section_path: Magnetic fields in a material
section_name: Magnetic fields in a material
paragraph_text: Magnetic fields in a material
  
  In vacuum,
  
  where μ is the vacuum permeability.
  
  In a material,
  
  The quantity μ M is called magnetic polarization.
  
  If the field H is small, the response of the magnetization M in a diamagnet or paramagnet is approximately linear:
  
  the constant of proportionality being called the magnetic susceptibility. If so,
  
  In a hard magnet such as a ferromagnet, M is not proportional to the field and is generally nonzero even when H is zero (see Remanence).