Abstract
Magnetoresistive devices are important components in a large number of commercial electronic products in a wide range of applications including industrial position sensors, automotive sensors, hard disk read heads, cell phone compasses, and solid state memories. These devices are commonly based on anisotropic magnetoresistance (AMR) and giant magnetoresistance (GMR), but over the past few years tunneling magnetoresistance (TMR) has been emerging in more applications. Here we focus on recent work that has enabled the development of TMR magnetic field sensors with 1/f noise of less than 100 pT/rtHz at 1 Hz. Of the commercially available sensors, the lowest noise devices have typically been AMR, but they generally have the largest die size. Based on this observation and modeling of experimental data size and geometry dependence, we find that there is an optimal design rule that produces minimum 1/f noise. This design rule requires maximizing the areal coverage of an on-chip flux concentrator, providing it with a minimum possible total gap width, and tightly packing the gaps with MTJ elements, which increases the effective volume and decreases the saturation field of the MTJ freelayers. When properly optimized using this rule, these sensors have noise below 60 pT/rtHz, and could possibly replace fluxgate magnetometers in some applications.
Highlights
Applications such as magnetocardiography (MCG), which require measurement of small lowfrequency magnetic fields, are among the most challenging target applications for magnetoresistive sensors
It is well known that at low bias voltage, white noise is dominated by Johnson noise, and that 1/f noise decreases as total sensor element size increases.[1]
SB1/2 is the equivalent magnetic field power spectral density (PSD) in units of T/Hz1/2, “Gain” is a dimensionless scale factor that represents the flux concentrator (FC) magnetic field gain, αMTJ the electronic Hooge parameter with units of um[2] representing the strength of the 1/f noise, AMTJ the area of a single MTJ element, ‘f’ the frequency, “Sen” is the slope of MTJ transfer curve in the absence of a FC with units of V/VB/T, kB is Boltzmann’s constant, T is the temperature in Kelvin, and Rtot is the total resistance of the magnetic field sensor
Summary
Applications such as magnetocardiography (MCG), which require measurement of small lowfrequency magnetic fields, are among the most challenging target applications for magnetoresistive sensors. These signals are in the tens of pT range in a frequency bandwidth of 0.1 to 100 Hz, where commercial magnetoresistive sensors exhibit 1/f noise that degrades signal-to-noise ratio. We find the magnetic noise contribution is small in our sensors This is a result of on-chip flux FCs increasing the effective magnetic volume of the MTJ elements.
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