Magnetic Sensors
Alan S. Edelstein
US Army Research Laboratory, Adelphi, MD 20783
Currently, we are primarily using anisotropic magnetoresitive (AMR) sensors and
fluxgate sensors for applications requiring high sensitivity vector sensors and optically
pumped sensors for applications requiring total field or pT sensitivity. There have been
several developments, which promise to provide new types of sensors that will either
supplement or replace the current sensors.
The work at NIST Boulder on chip scale atomic magnetometers1 has shown that it is
possible to make optically pumped magnetometers that are much smaller and consume
much less power. This is accomplished by constructing a stack consisting of the light
source, optics, optical cell, heater, and detector around a chip.
Several approaches offer the possibility of improved vector magnetometers.
Magnetoelectric sensors2, 3 have a piezoelectric layer sandwiched between two
magnetostrictive layers. In the presence of a magnetic field, the magnetostrictive layers
apply a stress to the piezoelectric material, which then generates a voltage. Thus, this
type of sensor generates its output without the need for any input voltage. The percentage
change observed in magnetoresitive sensors has increased dramatically in recent years. It
increased from 3.5% in AMR sensors, to 12.8% in giant magnetoresistance sensors4, to
80% in magnetic tunnel junctions (MTJ) with Al2O3 barriers, to 400% in MTJ of the
form5, 6 Fe(001)/MgO(100)/Fe(100). In these MTJ the barrier acts as a filter that
preferentially allows tunneling for majority spin electrons. The high magnetoresistance
values, however, do not necessarily lead to better sensors at low frequency due to the 1/f
noise that arises from trapping sites in the barrier and magnetization fluctuations of the
ferromagnetic layers. The problem of 1/f noise in small magnetoresistive sensor is
largely solved by using the MEMS flux concentrator.7 In the MEMS flux concentrator,
the sensor is place near flux concentrators on MEMS structures whose motion modulates
the signal at the position of the sensors. This shift the operating frequency for detecting
low frequency signals to a range where 1/f noise is orders of magnitude smaller. The
figure shows the result of shifting a 25 Hz to kHz frequencies. The signal now appears
as sidebands around the resonant frequency of the MEMS structure. This sensor should
have been able to detect signals of a few pT/Hz1/2 at 1 Hz.
1.
Schwindt, P. D. D.; Knappe, S.; Shah, V.; Hollberg, L.; Kitching, J., Appl. Phys.
Lett. 2004, 85, 6409.
2.
Dong, S.; et al., Appl. Phys. Lett. 2005, 86, 102901.
3.
Dong, S.; Zhai, J.; Bai, F.; Li, J.-F.; Viehland, D., Appl. Phys. Lett. 2005, 87,
62502.
4.
Carey, M. J.; et al., Appl. Phys. Lett. 2002, 81, 1044.
5.
Parkin, S.et al. Nature Materials 2004, 3, 862-867.
6.
Yuasa, S.; Nagahama, T.; Fukushima, A.; Suzuki, Y.; Ando, K.,. Nature
Materials 2004, 3, 868-871.
7.
Edelstein, A. S.; et al., e., J. Appl. Phys. 2009, 105, 07E720.
Figure (a) 25 Hz signal appearing as sidebands shifted 25 Hz from the resonant
frequency of the MEMS structure when the resonant structure is driven at 50, 70, and 90
V. (b) Absence of the sidebands when the drive signal is moved off the resonant
frequency of the MEMS structure