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The Scanning TMR Microscope for Biosensor Applications.

Vyas KN, Love DM, Ionescu A, Llandro J, Kollu P, Mitrelias T, Holmes S, Barnes CH - Biosensors (Basel) (2015)

Bottom Line: We present a novel tunnel magnetoresistance (TMR) scanning microscope set-up capable of quantitatively imaging the magnetic stray field patterns of micron-sized elements in 3D.By incorporating an Anderson loop measurement circuit for impedance matching, we are able to detect magnetoresistance changes of as little as 0.006%/Oe.By 3D rastering a mounted TMR sensor over our magnetic barcodes, we are able to characterize the complex domain structures by displaying the real component, the amplitude and the phase of the sensor's impedance.

View Article: PubMed Central - PubMed

Affiliation: Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, CB3-0HE Cambridge, UK. kunalnvyas@gmail.com.

ABSTRACT
We present a novel tunnel magnetoresistance (TMR) scanning microscope set-up capable of quantitatively imaging the magnetic stray field patterns of micron-sized elements in 3D. By incorporating an Anderson loop measurement circuit for impedance matching, we are able to detect magnetoresistance changes of as little as 0.006%/Oe. By 3D rastering a mounted TMR sensor over our magnetic barcodes, we are able to characterize the complex domain structures by displaying the real component, the amplitude and the phase of the sensor's impedance. The modular design, incorporating a TMR sensor with an optical microscope, renders this set-up a versatile platform for studying and imaging immobilised magnetic carriers and barcodes currently employed in biosensor platforms, magnetotactic bacteria and other complex magnetic domain structures of micron-sized entities. The quantitative nature of the instrument and its ability to produce vector maps of magnetic stray fields has the potential to provide significant advantages over other commonly used scanning magnetometry techniques.

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(a) The Anderson loop measurement circuit, developed by NASA [19], is a precision current source used to supply a series of impedances, Z1 to Zn, and a reference impedance, Zref . The potential difference across each impedance is amplified by a set of instrumentation amplifiers. The reference signal is subtracted using a set of differencing amplifiers to give us a series of outputs, proportional to the difference between the reference impedance and each measured impedance, respectively; (b) Block diagram showing how all of the various pieces of equipment are connected. Note that the lock-in frequency can be defined by the Keithley power source using a trigger line or by its own internal oscillator.
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biosensors-05-00172-f002: (a) The Anderson loop measurement circuit, developed by NASA [19], is a precision current source used to supply a series of impedances, Z1 to Zn, and a reference impedance, Zref . The potential difference across each impedance is amplified by a set of instrumentation amplifiers. The reference signal is subtracted using a set of differencing amplifiers to give us a series of outputs, proportional to the difference between the reference impedance and each measured impedance, respectively; (b) Block diagram showing how all of the various pieces of equipment are connected. Note that the lock-in frequency can be defined by the Keithley power source using a trigger line or by its own internal oscillator.

Mentions: Figure 2a shows how a generalised Anderson loop measurement system works. In our pre-amplifier design, a Keithley 6221 AC precision current source is used to supply a small current (typically 1.5 µA) through our Anderson loop, which contains a reference impedance and up to four TMR sensors. The potential difference across the reference impedance and any two of the sensors connected is amplified by a set of instrumentation amplifiers, which amplify the potential difference between two inputs, as illustrated in Figure 2a. After the amplification stage, the reference signal is subtracted from the sensor signal using an op-amp (INA105OKP) before being passed on to our lock-in amplifier. The reference is subtracted in order to negate any fluctuations in the current passing through our loop and to provide as small a background level as possible for any signals that we detect. The impedances of the purchased TMR sensors, from Micro Magnetics Inc. (http://www.micromagnetics.com), vary hugely, typically from 5 to 35 kΩ; therefore, we have incorporated the ability to switch between five different reference impedances: Zref = 0.1/2.1/5.1/10.1/20.1 kΩ. Whilst the reference signal has a fixed amplification of 10×, the sensor amplification can be adjusted to match the reference signal amplitude through the tuning of a variable resistor, so that two sensors with differing impedances can be matched with the same reference impedance. In our pre-amp, the variable resistor can be tuned from 1 to 6 kΩ, corresponding to a gain range of 21–4.3, respectively. This effectively means that we can match an impedance that is between half as big and twice as big as each reference impedance.


The Scanning TMR Microscope for Biosensor Applications.

