Hard-Magnetic Materials

(also high-coercivity materials), magnetic materials that are magnetized to saturation and experience a reversal in polarity in comparatively strong magnetic fields, with an intensity of thousands or tens of thousands of amperes per meter (10² -10³ oersteds). Hard-magnetic materials are characterized by high values of the coercive force H_c, residual induction B_r, and magnetic energy (BH )_max in the demagnetization region—the back of a hysteresis loop (see Table 1). After magnetization, hard-magnetic materials remain permanent magnets because of the high values of B_r and H_c. The large coercive force of hard-magnetic materials may be caused by restraint of the displacement of domain boundaries because of the presence of foreign inclusions or strong deformation of the crystal lattice or by the dropping out in a weakly magnetic matrix of small single-domain ferromagnetic particles that have strong crystalline anisotropy or form anisotropy.

Hard-magnetic materials are classified according to various characteristics, such as industrial characteristics and the physical nature of the coercive force. The hard-magnetic materials that have acquired the greatest importance in technology include foundry and powder (nonwrought) magnetic materials of the Fe-Al-Ni-Co type; wrought alloys of the Fe-Co-Mo, Fe-Co-V, and Pt-Co types; and ferrites (hexaferrites and cobalt ferrite). Also used as hard-magnetic materials are compounds of rare earths (particularly light elements) with cobalt; magnetoplastics and magnetoelastics made of powders of Alni, Alnico, and ferrites with a plastic and rubber bond; and materials made of powders of Fe, Fe-Co, Mn-Bi, and SmCo₅.

The high coercive force of foundry and powder hard-magnetic materials (which include materials such as Alnico and Magnico) is caused by the presence of finely divided strongly magnetic particles of elongated shape in a weakly magnetic matrix. Cooling in a magnetic field leads to preferential orientation of the longitudinal axes of the particles. Similar hard-magnetic materials that are single crystals or alloys created by directed crystallization have increased magnetic properties; their maximum energy (BH)_max reaches 10⁷ gauss · oersted. Hard-magnetic materials such as Fe-Al-Ni-Co are very hard; they can be worked only with an abrasive tool or by the spark-erosion method and can be bent at high temperatures. Products made of such hard-magnetic materials are manufactured by shape casting or powder metallurgy.

Wrought alloys (the most important of which are the Comols and Vicalloys) are more plastic and submit much more readily to mechanical working. Age-hardenable alloys such as Fe-CoMo (Comols) enter a state of high coercivity (magnetic hardness) as a result of tempering after hardening. During tempering dissociation of the solid solution takes place, and the molybdenum-rich phase is separated. Alloys such as Fe-Co-V (Vicalloys) are subjected to cold plastic deformation with great cogging and subsequent tempering to impart the properties of hard-magnetic materials. The high-coercivity state of alloys of the Pt-Co type arises because of the appearance of an ordered tetragonal phase having an energy of anisotropy of 5 X 10⁷ ergs/cm³. The permanent magnets used in measuring instruments (such as DC ammeters and voltmeters), micromotors and hysteresis electric motors, and clock mechanisms are manufactured from hard-magnetic foundry, powder, and wrought materials. The hexaferrites, that is, ferrites with a hexagonal crystal lattice (such as BaO·6Fe 2O₃ and SrO·6Fe₂CO₃) are hard-magnetic materials. In addition to hexaferrites, the ferrite of cobalt, CoO·Fe₂O₃ (which has a spinel structure), in which uniaxial anisotropy—the cause of high coercive force—is formed in the magnetic field after heat treatment, is used as a hard-magnetic material. Hard-magnetic ferrites are used under sparse magnetic fields and in the UHF band. Products made of ferrites are manufactured using methods of powder metallurgy.