Use of an electromagnet 11 with a magnetic core 3 formed from ferromagnetic or ferrimagnetic material to emit a controlled magnetic field is disclosed, possibly for the purpose of mine sweeping. When the electromagnet is not in use the magnetic core of the electromagnet is stored at a temperature above the Curie temperature of the magnetic core, thus de-magnetising the core. The core may be formed of a single crystal ferrite and may have a Curie temperature that lies in the range of 50˚C to 100˚C. A storage means, used for storing the core, may form part of a mine countermeasures vessel (51, Fig 1). A method of storing an electromagnet by heating the core to a temperature above the Curie temperature of the magnetic core material is also disclosed. An electromagnet with a magnetic core formed from ferromagnetic or ferrimagnetic material with a Curie temperature that lies in the range of 0˚C to 100˚C is also disclosed.