EuO Thin Films Research Articles

Sign of the superexchange coupling between next-nearest neighbors in EuO

The sign of the superexchange coupling ${J}_{2}$ between next-nearest neighboring Eu${}^{2+}$ magnetic moments in EuO is a matter subject to debate. We have obtained evidence that this coupling is of antiferromagnetic nature (${J}_{2}<0$). EuO thin films grown at different temperatures suggest that lattice expansion results in enhancement of ${T}_{C}$ as clearly observed in stoichiometric EuO films grown on CaF${}_{2}$ substrates. Resonant photoemission spectroscopy provides compelling evidence of strong hybridization between O 2$p$ and Eu 5$d$6$s$6$p$ weighted bands, suggesting that strong superexchange may be mediated by oxygen, thus consistent with the observed antiferromagnetic behavior between the next-nearest neighboring Eu atoms via nearest neighbor oxygen in EuO.

Ultrafast optical control of magnetization in EuO thin films

All-optical pump-probe detection of magnetization precession has been performed for ferromagnetic EuO thin films at 10 K. We demonstrate that the circularly polarized light can be used to control the magnetization precession on an ultrafast time scale. This takes place within the 100-fs duration of a single laser pulse, through combined contribution from two nonthermal photomagnetic effects, i.e., enhancement of the magnetization and an inverse Faraday effect. From the magnetic field dependencies of the frequency and the Gilbert damping parameter, the intrinsic Gilbert damping coefficient is evaluated to be \ensuremath{\alpha} \ensuremath{\approx} 3 \ifmmode\times\else\texttimes\fi{} 10${}^{\ensuremath{-}3}$.

Ultrafast Time-Resolved Faraday Rotation in EuO Thin Films

We have investigated the ultrafast spin dynamics in EuO thin films by time-resolved Faraday rotation spectroscopy. The photoinduced magnetization is found to be increased in a transient manner, accompanied with subsequent demagnetization. The dynamical magnetization enhancement showed a maximum slightly below the Curie temperature with prolonged tails toward both lower and higher temperatures and dominates the demagnetization counterpart at 55 K. The magnetization enhancement component decays in ~1 ns. The realization of the transient collective ordering is attributable to the enhancement of the f-d exchange interaction.

Lutetium-doped EuO films grown by molecular-beam epitaxy

The effect of lutetium doping on the structural, electronic, and magnetic properties of epitaxial EuO thin films grown by reactive molecular-beam epitaxy is experimentally investigated. The behavior of Lu-doped EuO is contrasted with doping by lanthanum and gadolinium. All three dopants are found to behave similarly despite differences in electronic configuration and ionic size. Andreev reflection measurements on Lu-doped EuO reveal a spin-polarization of 96% in the conduction band, despite non-magnetic carriers introduced by 5% lutetium doping.

NO-assisted molecular-beam epitaxial growth of nitrogen substituted EuO

We have investigated a method for substituting oxygen with nitrogen in EuO thin films, which is based on molecular beam epitaxy distillation with NO gas as the oxidizer. By varying the NO gas pressure, we produce crystalline, epitaxial EuO1-xNx films with good control over the films’ nitrogen concentration. In situ x-ray photoemission spectroscopy reveals that nitrogen substitution is connected to the formation Eu3+4f6 and a corresponding decrease in the number of Eu2+4f7, indicating that nitrogen is being incorporated in its 3− oxidation state. While small amounts of Eu3+ in over-oxidized Eu1-δO thin films lead to a drastic suppression of the ferromagnetism, the formation of Eu3+ in EuO1-xNx still allows the ferromagnetic phase to exist with an unaffected Tc, thus providing an ideal model system to study the interplay between the magnetic f7 (J = 7/2) and the non-magnetic f6 (J = 0) states close to the Fermi level.

Spectroscopic observation of strain-assistedTCenhancement in EuO upon Gd doping

The origin of the Curie temperature enhancement of EuO upon Gd doping is studied by using soft x-ray absorption spectroscopy on epitaxial pure and Gd-doped EuO thin films. Temperature- and doping-dependent changes in the oxygen K edge spectra provide information about the correlation of magnetism and lattice. Band-structure calculations reveal that these spectral changes as well as the increase of the Curie temperature to 125 K for Gd-doped EuO cannot be explained by electron doping alone. The compression of the crystal lattice due to the incorporation of the smaller Gd${}^{3+}$ ions plays also an important role.

