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Register a new userAging of poled ferroelectric ceramics due to relaxation of random depolarization fields by space-charge accumulation near grain boundaries
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Migration of charged point defects triggered by the local random depolarization field is shown to plausibly explain aging of poled ferroelectric ceramics providing reasonable time and acceptor concentration dependences of the emerging internal bias field. The theory is based on the evaluation of the energy of the local depolarization field caused by mismatch of the polarizations of neighbor grains. The kinetics of charge migration assumes presence of mobile oxygen vacancies in the material due to the intentional or unintentional acceptor doping. Satisfactory agreement of the theory with experiment on the Fe-doped lead zirconate titanate is demonstrated.
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Nonlinear screening of electric depolarization fields, generated by a stripe domain structure in a ferroelectric grain of a polycrystalline material, is studied within a semiconductor model of ferroelectrics. It is shown that the maximum strength of local depolarization fields is rather determined by the electronic band gap than by the spontaneous polarization magnitude. Furthermore, field screening due to electronic band bending and due to presence of intrinsic defects leads to asymmetric space charge regions near the grain boundary, which produce an effective dipole layer at the surface of the grain. This results in the formation of a potential difference between the grain surface and its interior of the order of 1 V, which can be of either sign depending on defect transition levels and concentrations. Exemplary acceptor doping of BaTiO3 is shown to allow tuning of the said surface potential in the region between 0.1 and 1.3 V.
In the paper we predict a distinctive change of magnetic properties and considerable increase of the Curie temperature caused by the strain fields of grain boundaries in ferromagnetic films. It is shown that a sheet of spontaneous magnetization may arise along a grain boundary at temperatures greater than the bulk Curie temperature. The temperature dependence and space distribution of magnetization in a ferromagnetic film with grain boundaries are calculated. We found that $45^circ$ grain boundaries can produce long-range strain fields that results in the width of the magnetic sheet along the boundary of the order of $ 0.5 div 1 mu m$ at temperatures grater than the bulk Curie temperature by about $10^2$ K.
Nano-crystallize materials have been known for decades to potentially owe the novel self-healing ability for radiation damage, which has been demonstrated to be especially linked to preferential occupation of interstitials at grain boundary (GB) and promoted vacancy-interstitial annihilation. A major obstacle to better understanding the healing property is the lack of an atomistic picture of the interstitial states near GBs, due to severely separation of the timescale of interstitial segregation from other events and abundance of interstitials at the GB. Here, we report a generic self-blocking effect of the interstitial cluster (SIAn) near the metallic GB in W, Mo and Fe. Upon creating a SIAn near the GB, it is immediately trapped by the GB during the GB structural relaxation and blocks there, impeding GBs further spontaneous trapping of the SIAn in the vicinity and making these SIAns stuck nearby the GB. The SIAn in the stuck state surprisingly owes an exceptionally larger number of annihilation sites with vacancies near the GB than the SIAn trapped at the GB due to maintaining its bulk configuration basically. Besides, it also has an unexpectedly long-ranged repelling interaction with the SIA in the bulk region, which may further affect the GBs trap of the SIAn. The self-blocking effect might shed light on more critical and extended role of the GB in healing radiation-damage in NCs than previously recognized the GBs limited role based on GBs trap for the SIA and resulted vacancy-SIA recombination.
We show that molecular dynamics (MD) simulations are capable of reproducing the drag of solute segregation atmospheres by moving grain boundaries (GBs). Although lattice diffusion is frozen out on the MD timescale, the accelerated GB diffusion provides enough atomic mobility to allow the segregated atoms to follow the moving GB. This finding opens the possibility of studying the solute drag effect with atomic precision using the MD approach. We demonstrate that a moving GB activates diffusion and alters the short-range order in the lattice regions swept during its motion. It is also shown that a moving GB drags an atmosphere of non-equilibrium vacancies, which accelerate diffusion in surrounding lattice regions.
The electric field control of functional properties is a crucial goal in oxide-based electronics. Non-volatile switching between different resistivity or magnetic states in an oxide channel can be achieved through charge accumulation or depletion from an adjacent ferroelectric. However, the way in which charge distributes near the interface between the ferroelectric and the oxide remains poorly known, which limits our understanding of such switching effects. Here we use a first-of-a-kind combination of scanning transmission electron microscopy with electron energy loss spectroscopy, near-total-reflection hard X-ray photoemission spectroscopy, and ab-initio theory to address this issue. We achieve a direct, quantitative, atomic-scale characterization of the polarization-induced charge density changes at the interface between the ferroelectric BiFeO3 and the doped Mott insulator Ca1-xCexMnO3, thus providing insight on how interface-engineering can enhance these switching effects.
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