Highcoercivity

High coercivity refers to magnetic materials that resist demagnetization by external magnetic fields. Coercivity, Hc, is the reverse field required to reduce the material’s magnetization to zero after it has been saturated. Materials with high Hc are called hard magnets or permanent magnets, as opposed to soft magnets that exhibit low coercivity and fast magnetization changes.

High coercivity arises from a combination of intrinsic and extrinsic factors. Intrinsic factors include large magnetocrystalline anisotropy energy and high saturation magnetization. Extrinsic factors involve microstructure: refined grain size, strong crystallographic texture, and effective pinning of domain walls or suppression of reverse-domain nucleation. Processing techniques such as grain-boundary diffusion, doping with heavy rare-earth elements (for example dysprosium or terbium in NdFeB magnets), and controlled cooling are used to enhance Hc, particularly at elevated temperatures. Temperature generally reduces coercivity, so designing for thermal stability is important.

Common high-coercivity materials include NdFeB magnets, SmCo magnets, and, historically, Alnico and ferrites in certain grades. NdFeB offers very high energy density but can be sensitive to temperature; Dy and Tb diffusion increases coercivity and temperature stability. SmCo provides excellent temperature stability and corrosion resistance but is more expensive and supply-sensitive.

Applications rely on high Hc to maintain magnetization in strong opposing fields and temperatures, including electric motors for automotive and industrial drives, wind turbines, generators, magnetic couplings, and magnetic storage components. Coercivity is a key parameter in the overall energy product BHmax, and remanence and coercivity must be balanced for a given application.