxidation state of Co ions in the lithiated phase, suggesting that its composition is close to LiCoF₃ (x ≈ 1). This compound exhibits a paramagnetic-to-antiferromagnetic transition at 36 K on cooling, accompanied by weak ferromagnetic ordering. The synthetic route based on low-temperature lithiation of metal fluorides host paves the way for obtaining a new LiNbO₃-type fluoride family.

A New Cation‐Ordered Structure Type with Multiple Thermal Redistributions in Co 2 InSbO 6

Cation ordering in solids is important for controlling physical properties and leads to ilmenite (FeTiO₃) and LiNbO₃ type derivatives of the corundum structure, with ferroelectricity resulting from breaking of inversion symmetry in the latter. However, a hypothetical third ABO₃ derivative with R32 symmetry has never been observed. Here we show that Co₂InSbO₆ recovered from high pressure has a new, ordered‐R32 A₂BCO₆ variant of the corundum structure. Co₂InSbO₆ is also remarkable for showing two cation redistributions, to (Co_0.5In_0.5)₂CoSbO₆ and then Co₂InSbO₆ variants of the ordered‐LiNbO₃ A₂BCO₆ structure on heating. The cation distributions change magnetic properties as the final ordered‐LiNbO₃ product has a sharp ferrimagnetic transition unlike the initial ordered‐R32 phase. Future syntheses of metastable corundum derivatives at pressure are likely to reveal other cation‐redistribution pathways, and may enable ABO₃ materials with the R32 structure to be discovered.

Pressure Control of the Structure and Multiferroicity in a Hydrogen-Bonded Metal-Organic Framework

Multiferroic materials with the cross-coupling of magnetic and ferroelectric orders provide a new platform for physics study and designing novel electronic devices. However, the weak coupling strength of ferroelectricity and magnetism is the main obstacle for potential applications. The recent research focuses on enhancing the coupling effect via synthesizing novel materials in a chemical route or tuning the multiferroicity in the physical way. Among them, pressure is an effective method to modify multiferroic materials, especially when the chemical doping has reached its tuning limit. In this work, we systemically studied the multiferroic properties in a hydrogen-bonded metal-organic framework (MOF) [(CH₃)₂NH₂]Ni(HCOO)₃ under high pressure. X-ray diffraction and Raman scattering reveal that a structural phase transition occurs in a pressure region of 6-9 GPa, and the crystal structure is greatly modified by pressure. With the ac magnetic susceptibility, pyroelectric current, and dielectric constant measurements, we obtain the multiferroic property evolution under high pressure and create a temperature-pressure phase diagram. Our study demonstrates that the pressure can modify the magnetic superexchange interaction and hydrogen bonding simultaneously in these perovskite-like MOFs. The multiferroic phase region has been expanded to higher temperature due to the pressure-enhanced spin-phonon coupling effect.

Magnetic Structures of Mn11Ta4O21 and Interpretation as an Hexagonal A-site Manganite

Mn₁₁Ta₄O₂₁ is presented as the first hexagonal A-site manganite. Based on simple rules, the structure is compatible with a 14H-layer (cchchch)₂ stacking sequence that is related to BaVO₃ and BaCrO₃ high-pressure polymorphs. The A-site overstoichiometry is explained through difference in ionic radii sizes between Ba and Mn. Magnetic properties show two transitions at T_N1 = 88 K and T_N2 = 56 K. Neutron powder diffraction evidence two magnetic structures with purely antiferromagnetic and ferrimagnetic orders below T_N1 and T_N2, respectively. A complementary description with 14H-(hhccccc)₂ sequence of only Mn octahedra provides a direct comparison with BaMnO_3-δ hexagonal perovskites and naturally explains the AFM order. Below T_N2 a magneto-elastic coupling along with uniaxial negative thermal expansion are observed.

Universal A-Cation Splitting in LiNbO3-Type Structure Driven by Intrapositional Multivalent Coupling

Understanding the electric dipole switching in multiferroic materials requires deep insight of the atomic-scale local structure evolution to reveal the ferroelectric mechanism, which remains unclear and lacks a solid experimental indicator in high-pressure prepared LiNbO₃-type polar magnets. Here, we report the discovery of Zn-ion splitting in LiNbO₃-type Zn₂FeNbO₆ established by multiple diffraction techniques. The coexistence of a high-temperature paraelectric-like phase in the polar Zn₂FeNbO₆ lattice motivated us to revisit other high-pressure prepared LiNbO₃-type A₂BB'O₆ compounds. The A-site atomic splitting (∼1.0-1.2 Å between the split-atom pair) in B/B'-mixed Zn₂FeTaO₆ and O/N-mixed ZnTaO₂N is verified by both powder X-ray diffraction structural refinements and high angle annular dark field scanning transmission electron microscopy images, but is absent in single-B-site ZnSnO₃. Theoretical calculations are in good agreement with experimental results and suggest that this kind of A-site splitting also exists in the B-site mixed Mn-analogues, Mn₂FeMO₆ (M = Nb, Ta) and anion-mixed MnTaO₂N, where the smaller A-site splitting (∼0.2 Å atomic displacement) is attributed to magnetic interactions and bonding between A and B cations. These findings reveal universal A-site splitting in LiNbO₃-type structures with mixed multivalent B/B', or anionic sites, and the splitting-atomic displacement can be strongly suppressed by magnetic interactions and/or hybridization of valence bands between d electrons of the A- and B-site cations.

