We develop new methods and instrumentation to quantitatively measure the magnetic moment and magnetic field strength produced by individual magnetic nanostructures.

Double-cross Hall sensor
with an active area
of 300×300 nm²

In order to detect magnetisation changes in a single magnetic particle at room temperature we develop nano-size Hall sensors (down to 200 nm bar size) exploring their main advantages: a high sensitivity (<1 µΤ/√Hz) and linear response to a strong magnetic field (up to 1 T). In this work we use 2DEG heterostructures [1], epitaxial InSb layers and metallic dicobalt octacarbonil probes exploring a strong extraordinary Hall effect. The work is performed in collaboration with NTT (Japan), Imperial College London (UK) and EMPA (Switzerland).

Nb SQUID with the loop size
of 300 nm and the
constriction width of 60 nm

Sub-micron SQUID devices, called 'nano-SQUIDs', are developed in order to perform low temperature measurements of small magnetic moments. We have pioneered the combination of modern fabrication techniques to achieve the required small SQUID loop inductance in Nb and Al. Using this type of SQUIDs, realisation of a sensor having an active area down to 50 nm and able to measure a minimum particle moment as small as m_min ~ 10² μ_B/√Hz is feasible [2-4].

A CryoAFM capable of low-temperature (down to 1 K) and high-vacuum operation, based on a tuning fork sensor and offering a spatial resolution of 10 nm is under development. The microscope will be capable of working in the magnetic-force mode on its own, or in combination with the Hall and SQUID nano-sensors.

Ultra-High vacuum
variable-temperature STM

Spin polarised STM combines high spatial resolution (down to the atomic scale) with a sufficient degree of magnetic sensitivity. This method exploits the quantum tunnelling effect between a magnetic tip and sample to measure topography, electronic structure and magnetic properties of the system. By using thin ferromagnetic or antiferomagnetic coatings of the tip, the magnetic dipole interaction between the sample and tip can be considerably reduced, thereby leading to more accurate magnetic maps of the surfaces. By choosing tips with either perpendicular or in-plane magnetisation, we can determine the 3D distribution of magnetic fields. The high spatial resolution allows studying 'fine' magnetic structures such as vortex cores, domain walls as well as band gaps in various magnetic materials.

References

[1] O. Kazakova, et al. Optimization of 2DEG InAs/GaSb Hall Sensors for Single Particle Detection. IEEE Tran. on Magn. 44, 4480 (2008).
[2] J. Gallop. SQUIDs: some limits to measurement. Superconductor Science &amp; Technology 16, 1575 (2003).
[3] J. Gallop et al. Miniature dc SQUID devices for the detection of single atomic spin-flips. Physica C 368, 109 (2002).
[4] L. Hao et al. Measurement and noise performance of nano-superconducting-quantum-interference devices fabricated by focused ion beam. Appl. Phys. Lett. 92, 192507 (2008).