“Ultra-sensitive” Magnetic Detecting Device Research Could Open the Door for New Application Areas

Magnetic sensors many uses in many industries. In the power industry, magnetic sensors are often used in monitoring the efficiency of rotating machines or for measuring magnetic fields for power distribution systems. Magnetic sensing can also be used in the medical sector for various applications such as testing for blood coagulation and early detection of heart valve bioprosthesis failure.

Recently, a team of Brown University researchers developed a new compact and “ultra-sensitive” magnetometer. The research was led by Yiou Zhang, a graduate student at Brown University and Kang Wang, a postdoctoral researcher. The research was originally published in Applied Physics Letters.

The effect of the magnet showing the magnetic field redirecting ferromagnetic material. Image courtesy of Brown University

Understanding the Anomalous Hall Effect (AHE)

Traditional magnetic sensors use the hall effect to detect voltages. The Hall Effect can be characterized as the perpendicular deflection of the direction of electrons due to the existence of a magnetic field.

The researchers found out that the tiny magnetic moment of each electron, also called electron spin, causes an effect similar to the hall effect called Anomalous Hall Effect. Although AHE causes a small voltage, instruments can still detect this voltage. The voltage is caused by the dispersion of electrons with different spins in different directions.

Thin ferromagnetic films formed of the material Co40Fe40B20, which contains cobalt, iron, and boron. For the sensor to work at its optimal conditions the film should be as thick as 0.9 nm. Any increase in the thickness of the film decreases the sensitivity of the sensor. Furthermore, any decrease in the thickness of the film causes the electron spins to change their positions on their own, meaning that the sensor will not be useful.

The film is treated under 280 degrees celsius for 1 hour and subjected to a large external magnetic field. This causes the electron spins to be aligned perpendicular to the film. Therefore, when the material senses an external magnetic field the electron spins are reoriented, producing a very small voltage. The process mentioned above increases the sensitivity 20 times more than traditional Hall Effect Sensors.  

Electrons with spin values of ½ and -½ in an External Magnetic Field

Possible Future Applications of AHE Magnetic Sensors

This research could be a window to developing and enhancing many future applications. Some of those are:

• Medical applications, such as looking for pathogens in fluid samples.
• Green energy applications, such as contactless current sensing.
• Magnetic imaging, where Xiao the chair of the physics department at Brown University and his colleagues are developing a high definition magnetic camera.

 

“Because the device is very small, we can put thousands or even millions of sensors on one chip,” Zhang said. “That chip could test for many different things at one time in a single sample. That would make testing easier and less expensive.”

The National Science Foundation supported this research.

To learn more about magnetic sensing and Hall Effect check out these articles:

• New Hall Effect Current Sensing IC from Melexis is Optimized for E-Mobility Applications
• Hall-Effect Current Sensors offer Enhanced Performance and Diagnostics
• Magnetic Sensing with Accuracy Linearity Dynamic Range And Sensitivity