Fermi level shifting, charge transfer and induced magnetic coupling at La0.7Ca0.3MnO3/LaNiO3 interface

Robust Two-Dimensional Ferromagnetism in Cr5Te8/CrTe2 Heterostructure with Curie Temperature above 400 K

The discovery of ferromagnetism in two-dimensional (2D) van der Waals crystals has generated widespread interest. The seeking of robust 2D ferromagnets with high Curie temperature (Tc) is vitally important for next-generation spintronic devices. However, owing to the enhanced spin fluctuation and weak exchange interaction upon the reduced dimensionalities, the exploring of robust 2D ferromagnets with Tc > 300 K is highly demanded but remains challenging. In this work, we fabricated air-stable 2D Cr5Te8/CrTe2 vertical heterojunctions with Tc above 400 K by the chemical vapor deposition method. Transmission electron microscopy demonstrates a high-quality-crystalline epitaxial structure between tri-Cr5Te8 and 1T-CrTe2 with striped moiré patterns and a superior ambient stability over six months. A built-in dual-axis strain together with strong interfacial coupling cooperatively leads to a record-high Tc for the CrxTey family. A temperature-dependent spin-flip process induces the easy axis of magnetization to rotate from the out-of-plane to the in-plane direction, indicating a phase-dependent proximity coupling effect, rationally interpreted by first-principles calculations of the magnetic anisotropy of a tri-Cr5Te8 and 1T-CrTe2 monolayer. Our results provide a material realization of effectively enhancing the transition temperature of 2D ferromagnetism and manipulating the spin-flip of the easy axis, which will facilitate future spintronic applications.

Ferromagnetic order controlled by the magnetic interface of LaNiO3/La2/3Ca1/3MnO3 superlattices

Interface engineering in complex oxide superlattices is a growing field, enabling manipulation of the exceptional properties of these materials, and also providing access to new phases and emergent physical phenomena. Here we demonstrate how interfacial interactions can induce a complex charge and spin structure in a bulk paramagnetic material. We investigate a superlattice (SLs) consisting of paramagnetic LaNiO₃ (LNO) and highly spin-polarized ferromagnetic La_2/3Ca_1/3MnO₃ (LCMO), grown on SrTiO₃ (001) substrate. We observed emerging magnetism in LNO through an exchange bias mechanism at the interfaces in X-ray resonant magnetic reflectivity. We find non-symmetric interface induced magnetization profiles in LNO and LCMO which we relate to a periodic complex charge and spin superstructure. High resolution scanning transmission electron microscopy images reveal that the upper and lower interfaces exhibit no significant structural variations. The different long range magnetic order emerging in LNO layers demonstrates the enormous potential of interfacial reconstruction as a tool for tailored electronic properties.

Coupled Co-Doped MoS2 and CoS2 as the Dual-Active Site Catalyst for Chemoselective Hydrogenation

Catalytic hydrogenation plays an important role in the industrial production of fine chemicals. Herein, we report a Co-doped MoS₂ and CoS₂ composite with a coupling interface and successfully apply it for the chemoselective hydrogenation of p-chloronitrobenzene to p-chloroaniline. The target catalyst 0.5CoMoS has ∼100% conversion and ∼100% selectivity. Experiments and theoretical calculations reveal that CoS₂ is more favorable for adsorbing and activating H₂ and provides active hydrogen (H_a) to Co-doped MoS₂ by the coupling interface. By matching the production and consumption rates of H_a, the maximization of the reaction yield was achieved. This work may promote the study of MoS₂-based catalysts for chemoselective hydrogenation.

