7. Comparison of GMR- and TMR-type magnetic biosensors
As shown in chapter 5.4 and 6.5, both GMR and TMR type magnetic biosensors produce signals which depend linearly on the coverage of magnetic markers on the surface of a sensor element. In this chapter, the response and applicability of both biosensor types is compared.
As shown in chapter 5.4 and 6.5, both GMR and TMR type magnetic biosensors produce signals which depend linearly on the coverage of magnetic markers on the surface of a sensor element. In this chapter, the response and applicability of both biosensor types is compared.
Figure 91 displays the response of both GMR and TMR based sensor elements to global unidirectional in-plane fields. In part a), the magnetoresistance curves are plotted, whereas part b) shows a comparison of the corresponding sensitivities (i.e.
the field derivatives of the magnetoresistance curves). Clearly, the TMR based sensor displays sharp sensitivity peaks of up to 0.3 % resistance change per A/m when the free magnetic layer switches its magnetization direction between parallel and antiparallel alignment to the pinning direction (blue curves). However, as demonstrated in chapter 6.5.1, it is not possible to actually use such a switching process for the detection of magnetic markers in our sensors, so the relevant sensitivity range for TMR type sensors is given more by its response to fields applied perpendicular to the pinning direction (black curves). In this configuration, the maximum sensitivity values are about two orders of magnitude lower (around 3 % per kA/m) and become comparable to the ones obtained for our GMR type sensor systems (red curves).
Figure 91 displays the response of both GMR and TMR based sensor elements to global unidirectional in-plane fields. In part a), the magnetoresistance curves are plotted, whereas part b) shows a comparison of the corresponding sensitivities (i.e.
the field derivatives of the magnetoresistance curves). Clearly, the TMR based sensor displays sharp sensitivity peaks of up to 0.3 % resistance change per A/m when the free magnetic layer switches its magnetization direction between parallel and antiparallel alignment to the pinning direction (blue curves). However, as demonstrated in chapter 6.5.1, it is not possible to actually use such a switching process for the detection of magnetic markers in our sensors, so the relevant sensitivity range for TMR type sensors is given more by its response to fields applied perpendicular to the pinning direction (black curves). In this configuration, the maximum sensitivity values are about two orders of magnitude lower (around 3 % per kA/m) and become comparable to the ones obtained for our GMR type sensor systems (red curves).
-8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 -3,0
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0 50 100 150 200 250 GMR sensor 300
TMR sensor perpendicular to pinning direction
sensitivity [% per kA/m]
in-plane field [kA/m]
parallel
sensitivity [% per kA/m]
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5 10 15 20 25 30 35 40
45 GMR sensor TMR sensor
parallel perpendicular to pinning direction
XMR ratio [%]
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(b) (a)
Figure 91: Comparison of the response of TMR and GMR sensor elements to unidirectional global in-plane fields
Figure 91: Comparison of the response of TMR and GMR sensor elements to unidirectional global in-plane fields
a) magnetoresistance ratio a) magnetoresistance ratio b) sensitivity
b) sensitivity
However, as the TMR type sensors are operated at an in-plane bias field that results in almost maximum sensitivity in the perpendicular field direction (see chapter 6.5.1), their response to magnetic markers is still larger than for GMR type sensors, for which most measurements are taken at zero in-plane bias field due to practical considerations (see chapter 5.4.2). This is demonstrated in Figure 92, which compares the response of exemplary GMR and TMR type sensor elements to an approximately equal surface coverage of Bangs 0.86 µm magnetic markers (the corresponding sensor elements are shown at identical scale in the electron micrographs of Figure 92). Both signal responses are compensated for drift and asymmetry according to the description in chapter 5.4.2 and constructed as differential measurements by subtracting the response of an uncovered reference sensor. The same is true for the displayed reference curves, which show the differential response of two uncovered GMR and TMR type sensor elements, However, as the TMR type sensors are operated at an in-plane bias field that results in almost maximum sensitivity in the perpendicular field direction (see chapter 6.5.1), their response to magnetic markers is still larger than for GMR type sensors, for which most measurements are taken at zero in-plane bias field due to practical considerations (see chapter 5.4.2). This is demonstrated in Figure 92, which compares the response of exemplary GMR and TMR type sensor elements to an approximately equal surface coverage of Bangs 0.86 µm magnetic markers (the corresponding sensor elements are shown at identical scale in the electron micrographs of Figure 92). Both signal responses are compensated for drift and asymmetry according to the description in chapter 5.4.2 and constructed as differential measurements by subtracting the response of an uncovered reference sensor. The same is true for the displayed reference curves, which show the differential response of two uncovered GMR and TMR type sensor elements,
Chapter 7: Comparison of GMR- and TMR-type magnetic biosensors Comparison of GMR- and TMR-type magnetic biosensors
113 er 7:
113 respectively. The magnetoresistance effects are normalized to the original resistance state at zero perpendicular field for each sensor type. For the TMR type sensor, measurements are taken at an in-plane bias field of 800 A/m (compare to chapter 6.5.1). Due to the smaller total current in the TMR setup, the noise level is slightly higher, resulting in a little larger total reference XMR effect (0.0127 % compared to 0.0075 % for the GMR reference curve). Still, as the magnitude of the TMR sensor response to magnetic markers is about 3.6 times larger than for the GMR sensor, the sensitivity ratio (total signal normalized to the corresponding reference effect) reaches a value of about 32 for the TMR sensor and only approximately 15 for the GMR type sensor at this coverage of magnetic markers. Thus, our TMR sensor design is about twice as sensitive as our GMR type sensor under the applied measurement conditions.
