Coercivity Mechanism in Hard and Soft Sintered Magnetic Materials

Abstract

The coercivity in soft and hard magnetic materials has different origin. The high coercivity of barium ferrite, SmCo 5 , Sm 2 Co 17 or Nd 2 Fe 14 B is due to high magnetocrystalline anisotropy, and the processing aims very small grain size (nanocrystalline). In the case of soft magnetic materials, the coercivity has origin in defects that are able to stop domain wall movement, as for example grain boundaries, inclusions or dislocations. Soft magnetic materials in general present large domain wall thickness (thousands of Angstroms for pure iron), whereas domain wall thickness is ~ 50 Angstroms for SmCo 5 and Nd 2 Fe 14 B. The differences between hard and soft magnetic behavior are commented and discussed. The domain wall energy and thickness can be used as parameters for classifying soft and hard magnetic behavior. Other examples of soft magnetic materials are the amorphous alloys and the nanocrystalline soft magnetic materials with grain size very below the single domain particle size. The soft behaviour in amorphous and soft nanocrystalline materials is also discussed.