A magnetic core is a piece of magnetic material with a high permeability used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices such as electromagnets, transformers, electric motors, generators, inductors, magnetic recording heads, and magnetic assemblies. It is made of ferromagnetic metal such as iron, or ferrimagnetic compounds such as ferrites. The high permeability, relative to the surrounding air, causes the magnetic field lines to be concentrated in the core material. The magnetic field is often created by a coil of wire around the core that carries a current. The presence of the core can increase the magnetic field of a coil by a factor of several thousand over what it would be without the core.The use of a magnetic core can enormously concentrate the strength and increase the effect of magnetic fields produced by electric currents and permanent magnets. The properties of a device will depend crucially on the following factors:
the geometry of the magnetic core.
the amount of air gap in the magnetic circuit.
the properties of the core material (especially permeability and hysteresis).
the operating temperature of the core.
whether the core is laminated to reduce eddy currents.
In many applications it is undesirable for the core to retain magnetization when the applied field is removed. This property, called hysteresis can cause energy losses in applications such as transformers. Therefore, 'soft' magnetic materials with low hysteresis, such as silicon steel, rather than the 'hard' magnetic materials used for permanent magnets, are usually used in cores.
https://en.wikipedia.org/wiki/Magnetic_core