High quality thin films of the ferromagnetic semiconductor EuO have been prepared and were studied using a new form of spin-resolved spectroscopy. We observed large changes in the electronic structure across the Curie and metal-insulator transition temperature. We found that these are caused by the exchange splitting of the conduction band in the ferromagnetic state, which is as large as 0.6 eV. We also present strong evidence that the bottom of the conduction band consists mainly of majority spins. This implies that doped charge carriers in EuO are practically fully spin polarized.

Exchange splitting and charge carrier spin-polarization in EuO

Metal (M) oxides are one of the most interesting and widely used solids, and many of their properties can be directly correlated to the local structural ordering within basic building units (BBUs) One particular example is the high-Ni transition metal layered oxides, potential cathode materials for Li-ion batteries whose electrochemical activity is largely determined by the cationic ordering in octahedra (eg, the BBUs in such systems) Yet to be firmly established is how the BBUs are inherited from precursors and subsequently evolve into the desired ordering during synthesis Herein, a multimodal in situ X-ray characterization approach is employed to investigate the synthesis process in preparing LiNi077Mn013Co010O2 from its hydroxide counterpart, at scales varying from the long-range to local individual octahedral units Real-time observation corroborated by first-principles calculations reveals a topotactic transformation throughout the entire process, during which the layered framework is retaine

Cationic Ordering Coupled to Reconstruction of Basic Building Units during Synthesis of High-Ni Layered Oxides

We propose a universal practical approach to realize magnetic second-order topological insulator (SOTI) materials, based on properly breaking the time reversal symmetry in conventional (first-order) topological insulators. The approach works for both three dimensions (3D) and two dimensions (2D), and is particularly suitable for 2D, where it can be achieved by coupling a quantum spin Hall insulator with a magnetic substrate. Using first-principles calculations, we predict bismuthene on EuO(111) surface as the first realistic system for a two-dimensional magnetic SOTI. We explicitly demonstrate the existence of the protected corner states. Benefitting from the large spin-orbit coupling and sizable magnetic proximity effect, these corner states are located in a boundary gap $\ensuremath{\sim}83\text{ }\text{ }\mathrm{meV}$, and hence can be readily probed in experiment. By controlling the magnetic phase transition, a topological phase transition between a first-order TI and a SOTI can be simultaneously achieved in the system. The effect of symmetry breaking, the connection with filling anomaly, and the experimental detection are discussed.

Universal Approach to Magnetic Second-Order Topological Insulator

Recent Advances in 2D Rare Earth Materials

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.

/pdf/high-quality-euo-thin-films-the-easy-way-via-topotactic-1abbku9fan.pdf

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

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.

Hetero-epitaxial EuO Interfaces Studied by Analytic Electron Microscopy

Hetero-epitaxial EuO interfaces studied by analytic electron microscopy

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.

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

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

Hetero-epitaxial EuO Interfaces Studied by Analytic Electron Microscopy

Hetero-epitaxial EuO interfaces studied by analytic electron microscopy

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