A device for controlling a magnetic flux in an electro- magnetic system, wherein the system comprises magnetically connected magnetic cores, between which a volume is arranged. The volume comprises a controllable magnetic flux region and the magnetic flux region contains a magnetic material that exhibits a relative permeability that may be varied by influencing the temperature of the material. The magnetic material contains a magnetic material, the Curie point of which lies within the temperature operating range of the device and exhibits paramagnetic properties within said temperature range. Also a method of controlling a magnetic flux is described.