Vyas KN, Love DM, Ionescu A, Llandro J, Kollu P, Mitrelias T, Holmes S, Barnes CH - Biosensors (Basel) (2015)

(a) The Anderson loop measurement circuit, developed by NASA [19], is a precision current source used to supply a series of impedances, Z1 to Zn, and a reference impedance, Zref . The potential difference across each impedance is amplified by a set of instrumentation amplifiers. The reference signal is subtracted using a set of differencing amplifiers to give us a series of outputs, proportional to the difference between the reference impedance and each measured impedance, respectively; (b) Block diagram showing how all of the various pieces of equipment are connected. Note that the lock-in frequency can be defined by the Keithley power source using a trigger line or by its own internal oscillator.
© Copyright Policy
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4493544&req=5

biosensors-05-00172-f002: (a) The Anderson loop measurement circuit, developed by NASA [19], is a precision current source used to supply a series of impedances, Z1 to Zn, and a reference impedance, Zref . The potential difference across each impedance is amplified by a set of instrumentation amplifiers. The reference signal is subtracted using a set of differencing amplifiers to give us a series of outputs, proportional to the difference between the reference impedance and each measured impedance, respectively; (b) Block diagram showing how all of the various pieces of equipment are connected. Note that the lock-in frequency can be defined by the Keithley power source using a trigger line or by its own internal oscillator.
Mentions: Figure 2a shows how a generalised Anderson loop measurement system works. In our pre-amplifier design, a Keithley 6221 AC precision current source is used to supply a small current (typically 1.5 µA) through our Anderson loop, which contains a reference impedance and up to four TMR sensors. The potential difference across the reference impedance and any two of the sensors connected is amplified by a set of instrumentation amplifiers, which amplify the potential difference between two inputs, as illustrated in Figure 2a. After the amplification stage, the reference signal is subtracted from the sensor signal using an op-amp (INA105OKP) before being passed on to our lock-in amplifier. The reference is subtracted in order to negate any fluctuations in the current passing through our loop and to provide as small a background level as possible for any signals that we detect. The impedances of the purchased TMR sensors, from Micro Magnetics Inc. (http://www.micromagnetics.com), vary hugely, typically from 5 to 35 kΩ; therefore, we have incorporated the ability to switch between five different reference impedances: Zref = 0.1/2.1/5.1/10.1/20.1 kΩ. Whilst the reference signal has a fixed amplification of 10×, the sensor amplification can be adjusted to match the reference signal amplitude through the tuning of a variable resistor, so that two sensors with differing impedances can be matched with the same reference impedance. In our pre-amp, the variable resistor can be tuned from 1 to 6 kΩ, corresponding to a gain range of 21–4.3, respectively. This effectively means that we can match an impedance that is between half as big and twice as big as each reference impedance.

Bottom Line: We present a novel tunnel magnetoresistance (TMR) scanning microscope set-up capable of quantitatively imaging the magnetic stray field patterns of micron-sized elements in 3D.By incorporating an Anderson loop measurement circuit for impedance matching, we are able to detect magnetoresistance changes of as little as 0.006%/Oe.By 3D rastering a mounted TMR sensor over our magnetic barcodes, we are able to characterize the complex domain structures by displaying the real component, the amplitude and the phase of the sensor's impedance.

View Article: PubMed Central - PubMed

Affiliation: Cavendish Laboratory, Department of Physics, University of Cambridge, JJ Thomson Avenue, CB3-0HE Cambridge, UK. kunalnvyas@gmail.com.

ABSTRACT
We present a novel tunnel magnetoresistance (TMR) scanning microscope set-up capable of quantitatively imaging the magnetic stray field patterns of micron-sized elements in 3D. By incorporating an Anderson loop measurement circuit for impedance matching, we are able to detect magnetoresistance changes of as little as 0.006%/Oe. By 3D rastering a mounted TMR sensor over our magnetic barcodes, we are able to characterize the complex domain structures by displaying the real component, the amplitude and the phase of the sensor's impedance. The modular design, incorporating a TMR sensor with an optical microscope, renders this set-up a versatile platform for studying and imaging immobilised magnetic carriers and barcodes currently employed in biosensor platforms, magnetotactic bacteria and other complex magnetic domain structures of micron-sized entities. The quantitative nature of the instrument and its ability to produce vector maps of magnetic stray fields has the potential to provide significant advantages over other commonly used scanning magnetometry techniques.

Show MeSH
Related in: MedlinePlus