High-Pressure, High-Temperature Synthesis and Characterization of Polar and Magnetic LuCrWO6

A new polar and magnetic oxide, LuCrWO₆, was synthesized under high pressure (6 GPa) and high temperature (1673 K). LuCrWO₆ is isostructural with the previously reported polar YCrWO₆ (SG: Pna2₁, no. 33). The ordering of CrO₆ and WO₆ octahedra in the edge-shared dimers induce the polar structure. The effective size of rare earth, Ln cation does not seem to affect the symmetry of LnCrWO₆. Second harmonic generation measurements of LuCrWO₆ confirmed the noncentrosymmetric character and strong piezoelectric domains are observed from piezoresponse force microscopy at room temperature. LuCrWO₆ exhibits antiferromagnetic behavior, T_N, of ∼18 K with a Weiss temperature of -30.7 K.

High-Pressure Synthesis and Ferrimagnetism of Ni3TeO6-Type Mn2ScMO6 (M = Nb, Ta)

The corundum-related oxides Mn₂ScNbO₆ and Mn₂ScTaO₆ were synthesized at high pressure and high temperature (6 GPa and 1475 K). Analysis of the synchrotron powder X-ray diffraction shows that Mn₂ScNbO₆ and Mn₂ScTaO₆ crystallize in Ni₃TeO₆-type noncentrosymmetric crystal structures with space group R3. The asymmetric crystal structure was confirmed by second harmonic generation measurement. X-ray absorption near-edge spectroscopies indicate formal valence states of Mn²⁺₂Sc³⁺Nb⁵⁺O₆ and Mn²⁺₂Sc³⁺Ta⁵⁺O₆, also supported by the calculated bond valence sums. Both samples are electrically insulating. Magnetic measurements indicate that Mn₂ScNbO₆ and Mn₂ScTaO₆ order ferrimagnetically at 53 and 50 K, respectively, and Mn₂ScTaO₆ is found to have a field-induced magnetic transition.

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.

Reversible Structural Transformation between Polar Polymorphs of Li2GeTeO6

Li₂GeTeO₆ prepared at ambient pressure adopts the corundum derivative ordered ilmenite structure (rhombohedral R3). When heated at 1073 K and 3-5 GPa, the as-made Li₂GeTeO₆ can convert into a LiSbO₃-derived Li₂TiTeO₆-type phase (orthorhombic Pnn2), which is the third LiSbO₃-derived double A₂BB'O₆ phase in addition to Li₂TiTeO₆ and Li₂SnTeO₆. This Pnn2 Li₂GeTeO₆ phase spontaneously reverts to the R3 phase if annealed up to 1023 K at ambient pressure. Although the crystal structural analyses and second harmonic generation measurements clearly demonstrate the polar nature of both the R3 and Pnn2 phases, P( E) and dielectric measurements do not show any convincing ferroelectric response. Given the large estimated spontaneous polarization (17 and 80 μC/cm²), the absence of ferroelectric behavior could be attributed to the random domain distribution and leakage due to Li-ion migration.

Simultaneous bifunctional application of solid-state lighting and ratiometric optical thermometer based on double perovskite LiLaMgWO6:Er3+ thermochromic phosphors

Both efficient light emitting diodes and highly sensitive (2.24% K⁻¹) ratiometric thermometer were obtained based on the LiLaMgWO₆:Er³⁺ thermochromic phosphor.

Low-Voltage Control of (Co/Pt) x Perpendicular Magnetic Anisotropy Heterostructure for Flexible Spintronics

The trend of mobile Internet requires portable and wearable devices as bio-device interfaces. Electric field control of magnetism is a promising approach to achieve compact, light-weight, and energy-efficient wearable devices. Within a flexible sandwich heterostructure, perpendicular magnetic anisotropy switching was achieved via low-voltage gating control of an ionic gel in mica/Ta/(Pt/Co) _x/Pt/ionic gel/Pt, where (Pt/Co) _x acted as a functional layer. By conducting in situ VSM, EPR, and MOKE measurements, a 1098 Oe magnetic anisotropy field change was determined at the bending state with tensile strain, corresponding to a magnetic anisotropy energy change of 3.16 × 10⁵ J/m³ and a giant voltage tunability coefficient of 0.79 × 10⁵ J/m³·V. The low voltage and strain dual control of magnetism on mica substrates enables tunable flexible spintronic devices with an increased degree of manipulation.