A.J. le Fèbre

Thermally induced switching field distribution of a single CoPt dot in a large array

Magnetic dot arrays with perpendicular magnetic anisotropy were fabricated by patterning Co(80)Pt(20)-alloy continuous films by means of laser interference lithography. As commonly seen in large dot arrays, there is a large difference in the switching field between dots. Here we investigate the origin of this large switching field distribution, by using the anomalous Hall effect (AHE). The high sensitivity of the AHE permits us to measure the magnetic reversal of individual dots in an array of 80 dots with a diameter of 180 nm. By taking 1000 hysteresis loops we reveal the thermally induced switching field distribution SFD(T) of individual dots inside the array. The SFD(T) of the first and last switching dots were fitted to an Arrhenius model, and a clear difference in switching volume and magnetic anisotropy was observed between dots switching at low and high fields.

Field emisssion at nanometer distances for high-resolution positioning

The dependence of the field emission effect on distance is applied for displacement sensing and high-resolution positioning. Silicon atomic force microscopy probes were used as a field emission source by applying voltages up to 400 V between this probe and a counter-electrode sample consisting of TiW sputtered on a silicon wafer. From current-voltage characteristics measured for distances varying from 50 to 950 nm, values for the field enhancement factor were determined which show a dependence on the electrode separation. This dependence can be correctly described by a model the authors developed using finite-element calculations and is determined by the emitter geometry and tip radius. Feedback to the probe position was used to maintain a constant current to apply this distance dependence for positioning. When increasing the applied voltage from 5 to 40 V for a constant current of 3 nA, the probe position is raised similar to 90 nm. The nonlinear sensitivity of this positioning method is determined by the varying field enhancement and can be fitted by the same calculated model. Using feedback, the field emitter can be positioned with high lateral resolution and scanned over a conducting surface. Increasing the bias voltage from 3 to 50 V results in an increase in the emitter-sample distance and a decrease in lateral resolution. Damage to the scanned surface has to be prevented by using a current-limiting resistor and by annealing the probe and sample under ultra high vacuum conditions before use

Field emission to control tip-sample distance in magnetic probe recording

An integrated method using field-emission to control the tip-sample distance for non-contact magnetic probe recording is presented, adopting the exponential relation between current and electric field as feedback. I/V characteristics that correspond well to field emission theory are measured using a probe coated with a 100 nm conductive diamond layer. By using feedback to control the tip-sample distance at constant current, the distance was increased by 2.8 nm per volt applied bias. The method was tested by scanning a probe coated with 20 nm chromium over a conducting nanopatterned sample, at bias voltages of 0.5V, 5.0V and 50.0V. The measurements confirm that field emission can be applied to control the tip-sample distance, with sufficient resolution and current stability for magnetic probe recording.

Field emission for cantilever sensors

Field emission provides an alternative sensing solution in scaled electromechanical systems and devices, when typical displacement detection techniques fail in submicron and nanodimenions. Apart from its independency from device dimension, it has also a high response, integration and high compatibility benefits. In this work, we propose using two modes of detection (fixed current and fixed bias) on two sensing methods: static sensing and dynamic resonance sensing. We measured the characteristic of the two modes and proved that field emission is a viable cantilever displacement detection technique. Customized tip on a fixed substrate has been fabricated and loaded to a UHV atomic force microscopy scanning tunneling microscopy system providing us a field emission environment with precise distance controls without the effects of cantilever bending. Thus, we are able to measure and determine the relationship of emission electric field to the electrode distance, as well as the relationship of the emission current to the electrode distance. The sensitivity obtained in our work for the static mode is 0.5 V/nm. In dynamic mode, we successfully measured a resonance of a piezoactuated cantilever at 162.2 kHz. Characterizing these relations enabled us to propose the possibility of using field emission as a cantilever displacement sensing technique.

Field emission for resonance sensing in MEMS/NEMS

In the past decades, there is a considerable interest in the sensor community to move from micron to nano-devices, typically scaling of resonators such as cantilever beams. The scaled beams give advantages in faster response and higher sensitivity; however the detection of their resonance becomes challenging as dimensions scale down. In our work, we demonstrate the use of field emission characteristics as a detection method for scaled resonators. The advantages of using field emission are several: it is geometrically scalable without loss of signal, it has a high bandwidth and it can be integrated using standard fabrication processes.