G. A. Held

Two-stage switching behavior of polymer stabilized cholesteric textures

The electro-optical behavior of polymer stabilized cholesteric texture cells has been investigated for three different polymers. The switching process was studied with respect to the electric field dependence of the diffuse reflectivity, diffuse transmittance, and the dynamics of the reorientation process. For certain polymer concentrations, a two-stage reorientation process was observed. This behavior is consistent with the cholesteric liquid crystal being divided between two distinct environments. In the first, the liquid crystal is strongly dominated by the polymer network, while in the second a bulklike behavior, comparable to the unstabilized cholesteric material, is observed. Scanning electron micrographs of the polymer networks further support this model. Measurements of the diffuse scattering indicate that the polymer influenced regions contribute largely to the observed back scattering, whereas the bulklike material contributes primarily to forward scattering.

Network morphology of polymer stabilized liquid crystals

Monomer solubility is identified as the primary factor determining network morphology in polymer stabilized cholesteric liquid crystal textures. Poorly soluble monomers form coarse structures composed of discrete, oblong grains, whereas soluble monomers yield smooth, continuous polymer networks. A crossover from smooth to grainy structure is observed as a function of monomer concentration. The grainy structure results from precipitation polymerization and the observed behavior is well described by the Flory–Huggins theory of polymer solubility.Monomer solubility is identified as the primary factor determining network morphology in polymer stabilized cholesteric liquid crystal textures. Poorly soluble monomers form coarse structures composed of discrete, oblong grains, whereas soluble monomers yield smooth, continuous polymer networks. A crossover from smooth to grainy structure is observed as a function of monomer concentration. The grainy structure results from precipitation polymerization and the observed behavior is well described by the Flory–Huggins theory of polymer solubility.

Quantum limit of magnetic recording density

Macroscopic quantum tunneling imposes an upper limit on the areal density of magnetic disk drives. We derive a general expression for this limit, and apply it to Co-alloy-based longitudinally recorded media, as well as to potential future media based on the high anisotropy material FePt. We estimate the temperature at which the switching rate due to quantum tunneling becomes comparable to the thermally activated switching rate. This is the lowest temperature to which a magnetic storage device can be cooled with the expectation of increasing the areal density by enhancing stability against spontaneous switching.

Network Morphology and Switching Transitions in Polymer Stabilized Cholesteric Textures

Abstract We study the fundamental properties of polymer stabilized cholesteric texture (PSCT) liquid crystal cells by (1) characterizing the polymerization process within the liquid crystal medium and, (2) identifying the effects of the resulting polymer network on the switching transition of the cholesteric. We show that monomer solubility is the primary factor determining network morphology-poorly soluble monomers yield coarse structures composed of discrete particles, whereas soluble monomers yield smooth, highly interconnected networks. Following network formation, we use confocal microscopy to obtain in situ images of both the polymer network and the liquid crystal domain structure as the cells are switched between the planar and focal conic textures.

X‐ray magnetic circular dichroism studies of multilayered thin films of 3d transition metals (invited) (abstract)

X‐ray magnetic circular dichroism (XMCD) is used to probe both the spin and orbital d moments of Co in Co/Pd, Co/Pt, and Co/Ni multilayers, and in a Au/Co‐wedge/Pd structure. While the spin moment per Co atom is found to be constant within the experimental error, the orbital moment shows a strong dependence both on the chemical environment and on the thickness of the Co layer. In particular, we find an orbital moment enhancement of up to a factor of three for a Co layer thickness of four atomic layers in the present Au/Co/Pd sandwich structure. This enhancement decays to the bulk Co value, roughly following a 1/dCo dependence, thus confirming its interfacial origin. The relevance of these observations with respect to the perpendicular magnetic anisotropy in these samples will be discussed.