Magnetic sensors, having high spatial and stray field resolutions, are key elements in many biomedical applications. One of the most promising ultra-sensitive and non-invasive magnetic detectors is a microsized Hall sensor. We fabricate and study linear arrays of Hall sensors (down to 5 x 5 µm²) made of an asymmetric 2DEG InAs/GaSb-based heterostructure. In our research we investigate magnetotransport properties of Hall sensors and show the route to optimisation of their performance [1]. In particular, we focus on examining noise characteristics of the sensor as it allows us to determine and improve the device sensitivity. We show that in investigated devices a magnetic field sensitivity of better than 0.5 µT/Hz^1/2 (corresponding to a magnetisation detection threshold of 2 x 10⁵ µB/Hz^1/2) is readily achievable at room temperature and at a frequency of around 3 kHz. The research is performed in collaboration with NTT Basic Research Laboratories, Japan.

(a) 100 x 20 µm2 Hall device with an attached single magnetic bead. (b) FIB-modified Hall device with the effective size 5 x 5 µm2. One FIB track is emphasised by a dashed contour for visibility.

(c) Hall voltage vs applied magnetic field for  InAs/GaSb Hall sensors.

Further minimisation of Hall sensors was achieved using epitaxial InSb thin films. Double-cross devices with the size 1.5 x 1.5 µm² have been fabricated using optical lithography and reactive ion etching. These devices have a high Hall coefficient, R_H ≈ 1000 V/AT. The adjacent crosses are characterised by very similar transport properties, with the difference between their Hall coefficients in the order of 1.5%. It demonstrates that such double-crosses can be successfully used for single magnetic particle detection, when a bead is placed on one of the crosses while the other one is used as a reference. The work is done in collaboration with Imperial College London (UK).

(a) Double cross InSb Hall sensor.
(b) Field dependence of the Hall voltage for two adjacent crosses (filled and open symbols).
(c) Rms voltage noise spectral density as a function of frequency for different bias currents, I_H = 0 – 70 µA.

We study the possibility of implementing a Hall sensor as an active part of a scanning probe microscope for detection of individual magnetic beads. The key parameter for bead detection is not simply the raw sensitivity of the sensor but the field resolution in the presence of large applied (polarising) background fields. The scanned micro-Hall probe sensor is particularly well adapted to measuring a very small field change of a few mT superimposed on a uniform background magnetic field of tens of µT. We have demonstrated that a simple scanned Hall probe system is capable of rapid detection of single paramagnetic beads down to 1 micron in size over a linear scan range more than 10⁴ times greater. The system has considerable potential for applications in the integration of nanoscale magnetics and molecular biology, promising applications in fundamental study of biological interactions as well as utilities in bioanalysis and biomedical applications. The work is done in collaboration with Imperial College London (UK).

(a) Magnetic image of a distribution of 1 µm magnetic beads in Si channels.
The image obtained using scanning Hall microscope. Scan size is 100 µm.
(b) AFM image of the sample with the scan size of 20 µm.

References

[1] O. Kazakova, et al. Optimization of 2DEG InAs/GaSb Hall Sensors for Single Particle Detection. IEEE Tran. on Magn. 44, 4480 (2008).