A magnetic core is made by ferromagnetic metal or ferrimagnetic compounds, the core with a high magnetic permeability, which popular used to confine and guide magnetic fields in electrical, electromechanical and magnetic devices. The magnetic field created by a current-carrying coil of wire around the core.

Comparing no magnetic core, use a magnetic core can increasing the strength of magnetic field in an electromagnetic coil by a factor of several hundred times .

However, considering the core loss, "Soft" magnetic materials with low coercivity and hysteresis, such as amorphous core, nanocryatalline, are usually used in cores.

During the  side effects, such as eddy currents and hysteresis can cause frequency-dependent energy losses, different working frequencies request use different core materials.

Vitreous metal

Amorphous metal is a variety of alloys (e.g. Metglas) that are non-crystalline or glassy. The materials with highly responsive to magnetic fields for low hysteresis losses, and they can also have lower conductivity to reduce eddy current losses. what's more, high mechanical strength and corrosion resistance are also positive for this application.  amorphous metail great for create high-efficiency transformers.

Nanocrystalline

Nanocryatalline alloy is a standard iron-boron silicon alloy, with addition of few amounts of copper and niobium. The powder grain sizes reacheds down to 10~100 nanometers. Nanocryatalline material has excelent performance at lower frequencies, such as: used in chokes for inverters and in high power applications.