J. De Boeck

Noninvasive magnetic imaging and magnetization measurement of isolated mesoscopic Co rings

A high-resolution scanning Hall probe microscope was used as a noninvasive technique to visualize the magnetization reversal in an array of micron-size Co rings. Two stable “onion” states at remanence and “vortex” states at switching fields were found. To rule out a possible influence of dipole–dipole interaction between ring elements on remagnetization processes, an isolated Co ring was deposited on top of a Hall magnetometer and extremely sharp transitions from onion to vortex and from vortex to onion state of opposite polarity were resolved. Our results were supported by MOKE magnetization measurements and micromagnetic simulations.

Optimization of the magnetic field of perpendicular ferromagnetic thin films for device applications

We studied the geometrical factors controlling the magnitude of the demagnetizing field and the fringing fields of ferromagnetic thin films with perpendicular, out‐of‐plane magnetization. The magnetic field emerging from the patterned ferromagnetic thin film can interact with carriers in an underlying semiconductor structure and generate a Hall voltage. Different geometries for use in practical device applications have been analyzed, using a simplified model as a design tool. The optimum geometry of the ferromagnetic thin film that will give maximum magnetic‐field strength in the underlying semiconductor consists of a grating‐type structure with periodicity of a few 100 nm to 1 μm. These geometries are suited for realization by holographic lithography.

On-chip magnetic particle transport: where physics, chemistry and biology meet

Magnetic biosensors are emerging as promising alternatives for classical fluorescent-based micro-arrays, replacing the fluorescent label by a super-paramagnetic particle. While on-chip detection of magnetic particles is firmly established, research groups now start to explore the unique ability of manipulating these particles by applying controlled magnetic forces. In this paper, we describe the importance of magnetic, hydrodynamic and electrostatic forces in the actuation of magnetic particles. The use of magnetic actuation for biological applications in which aqueous solutions with high ionic strengths are often used, increases the need to study the effect of electrostatic forces into greater detail. We conclude that for transport of magnetic particles, suspended in aqueous solutions with a high ionic strength, the electrostatic force starts to become an important factor. Therefore, a quantitative study of air forces acting on the magnetic particle is necessary for a further development of magnetic force generating structures.

Arrays of nano-rings for magnetic storage applications

We present the results of systematic characterization of arrays of small permalloy and cobalt ring elements with SQUID magnetization to determine the magnetic moment, with atomic force microscopy (AFM) and magnetic force microscopy (NUM) to determine the topography and the magnetic patterns inside the rings, and with high resolution scanning Hall probe, Lorentz microscopy and magneto-optical imaging to visualize the moment reversal process during a magnetization cycle.

Embedded Molecular Beam Epitaxy for a Coplanar Gallium-Arsenide on Silicon Technology

Heteroepitaxy of GaAs on Si has gained interest over the past years because of the promising combination of optoelectronic and high speed properties of GaAs with the well established high density silicon technology. Devices with performances comparable to those of their GaAs on GaAs counterparts are frequently reported (1,2,3). Although the high density of threading dislocations together with the remaining stress in the device layers are limiting factors for the lifetime and reliability of the minority carrier devices, some successes in the monolithic integration of e.g. Si MOSFET’s and double heterostructures LED’s are achieved (4). A serious problem that has not been adressed properly is the strong non-planarity of the surface after growth and definition of the GaAs device islands. This leads to severe problems during the following processing steps. A coplanar surface is desirable from a process point of view. One way to achieve such a coplanar surface is to recess wells several microns deep into the Si substrate and refill them with the epitaxial layer, such that the GaAs surface levels with the Si surface after the GaAs growth. Since the III–V devices are to be fabricated in the windows the embedded layer quality has to be at least comparable with the large area GaAs on Si. The possible introduction of surface roughness and the loss of the intentional misorientation after recessing the Si can lead to dramatic degradation of the morphology and the introduction of antiphase domains. It has been anticipated that the exposure of different crystallographic orientations and the more severe geometrical constraints for the embedded GaAs film may disturb the single crystal quality of the GaAs at the well edges (5).

EXCHANGE-BIASED MAGNETIC TUNNEL JUNCTIONS PREPARED BY IN-SITU NATURAL OXIDATION

Magnetic tunnel junctions, showing spin-dependent tunneling, are considered for future implementation in high density magnetic memories. A low device resistance is a key criterium for the implementation. In this paper, we discuss the transport properties of low-resistance tunnel barriers realized by in-situ natural oxidation of thin Al layers (< 1.3 nm). The resistance and magnetoresistance of the tunnel junctions is evaluated for different Al thickness and different oxidation times, showing tunnel magnetoresistance values of 20% for 1 kΩ.μm2. The voltage bias and temperature dependence of the transport properties is addressed, as well as the influence of thermal post-treatment.

GaAs ON Si: Device Applications

Device applications of GaAs on Si technology can be divided into two categories. In the first case Si is used as a passive substrate to take advantage of its thermomechanical superiority over bulk GaAs. The second application area aims at the functional combination of both GaAs and Si devices in a monolithic circuit. In both domains, significant technological achievements have been reported. Present results indicate that most GaAs-based devices can be processed and operated on GaAs/Si without dramatic loss of performance. A major exception is the semiconductor laser, which suffers from severe reliability problems. The implications of this situation for future developments of the GaAs/Si technology will be discussed.