Europium Monoxide Research Articles

Proximity coupling induced two dimensional magnetic order in EuO-based synthetic ferrimagnets

Proximity effects allow for the adjustment of magnetic properties in a physically elegant way. If two thin ferromagnetic (FM) films are brought into contact, electronic coupling alters their magnetic exchange interaction at their interface. For a low-TC\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_C$$\\end{document} rare-earth FM coupled to a 3d transition metal FM, even room temperature magnetism is within reach. In addition, magnetic proximity coupling is particularly promising for increasing the magnetic order of metastable materials such as europium monoxide (EuO) beyond their bulk TC\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$T_C$$\\end{document}, since neither the stoichiometry nor the insulating properties are modified. We investigate the magnetic proximity effect at Fe/EuO and Co/EuO interfaces using hard X-ray photoelectron spectroscopy. By exciting the FM layers with circularly polarized light, magnetic dichroism is observed in angular dependence on the photoemission geometry. In this way, the depth-dependence of the magnetic signal is determined element-specifically for the EuO and 3d FM parts of the bilayers. In connection with atomistic spin dynamics simulations, the thickness of the EuO layer is found to be crucial, indicating that the observed antiferromagnetic proximity coupling is a short-ranged and genuine interface phenomenon. This fact turns the bilayer into a strong synthetic ferrimagnet. The increase in magnetic order in EuO occurs in a finite spatial range and is therefore particularly strong in the 2D limit—a counterintuitive but very useful phenomenon for spin-based device applications.

Modulation of the Transient Magnetization of an EuO/Co Bilayer Tuned by Optical Excitation

AbstractThe magnetic proximity effect provides a promising way to increase the low Curie temperature (TC) of europium monoxide (EuO) toward or even above room temperature, while keeping its stoichiometry and insulating properties. This work studies EuO/Co bilayers using static and time‐resolved magneto‐optical Kerr effect measurements, and explores the influence of magnetic proximity on TC and on the spin dynamics in EuO. Excitation above the EuO bandgap results in an ultrafast enhancement of the EuO magnetization followed by a demagnetization within nanoseconds. This behaviour is also visible upon selectively photoexciting Co in the EuO/Co bilayer placed in an out‐of‐plane magnetic field, which is attributed to propagation of a superdiffusive spin current from Co into EuO. As the spin dynamics of Co shows a transient thermal demagnetization, the bilayer provides a system where the transient magneto‐optical signal can be tuned in amplitude and sign by varying external parameters such as the sample temperature or pump fluence. Moreover, in a strong excitation regime it is possible to measure the magnetic hysteresis of the underlying EuO, which is present up to room temperature – giving experimental evidence for the presence of a tuneable magnetic proximity coupling between Co and EuO.

Synthesis and Crystal‐Structure Analysis of the K2NiF4‐Type Hydride Oxides LiLnEuH2−xO2 (Ln=La, Ce, Pr, Nd, Sm) and LiEu2H3O by Neutron and X‐Ray Diffraction

AbstractThe hydride oxides LiLnEuH2−xO2 (Ln=La, Ce, Pr, Nd and Sm) were synthesized by reaction of the lanthanide sesquioxides with europium monoxide, europium dihydride and lithium hydride under inert conditions at 750 °C as black powders. They crystallize in the tetragonal K2NiF4‐type structure (space group: I4/mmm) with a mixed Ln3+/Eu2+ occupation. The crystal structures of the europium representatives LiLaEuH2−xO2 and LiLaEuD2−xO2 were analyzed by powder neutron diffraction data at short wavelengths (λ=70 pm). Hydrogen (deuterium) and oxygen atoms occupy distinct crystallographic sites with considerable vacancy concentrations on the hydrogen positions (a=363.80(8) pm, c=1323.3(3) pm, c/a=3.637 for LiLaEuH1.26(4)O2 and a=363.43(5) pm, c=1321.6(2) pm, c/a=3.636 for LiLaEuD1.41(2)O2). Moving from the mixed Ln/Eu occupation in LiLnEuH2O2 to Ln=Eu2+, we obtained the mixed‐anionic phase LiEu2H3O, which crystallizes in the same structure type with a=370.04(2) pm, c=1317.32(8) pm and c/a=3.560.

