EuO Thin Films Research Articles

Ferromagnetic semiconductor EuO thin films characterized by vacuum electrochemical process with ionic liquid

In this study, the electrochemical impedance analysis was demonstrated for thin film EuO, a ferromagnetic semiconductor, by taking the advantage of our original instrument, which enables to perform the entire process from thin film deposition to electrochemical measurement with ionic liquid in a vacuum. Effects of whether surface protecting layers were insulative or metallic, as well as their thickness on the electrochemical measurement of EuO thin films were examined by comparing the choices of amorphous alumina and Ag metal as the protecting layer. As a result, the relative permittivity of EuO films, even with a thickness of 2 nm, was successfully estimated and the averaged value was 6.6 within an error of 10 %. Furthermore, the donor density accordingly estimated was found well correlated with the Curie temperature TC of the EuO films.

Temperature-dependent spin-resolved electronic structure of EuO thin films

The electronic structure of the ferromagnetic semiconductor EuO is investigated by means of spin- and angle-resolved photoemission spectroscopy (spin-ARPES) and density functional theory. EuO exhibits unique properties of hosting both weakly-dispersive nearly fully polarized Eu $4f$ bands, as well as O $2p$ levels indirectly exchange-split by the interaction with Eu nearest neighbors. Our temperature-dependent spin-ARPES data directly demonstrates the exchange splitting in O $2p$ and its vanishing at the Curie temperature. Our calculations with a Hubbard $U$ term reveal a complex nature of the local exchange splitting on the oxygen site and in conduction bands. We discuss the mechanisms of the indirect exchange in the O 2p levels by analyzing orbital-resolved band characters in ferromagnetic and antiferromagnetic phases. The directional effects due to spin-orbit coupling are predicted theoretically to be significant in particular in the Eu 4f band manifold. The analysis of the shape of spin-resolved spectra in the Eu $4f$ spectral region reveals signatures of hybridization with O $2p$, in agreement with the theoretical predictions. We also analyze spectral changes in the spin-integrated spectra throughout the Curie temperature and demonstrate they derive from both the magnetic phase transition and effects due to sample aging, unavoidable for this highly reactive material.

Epitaxial Growth of EuF2 and EuO Thin Films Based on Spontaneous Anion Diffusion from Substrates

We report the epitaxial growth of EuF2 (111) thin films on F-ion-conducting CaF2 (111) substrates. We achieved the fluorination of the Eu metal without introducing fluorination gas into the vacuum chamber during magnetron sputtering; instead, F atoms were supplied by the substrates. This growth method also allows EuO (100) epitaxial thin films to be fabricated on O-ion-conducting yttria-stabilized zirconia substrates. X-ray diffraction, transmission electron microscopy, and optical absorption spectra verified that the proposed method yields high-quality EuF2 and EuO thin films. Therefore, this study presents a novel thin-film growth method based on spontaneous anion diffusion from substrates.

Growth and Characterisation Studies of Eu3O4 Thin Films Grown on Si/SiO2 and Graphene.

We report the growth, structural and magnetic properties of the less studied Eu-oxide phase, EuO, thin films grown on a Si/SiO substrate and Si/SiO/graphene using molecular beam epitaxy. The X-ray diffraction scans show that highly textured crystalline EuO(001) films are grown on both substrates, whereas the film deposited on graphene has a better crystallinity than that grown on the Si/SiO substrate. The SQUID measurements show that both films have a Curie temperature of ∼ K, with a magnetic moment of ∼ emu/cm at 2 K. The mixed valence of the Eu cations has been confirmed by the qualitative analysis of the depth-profile X-ray photoelectron spectroscopy measurements with the : ratio of . However, surprisingly, our films show no metamagnetic behaviour as reported for the bulk and powder form. Furthermore, the microscopic optical images and Raman measurements show that the graphene underlayer remains largely intact after the growth of the EuO thin films.

Exploring the intrinsic limit of the charge-carrier-induced increase of the Curie temperature of Lu- and La-doped EuO thin films

