We review our investigations on nanostructured ferromagnets fabricated on semiconductor heterostructures. We show that ballistic electrons in the semiconductor are in particular sensitive to local stray fields. The reversal of the ferromagnets give rise to characteristic magnetoresistance traces in the ferromagnet/semiconductor hybrid structures. We use these stray-field effects for detailed studies on the magnetic properties of rectangular micromagnets, nanodisks and nanorings. The nanostructures might be favorable candidates for ferromagnetic contacts in future spin-injection experiments in lateral feromagnet/semiconductor hybrid structures.
