Polarons, bipolarons, and solitons in conducting polymers

Exchange anisotropy — a review

The gas sensors fabricated by using conducting polymers such as polyaniline (PAni), polypyrrole (PPy) and poly (3,4-ethylenedioxythiophene) (PEDOT) as the active layers have been reviewed. This review discusses the sensing mechanism and configurations of the sensors. The factors that affect the performances of the gas sensors are also addressed. The disadvantages of the sensors and a brief prospect in this research field are discussed at the end of the review.

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Gas Sensors Based on Conducting Polymers

High K/sub u/, uniaxial magnetocrystalline anisotropy, materials are generally attractive for ultrahigh density magnetic recording applications as they allow smaller, thermally stable media grains. Prominent candidates are rare-earth transition metals (Co/sub 5/Sm,...), and tetragonal intermetallic compounds (L1/sub 0/ phases FePt, CoPtY,...), which have 20-40 times higher K/sub u/ than today's hexagonal Co-alloy based media. This allows for about 3 times smaller grain diameters, D, and a potential 10-fold areal density increase (/spl prop/1/D/sup 2/), well beyond the currently projected 40-100 Gbits/in/sup 2/ mark, Realization of such densities will depend on a large number of factors, not all related to solving media microstructure problems, In particular it is at present not known how to record into such media, which may require write fields in the order of 10-100 kOe. Despite this unsolved problem, there is considerable interest in high Ku alternative media, both for longitudinal and perpendicular recording. Activities in this area will be reviewed and data on sputtered and evaporated thin FePt films, with coercivities exceeding 10000 Oe will be presented.

High K/sub u/ materials approach to 100 Gbits/in/sup 2/

In current longitudinal magnetic recording media, high areal density and low noise are achieved by statistical averaging over several hundred weakly coupled ferromagnetic grains per bit cell. Continued scaling to smaller bit and grain sizes, however, may prompt spontaneous magnetization reversal processes when the stored energy per particle starts competing with thermal energy, thereby limiting the achievable areal density. Charap et al. have predicted this to occur at about 40 Gbits/in/sup 2/. This paper discusses thermal effects in the framework of basic Arrhenius-Neel statistical switching models. It is emphasized that magnetization decay is intimately related to high-speed-switching phenomena. Thickness-, temperature- and bit-density dependent recording experiments reveal the onset of thermal decay at "stability ratios" (K/sub u/V/K/sub B/T)/sub 0//spl sime/35 /spl plusmn/ 2. The stability requirement is grain size dispersion dependent and shifts to about 60 for projected 40 Gbits/in/sup 2/ conditions and ten-year storage times. Higher anisotropy and coercivity media with reduced grain sizes are logical extensions of the current technology until write field limitations are reached. Future advancements will rely on deviations from traditional scaling. Squarer bits may reduce destabilizing stray fields inside the bit transitions. Perpendicular recording may shift the onset of thermal effects to higher bit densities. Enhanced signal processing may allow signal retrieval with fewer grains per bit. Finally, single grain per bit recording may be envisioned in patterned media, with lithographically defined bits.

Thermal effect limits in ultrahigh-density magnetic recording

Presentation des resultats de calculs de structure de bandes en liaison forte pour une chaine polypyrrole deformable, en fonction du dopage. Les etats qui sont produits sont des polarons et des bipolarons dont le spectre d'excitation explique les donnees d'absorption optique

Polarons and bipolarons in polypyrrole: Evolution of the band structure and optical spectrum upon doing

A chemical-ordering-induced peak in the polar magneto-optical Kerr effect spectra of FePt compounds is found to be in quantitative agreement with predictions based on ab initio magneto-optical calculations. Experimental spectra of epitaxial molecular-beam-epitaxy grown FePt(001) films reveal a more than twofold enhancement of the Kerr rotation if full chemical ordering is present. A spectral feature at 2-eV photon energy shows a peak Kerr rotation of up to \ensuremath{\sim}0.8\ifmmode^\circ\else\textdegree\fi{}, which is strongly correlated with the presence of perpendicular magnetic anisotropy in ordered compound films.

Enhanced magneto-optical Kerr effect in spontaneously ordered FePt alloys: Quantitative agreement between theory and experiment.

Evidence for superconductivity in (BEDT-TTF) 4 (ReO 4) 2 [where BEDT-TTF is bis(eth-ylenedithiolo)tetrathiafulvalenel near 2 K for pressures above 4 kbar is reported. This is the first unambiguous observation of superconductivity in a sulphur-donor organic system. At higher pressures the transition temperature decreases rapidly, dT c,/dP~- 0.3 K/kbar. At low pressures this material exhibits a first-order metal-insulator transition, postulated to result from a change in the arrangement of the anions which are ordered at room temperature.

Superconductivity in a new family of organic conductors

Ferromagnetic resonance (FMR) spectra of Permalloy thin films exchange-coupled to iron-manganese films are analyzed. Studies were made on bilayer, ferromagnetic-antiferromagnetic (FA) and trilayer (AFA) structures, as a function of both F and A layer thicknesses in the range 20--800 A\r{}. Data are presented at a frequency of 9.3 GHz for both in-plane and perpendicular directions of the applied field, and at 34.1 GHz, in-plane. Analysis of these data enables extraction of the magnetization, gyromagnetic ratio, and an exchange shift due to spin-wave stiffness and perpendicular-surface anisotropy, as a function of layer thickness. The azimuthal dependence of the in-plane resonance is used to determine the magnitude of the exchange anisotropy (bias field). The magnetization and gyromagnetic ratio show little dependence on the thickness of either the F or A layer down to 50 A\r{}, implying that the interfaces are sharp on a scale of a few lattice constants. Within this interfacial region the magnetization is reduced as a result of interaction with the antiferromagnet. We suggest that the perpendicular-surface anisotropy is created by exchange coupling to the antiferromagnet whose easy axes are not in the plane of the interface. Finally, we suggest a model for exchange anisotropy in which the antiferromagnetic domain pattern is not totally locked, but adjusts in response to the ferromagnetization. Such a model qualitatively explains the bias field exerted by the antiferromagnetic layer deposited before the ferromagnet, the field-training effect, the FMR linewidth, and the magnitude of the bias field.

J. C. Scott

Papers

Polarons and bipolarons in polypyrrole: Evolution of the band structure and optical spectrum upon doing

Enhanced magneto-optical Kerr effect in spontaneously ordered FePt alloys: Quantitative agreement between theory and experiment.

Superconductivity in a new family of organic conductors

Ferromagnetic resonance studies of exchange-biased Permalloy thin films.

Magnetic disorder in the exchange bias bilayered FeNi-FeMn system