Raising the Curie temperature ${T}_{\mathrm{C}}$ of the highly spin-polarized semiconductor EuO by doping it with rare-earth elements is a strategy to make EuO more technologically relevant to spintronics. The increase of ${T}_{\mathrm{C}}$ with free carrier density $n$ and the surprisingly low dopant activation $p$, found in Gd-doped EuO thin films [Mairoser et al., Phys. Rev. Lett. 105, 257206 (2010)], raised the important question of whether ${T}_{\mathrm{C}}$ could be considerably enhanced by increasing $p$. Using a low-temperature growth method for depositing high-quality Lu-doped EuO films we attain high dopant activation ($p$) values of up to 67%, effectively more than doubling $p$ as compared to adsorption-controlled growth of Lu- and Gd-doped EuO. Relating $n, p$, and lattice compression of La- and Lu-doped EuO films grown at different temperatures to the ${T}_{\mathrm{C}}$ of these samples allows us to identify several different mechanisms influencing ${T}_{\mathrm{C}}$ and causing an experimental maximum in ${T}_{\mathrm{C}}$. In addition, scanning transmission electron microscopy in combination with electron energy loss spectroscopy measurements on La-doped EuO indicate that extensive dopant clustering is one, but not the sole reason for dopant deactivation in rare-earth doped EuO films.

Pulsed Laser Deposition of Rocksalt Magnetic Binary Oxides

Magnetic binary oxides with the rocksalt structure constitute an important class of materials for potential applications as electronic or electrochemical devices. Moreover, they often become a theoretical playground, due to the simple electronic and crystal structures, in the quest for novel phenomena. For these possibilities to be realized, a necessary prerequisite would be to grow atomically ordered and controllably-strained binary oxides on proper substrates. Here we systematically explore the use of pulsed laser deposition technique (PLD) to grow three basic oxides that have rocksalt structure but different chemical stability in the ambient atmosphere: NiO (stable), MnO (metastable) and EuO (unstable). By tuning laser fluence FL, an epitaxial single-phase NiO thin-film growth can be achieved in a wide range of growth temperatures 10 ≤ TG ≤750 °C. At the lowest TG, the out-of-plane strain raises to 1.5%, which is five times higher than in NiO film grown at 750 °C. MnO thin films that had long-range order were successfully deposited on the MgO substrates after appropriate tuning of deposition parameters. The growth of MnO phase was strongly influenced by FL and the TG. EuO films with satisfactory quality were deposited by PLD after oxygen availability had been minimized. Synthesis of EuO thin films at rather low TG = 350 °C prevented thermally-driven lattice relaxation and allowed growth of strained films. Overall, PLD was a quick and reliable method to grow binary oxides with rocksalt structure in high quality that can satisfy requirements for applications and for basic research.

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.

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.

Valence state of Sm in single-crystalline EuO thin films

Samarium has two stable valence states, 2+ and 3+, which coexist in many compounds forming spatially homogeneous intermediate valence states. We study the valence state of samarium when incorporated in a single crystalline EuO thin film which crystallizes in a fcc structure similar to that of the intermediate valence SmO, but with a larger lattice constant. Due to the increased lattice spacing, a stabilization of the larger Sm2+ ion is expected. Surprisingly, the samarium incorporated in SmxEu1−xO thin films shows a predominantly trivalent character, as determined by x-ray photoelectron spectroscopy and magnetometry measurements. We infer that the O2− ions in the EuO lattice have enough room to move locally, so as to reduce the Sm-O distance and stabilize the Sm3+ valence.

Elevated Curie temperature and half-metallicity in the ferromagnetic semiconductorLaxEu1−xO

Here we study the effect of La doping in EuO thin films using SQUID magnetometry, muon spin rotation ($\mu$SR), polarized neutron reflectivity (PNR), and density functional theory (DFT). The $\mu$SR data shows that the La$_{0.15}$Eu$_{0.85}$O is homogeneously magnetically ordered up to its elevated $T_{\rm C}$. It is concluded that bound magnetic polaron behavior does not explain the increase in $T_{\rm C}$ and an RKKY-like interaction is consistent with the $\mu$SR data. The estimation of the magnetic moment by DFT simulations concurs with the results obtained by PNR, showing a reduction of the magnetic moment per La$_{x}$Eu$_{1-x}$O for increasing lanthanum doping. This reduction of the magnetic moment is explained by the reduction of the number of Eu-4$f$ electrons present in all the magnetic interactions in EuO films. Finally, we show that an upwards shift of the Fermi energy with La or Gd doping gives rise to half-metallicity for doping levels as high as 3.2 %.

Topological Hall effect in thin films of the Heisenberg ferromagnet EuO

We report on the topological Hall effect (THE) in centrosymmetric EuO thin films. This THE signal persists down to the lowest temperature in the metallic region below 50 K for the films thinner than 200 nm. The signal rapidly disappears by tilting the applied magnetic field from surface normal, suggestive of noncoplanar spin configuration such as two-dimensional skyrmions. This observation possibly substantiates the theoretical proposal of magnetic skyrmions in 2D Heisenberg ferromagnets in marked contrast to better established $B20\text{\ensuremath{-}}\mathrm{type}$ chiral helimagnets.

