Crystal-field level inversion in lightly Mn-doped Sr3Ru2O7

Double exchange interaction in Mn-based topological kagome ferrimagnet

Recently discovered Mn-based kagome materials, such as RMn6Sn6 (R = rare-earth element), exhibit the coexistence of topological electronic states and long-range magnetic order, offering a platform for studying quantum phenomena. However, understanding the electronic and magnetic properties of these materials remains incomplete. Here, we investigate the electronic structure and magnetic properties of GdMn6Sn6 using x-ray magnetic circular dichroism, photoemission spectroscopy, and theoretical calculations. We observe localized electronic states from spin frustration in the Mn-based kagome lattice and induced magnetic moments in the nonmagnetic element Sn experimentally, which originate from the Sn- p and Mn- d orbital hybridization. Our calculations also reveal ferromagnetic coupling within the kagome Mn-Mn layer, driven by double exchange interaction. This work provides insights into the mechanisms of magnetic interaction and magnetic tuning in the exploration of topological quantum materials.

Realizing Room-Temperature Ferromagnetism in Molecular-Intercalated Antiferromagnet VOCl

2D van der Waals (vdW) magnets are gaining attention in fundamental physics and advanced spintronics, due to their unique dimension-dependent magnetism and potential for ultra-compact integration. However, achieving intrinsic ferromagnetism with high Curie temperature (TC) remains a technical challenge, including preparation and stability issues. Herein, an applicable electrochemical intercalation strategy to decouple interlayer interaction and guide charge doping in antiferromagnet VOCl, thereby inducing robust room-temperature ferromagnetism, is developed. The expanded vdW gap isolates the neighboring layers and shrinks the distance between the V-V bond, favoring the generation of ferromagnetic (FM) coupling with perpendicular magnetic anisotropy. Element-specific X-ray magnetic circular dichroism (XMCD) directly proves the source of the ferromagnetism. Detailed experimental results and density functional theory (DFT) calculations indicate that the charge doping enhances the FM interaction by promoting the orbital hybridization between t2 g and eg. This work sheds new light on a promising way to achieve room-temperature ferromagnetism in antiferromagnets, thus addressing the critical materials demand for designing spintronic devices.

Spin-Polarization Strategy for Enhanced Acidic Oxygen Evolution Activity

Spin-polarization is known as a promising way to promote the anodic oxygen evolution reaction (OER), since the intermediates and products endow spin-dependent behaviors, yet it is rarely reported for ferromagnetic catalysts toward acidic OER practically used in industry. Herein, the first spin-polarization-mediated strategy is reported to create a net ferromagnetic moment in antiferromagnetic RuO2 via dilute manganese (Mn2+ ) (S = 5/2) doping for enhancing OER activity in acidic electrolyte. Element-selective X-ray magnetic circular dichroism reveals the ferromagnetic coupling between Mn and Ru ions, fulfilling the Goodenough-Kanamori rule. The ferromagnetism behavior at room temperature can be well interpreted by first principles calculations as the interaction between the Mn2+ impurity and Ru ions. Indeed, Mn-RuO2 nanoflakes exhibit a strongly magnetic field enhanced OER activity, with the lowest overpotential of 143 mV at 10 mA cmgeo -2 and negligible activity decay in 480 h stability (vs 200 mV/195 h without magnetic field) as known for magnetic effects in the literature. The intrinsic turnover frequency is also improved to reach 5.5 s-1 at 1.45 VRHE . This work highlights an important avenue of spin-engineering strategy for designing efficient acidic oxygen evolution catalysts.

