The interest in the magnetic properties of extremely small but still ferromagnetic particles is twofold:

• The study of magnetic properies of extremely small particles is of fundamental interest. These particles are close to their superparamagnetic limit.
• From an applications point of view it's an explicit way to higher data storage densities. The study of their properties is therefore of direct interest.

The iron pillars are grown by STM assisted chemical vapor deposition. They are grown directly onto a Hall magnetometer prepared by photolithography and wet chemical etching from a 2DES (GaAs/GaAlAs) to enable magnetic measurements [16,21]. The STM chamer is flooded by vaporous iron pentacarbonyl which decomposes at the STM's tip. As the particle grows the tip is retracted by keeping the tunneling current constant. If the desired height is reached the tip is retracted completely. Advantages: particle size (diameter 5 - 20 nm, height up to μm) and their arrangement (square, hexagonal, ...) can easily be controlled by steering the tip positioning of the particles with respect to the sensing Hall cross can be easily controlled; important for enabling the Hall measurements different substrate materials can be used: non-magnetic, magnetic, superconducting the particles are surrounded by a carbon oxide layer; no aging effects Disadvantage: particles grown one by one, time consuming

Note the different sizes, shapes and arrangements of the particles as well as the different substrate materials (the microSQUID was kindly supplied by W. Wernsdorfer). Some of the arrays were grown onto permalloy to investigate interactions between the particles' magnetization.