Impact of Surface Defects on LaNiO3 Perovskite Electrocatalysts for the Oxygen Evolution Reaction

Perovskite oxides are regarded as promising electrocatalysts for water splitting due to their cost-effectiveness, high efficiency and durability in the oxygen evolution reaction (OER). Despite these advantages, a fundamental understanding of how critical structural parameters of perovskite electrocatalysts influence their activity and stability is lacking. Here, we investigate the impact of structural defects on OER performance for representative LaNiO₃ perovskite electrocatalysts. Hydrogen reduction of 700 °C calcined LaNiO₃ induces a high density of surface oxygen vacancies, and confers significantly enhanced OER activity and stability compared to unreduced LaNiO₃ ; the former exhibit a low onset overpotential of 380 mV at 10 mA cm^-2 and a small Tafel slope of 70.8 mV dec^-1 . Oxygen vacancy formation is accompanied by mixed Ni²⁺ /Ni³⁺ valence states, which quantum-chemical DFT calculations reveal modify the perovskite electronic structure. Further, it reveals that the formation of oxygen vacancies is thermodynamically more favourable on the surface than in the bulk; it increases the electronic conductivity of reduced LaNiO₃ in accordance with the enhanced OER activity that is observed.

Towards Control of the Size, Composition and Surface Area of NiO Nanostructures by Sn Doping

Achieving nanostructures with high surface area is one of the most challenging tasks as this metric usually plays a key role in technological applications, such as energy storage, gas sensing or photocatalysis, fields in which NiO is gaining increasing attention recently. Furthermore, the advent of modern NiO-based devices can take advantage of a deeper knowledge of the doping process in NiO, and the fabrication of p-n heterojunctions. By controlling experimental conditions such as dopant concentration, reaction time, temperature or pH, NiO morphology and doping mechanisms can be modulated. In this work, undoped and Sn doped nanoparticles and NiO/SnO2 nanostructures with high surface areas were obtained as a result of Sn incorporation. We demonstrate that Sn incorporation leads to the formation of nanosticks morphology, not previously observed for undoped NiO, promoting p-n heterostructures. Consequently, a surface area value around 340 m2/g was obtained for NiO nanoparticles with 4.7 at.% of Sn, which is nearly nine times higher than that of undoped NiO. The presence of Sn with different oxidation states and variable Ni3+/Ni2+ ratio as a function of the Sn content were also verified by XPS, suggesting a combination of two charge compensation mechanisms (electronic and ionic) for the substitution of Ni2+ by Sn4+. These results make Sn doped NiO nanostructures a potential candidate for a high number of technological applications, in which implementations can be achieved in the form of NiO-SnO2 p-n heterostructures.

Field induced crossover in critical behaviour and direct measurement of the magnetocaloric properties of La0.4Pr0.3Ca0.1Sr0.2MnO3

La0.4Pr0.3Ca0.1Sr0.2MnO3 has been investigated as a potential candidate for room temperature magnetic refrigeration. Results from X-ray powder diffraction reveal an orthorhombic structure with Pnma space group. The electronic and chemical properties have been confirmed by X-ray photoelectron spectroscopy and ion-beam analysis. A second-order paramagnetic to ferromagnetic transition was observed near room temperature (289 K), with a mean-field like critical behaviour at low field and a tricritical mean-field like behaviour at high field. The field induced crossover in critical behaviour is a consequence of the system being close to a first-order magnetic transition in combination with a magnetic field induced suppression of local lattice distortions. The lattice distortions consist of interconnected and weakly distorted pairs of Mn-ions, where each pair shares an electron and a hole, dispersed by large Jahn-Teller distortions at Mn3+ lattice sites. A comparatively high value of the isothermal entropy-change (3.08 J/kg-K at 2 T) is observed and the direct measurements of the adiabatic temperature change reveal a temperature change of 1.5 K for a magnetic field change of 1.9 T.

Polarity and Spin-Orbit Coupling Induced Strong Interfacial Exchange Coupling: An Asymmetric Charge Transfer in Iridate-Manganite Heterostructure

Charge transfer is of particular importance in manipulating the interface physics in transition-metal oxide heterostructures. In this work, we have fabricated epitaxial bilayers composed of polar 3d LaMnO₃ and nonpolar 5d SrIrO₃. Systematic magnetic measurements reveal an unexpectedly large exchange bias effect in the bilayer, together with a dramatic enhancement of the coercivity of LaMnO₃. Based on first-principle calculations and X-ray absorption spectroscopy measurements, such a strong interfacial magnetic coupling is found closely associated with the polar nature of LaMnO₃ and the strong spin-orbit interaction in SrIrO₃, which collectively drive an asymmetric interfacial charge transfer and lead to the emergence of an interfacial reentrant spin/superspin glass state. Our study provides a new insight into the charge transfer in transition-metal oxide heterostructures and offers a novel means to tune the interfacial exchange coupling for a variety of device applications.