Chemical stability of the magnetic oxide EuO directly on silicon observed by hard x-ray photoemission spectroscopy

We present a detailed study of the electronic structure and chemical state of high-quality stoichiometric EuO and O-rich ${\mathrm{Eu}}_{1}{\mathrm{O}}_{1+x}$ thin films grown directly on silicon without any buffer layer using hard x-ray photoemission spectroscopy (HAXPES). We determine the EuO oxidation state from a consistent quantitative peak analysis of $4f$ valence band and $3d$ core-level spectra. The results prove that nearly ideal, stoichiometric, and homogeneous EuO thin films can be grown on silicon, with a uniform depth distribution of divalent Eu cations. Furthermore, we identify the chemical stability of the EuO/silicon interface from Si $2p$ core-level photoemission. This work clearly demonstrates the successful integration of high-quality EuO thin films directly on silicon, opening up the pathway for the future incorporation of this functional magnetic oxide into silicon-based spintronic devices.

Oxygen off-stoichiometry and phase separation in EuO thin films

We report on our study on the influence of the growth conditions on the europium/oxygen stoichiometry, morphology, magnetic properties, and electrical conductivity of EuO thin films. SQUID magnetometry and x-ray photoelectron spectroscopy were utilized as complementary techniques to determine the oxygen content of EuO${}_{1\ifmmode\pm\else\textpm\fi{}x}$ thin films grown by molecular beam epitaxy with and without the employment of the so-called Eu distillation process. We found indications for phase separation to occur in Eu-rich as well as in over-oxidized EuO for films grown at substrate temperatures below the Eu distillation temperature. Only a fraction of the excess Eu contributes to the metal-insulator transition in Eu-rich films grown under these conditions. We also observed that the surfaces of these films were ill defined and may even contain more Eu excess than the film average. Only EuO films grown under distillation conditions are guaranteed to have the same magnetic and electrical properties as stoichiometric bulk EuO, and to have surfaces with the proper Eu/O stoichiometry and electronic structure.

La-doped EuO: A rare earth ferromagnetic semiconductor with the highest Curie temperature

We report the fabrication of single-crystalline La-doped EuO thin films with a Curie temperature (TC) of about 200 K, the highest among rare-earth compounds without transition metals. From first-principle band calculation and x-ray diffraction measurement, the observed increase in TC cannot be explained only by the increase in hybridization intensity due to lattice contraction and the increase in up-spin electrons of the Eu 5d state caused by the electron doping. Hybridization between the Eu 4f and donor states and/or Ruderman–Kittel–Kasuya–Yoshida interaction mediated by the doped La 5d state is a possible origin of the increase in TC.

Single Crystal Growth and Magnetic, Optical, and Photoelectrical Properties of EuO Thin Film

We report on the single-crystal growth of EuO thin films [fcc 1×1 (100)] and their magnetic, optical, and photoelectrical properties to examine the consistency with the bulk materials. EuO thin films were fabricated on a BaO buffer layer created on a SrTiO3 substrate. The obtained EuO thin film was found to be a ferromagnetic semiconductor with a Curie temperature of 71 K and an energy gap of about 1 eV. We were able to observe a clear angle dependence of the photoemission spectrum, indicating good single-crystalline properties.

Field and temperature dependence of the magnetization in ferromagnetic EuO thinfilms

Ferromagnetic EuO exhibits a metal–insulator transition showing a very largecolossal magneto-resistance. Recently, it became possible to grow epitaxial films ofEuO1−x on Si with spin polarization above 90%. The direct integration of EuO with Si will allowthe fabrication of model systems for studying devices in the field of spintronics. In order todetermine non-destructively the magnetic properties of thin films of EuO on Si, we havemeasured the critical angle of reflection by using polarized neutrons. The results confirmthat the magnetic moment in the films is consistent with the bulk value. In addition, weshow that the change of magnetization is not caused by repopulation of domains but bydomain rotation.

Angle-resolved photoemission study on EuO thin films

We report the electronic structure of single-crystalline EuO thin films studied by angle-resolved photoemission spectroscopy and band structure calculation. We have found that the Eu 4f states have sizable energy dispersion at both the Γ and X points. This result indicates that the hybridization between the Eu 4f states and the other electronic states such as the O 2p and Eu 5d ones is important.

