H. F. Tian

Microstructure and ordering of iron vacancies in the superconductor system KyFexSe2 as seen via transmission electron microscopy

Structural investigations by means of transmission electron microscopy (TEM) on KFexSe2 with 1.5 \leq x \leq 1.8 have revealed a rich variety of microstructure phenomena, the KFe1.5Se2 crystal often shows a superstructure modulation along the [310] zone-axis direction, this superstructure can be well interpreted by the Fe-vacancy order within the a-b plane. Increase of Fe-concentration in the KFexSe2 materials could not only result in the appearance of superconductivity but also yield clear alternations of microstructure. Structural inhomogeneity, the complex superstructures and defect structures in the superconducting KFe1.8Se2 sample have been investigated based on the high-resolution TEM.

Charge-stripe order in the electronic ferroelectric LuFe2O4

The structural features of the charge ordering states in LuFe(2)O(4) are characterized by in situ cooling transmission electron microscopy observations from 300 K down to 20 K. Two distinctive structural modulations, a major q1=(1/3,1/3,2) and a weak q2 = q1/10+(0,0,3/2), have been well determined at the temperature of approximately 20 K. Systematic analysis demonstrates that the charges at low temperatures are well crystallized in a charge-stripe phase, in which the charge-density wave behavior in a nonsinusoidal fashion results in elemental electric dipoles for ferroelectricity. It is also noted that the charge ordering and ferroelectric domains often change markedly with lowering temperatures and yield a rich variety of structural phenomena.

Phase transition, superstructure and physical properties of K2Fe4Se5

The structural features and physical properties of the antiferromagnetic K0.8Fe1.6Se2 (so-called K2Fe4Se5 phase) have been studied in the temperature range from 300 K up to 600 K. Resistivity measurements on both single crystal and polycrystalline samples reveal a semiconducting behavior. Structural investigations of K2Fe4Se5 by means of transmission electron microscopy (TEM) and powder X-ray diffraction (XRD) demonstrate the presence of a well-defined superstructure within the a-b plane originating from a Fe-vacancy order along the [1−10] direction (indexed based on the supercell with space group of I4/m). Moreover, in situ heating structural analysis shows that K0.8Fe1.6Se2 undergoes a transition of the Fe-vacancy order to disorder at about 600 K. The phase separation and the Fe-vacancy ordering in the superconducting materials of KxFe2-ySe2 (0.2 ≤y ≤ 0.3) has been briefly discussed.

Scanning secondary-electron microscopy on ferroelectric domains and domain walls in YMnO3

Ferroelectric domain structures in YMnO3 single crystals on the hexagonal polar surface have been investigated by scanning electron microscopy in secondary electron emission mode. The experimental results demonstrate that the domain, as well as domain walls, can be clearly revealed under the operation voltages ranging from 0.6 to 3 kV. Evolution of domain contrasts arising from electron-beam irradiation can be mainly explained by the pyroelectric effect and related charging process. A rich variety of microstructure features of ferroelectric domains can be clearly revealed in YMnO3 by this high-resolution technique.

Martensitic transformation and magnetic domains in Mn50Ni40Sn10 studied by in-situ transmission electron microscopy

The structural features, martensitic transformation, and magnetic domains in the Mn50Ni40Sn10 Heusler alloy are investigated by in-situ transmission electron microscopy and Lorentz microscopy. A rich variety of microstructural phenomena arising from structural and magnetic transitions are demonstrated at low temperatures. The “5M” or “6M” superstructure modulations and needle-shaped martensites have been clearly observed in the martensitic phase. Structural evolutions of magnetic domains are revealed under in-situ applied magnetic fields using Lorentz microscopy. The structural domains and reversibility of the phase transitions are analyzed based on our experimental observations and discussed in comparison with results of low-temperature magnetization.

Nonlinear transport properties and Joule heating effect in charge ordered LuFe2O4

The nonlinear current-voltage (I-V) relation of charge ordered (CO) LuFe2O4 has been studied in the temperature range from 220 to 520 K. Strong nonlinear effects are identified in the CO state and disappear in high-temperature charge disordered state. Alternation of the CO state under applied electric field is directly revealed by in situ transmission electron microscopy observations. Careful investigations of time-dependent resistance under the applied electric field near the threshold value (Eth) often show three distinct steps for the nonlinear transitions. The CO collapse likely at a critical conductivity yields a sharp drop in resistance.