Octahedral Distortion and Displacement-Type Ferroelectricity with Switchable Photovoltaic Effect in a 3d^{3}-Electron Perovskite System

Because of the half-filled t_{2g}-electron configuration, the BO_{6} octahedral distortion in a 3d^{3} perovskite system is usually very limited. In this Letter, a perovskitelike oxide Hg_{0.75}Pb_{0.25}MnO_{3} (HPMO) with a 3d^{3} Mn^{4+} state was synthesized by using high pressure and high temperature methods. This compound exhibits an unusually large octahedral distortion enhanced by approximately 2 orders of magnitude compared with that observed in other 3d^{3} perovskite systems like RCr^{3+}O_{3} (R=rare earth). Essentially different from centrosymmetric HgMnO_{3} and PbMnO_{3}, the A-site doped HPMO presents a polar crystal structure with the space group Ama2 and a substantial spontaneous electric polarization (26.5  μC/cm^{2} in theory) arising from the off-center displacements of A- and B-site ions. More interestingly, a prominent net photocurrent and switchable photovoltaic effect with a sustainable photoresponse were observed in the current polycrystalline HPMO. This Letter provides an exceptional d^{3} material system which shows unusually large octahedral distortion and displacement-type ferroelectricity violating the "d^{0}-ness" rule.

High-Pressure Synthesis of Two Polymorphic HgMnO3 Phases and Distinct Magnetism from 2D to 3D

An ilmenite-like monoclinic phase of HgMnO₃ with space group P2₁/c was prepared using high-pressure and high-temperature methods at 18 GPa and 1473 K. The MnO₆ octahedra form a two-dimensional (2D) network in the bc plane, leading to a long-range antiferromagnetic ordering with a low Néel temperature of T_N ∼ 32 K. As the synthesis pressure increases to 20 GPa, a new perovskite-like rhombohedral phase with space group R3̅c was found to occur. The rhombohedral phase exhibits a three-dimensional (3D) network for the MnO₆ octahedra, giving rise to an antiferromagnetic ordering at T_N ∼ 60 K. X-ray absorption spectroscopy confirms the invariable Mn⁴⁺ charge state in these two polymorphic phases, in agreement with the Curie-Weiss and bond valence sum analysis. HgMnO₃ provides an interesting example to study the magnetic properties from 2D to 3D by varying synthesis pressure.

High-pressure synthesis and spin glass behavior of a Mn/Ir disordered quadruple perovskite CaCu3Mn2Ir2O12

A new 3d-5d hybridized quadruple perovskite oxide, CaCu₃Mn₂Ir₂O₁₂, was synthesized by high-pressure and high-temperature methods. The Rietveld structure analysis reveals that the compound crystallizes in an [Formula: see text]-type perovskite structure with space group Im-3, where the Ca and Cu are 1:3 ordered at fixed atomic positions. At the B site the 3d Mn and the 5d Ir ions are disorderly distributed due to the rare equal  +4 charge states for both of them as determined by x-ray absorption spectroscopy. The competing antiferromagnetic and ferromagnetic interactions among Cu²⁺, Mn⁴⁺, and Ir⁴⁺ ions give rise to spin glass behavior, which follows a conventional dynamical slowing down model.

Spin, orbital and topological order in models of strongly correlated electrons

Different types of order are discussed in the context of strongly correlated transition metal oxides, involving pure compounds and [Formula: see text] and [Formula: see text] hybrids. Apart from standard, long-range spin and orbital orders we observe also exotic noncollinear spin patterns. Such patters can arise in presence of atomic spin-orbit coupling, which is a typical case, or due to spin-orbital entanglement at the bonds in its absence, being much less trivial. Within a special interacting one-dimensional spin-orbital model it is also possible to find a rigorous topological magnetic order in a gapless phase that goes beyond any classification tables of topological states of matter. This is an exotic example of a strongly correlated topological state. Finally, in the less correlated limit of 4d ⁴ oxides, when orbital selective Mott localization can occur it is possible to stabilize by a 3d ³ doping one-dimensional zigzag antiferromagnetic phases. Such phases have exhibit nonsymmorphic spatial symmetries that can lead to various topological phenomena, like single and multiple Dirac points that can turn into nodal rings or multiple topological charges protecting single Dirac points. Finally, by creating a one-dimensional [Formula: see text] hybrid system that involves orbital pairing terms, it is possible to obtain an insulating spin-orbital model where the orbital part after fermionization maps to a non-uniform Kitaev model. Such model is proved to have topological phases in a wide parameter range even in the case of completely disordered 3d ² impurities. What more, it exhibits hidden Lorentz-like symmetries of the topological phase, that live in the parameters space of the model.