Europium oxide: Growth guide for the first monolayers on oxidic substrates

Interfacial oxygen exchange at oxide interfaces bears huge potential in stabilizing metastable or novel phases of functional oxides down to the monolayer limit. By taking advantage of active oxygen supply of the substrate material, waiving any external oxygen dosage, high-quality, crystalline ultrathin films of the Heisenberg ferromagnet europium monoxide (EuO) were stabilized on YSZ (001). This so-called redox-assisted growth mode (or, vice versa, the extreme case of a distillation growth) was monitored end to end by in situ x-ray photoelectron emission spectroscopy and electron diffraction techniques. The evolution of Eu $3d$ core levels allows us to disentangle the processes of interfacial oxygen diffusion and vacancy formation in stabilizing the very first monolayers of EuO on YSZ (001). An expedient background correction analysis is presented, which allows us to quantify the critical ${\mathrm{Eu}}^{3+}/{\mathrm{Eu}}^{2+}$ ratio in the ultrathin film regime. We concluded on the key mechanisms of redox-assisted EuO/YSZ (001) thin film synthesis, which merge in a universal three-process growth model that may serve as guideline for redox-assisted synthesis of metastable low-dimensional oxides.

Metal-insulator transition temperature in EuO1−x films as a function of exposure time in air

Oxygen-deficient Europium monoxide EuO1−x shows a metal-to-insulator transition near the Curie temperature (TC). A systematic transport study of EuO1−x thin films as a function of exposure time in air reveals a gradually decreased TC from 140 K to 70 K as the concentration of the oxygen vacancies decreases, which is accompanied by a drastic increase in the resistance. We also find an unusual enhancement of magnetic anisotropy in the transport measurements, which results from the strong spin-orbit coupling at the interface between the film and substrate and possible presence of spin-textures like skyrmions in EuO1−x.

Europium monoxide as a basis for creating a high-temperature spin injector in the semiconductor spintronics

The results of the creation of a high-temperature spin injector based on EuO: Fe composite material are discussed. Their magnetic, electrical, structural and resonance parameters are given in a wide range of temperatures and an external magnetic field. A model calculation of the electronic spectrum of the solid solution Eu–Fe–O, responsible for the manifestation of the outstanding properties of the composite, is performed. The possibility of creating semiconductor spin electronics devices capable of operating at room temperature is shown.

Europium monoxide as a basis for creating a high-temperature spin injector in the semiconductor spintronics

The results of the creation of a high-temperature spin injector based on EuO: Fe composite material are discussed. Their magnetic, electrical, structural and resonance parameters are given in a wide range of temperatures and an external magnetic field. A model calculation of the electronic spectrum of the solid solution Eu–Fe–O, responsible for the manifestation of the outstanding properties of the composite, is performed. The possibility of creating semiconductor spin electronics devices capable of operating at room temperature is shown.

Positive exchange bias of EuO1−x films

We have investigated the magnetic properties of oxygen-deficient europium monoxide EuO1−x films prepared on (0 0 1)-orientation Si single crystal substrate via laser-MBE. Stoichiometric EuO and reduced EuO1−x films are deposited by adjusting the annealing conditions of Si wafers pre-deposition. An enhanced Curie temperature (TC) from 69 K to 150 K is observed by introducing oxygen vacancies to EuO, which is due to the formation of magnetic polarons. The EuO1−x films exhibit positive exchange bias when the films enter the Heisenberg ferromagnetic phase at a lower temperature, which suggests that an antiferromagnetic coupling between the magnetic polarons and Eu2+ 4f spins which is ordered in the typical Heisenberg ferromagnet.

Strain-Induced Increase of Dielectric Constant in EuO Thin Film

Recently, lattice dynamics of the highly strained Europium monoxide, as a promising candidate for strong ferroelectric-ferromagnet material, applied in the next-generation storage devices, attracted huge attention in the solid-state electronics. Here, the authors investigate the effect of tensile strain on dielectric properties of pulsed laser deposited EuO thin films from 10 to 200 K. A nearly 3% out-of-plane lattice compression is observed as the film thickness was reduced to ∼8 nm, which could originate from the lattice mismatch between film and a LaAlO3 substrate. The temperature and frequency dependence of capacitance and loss factor of the films reveals the dominant role of electronic and ionic polarization below 100 K. The interdigitated capacitor fabricated on strained film shows almost 50% increase of capacitance compared to the relaxed one, which corresponds to a considerable increase of dielectric constant induced by in-plane tensile strain. Softening of the in-plane polarized transverse optical (TO) phonon modes of EuO lattice due to tensile strain might have the major contribution to this behavior according to the Lyddane–Sachs–Teller relation.

Redox-controlled epitaxy and magnetism of oxide heterointerfaces: EuO/SrTiO3

We demonstrate a route to prepare thin films of the ferromagnetic insulator europium monoxide. Key is a redox-controlled interface reaction between metallic Eu and the substrate ${\mathrm{SrTiO}}_{3}$ 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.

