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Single nanomagnets made visible

Smallest magnetic particles are newly deployed in medicine for diagnostics and therapy. In particular, the magnetic properties of so-called magnetosomes – magnetite particles encased by a lipid membrane – are of interest for these applications. PTB has now succeeded in imaging these very small particles with a size of approx. 40 nm and in showing that single particles really behave like nanomagnets.

Image of single magnetosomes taken with the magnetic force microscope. On the left: topographic image, on the right: magnetic field.

Besides its significance for data storage technology, nanometrology for magnetic quantities is also increasingly gaining importance in medicine. At PTB, a magnetic force microscope has been further developed for this field. This microscope scans line by line with an extremely sharp magnetic tip. In a first step, it scans the surface of a sample and in a second cycle the magnetic field at a small distance to this surface. The radius of the stylus tips is smaller than 20 nm. This set up allows investigating a great variety of sample geometries at highest resolution. Biological samples can also be analyzed under ambient conditions.

Complete magnetosome ensembles were manufactured at the Max Planck Institute for Microbiology in Bremen. The magnetic properties were measured with the aid of spatially integrating measuring techniques. Finally, single nanoparticles have now been investigated with PTB's magnetic force microscope. Before the measurements were performed, the samples were magnetized perpendicular to the sample plane. In a surface image several single particles with a diameter of approximately 40 nm can be recognized as light spots. These are single magnetosomes.

In the image of the magnetic field, the particles appear as white spots surrounded by a darker ring. This appearance corresponds to that of a bar magnet standing perpendicular to the surface, its pole being viewed from above. This demonstrates for the first time that single magnetosomes are single-domain nanomagnets. This insight opens up new fields of application: contrary to conventional contrast agents, the particles, which also reach smallest capillaries, can be directed to any place in the body with the aid of magnetic fields. This considerably increases the spatial resolution when contrast agents are used and also improves the therapeutic effectiveness.

Contact at PTB:

Phone: +49-531-592-0