Rita Macedo

OPTIMIZATION AND INTEGRATION OF MAGNETORESISTIVE SENSORS

This paper addresses challenging issues related to the integration of magnetoresistive (MR) sensors in applications such as magnetic field mapping, magnetic bead detection in microfluidic channels, or biochips. Although sharing the same technological principle for detection (magnetoresistance effect), each application has unique specifications in terms of noise, sensitivity, spatial resolution, electrical robustness or geometric constraints. These differences are of high impact for manufacturing, because some strategies used for sensor optimization compromise the freedom for device architecture.

Ion beam assisted deposition of MgO barriers for magnetic tunnel junctions

This work reports for the first time results on MgO tunnel junctions prepared by ion beam. The MgO barrier was deposited from a ceramic MgO target using an assisted beam, following the deposition and assisted beam phase diagram which relate the beam profile with the current and energy. The deposition rate for MgO is calculated with and without assisted beam, and compared with the experimental values. The MgO film growth on Ta∕CoFeB∕MgO simple stacks was optimized aiming at a (002) preferred orientation for the MgO growth, measured by x-ray diffraction. The optimum assist beam energy was tuned for each deposition beam condition (+800,+1000,+1200V), using assist beams of 40mA (∼130μA∕cm2) with 0to+600V. Without assist beam, no texture is observed for the MgO, while the (002) orientation appears for assisted deposition. The optimum range of assist voltages is large, being limited by the onset of etching at high voltages, reducing the deposition rate. Magnetic tunnel junctions were deposited with the structur...

Self-powered, hybrid antenna-magnetoresistive sensor for magnetic field detection

A self-powered hybrid sensor integrating a resonant antenna with a magnetoresistive spin valve sensor was microfabricated. The device is activated by a radiofrequency (rf) external electromagnetic source. This hybrid sensor is capable of detecting dc and ac external magnetic fields in the thermal noise regime. For an excitation rf field of 17 dB m at 130 MHz, the sensitivity to a transverse magnetic field normalized by the current induced in the device was 812 V (T A)−1 for dc magnetic field detection and 918 V (T A)−1 for the ac field measurement.A self-powered hybrid sensor integrating a resonant antenna with a magnetoresistive spin valve sensor was microfabricated. The device is activated by a radiofrequency (rf) external electromagnetic source. This hybrid sensor is capable of detecting dc and ac external magnetic fields in the thermal noise regime. For an excitation rf field of 17 dB m at 130 MHz, the sensitivity to a transverse magnetic field normalized by the current induced in the device was 812 V (T A)−1 for dc magnetic field detection and 918 V (T A)−1 for the ac field measurement.

Switching Field Variation in MgO Magnetic Tunnel Junction Nanopillars: Experimental Results and Micromagnetic Simulations

The switching field dependence on the size of nanometric magnetic tunnel junctions was studied. CoFe/Ru/CoFeB/MgO/CoFeB nanopillars were fabricated down to 150 × 300 nm2 and characterized, revealing a squared transfer curve with a sharp transition between magnetic states. A micromagnetic finite element tool was then used to simulate the magnetic behavior of the studied nanopillar. The simulations indicated a single-domain like state at remanence, also displaying a sharp transition between parallel/antiparallel free-layer configurations. Overall, the experimentally measured switching fields (Hsw) were smaller than those obtained from simulations. Such trend was consistent with the presence of a particular free layer profile, signature of the two angle etching step used for pillar definition. Further decrease of experimental Hsw was attributed to local defects and thermal activated processes. This study was able to validate this particular simulation tool for the control of the nanofabrication process.

Plasma Channels for Electron Accelerators Using Discharges in Structured Gas Cells

We present a new scheme to produce preformed plasma waveguides to extend the acceleration length of laser-plasma accelerators. The plasma is produced in a hydrogen background by a high-voltage discharge between two hollow conic electrodes through a sequence of thin dielectric plates with apertures that fix the initial plasma diameter and position. The thickness of the dielectric plates is close to their aperture diameter and negligible when compared with the distance between them. This results in a fully ionized plasma cylinder that expands almost freely, evolving into a radial parabolic density profile with a minimum on axis as required for high-intensity guiding. In this paper, besides the description of the guiding device and its operation, we report the plasma density measurements of the first prototype with 16.6-mm length obtained by Mach-Zehnder interferometry as well as laser guiding results.

