Characterization of low temperature dielectric processes in Poly(dicyclohexyl-itaconate)

Thermally stimulated depolarization current experiments (TSDC) have been carried out on poly(dicyclohexyl itaconate) (PDCHI) in the glassy state. The polymer exhibits three relaxation zones in the interval of temperature studied: a δ-relaxation at ∼130 K, a complex γ-process, in the temperature interval of 140 K to 200 K, and the β-process in the range of 210-270 K. The loss factor of PDCHI in the glassy zone has been reproduced from the partial depolarization data by using the elementary relaxation times and activation energies. The results show the existence of at least two relaxation processes in γ- and β-zones. A tentative explanation of the molecular origin of the observed secondary relaxations has been done by means of molecular mechanics (MM). At this respect, we perform a comparison of the relaxational data of PDCHI and those corresponding to PCHMA. This comparison allows us to confirm the relevant role played by the cyclohexyl ring in the δ- and γ-relaxations.     

Study of the ferrodistorsive orbital ordering in NaNiO2 by neutron diffraction and submillimeter wave ESR

NaNiO₂ has been studied by neutron-powder diffraction, magnetic susceptibility and submillimeter wave ESR. The monoclinic structure at room temperature is characterised by a ferrodistorsive orbital ordering due to the Jahn-Teller (JT) effect of the Ni³⁺ ions in the low spin state. NaNiO₂ undergoes a structural transition at around 480 K, above which the orbital ordering disappears. The high temperature phase is rhombohedral with the layered -NaFeO₂ structure ( space group). The magnetic susceptibility exhibits hysteresis and we observe a change of the Curie-Weiss law parameters above the JT transition. The anisotropy of the g-factor at 200 K can be attributed to the JT effect which favours the orbital occupation. Finally, the interplay between the magnetic and structural properties of NaNiO₂ and Li_1-xNi_1+xO₂ is discussed. Received 29 May 2000     

Magnetic interactions in the monoclinic ludwigite Cu FeO BO

The system Cu₂FeO₂BO₃ is an oxyborate belonging to the family of the ludwigites. In this paper we present AC susceptibility, magnetization measurements and M?ssbauer spectroscopy on this material which allows for a complete characterization of its complex magnetic behavior. We find an hierarchy of interactions which clearly defines three regimes with decreasing temperature. These are associated with, the freezing of the Fe moments, the antiferromagnetic ordering of the Cu sub-lattice and finally the coupling between both systems. Received 25 September 1998     

Influence of Co doping content on its valence state in Zn1−xCoxO (0 ≤ x ≤ 0.15) thin films

Zn_1−xCo_xO (0 ≤ x ≤ 0.15) thin films grown on Si (1 0 0) substrates were prepared by a sol-gel technique. The effects of Co doped on the structural, optical properties and surface chemical valence states of the Zn_1−xCo_xO (0 ≤ x ≤ 0.15) films were investigated by X-ray diffraction (XRD), ultraviolet-visible spectrometer and X-ray photoelectron spectroscopy (XPS). XRD results show that the Zn_1−xCo_xO films retained a hexagonal crystal structure of ZnO with better c-axis preferred orientation compared to the undoped ZnO films. The optical absorption spectra suggest that the optical band-gap of the Zn_1−xCo_xO thin films varied from 3.26 to 2.79 eV with increasing Co content from x = 0 to x = 0.15. XPS studies show the possible oxidation states of Co in Zn_1−xCo_xO (0 ≤ x ≤ 0.05), Zn_0.90Co_0.10O and Zn_0.85Co_0.15O are CoO, Co₃O₄ and Co₂O₃, with an increase of Co content, respectively.     

Spin filtering in a nonuniform quantum wire with Rashba spin-orbit interaction

We investigate theoretically the spin-polarized electron transport for a wide-narrow-wide (WNW) quantum wire under the modulation of Rashba spin-orbit interaction (SOI). The influence of both the structure of the quantum wire and the interference between different pairs of subbands on the spin-polarized electron transport is taken into account simultaneously via the spin-resolved lattice Green function method. It is found that a very large vertical spin-polarized current can be generated by the SOI-induced effective magnetic field at the structure-induced Fano resonance even in the presence of strong disorder. Furthermore, the magnitude of the spin polarization can be tuned by the Rashba SOI strength and structural parameters. Those results may provide an effective way to design a spin filter device without containing any magnetic materials or applying a magnetic field.     

