Redox-controlled epitaxy and magnetism of oxide heterointerfaces: EuO/SrTiO3_3

We demonstrate a novel route to prepare thin films of the ferromagnetic insulator Europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate SrTiO$_3$ as the supplier of oxygen. The process allows tuning the electronic, magnetic and structural properties of the EuO films. Furthermore, we apply this technique to various oxidic substrates and demonstrate the universality and limits of a redox-controlled EuO film synthesis.Comment: 7pages, 6 figures, and supplemetar

Does Exchange Splitting persist above TCT_C? A spin-resolved photoemission study of EuO

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy and density functional theory (GGA+$U$). Our spin-resolved data reveals that, while the macroscopic magnetization of the sample vanishes at the Curie temperature, the exchange splitting of the O 2$p$ band persists up to $T_{C}$. Thus, we provide evidence for short-range magnetic order being present at the Curie temperature

Interfacing EuO in confined oxide and metal heterostructures

EuO is a ferromagnetic insulator, a rare material class combining ferromagnetic properties and insulating electronic behavior. We synthesize EuO on SrTiO$_{3}$ (001) and other oxides using a novel approach, the redox reaction with the substrate. For this we develop aquantitative XPS fitting routine on the basis of a set of Eu $\textit{3d}$ reference spectra. The redox growth is possible for T$_{S}$ = 300 $^\circ$C-600 $^\circ$C, where we detect stoichiometric EuO exceptfor a small interfacial region. The growth rate was evaluated and we find a Mott-Cabrera like growth limited by ionic oxygen conductivity of the substrate. The crystal structureis analyzed and epitaxial integration of EuO(110)/SrTiO$_{3}$ (100) and EuO(001)/SrTiO$_{3}$(001) is obtained. We detect bulk-like magnetic properties and a maximal thickness of $\textit{d} \approx$ 15 nm for this growth method. The proposed growth method reduces the complexity, as the necessity to calibrate the oxygen pressure is circumvented. For films $\textit{d}$ > 15 nm the well known adsorption limited deposition method can be employed after the redox growth. The electric properties of EuO/oxide and EuO/metal heterostructures are analyzed. Performing ARPES on the EuO/SrTiO$_{3}$ interface, we detect a 2DEG which shows comparable properties as the classical 2DEGs like LAO/STO. Our approach provides two novel prospects. First, the preparation of a 2DEG is achieved by a redox-controlled interface reaction and, second, the integration of a ferromagnetic insulator with the 2DEG. Hereby, a 2DEG is prepared in direct contact with a ferromagnet. This interface could be interesting to study the transport properties and elucidate whether the 2DEG is spin-polarized. The integration of EuO with BaTiO$_{3}$ was studied in a second experiment. Again, a 2DEG is created by the redox process between Eu metal and BaTiO$_{3}$. This interface could be of special interest, as other studies have shown BaTiO$_{3}$ to retain its ferroelectric properties at the 2DEG interface, while we find that $\textit{d}_{EuO}$ = 2ML exhibits sizable magnetic properties. This combination with a 2DEG at the interface could pave the way towards a multiferroic device, as the EuO/BaTiO$_{3}$ interface could influence the magnetic properties as a function of the ferroelectric polarization. The EuO/Pt interface reveals the opposite electric effect. Here a 2DHG is predicted by theoretical modeling and we measure, with element specific HAX-XMCD and volumetric magnetometry, an enhanced Curie temperature. This can be interpreted as a first sign of a magnetic interaction at the EuO/Pt interface, which leads to the 2DHG. The magnetic properties of Co/EuO heterostructures are studied with XMCD. We find that the sum rules can be applied to this rare earth material. We use this to determine the exchange length at the Co/EuO interface to 2$\lambda_{AFM}$ = (5.6 ± 1.4)nm, which shows, that the effect is localized to the interface and only thin films of EuO will experience the Co/EuO exchange. Measuring a hysteresis loop at room temperature we observe ferromagnetic properties of EuO, far above its bulk $\textit{T}_{C}$ = 69K. We utilize the hysteresis loop to obtain the Co/EuO exchange coupling strength $\textit{J}$ = 0.278 meV $\approx$ 5$\textit{J}_{bulk}$. We interpret this behavior as a magnetic proximity effect. A EuO ultra-thin film of $\textit{d}_{EuO}$ = 2ML is prepared at room temperature and the Co/EuO interface is also studied. We obtain comparable results and M(300K) = 1 μB/f.u.. This shows that a ignificant magnetic moment is retained at room temperature even for ultra-thin EuO films. In conclusion, we have developed a novel route to synthesize high quality EuO by utilizing a redox reaction with the substrate. This further enabled us to observe fascinating interfacial phenomena in oxide and metal heterostructures ranging from two-dimensional conductivity to magnetic proximity effect induced room temperature ferromagnetism in EuO. This can open up new directions in EuO related research

Thermodynamic stability and control of oxygen reactivity at functional oxide interfaces: EuO on ITO