Direct epitaxial integration of the ferromagnetic semiconductor EuO with silicon for spintronic applications.

Following a remarkable success of metallic spintronics, tremendous efforts have been invested into the less developed semiconductor spintronics, in particular, with the aim to produce three-terminal spintronic devices, e.g., spin transistors. One of the most important prerequisites for such a technology is an effective injection of spin-polarized carriers into a nonmagnetic semiconductor, preferably one of those currently used for industrial applications such as Si-a workhorse of modern electronics. Ferromagnetic semiconductor EuO is long believed to be the best candidate for integration with Si. Although EuO proved to offer optimal conditions for effective spin injection into silicon and in spite of considerable efforts, the direct epitaxial stabilization of stoichiometric EuO thin films on Si without any buffer layer has not been demonstrated to date. Here we report a new technique for control of EuO/Si interface on submonolayer level. Using this technique we solve a long-standing problem of direct epitaxial growth on silicon of thin EuO films which exhibit structural and magnetic properties of EuO bulk material. This result opens up new possibilities in developing all-semiconductor spintronic devices.

Growth of EuO/Si and EuO/SrO/Si heteroepitaxial structures by molecular-beam epitaxy

Epitaxial EuO thin films with thickness up to 60 nm have been grown by molecular beam epitaxy both on SrO sublayers and directly on Si (001) substrates. Crystal structure has been controlled in situ by reflection high energy electron diffraction. Ex situ studies by X-ray diffraction and Rutherford backscattering have confirmed high crystalline quality of the films.

Photoinduced sign inversion of the anomalous Hall effect in EuO thin films

We report on the sign inversion of the anomalous Hall effect (AHE) in EuO thin films along with photoirradiation as well as a temperature scan across \ensuremath{\sim}25 K that is well below the Curie temperature (${T}_{\mathrm{C}}\ensuremath{\sim}80$ K). The former gives an enhancement of the mobile electron density ($n$) by more than 30%, but the latter gives a negligible modification of $n$ of only 3% with a significant enhancement in mobility. It is found, in addition to the universal scaling law between longitudinal conductivity (${\ensuremath{\sigma}}_{xx}$) and anomalous Hall conductivity (${\ensuremath{\sigma}}_{\mathrm{AH}}$) as $|{\ensuremath{\sigma}}_{\mathrm{AH}}|\ensuremath{\propto}{\ensuremath{\sigma}}_{xx}^{1.6}$, that there is a critical value of about 10${}^{2}$ S cm${}^{\ensuremath{-}1}$ in ${\ensuremath{\sigma}}_{xx}$ that gives a boundary in the sign inversion of ${\ensuremath{\sigma}}_{\mathrm{AH}}$. If $n$ solely governs the sign of ${\ensuremath{\sigma}}_{\mathrm{AH}}$, the phenomenon could be explained by a Fermi level shift across the singularity in the band structure. However, our band calculation shows that, within any realistic adjustment of band parameters, the sign inversion of AHE never occurs. Thus, we conclude that other mechanisms of AHE are necessary to account for the AHE of EuO.

Growth and characterization of Sc-doped EuO thin films

The preparation of 3d-transition metal-doped EuO thin films by molecular beam epitaxy is investigated using the example of Sc doping. The Sc-doped EuO samples display a good crystalline structure, despite the relatively small ionic radius of the dopant. The Sc doping leads to an enhancement of the Curie temperature to up to 125 K, remarkably similar to previous observations on lanthanide-doped EuO.

Realizing the quantum anomalous Hall effect in materials with in-plane magnetization

