1. Inverse Square Sensitivity
Noise Spectroscopy of Spin-based and Semiconductor-Based
Magnetoresistive Systems
Edmund R. Nowak, Department of Physics & Astronomy, University of Delaware
The most likely technology for advancing small, low power magnetic sensors is based on magnetic tunnel junctions
(MTJs). MTJs are devices that utilize the spin of the electron, in addition to its charge, to achieve very large
changes in resistance (greater than 400% at room temperature) in response to small magnetic fields. Based on our
experimental studies of noise in these devices, we have developed a theoretical model (below) and incorporated
it into a spreadsheet for easy evaluation of the expected performance of sensors. Our current effort is directed at
understanding the underlying mechanisms responsible for magnetic 1/f noise, the last term below, which is expected
to set the ultimate sensitivity of practical sensors based on MTJs.
Noise Power
(Detection Limit)
(T/Hz½)2
Amplifier Noise
Johnson and Shot Noise
Electronic
1/f Noise
Thermal Magnetic
White Noise
Inverse Square Sensitivity
Magnetic
1/f noise
Material Parameters
αelec : Electronic 1/f noise strength
αmag : Magnetic 1/f noise strength
f : Detection frequency
αG : Damping parameter
Ω : Free layer volume
γ : Gyromagnetic ratio
[RAP] : Resistance Area Product
Bsat : Saturation field (T)
ΔR/R : Fractional resistance change
MS : Free layer intrinsic magnetization
Recent advances in tunneling magnetoresistance, free-layer saturation field, and MEMS oscillating flux concentrators suggest that it may be possible to extend small, inexpensive, low-power, ultra-sensitive magnetic sensors, which is currently dominated by fluxgates, optically pumped magnetometers and SQUIDS for low frequency applications.
Wheatstone bridge
N MTJ’s
Device Architecture
N shielded MTJ’s
Device Parameters
N : # of Sensing Junctions
VJ : Junction Voltage
R : Junction Resistance
A : Junction Area