Although applications for magnetic sensors cover a vast field, ranging from the gigantic magnetic resonance imaging or MRI systems to sensing tiny gear-teeth, they are one of the most overlooked or misunderstood among the modern sensors in use. Researchers are constantly on the lookout for increasingly small but more sensitive magnetic-field sensors. However, sensitivity alone is not the only qualifying parameter for such sensors—low-level transducers require to be low-noise as well.
That is exactly what researchers at Brown University have developed. Their magnetic sensor is not only sensitive, it exhibits a very low noise level. With support from the National Science Foundation, the researchers have developed a device that, as a part of an arrangement of a magnetic immunoassay, looks for pathogens in fluid systems using magnetism. They claim that as the device is extremely small, millions of such sensors can fit on a single chip.
The basic principle behind the sensor is the Hall effect. In a Hall effect sensor, passing a direct current through it when the sensor is perpendicular to a magnetic field, causes the development of a voltage at right angles to the current path. The presence and magnitude of the magnetic field directly influence the presence and magnitude of the voltage.
The researchers at Brown University have developed a variation of the Hall effect sensor and have named it the Anomalous Hall Effect or AHE, and this occurs in ferromagnetic materials only. The difference between the two effects is that while the conventional Hall effect is the result of charge on electrons, the anomalous Hall effect is due to electron spin.
As electrons with various spins orient themselves in different directions, the AHE detects this with a small but definite voltage. Incidentally, magnetic fields cause many interesting phenomena on atomic particles. For instance, MRI systems capture signal source emissions related to the magnetic moment of the hydrogen nucleus.
The researchers fabricated the device as an ultra-thin film made of ferromagnetic materials like boron, iron, and cobalt, with electron spins arranged in in-plane anisotropy—meaning, the electron spins align themselves in the plane of the film. However, exposing the film to a high temperature and a strong magnetic field can change the spin of the electrons to perpendicular anisotropy, and their alignment turns perpendicular to the film.
Equalizing the two anisotropies results in a reorientation of the electron spins when the material encounters any external magnetic field, providing a reorientation voltage across the AHE. Compared to a conventional Hall-effect sensor, an AHE sensor is about 20X more sensitive.
The thickness of the AHE device offers a tradeoff in performance. A thick film requires a strong magnetic field to reorient the spins, resulting in a reduction in sensitivity. However, in a thin film, the electrons tend to reorient their spins by themselves, reducing the usefulness of the sensor. The researchers tried many thicknesses and found 0.9 nm thickness worked the best.
As magnetic anisotropy is highly dependent on temperature, researchers are using temperature to fine-tune a single magnetic AHE sensor, thereby achieving very low levels of intrinsic noise during its operation.