Europium monoxide nanocrystalline thin films with high near-infrared transparency

Nanocrystalline textured EuO thin films are prepared by an oxygen loss process from a pure Eu2O3 bulk ceramic target through pulsed laser deposition in vacuum at room temperature. X-ray diffraction spectra evidence a well-defined diffraction peak corresponding to the EuO phase textured along the (1 1 0) direction. Analysis of the XRD peak profile indicates that the films are nanocrystalline (average crystallite size of 11 nm) with a compressive residual strain. The formation of stoichiometric EuO is further confirmed by a strong signal from Eu2+ in the X-ray photoelectron spectra. The complex refractive index in the near infrared has been determined by spectroscopic ellipsometry and shows that the EuO films have a high transparency (k < 10−3) and a refractive index of 2.1. A band-gap shift of 0.25 eV is found with respect to the EuO bulk. These films, deposited by an accessible and efficient method, open a new route to produce EuO films with optical quality, suitable for NIR optoelectronic components.

(Invited) Light on EuOx Nanostrutured Films

Europium monoxide (EuO) is a well-known ferromagnetic semiconductor with single crystal band-gap similar to Si (1.12 eV) and is compatible with the actual semiconductor technology, and suited for the development of integrated spintronics components. However, very little is known of the optical properties of this notable material in thin film configuration, and especially nanostructured. Only very recently some evidence on possible quantum confinement in EuO nanostructures has been reported [1]. In part, this is because EuO high quality films and nanostructures are very difficult to obtain and preserve. Indeed the only data available on its dielectric function are for single crystal EuO and dates from the 70’s from reflectance measurements. Recently we have been able to prepare high quality textured nanocrystalline EuOx films by pulsed laser deposition in vacuum at room temperature through a reduction process. Because the EuO films rapidly oxidize under exposure to ambient, we have encapsulated them with a 10 nm thick Al2O3 layer. This has allowed us to preserve the EuOx nanostructures and to study both their linear optical and luminescent properties. In this contribution, we will show the optical dielectric function of thin nanocrystalline EuOx films in the NUV-VIS-NIR, using spectroscopic ellipsometry measurements with perfect Kramers-Kronig consistency. The results show how tweaking the EuOx composition and nanostructure modifies its dielectric function, and therefore it enables the control of the excitonic response modifying the band-gap of the films. We will show the photoluminescent emission of these films, and discuss its excitonic or ionic (Eu2+) origin. We believe that the knowledge of the optical properties of this unique material will open a new route for the integration with commercial semiconductors (Si, GaAs, GaN) for novel spintronics and photonics combined devices. [1] G.M. Prinz, T. Gerber, A. Lorke, M. Müller, Quantum confinement in EuO heterostructures, Appl. Phys. Lett. 109 (2016). doi:10.1063/1.4966223.

On solubility of europium monoxide in melts of (NaBr + NaI) system at T = 973 K

Equilibria of EuO dissolution and dissociation in molten (NaBr+NaI) mixtures of 0.77:0.23 and 0.31:0.69 compositions at T=973K were studied by potentiometric titration method using Pt(O2)|ZrO2(Y2O3) indicator electrode. The solubility product indices of EuO are (7.81±0.08) and (8.43±0.16) in the melts of 0.77:0.23 and 0.31:0.69 compositions. The corresponding dissociation constant indices are (4.96±0.04) and (5.54±0.06), respectively (all the parameters are in molality). Non-dissociated EuO is the prevailing form in all the saturated solutions of europium monoxide. The decrease of the iodide ion concentration in the melts results in strengthening of EuO dissociation that is explained by introduction of harder Pearson’s base (Br−) in sodium iodide melt. In its turn this increases the fixation degree of Eu2+ in mixed halide complexes. The total solubility of EuO decreases going from NaI melt to the (bromide+iodide) mixtures that is caused by the decrease of ‘physical’ solubility of non-dissociated oxide which occupies hollow spaces of enough large size in the ionic solvents. The quantity of these hollow spaces diminishes at the sequential Br−→I− substitution.

High-quality EuO thin films the easy way via topotactic transformation.

Epitaxy is widely employed to create highly oriented crystalline films. A less appreciated, but nonetheless powerful means of creating such films is via topotactic transformation, in which a chemical reaction transforms a single crystal of one phase into a single crystal of a different phase, which inherits its orientation from the original crystal. Topotactic reactions may be applied to epitactic films to substitute, add or remove ions to yield epitactic films of different phases. Here we exploit a topotactic reduction reaction to provide a non-ultra-high vacuum (UHV) means of growing highly oriented single crystalline thin films of the easily over-oxidized half-metallic semiconductor europium monoxide (EuO) with a perfection rivalling that of the best films of the same material grown by molecular-beam epitaxy or UHV pulsed-laser deposition. As the technique only requires high-vacuum deposition equipment, it has the potential to drastically improve the accessibility of high-quality single crystalline films of EuO as well as other difficult-to-synthesize compounds.

