[ Instrument R&D ] On June 9th, a team of physicists at Brown University developed a new compact ultra-sensitive magnetic sensor. This new device may be useful in various applications involving weak magnetic fields.
Xiao Gang, head of the Brown Physics Department and senior author of the journal Physics, said: "From our electronic devices to the beating heart, almost everything around us generates magnetic fields. We can use these magnetic fields to obtain information about the system. We found a class of ultra-sensitive sensors, but they are small, cheap to manufacture, and consume little power. We think these new sensors may have many potential applications."
The new device was elaborated in a paper published in the Applied Physics Letters. Brown graduate student Yiou Zhang and postdoctoral researcher Wang Kang are the main authors of the study.
In general, the traditional method of inducing a magnetic field is through the so-called Hall effect. When a conductive material carrying a current contacts the magnetic field, the electrons in the current will deflect perpendicular to the direction of its flow. This produces a small vertical voltage that Hall sensors can use to detect the presence of magnetic fields.
The new device utilizes a close relative of the Hall effect, called anomalous Hall effect (AHE), which is produced in ferromagnetic materials. The Hall effect is due to the charge of the electrons, and AHE is due to the electron spin (the tiny magnetic moment of each electron). This effect causes electrons with different spins to be dispersed in different directions, resulting in very Small but detectable voltage.
At the same time, the new equipment uses ultra-thin ferromagnetic films made of cobalt, iron and boron atoms. The spins of electrons tend to be arranged in the plane of the thin film, and this characteristic is called in-plane anisotropy. After processing the thin film in a high-temperature furnace and under a strong magnetic field, the spin of electrons tends to be perpendicular to the thin film, the so-called vertical anisotropy. When the two anisotropies have the same intensity, if the material is in contact with an external magnetic field, the electron spins are easily reoriented, and the reorientation of the electron spins can be detected by the AHE voltage.
The key to making the equipment work properly is the thickness of the cobalt-iron-boron film. A film that is too thick requires a stronger magnetic field to redirect electron spins, which reduces sensitivity; if the film is too thin, the electron spins may redirect themselves, which will cause the sensor to malfunction. The researchers found that the preferred thickness is 0.9 nanometers and the thickness is about 4 or 5 atoms.
No strong magnetic field is required to flip the spin in the film, which makes the device very sensitive. The researchers say that, in fact, its sensitivity is 20 times higher than traditional Hall-effect sensors. Researchers believe that the device may have a wide range of applications. One example that may be useful to doctors is magnetic immunoassay, which uses magnetism to find pathogens in liquid samples.
The researchers said: "Since the device is very small, we can place thousands or even millions of sensors on a chip. This chip can test many different things in one sample at a time, which will make testing easier and more costly. low."
Another application may be part of the ongoing project of the Shaw Laboratory funded by the National Science Foundation. Xiao and his colleagues are developing an electromagnetic camera that can image high-definition magnetic fields generated by quantum materials. Such detailed magnetic profiles will help researchers better understand the properties of these materials. "Like a normal camera, we want our magnetic camera to have as many pixels as possible. Each magnetic pixel in our camera is a separate magnetic sensor. The sensor must be small and not consume too much power, so this This new sensor is very useful in our cameras."
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