Effects such as GMR occur in artificial multilayer structures. Here two magnetic layers, for example of iron, communicate (couple) with each other via an intermediate film. In the case of GMR, the mid-layer consists of a nonferromagnetic, electrically conducting material, for instance chromium. This layer has to be only a few nanometres in thickness and its interfaces must be of the very best quality. Such a "sandwich" structure changes its resistance by more than 50 percent while exposed to an external magnetic field. GMR stacks are found in highly sensitive magnetic field sensors, for example for reading out computer hard disks. In future these structures will be employed in anti-lock brake systems and in many other automotive applications or to read out biochips.