Granular Co-C nano-Hall sensors by focused-beam-induced deposition

We investigated the performance of Hall sensors with different Co-C ratios, deposited directly in nanostructured form, using Co-2(CO)(8) gas molecules, by focused-electron or ion-beam-induced deposition. Due to the enhanced intergrain scattering in these granular wires, the extraordinary Hall effect can be increased by two orders of magnitude with respect to pure Co, up to a magnetic field sensitivity of 1 Omega T-1. We show that the best magnetic field resolution at room temperature is obtained for Co ratios between 60% and 70% and is better than 1 mu T Hz(-1/2). For an active area of the sensor of 200 x 200 nm(2), the room temperature magnetic flux resolution is phi(min) = 2 x 10(phi 0)(-5) in the thermal noise frequency range, i.e. above 100 kHz.

Micro and nano tools for magnetic field imaging

Schahrazede-Lila Mouaziz

Scanning probe microscopy has become, nowadays, a branch of microscopy widely used to image surfaces of different specimens. Among the various applications, the magnetic imaging is one of the most exciting scanning probe microscopy techniques. It allows the study of topography of samples at the nanometer scale. The magnetic force microscopy is the best known and the most employed magnetic imaging technique. It has been successfully used for the imaging of various samples but with the major inescapable limitations that it is confined to the surface, non-quantitative and invasive. Techniques such as the scanning Hall probe microscopy (SHPM) and the magnetic resonance force microscopy (MRFM) have emerged, several years later, as promising approaches to overcome these limitations. In this thesis, we present the design, the fabrication and the characterization of, first, micro-Hall sensors on SU-8 cantilevers used for SHPM and, second, of ultrasensitive Si cantilevers used for MRFM experiments. A microfabrication technique has been developed to produce hybrid devices consisting of micro-nano Hall sensors integrated on SU-8 cantilevers. Two materials have been used to produce the Hall sensors. First, bismuth (Bi) was chosen for its low carrier charge density and its negligible surface charge depletion effects so that it should provide sensitive sensors. The second material tested was nickel (Ni), chosen for the dominant effect called the extraordinary Hall effect. The active areas of 5×5, 2×2 and 1×1 µm2 were realized and reproduced with the standard photolithography. Producing submicron Hall sensors required the use of the focused ion beam (FIB) that allowed a decrease of the size of the sensors down to 80×80 nm2. The delicate transferring of the nano sensors on the polymer cantilevers was successful. Bi micro-Hall sensors have shown a fragility making them delicate to handle. However, the Ni micro-Hall sensors had an excellent functionality. Their characterization has shown sensors with a low sensitivity compensated by the high applied current providing a high magnetic field resolution. The magnetic field images were carried out using the Ni micro-Hall sensors. They had a high stability during the imaging experiments. The spatial resolution was set by the sensors active area limited to the micro-meter scale. However, the magnetic field resolution is better than 1 T/√Hz a result comparable to the high resolution offered by the semiconductors. Single crystal silicon cantilevers have been fabricated for use in MRFM experiments. Also, cantilevers with integrated micro-magnets have been successfully produced. The developed microfabrication technique has allowed a high yield production (up to 70%). The softest fabricated cantilevers are 0.34 µm thick, 500 µm long and 10 µm wide. These cantilevers have been characterized under vacuum conditions (1×10-6 mbar). The resonance frequencies and the quality factors were measured using the frequency sweep or noise methods and both provided the same results. The Rayleigh method that we have adapted to the cantilevers geometry, made it possible to estimate the theoretical resonance frequencies with a good approximation and the calculated values fit well with the measured ones. All the cantilevers have a quality factor above 20000 and the force sensitivity achieved was between 10-16 and 10-17 N/√Hz A mathematical model has been developed based on the motion equation of a harmonic oscillator under the influence of a magnetic field. The model was studied to find the expression of the resonance frequency of cantilevers with integrated magnetic tips in the presence of a parallel or a perpendicular magnetic field. The shift in frequency due to the presence of the applied magnetic field depended on the magnetic tip magnetization. The measurement of the frequency shift should permit the calculation of the magnetization of these micro-magnets directly integrated on the cantilevers. The probes that we have realized represent micro and nano tools suitable for the magnetic field imaging that can be done with the SHPM and MRFM techniques.

EPFL2007

Single Superparamagnetic Bead Detection and Direct Tracing of Bead Position Using Novel Nanocomposite Nano-Hall Sensors

Laurent Bernau, Giovanni Boero, Ivo Utke

We show that nanocomposite materials consisting of superparamagnetic nanoparticles embedded in a matrix are very good candidates for a novel generation of Hall nanosensors with high spatial and magnetic resolution, capable of detecting superparamagnetic beads suspended at different heights on top of the sensor. Two detection schemes were used: 1) static-the bead was centered on top of the sensor and detected with a combination of ac and dc orthogonal magnetic fields, and 2) dynamic-using nanomanipulation in an electron microscope for moving the bead precisely at different heights above the sensor and using only one magnetic field (ac). The Hall sensors were directly written by focused-electron-beam-induced deposition and the active area was refined by focused ion beam milling. A magnetic field resolution of about 300 mu T(Hz)(-1/2) and a spatial resolution of 230 nm(Hz)(-1/2) were measured. Both resolutions could be improved by at least an order of magnitude with the suppression of the parasitic inductive signals.

Institute of Electrical and Electronics Engineers2013

Bismuth Micro and Nano Hall Sensors on Photo-plastic Cantilevers for Scanning Hall Probe Microscopy

Giovanni Boero, Jürgen Brugger, Schahrazede-Lila Mouaziz, Radivoje Popovic

Conventional Hall devices are widely employed to measure magnetic fields produced by millimetre sized (or larger) current-caring conductors or permanent magnets with a standard active area of about (100 um)^2. Micro-Hall sensors have recently emerged as a powerful and sensitive tool for the investigation of local magnetic field properties of the micro and nano-structures. The conventional Hall effect devices are based on non-magnetic materials (typically semiconductors e.g: AlGaAs/GaAs and InSb or semimetals such as Bi). Bismuth is a semi-metal that presents a very long mean free path, a large spatial extension of the electron wave function, and a low electron density. This explains the amazing galvanomagnetic properties of bismuth nanostructures. Recently, Hall sensors having dimensions down to (100 nm)^2 have been fabricated. By mounting them on micro-scanning systems, a new magnetic imaging method, called Scanning Hall Probe Microscopy (SHPM), has been introduced for simultaneous magnetic and topography imaging. Miniaturized Hall sensors are employed to measure highly inhomogeneous magnetic fields such as those produced by ferromagnetic nanoparticles, vortices in superconductors, magnetic domains in ferromagnetic materials, magnetic force microscopy tips, and magnetic recording media. We have developed an alternative route to fabricate micro and nano Hall sensors on cantilevers. The approach was to incorporate bismuth Hall sensors of dimensions down to 100 nm on photo-plastic cantilevers made with SU-8, a material already used for the fabrication of atomic force microscopy (AFM) and near field scanning optical microscopy (NSOM). The smallest active area of the Hall sensors obtained by the focused Ion beam (FIB) is (60x60) nm^2. The electrical characteristics of our bismuth Hall sensors are comparable to the best results reported up to now. For instance, a device with an active area about (0.1x0.1) um^2 the current sensitivity, the Hall coefficient and the field resolution measured are 2 V/AT, 180x10^-9 m3/C and 120 uT/Hz^1/2, respectively.