Rare earth doping is the key strategy to increase the Curie temperature (T(C)) of the ferromagnetic semiconductor EuO. The interplay between doping and charge carrier density (n), and the limit of the T(C) increase, however, are yet to be understood. We report measurements of n and T(C) of Gd-doped EuO over a wide range of doping levels. The results show a direct correlation between n and T(C), with both exhibiting a maximum at high doping. On average, less than 35% of the dopants act as donors, raising the question about the limit to increasing T(C).

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|YAlO 3 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. a These authors contributed equally to this work.

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.

We report on the magnetic field dependence of the apparent ferromagnetic ordering temperature (T F ) of the ferromagnetic semiconductor EuO doped with 8% Gd, La, or Lu. Chemical doping is a common method to increase the T F of EuO. Recent findings demonstrate that in thin films only a fraction of the dopants donate electrons into the conduction band. We show that the T F of doped EuO determined by the standard procedure drastically increases with applied magnetic fields. The comparison of measured data to theoretical models is in agreement with large fractions of dopant electrons being localized and the presence of magnetic disorder. and thin films. 8,[12][13][14][20][21][22][23][24][25] Despite exploiting the same physical mechanism to increase T C , the reported improvements vary strongly from experiment to experiment even for identical dopant elements and comparable dopant concentrations. These discrepancies might partially be explained by different magnetic background fields and different methods used to extract T C from the magnetic data including several superconducting quantum interference device (SQUID) magnetometry-based methods, x-ray magnetic dichroism (XMCD), second harmonic generation (SHG), magneto-optic Kerr rotation, and neutron reflectometry (see temperature (T F ) for meaurements performed in non-zero magnetic background fields.To address these questions and to provide a database for the comparison of experiments, in this Letter we investigate the dependence of T F of 8% rare-earth-doped EuO (Eu 0.92 B 0.08 O, with B = Gd, La, Lu) on applied external magnetic fields. The doping concentration was chosen to be in the range of the maximum reported and theoretically predicted T F values. 15,16,[21][22][23]26,27,34,35 The dopants were chosen to provide a spectrum of ionic radii, electron configurations, and to investigate possible differences between magnetic (Gd) and non-magnetic (La, Lu) dopants. To analyze systematically changes originating from applied external magnetic fields, we kept film thickness, microstructure, and oxygen content constant.The films were grown using reactive oxide molecular-beam epitaxy (Veeco 930 and GEN10MBE systems) on YAlO 3 single crystal substrates oriented within ±0.5• of (110). 37 Europium and the dopant elements were co-evaporated from effusion cells. The respective fluxes were calibrated to the desired Eu/dopant ratio using a quartz crystal microbalance. The total metal flux was set to 1.1 × 10 14 atoms /cm 2 s. The films were deposited in O 2 partial pressuresTorr above the vacuum chamber background pressure of 2 × 10 −9 Torr.To minimize additional charge carrier doping originating from oxygen vacancies, the films 3 were grown in the adsorption controlled growth regime at a substrate temperature of T sub = 350• C. 24,37 To prevent their oxidation and to allow ex situ analysis, all films were capped with about 20 nm of amorphous silicon. After growth, ex situ four-circle x-ray diffraction (XRD) was used to characterize the structural quality of all films....