Permanent magnets retain their magnetic properties once any external magnetising field has been removed. Materials that exhibit this behaviour include iron, nickel and cobalt. Some of the strongest magnetic fields can be achieved with alloys such as samarium cobalt (SmCo) or neodymium iron boron (NdFeB). Electromagnetic fields are generated around any current carrying wire. Such fields can be switched on and off and tuned depending on the applied current. Furthermore, time-varying fields can be obtained with alternating currents (ac). Care has to be taken to insulate the wires and to avoid excessive heating at high currents. Strong magnetic fields can be achieved by using an electromagnet with a highly permeable core material such as soft iron or mMetal.