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Hafnium oxide plays an important role as a dielectric material in various thin-film electronic devices such as transistors and resistive or ferroelectric memory. The crystallographic and electronic structure of the hafnia layer often depends critically on its composition and defect structure. Here, we report two novel defect-stabilized polymorphs of substoichiometric HfO

Utilizing the molecular beam epitaxy technique, a nanoscale thin-film magnet of c-axis-oriented Sm2Co17 and SmCo5 phases is stabilized. While typically in the prototype Sm(Co, Fe, Cu, Zr)7.5-8 pinning-type magnets, an ordered nanocomposite is formed by complex thermal treatments, here, a one-step approach to induce controlled phase separation in a binary Sm-Co system is shown. A detailed analysis of the extended X-ray absorption fine structure confirmed the coexistence of Sm2Co17 and SmCo5 phases with 65% Sm2Co17 and 35% SmCo5. The SmCo5 phase is stabilized directly on an Al2O3 substrate up to a thickness of 4 nm followed by a matrix of Sm2Co17 intermixed with SmCo5. This structural transition takes place through coherent atomic layers, as revealed by scanning transmission electron microscopy. Highly crystalline growth of well-aligned Sm2Co17 and SmCo5 phases with coherent interfaces result in strong exchange interaction, leading to enhanced magnetization and magnetic coupling. The arrangement of Sm2Co17 and SmCo5 phases at the nanoscale is reflected in the observed magnetocrystalline anisotropy and coercivity. As next-generation permanent magnets require designing of materials at an atomic level, this work enhances our understanding of self-assembling and functioning of nanophased magnets and contributes to establishing new concepts to engineer the microstructure for beyond state-of-the-art magnets.

Low-frequency noise in Y2O3-based resistive random-access memory devices with analog switching is studied at intermediate resistive states and as a function of dc cycling. A universal 1/f(alpha)-type behavior is found, with a frequency exponent of alpha approximate to 1.2 that is independent of the applied reset voltage or the device resistance and is attributed to the intrinsic abundance of oxygen vacancies unique to the structure of yttria. Remarkably, the noise magnitude in the high resistive state systematically decreases through dc training. This effect is attributed to the stabilization of the conductive filament via the consumption of oxygen vacancies, thus reducing the number of active fluctuators in the vicinity of the filament.

In this work, analysis and simulation of all experimentally observed switching modes in hafnium oxide based resistive random access memories are carried out using a simplified electrical conduction model. To achieve switching mode variation, two metal-insulator-metal cells with identical stack combination, but varying oxygen stoichiometry of the hafnia layer, namely, stoichiometric vs highly deficient, are considered. To access the individual switching modes, the devices were subjected to a variety of cycling conditions comprising different voltage and current ranges. For modeling the device behavior, a single or two antiserially connected memdiodes (diode with memory) were utilized. In this way, successful compact simulation of unipolar, bipolar, threshold, and complementary resistive switching modes is accomplished confirming the coexistence of two switching mechanisms of opposite polarity as the basis for all observable switching phenomena in this material. We show that only calibration of the outer current-voltage loops with the memdiode model is necessary for predicting the device behavior in the defined region revealing additional information on the switching process. The correspondence of each memdiode device with the conduction characteristics of the individual top and bottom metal-oxide contacts allows one to assess the role played by each interface in the switching process separately. This identification paves the path for a future improvement of the device performance and functionality by means of appropriate interface engineering.

This work investigates the transition from digital to gradual or analog resistive switching in yttrium oxide-based resistive random-access memory devices. It is shown that this transition is determined by the amount of oxygen in the functional layer. A homogeneous reduction of the oxygen content not only reduces the electroforming voltage, allowing for forming-free devices, but also decreases the voltage operation window of switching, thereby reducing intra-device variability. The most important effect as the dielectric becomes substoichiometric by oxygen engineering is that more intermediate (quantized) conduction states are accessible. A key factor for this reproducibly controllable behavior is the reduced local heat dissipation in the filament region due to the increased thermal conductivity of the oxygen depleted layer. The improved accessibility of quantized resistance states results in a semi-gradual switching both for the set and reset processes, as strongly desired for multi-bit storage and for an accurate definition of the synaptic weights in neuromorphic systems. A theoretical model based on the physics of mesoscopic structures describing current transport through a nano-constriction including asymmetric potential drops at the electrodes and non-linear conductance quantization is provided. The results contribute to a deeper understanding on how to tailor materials properties for novel memristive functionalities.

Solid electrolytes hold the promise of improved safety and superior electrochemical stability in energy storage systems. Among those, electrolytes with phosphate anions are expected to be more stable at high operating voltages, thereby providing even higher energy density. The key challenge is to control the boundary conditions at the cathode/electrolyte interface, which impact drastically the functionality of the energy storage devices. Here, the evolution of the chemical composition and electronic properties of the interface forms upon consequent deposition of solid electrolyte (lithium phosphorous oxynitride [LiPON]) onto the 5 V LiCoPO4 and LiNiPO4 carbon‐free thin film cathode materials is in situ studied by comprehensive electron spectroscopy experiments combined with the energy band diagram approach. It is demonstrated that the driving forces for interfacial reactivity are the band bending direction and the double layer potential drop at the electrode–electrolyte interface coupled to an unfavorable electrochemical potential shift of involved electronic states upon contact formation. The probability for interfacial chemical reactions is essentially increased at small energy differences in the ionization potentials of the cathode material and electrolyte, whereas a large energy difference ensures their chemical compatibility.

The chemical compatibility between carbon free olivine-LiNiPO4 thin films used as cathode material and LiPF6/DMC/FEC/0.2%- TMB liquid electrolyte was studied by soft-photoelectron spectroscopy, Ni L- and O K- XANES combined with electrochemical experiments. The stability of the electrolyte-electrode interface at a high charging potential strongly depends on the temperature of LiNiPO4 preparation. For the films grown at 740°C, the chemical composition of the interface is not changed even at a charging potential of 5.2 V, whereas the electrolyte-electrode interface of LiNiPO 4 grown at lower temperatures (675°C) decomposes at 5.1 V, involving the cathode material in the chemical reaction. © The Author(s) 2018.