Showing papers by "Etienne Snoeck published in 2002"
TL;DR: This work demonstrates the long-range ordering of nanoparticle assemblies that adopt the helical configuration of the cholesteric liquid crystalline phase and demonstrates how such an assembly process could be modulated, providing a versatile route to new materials systems.
Abstract: Patterning nano-objects is an exciting interdisciplinary research area in current materials science, arising from new optical and optoelectronic properties and the need to miniaturize electronic components. Many techniques have been developed for assembling nanoparticles into two- and three-dimensional arrays. Most studies involving liquid crystals as templates have dealt with colloidal particles and nematic and smectic phases. Here, we demonstrate the long-range ordering of nanoparticle assemblies that adopt the helical configuration of the cholesteric liquid crystalline phase. Because we used glass-forming cholesterics, the nanostructures could be examined by transmission electron microscopy. The platinum nanoparticles form periodic ribbons that mimic the well-known 'fingerprint' cholesteric texture. Surprisingly, the nanoparticles do not decorate the original cholesteric texture but create a novel helical structure with a larger helical pitch. By varying the molar fraction of cholesterol-containing mesogen in the liquid crystal host, we show that the distance between the ribbons is directly correlated to the pitch. Therefore this inherent lengthscale becomes a simple control parameter to tune the structuring of nanoparticles. These results demonstrate how such an assembly process could be modulated, providing a versatile route to new materials systems.
150 citations
TL;DR: In this paper, high-quality epitaxial thin films have been prepared by molecular-beam epitaxy and the growth and morphology of the films were extensively studied using electron diffraction and scanning tunneling microscopy.
Abstract: NiMnSb(001) high-quality epitaxial thin films have been prepared by molecular-beam epitaxy. The growth and morphology of the films have been extensively studied using electron diffraction and scanning tunneling microscopy. We report the presence of various surface structures on the free NiMnSb(001) surface. The structural properties of the prepared single-crystalline NiMnSb films have been analyzed by transmission electron microscopy and x-ray absorption. The typical $C1b$ structure is observed at a large scale with a very good chemical ordering in the unit cell. The magnetic properties have been investigated by x-ray magnetic circular dichroism and a superconducting quantum interferometer device and are compatible with the predicted half-metallic character. These NiMnSb layers have been integrated in single-crystalline NiMnSb/MgO/NiMnSb heterostructures for preparation of magnetic tunnel junctions with half-metallic electrodes.
34 citations
10 Dec 2002
TL;DR: In this paper, a spin-dependent tunnel junctions with the structure (Ta 70 /spl Aring/NiFe 70 ) were fabricated on top of 600-pl Aring/-thick ion-beam-smoothed low-resistance Al electrodes.
Abstract: Spin-dependent tunnel junctions with the structure (Ta 70 /spl Aring//NiFe 70 /spl Aring//MnIr 80 /spl Aring//CoFe 35 /spl Aring//HfAlO/sub x//CoFe 35 /spl Aring//NiFe 40 /spl Aring//TiW(N) 150 /spl Aring/) were fabricated on top of 600-/spl Aring/-thick ion-beam-smoothed low-resistance Al electrodes. HfAlO/sub x/ barriers were formed by natural oxidation (5 min at 1 torr in pure O/sub 2/) of 5-/spl Aring/-thick (2-/spl Aring/ Hf+3-/spl Aring/ Al) films or 6-/spl Aring/-thick (2-/spl Aring/ Hf+4-/spl Aring/ Al) films. Resistance/spl times/area (R/spl times/A) products of 0.65 /spl Omega//spl times//spl mu//sup 2/ and 2.1 /spl Omega//spl times//spl mu/m/sup 2/ were achieved with 9.5% and 13.5% tunnel magnetoresistance signal (TMR), respectively. Current inhomogeneity effects on the measured (R/spl times/A) products and TMR values were calculated in particular for junctions with resistance below 1 /spl Omega//spl times//spl mu/m/sup 2/. Transmission electron microscopy indicates that HfAlO/sub x/ forms a continuous amorphous barrier that follows conformally the topography of the bottom electrode. X-ray photoelectron spectroscopy analysis indicates that 2.5% metallic Hf is left inside the barrier closer to the bottom electrode. These low-resistance tunnel junctions are attractive for read-head applications at recording densities above 100 Gbit/in/sup 2/.
21 citations
TL;DR: In this paper, the influence of the substrate orientation on the surface morphology of the deposited layers was investigated by reflection high energy electron diffraction (RHEED), atomic force microscopy (AFM), X-ray diffraction and transmission electron microscopy.
18 citations
TL;DR: In this paper, a spin-dependent tunnel junctions with ZrAlOx barriers were fabricated with low resistance×area product 4'Ω×μm2, and tunnel magnetoresistance of 15.2%.
