Magnet
A permanent magnet is an object made from a material that is magnetized and creates its own persistent magnetic field. An everyday example is a refrigerator magnet used to hold notes on a refrigerator door. Materials that can be magnetized, which are also the ones that are strongly attracted to a magnet, are called ferromagnetic (or ferrimagnetic). These include the elements iron, nickel and cobalt, some alloys of rare-earth metals, and some naturally occurring minerals such as lodestone. Although ferromagnetic (and ferrimagnetic) materials are the only ones attracted to a magnet strongly enough to be commonly considered magnetic, all other substances respond weakly to a magnetic field, by one of several other types of magnetism.
Terahertz metamaterial
Currently, a fundamental lack in naturally occurring materials that allow for the desired electromagnetic response has led to constructing new artificial composite materials, termed metamaterials. The metamaterials are based on a lattice structure which mimics crystal structures. However, the lattice structure of this new material consists of rudimentary elements much larger than atoms or single molecules, but is an artificial, rather than a naturally occurring structure. Yet, the interaction achieved is below the dimensions of the terahertz radiation wave. In addition, the desired results are based on the resonant frequency of fabricated fundamental elements. The appeal and usefulness is derived from a resonant response that can be tailored for specific applications, and can be controlled electrically or optically. Or the response can be as a passive material.