Electronic structure of Pr2MnNiO6 from x-ray photoemission, absorption and density functional theory

The electronic structure of double perovskite Pr2MnNiO6 is studied using core x-ray photoelectron spectroscopy and x-ray absorption spectroscopy. The 2p x-ray absorption spectra show that Mn and Ni are in 2+ and 4+ states respectively. Using charge transfer multiplet analysis of Ni and Mn 2p XPS spectra, we find charge transfer energies {\Delta} of 3.5 and 2.5 eV for Ni and Mn respectively. The ground state of Ni2+ and Mn4+ reveal a higher d electron count of 8.21 and 3.38 respectively as compared to the atomic values of 8.00 and 3.00 respectively thereby indicating the covalent nature of the system. The O 1s edge absorption spectra reveal a band gap of 0.9 eV which is comparable to the value obtained from first principle calculations for U-J >= 2 eV. The density of states clearly reveal a strong p-d type charge transfer character of the system, with band gap proportional to average charge transfer energy of Ni2+ and Mn4+ ions.Comment: 18 pages, 9 figure

Ferromagnetism and ferroelectricity in a superlattice of antiferromagnetic perovskite oxides without ferroelectric polarization

Abstract We study the structural, electronic, and magnetic properties of the SrCrO3/YCrO3 superlattice and their dependence on epitaxial strain. We discover that the superlattice adopts A-type antiferromagnetic (A-AFM) ordering in contrast to its constituents (SrCrO3: C-AFM; YCrO3: G-AFM) and retains it under compressive strain while becoming ferromagnetic (5 μ B per formula unit) at +1% strain. The obtained ferroelectric polarization is significantly higher than that of the R2NiMnO6/La2NiMnO6 (R = Ce to Er) series of superlattices [Nat. Commun. 5, 4021 (2014)] due to a large difference between the antipolar displacements of the Sr and Y cations. The superlattice is a hybrid-improper multiferroic material with a spontaneous ferroelectric polarization (13.5 μC/cm2) approaching that of bulk BaTiO3 (19 μC/cm2). The combination of ferromagnetism with ferroelectricity enables multistate memory applications. In addition, the charge-order-driven p-type semiconducting state of the ferromagnetic phase (despite the metallic nature of SrCrO3) is a rare property and interesting for spintronics. Monte Carlo simulations demonstrate a magnetic critical temperature of 90 K for the A-AFM phase without strain and of 115 K for the ferromagnetic phase at +5% strain, for example

ZnSe and ZnTe as tunnel barriers for Fe-based spin valves

International audienceOwing to their use in the optoelectronic industry, we investigate whether ZnSe and ZnTe can be utilised as tunnel barrier materials in magnetic spin valves. We perform ab initio electronic structure and linear response transport calculations based on self-interaction-corrected density functional theory for both Fe/ZnSe/Fe and Fe/ZnTe/Fe junctions. In the Fe/ZnSe/Fe junction the transport is tunneling-like and a symmetry-filtering mechanism is at play, implying that only the majority spin electrons with Δ1 symmetry are transmitted with large probability, resulting in a potentially large tunneling magnetoresistance (TMR) ratio. As such, the transport characteristics are similar to those of the Fe/MgO/Fe junction, although the TMR ratio is lower for tunnel barriers of similar thickness due to the smaller bandgap of ZnSe as compared to that of MgO. In the Fe/ZnTe/Fe junction the Fermi level is pinned at the bottom of the conduction band of ZnTe and only a giant magnetoresistance effect is found. Our results provide evidence that chalcogenide-based tunnel barriers can be utilised in spintronics devices

Spin reorientation in NdFe 0.5 Mn 0.5 O 3 : Neutron scattering and ab initio study

International audienc

Empowering the Nickel Iron Oxyhydroxide through a Heterostructure Cocatalyst for Superior Alkaline and Saline Water Reduction

Hydrogen production using an electrochemical cell might be an alternative to conventional fuel systems (i.e., fossil fuels). The NiFe-layered hydroxide (NiFeOxHy) is one of the most popular electrocatalysts for water-splitting applications. However, its electrochemical activity is limited in the hydrogen evolution reaction (HER) because of its minimal active sites and modest charge transfer rates. To overcome the limitations in NiFeOxHy, silver sulfide (Ag2S) and graphitic carbon nitride (g-C3N4) were added as heterostructure cocatalysts. The heterostructure cocatalyst (Ag2S/g-C3N4) addition in NiFeOxHy is found to improve the electrochemical double layer capacitance (EDLC) seven times greater than that of pristine NiFeOxHy. The addition of the heterostructure cocatalyst in the NiFeOxHy sample enables hydrogen production with a minimum overpotential of η10 = 43 mV and Tafel slope of 131 mV/dec in saline water conditions. X-ray photoelectron spectroscopy reveals that the heterostructure cocatalyst effectively donates electrons to NiFeOxHy, which enhance the resultant charge transfer ability of the electrocatalyst. In addition, the density functional theoretical (DFT) analysis suggests that the exposed sulfur (S) sites within the Ag2S/g-C3N4 heterostructure cocatalyst serve as the prominent catalytic center for H* interactions. This study highlights the alteration of charge transfer dynamics in NiFeOxHy via heterostructure cocatalyst addition as an effective way to facilitate enhanced alkaline and saline water reduction

Wafer-scale single-crystal monolayer graphene grown on sapphire substrate

High-quality wafer-scale single-crystal monolayer graphene is achieved on sapphire substrate, by epitaxially growing graphene at the Cu(111)/sapphire interface and then detaching Cu film via immersion in liquid nitrogen and rapid heating. The growth of inch-scale high-quality graphene on insulating substrates is desirable for electronic and optoelectronic applications, but remains challenging due to the lack of metal catalysis. Here we demonstrate the wafer-scale synthesis of adlayer-free ultra-flat single-crystal monolayer graphene on sapphire substrates. We converted polycrystalline Cu foil placed on Al2O3(0001) into single-crystal Cu(111) film via annealing, and then achieved epitaxial growth of graphene at the interface between Cu(111) and Al2O3(0001) by multi-cycle plasma etching-assisted-chemical vapour deposition. Immersion in liquid nitrogen followed by rapid heating causes the Cu(111) film to bulge and peel off easily, while the graphene film remains on the sapphire substrate without degradation. Field-effect transistors fabricated on as-grown graphene exhibited good electronic transport properties with high carrier mobilities. This work breaks a bottleneck of synthesizing wafer-scale single-crystal monolayer graphene on insulating substrates and could contribute to next-generation graphene-based nanodevices