D. Pescia

An inverse transition of magnetic domain patterns in ultrathin films

Inverse freezing and inverse melting are processes where a more symmetric phase is found at lower temperatures than at higher temperatures. Such inverse transitions are very rare. Here we report the existence of an inverse transition effect in ultrathin Fe films that are magnetized perpendicular to the film plane. The magnetization of these films is not uniform, but instead manifests itself as stripe domains with opposite perpendicular magnetization. Predictions relating to the disordering of this striped ground state in the limit of monolayer film thicknesses are controversial. Mean-field arguments predict a continuous reduction of the stripe width when the temperature is increased; other studies suggest that topological defects, such as dislocations and disclinations, might penetrate the system and induce geometrical phase transitions. We find, from scanning electron microscopy imaging, that when the temperature is increased, the low-temperature stripe domain structure transforms into a more symmetric, labyrinthine structure. However, at even higher temperatures and before the loss of magnetic order, a re-occurrence of the less symmetric stripe phase is found. Despite the widespread theoretical and experimental work on striped systems, this phase sequence and the microscopic instabilities driving it have not been observed before.

Ultrafast generation of magnetic fields in a Schottky diode.

For the development of future magnetic data storage technologies, the ultrafast generation of local magnetic fields is essential. Subnanosecond excitation of the magnetic state has so far been achieved by launching current pulses into micro-coils and micro-striplines and by using high-energy electron beams. Local injection of a spin-polarized current through an all-metal junction has been proposed as an efficient method of switching magnetic elements, and experiments seem to confirm this. Spin injection has also been observed in hybrid ferromagnetic–semiconductor structures. Here we introduce a different scheme for the ultrafast generation of local magnetic fields in such a hybrid structure. The basis of our approach is to optically pump a Schottky diode with a focused, ∼150-fs laser pulse. The laser pulse generates a current across the semiconductor–metal junction, which in turn gives rise to an in-plane magnetic field. This scheme combines the localization of current injection techniques with the speed of current generation at a Schottky barrier. Specific advantages include the ability to rapidly create local fields along any in-plane direction anywhere on the sample, the ability to scan the field over many magnetic elements and the ability to tune the magnitude of the field with the diode bias voltage.

Scale invariance of a diode-like tunnel junction

We measure the current vs voltage (I-V) characteristics of a diodelike tunnel junction consisting of a sharp metallic tip placed at a variable distance d from a planar collector and emitting electrons via electric-field assisted emission. All curves collapse onto one single graph when I is plotted as a function of the single scaling variable Vd^{-\lambda}, d being varied from a few mm to a few nm, i.e., by about six orders of magnitude. We provide an argument that finds the exponent {\lambda} within the singular behavior inherent to the electrostatics of a sharp tip. A simulation of the tunneling barrier for a realistic tip reproduces both the scaling behavior and the small but significant deviations from scaling observed experimentally.

Bifurcation in precessional switching

We explore the precessional motion of the magnetization vector in a model magnetic element. We find that the Landau–Lifshitz equation governing this motion allows trajectories of the magnetization vector to bifurcate. This yet unknown phenomenon is accompanied by a slowing down of the precessional motion and an abrupt shrinking of the size of the trajectory of the precessing magnetization. We discuss the implication of bifurcation for future devices using precessional switching and suggest how magnetic elements showing the classical phenomenon of bifurcation can be tuned to act as quantum bits.

Static and dynamic properties of single-chain magnets with sharp and broad domain walls

We discuss time-quantified Monte Carlo simulations on classical spin chains with uniaxial anisotropy in relation to static calculations. Depending on the thickness of domain walls, controlled by the relative strength of the exchange and magnetic anisotropy energy, we found two distinct regimes in which both the static and dynamic behavior are different. For broad domain walls, the interplay between localized excitations and spin waves turns out to be crucial at finite temperature. As a consequence, a different protocol should be followed in the experimental characterization of slow-relaxing spin chains with broad domain walls with respect to the usual Ising limit.

Self focusing of field emitted electrons at an ellipsoidal tip

In models of field emission the needle is usually terminated by a hemispherical cap. Here we choose to terminate it with a hemiellipsoidal cap and use a three-dimensional Wentzel–Kramers–Brillouin method for the computations. This has two important consequences: as the ellipsoid becomes more elongated, (a) the effective emission area is decreased and (b) the quantum mechanically computed electron paths converge toward the needle axis. Both mechanisms produce a self-focusing of the field emitted electrons.

Evidence of nonplanar field emission via secondary electron detection in near field emission scanning electron microscopy

Nonplanar field emission from electrochemically etched tungsten field emitters has been observed using near field emission scanning electron microscopy. Close-proximity field emission in adequate ultrahigh vacuum conditions was implemented to attain Fowler–Nordheim plots using typical imaging parameters. The emission radii deduced via a detailed, spherical surface field emission theory, by [Edgcombe and de Jonge, J. Phys. D 40, 4123 (2007)], reveal that our sharpest tip asperities are less than a nanometer. This yields a spatial resolution on the order of one nanometer.

Fundamental Aspects of Near-Field Emission Scanning Electron Microscopy

Abstract In a previous publication (Kirk, 2010) the experimental technique of imaging near-field emission scanning electron microscopy (NFESEM) imaging was introduced. In NFESEM, a sharp tip in positioned at distances of a few 10nm from a metallic surface. Above a threshold voltage, electrons are field emitted from the tip. The field-emitted current is used, while scanning the tip across the surface at a well-defined, constant distance, to generate a topographic image of the surface with subnanometer vertical spatial resolution and a few-nanometer lateral spatial resolution. In this review, we discuss the fundamental physical processes that occur in NFESEM and provide some quantitative results. It is our goal to provide sufficient background information to allow NFESEM-based instruments to be developed in other laboratories.

Ultrathin magnetic particles

We have fabricated ultrathin Co particles with various shapes, variable thicknesses δ (2 ML<δ<22 ML), and lateral size L ranging from 100 μm to ≈100 nm. We find that all particles are magnetized in-plane at room temperature and are in a single domain state, independently of shape and size—with some remarkable exceptions. We also find that the magnetic state of a particle can be manipulated without influencing the state of the neighbors.

Quantum oscillations in a confined electron gas

When metals are structured on nanometre length scales, their electrons are subject to confinement effects: the response of a confined electron gas is governed by Friedel oscillations of the electron density and Rudermann–Kittel–Kasuya–Yosida oscillations of the spin density. Spatial oscillations of electron density have been observed directly at surfaces (in the vicinity of defects and steps) by scanning tunnelling spectroscopy. But it has proved more difficult to probe such oscillations in bulk materials and over large distances. Here we report the detection of quantum oscillations in a three-dimensional electron gas confined to a half space by a surface. To facilitate this detection, we have inserted an atomically thin ferromagnetic cobalt film at a variable distance τ from the surface of a copper single crystal. The cobalt film induces a total spin polarization P in the conduction electrons of the copper and, by virtue of the confining effects of the copper–vacuum interface, P varies as a function of τ. Our measurements of P reveal both quantum oscillations (the wavelengths of which are governed by the extremal diameters of the copper Fermi surface) and a decay with τ that are consistent with theoretical expectations. These observations show that a consequence of improving the quality of nanostructured materials is that long-range quantum interactions can emerge more effectively, so that even distant boundaries and defects can become pivotal in determining physical properties.