Abstract: Spin-dependent tunnel junctions with ZrAlOx barriers were fabricated with low resistance×area product 4 Ω×μm2, and tunnel magnetoresistance of 15.2%. Barrier fabrication was done by natural oxidation (5 min, at oxidation pressures ranging from 0.5 to 10 Torr). The junctions were deposited on top of 600 A thick, ion beam smoothed, low resistance, Al electrodes. X-ray photoelectron spectroscopy analysis indicates the presence of AlOx, ZrO2, some remnant metallic Zr, but no metallic Al in the as-deposited barriers. High resolution transmission electron microscopy indicates that ZrAlOx forms an amorphous barrier that is smoother than pure crystalline ZrOx or pure amorphous AlOx barriers. These low resistance tunnel junctions are attractive for read head applications above 100 Gbit/in2 where competitive signal to noise ratios imply resistance×area product below a few Ω×μm2, and tunneling magnetoresonance signals near or above 20%.
12 citations
TL;DR: In this paper, NiO layers epitaxially grown on a stabilized MgO(1.1) substrate at various temperatures between 700°C and 900°C were investigated using reflection high energy electron diffraction, atomic force microscopy, X-ray diffraction and high resolution transmission electron microscopy.
10 citations
TL;DR: In this article, sputtered epitaxial NiO-Co samples on MgO (110) substrates were sputtered with a saw-tooth-like surface morphology with terraces aligned along the direction and either (100) or (010) termination planes.
Abstract: We have sputtered epitaxial NiO–Co samples on MgO (110) substrates. NiO epitaxially grows on the isostructural fcc MgO substrate but, due to surface energy minimization, its surface morphology is saw-tooth-like with terraces aligned along [001] direction and either (100) or (010) termination planes. The obtained nanostructures are 80–200 A wide facets with micron-size length. Subsequently deposited Co layers adopt a fcc structure conformal with the NiO nanofacets. It consists thus in a set of connected nanostripes as evidenced by complementary structural characterizations. Shape anisotropy induces a strong easy axis along the stripe edges [001] and a hard axis along the [-110] MgO direction. Magnetization loops recorded along [001] have a total squareness. Thermal treatments were done in zero field for investigating the NiO/Co exchange thermal stability and activation. We observed thermally assisted exchange bias field (HE) variations on 1000 Oe field treated samples for various temperatures between 300 K...
6 citations
TL;DR: In this article, a remote O/sub 2/ ion source was used for the formation of nano-oxide layers, and the oxidation efficiency was measured in CoFe-oxide films, and a decrease of the oxide layer with the pan angle and oxidation pressure was observed.
Abstract: In this paper, a remote O/sub 2/ ion source is used for the formation of nano-oxide layers. The oxidation efficiency was measured in CoFe-oxide films, and a decrease of the oxide layer with the pan angle and the oxidation pressure is observed. For the same oxidation pressure, the oxidation efficiency depends on the O/sub 2/ content in the Ar-O/sub 2/ plasma. These results were applied in optimizing the fabrication of Al/sub 2/O/sub 3/ barrier for tunnel junctions. This method was also used to fabricate junctions with Fe-oxide layers inserted at the Al/sub 2/O/sub 3/-CoFe interface. TEM and magnetization data indicate that after anneal at 385/spl deg/C, a homogeneous ferromagnetic Fe-oxide layer (Fe/sub 3/O/sub 4/?) is formed.
6 citations
TL;DR: In this article, the authors studied NMR spectra at 1.5 K in a series of [Co 80 Fe 20 (t )/Al 2 O 3 (3nm)] 10 multilayers.
5 citations
TL;DR: In this paper, the NiMnSb(0, 0, 1) high quality epitaxial thin films have been prepared by MBE and the growth and morphology of the films were studied by RHEED and STM, and the magnetic properties were investigated by XMCD and SQUID.
5 citations
TL;DR: In this paper, a reactive facing target radio frequency sputtering on SrTiO3(001) substrates was used to grow La1/4Ca3/4MnO3-La2/3Sr1/3MnNO3 (LCMO-LSMO) thin films with perovskite structure.
Abstract: We have grown La1/4Ca3/4MnO3–La2/3Sr1/3MnO3 (LCMO–LSMO) thin films by reactive facing target radio frequency sputtering on SrTiO3(001) substrates. After 1 bar oxygen annealing at 800 °C, the films have the desired oxygen stoichiometry and perovskite structure. Transport properties of single LCMO films reveal the charge order (CO) transition between 100 and 180 K depending on the LCMO layer thickness while LSMO films have a Curie temperature greater than 330 K. Kerr effect measurements show evidence of an increase of the LSMO coercive field HC from ∼50 Oe at room temperature up to 110 Oe at 40 K and a significant decrease in the squareness of the hysteresis loop. This effect is accompanied by a slight shift of the hysteresis loop HE due to exchange bias between the antiferromagnetic charge ordered LCMO layer and the LSMO ferromagnet which reaches 20 Oe at 40 K. Both the onset of HE and the drop in squareness are observed to be close to the charge order transition of a similar LCMO single layer and are assu...
TL;DR: In this paper, the current-in-plane (CIP) transport properties of CoFe/Al 2 O 3 across a 10-μm gap between lithographed Al contacts are investigated.