Hard and Soft Magnets
Hard and Soft Magnets
A hard/permanent magnet can alone support a magnetic flux in a gap of a device; a soft magnet requires the assistance of an external electrical/magnetic input to do so. A hard/permanent magnet requires that MHc > Ms, the field it produces in a gap is proportional to its energy density product, (BH), and the demagnetizing portion of the hysteresis loop determines its performance for the intersection of the load line with the demagnetizing curve (matching magnet shape with materials selection) maximizing its energy product, (BH)max. Permanent magnets have maximum coercive fields well below the theoretical limit (Brown’s paradox), due to the role of microstructure in determining the magnetization reversal process; nevertheless, permanent magnets are developed by maximizing all three parameters, Ms, HC, and K1, as well as TC, to achieve high operating temperatures. Thus, the best permanent magnets contain RE elements for the localized, 4f-electron, single-ion anisotropy, and transition metals (TM) for their higher TC and higher Ms. Soft magnets require large saturation magnetization and initial permeability, small coercive forces, and low hysteresis and eddy current losses. To maximize magnetization, look to the Slater–Pauling curve and work around variations of Fe–Co alloys. Many soft magnets also require very small magnetostriction to avoid related mechanical stress cycling; examples are crystalline alloys of Fe–Si and Fe–Ni, and amorphous alloys of (Fe–Co–Ni) with (Si, B). Nanostructured materials with grain size, D, smaller than the exchange correlation length, lead to an effective directional averaging of the magnetocrystalline anisotropy and their coercivity scales as D6. Intrinsic magnetization may be augmented by the exchange-spring mechanism, using a composite microstructure that benefits from the best attributes of the constituent phases; the hard phase furnishes the high Hc and K1 values, while the soft phase contributes a large Ms value.
Keywords: demagnetizing field, energy density product, Brown’s paradox, hard magnets, soft magnets, eddy current losses, nanostructured soft magnets, RE-TM permanent magnets, nanocrystalline materials
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