Hybrid Integration of Magnetoresistive Sensors with MEMS as a Strategy to Detect Ultra-Low Magnetic Fields

In this paper, we describe how magnetoresistive sensors can be integrated with microelectromechanical systems (MEMS) devices enabling the mechanical modulation of DC or low frequency external magnetic fields to high frequencies using MEMS structures incorporating magnetic flux guides. In such a hybrid architecture, lower detectivities are expected when compared with those obtained for individual sensors. This particularity results from the change of sensor’s operating point to frequencies above the 1/f noise knee.

Nanoscale Probing of Local Electrical Characteristics on MBE-Grown Bi2Te3 Surfaces under Ambient Conditions

The local electrical characteristics on the surface of MBE-grown Bi2Te3 are probed under ambient conditions by conductive atomic force microscopy. Nanoscale mapping reveals a 10-100× enhancement in current at step-edges compared to that on terraces. Analysis of the local current-voltage characteristics indicates that the transport mechanism is similar for step-edges and terraces. Comparison of the results with those for control samples shows that the current enhancement is not a measurement artifact but instead is due to local differences in electronic properties. The likelihood of various possible mechanisms is discussed. The absence of enhancement at the step-edges for graphite terraces is consistent with the intriguing possibility that spin-orbit coupling and topological effects play a significant role in the step-edge current enhancement in Bi2Te3.

Optimization of exposure parameters for lift-off process of sub-100 features using a negative tone electron beam resist

A thorough study of exposure parameters for electron beam lithography using AR7520 negative tone electron beam resist is here presented. We optimized the beam voltage, apertures diameter and resist thickness in order to achieve the smaller dimensions possible for each resist thicknesses. Monte Carlo simulations of the electrons scattering process correlated the experimental results indicating a less efficient energy deposition into the resist layer for larger beam energies and resist thicknesses, thus resulting in larger doses required to expose a selected dot size. Furthermore, for the particular exposure conditions used we determined a forward scattered electrons range between 50 nm and 170 nm, depending on the dot nominal size. On the other hand, a reduced backscattering electrons range was observed showing a constant value of ~ 560 nm, being therefore more significant when larger dimensions are exposed in a point-by-point exposure, and thus supporting the smaller doses observed for larger sizes. Finally, a baking step is used to further improve the etch resistance of the resist, which allied to the optimized exposure parameters, opens a pathway to achieve sub-100nm critical dimensions for the reproducible fabrication of nanometric devices using a simple lift-off method.

Hybrid GMR Sensor Detecting 950 pT/sqrt(Hz) at 1 Hz and Room Temperature

Advances in the magnetic sensing technology have been driven by the increasing demand for the capability of measuring ultrasensitive magnetic fields. Among other emerging applications, the detection of magnetic fields in the picotesla range is crucial for biomedical applications. In this work Picosense reports a millimeter-scale, low-power hybrid magnetoresistive-piezoelectric magnetometer with subnanotesla sensitivity at low frequency. Through an innovative noise-cancelation mechanism, the 1/f noise in the MR sensors is surpassed by the mechanical modulation of the external magnetic fields in the high frequency regime. A modulation efficiency of 13% was obtained enabling a final device’s sensitivity of ~950 pT/Hz1/2 at 1 Hz. This hybrid device proved to be capable of measuring biomagnetic signals generated in the heart in an unshielded environment. This result paves the way for the development of a portable, contactless, low-cost and low-power magnetocardiography device.

Hybrid antenna―magnetoresistive sensor for radio frequency field detection

A self-powered, hybrid sensor integrating a resonant microfabricated antenna with a spin valve sensor was fabricated. The device was activated by a radio frequency (RF) external electromagnetic source. This hybrid device was designed to behave as a series resonant circuit at 130 MHz. The self-powered sensor (powered by the RF field through the antenna) was capable of measuring the amplitude of the perpendicular RF excitation field crossing the antenna, down to 2.5 μT when excited with a RF field of 130 MHz.