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Magnetic, Magnetoresistive, and Magnetostrictive Applications Intermetallic compounds are employed for a wealth of applications requiring both soft and hard magnetic materials. Soft magnetic materials are loosely defined as materials exhibiting a high value of the magnetic permeability /x and a low value of the coercivity Hc. High coercivity and large values of the maximum energy product [B • H]max are the defining characteristics of hard magnetic materials. Intermetallic compounds are also prevalent in applications based on magnetoresistivity and magnetostriction. Magnetoresistive materials must exhibit a change in bulk resistivity when the applied magnetic field changes. Magnetostriction involves the change in physical dimension of a material sample with the changing applied magnetic field. It is well-known that for soft magnetic materials, the required higher values of

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the magnetic permeability /x and lower values of the coercivity Hc are obtained from materials with low magnetocrystalline anisotropy Kt and saturation magnetostriction As. For the ordered A3B and AB phases in intermetallic compounds, all of these quantities depend strongly on the degree of crystalline order in the material. For example Bozorth showed that rapidly cooling Ni3Fe from above 600°C (above the order-disorder transition temperature) reduced the crystalline order in the material, thereby minimizing K\ and As.2 This material (Permalloy) was shown to exhibit very high permeability, typically 104-105, and was found to be improved by the addition of Mo (Supermalloy) and Nb (Nb-Permalloy). Also Masumoto et al. discovered that high permeability could be obtained in an Fe3Al phase (Alperm or Alfenol).3 Fe3Si and Fe3Al are completely miscible and exhibit the same crystal structure, and the ordered Fe(Al,Si) phase can exhibit permeabilities as high as /a, = 3.0 X 104 and /xmax = 1.2 X 105. This alloy was named Sendust by Masumoto in 1936 because the extreme brittleness of the alloy made it easy to pulverize. In 1929 the product Permendur (Fe-Co) was developed by Elmen based on the appearance of a peak permeability at the equiatomic composition of ordered FeCo.4 Soft magnetic materials are often used for example as magnetic cores in electrical devices, magnetic shields, and magnetic recording heads. Recently the development of soft magnetic materials in the form of very small particles and thin films has become of interest as well. Most hard magnetic materials (also

called permanent magnetic materials) are based on intermetallic compounds. As a result, they tend to be mechanically hard and brittle, and cannot be coldworked into forms required for most practical applications. While several ductile hard magnets exist (Cunife and Vicalloy for example), the coercivity of these materials tends to be low, originating from the magnetocrystalline anisotropy of the individual magnetic sublattices in the material. The Alnico alloy is a very useful hard magnet. This material contains rodshaped precipitated particles of a-Fe, which lead to shape-induced