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(a) SEM image of the central region of an InAs quantum well micro-Hall sensor with six Hall crosses and a superparamagnetic bead positioned on one of them. The Hall cross size was estimated from SEM analysis to be ; (b) typical Langevin response of a superparamagnetic bead to an applied magnetic field and a sketch of the physical principle underlying the detection method. , , and represent rms values of the corresponding ac quantities.
(a) SEM image of two adjacent Hall crosses adapted to show the actual detection measurement configuration; (b) ac Hall voltage as a function of time for the two crosses shown in part (a) of the figure. The drop in the signal from one cross upon applying the static field is due to the presence of the bead. The direct response of the Hall cross to the ac excitation field was eliminated with a zero offset on the lock-in amplifier.
(a) SEM image of two adjacent Hall crosses adapted to show the detection circuit in the Hall gradiometry configuration; (b) Hall voltage signal as a function of time from the Hall gradiometer shown in (a): The reference current applied to balance the gradiometer was .
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