Advances in magnetometry through miniaturization

Abstract

Recent innovations may lead to magnetic sensors that are smaller, more sensitive, and/or cost less than current magnetometers. Examples of this are the chip scale atomic magnetometer, magnetic tunnel junctions with MgO barriers, and a device for minimizing the effect of 1∕f noise, the microelectromechanical system (MEMS) flux concentrator. In the chip scale atomic magnetometer, researchers have been able to fabricate the light source, optics, heater, optical cell, and photodiode detector in a stack that passes through a silicon wafer. Theoretical and subsequent experimental work has led to the observation of magnetoresistance values of 400% at room temperature in magnetic tunnel junctions with MgO barriers. This large magnetoresistance occurs because electrons in the majority band can tunnel more easily through the MgO barrier than electrons in the minority band. The MEMS flux concentrator has the potential to increase the sensitivity of magnetic sensors at low frequencies by more than an order of magnitude. The MEMS flux concentrator does this by shifting the operating frequency to higher frequencies where the 1∕f noise is much smaller. The shift occurs because the motion of flux concentrators on MEMS flaps modulates the field at kilohertz frequencies at the position of the sensor. Though miniaturization is generally beneficial, trade-offs are necessary because some properties, such as noise, worsen with decreasing size.