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.
Articles you may be interested inSelf-sensing cantilevers with integrated conductive coaxial tips for high-resolution electrical scanning probe metrology
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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 bias͒ 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.
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.Field emission sensing has been studied before in pressure sensors [1], data storage distance control [2] and RF MEMS switches [3]. Based on similar principles, we use the exponential relation of the emission current to the electric field E, which is a function of distance E = γ (V/d) (where γ is the field enhancement factor, V the bias voltage and d the electrode-tip distance), to sense the amplitude displacement of the cantilevers, and further extract the resonance information. Therefore, the displacement of a vibrating cantilever at its resonance frequency can be transduced into electrical current signals via field emission.To understand the relationship between field emission and distance, we fabricated silicon tips with tip radius of 10~25 nm on fixed substrates (see Fig.1) and coated them with 6 nm Cr and 40 nm Au. The measurements are carried out in an AFM system under UHV condition, the testing setup is the same as in [4]. The AFM system together with the fixed silicon tip allows precise control of the electrode-tip distances, ruling out the effect of cantilever bending of typical AFM probes. The measurement are carried out in two ways: in the first one we fix the bias voltage, and slowly vary the tip distance with a calibrated piezo stage; in the second, we change the bias voltage, measuring the displacement of the piezo stage while maintaining a constant emission current by using feedback to the tip position. Fig. 2 shows the measurement of emission current vs. relative sample-tip distance, under different fixed bias voltages. The measurement shows not only the exponential-like relationship of the emission current and the distance, but also the sensitivity change under different bias voltage and initial distances. Fig. 3 shows the measurement of bias voltage vs. relative sample-tip distance, at a fixed emission current of 3 nA. The bias vs. distance shows a linear relationship, the sensitivity of such detection method is extrapolated to be as high as 0.5 V/nm. Finally, to measure the resonance detection by field emission, we use a commercial highly doped silicon AFM probe as the emitter. The AFM probe is actuated by piezo, and its tip is biased at 80 V. The probe is held ~100 nm away from the electrode in resonance and the resonance curve was obtained (see Fig. 4). The field emission current induced by the cantilever's mechanical resonance is fed to a low-noise current am...
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