Magnetic tunnel junction sensors with pTesla sensitivity for biomedical imaging

Abstract

Ultrasensitive magnetic field sensors at low frequencies are necessary for several biomedical applications. Suitable devices can be achieved by using large area magnetic tunnel junction sensors combined with permanent magnets to stabilize the magnetic configuration of the free layer and improve linearity. However, further increase in sensitivity and consequently detectivity are achieved by incorporating also soft ferromagnetic flux guides. A detailed study of tunnel junction sensors with variable areas and aspect ratios is presented in this work. In addition, the effect in the sensors transfer curve, namely in their coercivity and sensitivity, as a consequence of the incorporation of permanent magnets and flux guides is also thoroughly discussed. Using sensors with a tunnel magnetoresistance of ~200 %, incorporating both permanent magnets and flux guides sensitivities of 220-260 %/mT were obtained for high aspect ratio sensors, increasing to values larger than ~2000%/mT for large areas and low aspect ratio sensors. Measured noise levels of the final device at 10 Hz yield 3.9×10 -17 V 2 /Hz, leading to an improved lowest detectable field of ~ 94 pT/ Hz 0.5 .

Magnetoresistive Sensor Development Roadmap (Non-Recording Applications)

“…As one of the most fundamental and critical performance parameters of MR sensors, sensitivity has exhibited a considerable growth in the last two decades [223,243,245,[248][249][250][251][252][253][254][255][256][257][258][259][260][261][262][263][264][265][266], as shown in Figure 6. The sensitivity [250,254] of MR sensors is defined in the linear operation range of the magnetic transfer curve as…”

“…For the TMR sensors with an AlOx barrier during the period of 1995-2002, TMR sensors with sensitivity from several %/mT to almost two hundred %/mT were fabricated [242,269,[278][279][280][281][282][283][284]. After replacing the AlOx barrier with the crystalline MgO barrier, a rapid increase of MR ratio was accomplished ( Figure 7) [269,270,[285][286][287], resulting in a notable enhancement of sensitivity to 300-1000 %/mT ( Figure 6) [245,250,251,253,255]. By integrating MFCs into the TMR sensors, the saturation field was greatly diminished and thus the sensitivity was significantly increased [249-251, 254, 263].…”

“…To fabricate high-performance MR sensors for measuring ultra-low magnetic field, researchers endeavor not only to boost their sensitivity but also to improve their detectivity which determines the smallest magnetic signal a sensor can detect [50, 222, 223, 243, 249-255, 257-260, 314-326], as shown in Figure 8. The detectivity [250] of an MR sensor is associated with its sensitivity and noise level, as expressed by…”

“…As discussed in Section A, incorporation of the MR sensor array with MFCs can dramatically improve its sensitivity [245,252], leading to a considerable increase of the sensor detectivity. On the other hand, the sensor detectivity can be greatly enhanced by reducing the sensor noise through optimization of sensor fabrication, such as enlarging the sensor area [250,315], modifying the annealing process [243,258,323], and softpinning the sensing layer [249,257]. Defect-free MR sensors with relatively large sensing area can greatly reduce the 1/f noise and a sensor detectivity of ~60 pT/Hz 0.5 has been successfully demonstrated at 10 Hz [257].…”

“…Secondly, though SQUIDs comprised of ceramic HTS materials could alleviate the size, weight and power requirements, they have been found to be difficult to work with because of anisotropic electrical properties and intrinsic noise [333]. Compared to the conventional MFCs using soft ferromagnetic materials [249,250,254,317,326], superconductor MFCs exhibit a much higher gain (100-1000), as reported in [28,222]. However, the application of superconductor MFC is restricted by its relatively low superconducting critical temperature (Tc) [28,222,223,, which is far below the room temperature, as shown in Figure 9.…”