Strain-Engineered Oxygen Vacancies in CaMnO3 Thin Films

We demonstrate a novel pathway to control and stabilize oxygen vacancies in complex transition-metal oxide thin films. Using atomic layer-by-layer pulsed laser deposition (PLD) from two separate targets, we synthesize high-quality single-crystalline CaMnO₃ films with systematically varying oxygen vacancy defect formation energies as controlled by coherent tensile strain. The systematic increase of the oxygen vacancy content in CaMnO₃ as a function of applied in-plane strain is observed and confirmed experimentally using high-resolution soft X-ray absorption spectroscopy (XAS) in conjunction with bulk-sensitive hard X-ray photoemission spectroscopy (HAXPES). The relevant defect states in the densities of states are identified and the vacancy content in the films quantified using the combination of first-principles theory and core-hole multiplet calculations with holistic fitting. Our findings open up a promising avenue for designing and controlling new ionically active properties and functionalities of complex transition-metal oxides via strain-induced oxygen-vacancy formation and ordering.

Valence-band offset and forward-backward charge transfer in manganite/NiO and manganite/LaNiO3 heterostructures

The valence-band offset (VBO) of the La(0.67)Sr(0.33)MnO(3)/NiO (LSMO/NiO), LaMnO(3)/NiO (LMO/NiO), LSMO/LaNiO(3) (LSMO/LNO) and LMO/LaNiO(3) (LSMO/LNO) heterostructures has been investigated using X-ray photoemission spectroscopy. The VBO values are calculated to be -0.72, -0.05, +1.43 and +1.51 eV for the LSMO/NiO, LSMO/LNO, LMO/LNO and LMO/NiO heterostructures, respectively. Hence, when compared with NiO and LNO, the valence band of LSMO is shifted to a lower binding energy, whereas that of LMO is shifted to a higher binding energy. In addition, the charge transfer at the interfaces has been depicted as Mn(3.3+) + 0.7e→ Mn(2.6+), Mn(3.3+) + 0.1e→ Mn(3.2+), Mn(3.0+)- 0.4e→ Mn(3.4+) and Mn(3.0+)- 0.5e→ Mn(3.5+) for the LSMO/NiO, LSMO/LNO, LMO/LNO and LMO/NiO heterostructures, respectively. Thus, the charge transfer procedure can be described as electron hopping from NiO and LNO to LSMO in the LSMO/NiO and LSMO/LNO heterostructures, and electron hopping from LMO to NiO and LNO in the LMO/NiO and LSMO/LNO heterostructures. Therefore, the charge transfer is dependent on the VBO, and the charge transfer direction can be determined from the negative or positive values of the VBO.

Understanding hydrothermal transformation from Mn2O3 particles to Na0.55Mn2O4·1.5H2O nanosheets, nanobelts, and single crystalline ultra-long Na4Mn9O18 nanowires

Manganese oxides are one of the most valuable materials for batteries, fuel cells and catalysis. Herein, we report the change in morphology and phase of as-synthesized Mn₂O₃ by inserting Na⁺ ions. In particular, Mn₂O₃ nanoparticles were first transformed to 2 nm thin Na_0.55Mn₂O₄·1.5H₂O nanosheets and nanobelts via hydrothermal exfoliation and Na cation intercalation, and finally to sub-mm ultra-long single crystalline Na₄Mn₉O₁₈ nanowires. This paper reports the morphology and phase-dependent magnetic and catalytic (CO oxidation) properties of the as-synthesized nanostructured Na intercalated Mn-based materials.