As a prototypical all-oxide heterostructure, the ferromagnetic insulator europium monoxide (EuO) issynthesized on transparent and conductive indium tin oxide (ITO) virtual substrates. Non-destructivehard X-ray photoelectron spectroscopy is employed to depth profile the chemical composition of themagnetic layer and the buried oxide–oxide interface. We find that thermally activated oxygen diffusionfrom ITO affects the EuO growth process. We present how to control the oxygen reactivity at the interfaceand discuss its origin in a thermodynamic analysis. Our complementary methodical strategy allowsfor a significant improvement of the EuO chemical quality with sizeable magnetic properties. Generally,our approach derives guidelines for the proper choice of oxide substrates and buffer layer materials forfunctional all-oxide heterostructures

Direct Evidence of Subsurface Oxygen Formation in Oxide-Derived Cu by X-ray Photoelectron Spectroscopy

Subsurface oxygen has been proposed to be crucial in oxide-derived copper (OD-Cu) electrocatalysts for enhancing the binding of CO intermediates during CO$_2$ reduction reaction (CO$_2$RR). However, the presence of such oxygen species under reductive conditions still remains debated. In this work, the existence of subsurface oxygen is validated by grazing incident hard X-ray photoelectron spectroscopy, where OD-Cu was prepared by reduction of Cu oxide with H$_2$ without exposing to air. The results suggest two types of subsurface oxygen embedded between the fully reduced metallic surface and the Cu$_2$O buried beneath: (i) oxygen staying at lattice defects and/or vacancies in the surface-most region and (ii) interstitial oxygen intercalated in metal structure. This study adds convincing support to the presence of subsurface oxygen in OD-Cu, which previously has been suggested to play an important role to mitigate the σ-repulsion of Cu for CO intermediates in CO$_2$RR

Two-dimensional electron system at the magnetically tunable EuO/SrTiO3{\mathrm{EuO}\text{/}\mathrm{SrTiO}}_{3} interface

International audienceWe create a two-dimensional electron system (2DES) at the interface between EuO, a ferromagnetic insulator, and SrTiO 3 , a transparent nonmagnetic insulator considered the bedrock of oxide-based electronics. This is achieved by a controlled in situ redox reaction between pure metallic Eu deposited at room temperature on the surface of SrTiO 3 —an innovative bottom-up approach that can be easily generalized to other functional oxides and scaled to applications. Additionally, we find that the resulting EuO capping layer can be tuned from paramagnetic to ferromagnetic, depending on the layer thickness. These results demonstrate that the simple, novel technique of creating 2DESs in oxides by deposition of elementary reducing agents [T. C. Rödel et al., Adv. Mater. 28, 1976 (2016)] can be extended to simultaneously produce an active, e.g., magnetic, capping layer enabling the realization and control of additional functionalities in such oxide-based 2DESs

Hard x-ray photoelectron spectroscopy of tunable oxide interfaces

The tunability of the oxygen content in complex oxides and heterostructures has emerged as a key to designing their physical functionalities. Controlling the interface reactivity by redox reactions provides a powerful means to deliberately set distinct oxide phases and emerging properties. We present routes on how to control oxygen-driven redox mechanisms in ultrathin ferro(i)magnetic and ferroelectric oxide films and across oxide interfaces. We address the growth and control of metastable EuO oxide phases, the control of phase transitions of binary Fe oxides by oxygen migration, the in operando determination of NiFe$_2$O$_4$/SrTiO$_3$ interface band alignments, as well as the role of interfacial oxide exchange in ferroelectric HfO$_2$-based capacitors—uncovered by the unique capabilities of photoelectron spectroscopy and, in particular, using hard x-rays

State of the Surface During CO Hydrogenation over Ni(111) and Ni(211) Probed by Operando X-ray Photoelectron Spectroscopy

The state of the surface near-region during CO hydrogenation of Ni(111) and Ni(211) single crystal surfaces was investigated using various gas mixtures between 150 and 500 mbar, 200 and 325 °C, by operando X-ray photoelectron spectroscopy. We report how higher temperatures and hydrogen content correlate with a movement of CO away from the on-top configurations and toward multicoordinated sites of the nickel surface and how a nickel carbide is formed in the surface near region, particularly at high partial pressures of CO and lower temperatures. The presence of the carbide affects the CO bonding and was observed to be reduced during hydrogen-rich conditions and temperatures above 250 °C

Hard x-ray photoelectron spectroscopy of tunable oxide interfaces

The tunability of the oxygen content in complex oxides and heterostructures has emerged as a key to designing their physical functionalities. Controlling the interface reactivity by redox reactions provides a powerful means to deliberately set distinct oxide phases and emerging properties. We present routes on how to control oxygen-driven redox mechanisms in ultrathin ferro(i)magnetic and ferroelectric oxide films and across oxide interfaces. We address the growth and control of metastable EuO oxide phases, the control of phase transitions of binary Fe oxides by oxygen migration, the in operando determination of NiFe2O4/SrTiO3 interface band alignments, as well as the role of interfacial oxide exchange in ferroelectric HfO2 -based capacitors—uncovered by the unique capabilities of photoelectron spectroscopy and, in particular, using hard x-rays.publishe