Soft x-ray magnetic circular dichroism study on Gd-dopedEuOthin films

We report on the growth and characterization of ferromagnetic Gd-doped EuO thin films. We prepared samples with Gd concentrations up to 11% by means of molecular beam epitaxy under distillation conditions, which allows a very precise control of the doping concentration and oxygen stoichiometry. Using soft x-ray magnetic circular dichroism at the Eu and Gd M4,5 edges, we found that the Curie temperature ranged from 69 K for pure stoichiometric EuO to about 170 K for the film with the optimal Gd doping of around 4%. We also show that the Gd magnetic moment couples ferromagnetically to that of Eu.

Properties of thin film europium oxide by x-ray magnetic circular dichroism

Magnetic circular dichroism was measured as function of temperature for the Eu M5 and M4 absorption edges in single crystal EuO thin films. The M5 edge is resolved into two major features, one with the same helicity dependence as the M4 edge, and one with opposite helicity dependence. While the ratio of the absorption peaks with similar helicity dependence remains constant as a function of temperature, the ratio of those with opposite dependence is found to change linearly with temperature below 75 K. The physical mechanism for this linear behavior in the absorption peak intensity is not well understood.

Preparation of Polycrystalline GdxEu1-xO Thin Films

Ferromagnetic semiconductor EuO and GdxEu1-xO (0.01<x<0.05) thin films were prepared by a reactive deposition method in an ultra-high-vacuum deposition system. Aluminum capping and a combination of Ar treatment and CaF2 capping are effective for protecting sample films. Both oxygen-deficient and Gd-doped EuO films were examined as electron-doped ferromagnetic semiconductors. The Gd doping looks more promising due to the chemical stability and the better electron doping controllability.

Epitaxial growth and magnetic properties of EuO on (001) Si by molecular-beam epitaxy

Epitaxial (001) EuO thin films have been grown on (001) Si utilizing an intermediate, epitaxial SrO buffer layer by molecular-beam epitaxy. Four-circle x-ray diffraction reveals nearly phase-pure samples. Magnetic measurements indicate that the EuO layer is ferromagnetic, with a transition temperature (68 K) close to the bulk value and a saturation magnetic moment of 4.7 Bohr magnetons per Eu atom. The magneto-optic Kerr effect observed is also comparable to bulk EuO. Such heterostructures have potential as a means to inject spin-polarized electrons into silicon for use in spintronics applications.

Preparation and Magnetic Properties of Gd Doped EuO Thin Films

Preparation and Magnetic Properties of Gd Doped EuO Thin Films

Preparation and Magnetic Properties of EuO Thin Films Epitaxially Grown on MgO and SrTiO 3 Substrates

Ferromagnetic EuO films were epitaxially grown on MgO and SrTiO 3 substrates aligned with EuO[100]//MgO[100], EuO(001)//MgO(001) and EuO[100]//SrTiO 3 [110], EuO(001)//SrTiO 3 (001). The single phase EuO film was obtained on MgO substrate. The higher order oxides, Eu 2 O 3 , was always present in the films on SrTiO 3 substrate due to the reaction of europium metal and grown EuO with oxygen in SrTiO 3 . The optimized flux ratio of oxygen to europium, J OXY / J Eu , was 0.39, for J Eu =2.83× 10 13 cm -2 · s -1 . The representative growth rate was 9.0×10 -2 Å/s on MgO. Magnetic moment per europium atom was 6.9± 0.3 µ B . The Curie temperature was 69 K for films on MgO. The T C higher than 69 K was observed for films on SrTiO 3 thicker than 50 Å. The thickness of 400 Å was enough for the coercive field to be as low as 50 Oe.

Synthesis and Magnetic Properties of Non-Equilibrium Eu-Rich EuO Thin Films

EuO thin films have been formed by reactive evaporation, in which O2 gas was introduced in the form of a molecular beam directed toward a substrate. In order to optimize preparation conditions, we varied the substrate temperature, T s, and the ratio of the Eu vapor to O2 gas deposition rates, R. The structure and magnetic properties of the films were measured by X-ray diffraction and a super-conducting interference device (SQUID) magnetometer. Non-stoichiometric EuO films can be produced more easily at T s=200° C than at T s=300° C. Choosing an appropriate value of R at T s=200° C, we obtain Eu-rich EuO films with the Curie temperature of about 150 K. The present method is based on the control of reaction kinetics, and is superior to the post-oxidation method of Eu-metals with respect to the formation of Eu-rich EuO films.

Preparation of EuO films by sol-gel technique

EuO thin films with the cubic NaCl structure were fabricated by a sol-gel technique. The composition and crystal structure of the films were investigated as a function of firing temperature. The preparation of the stock solution and the coating and firing of the films are described. The firing of the gel film and the reduction of the Eu2O3 phase are also discussed.