Near-Room-Temperature Ferrimagnetic Ordering in a B-Site-Disordered 3d-5d-Hybridized Quadruple Perovskite Oxide, CaCu3Mn2Os2O12

A new 3d-5d hybridization oxide, CaCu₃Mn₂Os₂O₁₂ (CCMOO), was prepared by high-pressure and high-temperature synthesis methods. The compound crystallizes to an A-site-ordered but B-site-disordered quadruple perovskite structure with a space group of Im3̅ (No. 204). The charge states of the transition metals are determined to be Cu²⁺/Mn^3.5+/Os^4.5+ by X-ray absorption spectroscopy. Although most B-site-disordered perovskites possess lower spin-ordering temperatures or even nonmagnetic transitions, the current CCMOO displays a long-range ferrimagnetic phase transition with a critical temperature as high as ∼280 K. Moreover, a large saturated magnetic moment is found to occur [7.8 μ_B/formula units (f.u.) at 2 K]. X-ray magnetic circular dichroism shows a Cu²⁺(↑)Mn^3.5+(↑)Os^4.5+(↓) ferrimagnetic coupling. The corner-sharing Mn/OsO₆ octahedra with mixed Mn and Os charge states make the compound metallic in electrical transport, in agreement with a specific heat fitting at low temperature. This work provides a rare example with high spin-ordering temperature and a large magnetic moment in B-site-disordered 3d-5d hybridization perovskite oxides.

Atomic-scale fingerprint of Mn dopant at the surface of Sr₃(Ru₁-xMnx)₂O₇

Chemical doping in materials is known to give rise to emergent phenomena. These phenomena are extremely difficult to predict a priori, because electron-electron interactions are entangled with local environment of assembled atoms. Scanning tunneling microscopy and low energy electron diffraction are combined to investigate how the local electronic structure is correlated with lattice distortion on the surface of Sr3(Ru1-xMnx)2O7, which has double-layer building blocks formed by (Ru/Mn)O6 octahedra with rotational distortion. The presence of doping-dependent tilt distortion of (Ru/Mn)O6 octahedra at the surface results in a C2v broken symmetry in contrast with the bulk C4v counterpart. It also enables us to observe two Mn sites associated with the octahedral rotation in the bulk through the "chirality" of local electronic density of states surrounding Mn, which is randomly distributed. These results serve as fingerprint of chemical doping on the atomic scale.

Electronic superlattice revealed by resonant scattering from random impurities in Sr3Ru2O7

Resonant elastic x-ray scattering (REXS) is an exquisite element-sensitive tool for the study of subtle charge, orbital, and spin superlattice orders driven by the valence electrons, which therefore escape detection in conventional x-ray diffraction (XRD). Although the power of REXS has been demonstrated by numerous studies of complex oxides performed in the soft x-ray regime, the cross section and photon wavelength of the material-specific elemental absorption edges ultimately set the limit to the smallest superlattice amplitude and periodicity one can probe. Here we show--with simulations and REXS on Mn-substituted Sr3Ru2O7--that these limitations can be overcome by performing resonant scattering experiments at the absorption edge of a suitably-chosen, dilute impurity. This establishes that--in analogy with impurity-based methods used in electron-spin-resonance, nuclear-magnetic resonance, and Mössbauer spectroscopy--randomly distributed impurities can serve as a non-invasive, but now momentum-dependent probe, greatly extending the applicability of resonant x-ray scattering techniques.

Imaging and manipulation of the competing electronic phases near the Mott metal-insulator transition

The complex interplay between the electron and lattice degrees of freedom produces multiple nearly degenerate electronic states in correlated electron materials. The competition between these degenerate electronic states largely determines the functionalities of the system, but the invoked mechanism remains in debate. By imaging phase domains with electron microscopy and interrogating individual domains in situ via electron transport spectroscopy in double-layered Sr(3)(Ru(1-x)Mn(x))(2)O(7) (x = 0 and 0.2), we show in real-space that the microscopic phase competition and the Mott-type metal-insulator transition are extremely sensitive to applied mechanical stress. The revealed dynamic phase evolution with applied stress provides the first direct evidence for the important role of strain effect in both phase separation and Mott metal-insulator transition due to strong electron-lattice coupling in correlated systems.