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Some low-temperature magnetic sensors are based on the Hall effect in thin metallic films. However, their sensitivity is more than one order of magnitude smaller than that of silicon-based Hall sensors operating at room temperature. Furthermore, in order to avoid significant Joule heating, only very small bias currents can be injected at low temperatures. Here, we show that a sensor based on a photoinduced Hall effect, in which charge is photogenerated in a semiconductor and injected into an adjacent metallic layer, can be used as a bias-free, cryogenic sensor. The system consists of a platinum thin film deposited on intrinsic-silicon substrate. The film forms a Schottky interface with the semiconductor. At room temperature, carriers photogenerated in the semiconductor are injected into the metal, because of rounding-off of the Schottky barrier, and are deflected by a magnetic field applied in the film plane. At cryogenic temperatures, and well below the freeze-out temperature for silicon, the photogenerated electrons can tunnel through the barrier, and the sensor recovers the same sensitivity as that obtained at room temperature.

Magnetostrictive properties of Iron-Gallium alloys, such as the saturation of magnetic induction and magnetostriction can be derived from experimental measurements. It is known that, in order to exhibit the largest saturation magnetostriction, Galfenol requires the application of a suitable external compressive stress. In order to avoid a bulky system to provide such stress, a built-in stress could be added to the material through a technological process known as stress annealing. In this paper an experimental method to evaluate the stress applied to Galfenol rods is presented. Then, the method can be exploited either to evaluate the built-in stress or to develop a stress sensor. A concept device exploiting the method and its experimental characterization are presented.

Because of its low-cost, silicon is the standard material for photovoltaic conversion. Yet, its band-edge absorption spectrum is narrower than the spectrum of the solar radiation, which reduces its conversion efficiency. In this paper, it is shown that the spectrum of absorbance of silicon can be extended to longer wavelengths by proximity to a two-dimensional (2D) semiconductor. Photo-induced Hall effect, together with standard absorption spectroscopy, was employed to estimate the increase of photo-conversion efficiency of a 2D-platinum-diselenide/intrinsic-silicon heterostructure. The system shows a significantly higher absorption in the infrared as compared to the single films. Angle resolved X-ray Photoelectron Spectroscopy (XPS) confirm that a change of the band structure occurs in the silicon substrate at the interface between the two semiconductors. The results are interpreted in the framework of band-gap narrowing due to hole-confinement in the Si, induced by electron-confinement in the 2D film. This allows us to claim that the increase of photo-conversion efficiency in the Pt/PtSe2/Si sample is due to an enhancement of the light absorbance of silicon near the interface. Possible application of the effect in photo-voltaic cells is discussed.

