Permeability:
Permeability, represented by the lowercase Greek mu (µ), is measured on a scale associative to a vacuum or free space. The perfect vacuum is allocated, by convention, a permeability figure of precisely 1. When current is forced via a wire loop or coil in air, then the flux density within and about the coil is around similar as it would be in a vacuum. And hence, the permeability of pure air is about equivalent to 1. When you place an iron core in the coil, the flux density raises by a factor ranging from a few dozen to some thousand times, based on the purity of the iron. The permeability of iron is as low as approximately 60 (impure) to as high as around 8,000 (i.e., highly refined).
When you use particular metallic alloys known as permalloys as the core material in electromagnets, you can increase the flux density, so the local strength of the field, by as much as 1 million (106) times. These substances therefore have permeability as great as 106.
When, for any reason, you feel forced to make an electromagnet that is as weak as possible, you can employ dry wood or wax for the core material. Generally, though, diamagnetic substances are used to keep magnetic objects separately while minimizing the interaction among them.