respectively. The magnetoresistance effects are normalized to the original resistance state at zero perpendicular field for each sensor type. For the TMR type sensor, measurements are taken at an in-plane bias field of 800 A/m (compare to chapter 6.5.1). Due to the smaller total current in the TMR setup, the noise level is slightly higher, resulting in a little larger total reference XMR effect (0.0127 % compared to 0.0075 % for the GMR reference curve). Still, as the magnitude of the TMR sensor response to magnetic markers is about 3.6 times larger than for the GMR sensor, the sensitivity ratio (total signal normalized to the corresponding reference effect) reaches a value of about 32 for the TMR sensor and only approximately 15 for the GMR type sensor at this coverage of magnetic markers. Thus, our TMR sensor design is about twice as sensitive as our GMR type sensor under the applied measurement conditions.
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0,40 Bangs 0.86 µm microspheres TMR sensor (5.6 % coverage) GMR sensor (5.2 % coverage)
reference signal TMR GMR
XMR ratio [%]
out-of-plane field [kA/m]
20 µm sensor area
Figure 92: Comparison of the response of TMR and GMR type sensor elements to an approximately equal surface coverage of Bangs 0.86 µm magnetic microspheres Figure 92: Comparison of the response of TMR and GMR type sensor elements to an approximately equal surface coverage of Bangs 0.86 µm magnetic microspheres
Still, due to the limitations of the applicable measurement regime (see chapter 6.5), our TMR type sensors are not that much more sensitive as implied by the huge sensitivity to global in-plane fields parallel to the pinning direction (see Figure 91). As GMR type sensors are much easier to fabricate, allow the application of higher currents, are more robust to harsh environmental conditions and effectively show a comparable sensitivity, they are chosen as the most prominent sensor type for gene expression type experiments, which require the simultaneous large scale detection of different DNA sequences at relatively high concentrations well beyond the single molecule regime. Here, the required sensor size is of the order of a typical probe DNA spot, which can be realized easily by suitable GMR type sensor designs, and corresponding molecular detection experiments are described in chapter 1.
Still, due to the limitations of the applicable measurement regime (see chapter 6.5), our TMR type sensors are not that much more sensitive as implied by the huge sensitivity to global in-plane fields parallel to the pinning direction (see Figure 91). As GMR type sensors are much easier to fabricate, allow the application of higher currents, are more robust to harsh environmental conditions and effectively show a comparable sensitivity, they are chosen as the most prominent sensor type for gene expression type experiments, which require the simultaneous large scale detection of different DNA sequences at relatively high concentrations well beyond the single molecule regime. Here, the required sensor size is of the order of a typical probe DNA spot, which can be realized easily by suitable GMR type sensor designs, and corresponding molecular detection experiments are described in chapter 1.
While GMR type sensors are best applied for large scale molecular detection experiments, TMR sensors can be used for the detection of single molecules, which presents the ultimate objective both for medical and biotechnological applications as well as fundamental research. For magnetic biosensors, the goal of single molecule detection corresponds to the requirement of resolving the presence of single magnetic markers. This can be accomplished by shrinking the size of the sensor to the dimensions of the relevant labels (Ref. 109), i.e. to the sub-µm size scale. In this regime, it becomes increasingly demanding to build GMR type sensors with sufficient resistivity to allow an easy readout of the signals, which is due to the highly While GMR type sensors are best applied for large scale molecular detection experiments, TMR sensors can be used for the detection of single molecules, which presents the ultimate objective both for medical and biotechnological applications as well as fundamental research. For magnetic biosensors, the goal of single molecule detection corresponds to the requirement of resolving the presence of single magnetic markers. This can be accomplished by shrinking the size of the sensor to the dimensions of the relevant labels (Ref. 109), i.e. to the sub-µm size scale. In this regime, it becomes increasingly demanding to build GMR type sensors with sufficient resistivity to allow an easy readout of the signals, which is due to the highly
conductive all-metallic structure of this sensor type and the resulting low resistance.
Contrary, the resistance of TMR type sensors increases with decreasing area of the tunneling barrier, and elements with a resistance in the kΩ range have been demonstrated at sub-µm length scales for MRAM applications (Ref. 241). Thus, an ideal type of magnetic biosensor would consist of an array of small TMR sensor elements which give a logical yes/no type of output signal, depending on whether a single magnetic marker is present at the surface or not. By combining such sensors with small ferromagnetic labels in an in-plane sensing geometry (see chapter 3.3), it would be possible to actually use the large signals obtained from the switching of the free magnetic layer in single-pinned TMR junctions to generate this type of logical output signal. Combined with on-chip manipulation of molecules via magnetic gradient fields applied to their labels, such a system presents a very promising path towards the control of single molecules.
Chapter 8: DNA-detection and comparison to fluorescent method