M. Op de Beeck

Auto-oscillation and narrow spectral lines in spin-torque oscillators based on MgO magnetic tunnel junctions

We demonstrate spin-torque induced coherent auto-oscillation in magnetic tunnel junctions of composition PtMn/CoFe/Ru/CoFeB/MgO/CoFeB and of low resistance-area product. At the generation threshold, we observe a strong line narrowing down to 6 MHz at 300 K and a dramatic increase in oscillator power, yielding spectrally pure oscillations with extremely low flicker noise. The induced auto-oscillations are observed even at zero applied field. The frequency of the oscillation mode, and its dependence with easy and hard axis fields are consistent with an acoustical excitation of the two layers of the synthetic ferrimagnet subsystem. Setting the synthetic ferrimagnet into auto-oscillation requires the current polarity that transfers electrons from the synthetic ferrimagnet to the free layer. In auto-oscillation mode, line jitter is observed such that it is the line envelope that is measured in most cases. The line properties for applied fields near the instability boundaries of the Stoner astroid of the free l...

Comb-shaped polymer-based Dry electrodes for EEG/ECG measurements with high user comfort

Soft, comfortable polymer-based dry electrodes are fabricated. Impedance and biopotential measurements are carried out to compare the performance of conventional gel electrodes with our dry electrodes. The impedance of our dry electrodes is reduced by adding more conductive additives to the polymer material. To further lower the impedance, two skin pretreatment techniques are evaluated regarding their influence on skin impedance. However, these techniques are found to have only temporary beneficial effects. Finally biopotential measurements (both ECG and EEG) are performed using our soft polymer electrodes. The ECG signal acquired with both gel and our polymer electrodes demonstrates high degree of similarity. Therefore, heart beat detection is straightforward. To enable monitoring of EEG signals with smaller amplitudes, our dry electrodes need to be combined with pre-amplifiers. Initial EEG tests show that the alpha waves are clearly identifiable with the dry electrodes when subjects close their eyes. Based on the results, combining with sophisticated signal acquisition electronics, the dry electrodes provide a high user comfort solution for high quality biopotential measurements, even on very hairy skin.

Effect of patterning on the saturation magnetization in MgO based nanopillars

We have studied the effect of nanopillar patterning on the saturation magnetization of the CoFeB free layer in MgO magnetic tunnel junctions. Before patterning, the free layer magnetization is measured by ferromagnetic resonance and is found to be close to the bulk value, with no detectable interface anisotropy. After patterning, the shape anisotropy and the frequency of the main spin wave mode indicate that the free layer magnetization is substantially reduced. Current dependent measurements indicate that this is not due to Joule heating. Size dependent measurements indicate that the magnetization reduction most likely arises from process damage during the etching step.

Auto-oscillation threshold and line narrowing in MgO-based spin-torque oscillators

We present an experimental study of the power spectrum of current-driven magnetization oscillations in MgO tunnel junctions under low bias. We find the existence of narrow spectral lines, down to 8 MHz in width at a frequency of 10.7 GHz, for small applied fields with clear evidence of an auto-oscillation threshold. Micromagnetics simulations indicate that the excited mode corresponds to an edge mode of the synthetic antiferromagnet.

Design and fabrication of a biomedical Lab-on-Chip system for SNP detection in DNA

A Lab-on-Chip system is proposed, capable of SNP (Single Nucleotide Polymorphism) detection in DNA. One of the core components is an advanced filter consisting of an ordered array of Si micro-pillars enabling fast and effective separation of 5 DNA segments with different length using chromatographic techniques. Also a dedicated micro-pump is fabricated based on conductive polymer actuation, generating the required high pressure to sustain the fluid flow through the total system. For the detector, a known detector principle is applied, but pronounced miniaturization is carried out in order to make a small and portable system.

A micro-PCR chamber suitable for integration into a monolithic silicon lab-on-a-chip platform

A polymerase chain reaction (PCR) chamber microfabricated into a silicon substrate is presented. The unique design features of the device provide excellent thermal isolation of the PCR chamber from the surrounding silicon substrate allowing for highly localized control of the PCR chamber temperature. Steady-state, thermal, finite element method simulations were conducted to optimize the microreactor design. Silicon microreactors were fabricated and experimentally characterized. The fabricated PCR reactor is demonstrated to have good thermal isolation between the microcavity and surrounding silicon chip. Successful amplification of human genomic DNA is also demonstrated.

2-Scale topography dry electrode for biopotential measurements

The design and fabrication of a novel 2-scale topography dry electrode using macro and micro needles is presented. The macro needles enable biopotential measurements on hairy skin, the function of the micro needles is to decrease the electrode impedance even further by penetrating the outer skin layer. Also, a fast and reliable impedance characterization protocol is described. Based on this impedance measurement protocol, a comparison study is made between our dry electrode, 3 other commercial dry electrodes and a standard wet gel electrode. Promising results are already obtained with our electrodes which do not have skin piercing micro needles. For the proposed electrodes, three different conductive coatings (Ag/AgCl/Au) are compared. AgCl is found to be slightly better than Ag as coating material, while our Au coated electrodes have the highest impedance.

Integrated fluidic system for bio-molecule separation

An integrated fluidic system has been fabricated, capable of separating a mixture of different bio-molecules into its components. It is composed of a filter and an actuator; the pressure generated by the actuator sustains the flow of the mixture through the filter. The actuator is made by stacking several layers of conductive polymer. Actuator strain in excess of 10% has been obtained, which corresponds to a fluid flow of 3 µL/min in the fabricated system. The filter consists of an ordered array of Si micro-pillars. A mixture composed of DNA fragments of different length (300 and 400 base-pair) has been effectively separated by using the fabricated filter and chromatographic techniques.

Tuning electrode impedance for the electrical recording of biopotentials

Tuning the electrode impedance through the DC biasing of iridium oxide is presented. Impedance reduction of up to two orders of magnitude was reproducibly observed in 20 µm diameter microelectrodes at a biasing of 1V.

High pressure pump as lab on chip component for micro-fluidic integrated system

We developed a miniaturized pump (8 mm diameter, 1 mm thickness) which generates a flow rate of 2 µL/min at a pressure of 3 MPa. It consists of a stack of several conductive polymer (CP) layers intercalated with electrolyte layers. The stack is housed in a polycarbonate case specially conceived for integration in a lab-on-chip device. The actuator operates at a bias lower than 2V. A maximum strain of 13% is measured in the single CP layer when it expands against atmospheric pressure; this strain is reduced by only a factor of 3 when pressure increases to 15 MPa. Using the stacked actuator, a maximum strain of 5% is measured.