Demagnetizing effects in granular hard magnetic bodies.

Hard magnetic materials are usually measured and used in an open magnetic circuit, for which demagnetizing effects are present. The applied field Ha has been conventionally corrected to the volume averaged internal field H by a formula H=Ha-NmM, where M is volume averaged magnetization and Nm is magnetometric demagnetizing factor calculated for the case of uniform magnetization as a function of body shape. Since Nm>0, such a correction always increases the slope of the descending M(Ha) curve without influencing the value of the coercive field with respect to M. However, we show by numerical calculations that if the hard magnetic material is granular and modeled as a rectangular prism, the magnetostatic interactions among the grains will result in a formula H=Ha-NghmM+ΔHc, where the demagnetizing factor for granular hard magnets Nghm can be either positive or negative and the usually positive coercivity increment ΔHc is correlated to collective magnetic reversal of longitudinal grain chains. [ABSTRACT FROM AUTHOR]