Mark Thomas Johnson

Orientational dependence of the exchange biasing in molecular‐beam‐epitaxy‐grown Ni80Fe20/Fe50Mn50 bilayers (invited)

The exchange biasing field (Heb) and coercive field (Hc) of molecular‐beam‐epitaxy‐grown Cu/Ni80Fe20/Fe50Mn50 samples in [111], [001], and [110] orientations have been investigated by longitudinal Kerr effect measurements. Ni80Fe20 and Fe50Mn50 were deposited as orthogonal wedge‐shaped layers on single‐crystal Cu substrates in a magnetic field, enabling the study of the thickness dependence of Heb and Hc on a single sample for each orientation. A strong dependence of Heb and Hc on the growth orientation is observed. The results are interpreted in terms of the observed noncollinear spin structure of the antiferromagnet and a comparison is given with the predictions from recent theoretical models.

Orientational and structural dependence of magnetic anisotropy of Cu/Ni/Cu sandwiches: Misfit interface anisotropy

Magnetic anisotropies and misfit strain relaxations have been investigated in Cu/Ni‐wedge/Cu (100) and (111) sandwiches deposited by molecular beam epitaxy on single‐crystal Cu substrates. Our results reveal a clear distinction in the nature of the measured anisotropy at Ni thicknesses below and above the critical value tc, where the growth becomes incoherent. Below tc, coherent lattice strain modifies only the volume anisotropy, while interface anisotropy is Neel type; above tc, magnetoelastic effects are found to contribute to the interface anisotropy.

20.2: Drive Waveforms for Active Matrix Electrophoretic Displays

Microencapsulated electrophoretic (MEP) displays are promising candidates for smart handheld applications, where ease of reading and ultra-low power consumption are of primary importance. A thorough study of the electro-optic response characteristics of MEP systems has led to the development of a variety of novel drive waveforms that can be used to generate 1- bit, multi-bit grayscale and full-color images on MEP displays.

Localized Kerr study of the magnetic properties of an ultrathin epitaxial Co wedge grown on Pt(111)

We have used the magneto‐optical Kerr effect to study the thickness dependence of the perpendicular magnetic anisotropy, coercivity, nucleation field and remanence in an ultrathin Co wedge (0–9 monolayers) grown epitaxially on Pt(111) and capped by a thin Pt overlayer. In addition, the spin‐reversal mechanism in the Co layers has been investigated using Kerr domain imaging. The sample showed perpendicular magnetization up to an extrapolated Co thickness of 15 monolayers (ML), with derived volume and interface anisotropy constants of −0.77 MJ/m3 and 1.15 mJ/m2, respectively. The Kerr rotation and Kerr ellipticity demonstrated linear dependences on the Co thickness (tCo), with sizeable extrapolated offsets at tCo=0, attributed to polarization of the Pt by the Co. The coercivity (Hc) and nucleation field both rose to peak values at tCo=1.5 ML, with a subsequent monotonic fall‐off for higher Co thicknesses. In the case of Hc, this fall‐off did not demonstrate the tCo−5/2 dependence shown by a similar Pd(111)/...

Perpendicular giant magnetoresistance of Co/Cu multilayers deposited under an angle on grooved substrates

We propose a novel experimental technique for investigating the giant magnetoresistance effect measured with the current perpendicular to the layer plane (the so‐called CPP geometry). Using holographic laser interference nanofabrication techniques and anisotropic etching the surface of semi‐insulating InP substrates is patterned into V‐shaped grooves of 0.2 μm width. Subsequently, a magnetic multilayer can be evaporated under an angle with the substrate normal, naturally resulting in a CPP‐like magnetoresistance configuration. The technique is illustrated for Co/Cu multilayers, for which we present magnetization and magnetoresistance experiments.

Perpendicular magnetic anisotropy of multilayers: recent insights

Abstract Extending our understanding of perpendicular magnetic anisotropy (PMA) beyond the phenomenological approach of volume ( K V ) and interface anisotropies ( K S ) requires caution. Experimentally, many factors such as roughness, formation of interface alloys, or patchiness of ultrathin layers may cause a reduction in PMA. Where stresses are present in multilayer growth, these may, under particular circumstances, contribute to both K V and K S , as demonstrated for the Cu/Ni/Cu system. Theoretically, the difficulty in modelling incoherently grown materials prevents a detailed quantitative comparison with most experimental systems, so that an evaluation of the underlying approximations common to all of the quantitative theoretical studies becomes very difficult.

A video-speed reflective display based on electrowetting: principle and properties

Electrowetting is presented as a novel principle for a reflective display. By contracting a colored oil film electrically, an optical switch is obtained with many attractive properties that make it very suitable for use as a reflective display, for instance, as electronic paper. Firstly, it has the high reflectivity (>40%) and contrast ratio (15) required for a paper-like optical appearance. In addition, the principle shows a video-rate response time (<10 msec) and has a clear route toward a high-brightness color display. 1 Finally, the electro-optical response is independent of cell-gap thickness, which will be very beneficial when moving toward a flexible display.

Light degradation and voltage drift in polymer light-emitting diodes

It is shown that the voltage drift and light degradation in polymer light-emitting diodes are related and can be explained by the formation of traps and the modification of the space charge in the bulk of the polymer. The energy released by nonradiative carrier recombination is believed to be the driving force for the generation of traps in poly(p-phenylene vinylene) conjugated polymers. A first-approximation model is derived for the voltage drift and the light decrease during operation, which is in good agreement with experimental observations for time and current density dependencies.

Spatially resolved magneto-optical investigation of the perpendicular anisotropy in a wedge-shaped ultrathin epitaxial Co layer on Pd(111)

Abstract We have used the polar Kerr effect to measure the thickness dependence of the perpendicular anisotropy of ultrathin Co in Pd/Co/Pd(111) sandwiches in the 3–9 monolayer range. The anisotropy was determined from polar Kerr measurements of the perpendicular component of the magnetisation both as a function of the strength and the angle of the applied field. The Co layers were grown epitaxially in ultrahigh vacuum on a Pd(111) single crystal and covered by a 20 A epitaxial Pd layer. The Co layer was deposited in the form of a wedge (0–9 monolayers) so that the thickness dependence of the anisotropy could be measured on a single sample.

Local structural and polar Kerr effect measurements on an ultrathin epitaxial Co wedge grown on Pd(111)

A Pd/Co double layer has been grown epitaxially in ultrahigh vacuum on a Pd(111) single crystal, with the Co in the shape of a wedge of thickness varying from 0 to 10 monolayers (ML). The polar magneto‐optical Kerr effect has been used to make a nearly continuous set of measurements of the magnetic properties as a function of Co thickness tCo. The coercive fields Hc and nucleation fields Hn show large, well‐defined maxima at tCo∼2.3 ML. Between 4 and 9 ML, we find that Hc falls with increasing thickness according to a tCo−5/2 dependence. The Kerr ellipticity and Kerr rotation depend linearly on Co thickness above ∼2 ML Co, but show appreciable offsets when extrapolated to tCo=0. These offsets are clear evidence of the polarization of Pd near the interfaces. Extensive in situ growth studies of the structure of the films are also reported.