Hetero-epitaxial EuO interfaces studied by analytic electron microscopy

With nearly complete spin polarization, the ferromagnetic semiconductor europium monoxide could enable next-generation spintronic devices by providing efficient ohmic spin injection into silicon. Spin injection is greatly affected by the quality of the interface between the injector and silicon. Here, we use atomic-resolution scanning transmission electron microscopy in conjunction with electron energy loss spectroscopy to directly image and chemically characterize a series of EuO|Si and EuO|YAlO3 interfaces fabricated using different growth conditions. We identify the presence of europium silicides and regions of disorder at the EuO|Si interfaces, imperfections that could significantly reduce spin injection efficiencies via spin-flip scattering.

Electronic transport through EuO spin-filter tunnel junctions

Epitaxial spin filter tunnel junctions based on the ferromagnetic semiconductor europium monoxide, EuO, are investigated by means of density functional theory. In particular, we focus on the spin transport properties of Cu(100)/EuO(100)/Cu(100) junctions. The dependence of the transmission coefficient and the current-voltage curves on the interface spacing and on the EuO thickness is explained in terms of the EuO density of states and the complex band structure. Furthermore we also discuss the relation between the spin transport properties and the Cu-EuO interface geometry. The level alignment of the junction is sensitively affected by the interface spacing, since this determines the charge transfer between EuO and the Cu electrodes. Our calculations indicate that EuO epitaxially grown on Cu can act as a perfect spin filter, with a spin polarization of the current close to 100%, and with both the Eu-5d conduction band and the Eu-4f valence band states contributing to the coherent transport. For epitaxial EuO on Cu a symmetry filtering is observed, with the \Delta_1 states dominating the transmission. This leads to a transport gap larger than the fundamental EuO band gap. Importantly the high spin polarization of the current is preserved up to large bias voltages.

Antiferromagnetic coupling in EuO1−x

The mechanism for the ferromagnetic order in oxygen-deficient europium monoxide EuO${}_{1\ensuremath{-}\mathrm{x}}$ at temperatures higher than 69 K (the Curie temperature ${T}_{\mathrm{c}}$ of stoichiometric EuO) remains controversial. We have investigated the magnetization of EuO${}_{1\ensuremath{-}\mathrm{x}}$ thin films prepared via pulsed laser deposition as a function of temperature and applied field. The ferromagnetic ordering above 69 K originates from the exchange coupling between the doped electrons and Eu 4$f$ moments and is described using a magnetic polaron model. Our data show that the magnetic polarons are coupled antiferromagnetically to the Eu 4$f$ local moments that dominate the magnetization below 69 K. The magnetic polaron state is stable below 69 K and seems to persist down to a relatively low temperature. An explanation is given to account for the antiferromagnetic coupling. We also describe the evolution of the phases of the magnetic orders as the temperature is varied in EuO${}_{1\ensuremath{-}\mathrm{x}}$.

First principles description of the insulator–metal transition in europium monoxide

Europium monoxide, EuO, is a ferromagnetic insulator. Its electronic structure under pressure and doping is investigated by means of density functional theory. We employ spin polarized electronic structure calculations including onsite electron–electron interaction for the localized Eu 4f and 5d electrons. Our results show that under pressure the ferromagnetism is stable, both for hydrostatic and uniaxial pressure, while the compound undergoes an insulator–metal transition. The insulator–metal transition in O deficient and Gd doped EuO is reproduced for an impurity concentration of 6.25%. A 10 monolayer thick EuO(100) thin film is predicted to be an insulator with a narrow band gap of 0.08eV.

Structural, magnetic and electrical properties of a perovskite containing divalent europium EuZrO 3

A perovskite containing divalent europium, EuZrO 3, was synthesized from europium monoxide (EuO) and zirconium dioxide (ZrO 2). EuO was prepared from sesquioxide and carbon. The perovskite obtained has an orthorhombic structure and is insulating at room temperature. At higher temperature, this compound presents a semi-conducting behavior. The magnetic properties were determined and indicate a paramagnetic compound.

Preparation of Gd-doped EuO1−x thin films and the magnetic and magneto-transport properties

Polycrystalline europium monoxide (EuO) films doped with and without Gd were preparedusing a reactive deposition method in a MBE system. Two different electron dopingmethods, Gd substitution and oxygen deficiency, enhance the Curie temperature(TC) and the temperature dependence of magnetization differently. Gd doping increasesTC up to 125 K, while oxygen deficient samples enhance it above 150 K. The enhancedTC is attributed to the increase in the indirect exchange interaction betweenEu2+ spins viaconduction electrons. The resistivities as a function of temperature for both quasi-stoichiometric (QS) EuOand QS-GdxEu1−xO show maxima near 70 and 110 K, respectively. The conduction band magnetic splittingexplains the resistivity behaviours. Both the temperature dependences of the largemagneto-resistance and the absorption edge are also interpreted by the conduction bandmagnetic splitting mechanism. The unusual blue shift observed in Gd-doped samples is notyet understood.