The clusters are island or pancake shaped. They tend to be about 3 monolayers high and the lateral size can be 3-80 nanometers. The micrograph below shows what happens when the islands are grown at different temperatures. The islands appear as white and this image is a cross sectional slice through two layers. It is clear that the upper islands are bigger than the lower ones.
Electrons move through the material by hoping from island to island.
At high temperatures the electrical properties are not very different than that of a semiconductor with impurities. Electrons themselves are very small magnets and can interact with the magnetic island. In the case of these ErAs islands, the electron's magnet and the magnetism of ErAs island line up in the same direction. This makes it harder for the electron to leave the island. If the electron leaves the island it is in, it has to cause the magnetism of it's new island home to line up with it's own magnet.