Interfacing EuO in confined oxide and metal heterostructures

Abstract

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