Flux pinning is the phenomenon where a magnet's lines of force (called flux) become trapped or "pinned" inside a superconducting material. This pinning binds the superconductor to the magnet at a fixed distance. Flux-pinning is only possible when there are defects in the crystalline structure of the superconductor (usually resulting from grain boundaries or impurities). Flux-pinning is desirable in high-temperature ceramic superconductors in order to prevent "flux-creep", which can create a pseudo-resistance and depress critical current density and critical field.