A synthetic crystalline ferrite containing yttrium and iron (Y3Fe6012). If a single-crystal sphere of YIG is immersed in a magnetic field, and RF energy is coupled into it via a magnetic loop, the crystal will resonate at a frequency linearly proportional to the magnetic field strength.

In practical YIG-tuned oscillators and filters, the magnetic field is derived from an electromagnet and the resonant frequency of the YIG sphere is proportional to the current flowing through the magnetic coil.