Analysis of ferroelectric switching in sodium nitrite:poly(vinyl alcohol) nanocomposite films

The Fourier transform infrared (FTIR) spectra and switching current response in sodium nitrite:poly(vinyl alcohol) nanocomposite films have been studied as a function of composition of NaNO₂. The switching current data fitted well to infinite-grain model (IGM) in the region t<t _s and to finite-grain model (FGM) in the region t≥t _s . The microscopic parameters like the dimensionality, the domain wall velocity, and the nucleation rate have been evaluated which provide more physical insight of the switching phenomena in the composite films. The polarization current and nucleation rate are optimum in 50 wt.% composite film and have been discussed in terms of grain size and strain variations with the composition. The effect of applied field and pulse width variation on the switching behavior of 50 wt.% composition has also been studied. The exponential field dependence of the domain wall velocity and the nucleation rate indicate that nucleation mechanism is responsible for switching phenomena in the composite films. The writing pulse width affects significantly on the switching behavior of the composite films.     

Polarization response explored by joint Hamiltonian and stochastic approach

The effect of thermal fluctuations on systems modeled by anharmonic nonconservative Hamiltonians is investigated in the framework of interacting lattice cells each cell obeying to Langevin dynamics. Representative examples addressed to critical phenomena in ferroelectrics include polarization response of a single lattice cell, ergodicity breaking, and birth of a domain, and the effect of nonlocal electroelastic interaction all derived combining the Fokker–Planck, imaginary time Schrödinger, and symplectic integration techniques.     

Properties of Field Aligned Current in Plasma Sheet Boundary Layers in Magnetotail: Cluster Observation

Field aligned current （FAC） distribution in the plasma sheet boundary layers （PSBLs） in the magnetotail is studied statistically by analysing magnetic field data from the Cluster 4-point measurements. The results show that the FAC distribution on the dusk side is not the same as that on the dawn side in the magnetotail. On the each side earthward and tailward, FA C occurrences are different; occurrence and average current density of FA Cs in the northern hemisphere are different from those in the southern hemisphere. This implies that the FACs have dusk-dawn side asymmetry, polarity asymmetry and inter hemisphere difference in the magnetotail. The present results give a good observation evidence for study on the FAC mechanism.     

Anisotropic Spin Splitting in Step Quantum Wells

By the method of finite difference, the anisotropic spin splitting of the AlxGa1-xAs/GaAs/Aly Ga1-yAs/AlxGal-xAs step quantum wells （QWs） are theoretically investigated considering the interplay of the bulk inversion asymmetry and structure inversion asymmetry induced by step quantum well structure and external electric field. We demonstrate that the anisotropy of the total spin splitting can be controlled by the shape of the QWs and the external electric field. The interface related Rashba effect plays an important effect on the anisotropic spin splitting by influencing the magnitude of the spin splitting and the direction of electron spin. The Rashba spin splitting presents in the step quantum wells due to the interface related Rashba effect even without external electric field or magnetic field.     

Phase-Transition and Magnetic Moment of the Gd3+ Ion in the Gd2Fe17 Compound

The structure and magnetic phase transitions of the Gd2Fe17 compound are investigated by using a differential thermal/thermogravimetric analyzer, x-ray diffraction, and magnetization measurements. The result shows that there are two phase structures for the Gd2Fe17 compound： the hexagonal Th2Nilr-type structure at high temperatures （above 1243℃）, and the rhombohedral Th2Zn17-type structure, respectively. A method to measure the magnetic moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound is presented. The moments of the Gd-sublattice and the Fe-sublattice in the Gd2Fe17 compound from 77 to 500 K are measured in this way with a vibrating sample magnetometer. A detailed discussion is presented.