Suppression of the current leakage in charge ordered Lu2Fe2Fe1−xMnxO7 (0<x≤0.86)

The ferroelectricity, magnetic properties, and charge-ordering modulations in the layered Lu2Fe2Fe1−xMnxO7 have been investigated for x ranging from 0 to 0.86. Experimental measurements reveal that substitution of Mn for Fe could evidently influence the low temperature magnetic transition. Moreover, ferroelectric analysis suggests that introduction of Mn ions in this layered system can efficiently suppress the conducting leakage and result in clear hysteresis loops in the polarization–electric field switching curves. The charge ordering states in Lu2Fe2Fe1−xMnxO7 characterized by two incommensurate modulations has been observed and discussed in association with the local spontaneous polarizations.

The origin of the weak ferroelectric-like hysteresis effect in paraelectric Ba0.5Sr0.5TiO3 thin films grown epitaxially on LaAlO3

Perovskite Ba0.5Sr0.5TiO3 (BST) thin films, with thickness of about 350 nm, have been epitaxially grown on (001) LaAlO3 substrates by pulsed-laser deposition. The good crystallography and epitaxy characteristics were confirmed using x-ray diffraction and transmission electron microscopy (TEM). The dielectric properties of the BST thin films were measured with a planar capacitor configuration in the temperature range of 77-300 K. The capacitance temperature characteristics, measured with no and several different DC biases, reveal that the BST films are in the paraelectric state at room temperature, with a very good dielectric tunability. However, a butterfly-shaped curve, typical for a ferroelectric material, was obtained from room temperature capacitance-voltage (C-V) measurements, suggesting a faint ferroelectric-like effect for the BST thin films. Careful dielectric property characterizations showed significant temperature and frequency dependence, probably indicating a behaviour of Maxwell-Wagner-type dielectric relaxation. As a result, the weak C-V hysteresis effect in our paraelectric BST thin films is believed to be ascribable both to oxygen vacancies and to the presence of other space charges, trapped at the grain boundaries and/or at the substrate/dielectric film interface, which give rise to local polar regions in the thin-film samples. Furthermore, this explanation is supported by cross-sectional TEM and off-axis electron holographic observations.

Microstructure and tetragonal-to-orthorhombic phase transition of AFe(2)As(2) (A=Sr,Ca) as seen via transmission electron microscopy

The structural properties of the SrFe2As2 and CaFe2As2 compounds have been extensively analyzed by transmission electron microscopy (TEM) from room temperature down to 20K. The experimental results demonstrate that the SrFe2As2 crystal, in consistence with previous x-ray data, has a tetragonal structure at room temperature and undergoes a tetragonal (T)-orthorhombic (O) phase transition at about 210K. Moreover, twinning lamella arising from T-O transition evidently appears in the orthorhombic phase. On the other hand, TEM observations of CaFe2As2 reveal the presence of a pseudo-periodic structural modulation with the periodicity of around 40nm at room temperature. This modulation is likely in connection with the local structural distortions within the Ca layer. In-situ cooling TEM observations of CaFe2As2 reveal the presence of complex domain structures in the low-temperature orthorhombic phase.

Electrostatic potential in manganite-based heterojunctions by electron holography

Electric potentials at the manganite-based heterojunctions, composed of La0.9Sr0.1MnO3 and 0.01wt% Nb-doped SrTiO3 (p-n junction) and La0.7Ce0.3MnO3 and 0.5wt% Nb-doped SrTiO3 (n-n junction), have been studied by means of off-axis electron holography. The experimental measurements demonstrate that evident electron diffusion from the n-type SrNbxTi1−xO3 to the manganite films occurs in both p-n and n-n heterojunctions. The p-n heterojunction shows a complex potential change across the interface due to the variation of dielectric constant and electronic structure. In situ cooling experimental measurement on n-n junctions reveals an apparent change of potential along with the ferromagnetic phase transition.