The requirement of self-powered stand-alone sensors or wireless sensor networks (WSN) with a “fit and forget” paradigm, that is, without the need of batteries, seems to have attracted the interest of technological research in last years. Activity in development of devices, Known as Energy Harvesters, able to transform ambient energy into a kind of usable energy were boosted. Among this quite large set of solutions, those able to transform mechanical vibrations into electric power and referred to as Kinetic Energy Harvesters (KEH) are among the most known and studied, [1]. Such special set of self powered systems found a noteworthy interest in the health monitoring of civil structures or in automotive or railway applications, where, in the latter, the KEH potentialities have been identified for low cost self power sensors in freight railway wagons, [2]. Among them, one solution is represented by the cantilever configuration exploiting magnetostrictive materials, [3], based on Fe-Ga (Galfenol) alloys, showing interesting mechanical and magnetostrictive properties, [4]. Vibrations energy spectrums are complex and usually distributed over a quite large frequency range, while, conversely, cantilever KEH work in a relatively narrow band and this requires both modelling and experimental effort in order to provide an “optimal” system tuning. In this paper as a first step, different configurations employing one or more Fe-Ga strips over an Al substrate have been developed and tested through the analysis of a converted RMS power vs. frequency in order to detect the geometric and physical parameters affecting resonance frequency, bandwidth and converted power by Villari effect, [2]. The selected cantilever is implemented by bonding two single Galfenol strips 120X15X0,3 mm to a 120X15X2 mm Aluminium foil. The latter guarantees a better mechanical resistance to cyclic loads and provides a larger stiffness yielding to a consequent increase of the resonance frequency. The device has been equipped by a 300 turns coil, as it can be observed in Fig. 1. Experimental tests were performed with an electrodynamic shaker (Fig. 2a) in the 50–200 Hz frequency interval with sinusoidal acceleration in the ${1-4 g}$ amplitude range [5]. The excitation level has been detected by a reference accelerometer, while the beam bending is measured by a strain gauge based system. The effect of a resistive load on the RMS converted power is shown in Fig. 2b) where the power response with different resistive loads and 4g acceleration is shown. The best result of 305mW is achivied with a 9,9 W. The low energy conversion required the design of a suitable magnets set in order to supply a sufficient magnetic bias to the active material with the aim to increase the converted power [6]. In order to increase the flux captured by the coil, two magnets were attached to the structure [7]. The field generated by two magnets was measured spanning from 43 kA/m on the upper strip down to 3 kA/m on the lower strip. In Fig. 2c) the effect of the magnetic bias on the RMS converted power is shown. However, it should be outlined that the applied bias is the result of an optimization procedure since, as known, high magnetic biases pushes the material towards saturation and energy conversion is no longer observed [6]. Considering the importance of the bias, tests were carried out by varying the number of magnets (Fig. 2d). In this case it is important to evaluate the position of the magnet on the beam as it influences the resonance frequency and the magnetoelatic coupling [8]. These results seem quite encouraging, due to the relatively high converted energy (37mW) and the simple device structure. However, a thorough and accurate modelling analysis in connection to the experimental data, in order to quantitatively show the dependence of the converted power on geometric and physical parameters will be discussed in the full paper.

A magnetic field sensor prototype is developed and tested in this work. The device exploits a Galfenol rod, i.e. a giant magnetostrictive Iron-Gallium alloy, integrated with a Fiber Bragg Grating. In particular, the full-scale range of the sensor can be modulated through the exploitation of the geometrically dependent effect of the demagnetizing field. Indeed, it pushes toward higher fields the magnetic saturation by producing a sort of magnetic shield in the material. As a consequence, the geometrical viewpoint is included into the frame of the entire design process, with the aim of investigate how it influences the detectability range and the performance of the sensor. Furthermore, a permanent magnet system providing a DC bias magnetic field has been designed and exploited to allow the device to be able to measure both negative and positive magnetic fields. © 2018 Elsevier B.V.

The paper aims to discuss the basic issues related to the analysis and design of magnetic sensors based on the employment of magneto-active materials. In particular, the basic idea is based on the integration of a Galfenol magnetostrictive alloy to a Fiber Bragg Grating (FBG) embedded into an optic fiber, able to sense the deformation of the material induced by magnetic field. The structure of the alloy and the characteristics of the fiber, make the device suitable to work also in harsh envi- ronments. One of the basic goals is to provide a sensor as simple as possible, with high field range detection and, at the same time, low reconstruction error. It has been observed that the increase of the field range could be achieved by exploiting the effects of the demagnetizing field, without exploit- ing the well-known magnetic hardening induced by the applied stress. In fact, the latter requires a clamping system, resulting in the increase of the sensor size. The demagnetizing field, conversely, provides a shielding of the external field, turning away the undesired approach to saturation. Finally, the employment of a material characterized by weak hysteresis phenomena avoids the use of complex compensation algorithm without losing accuracy. Some result of its characteristics and performances are provided.

This paper investigates the new developments of a class of magnetic field sensors based on the integration of Iron-Gallium magnetostrictive alloys (Galfenol) and fiber Bragg gratings used to detect the magneto-induced mechanical strain. This kind of sensor has the advantage of being able to work also in harsh environments, but on the other hand cannot detect fields beyond 10 kA/m, because of the magnetic softness of the active material. A simple solution consists in the exploitation of the demagnetizing field experienced by the ferromagnetic alloy by effect of its magnetization, generated by the application of the external magnetic field. Since the demagnetizing field effect depends only on geometrical parameters, the use of samples with different aspect ratios allows us to check how the shape of the active material can be used as a control parameter of the sensor detection range. © 1965-2012 IEEE.