Much stronger magnetic fields can be produced if a " magnetic core " of a soft ferromagnetic (or ferrimagnetic) material, such as iron, is placed inside the coil. A core can increase the magnetic field to thousands of times the strength of the field of the coil alone, due to the high magnetic permeability μ of the material. Not all electromagnets use cores, so this is called a ferromagnetic-core or iron-core electromagnet.