When an electric current flows through a confined two-dimensional (2D) electron gas, which is under an external high magnetic field (H) along the z direction, themagnetic fieldwill deflect the current. A transverse quantizedHall conductance can then be observed across the sample. This effect, known as the quantum Hall effect (QHE), was discovered in 1980 by Klaus von Klitzing who won the 1985 Nobel Prize in Physics for this fundamental discovery. However, the large externalmagnetic field required to observe the QHE strongly hampers its practical application. In the last few years, people have realized that even without the externalH, the quantizedHall conductance could also be induced by magnetization, dubbed as the quantum anomalous Hall (QAH) effect.Thiswas first observed this year in magnetically doped topological insulator thin films. As the QAH system carries dissipationless currents, its realization could pave the way for the field of dissipationless electronics. It is commonly believed that the QAH effect, just like the QHE, also requires out-of-plane magnetization. Recently, Chao-Xing Liu and his colleagues from the Pennsylvania State University predicted that, for the QAH system, the out-of-plane magnetization is not a must, whereas the most essential point is to break all the reflection symmetries [1]. By using general symmetry analysis, Liu et al. found that the presence of the reflection symmetry (RS), or time reversal symmetry in a 2D lattice, can bring the Hall conductance to zero. For a 2D system with in-plane magnetization, RS, with the reflection plane normal to the magnetization direction, is still preserved. Therefore, the remaining RS needs to be broken in order to achieve a non-zero Hall conductance. Following this idea, Liu et al. proposed the experimental setups in two realistic systems, including magnetically doped Bi2Te3 thin films and HgMnTe quantum wells. For magnetically doped Bi2Te3 thin films, they demonstrated that when in-plane magnetization is aligned along proper directions, the warping effects, which originate from the microscopic crystalline structure, can break the remaining RS and yield the QAH effect. More interestingly, they revealed that the sign of the quantized Hall conductance depends on the angle between the magnetization and the crystalline orientation. Thus, by rotating the inplane magnetization, one can easily distinguish the in-plane-magnetizationinduced QAH effect from that caused by out-of-plane magnetization. Moreover, Liu et al. predicted that the similar effect also occurs in HgMnTe quantum wells with the shear strain, indicating that the proposed effects should exist generally. Furthermore, based on two concrete examples, Liu et al. suggested that in a QAH system, a pseudo-scalar can always be constructed to break all the RSs in 2D point groups. Guided by this strategy, it is possible to search newQAH systems in hybrid structures with the ferromagnetic insulator substrates, such as EuO and GdN thin films, which host in-plane magnetization. Liu et al.’s proposal completely excludes the orbital effect from Landau levels induced by the out-of-plane magnetic field and provides a clear experimental signature to distinguish the orbital effect of magnetic fields from the exchange effect of magnetic ions.

Three-dimensional angle-resolved photoemission spectra of EuO thin film

We report the electronic structure of EuO thin films not only parallel to but also perpendicular to the (001) plane using synchrotron radiation angle-resolved photoemission spectroscopy. As a result, the electronic structure perpendicular to the (001) plane is consistent with in-plane electronic structure in spite of the thin films. This result implies that the electronic structure of EuO thin films is consistent with that of the bulk materials.

Structure and magnetic properties of ultra thin textured EuO films on graphene

We present a straightforward and reproducible method to grow stoichiometric and single phase (100) textured EuO thin films on epitaxial graphene. Depending on coverage, either separated EuO grains or fully closed layers can be prepared. Room temperature preparation followed by annealing in Eu vapor leads to a random distribution of the in-plane orientation, whereas growth under distillation conditions at 720 K induces a fixed orientation with respect to the substrate. Magneto-optical Kerr effect (MOKE) shows that the films are ferromagnetic with an enhanced Curie temperature.

A magnetic polaron model for the enhanced Curie temperature of EuO1−x

The investigation of a series of oxygen-deficient EuO thin films provided strong evidence that the doped electrons form magnetic polarons with the nearby Eu2+ 4f spins; this is responsible for the enhanced Curie temperature observed near 140 K. Unlike in the previous magnetic polaron models proposed for the metal-to-insulator transition in EuO, the exchange coupling J between the doped electron and its neighboring 4f spins is antiferromagnetic. The model explains satisfactorily the fact that the ordering temperature of the magnetic polarons occurs at ∼140 K, independently of the oxygen vacancy concentration, and the contradiction that electron doping increases Tc and yet reduces the red shift in the optical absorption. The magnetic polarons are coupled antiferromagnetically to the Eu2+ local moments that are ordered in the Heisenberg ferromagnet below 69 K. This coupling was observable in the vicinity of 69 K. We discuss how, with increasing concentration of the oxygen vacancies, their behaviors evolve from those of isolated superparamagnetic polarons to those of percolating magnetic polarons with a finite coercivity.

Effect of film thickness and biaxial strain on the curie temperature of EuO

The effects of film thickness and epitaxial strain on the magnetic properties of commensurate EuO thin films grown on single crystalline (001) yttria-stabilized zirconia (YSZ) and (110) LuAlO3 substrates are presented. Magnetic measurements show a reduction in the Curie temperature (TC) for EuO/YSZ films thinner than ∼10 nm. Additionally, the EuO/LuAlO3 films exhibit a systematically lower TC than the corresponding EuO/YSZ films. This further reduction in TC is attributed to the effect of biaxial tensile strain arising from lattice mismatch: 0.0% for EuO/YSZ and +1.0% for EuO/LuAlO3.