Ferroelectronics Lab

Understanding and utilizing non-volatile properties of materials

New Publication! “Investigation of the Influence of Growth Conditions on the Local Structure in High Entropy Oxides Using S/TEM”

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Abstract: Recently, the chemically disordered multi-cation oxide class of materials called High Entropy Oxides (HEOs) has been widely explored due to their tunable functional properties because of their higher configurational entropy and to understand their fundamental phase formation. These HEOs have shown potential applications as thermoelectrics, ionic conductors, electrocaloric materials, etc. Since these systems have multiple aliovalent cations in a single lattice, it necessitates the understanding of elemental distribution and structure of these novel oxides. This would help us understand how the HEOs navigate the configurational space and enable us to establish a correlation between structure and property. In this study, we are investigating the influence of oxygen partial pressure during pulsed laser deposition on the structure, and chemical environment on the HEO thin films. (Mg0.2Ni0.2Co0.2Cu0.2Zn0.2)O, commonly known as ‘J14’, is the prototype HEO that has been widely studied. We are currently exploring the seven component HEO system with the composition: (Mg0.167Ni0.167Co0.167Cu0.167Zn0.167(Cr, Mn)0.167)O, referred to as ‘J14MnCr’. The goal of this study is to understand the influence of addition of Cr and Mn to J14 on the structure and how the growth conditions modulate the structure and tune magnetic properties. Due to the presence of seven cations with some cations that can adapt multiple valences, it is necessary to probe the structural nuances and chemical environment in these systems. In this study, Scanning/Transmission Electron Microscope is used to characterize the HEOs as S/TEM allows us to probe the structure, composition, and valence variation at nanoscale regime.

We have probed the J14MnCr thin film grown at 5mTorr and 50mTorr oxygen partial pressure. We have observed from Selected Area Electron Diffraction (SAED) and dark field TEM experiments that there is a change in the crystal structure in the films grown at 5mTorr vs. 50mTorr. At 50mTorr, the SAED reveals the formation of rock salt structure with additional secondary phase. The increase in oxygen partial pressure during synthesis has led to the nucleation of a new phase with a different structure which is speculated to influence the magnetic property. We have further performed STEM experiments and will be implementing unsupervised machine learning to detect the local structural variation. STEM-EELS experiments have also been performed to investigate the local changes in structure and to correlate it to changes in the valences of the cations and chemical environment. These S/TEM experiments enable us to understand the influence of processing conditions on microstructures and chemical environment, informing us about the structural nuances at the nanoscale and allowing us to tune the structure for desired properties.

Full text available from Microscopy and Microanalysis

New publication! “Efficient Data Processing Using Tunable Entropy-Stabilized Oxide Memristors“

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Abstract: Memristive devices are of potential use in a range of computing applications. However, many of these devices are based on amorphous materials, where systematic control of the switching dynamics is challenging. Here we report tunable and stable memristors based on an entropy-stabilized oxide. We use single-crystalline (Mg,Co,Ni,Cu,Zn)O films grown on an epitaxial bottom electrode. By adjusting the magnesium composition (XMg = 0.11–0.27) of the entropy-stabilized oxide films, a range of internal time constants (159–278 ns) for the switching process can be obtained. We use the memristors to create a reservoir computing network that classifies time-series input data and show that the reservoir computing system, which has tunable reservoirs, offers better classification accuracy and energy efficiency than previous reservoir system implementations.

Full text available from Nature Electronics

New Publication! “High temperature stability of entropy-stabilized oxide (MgCoNiCuZn)0.2O in air”

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Abstract: Entropy-stabilized oxides are single-phase, multicomponent oxides that are stabilized by a large entropy of mixing, ΔS, overcoming a positive enthalpy. Due to the −TΔS term in the Gibbs’ free energy, G, it can be hypothesized that entropy-stabilized oxides demonstrate a robust thermal stability. Here, we investigate the high temperature stability (1300–1700 °C) of the prototypical entropy-stabilized rocksalt oxide (MgCoNiCuZn)0.2O in air. We find that at temperatures >1300 °C, the material gradually loses Cu and Zn with increasing temperature. Cu is lost through a selective melting as a Cu-rich liquid phase is formed. Zn is sublimed from the rocksalt phase at approximately similar temperatures to those corresponding to the Cu loss, significantly below both the melting temperature of ZnO and its solubility limit in a rocksalt phase. The elemental loss progressively reduces the entropy of mixing and results in a multiphase solid upon quenching to room temperature. We posit that the high-temperature solubility of Cu and Zn is correlated providing further evidence for entropic stabilization over general solubility arguments.

Full text available from Applied Physics Letters

New Publication! “Effects of local compositional and structural disorder on vacancy formation in entropy-stabilized oxides from first-principles”

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Abstract: Entropic stabilization has evolved into a strategy to create new oxide materials and realize novel functional properties engineered through the alloy composition. Achieving an atomistic understanding of these properties to enable their design, however, has been challenging due to the local compositional and structural disorder that underlies their fundamental structure-property relationships. Here, we combine high-throughput atomistic calculations and linear regression algorithms to investigate the role of local configurational and structural disorder on the thermodynamics of vacancy formation in (MgCoNiCuZn)O-based entropy-stabilized oxides (ESOs) and their influence on the electrical properties. We find that the cation-vacancy formation energies decrease with increasing local tensile strain caused by the deviation of the bond lengths in ESOs from the equilibrium bond length in the binary oxides. The oxygen-vacancy formation strongly depends on structural distortions associated with the local configuration of chemical species. Vacancies in ESOs exhibit deep thermodynamic transition levels that inhibit electrical conduction. By applying the charge-neutrality condition, we determine that the equilibrium concentrations of both oxygen and cation vacancies increase with increasing Cu mole fraction. Our results demonstrate that tuning the local chemistry and associated structural distortions by varying alloy composition acts an engineering principle that enables controlled defect formation in multi-component alloys.

Full text available from npj computational materials

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Ferroelectronics Lab
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E: jtheron@umich.edu