Tuning the electron emission properties of films containing carbon nanotubes is demonstrated. Microcontact printing is used to pattern silicon substrates with catalyst, which facilitates the growth of the carbon nanotubes by the chemical vapor deposition of acetylene. The catalyst is applied in liquid form to an elastomeric stamp prior to transfer to the substrate, and varying the concentration of the catalyst on the solution (“the ink”) directly influences the density of nanotubes on the patterned film.

ARTICLES 546nature materials | VOL 2 | AUGUST 2003 | www.nature.com/naturematerials E xploring the ultimate density limits of magnetic information storage, whether on computer hard disks or in MRAMs (magnetic random access memories), requires elaborate tuning of the preferred (easy) magnetization axis, of the magnetic anisotropy energy, and of the magnetic moment in the units used to store a bit. These units are single-domain particles (with diameter d < 20 nm) where the magnetic moments of all atoms are ferromagnetically aligned 1 to form the overall magnetic moment of the particle M, which is also called the macrospin. The preferred orientations of M, and the anisotropy energy barriers K separating them, are given by the delicate balance between several competing energies. These are the magnetocrystalline bulk anisotropy, its surface and step counterparts, and the shape anisotropy, or demagnetizing energy, resulting from the interaction of M with its own dipolar stray field. Unravelling the anisotropy's origin is far from trivial due to the competition between these energies 2 . This is unfortunate because the anisotropy is one of the key quantities: it defines the stability of the magnetization direction against thermal excitation,and therefore the minimum particle size for which non-volatile information storage may be achieved (at 300 K this requires K ≥ 1.2 eV). A further key parameter is the modulus of M, M defining the dipolar stray field used to read and write, but also mediating interactions between adjacent bits. These interactions are minimized for out-of-plane magnetization, and because the ultimate limit of single-particle bits may only be achieved for uniaxial systems, uniaxial out-of-plane systems are best suited to explore the ultimate density limit of magnetic recording 3,4 . Current studies attempting to identify the origin of magnetic anisotropy mainly deal with two model systems. These are colloids or three-dimensional (3D) nanoparticles, and 2D nanostructures created by molecular-beam epitaxy at single-crystal surfaces. For colloid particles, remarkable progress has been achieved in monodispersity 5 , their self-assembly into 2D superlattices 6,7 and in the accomplished anisotropy energies per constituent atom 8 . Despite their promising properties for applications,3D nanoparticles present several difficulties for tracing back the origin of anisotropy. First, although the magnetism of a single particle can be addressed 9 , it is almost impossible to study the morphology of the very same particle in conjunction with its magnetism. Second, the particles frequently have a few atomic layers of oxide at their surface, which is not ferromagnetic

We report on the magnetic properties of two-dimensional Co nanoparticles arranged in macroscopically phase-coherent superlattices created by self-assembly on Au(788). Our particles have a density of 26 Tera=in 2 (1 Tera 10 12 ), are monodomain, and have uniaxial out-of-plane anisotropy. The distribution of the magnetic anisotropy energies has a half width at half maximum of 17%, a factor of 2 more narrow than the best results reported for superlattices of three-dimensional nanoparticles. Our data show the absence of magnetic interactions between the particles. Co=Au788 thus constitutes an ideal model system to explore the ultimate density limit of magnetic recording. DOI: 10.1103/PhysRevLett.95.157204 PACS numbers: 75.75.+a, 75.30.Gw, 81.16.Dn The bit density on magnetic hard disks has been increasing at a constant pace for many years [1]. Besides the technological challenges, we face today the question of where downscaling ends from fundamental physics. This question can be addressed by studying periodic lattices of ferromagnetic monodomain particles, where each particle stores one magnetic bit. The high density requires out-ofplane magnetic anisotropy to minimize dipolar interactions among adjacent particles. Further, to optimize the signalto-noise ratio in read or write processes, the magnetic properties have to be uniform; i.e., the particles have to be uniaxial and the distributions of moments M, and magnetic anisotropy energies (MAEs), K, have to be narrow.Chemical synthesis of self-assembled colloid particles has led to excellent size distributions with a half width at half maximum (HWHM) of 7% in diameter and 21% in volume, respectively [1,2]. However, these are accompanied by systematically much wider K distributions [3,4] partly caused by the random orientation of the particle's easy axes [4,5] causing strong dipolar interactions. Colloid particles have obvious practical advantages, one of them being the high blocking temperatures [6]. However, for the outlined reasons, the ultimate density limit could not be explored with such systems so far. Alternatively, massselected magnetic 3D clusters [7] are monodisperse and may be soft-landed onto surfaces [8], but cannot be arranged into ordered arrays of equidistant magnetic units.Self-assembly during atomic beam epitaxy on periodic strain relief patterns on single crystal surfaces has been shown to provide well ordered superlattices of 2D islands with narrow size distributions [9]. While such lattices are generally not in phase from terrace to terrace, a combination of epitaxial strain relief patterns and vicinal surfaces has been shown to lead to macroscopically phase-coherent lattices [10]. Former studies of the magnetic properties of metal islands on single crystal surfaces revealed that the spin-orbit interaction with the substrate may induce a strong enough magnetocrystalline anisotropy to yield a common out-of-plane easy magnetization axis for all islands [11][12][13][14]. Therefore we have today the tools to fabricate the required lattices of uni...

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We report on localized measurement of the longitudinal and transverse flow velocities in a colloidal suspension using optical coherence tomography. We present a model for the path-length resolved autocorrelation function including diffusion and flow, which we experimentally verify. For flow that is not perpendicular to the incident beam, the longitudinal velocity gradient over the coherence gate causes additional decorrelation, which is described by our model. We demonstrate simultaneous imaging of sample morphology and longitudinal and transverse flow at micrometer scale in a single measurement.